I have said it before and I have seen others say similar but why not limit somethings but leave others free. In terms of engines why not limit each car to a maximum fuel tank size, say of 100 litres with no refueling in the race but each team can run whatever engine they want. If they want to make it green, after a few years lower to 90 litres, 80 litres etc. etc. Let them run inline engines, V's, n/a's, turbo/super chargers ....