Around the time of World War II, fuel rations caused Europeans to look to alternative means to power their cars. Some attached hulking tanks to the back of their car that burned wood chips as fuel.

Several years ago, George Mason University graduate student Jason Force had a modern idea for the technology: Because grass pellets are just as good as wood chips for this kind of system, why not create a lawn mower with it? It could power itself with the grass it harvested. It could also be self-guiding, like a Roomba, so users would not longer have to mow their own lawn.

He got to work on the self-guiding aspect five years ago. The idea grew into a startup called EcoMow Technologies. Its team began working on the chemical engineering last year.

Force tested the robot among consumers and found that they loved the idea. But investors…