mysterious fuel evaporation
#1
06-02-2002, 04:25 PM
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I would like to draw your attention and ask for your comments regarding a puzzling phenomenon that I have encountered while using my Sportsman 700 , I usually fill my ATV's tank to its fullest capacity. I do that for practical reasons, as the ATV hogs 15 miles per gallon, and sometimes does even worse than that. Since I fill the tank to its fullest capacity, it allows for a very easy tracking of the quantity of fuel in the tank.
I therefore was quite surprised to learn thatafter 7 days 0.24 of a gallon (0.9 liter) mysteriously "evaporated" from the tank. To make a long story short, I have filled the tank to its fullest capacity last week, parked my ATV in its designated place (a well protected, shaded cover, to which no one but me has access), only to find, seven days later, that 0.24 of a gallon were missing.
No one has moved, or touched the ATV at that time. I have thoroughly checked, and can assure you that there is no fuel leak, that the fuel tank cap was properly secured to its place, and that at all times, the fuel velve was not shut off.
Adding to the mystery, one of my ATV group companions, who also owns the Sportsman 700 , noticed the same phenomenon with his ATV. Is this a known problem? Is this a problem at all? If so, is there a way we can resolve it?
I would appreciate any comment regarding the issue, and thank you in advance for your kind attention to this matter.
#2
06-02-2002, 04:28 PM
#3
06-02-2002, 05:51 PM
Chew on this, there is no phenomenon going on, there is no preasure on the tank (other than head psi, and that is less than .5 psi in your tank or (unless your vent is plugged) the fuel has to flow through the fuel pump (diaphram type)so the likely hood of it draining into the carb and into the jug is unlikely,(even with a worn neddle & seat) you have one of two options here, evaporation through the vent or the net volume loss from cooler ambeiant temps. Gasoline, like all hydrocarbins are a funny thing in that they expand and contract more that most liquids we come in contact with on a regular base. This is why all bulk fuel is sold on a Net or Gross volume, 10 gallons of gas will always weigh the same ( hot or cold)but if it's real cold out the volume will shrink considerably and likewise whan it real hot, (your gas can looks round)this is also why all bulk fuel is sold at a tempreture corrected volume. 60 degrees is what fuel is corrected to. (more than you needed to know I bet, sorry)
#5
06-02-2002, 09:22 PM
I remember reading a post sometime ago where a guy found the shifter rubbing the gas tank. He said that it was close to putting a hole into it, but he found it b4 that happened. He said he had to pull the shifter off to see it. I know you said that you didn't have a leak, but this one could be tough to find if you haven't pulled the shifter. Thought I would just pass this along. Just shoutin' out ideas!
#6
06-02-2002, 09:30 PM
One other idea, I was filling a Buddy's Honda dirt bike the other day and it had a "drop tube" that went down about 2 1/2" into the tank, when I filled it up (overflowed onto the ground)I watched the gas slowly drop into the tank, filling the air space above the drop tube.
????? Does the 700 have anything like this? I know my old 500 does not.
????? Does the 700 have anything like this? I know my old 500 does not.
#7
06-02-2002, 09:34 PM
What does it do if you turn off the gas and run the gas out of the carb? It is probably going down into the bowl of the carb and running through the lines when you have the gas on, which is why you should always turn the gas off when you aren't using the machine. It could also be because it was filled up too much and it just evaporates out of the overflow. Just a thought...
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#8
06-03-2002, 04:04 PM
I just came back from the dealership with my 700 and I asked the mechanic about the fuel loss problem when filling the gas tank on the 700. He says that their is a vent tube comeing out of the tank about 1/2 to 1 inch below the bottom of the neck in the fuel tank. This vent hose runs from the tank, inside the headlight pod and down the inside of the steering shaft. If you fill the tank all the way up to the bottom of the neck, the fuel will run out the vent tube and down the inside of the steering shaft until the fuel level draws even or below the vent tube. I have seen the vent tube before when I had my headlight pod off. Makes sense to me. I guess we can only fill them to 1/2 to 1 inch below the neck. BUT I WANT A BIGGER TANK!!
#9
06-03-2002, 09:59 PM
Well . . . here it is.
In summary, the vapor pressure limits for the winter months are relatively higher values to ensure sufficient fuel vaporization at cold temperatures. This will assist with engine starting while appropriate distillation temperatures ensure effective vaporization during engine warm-up. Conversely, the summer specifications include lower vapor pressure limits, higher distillation temperatures and higher temperatures for a vapor/liquid ratio of 20 to ensure a less volatile fuel and help prevent vapor lock at higher temperatures.
------------------------------
All gasolines are seasonally adjusted to control vapour pressure. Summer-blended gasolines have lower vapour pressure to reduce evaporative emissions at high temperatures, while winter blends have higher vapour pressure to enhance cold starting and driveability.
---------------------------------
Volatility is the tendency of petrol to vaporize, and is critical to engine performance. Petrol must vaporize readily to allow easy starting and driveability when the engine is cold (to prevent carburetor icing), but not so much that it begins to evaporate in fuel lines when the engine is hot (called vapor lock, which impedes fuel flow).
Fuel volatility must therefore change with location and season. Volatility also affects vapor emissions. For environmental and health reasons, petrol must not be so volatile that evaporation from the fuel tank is excessive in hot weather. Volatility is controlled by distillation and vapor pressure specifications.
----------------------------------
Reid Vapour Pressure (RVP) is the direct measure of volatility at ambient temperatures. For environmental reasons, many countries specify RVP limits to reduce evaporation losses from vehicles and during bulk storage and distribution of gasoline. However, lower RVP affects refining operations by reducing the total petrol yield from crude oil, and thereby increasing overall production costs.
