1. Field of the Invention
The present invention relates to an air-compressing, self-ignition or spark-ignition, four-stroke internal combustion engine having direct fuel injection, turbo-charging, and load-dependent internal exhaust gas recirculation. This exhaust gas recirculation is effected, at least in certain operating ranges, by intervention in the gas change system. Within this gas change system, the mixture formation is essentially controlled by means of the high-speed rotary motion of the fresh air charge, which is produced in an intake channel and is maintained in a combustion chamber having the shape of a solid of revolution.
2. Description of the Prior Art
Diesel engines have qualitative governing or control. There is no throttling and, consequently, volume flow is high. Owing to the difficulty of the task of achieving mixture formation immediately before combustion in a minimum of time, provision is made for more air to be drawn into the combustion chambers of the engine than is actually necessary for the combustion of the fuel injected into the combustion chambers. Generally, the greater the proportion of the inducted air, the lower the load of the engine. Moreover, combustion in the low load range is sluggish and takes place at a low temperature level. Therefore, this is the range where the exhaust gases are unfavorable with respect to the unburnt substances; above all, the great amount of oxygen is conducive to the formation of nitric oxides. This is particularly so in the case of direct-injection engines.
In recent years, it has become general practice, in turbo-charged internal combustion engines, to return a portion of the exhaust gases to the intake in order to reduce emission of pollutants (nitric oxides and hydro-carbons).
Exhaust gas recirculation reduces the oxygen content of the air for combustion and, consequently, the effective excess air in the fresh gases. In other words, the reaction kinetics of combustion are interfered with by means of the oxygen concentration of the cylinder charge, whereby the combustion process and the exhaust gas composition are influenced.
Another very important aspect of exhaust gas recirculation is the reduction of the ignition lag, which includes the time from the start of injection of the fuel to the start of combustion. Ignition lag is a consequence of the higher final compression temperature which results from the higher fresh gas inlet temperature. In addition to other advantages, e.g. the reduction of ignition noise, the shortening of the ignition lag results in an improvement in combustion which, in turn, decreases the emission of pollutants.
In an exhaust gas recirculation control system for diesel engines, it is desirable that the amount of air replaced by exhaust gas should be proportional to the amount of surplus air relative to actual air requirements for the combustion of the injected fuel, such that a maximum amount of surplus air is removed from the air flow supplying the engine cylinder without causing unsteady combustion of the fuel in the cylinders. The objective is to achieve maximum efficiency throughout the operating spectrum of the engine with respect to control of nitric oxide emission.
The maximum rate of exhaust gas recirculation is required in the lower part load range because this is where the greatest amount of excess air exists. In the full-load range, however, a high rate of exhaust gas circulation would reduce the output which the engine can attain because there is only a small amount of excess air. For this reason, it is necessary for the rate of exhaust gas recirculation to be controlled so that the proportion of recirculated exhaust gas decreases as the load of the engine increases until no exhaust gas at all is recirculated at full load.
A distinction is made between external and internal exhaust gas recirculation. In the case of external recirculation, the exhaust gases are returned from the exhaust gas ducts via pipes and control devices into the intake channel. In contrast to this, internal exhaust gas recirculation can be implemented in a simpler manner by appropriate intervention in the gas change system. Therefore, internal recirculation offers certain advantages over the external concept; especially, it affords advantages with respect to hydro-carbon emission in the low-load range (due to the hotter exhaust gas). Last but not least, it will also expedite cold starting and warming-up of the engine.
Intervention in the gas change system in the case of internal exhaust gas recirculation can be effected in various ways (cf. German Auslegeschrift No. 12 22 735 and German Pat. No. 12 42 044), the means employed including, inter alia, double-rise cams for controlling the gas change valves (cf. for instance, German Auslegeschrift No. 14 01 228, German Offenlegungsschrift No. 21 25 368, German Offenlegungsschrift No. 26 38 651, German Offenlegungsschrift No. 27 10 189, German Pat. No. 17 51 473).
Of the references mentioned, only the German Pat. No. 12 42 044 covers a turbo-charged internal combustion engine.
The present invention relates to this type of internal combustion engine, which uses internal exhaust gas recirculation in a similar manner to that already described.
An object of the present invention is to improve such an internal combustion engine of the aforementioned general type in such a way that, on the one hand, exhaust gas recirculation does not cause any weakening of the air swirl energy (charge swirl energy) at low loads, which would spoil the desired exhaust gas improvements, and that, on the other hand, assisted cylinder scavenging would be obtained at high loads.