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E85 Fuel Use in Gasoline Engines
E85 is an alcohol fuel mixture that typically contains a mixture of up to 85% denatured fuel ethanol and gasoline or other hydrocarbon by volume. On an undenatured basis, the ethanol component ranges from 70% to 83%. E85 as a fuel is widely used in Sweden and is becoming increasingly common in the United States, mainly in the Midwest where corn is a major crop and is the primary source material for ethanol fuel production; however as yet, there are still only about 800 filling stations selling E85 to the public in the US, and, until recently, only one in Canada[1], [2] (there are now three).
Use in flexible-fuel engines
E-85 ethanol is used in engines modified to accept higher concentrations of ethanol. Such flexible-fuel engines are designed to run on any mixture of gasoline or ethanol with up to 85% ethanol by volume. The primary differences from non-FFVs is the elimination of bare magnesium, aluminum, and rubber parts in the fuel system, the use of fuel pumps capable of operating with electrically conductive (ethanol) instead of non-conducting dielectric (gasoline) fuel, specially-coated wear-resistant engine parts, fuel injection control systems having a wider range of pulse widths (for injecting approximately 40% more fuel), the selection of stainless steel fuel lines (sometimes lined with plastic), the selection of stainless steel fuel tanks in place of terne fuel tanks, and, in some cases, the use of acid-neutralizing motor oil. For vehicles with fuel-tank mounted fuel pumps, additional differences to prevent arcing, as well as flame arrestors positioned in the tank's fill pipe, are also sometimes used. There is increasing evidence to suggest that using E85 in engines contributes in increased fuel inefficiency in vehicles.[3] Comparisons to regular gasolineDepending on composition and source, E85 has an octane rating of 100 - 105 [4] compared to regular gasoline's typical rating of 87 - 93. This allows it to be used in higher compression engines which tend to produce more power per unit of displacement than their gasoline counterparts. Since the reciprocating mass of the engine increases in proportion to the displacement of the engine E-85 has a higher potential efficiency for an engine of equal power. One complication is that use of gasoline in an engine with a high enough compression ratio to use E-85 efficiently would likely result in catastrophic failure, whereas use of E-85 in an engine designed for gasoline will result only in a loss of efficiency. Therefore, E85 gets more fuel consumption in flex fuel type vehicles because they are designed with lower compression ratios. So in order to save money at the pump with a flex fuel vehicle the price difference between gasoline and E-85 must be great enough to offset this. Currently E85 is about 5-10% less expensive in most areas.[5] More than 20 fueling stations across the Midwest are selling E85 25%-40% cheaper than gasoline.[6] E85 also gets less MPGs. It has been tested and proven that a car that would get 19 city/21 Hwy using normal gas (regular), would get 9 city/11-13 hwy using E85. References
See also
Notes
References
Handbook for Handling Storing and Dispensing E85 National Rewnewable Energy Laboratory, April 2006
Flex-Fuel Bait and Switch - See how many flex-fuel cars are on the road and how many stations offer E85.
External links
The use of pure E85 in standard gasoline car engines will void the manufacturer's warranty, may disrupt oxygenation control in fuel-injected cars, and may result in power loss. It may also interfere with proper operation of the catalytic converter. E85 can also cause engine damage, since prolonged exposure to high concentrations of ethanol may corrode metal and rubber parts in older engines (pre-1995) designed primarily for gasoline. Post-1995 fuel-injected cars are designed to accept E10 fuel, and may be tolerant of higher concentrations of ethanol to varying degrees, usually up to at least 20%. Another risk is that of water contamination, which can produce engine wear directly and through corrosive formic acid in the combustion process. Oil and acid neutralizer additives can counteract these risks. It may not be possible to continue the use of non-additive (ND) oils in older engines. After-market conversion kits for converting standard engines to operate on E85, which are not legal to manufacture in the United States without EPA approval, are widely available online. Converted vehicles usually require emissions certification, which is often not economically feasible, in order to be resold or traded.
