Honda Working on Variable Compression Ratio Engines

It’s been years since Honda brought us VTEC, and frankly, some of us have been wondering when the company was going to give the internal combustion engine another developmental kick in the pants. This may be it.

This year at the SAE World Congress Honda let loose a paper on their development of a variable compression ratio (VCR) engine. Like variable valve and cylinder actuation technologies, this promises an increase in power and fuel economy if implemented. Higher compression ratios are key in increasing the fuel efficiency of internal combustion engines because, as a general rule, the higher the compression ratio the better the thermal efficiency of an engine is.

However, the need to run high octane fuel has kept high compression engines out of most economy cars. Honda’s system will switch on the fly between high and low compression to give the best of both worlds:

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. As part of the study, the team also demonstrated switching durability of the dual piston mechanism of more than one million cycles.

There are other technologies such as HCCI that attempt to greatly increase the volumetric efficiency of gasoline-burning engines, but their main drawback is that they do not operate over the full range of the engine’s use and are often costly because of the complexity of their designs.

Honda thinks that it’s solved this problem. The company claims that their current dual-piston technology can be used in almost any engine without the need to modify any major components, such as the engine block.

While the concept is a bit complicated, it is interesting because it’s limited to a change in the piston design. By using two pistons, one outside of the other, actuated by hydraulic pressure, Honda is able to change the engine’s compression ratio on the fly:

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. The outer piston can be raised or lowered by 3.5 mm with reference to the piston pin. This in turn changes the compression ratio by 4.6, i.e., from 9.6 to 14.2 and back again.

Incredibly impressive technology to say the least. While I wonder if we’ll ever see it in production cars, most people probably thought Variable Valve Timing Electronic Lift Control would never see the light of day. But then again, other interesting concepts like the dual-rod, oval piston engine never made it anywhere near production.

Source: GCC via our forums

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