For my money, a gas-engine hybrid is not just a waste but a frivolity. Diesel engine autos used to get roughly double (and sometimes more than double) the mileage of the same model with a gas engine. I know the gap has shrunk quite a bit, but I think that's mainly because there's more money in gas engines, so more research is done on increasing the mileage of gas-powered cars.
I'm not all that sure though, someone please correct me if I'm wrong on any of this.
Anyhoo, I suspect that a given car powered by a well-made diesel should get at least 20% better mileage. But there's more.
Everyone knows that the output of an engine varies with RPM, but so does the efficiency. What I can't understand is why someone doesn't make a hybrid that runs off batteries, and uses a diesel engine which runs at a constant RPM, the RPM at which it produces the most power per unit of fuel, to charge the batteries only rather than to provide any power to the wheels.
I believe this would work well for a number of reasons.
First, it would eliminate some of the limitations of diesel engines in cars--the relatively sluggish performance, the relatively narrow RPM range and maybe some of the noise and vibration, because the motor mounting system could be tuned to maximize its vibration-damping performance at the specific RPM at which the engine runs. The intake and exhaust systems could also be tuned to that RPM which might result in even greater efficiency.
In addition, the increased efficiency from the diesel by running it at its optimal RPM may offset some of the power conversion losses in changing the mechanical power of the diesel to electricity and storing it.
I could be wrong of course, but I suspect that such a car, if engineered carefully, could actually offset the increased purchase price due to the electric storage and drive system in fuel savings within a few years. I would hope two years, three at the most. IMO if it takes more than three years to make back the price difference, most people wouldn't consider it worthwhile.
If the diesel-electric got 50% more MPG (real-world mileage, not EPA ratings) than its gas-engine counterpart, and the gas-engine car got 36 MPG then the D-E should get 54 MPG and that would result in about $300 per year savings given annual mileage of 12,000 and a gas price of $2.70 per gallon. So that means the added cost of the electrics would have to be no more than $900. Seems pretty unlikely. However, there would be some inherent savings on the engine which would be smaller and simpler, also there'd be no transmission or differential, which would further help to offset the price increase from the electric drive system. In addition, if you were driving a larger vehicle which got poorer mileage, the savings would be greater even given the same percentage of fuel savings, annual mileage and price of fuel.
A vehicle that got 10 MPG with a gas engine, for example, would use 1200 gallons per year, and if switching to the diesel-electric powerplant increased efficiency 50%, they'd use only 800 gallons, saving over $3200 over three years using the gas price of $270 per gal. Of course, the larger vehicle would require more batteries and a bigger electric drive system, so the price increase to the sticker would be more, offsetting some of the savings.
What I'd like to find is a formula which could convert the price of electricity per KwH into dollars per mile for a given vehicle. If it's substantially less than the price of gas, then simply adding a plug-in charging system would further boost the money savings per year and it doesn't seem like it would add much to the cost of the vehicle.