New Owner Question

As I understand it (and I could be wrong), the ICE in the Prius has some technology that allows it to do this much more efficiently than a standard engine. For example, the Prius keeps in storage some hot fluid that it pumps into the ICE at startup - this makes the engine warm up MUCH faster. Also, bear in mind that when the engine DOES come on, it isn't being charged with being the 100% power supplier to the car - it's being assisted by the electric motor.

Just my 2 cents....

Yoda

 

The Prius has 2 motor/generators on board. When the vehicle is using its auto stop functionality the smaller of the M/Gs is spun in such a way that it starts up engine movement, like a starter, but the RPM of the engine when ignition actually occurs is much higher than with a normal starter. Common knowledge is that ignition engages at somewhere between 700 and 1000 rpm, which is a very comfortable operating speed for that engine. When we start a regular car the ignition occurs at usually a very low RPM, like 100 or so. Then the natural combustion process brings the vehicle up to speed. To start a regular engine you need to have a fairly rich mixture. With the Prius, because the engine is already turning over at a rpm comparable to operating RPM, much less fuel is needed to start ignition.

 

During driving, the Prius doesn't allow the ICE to get cold enough to be inefficient once it has reached optimum temperature at the beginning of the trip. Now if you park for a while, even in READY mode, then the ICE can cool down enough to become less efficient. I believe that happened to me when I was stopped for a minor traffic violation last fall. Noticed MPG for the next five minutes was lower than usual.

Even with some cooldown, you gain far more not idling needlessly than some loss restarting. Short trips where you power off however, will still yield lower MPG.