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

It increases friction

When we increase the number of parts in a system and thereby increase its apparent complexity, we imagine that the system will dissipate more energy because of friction. This may be true but it may also be false. A sledge runner is very simple but it induces higher friction than a wheel mounted on ball bearings that has several dozen moving parts. This is why trains are not mounted on runners but on wheels mounted on bearings.

The gearwheel and rack rolling surfaces eliminate
piston-to-liner friction and replace it with pure rolling

In 2004, the IMPACT project made it possible to
compare friction losses generated by a
modern conventional engine with those
generated by the MCE‑5 VCRi

A tribometer reproducing gear teeth contact
during operation was intensively used to identify
the best pairs of materials for MCE‑5 gears

MCE‑5 VCRi final FMEP is measured using
multi-cylinder engines and is then compared
to that of reference conventional engines

There are three basic principles to limit engine friction: favoring as much as possible rolling to sliding, favoring hydrodynamic lubrication (friction coefficient = 0.00x) and limiting stress when friction occurs in a mixed or boundary lubrication (0.x).

The MCE‑5 VCRi engine is based on these three basic principles and implements them in the best possible conditions.

Rod side thrust is no longer applied to the cylinder via piston sliding but via pure rolling thanks to a roller, gearwheel rolling surfaces and rack rolling surfaces. Small racks vertically synchronize the position of the roller.

The piston is thus “protected” from the radial stress exerted by the rod and no longer ensures its own guiding in axial orientation. The MCE‑5 VCRi system thereby separates two functions that were previously linked: application of gas thrust to the piston and transformation of this thrust into continuous crankshaft rotation. The resulting operating conditions reduce energy losses due to piston friction. This is crucial: a conventional engine’s piston-ring-liner assembly generates between 40 and 60% of total friction losses. Reducing the losses of this assembly gives new maneuvering room to add the components required for VCR that inevitably cause friction. This was the strategy selected for MCE‑5 VCRi.

The MCE‑5 VCRi gears enable VCR control but these gears also create a new source of friction. One of the main challenges was to reduce this source of friction as much as possible. Spur gears were selected because they don’t generate any axial stress. Chevron gears were not considered since they are too big, heavy and complex. Spur gears provide excellent efficiency, in the order of 0.98. This efficiency is acceptable for a gearbox but is insufficient for the MCE‑5 VCRi: an efficiency of 0.997 is necessary. Between an efficiency of 0.98 and an efficiency of 0.997, the energy loss is divided by 6.6. To reach the objective of 0.997, the smallest possible gear module was selected. The smaller the module, the more the operation is close to pure rolling as it takes place on the gear pitch circle. What’s more, their contact ratio (percentage of meshing when 2 pairs of teeth are in contact simultaneously) was reduced by reducing the tooth height, thereby reducing the sliding proportion of the gear meshing.

Spur gears do not only have advantages: they have a poorer gear meshing continuity than do helical gears, and because of this we avoid using them at high loads and high speeds. This is advantageous for MCE‑5 VCRi. High loads occur at very low rotation speeds (close to TDC and BDC) guaranteeing silent operation. In order to maximize the efficiency of the MCE‑5 VCRi gear, geometric teeth corrections were made to provide the best possible mechanical efficiency and gear mesh continuity. This leads notably to a contact ratio that varies according to load.

In the end, the total friction losses of the MCE‑5 VCRi are significantly lower than those of the most modern conventional engines used in the same conditions. The MCE‑5 VCRi has a real tendency to reduce friction a low speeds, below 3500 rpm. Above this engine speed, the engine friction increases due to inertia forces relayed through the teeth. However, these engine speeds are not common targets in normal use (downspeeding). Moreover, high speeds at high power are not very sensitive to a slight increase in friction losses (high IMEP/FMEP ratio). Thanks to its high-efficiency gears, the MCE‑5 VCRi is not very load sensitive, which means that at high loads, it has significantly less friction losses than a conventional engine subjected to the same load. This is exactly the quality sought in a highly-loaded VCR engine (hard downsizing).

In conclusion, the appearance of MCE‑5 VCRi technology is misleading: MCE‑5 VCRi is an effective solution to reduce friction losses in highly-loaded engines. This is an additional source of efficiency resulting directly from MCE‑5 VCRi technology.

The mechanical architecture of the MCE‑5 VCRi can bear very high loads without
increasing friction losses

In 2007, a friction-optimized MCE‑5 VCRi engine was put on the test bench. This engine revealed
the potential of the MCE‑5 VCRi to reduce FMEP in comparison with most modern conventional engines