Hi,
While walking the dog, I realized I made a mistake:
Quote:
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Originally Posted by bwilson4web
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Now there are also mechanical losses from the gears. This loss is a function of RPM and transaxle oil temperature. At maximum 5,000 rpm, the mechanical energy loss is 1.1 kW or about 1.3 hp, pp. 26-28 (3). The axle rpm can be converted to miles per hour but I don't have that handy. Regardless, we can calculate the hp based upon the rpm and torque. Divide the 1.3 hp by the power, 6.6 hp and you'll have the Prius transaxle mechanical efficiency at maximum RPM of ~20%. At a more sedate, 2,000 rpm typical of highway cruising, the efficiency improves to ~10%.
So let's review:
- <<25% efficiency at engine redline (HOLY CRAP! ARE YOU CRAZY!)
- <<15% efficiency at ordinary cruise speeds, ~55 mpg
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The gear losses are a combination of friction and thrashing the transaxle oil losses. We've made an assumption that the friction losses will track with the power-in and power-out. However, thrashing the transaxle oil losses are independent of the power-in and power-out. I don't know how to factor this out except by repeating the test at different power levels. In simple terms, we are looking at the worst possible, highest values.
One other observation is the Prius MG1 is effectively a torque and power meter for the ICE. This means if I want to map a maximum ICE efficiency range, a combination of fuel flow and MG1 data would allow us to map it out.
LATE EDIT
In gear losses, we are used to having two fixed axles that see a lateral force proportional to the applied torque. However, the power split device is a planetary gear set. This means the MG1 and ICE torque are co-axial and not subject to the ordinary lateral forces of regular gears. The silent chain power take-off from the ring gear would induce some lateral force, the chain tension, but compared to a friction drive, this would be very small. Bottom line, PDS friction losses are even lower than ordinary gears.
Bob Wilson