Originally Posted by alan_in_tempe

The electric motors are each over 50 HP, but usually one is working as a generator, supplying power to the other one acting as a motor. (MG1 generating power for MG2 is equivalent to low gear, where MG2 generating power for MG1 is equivalent to high gear or overdrive - the electric motors are the car's transmission, as well as other things such as the starter and battery generator.) The drive battery can provide over 40 HP of current to the motors which can add to the ICE power.

To start up the ICE requires about 1 HP from the drive battery, so starting from a stop with the ICE off gives you about 40 HP of acceleration for a moment until the computer decides to start the engine, when you get about 39 HP worth of acceleration for about a second while the ICE starts, and then you get additional power from the ICE which gradually builds as the ICE speeds up for about a second to give you up to nearly 200 HP. That full power is only available for a few 10s of seconds until the battery charge goes down and then only the ICE power is driving the car.

I'm guessing the reported startup "stumble" is going from (about) 40 HP nearly instantly, down to 39 HP for about a second, to over 100 HP all within a couple seconds, where a non-hybrid conventional vehicle starting with the ICE already running has a longer delay to get the ICE spun up to less than 40 HP, but rapidly increases to over 100 HP. Both systems will likely cross that 100 HP on their way to full power about the same time, but the hybrid will give more power initially (thus better 0-60 miles per hour times for equivalent weight vehicles for a hybrid with less total ICE+battery HP than a non hybrid with more total ICE HP).

Once the ICE is into it's power range, the hybrid power will be perfectly smooth regardless of the vehicle speed or acceleration, while the non-hybrid must suffer those horrible, stumbling, intermittent losses of power and efficiency during those antiquated events known as "gear shifts".

-- Alan