Re: To Darryl-80 mpg diesel hybrid-real or not?
The best design is one where the engine (when it's running) runs at one speed (the speed at which it gets the best efficiency). Connecting the engine to the wheels directly was never a good idea. Doing so in the Prius was a bad idea even with the high-tech transmission. From day one when cars were first built we should have had the engine running a generator which in turn powered electric motors, as is done with diesel locomotives. This would have done away with cumbersome multispeed mechanical transmissions and resulted in better acceleration for a given engine power. When you have a 200 HP engine accelerating a car through a regular transmission, on average about 60% of the power actually ends up being used for acceleration. The reasons are several. First, because the RPM varies, the engine spends a good deal of time out of its peak power RPM. Second, energy is spent speeding up the engine with each gear change. Third, the gear changes cost time. If you do it the other way, when you floor it the engine goes right to its peak power RPM, the generator puts out full power, and the electric motors convert most of that power into acceleration. The same car might only require 120 HP instead of 200 HP for the same performance. By storing energy, you lower the peak power requirements even more. Ideally, you only need an engine which supplies the average power used by the vehicle. This might be less than 25 HP for most driving. In fact, in a properly designed car, 25 HP should be good to maintain a speed of 125 mph, more than adequate for any sensible driving. Energy storage will take care of the power bursts for acceleration. In regular driving the 25 HP engine might be running less than half the time.
Using only electric motors to drive the car has many advantages. Regenerative braking for one. The ability to electronically control speed to a very high precision for another. And the ability to control torque for different driving conditions if you use a motor on each wheel. All cars should, regardless of power source, be designed with 4-wheel electric motor drive, energy storage for acceleration, regenerative braking, and solar cells to supplement the power plant (these can generate a good portion of the vehicle's energy needs when it is parked). Ideally, in light use, the solar cells can supply 100% of the power, making recharging and refueling unnecessary. Powering the vehicle directly off the power grid via inductive pickup from high-frequency AC power cables embedded in the roadway is another possibility. This gives all the advantages of a straight electric without the range or recharging problems.
As for the technical limits on fuel economy, I've heard that 100 mpg @ 100 mph is very feasible. 200 mpg might represent an upper limit for a 4-passenger driveable vehicle. I'd like to see fossil fuels done away with in the near future, but in the mean time we can do a lot better than most people think.