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

It protects the cylinder

If nothing is done to prevent it, a VCR engine adds new constraints to the cylinder: the heat flow crossing it is higher due to the higher specific power, the side forces applied by the conrod are higher due to the increase in BMEP (Brake Mean Effective Pressure) and increased in-cylinder pressure easily reaching 130 bar. Another characteristic of VCR engines: the vertical position of piston turnaround varies with potential consequences for the ring pack.

The average radial stress on the MCE‑5 VCRi piston
is between 10 to 100 times lower than
that of a conventional engine

For highly-loaded gasoline engines, high BMEP at low speeds is essentially obtained by a horizontally-extended PV area and not an increase in absolute pressure.

In other words, the pressure is not much higher but “lasts” much longer. This results in stronger forces applied by the conrod on the piston skirt and cylinder. Indeed, the conrod is already strongly inclined while there is still high pressure in the cylinder. On a hard downsized VCR engine at high speeds, in addition to this effect there is a significant increase in the absolute pressure in addition to the increase in the sliding speed. What’s more, the piston slap – a violent impact of the piston against the bore when the conrod changes direction – worsens and may cause a local alteration of the micro geometry of the cylinder surface or even the ovalization by wear of the cylinder at the level of the first ring.

The increase in specific torque of hard-downsized engines is mainly obtained by an horizontally
expanded in-cylinder pressure area, which increases the radial stress applied to the cylinder

Side forces caused by rod thrust are converted
into radial stress and friction between pistons
and liners on conventional engines. They are taken
up by a pure rolling system on the MCE‑5 VCRi

The design challenges for the MCE‑5 VCRi piston and
cylinder are considerably reduced in comparison
with state-of-the-art engines. This leads to
improved performance and new opportunities

MCE‑5 VCRi provides new opportunities to manage these constraints: the thrust load from the conrod is no longer applied to the cylinder but to a pure rolling system. As a result, the cylinder is subjected to less bending stress.

MCE‑5 VCRi also eliminates piston slap, which improves the operation of the ring pack in the upper part of the cylinder: the rings no longer tilt when the piston moves at very low speeds in the hottest, driest and most polluted part of the cylinder.

Though it’s difficult to precisely quantify the advantages expected from MCE‑5 VCRi’s natural protection of the cylinder, we can however list the opportunities. For example, a reduction in cylinder distortion is conceivable. In a conventional engine, this distortion is mainly due to three effects: the first is a thermal effect due to local temperature variations. The second is a mechanical effect due to the bolting of the cylinder head, to gas pressure and to conrod side force. The third is an effect due to the abrasive wear of the cylinder, particularly in the zone affected by piston slap (local ovalization).

MCE‑5 VCRi technology reduces the horizontal forces from the conrod on average by a factor of 10 in the most unfavorable cases (low compression ratio with a high offset of the synchronized roller). As a result, the top of the engine’s cylinder is subjected to less bending stress making it conceivable to reduce the bolting load and to reduce the thickness in the hottest upper part of the cylinder to favor the circulation of the coolant over 360°. Current engine cylinders are in contact with each other and hence cooling is not uniform. A slit or a hole is made in the top part of the cylinders of highly-loaded engines: these modifications improve cooling and limit cylinder distortion. In the case of MCE‑5 VCRi, it is probably possible to make independent and juxtaposed liners, cooled over 360°, at least in the upper part of cylinder.

The potential reduction in bolting, the improvement in cooling over 360° and the elimination of piston slap will improve the geometry of the MCE‑5 VCRi cylinders. It will be an opportunity to reduce ring tension and hence engine friction, while making ring operation more durable.

Piston slap elimination is also a factor making VCR engines more durable for another reason. Variations in piston altitude in VCR engines extend the cylinder zone affected by piston slap. Because of this, at high compression ratios, the rings can pass at high speed over the sections that were altered by piston slap when the engine was running at a low compression ratio. This problem is eliminated in MCE‑5 VCRi.

In conclusion, due to its ability to naturally protect its cylinder at high loads, MCE‑5 VCRi technology has real potential to improve the operating conditions, life cycle and energy efficiency of the piston-rings-cylinder assembly.