Poster Presentation

Prof. Dr. Victor Gheorghiu, University of Applied Sciencies, Germany

In conventional engines, because the volumetric compression and expansion strokes are nearly identical and the cylinder filling is completely, the effective compression ratio and the effective expansion ratio are basically identical. Consequently, any attempt to increase the expansion ratio (also increases the volumetric compression ratio) or to enhance the charge pressure level, inevitably resulting in knocking (by SI engines) respectively in excessive combustion pressure (by CI engines) and placing a limit on increases in the expansion ratio.

As well known the thermal conversion efficiency increases when the effective compression ratio grows up and/or the effective expansion becomes completed. Because in the case of the Atkinson cycle the expansion ratio is increased in comparison to the effective compression ratio and the latter is at a high level, this cycle should theoretically yield higher thermal conversion efficiency. Usual for realizing a modified Atkinson cycle, the volumetric compression ratio is increased and at the same time the timing of the intake valve closing is delayed (like Toyota uses in its Prius II has done). Consequently in the initial stage of the compression stroke, some of the air that has entered the cylinder is returned to the intake manifold, in effect delaying the start of compression. In this way, the expansion ratio is increased without increasing the effective compression ratio. This implementation of the modified Atkinson cycle is not the optimal solution because the oscillating air stream from respectively to intake manifold through intake valve port considerably reduces the thermal conversion efficiency of the cycle. The potential of this implementation is investigated here in a number of different variants and shows that the benefits in the form of increased efficiency would be minimal in this kind of Atkinson cycle implementation. Furthermore in the case of aspirated engines, that leads to a lower imep of the cycle. For this reason an engine with a bigger displaced volume becomes necessary for achieving the same power as a conventional one.

As additional variants, the use of a new kind of crankshaft drive will be presented which permits different size strokes for compression and expansion. In this case the improvements in the efficiency are clearly visible, even without supercharging of the engine.

In addition the improvement potential of the thermal conversion efficiency for supercharged SI and Diesel engines will be analyzed and evaluated in detail. The lower imep disadvantage from aspirated engines is negated by supercharged engines because of the possibility to increase the charge pressure while keeping the charge temperature down. Thus it becomes possible to achieve simultaneous a higher imep and a higher thermal conversion efficiency as in the case of the classic supercharged engines without exceeding the usual maximum of pressure and temperature on the cycle. This behavior is detailed demonstrated in the present paper.

This abstract is supplemented by a PDF, which can be viewed here.

Session: IC Engines, Goals and Development