One of the neatest HEV powertrains I know uses a very simple construction of a planetary gear system being driven by two power sources: First, the ICE, and, second, an electric motor. Power is diverted off the ICE to drive a generator, which in combination with a large battery, powers the electric motor (see figure below). The diversion of power to the generator is regulated by electronics that determine how much the generator provides charge and, as a result, how much of the ICE torque is directly sent to the wheels. The amount of power the generator demands from the ICE is related to the amount of power it generates.
A popular style hybrid electric vehicle powertrain
By regulating the split between mechanical and electrical motive sources, the ICE is allowed to run at its optimum speed/torque for extended periods of time. This improves the efficiency of the ICE dramatically. There is a sweet spot in the engine rpm/torque curves that provides the best efficiency (see figure below). Hybrid technology allows the engine to stay in that sweet spot longer, thus providing much better fuel economy.
ICE efficiency curves
Secondly, the gasoline engine can now be much smaller because the torque needed to accelerate is shared between the electric motor (and its accompanying battery storage) and the ICE.
Doing the math on one typical hybrid powertrain (from a popular brand’s website), we get:
• ICE power = 98 hp
• Electric motor power = 80 hp
• Battery capability = 36 hp
• Total available power = 134 hp
Note, that the total available power is just the ICE and what the battery can deliver. To me this seems a bit simplistic because 134 hp is the absolute most power that can potentially be in the system at any one time. Between the 201.6V battery and the wheels is first a 650V DC-DC converter and then a three phase inverter and then the motor-transmission. To assume that the heat generated by these elements is negligible is optimistic. Unless these smart guys have invented the next best thing to perpetual motion, lossless energy conversion (from battery to motion), they are being a bit optimistic.
Am I picking nits? Yes, I guess so. These hybrid engineers have done a lot to minimize the losses between the battery and the wheels. For instance, the DC–DC converter and 650V inverter-motor waste much less energy than a 200V inverter-motor would. Using a battery and electric motor to boost the powertrain output during high loads and taking advantage of times when the engine load is not so high to replace that energy drained from the battery improves the efficiency of the ICE. Improved efficiency also means lower exhaust gas emissions for the same output power. And that’s a good thing.
(Part 2 of this feature discusses regenerative braking and a future beyond hybrids.)
is principal engineer at STMicroelectronics