Internal-combustion engines – Charge forming device – Heating of combustible mixture
Reexamination Certificate
1999-12-22
2001-10-02
McMahon, Marguerite (Department: 3747)
Internal-combustion engines
Charge forming device
Heating of combustible mixture
Reexamination Certificate
active
06295973
ABSTRACT:
TECHNICAL FIELD
The invention relates to a controller for the timing of auto-ignition for homogeneous-charge, compression-ignition engines.
BACKGROUND OF THE INVENTION
It is known practice to design four-stroke cycle internal combustion engines to accommodate homogeneous-charge, compression-ignition (HCCI) combustion wherein light-load operation can be achieved with minimal throttling. This results in fuel economy comparable to the fuel economy of a diesel engine of the same displacement. A homogeneous charge of fuel and air is used in a HCCI engine in a manner similar to the use of air-fuel mixture in a spark-ignited engine, but the homogeneous charge is compressed to auto-ignition. The homogeneous-charge, compression-ignition engine thus has characteristics that are comparable in some respects to an engine with a diesel cycle.
The temperature of an air-fuel mixture in the combustion chamber of a homogeneous-charge, compression-ignition engine (HCCI) is high enough to initiate auto-ignition. The homogeneous air-fuel mixture is created either in the intake manifold or in the cylinder by early fuel injection and fast fuel-air mixing. A homogeneous air-fuel mixture in the intake manifold may be achieved, as in the case of a conventional auto cycle engine, by using a fuel-aspirating carburetor or by using a low-pressure fuel injection pump and nozzle. No spark ignition is necessary when the HCCI engine is operating in a specified operating region of the load and engine speed relationship.
It is known design practice also to expand the useful operating region of the load and engine speed relationship by using a hybrid ignition controller wherein spark ignition can be relied upon during operation at high loads and at high engine speeds with moderate loads. Thus, at lower loads, the engine can be operated in HCCI combustion mode with high dilution of the air-fuel mixture using a high air-fuel ratio or a high exhaust gas recirculation rate in order to limit the rate of combustion of the homogeneous air-fuel mixture. If the mixture is too rich, on the other hand, the rate of combustion becomes too fast and engine knocking or detonation may occur.
HCCI engines are characterized by minimal variation in the combustion of the air-fuel mixture since the initiation of combustion takes place throughout the entire mixture rather than at a single point from which a flame front develops. Instabilities of flame propagation are avoided.
An HCCI engine has reduced levels of nitrous oxide (NO
x
) in the exhaust gases. This is due to the low combustion temperature of the diluted mixture. It is characterized also by reduced soot or particulates in the emission due to the premixed lean mixture.
The thermal efficiency of an HCCI engine is higher than the thermal efficiency typically associated with spark ignition engines of known designs. This is due to the high compression ratio that can be used. It is due also to the unthrottled operation of the air-fuel mixture at the intake manifold, which reduces engine pumping losses. High specific heat ratios, reduced radiation heat loss and reduced cycle-to-cycle variations in combustion are further characteristics of HCCI engines where combustion does not rely upon in-cylinder air flow conditions.
The limitations of HCCI engines of known design relate to the control of the timing of the auto-ignition event and the combustion rate for the air-fuel mixture in the combustion chamber. Since combustion begins with auto-ignition of a premixed air-fuel mixture, the ignition may occur at any time during the compression process. If the engine load increases, auto-ignition tends to advance and the combustion rate tends to increase due to the rich mixture characteristic of an increased load. Thermal efficiency may decrease due to early heat release before top dead center. This results in roughness of the engine due to rapid and early combustion. NO
x
emissions also increase due to increased burnt gas temperature of the less diluted mixture. When the engine load decreases, on the other hand, auto-ignition tends to be retarded, which may result in misfire.
Although it is possible to control auto-ignition by changing the temperature of the air-fuel mixture at the intake manifold using electrical heater devices to promote auto-ignition, such heaters are impractical for high volume production engines used in the automotive industry.
BRIEF DESCRIPTION OF THE INVENTION
It is an objective of the invention to promote efficient combustion in a HCCI engine and to control auto-ignition timing by controlling intake air temperature. This objective is achieved by using a split air intake system to heat the intake air and to adjust the intake air temperature according to the operating conditions. The temperature adjustment of the air-fuel mixture is fast enough to accommodate rapid changes in engine load.
The invention uses hot exhaust gases and engine coolant as thermal energy sources for heating the intake air mixture. According to one embodiment of the invention, the heating of the intake air is controlled by a variable conductance heat pump wherein thermal energy is transferred from the exhaust port to the intake port.
According to another embodiment of the invention, the transfer of thermal energy can take place with a rapid response to the intake air-fuel mixture temperature change by using a dual intake system with two intake passages. When air flows through one passage, it is heated by hot exhaust gases or engine coolant, or both, using heat exchangers. The air or air-fuel mixture in the other passage is unheated. The flow of air in the two passages is mixed at the intake port of the cylinder (or cylinders is in the case of a multiple-cylinder engine). The temperature of the air or the mixed air-fuel flow depends on the mass flow rates in the two passages, one flow rate being the flow rate for the hot gases and the other flow rate being the flow rate for the cooler gases. The relative mass flow rates of the hot and cooler gases are controlled by a flow distribution valve, or by dual intake valves which can change the gas flow through each of the two passages.
During operation in the region of the load-speed characteristic where HCCI combustion occurs, the gases in the intake manifold pass mainly through the heated passage, which results in higher intake air temperatures to promote auto-ignition. Variation in air-flow distribution of the two passages will vary the intake air temperature when the operating conditions are changed.
When the load of the engine increases, HCCI combustion becomes unacceptable because the combustion rate may be too high due to the less-diluted mixture. The engine then may operate under high load conditions with a conventional spark ignition combustion mode. To avoid detonation (knocking) when the engine operates in the spark ignition combustion mode, the intake air temperature should be as low as possible so that the effective compression ratio and the thermal efficiency can be as high as possible. To reduce intake air temperature, the heated air passage closes and the unheated passage opens.
When the engine load decreases, the combustion mode can be switched back from the spark ignition mode to the HCCI operating mode. At that time the intake air temperature should be boosted to promote auto-ignition.
To increase air temperature, the heated passage opens, and the unheated passage closes. There is no thermal inertia involved in this operating sequence. The response of the temperature change of the intake air is fast enough to accommodate rapid changes in engine load.
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“Compression —Ignited Homogeneous Charge Combustion”, by Paul M. Najt et al, SAE Technical Paper No. 830264, 1983, pp. 1-1
Drouillard Jerome R.
Ford Global Technologies Inc.
McMahon Marguerite
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