ExperimentalE85 has an octane rating of 105, which is higher than typical commercial gasoline mixtures (octane ratings of 85 to 98); however, it does not burn as efficiently in traditionally-manufactured internal-combustion engines. Additionally, E85 contains less energy per volume as compared to gasoline. Although E85 contains only 72% of the energy on a gallon-for-gallon basis compared to gasoline, experimenters have seen slightly better fuel mileage than the 28% this difference in energy content implies. For example, recent tests by the National Renewable Energy Lab on fleet vehicles owned by the state of Ohio showed about a 25% reduction in mpg [1] (see table on pg 5) comparing E85 operation to reformulated gasoline in the same flexible fuel vehicle. Results compared against a gasoline-only vehicle were essentially the same, about a 25% reduction in volumetric fuel economy with E85. The main attractions of burning E85, of course, are the common benefits of renewable energy sources, such as increased economic benefits for rural populations, less reliance on foreign energy and, with further research into increasing production efficiency, less carbon emissions per unit as compared to conventional fossil fuels. Modern cars (i.e., most cars built after 1988) have fuel-injection engines with oxygen sensors that will attempt to adjust the air-fuel mixture, but the oxygen sensor only changes the air to fuel ratio at idle, and at light cruising speeds. Since the computer can not add more fuel without the input from the oxygen sensor at high loads, there will be significant power losses in modern cars. Use of any fuel with greater than 10% alcohol added will void any car manufacturer's warranty.[2] Operating fuel-injected non-FFVs on more than 50% ethanol will generally cause the Malfunction Indicator Lamp (MIL) to illuminate, indicating that the electronic control unit (ECU) believes that it can no longer maintain closed-loop control of the internal combustion process due to the presence of more oxygen in E85 than in gasoline. Once the MIL illuminates, adding more ethanol to the fuel tank becomes rather inefficient. For example, running 90% ethanol in a non-FFV (Flexible Fuel Vehicle) will reduce fuel economy by 33% or more relative to what would be achieved running 100% gasoline. Even more importantly, continuing to operate the non-FFV with the Malfunction Indicator Lamp (MIL) illuminated may also cause damage to the catalytic converter as well as to the engine pistons if allowed to persist. To run a non-FFV with amounts of ethanol high enough to cause the MIL to illuminate risks severe damage to the vehicle, that may outweigh any economic benefit of E85. Under stoichiometric combustion conditions, ideal combustion occurs for burning pure gasoline as well as for various mixes of gasoline and ethanol (at least until the MIL illuminates in the non-FFV) such that there is no significant amount of uncombined oxygen or unburned fuel being emitted in the exhaust. This means that no change in the exhaust manifold oxygen sensor is required for either FFVs or non-FFVs when burning higher percentages of ethanol. This also means that the catalytic converter on the non-FFV burning ethanol mixed with gasoline is not being stressed by the presence of too much oxygen in the exhaust, which would otherwise reduce catalytic converter operating life. Nonetheless, even when the MIL does not illuminate on the non-FFV burning an ethanol-gasoline mixture, proper catalytic operation of the catalytic converter for a non-FFV burning higher percentages of ethanol may not be achieved as soon as necessary to prevent the emission of some pollution products resulting from burning the gasoline contained in the mixture, especially upon initial cold engine start. This is because the catalytic converter needs to rise to an internal temperature of approximately 300 °C before it can 'fire off' and commence its intended catalytic function operation. When burning large concentrations of ethanol in a non-FFV, the cooler burning characteristics of ethanol fuel than gasoline fuel may delay reaching the 'fire-off' temperature in a non-FFV as quickly as when burning gasoline. Any additional pollution, however, is only going to be emitted for a very short distance when burning E85 in a non-FFV, as the catalytic converter will nonetheless still 'fire off' quite quickly and commence catalytic operation shortly. It is not known whether the small amount of pollution emitted prior to catalytic converter 'fire off' may actually be reduced even during the cold startup phase, as well as once catalytic operation commences, when burning E85 in a non-FFV. Likewise, even once the catalytic converter 'fires off', operation with the MIL illuminated will still result in excess amounts of nitrous oxide being released, greater than when operating the engine on gasoline. The solution is simply to add gasoline, and extinguish the Malfunction Indicator Lamp (MIL), at which time exhaust pollutants will return to within normal limits. For non-FFVs burning E85 once the MIL illuminates, it is the lessened amount of fuel injection than what is needed that causes the