The gasoline engine in the Volt does not power the drive wheels. It powers a generator to recharge the batteries. A diesel would be no more efficient at this task. Here’s why:
Automotive engines operate over a wide range of RPMs. When you are sitting at a stoplight, your engine is turning at somewhere around 750 RPM, but when you’re accelerating, some engines turn faster than 8000 RPM. Engine power and torque increase with RPM. This is why you need a transmission.
Diesel engines typically turn at low RPMs even at their maximum because the cylinder design is different from that of a gas engine. The fuel burns slowly, allowing for a very long power stroke. The result is enormous torque – up to twice that of a comparable-displacement gas engine, which is what you need to get a vehicle moving. The torque advantage is one of the things that makes diesels so economical. While a gas engine is screaming to produce enough power to move the car, a diesel can turn lazily and still get you away from the stoplight. Lower RPMs means lower internal resistance, so more of the engine’s power is available for propulsion.
For running a generator, though, you don’t need to operate the engine over a wide RPM range. The engine can be tuned to turn at a rate that is optimal for the generator. When the engine kicks in, it spins up to that RPM, and just stays there. A diesel has no real advantage under those operating conditions.