---------------------------------
High Volatility
Risk of vapor lock
- Engine will stall or won't start
- Poor driveability
- Engine stall
- Rough idle
- Poor acceleration
Increased evaporative emissions
--------------------------------------
Volatility
Gasoline volatility, which is the ability to evaporate, greatly affects a vehicle’s "driveability." Driveability is a measure of how smoothly a vehicle performs when starting, idling, accelerating, and running at constant speed.
Gasoline must be delivered to the carburetor or fuel injectors as a liquid. Too volatile a fuel can cause vapor in the fuel distribution system (vapor lock). This results in insufficient fuel supply, leading to rough engine operation or stalling. Vapor lock may occur when winter-grade gasoline is used on an unseasonably warm day. Some vehicles are more sensitive to vapor lock than others, and generally, vehicles with fuel injection systems are more resistant to vapor lock.
The carburetor or fuel injector meters fuel in the proper proportion to the air throttled into the intake manifold where much of the fuel is vaporized before it enters the combustion chamber. A spark-ignition engine requires that air and fuel be premixed and vaporized before combustion. Poor vaporization leads to long cranking times when the engine is cold, rough idle and stall, poor driveability during warm-up, and increased exhaust emissions resulting from misfire or incomplete combustion.
Finally, evaporation of raw gasoline from the tank and fuel system contribute to air pollution. Reducing gasoline volatility helps control these emissions, and it also reduces fuel vapor losses when the tank is being filled.
Because of these conflicting requirements, and because of the variety of climatic conditions, gasoline volatility must be carefully controlled in the manufacturing process. Volatility is adjusted seasonally and geographically to account for weather extremes.
There are several ways to characterize volatility. One is distillation, which determines at what temperature specific fractions of liquid gasoline turn to vapor. Distillation properties can predict both hot and cold temperature performance. Another way is vapor pressure, which is a measure of the force on a closed container at a specific temperature. Vapor pressure relates to hot-temperature performance and evaporative losses. Using predictive models of some of these properties, Shell seasonally blends gasolines to assure good driveability performance under all conditions.
---------------------------------------------
Twenty years ago vapor-lock was a problem with many engines in heavy-duty use. Then we got universal fuel injection, and vapor-lock virtually disappeared, Now we have reformulated gasoline, and vapor-lock has returned. Is there an effective way to combat vapor-lock? Well, some sidewalk engineers say to add one gallon of diesel fuel for each nine gallons of gas, and ;you'll eliminate vapor-lock. And it works. No more vapor-lock. What the sidewalk engineers don't tell you is that you also decrease the octane rating by 10 percent - from 89 down to 80 - and detonation begins destroying your engine. Diesel fuel works very well in diesel engines, but keep it out of your gasoline engine. If you start getting vapor-lock, start using the old time-tested ideas for keeping the fuel cool.
---------------------------------------------
Gasoline is a complex blend of carbon and hydrogen compounds. Additives are then added to improve performance. All gasoline is basically the same, but no two blends are identical. The two most important features of gasoline are volatility and resistance to knock (octane). Volatility is a measurement of how easily the fuel vaporizes. If the gasoline does not vaporize completely, it will not burn properly (liquid fuel will not burn).
If the gasoline vaporizes too easily the mixture will be too lean to burn properly. Since high temperatures increase volatility, it is desirable to have a low volatility fuel for warm temperatures and a high volatility fuel for cold weather. The blends will be different for summer and winter fuels. Vapor lock which was a persistent problem years ago, exists very rarely today. In today's cars the fuel is constantly circulating from the tank, through the system and back to the tank. The fuel does not stay still long enough to get so hot that it begins to vaporize. Resistance to knock or octane is simply the temperature the gas will burn at. Higher octane fuel requires a higher temperature to burn. As compression ratio or pressure increases so does the need for higher octane fuel. Most engines today are low compression engines therefore requiring a lower octane fuel (87). Any higher octane than required is just wasting money. Other factors that affect the octane requirements of the engine are: air/fuel ratio, ignition timing, engine temperature, and carbon build up in the cylinder. Many automobile manufacturers have installed exhaust gas recirculation systems to reduce cylinder chamber temperature. If these systems are not working properly, the car will have a tendency to knock. Before switching to a higher octane fuel to reduce knock, make sure to have these other causes checked.
----------------------------------------------------
Vapor Pressure by ASTM D5191 – Vapor Pressure is a physical measure of gasoline volatility. High vapor pressures and a low distillation temperature for 10% evaporated both help cold starting. Under hot-operating conditions, high vapor pressure also contributes to vapor lock and increases vapor formation in fuel tanks and carburetors. The amount of vapors formed in fuel tanks and carburetors, which must be contained by evaporative loss control system, are related to the vapor pressure and distillation temperatures. In order to reduce overloading the vehicle's control systems, the Environmental Protection Agency has placed maximum limits on gasoline's vapor pressure during the ozone season (May 1 - September 15). Also, the Washoe County Health Department has placed even lower vapor pressure requirements for ozone non-attainment areas of Washoe County during the period of June 1 - September 15. Hydrocarbon emissions are precursors to the formation of ozone (smog).
To assure that fuels have the proper volatility characteristics, refineries adjust gasoline seasonally. During the cold, winter months, fuel will be more volatile. This helps good "cold" start and warm-up performance. During the hot summer months, the fuel tends to be less volatile. This will help to minimize the incidence of vapor lock and hot driveability problems.
Since there are six seasonal classes of gasoline, only a sufficient quantity of fuel that can be consumed in a short period of time should be sold to a consumer/user.
The Environmental Protection Agency regulations allow 1.0 psi higher vapor pressure for gasoline-ethanol blends containing 9 to 10 volume % ethanol. The exception is that the Nevada Department of Agriculture has placed even lower vapor pressure requirements for ozone non-attainment areas of Clark County during the period of October 1 – March 31.
