Internal-combustion engines – Combustion chamber means combined with air-fuel mixture... – Having a single combustible mixture inlet combined with...
Reexamination Certificate
2001-05-08
2004-02-03
Mohanty, Bibhu (Department: 3747)
Internal-combustion engines
Combustion chamber means combined with air-fuel mixture...
Having a single combustible mixture inlet combined with...
C123S295000, C123S527000
Reexamination Certificate
active
06684852
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an improved internal combustion engine for increasing fuel efficiency while reducing exhaust emissions and a method of operating such an engine. In particular, the present invention is directed to such an engine operable in a premixed charge compression ignition mode.
2. Description of Related Art
Relatively recently, because of the increased regulatory pressure for fuel efficient and low emissions engines, some engine designers have directed their efforts to one type of an internal combustion engine which utilizes premixed charge compression ignition (PCCI). Researchers have used various other names in referencing PCCI combustion including homogeneous charge compression ignition (HCCI) as well as others such as “ATAC” which stands for “Active Thermo-Atmosphere Combustion.” (SAE Technical Paper No. 790501, Feb. 26-Mar. 2, 1979), “TS” which stands for “Toyota-Soken” (SAE Technical Paper No. 790840, Sep. 10-13, 1979), and “CIHC” which stands for “compression-ignited homogeneous charge” (SAE Paper No. 830264, 1983). All of these terms are hereinafter collectively referred to as PCCI.
Generally, conventional internal combustion engines are either a diesel or a spark ignited engine, the diesel engine controlling the start of combustion (SOC) by the timing of fuel injection while a spark ignited engine controls the SOC by the spark timing. Initially, it should be understood that SOC refers to the point in time at which a charge within the cylinder begins to ignite. The major advantage that a spark ignited natural gas or gasoline engine has over a diesel engine is its ability to achieve extremely low NOx and particulate emissions levels. The major advantage that diesel engines have over premixed charge spark ignited engines is in its higher thermal efficiency. One key reason for the higher efficiency of diesel engines is its ability to use higher compression ratios than premixed charge spark ignited engines since the compression ratio in premixed charge spark ignited engine must be kept relatively low to avoid knock.
A second key reason for the higher efficiency of diesel engines lies in the ability to control the diesel engine's power output without a throttle. This eliminates the throttling losses of premixed charge spark ignited engines and results in significantly higher efficiency at part load for diesel engines. Typical diesel engines, however, cannot achieve the very low NOx and particulate emissions levels which are possible with premixed charge spark ignited engines. Due to the mixing controlled nature of diesel combustion, a large fraction of the fuel exists at a very fuel rich equivalence ratio which is known to lead to particulate emissions. Premixed charge spark ignited engines, on the other hand, have nearly homogeneous air fuel mixtures which tend to be either lean or close to stoichiometric, resulting in very low particulate emissions. Another consideration is that the mixing controlled combustion in diesel engines occurs when the fuel and air exist at a near stoichiometric equivalence ratio which leads to high temperatures. The high temperatures, in turn, cause high NOx emissions. Premixed charge spark ignited engines, on the other hand, either have much lower NOx emissions or their NOx emissions can be reduced to very low levels with a three-way catalyst.
Another type of engine that has been recent focus of research and has been proposed and studied in the prior art is a direct injection natural gas engine that utilizes compression ignition. In such an engine, highly pressurized natural gas is injected directly into the combustion chamber during or after compression so that the heat generated by compression ignites the injected natural gas in a manner similar to that of diesel injection applications. Such combustion is typically aided by a glow plug and/or a pilot injection and the direct injection natural gas engine allows higher compression ratios than a comparable spark ignition natural gas engine. Hence, the gross thermal efficiency of a direct injection natural gas engine is known to be higher than that of a spark ignition natural gas engine. However, a direct injection natural gas engine requires the natural gas to be compressed to very high pressures such as 3000 psi or greater which is very difficult to attain. This required compression process requires a substantial amount of work which reduces the brake thermal efficiency of the direct injection natural gas engine. Consequently, whereas the emission performance in a direct injection natural gas engine has been found to be better than a conventional diesel engine, the higher emissions (as compared to a spark ignited engine) as well as complexity and high cost has minimized the commercial appeal.
Unlike the above described internal combustion engines, engines operating on PCCI principles rely on autoignition of a relatively well premixed fuel/air mixture to initiate combustion. More specifically, in PCCI engines, the fuel and air are mixed in the intake port or in the cylinder, long before ignition occurs. The extent of the homogeneity of the mixture may be varied depending on the combustion characteristics desired. Some engines may be designed and/or operated to ensure that the fuel and air are mixed into a homogeneous, or nearly homogeneous, state. Also, an engine may be specifically designed and/or operated to create a somewhat less homogeneous charge having a small degree of stratification. In both instances, the mixture exists in a premixed state well before ignition occurs and is compressed until the mixture autoignites. Thus, PCCI combustion event is characterized in that: 1) the majority of the fuel is sufficiently premixed with the air to form a combustible mixture throughout the charge at the time of ignition; and 2) ignition is initiated by compression ignition. In addition, PCCI combustion is also preferably characterized in that most of the mixture is significantly leaner than stoichiometric to advantageously reduce emissions, unlike the typical diesel engine cycle in which a large portion, or all, of the mixture exists in a rich state during combustion. Because an engine operating on PCCI combustion principles has the potential for providing the excellent fuel economy of the diesel engine while providing NOx and particulate emissions levels that are much lower than that of current spark ignited engine, it has also recently been the subject of extensive research and development.
It is now known that for efficient, low emission PCCI combustion, it is important to have the combustion event occur at an appropriate crank angle during the engine cycle. In this regard, it has further been found that the timing of the start of combustion (SOC) and the combustion rate (therefore combustion duration) in a PCCI engine primarily depend on various combustion history values such as the temperature history; the pressure history; fuel autoignition properties (e.g. octane/methane rating or activation energy); and trapped cylinder charge air composition (oxygen content, EGR, humidity, equivalence ratio etc.). However, it should be understood that the term PCCI does not exclude the use of ignition timing mechanisms such as pilot injections and spark ignition known in the art that are used to precisely time the ignition of the premixed charge. Whereas the premixed charge may combust due to compression, such ignition timing mechanisms aid in initiating the SOC of the premixed charge at a precise time to ensure desirable combustion characteristics. This is in contrast to non-PCCI engines such as conventional gasoline engines with spark ignition in which the premixed charge of gasoline and air would not ignite at all without the spark.
A premixed charge compression ignition engine with optimal combustion control with various control features for controlling SOC and the combustion rate is disclosed in the patent application Ser. No. 08/916,437 filed on Aug. 22, 1997, currently assigned to the Assignee of the present invent
Pierz Patrick M.
Wright John F.
zur Loye Axel O.
Brackett, Jr. Tim L.
Cummins Inc.
Mohanty Bibhu
Nixon & Peabody LLP
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