The Prius utilizes the Atkinson cycle rather than the conventional Otto cycle, which is more common in traditional gasoline engines. The Atkinson cycle is designed for higher efficiency, sacrificing some power output for improved fuel economy.
The Atkinson cycle has a longer expansion stroke compared to the compression stroke, resulting in a more efficient conversion of heat to mechanical work. However, this cycle tends to have a narrower range of operation, with less torque and power available at lower RPMs compared to an Otto cycle engine.
Perhaps a more efficient cycle is possible by reducing the RPM/torque range even further, but I am under the impression that a lot has been left on the table efficiency wise.
The Atkinson cycle has a longer expansion stroke compared to the compression stroke, resulting in a more efficient conversion of heat to mechanical work. However, this cycle tends to have a narrower range of operation, with less torque and power available at lower RPMs compared to an Otto cycle engine.
Perhaps a more efficient cycle is possible by reducing the RPM/torque range even further, but I am under the impression that a lot has been left on the table efficiency wise.