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

VCR actuator

The MCE‑5 VCRi VCR actuator, also called the “VCR control jack”, is made up of a hydraulic power stage and an electromechanical control stage. The hydraulic stage comprises a follower jack that is displaced by gas pressure and engine inertia, making a hydraulic pump unnecessary. This makes energy consumption minimal, only a few dozen watts on average in NEDC. The jack acts as a hydraulic “ratchet”. The system comprises two balls, each with a double function: they act as a simple valve when they are lifted from their seat by a push-rod, and act as a check valve when resting in their seat. It’s the check function of these balls that produces the ratchet effect. The choice of balls was driven by the need for robustness but also because they ensure a large passage section and excellent sealing, while being highly tolerant to the impurities found in engine oil (particles, combustion residues).

Each cylinder of the MCE‑5 VCRi has an independant
VCR actuator, which opens the way to various
industrial and functional strategies

The MCE‑5 VCR actuator uses gases pressure and
inertia forces to move up and down, thanks
to balls that can be lifted from their seat

When an increase in compression ratio is required, the ball in the lower chamber of the VCR actuator is maintained in the open position by its push-rod for as long as required for the compression ratio set point to be reached. A 2D Hall-effect sensor indicates the position of the control rack to more or less 0.02 mm, so that when the set point position is reached, the ball is immediately returned to its seat. For the opposite case (to move from the current ratio to a lower one), the operation is exactly opposite. A Hall-effect passage sensor was also developed to deduce the position of the combustion piston at TDC (Top Dead Center) from its point of passage in front of the sensor. A target thereby passes in front of the sensor, to be specific, the lower edge of the piston combustion rack. Depending on the crankshaft angle at which piston passage was detected, it is possible to accurately deduce its final altitude at TDC.

The ball-lift compression ratio control system in MCE‑5 VCRi prototype engines is the “proportional” type, i.e. the balls can be lifted from their seat at any value between 0 and 1 mm. This result is obtained using a brushless electric motor equipped with 3 Hall targets that deliver three “signals” per rotation. A set of gears and push-rods that screw or unscrew make it possible to reach the sought-after result. Various functions for resetting, for compression ratio drift measurement and for “hydraulic zero” (accidental absence of oil in the jack) are used to manage all situations. Other versions of the system are being studied with direct acting electromagnetic coils or staged hydroelectromagnetic ball lifters (using the hydraulic prepressurization circuit of the VCR jacks to lift the balls). The final industrialized solution will be the one that provides the best compromise between cost and efficiency, at comparable robustness.

The VCR control piston has physical stops at each end of its maximum stroke. These stops are never reached during normal operation. They define the engine’s maximum compression ratio. During assembly, the engine is adjusted so that the combustion piston can never collide with the cylinder head, even when the maximum CR stop is reached. The adjustment is made by inserting a washer between the upper control rack rod and the piston of the VCR actuator. The physical stops guarantee the same maximum altitude for all the engine pistons at TDC when they are in contact. The adjustment washer does not modify piston stroke that remains unchanged from stop to stop, except the dispersion in terms of the height of the VCR control pistons and of the thickness of the stops, as well as the depth of the VCR actuator chambers in the crankcase.

The MCE‑5 VCRi VCR actuator uses the same oil as is used to lubricate the engine. This oil is pre-pressurized to between 40 and 60 bar. A small pressure accumulator housed in the engine lubrication sump ensures the pre-pressurization. Once the pressure is accumulated by a miniature electric pump, no further energy consumption is required. This miniature pump is only there to compensate for any possible leaks in the entire VCR system. This pre-pressurization reduces oil compressibility for greater VCR actuator stability, eliminates risks of cavitation in the actuator chambers and makes the maneuvering forces of the VCR actuator asymmetrical to help it when increasing the compression ratio.

The MCE‑5 VCRi VCR actuator has two seals: one on the piston and one on the control rack rod. They are composite seals with an external Teflon ring loaded with friction and wear resistant particles and an inner Viton® O-ring. The rod seal is particularly tolerant to excentering. This is necessary since the control rack can move freely with respect to the actuator piston, which behaves like a ball joint. It can rotate freely in the 3 axes, which is vital for an even distribution of contact pressure between the control rack teeth and the gear wheel teeth. The piston of the VCR actuator is housed in a bore in the crankcase. There is a hydraulic jack-type steel liner within the bore. The radial stress exerted by the piston on the liner never changes direction: it is always directed towards the outside of the crankcase. Hence, there is no piston tilting or any noise emissions at the piston / cylinder interface of the MCE‑5 VCR actuator. To reduce the Hertz pressure at the point of contact between the piston and the liner, the piston is shaped like a barrel with a radius of roughly 500 mm.

The MCE‑5 VCR actuator is managed by the EMS (Engine Management System) via the central engine control unit and specific electronic control units. In the prototype version, the brushless motors are controlled via long cables. In future, each cylinder’s individual control system (direct acting electromagnetic coils or staged hydro-electromagnetic ball lifters or brushless motor) will have its own local controller, with a rapid bus between the EMS and the local controllers.

Current MCE‑5 VCRi prototypes are equipped
with a proportional-type ball lifter

A Hall 2D position sensor makes it possible to
measure the vertical position of the VCR
control rack with a precision of ± 0.02 mm

The main characteristics of the MCE‑5 VCRi VCR actuator