Variable compression engine reduces fuel use 30%?

[SVOboy]

From ABG: http://www.autobloggreen.com/2008/04...-ratio-engine/

Meet the MCE-5 VCR engine, coming at you from France. VCR stands for Variable Compression Rate, and its creators claim it will replace conventional Fixed Compression Rate engines. According to the press release (found after the jump), what is special about this engine is that it's based on a combination of rod-crank mechanism and long-life gears, integrating both power transmission and compression ratio control for each cylinder.

MCE claims that this allows ratios from 7:1 to 20:1, which adapts the engine for most usage requirements. Oh, and this reduces fuel consumption by 30 percent, as two other prototypes of VCR engines showed by Saab and Audi in 2000. MCE also claims the design is more durable and robust, and will have lower maintenance costs.

This engine has no impact on the rest of the components of a vehicle, so it's potentially installable in every sort of vehicle. MCE's current prototype offers 160 kW (218 HP) with a 1.5-liter engine.

Sounds interesting, at least.

[tasdrouille]

Here's additional information at autospeed:

The technologies and control of variable compression ratio engines.

[Lazarus]

Very interesting. I wonder how it will stack up with emissions testing though?

[LostCause]

Great articles SVOboy and tasdrouille.

What I don't understand is how the heightened compression ration will be used. If knock isn't going to be eliminated by direct injection or some other technique, is there something I'm missing? I suppose an engine may not knock under low load (cruising) and only under acceleration...

I also wonder why variable compression can't be achieved with variable valve timing. Toyota's "Atkinson" cycle engine uses late valve closure to reduce the compression ration, so why not apply variable valve timing to choose high, low, or gradations in between as technology becomes more advanced. Seems simpler than linkages, moveable heads, variable height pistons, etc.

- LostCause

[Andyman]

LostCause:

What you're missing is that the compression ratio will be high during low loads and low during high loads. The compression pressure will remain almost constant while the engine load changes. The higher compression ratio will give much better efficiency during low power operation. The peak pressures will be limited to avoid problems with knocking and preignition.

Variable valve timing will change the effective compression ratio but it can't give the engine high compression during light loads. A really efficient engine would have both variable compression ratio and variable valve timing. It could stay efficient over a wide range of power levels, unlike normal engines. The low combustion pressure and high pumping loss makes them less efficient at low load operation. Variable valve timing reduces pumping loss and variable compression ratio allows higher combustion pressure at low loads.

The disadvantage of the variable compression ratio is that it might be too expensive for some economy cars. I think that a reasonable compromise is to use a high fixed compression ratio such as 14:1 with variable valve timing, variable intake air temperature, and variable fuel mixture. The engine could use high octane fuel only for high power levels and low octane fuel most of the time. Some water injection may also be good for eliminating knock when making high power.

[Peter7307]

An interesting idea but to get the same result for a lower cost and with already proven technology a turbo / supercharger combination would be a better alternative from the car makers point of view.
No licence fees to pay either.

Pete.

[Duffman]

No not the same results Peter,
Forced Induction gives you variable compression but a fixed expansion ratio. This engine does both. Its the fact that the expansion ratio is larger than the compression ratio in the atkinson cycle that makes it more fuel efficient.