17 September 2005

Plug-in Drivetrain

When we talk about the plug-in hybrid, there's two potential means to get power to the wheels. The first is to have a hybrid transmission that couples mechanically to both a combustion engine and an electric motor. This is the type of drivetrain in use by current hybrids, such as the Toyata Prius:


The second option is to use the combustion engine merely as a generator for the storage system. In this case, 100 % of the power to the wheels is supplied by electric motors. The electric motor would be sized to be able to handle the current from the batteries and the generator at the same time in order to maximize acceleration.

The question is, which is better for the plug-in? There's no doubt for a regular hybrid -- where most of the power is delivered through the combustion engine -- that the mechanical transmission is more efficient in delivering power to the wheels. The reason is because electric generators are not very efficient. However, combustion engines also like to operate under an optimal load -- the constant accelerations of city driving lead to inefficiencies in the overall efficiency of a car. Of course, plug-in hybrids with only a very short all-electric range can also run the engine at more ideal rates, using the battery to makeup shortfalls.

Diesel generators can have efficiencies of 37 %, Lithium-ion batteries round-trip efficiencies of 90 %, and electric motors can easily break 95 % efficiency. This means an all-electric drivetrain hybrid could have a tank-to-wheel efficiency of 32 %. In reality performance will be worse due to transients from starting. The typical tank-to-wheel efficiency of a diesel is about 29 %, so they are definitely comparable. In contrast, conventional spark ignition engines have a tank-to-wheel efficiency of about 22 %. In the end, it probably comes down to complexity and weight, both of which contribute greatly to cost. The electric drivetrain is likely lighter than its hybrid continuously-variable cousin. Assuming the cost of permanent magnets doesn't skyrocket, it should be a more inexpensive solution in the future, even if it isn't today.

In the case of a plug-in hybrid that gets about 75 % of its energy requirements from electricity and 25 % from carbon fuels, we will probably see a transition in the hybrid industry from the coupled mechanical-electrical transmissions of today to pure electric drive. Some more information on vehicle technology can be found at the DoE's Energy Efficiency and Renewable Energy web page:


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