Originally Posted by Droid13

True, but the 12.5:1 ratio is only a physical ratio. Atkinson engines typically don't experience the high compression pressures found in regular high ratio engines in most every day driving because they limit the intake volume. That is the whole purpose of atkinson, less intake, but more physical room for the combustion process to squeeze out a little more energy for a given amount of fuel. The high physical ratio in atkinson engines is really just meant to be able to bring up the low intake volume to more reasonable compression pressures for ignition.

Just like a turbo charger artificially increases compression pressure for a given physical compression ratio, atkinson decreases compression pressure, you could call it an anti-turbo charger. This is why atkinson engines are more efficient, but greatly lack the torque of equivalent sized engines (and in steps the beefy electric motor to make up the torque).

Originally Posted by HKPolice

These hybrid engines are not permanently in atkinson mode. VVT-i allows them to switch to otto cycle when more power is required.