----------------------------------------------------
Distillation by ASTM D3710 or ASTM D86 –The "distillation" standard is one of several tests used to address gasoline's vaporization characteristics. Gasoline is metered in liquid form, through the fuel injectors (or carburetor), and mixed with air and atomized before entering the cylinders. Therefore, it is very important that a fuel's tendency to evaporate is controlled to certain standards. A fuel's ability to vaporize or change from liquid to vapor is referred to as its volatility. In gasoline, the distillation characteristics, along with vapor pressure, define and control starting, warm-up, acceleration, vapor lock, crankcase oil dilution, and, in part, fuel economy and carburetion icing.
The tendency of a fuel to vaporize is also characterized by determining a series of temperatures at which various percentages of the fuel have evaporated (boiling temperatures), as described in ASTM D86, Test Method for Distillation of Petroleum Products. The temperatures at which 10%, 50%, and 90% evaporation occurs are often used to characterize the volatility of gasoline.
The 10% evaporated temperature is directly affected by the seasonal blending of the gasoline. This temperature must be low enough to provide easy cold starting, but high enough to minimize the vapor lock and hot weather driveability problems. Most cool weather driveability problems occur from the use of summer season gasoline in the winter months. This is especially true in premium grades, which normally have a high 10% evaporated temperature.
The 50% evaporated temperature must be low enough to provide good warm-up and cool weather driveability without being so low as to contribute to hot driveability and vapor locking problems. This portion of the gasoline greatly affects fuel economy on short trips.
The 90% and end-point evaporation temperatures must be low enough to minimize crankcase and combustion chamber deposits, as well as spark plug fouling and the dilution of engine oil.
If the end-point temperature exceeds the ASTM maximum requirement, it is usually because of the presence of a distillate fuel such as No. 2 diesel. This contamination can be directly attributable to the delivery of diesel prior to the delivery of the gasoline. This problem is avoided if care is taken in handling the product.
----------------------------------------
Volatility Adding ethanol to a conventional gasoline not designed for alcohol blending can produce a blend which is too volatile. Ethanol can increase the vapor pressure 1.0 psi for a conventional gasoline with a vapor pressure of 9.0 psi. This increase, coupled with the corresponding reduction in mid-range distillation properties, lowers the V/L=20 temperature, reducing vapor lock protection. Blend volatility is increased only slightly by MTBE and not at all by ETBE and TAME.
Ethanol is not the oxygenate of choice for summer RFGs. The volatility increase makes it more difficult to meet the very tight vapor pressure limits of these gasolines.
------------------------------------------
Volatility
Volatility affects evaporative emissions and drive-ability. It is this property that must change with location and season. Fuel for mid-summer would be difficult to use in mid-winter. Incorrect fuel may result in difficult starting in cold weather, carburetor icing, vapor lock in hot weather, and crankcase oil dilution.
Volatility is controlled by distillation to Reid vapor pressure specifications. The higher boiling fractions of the gasoline have significant effects on the emission levels of undesirable hydrocarbons and aldehydes. Also, a reduction of 4 0C in the final boiling point will reduce the levels of benzene, butadiene, formaldehyde and acetaldehyde by 25%, and will reduce HC emissions by 20% .
Reid Vapor Pressure (RVP)
RVP is a measure of how quickly fuel evaporates.
RVP reduction provides the majority of VOC emission reductions from RFG.
------------------------------------------------
Why are there seasonal changes in Gasoline?
Only gaseous hydrocarbons burn, consequently if the air is cold, then the fuel has to be very volatile. But when summer comes, a volatile fuel can boil and cause vapor lock, as well as producing high levels of evaporative emissions. The solution was to adjust the volatility of the fuel according to altitude and ambient temperature. The oil companies without informing the public of the changes have automatically performed this volatility change for decades. It is one reason why storage of gasoline through seasons is not a good idea. Gasoline volatility is being reduced as modern engines, with their fuel injection and management systems, can automatically compensate for some of the changes in ambient conditions - such as altitude and air temperature, resulting in acceptable driveability using less volatile fuel.
-------------------------------------------------
Why is my Gas Mileage worse in the winter?
Two big factors can cause your gas mileage to be worse in the winter.
The first is temperature; the colder the engine temperature, the more fuel it needs. With a colder engine when starting and longer warm up time, more fuel will be used than in the same trip in the summer. This is especially noticeable in short trip driving.
The second is the season blend. Summer fuel has a higher energy content than the winter blend. Winter grades are made more volatile (less dense) to aid in cold start and warm up performance and typically contain 108,500 to 114,000 btu/gallon. Summer grades are of much lower volatility to minimize evaporative emissions and hot start/hot driveability problems. Summer grades will typically contain 113,000 to 117,000 btu/gallons. So the energy content, and therefore the fuel economy, can vary 3.4% to 5.0% just based on the energy content of the fuel.
The lower energy content of winter fuels and the other wintertime influences on fuel economy can easily lead to reductions of 10-20% in miles per gallon during the coldest winter months.
----------------------------------------------------
Volatility:
Gasoline volatility affects the driveability of the vehicle. Driveability is a measure of how smoothly a vehicle performs when starting, idling, accelerating and running at constant speed.
Gasoline must be sent to the fuel injector as liquid. Too volatile fuel can cause a vapor lock that results in insufficient fuel supply, leading to stalling of the engine.
Evaporation of raw gasoline from the tank causes air pollution. Certain amount of volatility is desirable as the fuel is vaporized and mixed with air before ignition.
Because of conflicting requirements and varying climatic conditions the volatility is carefully monitored during the manufacturing process.
The various methods to characterize volatility are distillation and vapor pressure.
-----------------------------------------------------------
I would like to try to enlighten you on some of the problems associated with today's gasoline in carbureted engines. These problems exist whether an engine is rebuilt to stock specifications or modified for increased power and include engines for tow vehicles, 4x4, and marine use. Some street performance and race engines share some of these problems but generally operate with a higher-grade fuel.
I am by no means a Petro-chemist but I do know engines and how different types of fuel will react in different engine combinations.
