 Four-stroke engine
For now, let's stick with the reciprocating piston engine, otherwise we'll never get a clear answer to any questions that may arise. The following section first describes the processes and
then the mathematical variables (don't worry!) that clearly describe such an engine.
A cycle of the four-stroke process begins with the intake stroke. It is called this because the cylinder chamber expands as the piston moves toward TDC. The pressure drops to just below atmospheric pressure,
which now fills the chamber with pure air in direct injection engines or with an air-fuel mixture in other engines.
If the engine is turbocharged or supercharged, the term 'intake stroke' loses its meaning. Incidentally, this cycle does not always guarantee the cooling effect of a temperature drop to approx. 100°C. However, this can be
corrected somewhat by intercooling. In some modes, e.g., gasoline direct injection, injection already takes place at this cycle.
After the reversal point, the piston moves toward TDC. In most cases, the valves are closed quite early in the compression stroke. The higher the speed of an engine, the longer one attempts to utilize the inertia of the fresh
gas column by leaving the intake valve open, thereby improving the filling.
However, there is also the opposite case, in which air is deliberately allowed to return to the intake system. More on Atkinson/Miller later. At some point, however, every four-stroke engine must compress, because that was
the purpose of its invention. What remains for ignition is now finally the filling. Depending on engine speed, temperature and other factors, this process is initiated at some point before top dead center (TDC).
The enormous increase in pressure caused by combustion drives the piston once again toward TDC. The work cycle must supply all the energy that the other cycles consume. And, of course, even more, because only
when everything that is still deducted in a combustion engine has been mathematically subtracted do the DIN kW begin to count at the flywheel.
Open the exhaust valves before the piston reaches TDC. This is because the exhaust gases need a certain amount of time to decide to exit. Similarly, the exhaust valves remain open even into the intake stroke in order to
utilize the energy of the escaping exhaust gases for the fresh gases. This is called 'valve overlap'.
If the engine control system is designed to be variable, the valves can be controlled in such a way that exhaust gas is deliberately left in the engine. Internal exhaust gas recirculation is the name given to this measure,
which does not exactly increase performance. It is part of the comprehensive exhaust gas detoxification of combustion engines.
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