I'm eyeing the transaxle for pre-heating because unlike the engine, it warms up slowly. The graph below shows the relationship between fuel economy and temperature for the Gen 1 Prius. I think this has to do mostly with the transaxle and high voltage battery, both of which are more efficient when warm.
I'm also going to install a high wattage circulation tank heater to try to get the Prius into stage 4 right on startup. Stage 4 simply put is when the engine shuts down anytime it's not needed. Normally the engine runs until it reaches 70˚C whether it's needed or not and runs rich while doing it.
I have not found anyone preheating their Prius transaxle yet so I think I'm blazing a trail here. Please let me know if you see anything out there.
From Bob's transaxle study:
2003 Prius - Cold Weather and Transaxle
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The ORNL/TM-2004/247 study reports a 20% transaxle energy loss at room temperatures "27 (C)" (pp. 32)
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So it must be worse at lower temperatures.
The following graph shows the effect of cold on mileage in the gen 1 prius:
I have a tank/circulation engine heater which I plan to install in the transaxle coolant circuit. The Prius has a separate cooling circuit in the transaxle/inverter which uses the same coolant as the engine. there is a drain plug in the bottom which I may be able to tap and a series of hoses on the upper side which feed the MG2 electric motor.
What I don't know yet is if the coolant has to be forced through with the coolant pump or if it will circulate by convection through the internal passages.
I'm also interested in preheating the transmission oil via the pan. For this I may have to fab something custom to get the heating I want.
Added chart for reference (Thx Bob):
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The following chart shows a series of parametric curves at different rpms showing the energy loss as a function of temperature within each rpm:
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Two lines, one at the 28 (C) value and one at the 70 (C) value were drawn to give an idea of energy loss at rpms between any two rpm curves. Not the highest value, the 70 (C), 158 (F), line is just above the highest temperatures so far measured in high-speed cruise tests. To keep the rpm curves separated, a 'null' entry was put in that displaces each curve to the right. The report 500 rpm data was dropped since the Prius ICE never runs at this speed and rpm above 5,500 since the ICE is speed limited below this value.
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