Right now, there are basically three noteworthy engines: The "fast" 3-phase, the "heat-management" advanced coolant, and biodiesel for its special ability. There was some discussion a while back about differentiating the others, but nothing came of it. But really, we already have a way to differentiate things. All our engines are basically the same weight (I think only Plain differs from the others.) And their weight is too small to be meaningful in any case. So. The solution seems easy to me: Raise the weight of all existing engines and the weight limit of all tanks by the same amount (these will cancel out and have no effect on their own, since you can't not use an engine.) Then, using the margin created by the larger engine weight, choose one of the two 'generic' engines and make it the "light" engine. Nuclear would be logical. Easy enough to do. The exact numbers might require some balancing; it's a bit tricky, since if you drop the weight of Nuclear below standard, you're basically giving 'free' additional weight capacity to people who use it. But I think that it certainly could be balanced, and would make for a comparatively easy-to-implement way to add more options for engines.