At the Institute for Internal Combustion Engines and Automotive Engineering at the University of Stuttgart, a single-cylinder gasoline engine with an actually isochoric combustion was developed and experimentally investigated regarding its thermodynamic behaviour.
The constant-volume cycle is the most efficient in terms of thermodynamics. In this case, heat is supplied isochorously, which means at a constant volume. Due to the kinematics of the crank drive of a conventional reciprocating piston engine, combustion would have to take place infinitely fast. However, the conversion of the air-fuel mixture into heat actually requires a certain amount of time. Therefore, the approach taken in the research project was to eliminate the volumetric change during the combustion period. By superimposing the displacement functions of two counter-rotating crankshafts with different speeds and strokes, a variable stagnation of the piston movement around the top dead center is realized. For this purpose, a crankcase with appropriate mechanics was developed, designed and manufactured. By using different kinematic configurations, the dwell time of the piston can be up to 70 degrees crank angle. In the experiment, different stagnation times of the piston were measured and a variation of the ignition timing was carried out. A direct comparison with a conventional crank drive in the same test engine makes it possible to determine the influences of the real isochoric conditions on combustion as well as wall heat transfer. For a more detailed analysis, cylinder pressure and surface temperatures in the combustion chamber were measured simultaneously. Therefore, a special measuring probe was used, that enables a crank angle resolved measurement of the temperatures at the combustion chamber surface.