Combustion engine

Details Combustion engine Combustion engine

1999-06-17

2001-04-10

Dolinar, Andrew M. (Department: 3747)

Internal-combustion engines

Precombustion and main combustion chambers in series

Having combustible mixture forming means

C123S179500, C123SDIG007, C123S625000, C123S02700A

Reexamination Certificate

active

06213086

ABSTRACT:

BACKGROUND OF THE INVENTION
A process for operating of an internal combustion engine running on fuel that is both externally ignitible and self-ignitible, especially gasoline, the operating region of the engine including self-ignition or compression-ignition regions and external-ignition or sparkignition regions, and a high compression ratio suitable for self-ignition of the fuel being provided, at least in compression-ignition regions, and fuel combustion being initiated by self-ignition of the fuel-air mixture in compression-ignition regions and by spark-ignition of the fuel-air mixture in spark-ignition regions, and part-load operation being assigned to the compression-ignition region, whilst full-load operation and/or operating modes with high engine load as well as cold starts are assigned to the sparkignition region.
DESCRIPTION OF THE PRIOR ART
From the publication “Homogeneous Charge Compression Ignition (HCCI) of Diesel Fuel”, Allen W. Gray et al., SAE 971676, it is known that combustion of a self-igniting lean fuel-air mixture will lead to extremely low NO
x
and particulate emissions due to the homogeneous concentration and temperature distribution. This method is referred to as HCCI process. It is further known that the use of diesel fuel for this type of combustion process will cause problems since the desired timing of initial ignition just before top dead center can only be effected with a low compression ratio and low effective mean pressure, on account of the diesel fuel's readiness to ignite. Compared to the conventional diesel process, the low compression ratio of about 10 required in this instance leads to considerable disadvantages with regard to specific fuel consumption and performance attained, which have so far prevented the use of this low-emission combustion process on a larger scale. Another problem specific to diesel fuel is its boiling range of about 170-360° C., which is not conducive to evaporation and thus homogenization of the cylinder charge and may lead to high levels of No
x
, particulate and unburnt hydrocarbon emissions in addition to the danger of diesel fuel accumulating in lubricating oil.
The use of gasoline for the HCCI process has considerable advantages on account of its low tendency to self-ignition and its lower boiling range of 30-190° C. The compression ratio may be increased to about 15 to 17, similar to the diesel engine. However, the effective mean pressure obtainable in this case again is restricted to the part-load region, as discussed in the paper “An Experimental Study on Premixed-Charge Compression Ignition Gasoline Engine” by Taro Aoyama et al., SAE 960081.
In DE 36 32 579 C2 an air-compression, spark-ignition engine operating at a high compression ratio similar to that of a diesel engine is described, in which a stratified charge is produced to ensure ignition of the air-fuel mixture. The principle of charge stratification will ensure that at the time of arc-over the mixture prevailing in the vicinity of the ignition source will remain ignitible for a time long enough to produce a flame that is sufficiently large for the cylinder charge to continue burning. Such a Stratified Charge Spark Ignition (SCSI) process does not yield the low emission levels of the HCCI process at part-load, but it will tolerate much higher mean pressures and is characterized by lower particulate emissions than the diesel engine.
DE 28 51 504 A1 discloses a method as described above for operation of an internal combustion engine with variable compression ratio, wherein the engine runs on a single type of fuel that is both externally ignitible and self-ignitible. With this engine the compression ratio is increased under part-load conditions to permit self-ignition and decreased under full-load conditions to permit spark-ignition. Via an injection nozzle opening into the main combustion chamber or into a prechamber fuel is supplied for compression-ignition operation, and an inhomogeneous fuel-air mixture is produced in the combustion chamber. As a consequence no HCCI operation takes place. The fuel supply for spark-ignition operation is effected by means of a carburetor in one variant and via a further injection nozzle opening into a secondary combustion chamber in another variant. Fuel combustion is initiated in the SI mode by means of a spark plug reaching into the secondary combustion chamber. In the SI mode with fuel injection and air intake under full-load conditions there is a charge concentration in the secondary combustion chamber relative to the main combustion chamber. This process, which is well known in the art, ensures stable operation and high efficiency. The extremely low emissions encountered with HCCI will not be obtained in this instance, however.
Another internal combustion engine that is suitable for operation by both spark-ignition and compression-ignition, is described in U.S. Pat. No. 4,126,106 A. During start-up and part-load operation a stratified charge is produced with direct fuel injection into the combustion chamber, and fuel combustion is initiated by spark-ignition of this stratified charge. At full load, however, the fuel is directly injected onto the hot walls of the combustion chamber, where it evaporates and ignition is effected by compression according to the diesel principle. The engine is operated at a compression ratio of less than 16:1. At higher engine loads the time span between the beginning of injection and the moment of self-ignition will not suffice for satisfactory preparation of the mixture, which will adversely affect the combustion process and emission levels.
A multi-purpose engine is described in U.S. Pat. No. 3,125,079 A, which can be operated on both the compression-ignition and spark-ignition principle, at a fixed compression ratio of 15:1. The fuel is radially injected into the combustion chamber by direct injection from a multiple jet nozzle. This method is not suitable for obtaining a high degree of charge stratification.
SUMMARY OF THE INVENTION
It is an object of this invention to overcome the above disadvantages and to improve the quality of the exhaust gases while maintaining high efficiency in an internal combustion engine for fuel that is both externally ignitible and self-ignitible.
According to the invention this object is achieved by producing an at least approximately homogeneous fuel-air mixture in the combustion chamber in the compression-ignition mode. The process according to the invention thus has all advantages of the HCCI process whilst avoiding its disadvantages by changing over to spark-ignition under conditions of high engine load. A homogeneous fuel-air mixture can also be generated in the combustion chamber in at least one spark-ignition region. The homogeneous fuel-air mixture may be stoichiometric or lean. Particularly low fuel consumption and emission levels are obtained in spark-ignition regions by producing a stratified charge in the combustion chamber. The process of the invention thus combines the advantages of the HCCI process and the SCSI process.
At higher engine loads there would not be sufficient time in the HCCI process between the beginning of injection and the moment of self-ignition to give satisfactory mixture preparation; as a consequence, the combustion process and emission levels would deteriorate upon self-ignition. As spark-ignition is employed in the full-load region and/or operating regions with high engine load these disadvantages are avoided. Emissions may thus be significantly reduced in both part-load and full-load regions.
Although the SCSI process has higher emission levels than the HCCI process, it offers two advantages as regards emission vis-a-vis the direct injection (DI) process of a conventional diesel engine. First, gasoline has a significantly lower tendency towards particulate formation than diesel fuel on account of its smaller molecule size and the higher vapor pressure resulting therefrom, which will enhance mixture formation. Second, spark-ignition offers the additional degree of freedom over the diesel process that th

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