MCE-5 VCRi: Pushing back the fuel consumption reduction limits

Racks

Along with the gearwheel, the racks are the most complex parts of MCE‑5 VCRi. These parts posed numerous problems that have today been fully resolved. The most complex of the two racks is the piston rack. This part is light, precise (5 quality class teeth according to the ISO 1328 standard) and comprises many functional surfaces: gear teeth, front central rolling surface, rear rolling surface, rear synchronization rack, piston support and bolting face, upper centering, internal thread for piston screw.

The MCE‑5 VCRi combustion rack is connected
to the aluminum piston via an interface
fixing sleeve and an internal stud bolt

Several years of Finite Element Analysis (FEA)
studies were necessary to design the MCE‑5 VCRi
racks, in correlation with test results

The morphology of the piston rack is quite different from that of the control rack: the first is mobile and the second is fixed, except for the compression ratio variation. Most of the work focused on the piston rack but the control rack naturally benefited from it. The weight of the later was recently optimized, dropping from 1400 to 900 grams with the same bending under load and maximum stress (MPa).

Another difficulty linked to the MCE‑5 VCRi racks is that they don’t leave any room for conventional machining tools: there is no place to insert trimming or grinding wheels. Since they are highly loaded, with roughly 57,000 N of tangential stress for an in-cylinder pressure of 130 bar, they are made of steel with excellent mechanical qualities that is carburized and hardened. The route sheet for these parts nevertheless remains simple and their cost reasonable.

It took almost five years to solve the challenge of manufacturing the MCE‑5 VCRi racks, with total success. In the end, MCE‑5 VCRi racks are manufactured using Near-Netshape forging (blanking, forging, cold sizing) which increases their fatigue strength, through the fibering of the material. After various conventional machining processes that follow forging, they are carburized and then hardened. The finishing of the teeth and the central rolling surface is done with precision electrochemical machining. This process makes it possible to approach the ideal geometry at ± 2 microns in close to 1 minute. Precision electrochemical machining is specific in that it does not cause any tool wear (stainless steel electrode) and has perfect repeatability. At the end of the manufacturing process, the gear teeth of the combustion rack and VCR actuation rack are subjected to shot peening and tribofinishing.

These two processes further improve the fatigue strength of the gear teeth and facilitate lubrication (series of lubricant “pockets” and hydrodynamic lift plateaus). These manufacturing processes are strategic: they make it possible to produce MCE‑5 VCRi racks according to production schedules, costs and functional safety factors that meet the most stringent expectations for automotive mass production.

As was the case for the combustion rack, advanced topological optimization tools
were used to design the MCE‑5 VCRi’s VCR control rack

The main characteristics of the MCE‑5 VCRi racks