Increasing Engine Efficiency and Power
Most of us are probably anxious to get rid of internal combustion engines (ICE) and move on to electric motors. Unfortunately, refining a technology until it is affordable and performs well enough for the mass market takes a while (read up on the history of computers or cell phones), and we might not have electric cars for a few more years. In the meantime, engineers are trying to squeeze more life out of ICEs (see 5 technologies to improve ICEs).
One promising technique is the use of variable compression ratios. Read on for details.
How Variable Compression Ratio Engines Work
In an internal combustion engine, the compression ratio tells you what the ratio is between the biggest and smallest volumes of the combustion chamber in the cylinder. The concept of modifying that ratio is very old (around the 1920s), but very few cars have actually used it (SAAB was working on it before GM took over, but that project has been shelved as far as I know).
The benefits of having a variable compression ratio is that you can control much more precisely combustion and adjust the variable to get the best performance for each situation (acceleration, cruising, deceleration, etc). It becomes especially potent - in theory - when combined with other technologies like turbochargers, direct injection, variable valve timing and lift, etc.
From a green point of view, this mean you could downsize an engine quite a bit while still getting enough power and achieving high thermal efficiency. It might even help with the use of (cellulosic) ethanol (which has different characteristics from gasoline), or to reduce emissions by optimizing combustion.