I was flipping through the NHW11 shop manuals for some light bedtime reading, but suddenly I'm not sleepy. The ICE oil pump lives in the timing chain cover and is driven by the crankshaft, so only when the ICE is running. Standard stuff, didn't really expect anything different, ok fine. The transaxle oil pump lives on the outside end of MG2. I think I must have been assuming that MG2 drove it. Or maybe MG1 somehow. But the drawings on pages HT-12 and HT-13 pretty clearly show an Oil Pump Drive Shaft that goes clear through MG2 and engages in - it looks like - the input shaft / planet carrier, which is the part that's splined to the ICE. So the transaxle oil pump also turns only when the ICE turns. In a hybrid?! Ok, so that sure explains why they warn against towing with the front wheels down. But what about stealth? What were they thinking? Do they just figure that because the stealth range is short, the ICE's going to start every mile or two and pump some oil around, and that's good enough? Has anybody seen the inside of one of these transaxles closely enough to see what parts rely on oil from the pump, as opposed to plain splash or something? Have I misread something, or is there some fact I've overlooked that would make this make sense? Rather curiously, -Chap
Hi Chap, Yes, this is one reason why engineering a PHEV Prius with extended battery-only range is not a trivial exercise. That oil pump problem is one of many to be solved.
Splash oiling can work pretty well. Our marine diesel only uses splash oiling - no oil pump. The valves are manually oiled through a drip oiler. That engine will still be running after our Prius is a pile of rust and plastic. The Prius doesn't run very long or fast without the ICE, and at speed the ICE spins even when it isn't running. Tom
The transaxle parts don't depend on a continuous *feed* of oil the same way as an engine does. Think of it like the oil you'd put into a regular manual transmission -- enough to bathe the gears and get thrown around when it's turning, but in most cases those don't need an actual pump to push it around. All the pump does is push oil down the center shaft where it then bleeds out side holes into the various bearings, most of which are ball or roller in the NHW20 and can run quite a while on their own without additional feed and without "going dry". . So far the PHEV folks have racked up many EV-only miles and not reported any problems, but if PHEV Priuses get more widespread and start going through many ICE-free drive cycles across the usual thermal/humidity/stress changes, then we'll see. . _H*