air fuel mixture to become too lean; that is, there is not enough fuel being injected into the combustion process, with the result that the oxygen content in the exhaust rises out of limits, and perfect (i.e., stoichiometric) combustion is lost if the percentage of ethanol in the fuel tank becomes too high. It is the loss of near-stoichiometric combustion that causes the excessive loss of fuel economy in non-FFVs burning too high a percentage of ethanol versus gasoline in their fuel mix. E85 gives particularly good results in turbocharged cars due to its high octane [3]. It allows the ECU to run more favorable ignition timing and leaner fuel mixtures than are possible on normal premium gasoline. Users who have experimented with converting OBDII (i.e., On-Board Diagnostic System 2, that is for 1996 model year and later) turbocharged cars to run on E85 have had very good results. Experiments indicate that most OBDII-specification turbocharged cars can run up to approximately 39% E85 (33% ethanol) with no MILs or other problems. (In contrast, most OBDII specification fuel-injected non-turbocharged cars and light trucks are more forgiving and can usually operate well with in excess of 50% E85 (42% ethanol) prior to having MILs occur.) Fuel system compatibility issues have not been reported for any OBDII cars or light trucks running on high ethanol mixes of E85 and gasoline for periods of time exceeding two years. (This is likely to be the outcome justifiably expected of the normal conservative automotive engineer's predisposition not to design a fuel system merely resistant to ethanol in E10, or 10% percentages, but instead to select materials for the fuel system that are nearly impervious to ethanol.) Fuel economy does not drop as much as might be expected in turbocharged engines based on the specific energy content of E85 compared to gasoline, in contrast to the previously-reported reduction of 23.7% reduction in a 60:40 blend of gasoline to E85 for one non-turbocharged, fuel-injected, non-FFV. The reason for this non-intuitive difference is that the turbocharged engine seems especially well-suited for operation on E85, for it in effect has a variable compression ratio capability, which is exactly what is needed to accommodate varying ethanol and gasoline ratios that occur in practice in an FFV. At light load cruise, the turbocharged engine operates as a low compression engine. Under high load and high manifold boost pressures, such as accelerating to pass or merge onto a highway, it makes full use of the higher octane of E85. It appears that due to the better ignition timing and better engine performance on a fuel of 100 octane, the driver spends less time at high throttle openings, and can cruise in a higher gear and at lower throttle openings than is possible on 100% premium gasoline. In daily commute driving, mostly highway, 100% E85 in a turbocharged car can hit fuel mileages of over 90% of the normal gasoline fuel economy. Tests indicate approximately a 5% increase in engine performance is possible by switching to E85 fuel in high performance cars. Experimenters who have made conversions to 100% E85 report that cold start problems at very cold temperatures can easily be avoided through adding 1 - 2 gallons of gasoline to the E85 in the tank, prior to the arrival of the cold weather. No significant ignition timing changes are required for a gasoline engine running on E85. RisksCorrosionE85 can cause damage, since prolonged exposure to high concentrations of ethanol may corrode metal and rubber parts in older engines (pre-1995) designed primarily for gasoline. The hydroxyl group on the ethanol molecule is an extremely weak acid, but it can enhance corrosion for some natural materials. For post-1995 fuel-injected engines, all the components are already designed to accommodate E10 (10% ethanol) blends through the elimination of exposed magnesium and aluminum metals and natural rubber and cork gasketed parts. Hence, there is a greater degree of flexibility in just how much more ethanol may be added without causing ethanol-induced damage, varying by automobile manufacturer. Anhydrous ethanol in the absence of direct exposure to alkali metals and bases is non-corrosive; it is only when water is mixed with the ethanol that the mixture becomes corrosive to some metals. Hence, there is no appreciable difference in the corrosive properties between E10 and a 50:50 blend of E10 gasoline and E85 (47.5% ethanol), provided there is no water present, and the engine was designed to accommodate E10. Nonetheless, operation with more than 10% ethanol has never been recommended by car manufacturers in non-FFVs. Operation on up to 20% ethanol is generally considered safe for all post-1988 cars and trucks. Water contaminationIn addition to corrosion, there is also a risk of increased engine wear for non-FFV engines that are not specifically designed for operation on high levels (i.e., for greater than 10%) of ethanol. The risk primarily comes in the rare event that the E85 fuel ever becomes contaminated with water. For water levels below approximately 0.5% to 1.0% contained in the ethanol, no phase separation of