Why didn't GM use a small Diesel instead of a gas engine in the Volt?

Actually diesel currently tends to be more expensive.

But I would assume they went with gas because it is your average everyday americans fuel of choice. Plus the engines don’t wear down as fast and run a lot cleaner (environmental concerns being one of the chief concerns with the volt being made).

Don’t be surprised though if you see volts featuring diesel secondary engines at some point though, especially in Europe.

Modern Diesel engines burn less fuel and burn fuel cleaner than petrol engines. They are also more reliable than petrol engines due to having fewer parts and being generally more robust. And in Europe, Diesel is cheaper than petrol, too. (that is why the upcoming VW L1 will have a 1l Diesel engine, and not a petrol engine.)
I suppose they did not use a Diesel engine because people can not get the right kind of Diesel those engines need, and because Diesel is more expensive in the US.
Maybe technical incompetence plays a role too, but then again they can milk Opel for the engineering..

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.

GM has been shy about putting diesel engines in cars ever since the debacle of the converted 350 V-8 in the 70s. Those unreliable pieces of crap turned an entire generation of Americans away from diesels. I love M-B diesel cars though, have 3 of them and a Unimog diesel.

@IchtheosaurusRex GA, beat me to it.

I have a Citroen C3, it is diesel and the mileage is great. In Australia diesel is more expensive than regular or even high octane petrol but it is still overall a very economical choice. Although in the larger cars diesel engines are much more expensive to purchase initially.

Bad reputation in the US. The situation in Europe is different.