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

It’s quick

In most cases, the variable compression ratio control speed required for a VCR engine is overestimated. In the first years of 2000, this overestimation led the designers of MCE‑5 VCRi to set the objective of 100ms to cover the whole compression ratio range from 15:1 to 6:1. Even if MCE‑5 VCRi could offer this type of responsiveness, it would be useless.

The forces generated by gas pressure and
the inertia of moving parts allow the rapid
movement of the MCE‑5 VCRi control jack
via a robust and reliable ball lifter device

The operation of the MCE‑5 VCRi control jack was
thoroughly studied on specific test benches before
being implemented on prototype engines

The most demanding case involves a request for full torque with maximum supercharging pressure starting from idle, with the butterfly valve closed. This requires rapidly moving from a 15:1 compression ratio (CR 15) to CR 6. To encounter this situation in real life, you have to imagine a motorist stopped on the side of the road who wants to immediately merge into high-speed traffic, or someone who wants to race away from a traffic light. These cases may exist but it’s not the engine that will cause the delay but rather the supercharging system. All VCR engines must be equipped with one or two turbochargers, which need a certain amount of time to build up pressure. The engine “waits” for its turbochargers for 1 to 1.5 seconds. Even when equipped with a more responsive mechanical supercharger, this delay is at least 1 second. MCE‑5 VCRi, which is currently designed to reduce the compression ratio from CR 15 to CR 6 in roughly 200ms, still “waits” for its turbocharging system, however this high VCR control speed is partially used to “boost up” the turbocharger: a drastic decrease in the compression ratio increases the exhaust gas enthalpy which favors turbine spin up.

If, on the other hand, we consider a rapid increase in the compression ratio from CR 6 to CR 15, we see that its usefulness is limited. An increase of this order in the compression ratio corresponds to a drastic drop in load. In this case, the engine and vehicle inertia will cause momentary injection cut-off, so that maximizing the compression ratio to obtain the best efficiency possible is useless: there’s more than enough time to increase the compression ratio.

Compression ignition (CAI/HCCI) has higher VCR control speed requirements. VCR must be controlled cycle to cycle to accompany the engine in load and speed transients. However, piston altitude variations at TDC (top dead center) are minimal, just a few tenths of a millimeter per cycle, since the load is low and the compression ratio is high. MCE‑5 VCRi has no problem serving this strategy.

Though the need for speed in VCR engines is fairly low, the MCE‑5 VCRi was tested with extremely rapid servo VCR control systems in order to characterize its “intrinsic” speed, i.e. its maximum rapidity, excluding any servo constraints caused by the electrical ball-lifter valves. This gives travel times that can go down as far as 30ms to cover the entire CR control stroke.

When we take into account the present and future needs for rapidity in VCR control, MCE‑5 VCRi is designed to meet them all, regardless of the scenario.

When used in the best possible configuration, the MCE‑5 VCRi control jack is extremely rapid.
Such high actuation speeds are useless in real life