gasoline and ethanol occurs. For contamination with 1% or more water in the ethanol, phase separation occurs, and the ethanol-water mixture will separate from the gasoline. This can be observed by pouring a mixture of suspected water-contaminated E85 fuel in a clear glass tube, waiting roughly 30 minutes, and then inspecting the sample. If there is water contamination of above 1% water in the ethanol, a clear separation of ethanol-water from gasoline will be clearly visible, with the colored gasoline floating above the clear ethanol-water mixture. For ethanol contaminated with larger amounts of water (i.e., approximately 11% water, 89% ethanol, equivalent to 178 proof ethanol), considerable engine wear will occur, especially during times while the engine is heating up to normal operating temperatures. For example, just after starting the engine, low temperature partial combustion of the water-contaminated ethanol mixture takes place and causes engine wear. This wear, caused by water-contaminated E85, is the result of the combustion process of ethanol, water, and gasoline producing considerable amounts of formic acid (HCOOH, also known as methanoic acid and sometimes written as CH2O2). In addition to the production of formic acid occurring for water-contaminated E85, smaller amounts of acetaldehyde (CH3CHO) and acetic acid (C2H4O2) are also formed for water-contaminated ethanol combustion. Of these partial combustion products, formic acid is responsible for the majority of the rapid increase in engine wear. Engines specifically designed for FFVs employ soft nitride coatings on their internal metal parts to provide resistance to formic acid wear in the event of water contamination of E85 fuel. Also, the use of lubricant oil (motor oil) containing an acid neutralizer is necessary to prevent the damage of oil-lubricated engine parts in the event of water contamination of fuel. Such lubricant oil is required by at least one manufacturer of FFVs even to this day (Chrysler). For non-FFVs burning E85 in greater than 23.5% E85 mixtures (20% ethanol), the remedy for accidentally getting a tank of water-contaminated E85 (or gasoline) while preventing excessive engine wear is to change the motor oil as soon as possible after either burning the fuel and replacing it with non-contaminated fuel, or after immediately draining and replacing the water-contaminated fuel. The risk of burning slightly water-contaminated fuel with low percentages of water (less than 1%) on a long commute is minimal; after all, it is the low temperature combustion of water contaminated ethanol and gasoline that causes the bulk of the formic acid to form; burning a slightly-contaminated mix of water (less than 1%) and ethanol quickly, in one long commute, will not likely cause any appreciable engine wear past the first 15 miles of driving, especially once the engine warms up and high temperature combustion occurs exclusively. For those making their own E85, the risk of introducing water unintentionally into their homemade fuel is relatively high unless adequate safety precautions and quality control procedures are taken. Ethanol and water form an azeotrope such that it is impossible to distill ethanol to higher than 95.6% ethanol purity by weight (roughly 190 proof); regardless of how many times distillation is repeated. Unfortunately, this proof ethanol contains too much water to prevent separation of a mixture of such proof ethanol with gasoline, or to prevent the formation of formic acid during low temperature combustion. Therefore, when making E85, it becomes necessary to remove this residual water. It is possible to break the ethanol and water azeotrope through adding benzene or another hydrocarbon prior to a final rectifying distillation. This takes another distillation (energy consuming) step. However, it is possible to remove the residual water more easily, using 3 angstrom (3A) synthetic zeolite pellets to absorb the water from the mix of ethanol and water, prior to mixing the now anhydrous ethanol with gasoline in an 85% to 15% by volume mixture to make E85. This absorption process is also known as a molecular sieve. The benefit of using synthetic zeolite pellets is that they are essentially comparable to using a catalyst, in being reusable and in not being consumed in the process, and the pellets require only re-heating (perhaps on a backyard grill, in a solar reflector furnace, or with heated carbon dioxide gas collected and saved from the fermentation process) to drive off the water molecules absorbed into the zeolite. Research has also been done at Purdue University on using corn grits as a desiccant. [4] Once the ground corn becomes water logged, the corn grits can be processed much as the zeolite pellets, at least for a number of drying cycles before the grits lose their effectiveness. Once this occurs, it is possible to run the now water-logged corn grits through the natural fermentation process and convert them into even more ethanol fuel. Air/Fuel mixture problemsRunning a non-FFV with a high percentage of ethanol will cause the air fuel mixture to be leaner than normal in carbureted or open loop fuel injection