Reliability, durability

Cylinder, piston and rings

As it is no longer subjected to piston slap and piston radial stress, the MCE-5 roller-guided piston reduces the cylinder wear and distortion: the piston is not only guided by a skirt, but also by a synchronized roller located lower in the engine block:

As a result, torque on crankshaft no longer results from forces applied to the cylinder by the piston, but from forces applied to rollers:

Complete explanation in pdf format (Explain_MCE.pdf).

The absence of piston radial stress and piston slap combined with a lower cylinder wear leads to a better piston and rings durability. As the piston is no longer self-oriented, it doesn’t tilt nor lean on the first-land (cylindrical piston surface located just above the first ring). As a result, cylinder wear due to abrasive particulates agglomerated on the first-land is widely reduced.

Both VCR and MCE-5 highly improve cylinder durability of high-loaded VCR engines:

MCE-5 and VCR cylinder wear reduction: 1) Maximum wear reduction: MCE-5: No piston slap; No piston first-land radial stress. VCR: Wear step effect reduction due to Compression Ratio control. 2) Low wear reduction: MCE-5: No piston radial stress.

These particular features avoid entering into the cylinder wear vicious circle: the more distorted the cylinder geometry, the more important the cylinder, piston and rings wear at each engine revolution. The MCE-5 roller-guided piston widely limits existing challenges for pistons design:

source: MAHLE presentation - GFC - 18 October 2004 - Paris

The following graphs permit comparing the piston radial stress generated by a conventional modern 2L SI engine (max. power 100 kW) to that generated by a MCE-5 1.5 L engine (max. power 160 kW):

Modern 2L SI engine: rod mass 570 g, piston+pin mass: 400 g, bore: 86 mm, stroke: 86 mm.
MCE-5 1.5 L engine: piston mass: 753 g, roller mass: 66 g, bore: 75 mm, stroke: 84 mm)

Thanks to its unique features, the roller-guided piston of the MCE-5 VCR engine block permits to guarantee the cylinder durability of high-loaded supercharged VCR engines on high mileage.

Crankshaft and bearings

In addition to cylinder durability improvement, the MCE-5 VCR engine block also responds to rod-crank mechanism reliability-durability: its rigid crankcase provides a precise and rigid bearing line in which is mounted a rigid crankshaft (crank radius divided by two):

Gears

The MCE-5 gears also respond to most stringent reliability and durability requirements:

	1)	Gear dimensionning and design are well-adapted to all load-speed operations, number of cycles and lubrication conditions;
	2)	The MCE-5 mechanism guarantees an optimum pressure distribution on the entire gear teeth surface (operational geometric conditions);
	3)	The MCE-5 kinematics permits to maintain an excellent lubricant film behaviour whatever the engine load and speed (favorable pressure x speed criterion on the entire load-speed range).

As a result, the MCE-5 long-life gears are designed for more than a billion cycles:

The first MCE-5 prototype components that have been taken as references to announce the MCE-5 reciprocating mass and to measure its performance have been designed according to an extremely secure approach: engineers considered that the engine would operate at max load during 1 Giga cycles and beyond.

In practice, not only engines rarely operate at full load, but their entire life rarely exceeds 700 Million cycles (in this case, MCE-5 gear teeth would be subjected to max load only on 350 Million cycles because max pressure occurs one time per 720° engine revolution).

As can be seen on the following graph (Wohler curve), the MCE-5 gear teeth fatigue resistance is oversized on both gear teeth bending and Hertz pressure resistance criteria:

In addition to this secure gear dimensionning, engineers decided to add new security margins when calculating mass-production costs:

	1)	MCE-5 gears will be made of high quality steels;
	2)	Gears will be made by forging, which improves their mechanical resistance (this is not the case for gears of the first MCE-5 prototype);
	3)	Gears’ fatigue resistance will be increased by shot peening (metal treatment for performance enhancing and extended life span) and lubricant film behaviour and resistance will be improved by tribofinishing (better surface texture).

But the most effective guarantee for the reliability and durability of the MCE-5 gears comes from their optimal geometrical operating conditions: contrary to most gears applications, the MCE-5 gears are subjected neither to shaft misalignment nor to shaft bending:

Shaft misalignment	Shaft bending

Indeed, MCE-5 gear-wheel and racks relative orientation is not given by shafts, but by the gear teeth surfaces themselves. This is due to the fact that both MCE-5 main piston and jack piston operate as ball joints, thus permitting racks' self-orientation for an optimal gear teeth contact:

Thanks to this exclusive feature, it is possible to replace the helix crowning commonly used in gear boxes to adapt to shaft bending and misalignment (helix crowning permits teeth surfaces to operate as ball joints) by a simple radius on both sides of gear teeth face width contact area. This arrangement widely reduces both Hertz pressure applied on gear teeth surfaces and lubricant film temperature:

Gearbox	MCE-5

However, on the MCE-5 first prototype, tests have been carried out for several hundreds of hours at 90% from max load with gears presenting a longitudinal gear teeth optimisation. Lubrication was provided by used oil.

But MCE-5 not only provides optimal geometrical conditions for gears, it also maintains a «pressure x speed criterion» that remains always favorable to an appropriate lubricant film behaviour. Indeed, pressure x speed criterion determines the lubricant film temperature: if oil flash temperature is reached, the lubricant film collapses. Combining high speed to high contact pressure leads to oil flash temperature. This is never the case of MCE-5 for which:

	1)	High gear teeth contact pressures are always combined with low gear rotational speeds (max pressures and max accelerations occur at TDC and BDC);
	2)	High speeds are always combined with low gear teeth contact pressures (mid piston stroke).

As can be seen on the following graphs, the MCE-5 gear system pressure x speed criterion remains always favorable to a moderate lubricant film temperature:

Conclusion

Reliability and durability count among the main strong points of the MCE-5 VCR technology. This guarantees a life span to future high-loaded downsized VCR engines that is comparable to that of long-life conventional engines, thanks to:

	1)	A rigid engine block
	2)	A rigid crankshaft
	3)	A rigid and precise bearing line
	4)	Long-life gears operating under optimum conditions
	5)	The exclusive MCE-5 roller-guided piston: No piston slap No piston radial stress Reduced cylinder wear and distortion.

(see: VCR engines’ requirements for mass-production: reliability, durability)