One of the biggest problems of today's fuel in carbureted engines is drivablity i.e. vapor lock-hesitation, loss of power, inconsistent throttle response, and black smoke all relate to vapor lock.
Gasoline today has a low vapor point, which means it turns gas into a vaporish state as a lower temperature. A carburetor is designed to meter fuel in a liquid state not a vapor. The vaporization should take place in the manifold for the engine to run correctly. This problem is aggravated by the fact that fuel blends vary depending on the time of year. We see those problems less in marine applications than towing because of engine temperatures. Marine engines can run consistently at 140 -150 degrees where a tow vehicle can run constantly at 210 - 220 degrees.
We have observed this problem for many years but has become worse. You might ask why we are forced to use this fuel? The answer is emissions. Every new vehicle built today is fuel injected. Fuel injection requires the fuel to be under pressure right up to the injector. When you pressurize a liquid it raises its boiling point. Most throttle body systems require 14psi and most multi-port injection systems require 40-45 psi. End of problem.
This doesn't spell death to the carburetor but is helping in its demise. There are a few things that can be done to make the problem livable, but not totally eliminate it short of retrofitting to a modern EFI system. An electronic fuel pump installed at or in the tank will help by providing pressurized fuel all the way to the carburetor. Remember that delivering fuel under pressure will help keep it from vaporizing so easily. Unfortunately, the fuel can still vaporize inside the carburetor because it is no longer under pressure. The only thing that can be done here is to keep the carburetor and fuel cooler. Heat shields under the carburetor, fresh air intakes to the carburetor from outside the vehicle (not hot air coming thru the radiator), and lower cooling system temperatures are about it.
The second problem we are faced with is detonation or pinging under load. Uncontrolled detonation can cause catastrophic engine damage in a big hurry. If you hear pinging or engine knock, get out of throttle, switch of higher-grade fuel, and reduce engine temperature. Myself or no other custom engine builder will warranty an engine that has detonation-induced damage. What we are doing to help prevent this problem when we build an engine, knowing that it is going to run pump gas, is to reduce cylinder pressure be reducing compression and on supercharged engines reduce boost.
This is a tough situation when we want to build more power. Obviously temperature, ignition timing, air fuel ratios, piston and chamber design, engine construction materials i.e. aluminum or cast iron, and of course the load that the engine will see all affect this problem. The thing to remember is that we can't just jack up the compression like we did 20 years ago with today's fuel.
When it comes to newer style engines with modern electronic controls this is again less of a problem. With sophisticated knock sensors and electronics the ECU's can modify ignition timing and fuel delivery to help reduce the problem. I presently own a '94 and '96 vehicle with these features but at times we still hear pinging under certain circumstances even with 92 octane unleaded.
Today's gasoline also doesn't store well. This is a problem that relates more to marine use but can be a problem with vehicles that sit unused for any period of time. Supposedly when fuel sits its oxygen compounds dissipate and what was once 92 octane is now 86. Heat and time seem to be the contributing factors here.
Today's fuel is also hard on carburetors. We rebuild more carburetors today then ever before. Not from dirt and varnish but from the fuel attacking various components requiring replacement. In light of all this t6he owner of the vehicle needs to assume a certain amount of responsibility and realize in an extreme duty application such as those discussed and deal with them as best you and your engine builder/tuner can.
In closing another matter that needs to be mentioned is that the stated octane value on the pump is not always what it should be. One way to deal with this is to buy fuel from the same station as much as possible and hopefully it is one that sells a lot of fuel keeping its stock fresh.
LARRY PETO HOME
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REFORMULATED GASOLINE, WHAT IS IT AND WHAT DOES IT MEAN TO GMC OWNERS?
by Frank Condos
By now most of us have had some experience with the newly mandated reformulated gas and we may or may not have noticed a difference. I'll try to limit this report to an explanation of the reported technical information and what it means and leave the politics to others. Most of the information bas been obtained from sources such as the EPA Office of Mobile Sources, The American Petroleum Institute and fuels consultants.
Reformulated gasoline (RFG) is blend of fuels designed to reduce Volatile Organic Compounds (VOC) that result from the evaporation of gas, the emissions of carbon monoxide (CO) and nitrous oxide (NOX). Summer evaporation is reduced by reducing the Reid Vapor Pressure, or the measure of volatility, during the summer months by about 15-20% (1 psi). This change reduces the amount of evaporation under ambient temperatures and also reduces tendency to form a tree vapor lock (more on this later).
CO is reduced in any engine by the introduction of oxygenates to the formula. These are compounds that carry an extra oxygen atom that is released during burning to help convert CO to CO2. As most people now know, the oxygenates are either ethel alcohol or Methyl Tertiary-butyl Ether (MTBE). The alcohol addition is used more often by refiners because of cost. On the average, about 6% by volume alcohol is used as compared to the earlier gasohol mixture of 10%. 11% MBTE is used. The NOX emissions are reduce by closely controlling the aromatics, sulfur and benzene in the fuel.
RFG is mandated by the EPA in most major metropolitan areas of the northeast, middle Atlantic states, the industrial areas of the mid-west and southern California. Other areas have also opted to join. Oxygenated fuels are required during the winter along the Rocky mountain area. In short, one third to one half of the fuel supply will be RFG or oxygenated as supply sources convert.
The effects of RFG on our GMCs can be divided into the following categories: performance, drive ability, and materials. Since RFG contains a bit less energy per gallon, fuel economy is off very slightly. On the order of 2-3%, probably not noticeable in light of all the other variables such as wind and temperature. There is an additional loss caused by the shift to a leaner bum in the combustion chamber by the introduction of additional oxygen. Assuming the engine operates at the optimum air/fuel ratio of 14.7:1 the ratio is shifted to about 15.1. Again with a slight loss in power, but I would guess most of our coaches operate on the rich side of 14.7 under load because of wear on the metering rods and jets. According to Joe Mondello, one could change the jet metering rod combination to off set this leaning by the new fuel, and he plans to experiment on the right combination.