engines, and cause closed loop fuel injection systems to adjust for the increase in oxygen content of the fuel mixture. A lean mixture, when leaner than stoichiometric, is unlikely to cause heat related engine damage because temperature decreases quickly once there is a surplus of air during the combustion event. The surplus air cools the burn, and lowers the exhaust gas temperature. The effects of surplus oxygen on the catalytic converter may be undesirable, and if too lean the engine will display roughness in operation. If the percentage of ethanol used results in sustained operation in the range between stoichiometric and best power mixture, problems may develop. In this range, between peak exhaust gas temperature and approximately 50 degrees rich of peak, combustion temperatures are at the highest possible, and may exceed the design temperatures for the engine. Detonation margins are reduced, and if operation at elevated temperatures is allowed to persist over considerable periods of time, heat related damage to valves and pistons can occur. Without in-depth knowledge of the engine's mixture control system and instrumentation to monitor exhaust gas temperature, cylinder head temperature, cylinder pressure, and/or exhaust oxygen content, it is difficult to know whether the engine is operating in the "red" zone, or an acceptable mixture zone. Closed loop fuel injection systems eliminate much of the risk. This is also why the check engine light will illuminate if you mix more than around 50% to 60% E85 by volume with your gasoline in a non-FFV. If this happens, just add more 87 octane regular grade gasoline as soon as possible to correct the problem. (Some premium blends contain up to 10% ethanol; to correct the problem as quickly as possible, always add regular grade gasoline, not premium grade gasoline.) These fuel/air mixture related problems will not happen in a properly-converted vehicle. Jurisdictions mandating ethanolStarting in 2013[5], at least one US state (Minnesota) already has legislatively mandated and planned to force E20 (20% ethanol) into their general gasoline fuel-distribution network. The city of Portland, Oregon will require E85 and biodiesel at all gas pumps instead of their petroleum equivalents by 2009. Details of how this will work for individual vehicle owners while maintaining automobile manufacturer warranties, despite exceeding the manufacturer's maximum warranted operation percentage of 10% of ethanol in fuel, are still being worked as of late-2005. However, the choice of transitioning to a 20% ethanol blend of gasoline is not without precedent; Brazil, in its conversion to an ethanol-fueled economy, determined that operation with up to 22% ethanol in gasoline was safe for the cars and trucks on the road in Brazil at the time, and the conversion to a 20% blend was accomplished with only minor issues arising for older vehicles. Recently, conversion to a 24% blend was accomplished in Brazil. After-market conversionsAfter-market conversion kits, for converting standard engines to operate on E85, are generally not legal in U.S. states subject to emissions controls, unless the converted vehicle is independently EPA certified. This is despite the fact that the exhaust emissions from any such converted cars are improved by utilizing higher percentages of ethanol in the gasoline blend. Unfortunately, EPA certification costs in excess of $23,000 and requires proof that the vehicle will maintain low emissions for at least 50,000 miles after the conversion. Most individuals won't give up their vehicles for the requisite 50,000 mile test period. Likewise, conversion kit manufacturers generally don't certify their kits due to the onerous and expensive burden of these laws. The kits would have to be tested with every model vehicle for which they are to be sold. If a kit is already certified as described, the EPA Federal Test Procedure for an individual's conversion costs $750. One can request a reduction of payment of down to 1% of the car's added retail value due to the conversion. A minimum fee may apply if the value added is not seen to be very high. Similarly, U.S. Federal law prohibits the manufacture of many such conversion kits for sale in the U.S. unless they are EPA certified. The origin of this ban dates to when conversion kits for using compressed natural gas were originally sold. The ban was enacted to prevent the sale of such conversion kits due to safety concerns. This ban on the manufacture of kits is at odds with the fact that these kits, once existing, are legal in all states but CA, and most states offer some sort of tax break for converting your vehicle [6] One Brazilian after-market kit is available legally in US States that do not have restrictive emission controls. The kit will permit the conversion of 4, 6, or 8 cylinder engines to operate from fuels ranging from pure gasoline to a mix of gasoline and ethanol to pure ethanol, including E85. It operates by modifying the fuel-injection pulses sent to the fuel injectors when in 'A', or ethanol mode instead of 'G', or gasoline mode. (In 'G' mode, no modification to the fuel-injection pulses is performed.) This