For more than a century, internal combustion engines have been relied upon as a principal source of power in a variety of applications. Of those engines, the most widely used are the reciprocating piston engines which are found in automobiles or other forms of transportation, as well as a variety of industrial and consumer applications. Such engines can be built in a variety of sizes, types and configurations depending on the power requirements of a particular application.
Of those variations, Diesel engines have a number of important advantages over gasoline engines. They provide reliability, long life, and good fuel economy, and are expected to remain the dominant heavy-duty transport power plants for many years. Diesel engines typically inject diesel fuel into the engine's combustion chamber when that chamber's piston is near the end of the compression stroke. The high pressure present in the chamber ignites the diesel fuel. Due to the uncontrolled nature of the mixing of diesel and air during combustion, a large fraction of the fuel exists at a very fuel-rich equivalence ratio. That is, the fuel and air in the combustion chamber are not necessarily a homogenous mixture. This typically results in incomplete combustion of the diesel fuel, which tends to result in high particulate emissions. Furthermore, the fuel-rich equivalence ratio can also lead to high flame temperatures residing in a small area in the combustion process, which results in increased NOx emissions. As tougher environmental standards are being enacted for diesel sources, users of diesel engines are looking for ways to lower emissions.
One solution is to reduce the amount of diesel injected into the combustion chamber, which reduces the equivalence ratio and works to reduce particulate and NOx emissions. However, it also reduces engine power.
Another solution is to partially or completely convert the engine for use with alternative fuels such as, compressed natural gas (CNG), liquid natural fuels (LNF) such as ethanol, and liquid or liquefied petroleum gas (LPG) such as propane. Utilization of such alternative fuels with diesel engines not only provides for more stable and complete combustion and thereby enhanced fuel economy, but also typically results in lower engine emissions. However, alternative fuels, and more particularly gaseous fuels, typically do not have the centane value required to allow for their ignition through compression. Accordingly, diesel engines must be modified to use such fuels. Methods for converting a diesel engine to consume alternative fuels typically fall into three categories. The first is to convert the engine to a spark-ignited engine; a second is to convert the engine to allow for the direct injection of gaseous-fuels into the combustion chamber; and a third is "fogging" or "fumigation" of the gaseous-fuel with all or a portion of the intake air charge entering the engine. As will be appreciated, the second and third methods utilize injected diesel (i.e., pilot diesel) to ignite the gaseous-fuel. In this regard, the combustion of the gaseous-fuel results in more complete combustion of the diesel. Furthermore, the combination of gaseous-fuel and diesel allows the engine to produce additional power while less diesel fuel is injected into the cylinders.
However, conversion to a spark-ignition system and/or a direct gaseous-fuel injection system for utilizing gaseous-fuels with a diesel engine each typically require substantial modification to the diesel engine. Such modifications may include replacement of cylinder heads, pistons, fuel injection system and/or duplication of many engine components (e.g., injection systems). Accordingly, these systems are typically expensive and often times unreliable.
On the other hand, fogging or fumigation type dual-fuel systems require little modification to existing engines. The mixture of gaseous-fuel with the intake air charge is introduced into each cylinder of the engine during the intake stroke. During the compression stroke of the piston, the pressure and temperature of the mixture are increased in the conventional manner. Near the end of the compression stroke, a smaller than normal quantity of diesel fuel from the engine's existing diesel fuel injection system is injected into the cylinder. The diesel ignites due to compression and in turn ignites the mixture of gaseous-fuel and intake air, which in turn, accelerates the flame front of the Diesel Fuel, enhancing the combustion process. As will be appreciated, such fumigation systems may be retrofit onto existing diesel engines with little or no modification of the existing engine. Furthermore, engines using such fumigation systems may typically be operated in a dual-fuel mode or in a strictly diesel mode (e.g., when gaseous-fuel is not available).