Drive ability is probably a bigger factor for us than the performance loss. The specifications for RFG reduce the vapor pressure about 1 psi from conventional gas during the summer. This change actually reduces the likelihood of vapor lock which is generally defined as a vapor bubble in the fuel line or fuel pump on the suction side. The bigger problem comes from what is more correctly called percolation, or boiling of the fuel in the carburetor bowel. RFG lowers the boiling point at which 50% of the fuel will boil away by 5-50 degrees F and the 90% point by 30 degrees on the average. It is this change, which is also true for the older gasohol, that causes the problem. The writer has observed a coach at the Denver elevation that suffered fuel starvation on a moderately warm day even with the electric booster pump on. Inspection with the carburetor top off and the engine hot, indicated that the fuel fed by booster pump boiled so vigorously that the bowl would not fill. This percolation problem is exacerbated by RFG.
If fuel starvation is a problem with your coach there are several solutions. First try different brands of gas since there are differences in formulation that can affect driveability. A flow through booster pump or full time electric pump near the tanks will eliminate the vapor lock issue. For those who prefer not to use electric fuel pumps, use some form of insulation or shielding of the lines from opposite the exhaust manifolds to the mechanical pump. The more serious percolation problem can be helped by installing laminated insulating gaskets between the carburetor and the manifold These gaskets are more effective at reducing the heat transfer to the carburetor than the stock spacer and can be obtained from stores specializing in speed equipment. Ducting cool air directly on to the carburetor or into the air cleaner could also help, Finally, one can plug the heat riser in the intake manifold but this will adversely affect cool weather driveability and fuel economy.
Some elastomeric materials are affected by the additives, particularly the alcohol. Since gasohol has been around for about ten years, most replacement
parts have compatible materials but some original equipment may have problems. Areas to watch for leakage include the fuel lines, the transfer valve and the carburetor float needle.
Whether or not one agrees with all the EPA policies, Reformulated Gas is here to stay and it is not just a California phenomena. With a little bit of attention, the GMC should be able to digest RFG.
Good Luck . . . Trader Jack
In summary, the vapor pressure limits for the winter months are relatively higher values to ensure sufficient fuel vaporization at cold temperatures. This will assist with engine starting while appropriate distillation temperatures ensure effective vaporization during engine warm-up. Conversely, the summer specifications include lower vapor pressure limits, higher distillation temperatures and higher temperatures for a vapor/liquid ratio of 20 to ensure a less volatile fuel and help prevent vapor lock at higher temperatures.
------------------------------
All gasolines are seasonally adjusted to control vapour pressure. Summer-blended gasolines have lower vapour pressure to reduce evaporative emissions at high temperatures, while winter blends have higher vapour pressure to enhance cold starting and driveability.
---------------------------------
Volatility is the tendency of petrol to vaporize, and is critical to engine performance. Petrol must vaporize readily to allow easy starting and driveability when the engine is cold (to prevent carburetor icing), but not so much that it begins to evaporate in fuel lines when the engine is hot (called vapor lock, which impedes fuel flow).
Fuel volatility must therefore change with location and season. Volatility also affects vapor emissions. For environmental and health reasons, petrol must not be so volatile that evaporation from the fuel tank is excessive in hot weather. Volatility is controlled by distillation and vapor pressure specifications.
----------------------------------
Reid Vapour Pressure (RVP) is the direct measure of volatility at ambient temperatures. For environmental reasons, many countries specify RVP limits to reduce evaporation losses from vehicles and during bulk storage and distribution of gasoline. However, lower RVP affects refining operations by reducing the total petrol yield from crude oil, and thereby increasing overall production costs.
---------------------------------
High Volatility
Risk of vapor lock
- Engine will stall or won't start
- Poor driveability
- Engine stall
- Rough idle
- Poor acceleration
Increased evaporative emissions
--------------------------------------
Volatility
Gasoline volatility, which is the ability to evaporate, greatly affects a vehicle’s "driveability." Driveability is a measure of how smoothly a vehicle performs when starting, idling, accelerating, and running at constant speed.
Gasoline must be delivered to the carburetor or fuel injectors as a liquid. Too volatile a fuel can cause vapor in the fuel distribution system (vapor lock). This results in insufficient fuel supply, leading to rough engine operation or stalling. Vapor lock may occur when winter-grade gasoline is used on an unseasonably warm day. Some vehicles are more sensitive to vapor lock than others, and generally, vehicles with fuel injection systems are more resistant to vapor lock.
The carburetor or fuel injector meters fuel in the proper proportion to the air throttled into the intake manifold where much of the fuel is vaporized before it enters the combustion chamber. A spark-ignition engine requires that air and fuel be premixed and vaporized before combustion. Poor vaporization leads to long cranking times when the engine is cold, rough idle and stall, poor driveability during warm-up, and increased exhaust emissions resulting from misfire or incomplete combustion.
Finally, evaporation of raw gasoline from the tank and fuel system contribute to air pollution. Reducing gasoline volatility helps control these emissions, and it also reduces fuel vapor losses when the tank is being filled.
Because of these conflicting requirements, and because of the variety of climatic conditions, gasoline volatility must be carefully controlled in the manufacturing process. Volatility is adjusted seasonally and geographically to account for weather extremes.
There are several ways to characterize volatility. One is distillation, which determines at what temperature specific fractions of liquid gasoline turn to vapor. Distillation properties can predict both hot and cold temperature performance. Another way is vapor pressure, which is a measure of the force on a closed container at a specific temperature. Vapor pressure relates to hot-temperature performance and evaporative losses. Using predictive models of some of these properties, Shell seasonally blends gasolines to assure good driveability performance under all conditions.
