Carnot limit is over 60-70 percent in fixed load power plants. 70% is something only could be attained with very exotic materials(expensive and difficult to manage) that could handle high temperatures.

In a mobile system you could get 50% with a fixed load. The problem with the car is that is has to work in different regimes.

That is the reason hybrid engines are more efficient. They work at a fixed load and store the energy in a battery or super cap.