At 4:29 PM 3/30/5, Stephen A. Lawrence wrote: >Why is it not better to use a gutsy electric motor, a small gasoline or >diesel engine, and a battery pack? That always seemed like the >"reasonable" way to build a hybrid -- take a tip from diesel electric >locomotives, that run the diesel engine at a very efficient constant >RPM, and use the electric wheel motors with their very wide dynamic >range to do the "impedence match" to the terrain. The battery pack gets >you onto the expressway and up the hills, and the petroleum based >motor/generator provides all the electricity that you need while cruising. > >Obviously my intuition here is wrong. Is there a 20 word explanation of >why?
I'm no expert on this, but I'll give it a shot. However 20 words is a bit too little. Here's a short answer: If big energy batteries weighed nothing, cost nothing, and had no bulk, you would need no gas motor. A longer answer is size, cost and weight of electric motor, control, and battery must be balanced to savings achieved by reduction in these parameters due to transmission and fueled motor size reduction and regenerative breaking. Optimizing depends on the driving conditions for the design. It appears a good match for present battery systems is roughly even sizes for the power of the two systems. In this way a combined gas motor/battery combination is available for big hill climbs or charging while driving. The motor does not need to be on while idling or even at all times when cruising. If you have a 30 hp gas motor and cruise at 60 mpg you have about 2/3 the motor output available for charging when cruising, so can cruise more than half the time without the gas motor running at all. The vehicle can perform at half power on either pure electric or pure gas motor. When stopped in traffic no motor need run at all. The answer also depends on what kind of transmission is used for the gas motor "impedence matching". If no shifting transmission is used for the gas motor, i.e. no gas power is put through a shifting transmission, but it can direct connect in high gear for hill climbs or high speed driving, then this doubles the hill climb horsepower without beefing up the transmission or incurring the overhead for electric power generation from gas during the cruise. This overdrive type connection doesn't help low speed acceleration, but low speed acceleration using electric motors is pretty good anyway. Acceleration could be improved using a fluidized or electronic clutch connection between the gas motor and the overdrive connection, or just by beefing up the transmission, but this is a cost/benefit trade-off. Another short answer: the gas motor doesn't need to run when you are cruising unless you need to charge or to go uphill. Regards, Horace Heffner
