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Originally Posted by MGBGT
Here's a tricky question about effect of heat, specific to the Prius:
The Prius ICE runs in very rich mode for the forst two minutes after a cold start to warmp up rapidly. AFAIKleaving this rich mode is in part dependent on mesured enine temperature. FE is usually abysmal during this time, which is why many short trips in the Prius tank the mileage. A block heater would help.
The Prius also uses an electric heating element to help heat the cabin, if the ICE is not yet warmed up sufficiently. Thus, if you use heat during the first 2 minutes, you will be using the electric heater element, which should suck up energy via the electric generator/motor.
However, this same process (electric heating) should increase the load for the ICE during startup, and may shorten the warmup period of rich running at low FE, saving fuel. The question is: how does this balance out.
Any thoughts?
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Without pre-heating, my approach is to keep the ICE power load as low as possible. I do this by driving through the neighborhood at speeds of 20-25
mph and traffic permitting, with the ICE in idle and the vehicle coasting in neutral. The instantanious MPG often doubles during these coasts. The distance varies but generally, it ranges from 1-2 miles. Still, there is no free lunch and the first 5 minutes are pretty poor.
I have yet to find an easily detectable indicator of the transition from poor to good MPG. The 'blue' engine cold light goes out too early but is better than nothing. Experiments using the access road on my morning commute indicates warm-up continues for several minutes. But my approach is to add a block heater and transaxle oil pan heater ("Don't curse the darkness, light a candle.")
Bob Wilson