[Big Dave]

The term is called compounding. It has been around for a long time. The gasoline aircraft engines of the mid-40s were sometimes compounded. The R-3360 engines used on B-29s were tubocompounded and were very efficient. Without the efficiency of the turbocompounded engines, missions from Saipan to Tokyo would have been impossible. The Napier Sabre emgine was used in British Shackleton patrol planes after the war was maybe the most fuel efficient internal combustion engine of all time.

Both th R-3360 and the Sabre were maintenance nightmares.

Using compounding works OK for aircraft and ship powerplants because those engines see very flat duty cycles. An airplane takes of at full power then throttles back to a cruise speed to get close then throttles back to nearly a glide for approach. Cars don't get used that way. They rip up and down through their operating range and rarely operate at rated power. Thus the Rankine or Brayton cycle compounding engine never gets a chance to stabilize and make meaningful power.

Another factor is that IC engines, when operated at constant speed is more efficient than you think. This is especially true of diesels. Big marine diesels are so efficient that there is not enough heat left in the exhaust gas to do much of anything. The owner would have to spend a lot of money for very little return.

Far and away the most successful compounding scheme has been the turbocharger. A Brayton cycle spins the supercharger for an IC engine.

The mostI'd expect of a steam compounding scheme for a car would be enough electricity to run the alternator and maybe the power steering. Its been tried with air conditioning and found wanting.

Stirlings are efficient and run on low delta-T but they are huge and expensive for the power they produce. A 20 HP Stirling would be as big as an office desk and weigh 1500 lb.

These ideas have been around a long time. The Stirling was invented just a little later than the steam engine, but the steam engine (although clearly less efficient) became dominant for a century and the Stirling was a curiosity. Then the IC engines came along and within seventy years relegated steam engines to a niche role.

Don't you ever bother to wonder why? Do you think that hundreds of thousands of engineers over the years were complete blockheads? Or that some deep, dark conspiracies kept thee ideas inthe shade?