There are a couple of key characteristics that make Stirling engines impractical for use in many applications, including in most cars and trucks.
Because the heat source is external, it takes a little while for the engine to respond to changes in the amount of heat being applied to the cylinder -- it takes time for the heat to be conducted through the cylinder walls and into the gas inside the engine. This means that:
- The engine requires some time to warm up before it can produce useful power.
- The engine can not change its power output quickly.
These shortcomings all but guarantee that it won't replace the internal-combustion engine in cars. However, a Stirling-engine-powered hybrid car might be feasible.
For more information on Stirling engines and related topics, check out the links below.