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

It has a wide control range

The compression ratio control range is a decisive factor in the energy performance of VCR. This point must be considered very carefully as it would be a shame to produce a VCR engine with limited compression ratio control that would reduce its potential and effectiveness. The question is: what are the minimum and maximum compression ratios required to serve all VCR strategies, without any exceptions?

With a 10.8 mm compression ratio control stroke
the MCE‑5 VCRi permits a CR variation
ranging from 6:1 to 15:1

A 6:1 minimum compression ratio eliminates
the risk of hard-knocking at very high loads
and minimizes the consequence of such knocking

The work performed by MCE‑5 DEVELOPMENT concluded that there are two distinct needs depending on whether the engine runs solely on SI (spark ignition) or on SI and CAI (Controlled Auto Ignition). This need is 6:1 to 15:1 in the first case and 6:1 to 18:1 in the second case.

Paradoxically, the essential part of VCR-generated fuel savings stem from the minimum compression ratio of 6:1 that safely allows very high specific torques at low engine speeds with very acceptable levels of BSFC (Brake Specific Fuel Consumption) at high loads. For example, an MPFI VCR engine that can deliver 28 bar of BMEP at 1500 rpm with a compression ratio of 7:1 delivers a BMEP of 35 bar with a 6:1 CR – the specific torque is increased by 25%. The impact of this increase in torque enables a new ratio of gears resulting in an additional drop of roughly 3.6% in fuel consumption in NEDC, with a manual transmission:

With regard to the maximum power, a 7:1 CR is acceptable in order to reach very high specific powers:

With regard to the maximum compression ratio, we note that in spark ignition, there are no further efficiency gains to be expected beyond 15:1 – we are on the flat part of the efficiency asymptote as a function of compression ratio.

The 18:1 CR is nevertheless useful when we use an engine in compression ignition (CAI-HCCI) to ignite at low loads while taking into account other factors such as the EGR (Exhaust Gas Recirculation) ratio. This type of compression ratio is difficult to achieve in a gasoline engine because of the pent-roof combustion chamber and the valve pockets. These two constraints bring the piston crown dangerously close to the cylinder head, which it must never come into contact with.

Current MCE‑5 VCRi prototypes have a 13 mm compression ratio control stroke for a total piston stroke of 84 mm. Only 10.8 mm of these 13 mm are used, the remainder is used for assembly and the initial adjustment (stroke remaining between the physical stops at min and max CR). This stroke length meets the most demanding requirements for compression ratio control range.

Simultaneously reaching high specific power and high specific torque significantly reduces CO2 emissions,
particularly for vehicles equipped with a manual gearbox (58% of worldwide market in 2008)

The ideal compression ratio map for a 2-stage turbo GDI VCR engine
proves the need for VCR control ranging from 6:1 and 15:1