I've had my 08 Prius about a week now. While test driving on the turnpike at 65 mph I asked the sales lady riding with me if when I lifted my foot off of the accelarator for an instant, if the engine stopped and restarted a couple of second later, she said yes!! I find this hard to believe that an engine would stop and restart within a couple seconds. What is the truth???
Actually, at 65mph the engine (called the ICE - Internal Combustion Engine in hybrid speak) will not completely stop but the fuel will be cut off when no power is needed. It will restart when you press the accelerator again. At speeds below 41 mph, the ICE will actually stop spinning and the fuel will be cut off when no power is needed. So the salesperson technically was not correct, but was correct about the engine stopping and starting in a few seconds - actually it can do this very quickly (less than a second) when required.
Once you get used to it, you can feel when the engine comes on. It's subtle, but noticeable if you pay attention. Tom
Above 42 MPH the ICE(Internal Combustion Engine) always is turning. It may shut the fuel flow, but it is always turning. Below 42 MPH, the ICE will actually stop depending on the situation. Read this. It will help you understand the workings of the HSD
On my way home, I decided to see how long it takes for the engine to restart. I have a ScanGauge, so I can see both the engine rpm and fuel flow (gal/hr). At about 38 mph on level ground, I lifted my foot so the engine would stop running and verified that the rpm was zero. Then I gave the accelerator (I like to think of it as a Torque pedal, but never mind) a fairly strong push and watched the ScanGauge. The rpm went above 800 virtually instantaneously, but it took about 1 second (one thousand one) for the fuel to flow. Tom is correct that you can feel the engine start, but it may take getting used to the car before you can feel it.
it's amazing engineering work, most engines would die quickly if started as many times as the prius one.
The way the Prius starts the engine is very smooth and is nothing like a regular start with a typical non-hybrid. When the HV ECU starts the engine, it first spins the engine up to speed (~ 1000 rpm, my ScanGauge usually shows about 980 rpm) using MG1. Because the oil pump is driven by the crankshaft, there is full oil pressure available to the lubricated parts. Then the spark and fuel are added to start the engine. Oil analysis results (and actual vehicles driven >200k miles) have shown that the engine life is not a concern with the Prius.
Actually before the spinup, the VVT overlaps the valve timing to lessen the compression, which add to the smooth startup!!
