It seems that a small diesel, operating in the optimum powerband at all times, would be ideal for a hybrid application. Let the batteries handle the low-end grunt, and let the small motor operate at a constant RPM with constant load at all times
You could even hop in the car and drive away on the batteries, and the diesel could start later, when it was good and ready.
The problem is, on-road diesels typically don't spend enough time off their design point to justify the expense and weight of a hybrid. They get better gas mileage (for most of us) without the weight.
What's wrong with Diesel-Electric like the trains run on?
It would be, without a doubt.
Running a generator at a constant speed when needed is a perfect application for a diesel engine. Diesels prefer to run under load at a constant RPM.