conversion kit modification serves to extend the control range over which the ECU can adjust the air-fuel ratio to achieve an oxygen sensor reading measured before the catalytic converter that falls within nominal stoichiometric ideal combustion limits. The general belief is that this conversion kit operates in its 'A' mode simply through lengthening the individual pulse-widths of fuel-injection pulses, thereby increasing fuel flow per injection pulse by roughly 30%, whereas in 'G' mode, it acts simply as a straight pass through for fuel-injection pulses. Due to the nature of this kit, it is fully reversible (see below for other approaches). Other than the one Brazilian after-market kit, no other pre-manufactured E85 conversion kits are known to exist. Nonetheless, it is still possible to modify existing non-FFV engines to operate on pure E85 without the use of this particular after-market kit. The primary method used to convert non-fuel-injected cars is two-fold. First, any non-compatible rubber parts and gaskets and terne gas tanks and terne fuel lines are replaced. Then, it remains necessary to increase the fuel rate of flow by roughly 25% - 30%. This can be accomplished in one of several different ways, depending on the specific details of the fueling system. In the early 80's auto makers were required to make vehicles ethanol compatible, so most new vehicles will handle E85 with no problem. If a car is converted though, the ethanol will clean out the gunk left over from the gasoline and plug the fuel filter. The fuel filter needs to be replaced after about 600 miles. For non-fuel-injected engines, this may be accomplished through increasing the diameter of the carburetor running jets to a size that is slightly larger in diameter. The theoretical change is not to increase the hole diameter by 25% to 30%, but rather to increase the area and hence the fuel flow rate by 25%-30%. Hence, the diameter of the jets must be increased by 11.8% to 14%, while keeping the general shapes at the opening of the jets as close to nearly the same as possible. (The idling jet must also be increased in diameter in addition to the running jet, primarily to accomplish successful starting in colder weather.) An excellent starting point, if one doesn't want to experiment with multiple test trials over the 25% to 30% range, is simply to increase the fuel flow by 27%, which just requires increasing the diameter of the jets by 13%. For older vehicles, an even simpler non-conversion 'conversion' is possible once any non-compatible rubber gas hoses and cork gaskets and such are all replaced with ethanol-resistant materials. For older vehicles with a manual choke, it is possible simply to leave the choke slightly engaged even when the motor is warmed up, and the conversion is complete. For converting later-model fuel-injected cars and trucks, fuel injection-pressure boosters can be installed, to increase fuel-injector fuel rate flow. It may be difficult to get your mixture right, plus there is a safety risk of more leaks in your fuel system. Likewise, if you do choose this method, you may lose some of your compatibility with running on pure gasoline, from moving the air fuel mix farther from optimum for what is needed for running on pure gasoline. The disadvantage of most of the above conversions is the conversion is permanent, without changing out or removing added parts. If any of these conversion techniques are used, especially in older vehicles in which there may be rust or other residue present in the fuel tank, it may be necessary additionally to replace the fuel filter within 400 to 600 miles, as ethanol has a tendency to release any trapped rust or gasoline fuel gum or residue, which can cause the fuel filter to become blocked. Once replaced, life expectancy of the new fuel filter should be normal, barring an exceptionally dirty gas tank or fuel system. Running E85 in a vehicle can actually improve fuel efficiency if the fuel delivery system was especially gummed up. This improvement remains if the vehicle is returned to operation on gasoline only. Air fuel ratio comparisonE85 fuel requires a richer air fuel mixture than gasoline for best results. Successful conversions generally require up to 60% more fuel flow than when the engine burns 100% gasoline. (In contrast, methanol conversions require even more fuel flow increase than ethanol conversions.) Flexible fuel vehicles additionally impose a wider range of air fuel ratios that must be achieved than what is required for vehicles that operate only on gasoline or ethanol. This is because a wider range of air fuel ratios is required to use all the varying percentages of ethanol and gasoline efficiently that may be present in the fuel tank at any given time. The nominal (chemically correct) air fuel ratio is 14.64:1 by mass (not volume) for burning 100% gasoline, but in practice the nominal air fuel ratio for most 100% gasoline fuel injection systems ranges from about 14.6 to 14.7 for a typical nominal value, depending on manufacturer, with the ratio of 14.7 being slightly preferred for increasing fuel economy under light load conditions. The following table shows the range of air fuel ratios typically used for burning gasoline, E85, and pure ethanol (E100) under an assortment of assumed operating conditions:
