Here’s the latest reader question, along with my reply!
Chris asks: I’ve often wondered why no one has ever put together a diesel hybrid. Wouldn’t this result in a car that averaged 65 or 70 MPG rather than the current average (with a gas engine) of around 50 MPG? Why isn’t this pursued?
My answer: It has been pursued! Diesel-electric locomotives are essentially this type of hybrid. The diesels make the electricity that powers the traction motors that turn the wheels.
So why not the wheels of cars?
Several reasons. First – and chiefly – because of the Emissions Religion. Hybrids are marketed as being “clean” as well as economical. Diesels have been very effectively portrayed as “dirty” by those opposed to efficient internal combustion power (see the VW Proscription for a case in point). Their actual emissions are so low as to be an irrelevance, especially in a hybrid vehicle because the engine would be not running much of the time. It doesn’t matter. Diesels have been demonized – and the regs are now so tight that getting a diesel “certified” is very difficult as well as very expensive.
Which brings up the other reason. 
Hybrids already cost too much relative to what is saved . . . on fuel. A diesel hybrid would cost thousands more. And then there is the cost of the diesel fuel – also substantially more than the cost of gasoline.
Thus, even if our hypothetical hybrid diesel averaged 70 MPG, it would not save money if it cost you $5k more to buy than a gas-powered hybrid and then cost you 30 cents or more per gallon to fill up.
You’d also have to factor in the fact that modern diesels cost more to maintain than modern gas engines; they have particulate traps and DEF injection, for instance. These further erode the benefit side of the cost-benefit equation.
And that’s prolly why no diesel hybrids.
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I always wonder why some (at least really heavy) semi trucks didn’t go the locomotive style diesel-electric (like decades ago, not today). Would put the weight down low, and the trailer wheels could be powered too by plugging them in.
I guess the truck transmission is still way cheaper than electric motors. A new locomotive is over a million a semi truck is about $300k.
Plus the added weight of the electric motor(s) and associated wiring would reduce the (legal) carrying capacity of said semi tractor. IIRC 80k pounds is the limit w/o jumping through extra hoops.
Still think it is a fine idea though, esp. for a small commuter car.