Spark ignited engines face multiple constraints in operation that limit their efficiency. Two such limitations are misfire and knock. Misfire is the phenomenon whereby the gasoline-air mixture does not ignite in the presence of a spark. Knock is the uncontrolled auto-ignition of the unburned air-fuel mixture during the combustion process.
Lean operation, which is defined as operation in which there is an excess amount of air for the amount of fuel introduced, has been known to increase the efficiency of an engine through the reduction of frictional losses, as well as decreased heat losses to the cylinder wall due to lower combustion temperatures. The NOx emissions decrease when excess amounts of air are used, and it is possible to decrease NOx emissions to the point where there is no need for NOx after-treatment. However, in general, this requires an air to fuel ratio that is so great that combustion stability (i.e. cycle-to-cycle variation) suffers, thereby resulting in misfire. A powerful ignition source could allow the operation of an engine at these large excess air-fuel ratios.
An alternative means of operation is to operate the engine at stoichiometric ratios, that is, with enough oxygen to combust all of the fuel, with no excess or shortage of air, and to introduce a high level of exhaust gas recirculation (EGR). The three way catalyst works well under these conditions, and this can be used to control emissions. EGR, and particularly cooled EGR, has been known to decrease the engine tendency to knock by decreasing the temperatures in the cylinder. Because of the effect on the ratio of specific heats, EGR operation is not as efficient as lean operation, but the simplicity of aftertreatment makes this an attractive alternative. Under EGR conditions of more than 15-25%, referred to as heavy EGR, the engine experiences the same problem of misfire as occurs during very lean operation. Again, a powerful ignition source could allow reliable operation at conditions of heavy EGR.
The effect of lean operation and EGR is to dilute the air/fuel mixture. Conditions with heavy EGR or very lean operation are characterized by high dilution of the air/fuel mixture.
In addition to misfire, the second limitation of spark ignited engines is knock, the uncontrolled auto-ignition of the unburned air-fuel mixture during the combustion process. Knock is more prevalent when the fuel-air mixture is under high compression. Thus, knock places severe limitations on the use of high compression ratio, and turbocharging, which limits the potential efficiency of the engine.
Otto cycle engines typically ignite the air-fuel mixture by an electric spark, such as a spark plug. However, where there is high dilution (either through very lean operation or heavy EGR), reliable ignition of the mixture may be compromised. Ignition can be more powerfully administered by injecting a small quantity of easily ignitable (i.e. high cetane number) diesel fuel that is stored in a separate tank. This fuel that is injected for ignition purposes is often called a “pilot fuel”, and the process is known as pilot-ignition. This process is capable of delivering far more ignition energy to a lean fuel-air mixture than is possible with a spark plug. As a result, the lean fuel operation of an Otto cycle engine becomes smoother and more reliable by pilot fuel injection that would be possible, relying only on spark ignition.
However, the use of conventional liquid diesel fuel as a pilot fuel in an Otto cycle engine is problematic. There must be considerable excess air (and oxygen) in the fuel-air mixture in order to avoid having an unacceptable portion of the pilot fuel decompose into soot. Also, providing enough excess air to eliminate soot generation is an unattractive solution because, in a given engine, excess air means less fuel is combusted, and therefore less power is produced. Even more important, excess oxygen in the exhaust makes it impractical to use the conventional, efficient and economical 3-way catalysts commonly used to clean the exhaust of stochiometrically fueled spark ignition engines. Therefore, expensive and inefficient diesel exhaust catalyst systems would need to be employed if conventional diesel fuels were to be used as pilot fuel.
Therefore, there exists a need for a method and system that allows lean operation and EGR, without requiring the use of expensive diesel exhaust systems. Such a method and system would require a powerful pilot fuel which does not create the soot associated with diesel pilot fuel.