Thermodynamic Evaluation of Variable Valve Timing 921479

The optimization of valve train actuation for internal combustion engines through the fixed geometry camshaft is a compromise between the required torque, the fuel consumption, idle characteristics and exhaust emissions. Some automobile manufacturers, however, have already incorporated in some models the variable valve actuation; in these systems, the duration of intake process, valve overlapping or both can be modified by the engine operation condition.
Although experimental tests are the decisive ones, their costs can be minimized if it is possible to adopt some mathematical modeling of the engine physical phenomena in order to obtain qualitative tendencies. The analysis of the engine performance modifications associated with a variable camshaft actuation can be obtained - at least as a first approximation - through thermodynamic simulation models. Then the number of necessary experimental work can be reduced.
In this work the effects of variable valve actuation on the indicated horsepower the specific fuel consumption and the volumetric efficiency are discussed, using a thermodynamic engine model. The engine speed and load effects over the residual gas fraction, over the weak-spring indicator diagram and over other performance parameters are analyzed. Thermodynamic irreversibilities in the exhaust and intake processes are also evaluated through exergetic analysis, complementary to the usual engine analysis.


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