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

It doesn’t change piston motion

Conventional engine piston motion is a real “stroke of luck” as it works well. When we move too far away from the piston motion induced by conventional conrod-crank mechanisms, we reduce engine efficiency. For example, the rod/crank ratio directly influences the natural filling of the cylinder according to engine speed: an engine with a long conrod is naturally more powerful at high speeds and an engine with a short conrod fills better at low speeds.

MCE‑5 VCRi's mechanical arrangement ensures
a total absence of variation in piston
stroke and in piston motion

Different investigations were carried out using cam engines to study the influence of piston motion on efficiency. After testing, the choice of a standard system is generally made since the alternatives don’t offer sufficiently significant advantages, and on the contrary, often have disadvantages.

Piston motion conditions cylinder filling but also the turbulence required by the flame to quickly propagate in the combustion chamber volume. The stopping of the piston at top dead center (TDC) determines the time left for combustion to develop before expansion. The stopping of the piston at bottom dead center (BDC) allows the evacuation of the burnt gas from the cylinder (exhaust) by producing the minimum back pressure, as the latter opposes the rise of the piston (undesirable negative work).

All things considered, the most favorable piston motion comes for the standard rod-crank mechanism. By maintaining this motion, we have sufficient piston speed near top dead center to create enough tumble in the cylinder during the intake phase. This entire charge motion is then transformed into fine turbulence near top dead center at the end of the compression stroke.

This turbulence considerably accelerates the flame, enabling “later and faster” combustion. Consequently, by favoring this rapid combustion, we reduce the engine’s sensitivity to knock and heat losses while increasing the “effective” expansion ratio, which results in better efficiency.

MCE‑5 VCRi piston motion is strictly identical to that of a conventional engine with the same
rod/crank ratio, regardless of its compression ratio (example: from 6:1 to 15:1)

"Multilink" VCR concepts can lead to an "exotic"
piston motion, which penalizes certain
engine operating points

Depending on the chosen configuration,
the overly long stop at TDC of certain "multilink"
engines increases knock sensitivity and heat losses

At bottom dead center at the end of expansion, it is preferable to have enough time to empty the cylinder of burnt gas while minimizing residual pressure working “against” the piston when it starts to rise (limiting EGBP – exhaust gas back pressure).

It is clear that the right objective is to use conventional engine piston motion. Yet, some VCR engines modify this motion to create “exotic” motions. This is the case of multilink engines that add rods and levers. These engines could use standard motion but at the cost of greater complexity, which results in added weight, size and friction. This type of engine generally behaves like a conventional engine with an infinitely long conrod. The stops at TDC become too long and those at BDC become too short.

The resulting piston motion penalizes combustion efficiency, particularly at high loads with spark ignition.

On these VCR engines, the thermodynamic losses caused by an overly slow piston at TDC can reach 8% or more at medium and high loads. This is due to an over-sensitization of the engine to knock and an increase in heat losses. In this case, VCR loses all or part of its advantages since medium and high loads are often used on a hard downsized VCR engine. On the other hand, the thermodynamic losses caused by this type of motion remain low if the engine uses compression auto ignition (CAI-HCCI), which only concerns low loads below 5-6 bar of BMEP (brake mean effective pressure). In this case, the gains made in CAI at low loads are largely used to compensate for the losses in SI at high loads. The result is so disappointing that it is finally preferable to continue to produce simpler and less expensive conventional engines.

Other VCR engines make piston motion dissymmetrical: the piston “rises” more quickly than it “descends”. The result of this type of approach is that these engines should run clockwise at low loads and counter-clockwise at high loads, which is impossible.

For all of these reasons, MCE‑5 VCRi maintains strictly conventional piston motion. Current MCE‑5 VCRi prototypes associate a low bore/stroke ratio (long stroke) favoring filling at low engines speeds, with a high conrod/crank ratio (long conrod) favoring filling at high engine speeds. A conventional gasoline engine with the same features would have exactly the same height, proof of MCE‑5 VCRi’s intrinsic compactness. Hence, MCE‑5 VCRi does not cause any fault attributable to non-conventional piston motion and allows combustion engineers to directly apply their know-how to VCR without any penalties.

In its SAE 2206-32-0098 publication, Honda underlines the significant consequences
of piston motion on engine thermal efficiency (above: impact of the rod/crank ratio)