01.07.2009

Some knowledge about E85 standard engines

E85 is a blend of standard gasoline or petrol and ethanol, with 85% being ethyl alcohol and the remaining 15% being a gasoline formulation. Flexible fuel vehicles (FFV) are designed to run on E85 but standard gasoline or petrol engines are not. Standard engines can be adapted to run on E85 but there are some potential problems.

Benefits of 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 that keeps more fuel dollars in the domestic economy, with further research into increasing production efficiency, less carbon emissions per unit as compared to conventional fossil fuels.

Fuel-injection engines

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.

Problems

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 not due to the presence of more oxygen in E85, but rather the fact that E85 has less carbon per volume, thus requiring more than the injectors can deliver, than 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. 

Corrosion

E85 can cause damage, since prolonged exposure to high concentrations of ethanol may corrode metal and rubber parts in older engines (pre-1985) 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-1985 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 contamination

Although water phase separation can be a significant problem in ethanol-blended gasoline fuels such as E10, contamination by small amounts of water does not lead to phase separation in E85 fuel. The fraction of water required to induce phase separation is higher than 20% (by weight).

Air/Fuel mixture problems

Running 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, could cause heat related engine damage because combustion chamber temperatures can increase with a surplus of air during the combustion event. Some aftermarket E85 conversion kits for modern fuel injected vehicles operate by altering the duty cycles of electronic injectors to help offset air/fuel mixture issues. 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.

Calculations

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:

C2H6O + 3 O2 = 2 CO2 + 3 H2O

Adding up the molar mass for ethanol:

(6 x 1.00794) + (2 x 12.0107) + (1 x 15.9994) = 46.0684 grams per mole of ethanol

1 mol x 46.0684 g/mol ethanol : 3 mol x 2 x 15.9994 g/mol oxygen

46.0684 : 95.9964 = 1:2.0838 for the fuel:oxygen ratio for perfect (i.e., stoichiometric) combustion.

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:

(2.0838/0.23133) : 1 = 9.0078 : 1

So, for E85 (summer blend), the required air fuel ratio can be estimated as:

0.85 x 9.0078 + 0.15 x 14.64 = 9.8526

Likewise, for E85 (winter blend), the required air fuel ratio can be estimated as:

0.70 x 9.0078 + 0.30 x 14.64 = 10.6975, which is closer to the gasoline air fuel ratio.

Discussion

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 g/ml 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 g/ml.

 

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