It should not be surprising that the most important attribute of today's hybrid propulsion system is that total driveline efficiency exceed 80%. When vehicle fuel economy is in excess of 40 mpg, a 100 W power loss due to core heating in the traction motor or its attendant power inverter represents a significant impact. Weight is another very important attribute, but its impact is not as noticeable until performance on grades is required. This chapter provides an assessment of the complete hybrid drive system and where the prominent loss mechanisms reside. Particular attention is paid to the traction M/G core and copper losses and the inverter conduction and switching losses. Mechanical friction contributions are noted, particularly with regard to non-conventional designs due to adding the hybrid components.

An illustration of a non-conventional contributor to friction would be the need for a large diameter bearing to support the otherwise cantilevered mass of a crankshaft mounted starter-alternator. In single clutch, or M/G to transmission torque converter arrangements, there is a tendency to have large shifts in M/G rotor centre of gravity from the crankshaft main journal bearing and the potential to have this rotor execute large deviations in whirl as the unsupported end has no means to resist crankshaft bounce and whirl. An auxiliary large diameter bearing is then incorporated on the outboard rotor side for support.