Hi

You really don't notice it when it is running because it is so quiet. You notice it shutting off when you first own the car because the quiet Prius gets quieter. I sometimes feel a slight vibration as it turns off. The experts on PC have said that the car's computer will cause the gas engine to turn over again if it doesn't stop the cylinders in the correct position. There is an optimal position for the cylinders to be in for quick and efficient engine startup. New people in your car usually think your engine has stalled (stalled quietly). The engine only starts back up again if the car needs to heat up fluids (for its or your comfort), if you need extra acceleration, or when you reach approximately 42 MPH. At 42 MPH you may not need the engine to run but there is a special gear that turns in place of the internal combustion engine (ICE) input and it starts turning too fast at that speed and needs the ICE to start supplying some torque. I think the power split device link shows all that to include animation. Pretty cool really.

 

Also, another important piece of technology with the Prius (and other hybrid cares) is that when the engine does come on again , it is not cranking back from zero rpm. That is, there is a seperate electric motor whose job it is to get the pistons cranking at ~1000 rpm BEFORE a spark is ever delivered.

This is importatant for several reasons. In normal cars the spark is delivered soon after the starter is engaged and is cranking the engine. In this case, the spark is delivered at very low RPMs and before the cylinder walls are primed with oil. This is very hard on engines (i.e., the explosiveness of the spark on slow moving piston combined with little or no lubrication in the cylinder).

Contrast that to a hybrid vehicle in which an electric motor turns the engine to 1000 rpm, in which lubrication to the cylinder walls also occurs, BEFORE the spark is delivered. This process practically eliminates the harsh starting environment of traditional cars. This also results in the igntion of the engine being barely noticeable to passengers in the car.

I'll try to find a link that explains the process in better technical detail than I am capable of.

 

Because the engine in a Prius is already spinning at idle speed before the valve timing and fuel injectors kick in means that there is no shock at startup like when you start a conventional car where the starter spins the crank at only 200 - 300RPM before the engine fires and instantly flings the crank up to about 1000RPM.

I would suggest going for a ride in a Prius and see for yourself.