1. Field of the Invention
The present invention relates to a system for electronically controlling a diesel engine during the cold-engine warm-up period, and specifically to a diesel-engine control system capable of reconciling slow initial combustion (slow early-stage combustion) and sharp middle-stage combustion, while controlling both a combustion temperature and an ignition delay duration of diesel fuel injected into the combustion chamber even during the cold-engine warm-up period.
2. Description of the Prior Art
In Diesel engines, when the combustion temperature is properly reduced by application of heavy exhaust gas recirculation (EGR) to reduce NO.sub.x (nitrogen oxides) emissions, the ignition delay duration between the start of fuel-injection and the start of ignition tends to be increased. As a result of this, the combustion temperature drops and additionally the combustion rate of the latter stage of the power stroke to the former stage increases. Also, owing to the increased ignition delay duration, some places in the engine cylinders lack a sufficient supply of oxygen. This results in the formation of particulate matter (PM) and produces unburned fuel (unburned HCs) and unburned gases such as carbon monoxide (CO). To improve such a trade-off relationship between the reduced NO.sub.x emissions and the increased particulate matter (soot that causes black smoke in the exhaust) and unburned HC and CO, recently, there has been proposed and developed a new combustion concept with respect to a conventional in-cylinder direct-injection diesel engine. In order to suppress the formation of NO.sub.x emissions and to simultaneously reduce PM (smoke), Japanese Patent Provisional Publication No. 8-86251 teaches the ignition-delay-duration control and the use of strong swirl. According to the Japanese Patent Provisional Publication No. 8-86251, the ignition delay duration is positively increased by lowering the combustion temperature depending on engine operating conditions, and additionally strong swirl motion is produced in the combustion chamber, so as to simultaneously reduce both NO.sub.x emissions and particulate matter (smoke). When the combustion temperature drops, NO.sub.x density can be reduced. At this time, if the ignition delay duration is increased, the exhaust smoke density can be reduced by virtue of the generation of swirl motion. As is generally known, the combustion process of a usual diesel engine comprises a premixed combustion duration (an initial combustion duration corresponding to the early stage of the combustion process) where the air-fuel mixtures premixed during the ignition delay duration rapidly burn and thus combustion takes place all at once, raising the combustion temperature, and a diffusion combustion (a main combustion often called a controlled combustion duration) where the burning velocity is limited by the diffusion rate of the diesel fuel and air and also the diffusion combustion is controlled depending on the rate of fuel injection, since the mixture is combusted as the fuel is injected. On diesel engines, the diffusion combustion follows the premixed combustion. The premixed combustion tends to produce little soot, as compared with a conventional diffusion combustion. As discussed above, in the case that swirl motion is created in the combustion chamber in addition to the positively increased ignition delay duration, such a swirl motion promotes mixing of the air and the fuel spray injected from the fuel injector nozzle. Owing to both the positively increased ignition delay period based on the combustion-temperature drop and the generation of the strong swirl motion, more of the combustion process tends to become the premixed combustion. This suppresses the formation of soot that causes black smoke in the exhaust.
Just after the engine starts to run, the engine, the combustion chamber, and the diesel fuel are all cold. During the initial warm-up period (or during cold engine operation), the ignition delay duration between the start of injection and the start of ignition is extended, and thus combustion is retarded. This results in the generation of white smoke (increased unburned hydrocarbon emissions in the exhaust) and also the combustion is apt to become unstable. To avoid this, in conventional diesel engines, generally, the fuel-injection timing is often advanced during the cold engine operation.
Japanese Patent Provisional Publication Nos. 6-108926 and 8-74676 disclose another control methods for exhaust-gas recirculation (EGR) amount to reduce exhaust emissions and to enhance driveability during the cold engine operation or during the engine warm-up period. The Japanese Patent Provisional publication No.6-108926 teaches the adjustment of an intake throttle opening based on the engine coolant temperature. On the other hand, the Japanese Patent Provisional publication No. 8-74676 teaches the adjustment of an exhaust-gas recirculation (EGR) valve opening based on the engine coolant temperature.
In general, diesel engines have the advantage of more superior fuel economy in comparison with spark-ignition gasoline engines, and particularly have the advantage of a high thermal efficiency at partial loads. In other words, specifically in direct-injection diesel engines, there is less heat being lost to the engine coolant, thus deteriorating the heating performance (or the warming-up performance) during the cold-engine operation, than in spark-ignition gasoline engines. To avoid the heating performance from being lowered during the cold-engine warm-up period in diesel engines, Japanese patent Provisional Publication No. 8-93510 teaches the adjustment of an exhaust-gas temperature. In the heater device disclosed in the Japanese patent Provisional Publication No. 8-93510, the exhaust temperature is adjusted by way of movement of the exhaust throttle valve depending on the engine operating conditions, and whereby the heating performance can be enhanced without undesirably increasing the exhaust smoke density.
In recent years, it is necessary to simultaneously reduce both NO.sub.x emissions and particulate matter (PM) for example smoke, from the viewpoint of exhaust-emission purification. That is, it is necessary to further reduce exhaust emissions produced during the cold-engine warm-up period. However, if the injection timing is compensated for such that the timing is advanced in order to prevent reduction in the driveability and generation of white smoke (unburnt hydrocarbons), in lieu of thereof NO.sub.x emissions are built up to a high level.
When reduction in NO.sub.x emissions is attempted with the increased EGR amount during the cold-engine warm-up period in the same manner as after the engine warm-up. the engine is apt to misfire owing to an increased cooling loss arisen from a lower engine-cylinder wall temperature than with after the engine warm-up period. This results in unstable combustion in the engine cylinder. Also, there is a possibility of the white smoke formation and the generation of nasty smell.
Furthermore, according to the combustion concept disclosed in the Japanese Patent Provisional Publication No. 8-86251, in the case that the ignition delay duration of the fuel injected into the combustion chamber is prolonged under low engine coolant temperatures with the combustion temperature dropped, the premixed combustion rate increases. Thus, there is a tendency for white smoke caused by unburnt fuel and/or soluble organic substance (SOF) contained in the particulate matter (PM) to increase.
Moreover, when the intake throttle opening and/or the exhaust throttle opening are adjusted for the purpose of improvement of heating performance during cold-engine operation, there is a possibility that the smoke emission density, such as white smoke or black smoke, increases. This deteriorates the stability of the engine under particular engine operations, for example during lower engine loads. The rising of the exhaust-gas temperature achieved through adjustment of the intake throttle opening and/or the exhaust throttle opening means deterioration in fuel consumption. Therefore, it is desirable to enhance the heating performance without deteriorating the fuel economy. Generally, there is a tendency for friction loss of the engine to increase during the cold engine operation or during the warm-up period. Thus, it is desirable to rapidly complete the engine warm-up operation so as to reduce both fuel consumption and exhaust emissions.