---------------------------------------------
Twenty years ago vapor-lock was a problem with many engines in heavy-duty use. Then we got universal fuel injection, and vapor-lock virtually disappeared, Now we have reformulated gasoline, and vapor-lock has returned. Is there an effective way to combat vapor-lock? Well, some sidewalk engineers say to add one gallon of diesel fuel for each nine gallons of gas, and ;you'll eliminate vapor-lock. And it works. No more vapor-lock. What the sidewalk engineers don't tell you is that you also decrease the octane rating by 10 percent - from 89 down to 80 - and detonation begins destroying your engine. Diesel fuel works very well in diesel engines, but keep it out of your gasoline engine. If you start getting vapor-lock, start using the old time-tested ideas for keeping the fuel cool.
---------------------------------------------
Gasoline is a complex blend of carbon and hydrogen compounds. Additives are then added to improve performance. All gasoline is basically the same, but no two blends are identical. The two most important features of gasoline are volatility and resistance to knock (octane). Volatility is a measurement of how easily the fuel vaporizes. If the gasoline does not vaporize completely, it will not burn properly (liquid fuel will not burn).
If the gasoline vaporizes too easily the mixture will be too lean to burn properly. Since high temperatures increase volatility, it is desirable to have a low volatility fuel for warm temperatures and a high volatility fuel for cold weather. The blends will be different for summer and winter fuels. Vapor lock which was a persistent problem years ago, exists very rarely today. In today's cars the fuel is constantly circulating from the tank, through the system and back to the tank. The fuel does not stay still long enough to get so hot that it begins to vaporize. Resistance to knock or octane is simply the temperature the gas will burn at. Higher octane fuel requires a higher temperature to burn. As compression ratio or pressure increases so does the need for higher octane fuel. Most engines today are low compression engines therefore requiring a lower octane fuel (87). Any higher octane than required is just wasting money. Other factors that affect the octane requirements of the engine are: air/fuel ratio, ignition timing, engine temperature, and carbon build up in the cylinder. Many automobile manufacturers have installed exhaust gas recirculation systems to reduce cylinder chamber temperature. If these systems are not working properly, the car will have a tendency to knock. Before switching to a higher octane fuel to reduce knock, make sure to have these other causes checked.
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Vapor Pressure by ASTM D5191 – Vapor Pressure is a physical measure of gasoline volatility. High vapor pressures and a low distillation temperature for 10% evaporated both help cold starting. Under hot-operating conditions, high vapor pressure also contributes to vapor lock and increases vapor formation in fuel tanks and carburetors. The amount of vapors formed in fuel tanks and carburetors, which must be contained by evaporative loss control system, are related to the vapor pressure and distillation temperatures. In order to reduce overloading the vehicle's control systems, the Environmental Protection Agency has placed maximum limits on gasoline's vapor pressure during the ozone season (May 1 - September 15). Also, the Washoe County Health Department has placed even lower vapor pressure requirements for ozone non-attainment areas of Washoe County during the period of June 1 - September 15. Hydrocarbon emissions are precursors to the formation of ozone (smog).
To assure that fuels have the proper volatility characteristics, refineries adjust gasoline seasonally. During the cold, winter months, fuel will be more volatile. This helps good "cold" start and warm-up performance. During the hot summer months, the fuel tends to be less volatile. This will help to minimize the incidence of vapor lock and hot driveability problems.
Since there are six seasonal classes of gasoline, only a sufficient quantity of fuel that can be consumed in a short period of time should be sold to a consumer/user.
The Environmental Protection Agency regulations allow 1.0 psi higher vapor pressure for gasoline-ethanol blends containing 9 to 10 volume % ethanol. The exception is that the Nevada Department of Agriculture has placed even lower vapor pressure requirements for ozone non-attainment areas of Clark County during the period of October 1 – March 31.
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Distillation by ASTM D3710 or ASTM D86 –The "distillation" standard is one of several tests used to address gasoline's vaporization characteristics. Gasoline is metered in liquid form, through the fuel injectors (or carburetor), and mixed with air and atomized before entering the cylinders. Therefore, it is very important that a fuel's tendency to evaporate is controlled to certain standards. A fuel's ability to vaporize or change from liquid to vapor is referred to as its volatility. In gasoline, the distillation characteristics, along with vapor pressure, define and control starting, warm-up, acceleration, vapor lock, crankcase oil dilution, and, in part, fuel economy and carburetion icing.
The tendency of a fuel to vaporize is also characterized by determining a series of temperatures at which various percentages of the fuel have evaporated (boiling temperatures), as described in ASTM D86, Test Method for Distillation of Petroleum Products. The temperatures at which 10%, 50%, and 90% evaporation occurs are often used to characterize the volatility of gasoline.
The 10% evaporated temperature is directly affected by the seasonal blending of the gasoline. This temperature must be low enough to provide easy cold starting, but high enough to minimize the vapor lock and hot weather driveability problems. Most cool weather driveability problems occur from the use of summer season gasoline in the winter months. This is especially true in premium grades, which normally have a high 10% evaporated temperature.
The 50% evaporated temperature must be low enough to provide good warm-up and cool weather driveability without being so low as to contribute to hot driveability and vapor locking problems. This portion of the gasoline greatly affects fuel economy on short trips.
The 90% and end-point evaporation temperatures must be low enough to minimize crankcase and combustion chamber deposits, as well as spark plug fouling and the dilution of engine oil.
If the end-point temperature exceeds the ASTM maximum requirement, it is usually because of the presence of a distillate fuel such as No. 2 diesel. This contamination can be directly attributable to the delivery of diesel prior to the delivery of the gasoline. This problem is avoided if care is taken in handling the product.
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Volatility Adding ethanol to a conventional gasoline not designed for alcohol blending can produce a blend which is too volatile. Ethanol can increase the vapor pressure 1.0 psi for a conventional gasoline with a vapor pressure of 9.0 psi. This increase, coupled with the corresponding reduction in mid-range distillation properties, lowers the V/L=20 temperature, reducing vapor lock protection. Blend volatility is increased only slightly by MTBE and not at all by ETBE and TAME.
