Stirling engine is indeed such an example, I have presented this concept to many audiences. I have one major feedback that is the power needed for the air compressor. The power saved may not be sufficient to cover power gained. I have done my calculation and found out that the power for compressor is just 10% of power gained.
Another question is "What about the CO2 and H2O as the product of combustion?" . The answer is that these combustion products are at peak temperature, thus, they do not rob the energy like the waste CO2 and H2O. In other words, the waste gas lower the combustion temperature from say 2000 C to 1500 C. If there is no waste gas then the temperature remains at 2000 C.
A legitimate concern is that this new engine needs huge developmental effort to bring it to market. That I agree. Perhaps over 100 million dollars.
By the way, I have created a new thread, "Chen engine, a more efficient and cleaner engine". I'll continue this discussion on that thread.
Regards,
Chengine