Internal-combustion engines – Convertible cycle
Reexamination Certificate
1999-03-15
2001-09-11
Kwon, John (Department: 3747)
Internal-combustion engines
Convertible cycle
C123S064000
Reexamination Certificate
active
06286466
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a piston internal-combustion engine having a control system for controlling the stroke sequence of the engine-operating process as a function of operating parameters, and to a process for controlling and automatically controlling the operating mode of a piston internal-combustion with a variable sequence of strokes as a function of operating parameters.
DE-A-196 31 799 describes a piston internal-combustion engine and a process in which a piston internal-combustion engine has six working strokes and a variable valve timing is operated by four strokes-during a warm-up phase and by six strokes after the operating temperature has been reached. The number of strokes can only be varied between a four and six stroke operation, with a pushing of the combustion products out of the combustion space of the cylinder in all cases taking place during the fourth stroke. The fifth stroke is a working stroke. At its start, water is sprayed into the cylinder in order to generate water vapor in the cylinder which is still hot from the preceding combustion. The pressure of this water vapor moves the piston further along. The sixth stroke is used for pushing out the water vapor.
DE-A 33 17 128 discloses an internal-combustion engine having a six-stroke cycle, in which the push-out stroke for pushing out the combustion products is always the sixth stroke.
Both known concepts have in common that the number of strokes of the internal-combustion engine is either fixed from the beginning or can be varied between only two values. The combustion products are pushed out of the cylinder no later than during the sixth stroke, irrespective of whether they still contain combustible parts or residual oxygen or not. As a result, a portion of the energy which can still be utilized in the cylinder is lost. Because power losses occur with each charge cycle as the result of valve movements and flows, premature charge cycles have a negative effect on the efficiency of the internal-combustion engine.
Modern directly injecting NFZ diesel engines reach an efficiency which is comparatively high for internal-combustion engines with effective efficiencies of up to 45% in the full-load range. In the partial load range, the efficiency is reduced to values of about 30% and below. This is the result of the lower pressure level and temperature level of the combustion, the high charge cycle losses caused by the pressure gradient from the exhaust gas side to the intake side and the high proportion of mechanical losses. Because internal-combustion engines in vehicles are operated in the partial load range for a considerable portion of their operation, this represents a serious disadvantage.
Measures which endeavor to obtain an improvement of the internal efficiency by raising the internal-combustion pressure level and the temperature level, however, result in a rise of the nitrogen oxide (NO
x
) emissions. Furthermore, the capability to be mechanically and thermally stressed with respect to the peak pressure and the internal-combustion temperature in the full-load operation represents the limit for the above-mentioned measures furthering the partial load operation.
SUMMARY OF THE INVENTION
An object of the present invention is to increase the efficiency of piston internal-combustion engines, particularly in the partial-load range, and in the process minimize the pollutant emissions.
With respect to the piston internal-combustion engine, this object has been achieved by providing that a control system is provided which controls the stroke sequence of the engine-operating process such that the process has a variable cycle of at least four strokes, and in that devices are provided so that a pushing of a charge out of a combustion space of the piston internal-combustion engine does not take place before at least one operating parameter has reached a limit value defined therefor it, the engine-operating process takes place in a variable cycle of at least four strokes, and the sequence of strokes is varied such that a pushing of a charge out of the combustion space of the piston internal-combustion engine does not take place before at least one operating parameter has reached a limit value defined therefor.
These measures have the advantageous effect that the number of charge cycles per n working strokes is reduced, resulting in the lowering of the charge cycle losses mainly in those ranges of the characteristic diagram in which a negative pressure gradient (charge pressure<exhaust gas counterpressure) exists in the four-stroke operation. If, for example, the pushing-out does not take place before the eighth stroke, the air flow rate will be reduced by approximately half and, together with it, the charge cycle work will be reduced by approximately the same amount. Since, instead of being operated once per two crankshaft. rotations in the four-stroke operation, each valve is operated only once per four crankshaft rotations in the eight-stroke operation, the power to be applied for this purpose is also reduced by 50%. A special advantage achieved with the present invention in contrast to the prior art is the fact that the number and sequence of the strokes, starting from the fourth stroke, are flexibly adapted to the momentary operating conditions of the internal-combustion engine.
Assuming the existence of the same emission concentrations, the lowering of the exhaust gas flow rate leads to the reduction of the pollutant emission. The renewed compression(s) and combustion(s) of the hot exhaust gas, the combustion temperature and the ignition pressure are at a higher level, and thus the HC emissions in the exhaust gas are lowered. The exhaust gas which occurs and remains in the combustion space during the first combustion and during all following combustions compensates the formation of NO
x
which increases because of higher process temperatures because the mechanisms are in effect which are known from the exhaust gas recirculation.
The lower spark gaps also have a positive effect on the vibration behavior and the generation of noise of the internal-combustion engine.
A particularly advantageous further feature of the present invention involves the fact that a control system is provided which controls the intake and exhaust valves of the internal-combustion engine such that a charge will not be pushed out of the combustion space and the exhaust valve will not be opened up before the air-fuel ratio &lgr; existing in the combustion space has reached a lower limit value &lgr;
limit
. Therefore, during the overstoichiometric combustion, the residual oxygen not yet used for a combustion can be used for further working strokes. The reason is that, after its first combustion, the air-exhaust gas mixture passes through further combustion cycles and, in this operating mode, the charge cycle takes place at the earliest during the sixth stroke, and depending on the amount of air excess, also later. During an acceleration operation, the increased mixture quantity can already be charged one crankshaft rotation after the preceding combustion stroke, so that the response behavior of the engine will increase in the transient operation.
A charge cycle as a function of the air-fuel ratio &lgr; is advantageous particularly in the case of diesel and Otto direct-injection lean-mixture engines. In the case of diesel engines, the air-fuel ratio &lgr; changes over the load range. In the full-load range, values of &lgr;=1.2 are reached, while, in the lower partial load range, the mixture is made lean to &lgr;=3 to &lgr;=7. At &lgr;=3, the exhaust gas contains a residual oxygen amount which would be sufficient for one additional combustion operation at &lgr;=7, for example, it would even be sufficient for four additional combustion operations.
When the engines are charged, the higher exhaust gas temperature also has an advantageous effect on the charge pressure buildup because, relative to the carried-through mass, the energy available at the exhaust gas turbochar
Braun Tillman
Gaertner Uwe
Marquardt Klaus-J.
Crowell & Moring LLP
Daimler-Chrysler AG
Kwon John
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