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Honda is developing a variable compression automobile engine: MPG up 7.4%

Source: Green Car Congress: Concept: Honda RD Developing Variable Compression Ratio Engine with Dual Piston Mechanism

Quote:

In testing in an otherwise conventional production 2.0-liter, 4-cylinder engine, the dual piston mechanism was able to adjust the engine from a CR of 9.6 to 14.2 and back again. Combining the high compression ratio with the Atkinson cycle, the engineering team demonstrated a 7.4% improvement in fuel economy in operation over the Japanese 10-15 cycle.

They realized the movement of the dual piston mechanism by inertia and external hydraulic pressure, with the result that...

Quote:

...in contrast to the complexity, size and power demands of earlier VCR concepts, [Honda engineer] Kato said, the new dual piston VCR could be installed without changing any of the major parts of the engine such as the engine block.

The ratios are set at "high" and "low", though, not a range between the two.

The engine was durability tested to 1 million cycles.

Honda sees an efficiency advantage to this approach vs. turbo/supercharging a smaller displacement engine.

Details are available in an SAE paper on the subject, presented at the SAE 2009 World Congress

SAAB did one many years ago but it seemed to me to be really complex.

There is a Veedubber in Cali that made an experimental one in his garage like a decade ago

ok... correction, has been working on it for a Decade in his garage LOL
Amazing Engine, Amazing Pizza! at TimboTrip.com

Anyone have any images of the principle of the idea?
I could see this being easy on a two stroke type engine just having the head with a moving part in it that would take up some combustion chamber, but with valves in the way....

Frank: didn't SAAB's use a movable head somehow?

rkcarguy: no image, but a pretty good description from the GCC article:

Quote:

The piston structure has an inner and an outer piston. The outer piston sits atop the inner piston, and constitutes the combustion chamber against a cylinder head; the inner piston has the function of a piston skirt, a lifter mechanism, and a lock mechanism....

Honda installed a small pump separate from that of engine lubricating oil pump. Oil passage was constructed from the external pump through the crankshaft and connecting rod to the piston.

To switch from low to high, the hydraulic pressure is turned on. This acts on the pin of a lock plate, freeing the outer piston. The outer piston rises the 3.5 mm by the resultant force of the piston inertia force and the cylinder pressure force. To go from high to low, the hydraulic pressure is turned off, the lifter plate rotates out, and the out piston lowers over the inner piston by the resultant force.

I do remember hearing about the SAAB engine a long time ago. Here is a link:

http://www.saabnet.com/tsn/press/000318p.html

Glad to see the issue being revisited. 7.4% is nothing to sneeze at.

Description works for me metrompg. Looks like it's VTEC pistons:)
VTEC activates a solenoid which pressurizes the pivot point of a 2nd set of rockers under a larger cam lobe, pretty ingenious.

I could see valve timing be used to increase and decrease "trapped" compression

VTEC for pistons - good analogy!