The term AFRst refers to the air fuel ratio under stoichiometric or ideal air fuel ratio mixture conditions. (See stoichiometry.) FARst refers to the fuel air ratio under stoichiometric conditions, and is simply the reciprocal of AFRst. Equivalence ratio is the ratio of actual fuel air ratio to stoichiometric fuel air ratio; it provides an intuitive way to express richer mixtures. Lambda (λ) is the ratio of actual air fuel ratio to stoichiometric air fuel ratio; it provides an intuitive way to express leanness conditions (i.e., less fuel, less rich) mixtures of fuel and air. Air fuel ratio is always computed on the basis of ratios of mass (not volume). The following is a computation of the theoretical E100 (100% ethanol, C2H6O) air fuel ratio, based on stoichiometric (perfect combustion) principles:
Adding up the molar mass for ethanol:
Now, oxygen is 20.9% of air by volume, or equivalently, 23.1% of air by mass, assuming that atmospheric gases behave as ideal gases. (See Earth's atmosphere.) Hence, the theoretical air fuel ratio for E100 (100% ethanol) is:
So, for E85 (summer blend), the required air fuel ratio can be estimated as:
Likewise, for E85 (winter blend), the required air fuel ratio can be estimated as:
The estimated required E85 summer blend air fuel ratio compares very closely to the value of 9.765 given in the table. In practice, though, the exact stoichiometric air fuel ratio for gasoline varies as a function of the exact blend of gasoline, which, in turn, is varied by time of year by refineries to increase or decrease volatility, prevent vapor locking, etc., for better matching seasonal climatic changes. Deviations from stoichiometric combustion computed values are required during non-standard operating conditions such as heavy load, or cold weather operation, in which case the mixture ratio can range from 10:1 to 18:1 for burning 100% gasoline. Slightly wider ranges than even this on the low end of the air fuel ratio, dropping to below 8:1, are required for burning all possible blends of E85 and gasoline efficiently under all conditions of engine loads and inlet air temperatures. At inlet air temperatures below 15 °C (59 °F), it is likewise not possible to start the typical internal combustion engine on pure ethanol (E100); for cold engine starts, starting the engine on gasoline and then shifting to E100 can be done. Similarly, for starting a vehicle on E85 summer blend in extremely cold weather, it is likewise required to add additional gasoline during at least the starting of the engine, before switching to burning the E85 summer blend. In practice, it is easier simply to add more pure gasoline to the fuel tank when extremely cold weather is expected, prior to the arrival of the cold weather, to avoid cold engine start difficulties. Fortunately for those converting non-FFVs to operate on E85, the wide range of inherent air fuel control required for burning pure gasoline is very nearly the same range required for burning many blends of E85 with gasoline up to approximately 60% E85, at least for non-extreme engine loads and non-extreme weather conditions. Hence, the common success seen in practice for burning many blends of E85 with gasoline even in non-FFVs at blends in excess of 50% E85, especially under light engine loads cruising under benign weather conditions. All of these theoretical stoichiometric combustion estimated values should be taken only as approximations to what may really be required for achieving perfect combustion. The lambda sensor is what ultimately confirms whether stoichiometric combustion is taking place in practice. Additionally, the ideal stoichiometric mixture typically burns too hot for any situation other than light load cruise. This is the target mixture that the ECU attempts to achieve in closed-loop fueling to get the best possible emissions and fuel mileage at light load cruise conditions. This mixture typically can give approximately 95% of the engine's best power, provided the fuel has sufficient octane to prevent damaging detonation (i.e., knock). The "max power rich" condition is the richest air fuel mixture (more fuel than best power) that gives both good drivability and power levels, within approximately 1% of the absolute best power on that fuel. The "max power lean" condition is the leanest air fuel mixture (less fuel than best power) that gives good drivability, acceptable exhaust gas temperatures to prevent engine damage, and power levels within approximately 1% of the absolute best power on that fuel. Lambda, typically used for referring to lean versus rich air fuel mixtures, is normally measured by the lambda sensor] (also known as an oxygen sensor.) Depending on seasonal blend variations E85 will weigh approximately 6.5 pounds per U.S. Gallon, having a liquid density of approximately 0.77 - 0.79 compared to gasoline which has typical values of 6.0 - 6.5 pounds per U.S. gallon and a density of 0.72 - 0.78. This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "E85 in Standard Engines" |
