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


The gearwheel is the most characteristic part of MCE‑5 technology. It’s a spur pinion with teeth that are involutes of a circle, similar to those used in gearboxes, however it has a truncated profile to define two active angular sectors. A 4-cylinder MCE‑5 has 4 gearwheels and 8 racks. Current MCE‑5 gearwheels can bear an in-cylinder pressure of 130 bar and transfer the resulting forces to the connecting rod. According to the present definition of MCE‑5 engine prototypes, the tangential s applied to the gear teeth is 57,000 N (5.7 tons).

The transmission gearwheel is the most emblematic
and recognizable part of the MCE‑5 VCRi

The MCE‑5 gearwheel operating conditions are quite different from those of gearbox pinions:

1. It’s rotation direction reverses with each rotation (piston rising, piston descending), and hence its rotation speed varies continuously (figure 1).

2. It’s lubricated with engine oil that is not formulated for gears (gearboxes use “extreme pressure” oil). This engine oil is moreover loaded in combustion residue and different impurities, contains gasoline and oxidizes between two oil changes in such a way as to lose part of its lubricating ability.

3. It’s not subjected to the same type of shocks as are observed in gearboxes:sudden release of the clutch, accelerator release,re-acceleration (reversing the direction of force along the entire transmission chain) or a collision with a curb. The MCE‑5 gearwheel is placed between the rod and the piston and is subjected to the piston’s inertia forces combined with the gas pressure forces in the combustion chamber. These forces are high but are not like a shock (low gradient).

The rotation speed of the MCE‑5 VCRi gearwheel always remains low, while the high tangential stress and
resulting high Hertz pressure are never combined with the highest instantaneous gearwheel rotational speeds

Finite Element Analysis and topological
optimization were at the heart of MCE‑5 VCRi
gearwheel design studies

Gearwheel inertia has virtually
no influence on MCE‑5 VCRi NVH

4. The pressures applied to the surface of its teeth (Hertz pressure) are comparable to those applied to the surface of the gearbox gear teeth.

5. The product of Pressure x Speed (p.s) during contact with the teeth always remains low (see figure 1): the high in-cylinder pressures and the high inertia forces are always produced at low speeds (near TDC and BDC), while the high rotation speeds are always produced at low forces (mid-stroke: low in-cylinder pressure, low inertia forces). This low p.s product enables a proper lubricant film to be maintained between the gears: the flash temperature is never reached regardless of the operating conditions (oil film vaporization temperature).

The gearwheel is articulated to the rod via a pin. This pin is mounted freely in the gearwheel and is stopped by axial lock rings. The pin housing bore is equipped with oil reservoirs and has a double “trumpet” shape to follow the distortion and ovalization of the pin. The gearwheel’s angular displacement with respect to the rod is higher than that of a conventional piston and its rod (up to twice as high) due to the gearwheel’s own movement in relation to the cylinder. This increase in displacement makes it easier to maintain hydrodynamic lubrication, but it increases the “speed” criteria in the p.s product. The development of the gearwheel pin’s operation took several years of studies and testing.

The MCE‑5 gearwheel runs at a low absolute speed (maximum speed = 2,590 rpm at engine speed 6000 rpm). Its vertical displacement is half that of the piston. It generates an inertia moment due to its alternating rotation whose value is roughly 9 lower than the moment of rotation applied to the engine by variations in crankshaft torque. The combination of the crankshaft torque variations and the gearwheel inertia moment gives an offset in the engine tilt timing (no influence on overall engine NVH – see figure 2).

The MCE‑5 gearwheel is dissymmetrical and has teeth with variable widths: the wide teeth bear the high gas pressure forces at TDC, while the narrow teeth bear the low forces. The narrow teeth also allow the gearwheel to penetrate into the engine cylinder, enabling a reduction in engine height of roughly 25 mm. MCE‑5 gearwheel teeth were designed to generate minimal friction. Their module and number are planned to cover piston stroke and VCR control stroke requirements while keeping high safety coefficients for fatigue strength under alternating stress (short teeth and optimized tooth root radii). To limit friction, the contact ratio is also minimized to reduce the specific sliding of the gear meshing.

The MCE‑5 gearwheel has a central rolling surface that cooperates with that of the piston and control racks. This surface maintains the center-to-center distance between the teeth at a value that ensures a functional clearance between the teeth of 30 to 70 microns. This low clearance eliminates any perceptible operating noise. What’s more, this surface takes up the side forces of the rod and hydraulic pushers that maintain the MCE‑5 parts in contact. The side forces applied to the gearwheel by the combustion piston are taken up by the teeth, while the radial forces are taken up by its rolling surfaces: the two functions are thus dissociated.

The general structure of the MCE‑5 gearwheel ensures minimum distortion under load with a minimum of material. Topological optimization calculations led to its characteristic “organic” shape that gives it maximum rigidity. The gearwheel’s geometry is also dictated by the forging processes planned for its mass production.

9 years of testing under different conditions have proven the extreme robustness of the MCE‑5 gearwheel. Its fatigue strength under alternating stress, contact pressure resistance (Hertz pressure), proper lubrication conditions, absence of seizing, pitting and wear (roughly 1 micron during a vehicle life of 300,000 km) make it a highly-reliable central part capable of ensuring reliable service for mass-produced vehicles.

The design of the MCE‑5 VCRi rod/gearwheel pin was a huge challenge, necessitating
long and complex studies. This link no longer poses any problem, even for
long endurance operation at extreme engine loads.

The MCE‑5 VCRi transmission gearwheel production route sheet provides an
excellent quality/cost ratio for this strategic component

The main characteristics of the MCE‑5 VCRi gearwheel