If we are talking about overall efficiency, I think the numbers you quoted (around 30%) look reasonable, except for the 90% jet engine. The 90% value may actually be a combustion efficiency number, it is much to high for the overall efficiency. There are lots of different types of "jet engines" and I'm not an expert on any of them. I don't have a source to verify the efficiencies of current engines, but I wouldn't be too surprised if the latest high efficiency engines (gas and diesel) were doing a little better. Maybe someone else knows.

Getting back to basics for a minute, someone pointed out that in an internal combustion engine about 1/3 of the energy is lost to the environment as heat (mostly though the cooling system), about 1/3 is lost out the exhaust (as hot gasses), and about 1/3 is turned into useful (shaft) power. These are round numbers, but close enough. Obviously, our goal is to minimize the lost energy.

If we look at the 1/3 that is lost as heat to the environment, we can theoretically get rid of the cooling system and operating the engine at higher temperatures, except it will melt . This is where the proposed ceramic (and ceramic coated engines) come in. The idea is to design an engine that will operate reliably at higher temperatures. I don't have any problem with this concept, but (as far as I know) no-one has developed a practical, cost effective design. I think a lot of the issues have to due with trying to manufacture complex ceramic parts (e.g., space shuttle tile problem). As materials improve and engines can be reliably operated at higher temperatures (for reasonable costs), this loss should inch down.

The 1/3 that is lost out the exhaust (as hot gasses) is pretty much a given. The second law of thermodynamics limits the maximum efficiency between a high temperature source (combustion temperatures) and a low temperature sink (ambient temperature). As we discussed earlier, this maximum efficiency is about 70% based on assumed temperatures of about 1300F and 70F. That means we are going to lose at least 30% no matter how well we design the engine (no free lunch there).

If we consider the 1/3 that is lost to cooling (material limits) and the 1/3 that is lost through the exhaust (thermodynamic limit), we don't have a lot left to work with. All modern engines are designed to provide maximum combustion efficiency (while still meeting emission and temperature limits). Current fuel injection control systems are very complex and are designed to continuously optimize the combustion process.

If we go back to the 50s and 60s there was clearly room for improvement in the carburetors. At that time, I can believe that someone could design a carburetor that would be a significant improvement over the stock designs. I also understand that a carburetor could be designed/adjusted to operate very lean, resulting in higher operating temperatures and an improvement in efficiency. However, if the engine was not designed for those operating conditions it would not be reliable. In addition, there are emissions consequences (i.e., high NOX) associated with high combustion temperatures that would have to be addressed.

With regard to the "vapor carburetor" design, I'm not sure I understand what it was trying to do. I suspect it was designed to operate the engine under very lean conditions, minimizing incomplete combustion and increasing operating temperatures. I expect this would result is an increase in efficiency (I don't know how much), but I also expect it would be very hard on the engine and would adversely affect some emissions. If that was the goal, the same thing could be accomplished today by reprogramming the fuel injection system on a modern engine.

If I understood your earlier posts correctly, you were attributing the increased efficiency of the "vapor carburetor" design to minimizing incomplete combustion. I doubt that was the major effect because stock engines should have very small loses due to that. If this design was causing the engine to run very lean (and hot) it could have increased efficiency in the short term. If that is the case, this design concept has been superseded by modern fuel injection (and engine management) systems that allow all these variables to be continuously monitored and controlled.

Personally, I think internal combustion engines have reached the point where improvements in efficiency will come very gradually, because all the easy stuff was done years ago. Engine management systems and materials will continue to improve, but I would be surprised to see a sudden leap in efficiency. I suspect we will see bigger improvements in the remainder of the vehicle (lighter weight, improved drive-trains, etc.), but that's a whole other subject.

What do you think?