What affects which engine types I can build?

[morik]

I was able to put a triple diesel engine in a self propelled artillery, but when I was making a medium tank the options only had up to double diesel.
I checked over my technologies and didn't see anything that looked relevant to engine options...

Can anyone tell me what to focus on/do to get access to larger engine types? Is it just not possible to put a triple diesel in a med tank? Or was there something special in the self propelled artillery (or maybe it was a transport truck type?) that allows bigger engines?

Is it research based? Or is it just different chassis affect which engine types are allowed, and I start with all the sizes already available?

[Elver]

It's entirely based on model type. Some chassis can handle bigger engines than others. Research will eventually add the option for electric/nuclear engines, but the types of those available will also be limited by model.