| The ICE is more efficient because the design constraints have changed. Here's a single example with the Prius: 1. The intake valve stays open for part of the compression stroke. 2. (1) means that the compression ratio for the compression stroke is lower than the compression ratio for the power stroke. 3. The efficiency of the engine is limited by the compression ratio of the power stroke 4. The compression ratio of the compression stroke is limited by engine knocking (if you compress a mixture of fuel and air too much, it will spontaneously combust) 5. So this engine can be made more efficient than an engine that closes the valve for the entire compression stroke 6. Some (non-hybrid) engines have variable valve timing and can do (1) some of the time, to a small extent. 7. Doing (1) to a larger extent makes the low-RPM torque very poor. 8. Electric motors have excellent low-RPM torque, so (7) is compensated for by having an electric motor run at low speeds. |