Ethanol is not the oxygenate of choice for summer RFGs. The volatility increase makes it more difficult to meet the very tight vapor pressure limits of these gasolines.
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Volatility
Volatility affects evaporative emissions and drive-ability. It is this property that must change with location and season. Fuel for mid-summer would be difficult to use in mid-winter. Incorrect fuel may result in difficult starting in cold weather, carburetor icing, vapor lock in hot weather, and crankcase oil dilution.
Volatility is controlled by distillation to Reid vapor pressure specifications. The higher boiling fractions of the gasoline have significant effects on the emission levels of undesirable hydrocarbons and aldehydes. Also, a reduction of 4 0C in the final boiling point will reduce the levels of benzene, butadiene, formaldehyde and acetaldehyde by 25%, and will reduce HC emissions by 20% .
Reid Vapor Pressure (RVP)
RVP is a measure of how quickly fuel evaporates.
RVP reduction provides the majority of VOC emission reductions from RFG.
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Why are there seasonal changes in Gasoline?
Only gaseous hydrocarbons burn, consequently if the air is cold, then the fuel has to be very volatile. But when summer comes, a volatile fuel can boil and cause vapor lock, as well as producing high levels of evaporative emissions. The solution was to adjust the volatility of the fuel according to altitude and ambient temperature. The oil companies without informing the public of the changes have automatically performed this volatility change for decades. It is one reason why storage of gasoline through seasons is not a good idea. Gasoline volatility is being reduced as modern engines, with their fuel injection and management systems, can automatically compensate for some of the changes in ambient conditions - such as altitude and air temperature, resulting in acceptable driveability using less volatile fuel.
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Why is my Gas Mileage worse in the winter?
Two big factors can cause your gas mileage to be worse in the winter.
The first is temperature; the colder the engine temperature, the more fuel it needs. With a colder engine when starting and longer warm up time, more fuel will be used than in the same trip in the summer. This is especially noticeable in short trip driving.
The second is the season blend. Summer fuel has a higher energy content than the winter blend. Winter grades are made more volatile (less dense) to aid in cold start and warm up performance and typically contain 108,500 to 114,000 btu/gallon. Summer grades are of much lower volatility to minimize evaporative emissions and hot start/hot driveability problems. Summer grades will typically contain 113,000 to 117,000 btu/gallons. So the energy content, and therefore the fuel economy, can vary 3.4% to 5.0% just based on the energy content of the fuel.
The lower energy content of winter fuels and the other wintertime influences on fuel economy can easily lead to reductions of 10-20% in miles per gallon during the coldest winter months.
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Volatility:
Gasoline volatility affects the driveability of the vehicle. Driveability is a measure of how smoothly a vehicle performs when starting, idling, accelerating and running at constant speed.
Gasoline must be sent to the fuel injector as liquid. Too volatile fuel can cause a vapor lock that results in insufficient fuel supply, leading to stalling of the engine.
Evaporation of raw gasoline from the tank causes air pollution. Certain amount of volatility is desirable as the fuel is vaporized and mixed with air before ignition.
Because of conflicting requirements and varying climatic conditions the volatility is carefully monitored during the manufacturing process.
The various methods to characterize volatility are distillation and vapor pressure.
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I would like to try to enlighten you on some of the problems associated with today's gasoline in carbureted engines. These problems exist whether an engine is rebuilt to stock specifications or modified for increased power and include engines for tow vehicles, 4x4, and marine use. Some street performance and race engines share some of these problems but generally operate with a higher-grade fuel.
I am by no means a Petro-chemist but I do know engines and how different types of fuel will react in different engine combinations.
One of the biggest problems of today's fuel in carbureted engines is drivablity i.e. vapor lock-hesitation, loss of power, inconsistent throttle response, and black smoke all relate to vapor lock.
Gasoline today has a low vapor point, which means it turns gas into a vaporish state as a lower temperature. A carburetor is designed to meter fuel in a liquid state not a vapor. The vaporization should take place in the manifold for the engine to run correctly. This problem is aggravated by the fact that fuel blends vary depending on the time of year. We see those problems less in marine applications than towing because of engine temperatures. Marine engines can run consistently at 140 -150 degrees where a tow vehicle can run constantly at 210 - 220 degrees.
We have observed this problem for many years but has become worse. You might ask why we are forced to use this fuel? The answer is emissions. Every new vehicle built today is fuel injected. Fuel injection requires the fuel to be under pressure right up to the injector. When you pressurize a liquid it raises its boiling point. Most throttle body systems require 14psi and most multi-port injection systems require 40-45 psi. End of problem.
This doesn't spell death to the carburetor but is helping in its demise. There are a few things that can be done to make the problem livable, but not totally eliminate it short of retrofitting to a modern EFI system. An electronic fuel pump installed at or in the tank will help by providing pressurized fuel all the way to the carburetor. Remember that delivering fuel under pressure will help keep it from vaporizing so easily. Unfortunately, the fuel can still vaporize inside the carburetor because it is no longer under pressure. The only thing that can be done here is to keep the carburetor and fuel cooler. Heat shields under the carburetor, fresh air intakes to the carburetor from outside the vehicle (not hot air coming thru the radiator), and lower cooling system temperatures are about it.
The second problem we are faced with is detonation or pinging under load. Uncontrolled detonation can cause catastrophic engine damage in a big hurry. If you hear pinging or engine knock, get out of throttle, switch of higher-grade fuel, and reduce engine temperature. Myself or no other custom engine builder will warranty an engine that has detonation-induced damage. What we are doing to help prevent this problem when we build an engine, knowing that it is going to run pump gas, is to reduce cylinder pressure be reducing compression and on supercharged engines reduce boost.
