Based on GM's video and discussions here, I've always assumed the "Mode 4" operation - where the ICE is actually providing some propulsion power to the wheels - only happens at high (70+ MPH) speeds. In recent discussion on a FaceBook thread, though, someone is claiming this is false and that it actually will engage that mode at lower (sub 40 MPH) speeds too. They have no concrete evidence or documentation of this, however, and are solely going on feel:
I was thinking about testing this using my OBD2 wifi adapter to watch the RPM's of the ICE and 2 electric motors but the MyGreenVolt app I use (iPhone version) doesn't seem to have the RPM's of the electric motors. Any idea on a way to test this person's theory? (or has anyone posted evidence to support/challenge it already - my searching hasn't found anything)If I drive in range extended mode, ICE running, 38 MPH at a steady speed (not accelerating), level road (not climbing a hill) and maintain this condition for over 30 seconds to a minute, the ICE will engage. I can feel it! It’s very subtle, similar to how an automatic transmission feels when the lockup clutch in the torque concert feels when it locks. At this point if I floor the accelerator pedal, there is a 1-2 second hesitation before the Volt starts accelerating. This delay is caused by the car transitioning between Mode 4 to Mode 3, disconnecting the ICE from the wheels and operating in pure generator mode. If I stab the throttle again within 10 seconds, the car will accelerate immediately. This indicates the car is in mode 3 still, with the ICE mechanically disconnected from wheels. However if I maintain steady speed at 38 MPH for over 30 seconds to a minute, it will again transition to mode 4, and the 1-2 second delay will be back if you stab the pedal. It will all happen below 40 MPH and above 35 MPH. It would be plain DUMB to limit this to over 70 MPH, because when operating in pure generator mode, there is quite a bit of conversion losses which lowers efficiency and MPG. They wanted to mechanically connect the ICE to wheels at a lower speed and have it operate in this mode for a much as the time as possible. Go try it for yourself!