MCE-5 DEVELOPMENT has been developing its variable compression ratio engine (VCRi) for over a decade aiming at reducing fuel consumption and pollutant emissions. In order to transmit power from the piston (combustion) to the crankshaft, the MCE-5 VCRi technology is based on three innovative components: a gear wheel and two racks. This gear mechanism ensures a very low friction compared to other continuous VCR solutions based on bearings. However, this transmission is used in nonstandard conditions: the direction of rotation is reversed repeatedly, and the parts are submitted to high and rapidly varying loads. To avoid interferences and alteration caused by high contact pressure at high load, and ensure a regular transmission at low load, the profile of the teeth is carefully considered. A crowning shape is placed on the teeth in the direction of the gear axis, and a correction is applied to the tooth active profiles in the area of tooth roots and tooth tips. A Design of Experiment (DOE) is realized on a light Finite Element 2D model in order to highlight the influence of the parameters of the correction on the maximum stress and the contact pressure on the teeth, and on the speed ratio between the gear and racks. A refined Finite Element 3D model is developed in order to obtain more accurate results concerning the contact pressure and the stress on the teeth, and to take into account the crowning shape. Endurance experiments were conducted on engine test benches before and after optimization of the tooth profile. Observations of the teeth show the benefice of this method, with a suppression of the wear and pitting on the surface contact.
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