This is a tough situation when we want to build more power. Obviously temperature, ignition timing, air fuel ratios, piston and chamber design, engine construction materials i.e. aluminum or cast iron, and of course the load that the engine will see all affect this problem. The thing to remember is that we can't just jack up the compression like we did 20 years ago with today's fuel.
When it comes to newer style engines with modern electronic controls this is again less of a problem. With sophisticated knock sensors and electronics the ECU's can modify ignition timing and fuel delivery to help reduce the problem. I presently own a '94 and '96 vehicle with these features but at times we still hear pinging under certain circumstances even with 92 octane unleaded.
Today's gasoline also doesn't store well. This is a problem that relates more to marine use but can be a problem with vehicles that sit unused for any period of time. Supposedly when fuel sits its oxygen compounds dissipate and what was once 92 octane is now 86. Heat and time seem to be the contributing factors here.
Today's fuel is also hard on carburetors. We rebuild more carburetors today then ever before. Not from dirt and varnish but from the fuel attacking various components requiring replacement. In light of all this t6he owner of the vehicle needs to assume a certain amount of responsibility and realize in an extreme duty application such as those discussed and deal with them as best you and your engine builder/tuner can.
In closing another matter that needs to be mentioned is that the stated octane value on the pump is not always what it should be. One way to deal with this is to buy fuel from the same station as much as possible and hopefully it is one that sells a lot of fuel keeping its stock fresh.
LARRY PETO HOME
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REFORMULATED GASOLINE, WHAT IS IT AND WHAT DOES IT MEAN TO GMC OWNERS?
by Frank Condos
By now most of us have had some experience with the newly mandated reformulated gas and we may or may not have noticed a difference. I'll try to limit this report to an explanation of the reported technical information and what it means and leave the politics to others. Most of the information bas been obtained from sources such as the EPA Office of Mobile Sources, The American Petroleum Institute and fuels consultants.
Reformulated gasoline (RFG) is blend of fuels designed to reduce Volatile Organic Compounds (VOC) that result from the evaporation of gas, the emissions of carbon monoxide (CO) and nitrous oxide (NOX). Summer evaporation is reduced by reducing the Reid Vapor Pressure, or the measure of volatility, during the summer months by about 15-20% (1 psi). This change reduces the amount of evaporation under ambient temperatures and also reduces tendency to form a tree vapor lock (more on this later).
CO is reduced in any engine by the introduction of oxygenates to the formula. These are compounds that carry an extra oxygen atom that is released during burning to help convert CO to CO2. As most people now know, the oxygenates are either ethel alcohol or Methyl Tertiary-butyl Ether (MTBE). The alcohol addition is used more often by refiners because of cost. On the average, about 6% by volume alcohol is used as compared to the earlier gasohol mixture of 10%. 11% MBTE is used. The NOX emissions are reduce by closely controlling the aromatics, sulfur and benzene in the fuel.
RFG is mandated by the EPA in most major metropolitan areas of the northeast, middle Atlantic states, the industrial areas of the mid-west and southern California. Other areas have also opted to join. Oxygenated fuels are required during the winter along the Rocky mountain area. In short, one third to one half of the fuel supply will be RFG or oxygenated as supply sources convert.
The effects of RFG on our GMCs can be divided into the following categories: performance, drive ability, and materials. Since RFG contains a bit less energy per gallon, fuel economy is off very slightly. On the order of 2-3%, probably not noticeable in light of all the other variables such as wind and temperature. There is an additional loss caused by the shift to a leaner bum in the combustion chamber by the introduction of additional oxygen. Assuming the engine operates at the optimum air/fuel ratio of 14.7:1 the ratio is shifted to about 15.1. Again with a slight loss in power, but I would guess most of our coaches operate on the rich side of 14.7 under load because of wear on the metering rods and jets. According to Joe Mondello, one could change the jet metering rod combination to off set this leaning by the new fuel, and he plans to experiment on the right combination.
Drive ability is probably a bigger factor for us than the performance loss. The specifications for RFG reduce the vapor pressure about 1 psi from conventional gas during the summer. This change actually reduces the likelihood of vapor lock which is generally defined as a vapor bubble in the fuel line or fuel pump on the suction side. The bigger problem comes from what is more correctly called percolation, or boiling of the fuel in the carburetor bowel. RFG lowers the boiling point at which 50% of the fuel will boil away by 5-50 degrees F and the 90% point by 30 degrees on the average. It is this change, which is also true for the older gasohol, that causes the problem. The writer has observed a coach at the Denver elevation that suffered fuel starvation on a moderately warm day even with the electric booster pump on. Inspection with the carburetor top off and the engine hot, indicated that the fuel fed by booster pump boiled so vigorously that the bowl would not fill. This percolation problem is exacerbated by RFG.
If fuel starvation is a problem with your coach there are several solutions. First try different brands of gas since there are differences in formulation that can affect driveability. A flow through booster pump or full time electric pump near the tanks will eliminate the vapor lock issue. For those who prefer not to use electric fuel pumps, use some form of insulation or shielding of the lines from opposite the exhaust manifolds to the mechanical pump. The more serious percolation problem can be helped by installing laminated insulating gaskets between the carburetor and the manifold These gaskets are more effective at reducing the heat transfer to the carburetor than the stock spacer and can be obtained from stores specializing in speed equipment. Ducting cool air directly on to the carburetor or into the air cleaner could also help, Finally, one can plug the heat riser in the intake manifold but this will adversely affect cool weather driveability and fuel economy.
Some elastomeric materials are affected by the additives, particularly the alcohol. Since gasohol has been around for about ten years, most replacement
parts have compatible materials but some original equipment may have problems. Areas to watch for leakage include the fuel lines, the transfer valve and the carburetor float needle.
Whether or not one agrees with all the EPA policies, Reformulated Gas is here to stay and it is not just a California phenomena. With a little bit of attention, the GMC should be able to digest RFG.
Good Luck . . . Trader Jack
#10
06-03-2002, 11:45 PM
