I'm sure some of you have noticed this - let's say you are going down a slope at 35mph and the ICE is not engaged (running off battery, not gas). Because you are on slope & pressing the gas pedal, naturally your speed increases. However as speed increases, in order to remain on battery you actually have to release the pedal slightly more and slope. Does anyone know the technical reason for this? Is this characteristic because of the fact that MG2 cannot be pushed to the limits due to the higher speeds, or for some other related reason? Just curious, as everytime I am on going down a slope I realize I have to be careful if I do not want to engage the ICE. Thanks!
EDIT: My answer below doesn't exactly answer your question it's just my observation so maybe it will lead you to an answer. I think MG2 can handle it just fine, based on scan gauge readings it seems to draw a lot more current under hard acceleration at low speed to help the ICE (but only for a few seconds) than when you are just using the battery at moderate throttle with the ICE off. Once the car is warm two things I know of that will start the ICE when you are running on the battery are speed over 46 MPH and a power demand that puts the HSI to the right side of the center line. The speed limitation is to prevent over speed of MG1, the power demand limitation is most likely to prevent us from putting to much strain on the battery for a or perhaps to save energy. I usually try to make it the last 3/4 of a mile or so to my house (mostly level) on the battery at around 30 to 35 MPH. I watch the HSI very closely and if I keep it to the left of center I can draw the battery down quite a ways, but if I let it touch the center marker the ICE starts.
It is an issue of speed. Aerodynamic drag is proportional to the square of the speed; the power required is proportional to the cube of the speed. In other words, to go twice as fast requires eight times the power. As you head down the hill, the hill isn't steep enough to overcome the extra drag. You need to add gas to keep the speed increasing. Even on a 6% downhill in the mountains, you need engine power to keep a Prius up to highway speed. Now the bigger question is *why* do you want to keep the ICE from running? Staying on battery power is bad for your mileage. Get the engine running and let it power your Prius efficiently. Tom
At higher speeds (above 46mph), you don't have to cross the half-way mark on the HSI - the ICE will fire up sooner. But you can get something of a glide in the first 25% of the HSI if you're careful. (whether or not it's useful / efficient is another question)
