I came to this site wanting to learn more on my MB.And hoping to help others with my mechanical experience.
This started on someone elses thread.And I'm hoping to clear up misunderstandings with this one.Especially if the misunderstandings are my own.So here goes.
A four stroke piston engine of currant design is about 33% thermally efficient.While a two stroke is slightly less at about 30%.While a jet engine directly utilising heat for purposes of thrust is extreemly thermally efficient.

Due to parasitic power loss a four stroke piston engine achieves maybe 25% mechanical efficiency.(educated guess)
A two stroke engine would be better,at maybe something like 38% mechanical efficiency.(educated guess)
A rotory engine would be best.I believe they are near 40% in mechanical efficiency.
A jet engine is extreemly low in mechanical efficiency,unless it is of the turbine design which directly takes mechanical advantage of the thrust being generated.

A two stroke engine is only about 27% efficient in combusting gasoline.And around 29% efficient in combusting diesel fuel.
A four stroke piston engine is about 30% efficient in combusting gasoline.And around 37% efficient in combusting diesel fuel?
A rotory engine is about 29% efficient in combusting gasoline.(due to compression losses on the ends of the rotor.)
While a jet engine is somewhere in the 90% range for efficiently combusting fuel.

This is where I'm starting this discussion.If any of my information is flawed,please tell me so.

Now I come to the point where most people were misunderstanding me before.The primary reason for automobiles having catalytic converters and air injection systems into the exhaust is to burn the roughly 70% unburned hydrocarbons leaving the cylinders.Once past the converter,the fuel is over 90% combusted.But in the exhaust system,you get 0% energy gains.Some claim you can't burn vaporised fuel.That it can't be ignited with a spark plug.But the fact is you can run an engine on propane and methane both.They are both vapor fuels.A jet engine uses high speed air flow to mostly vaporize it's fuel.Which is high grade kerosene.I realise that an internal combustion engine can't run on completly vaporized gasoline because of tar and varnish residues left behind after complete vaporization.

In past times.It was believed you couldn't run a fuel air mixture in a carburated engine any leaner then 13.5:1.Any less would cause damage,and it did.Then electronic fuel injection came along.Instead of partially misted fuel from the carburator,we now had well misted high pressure fuel leaving the throttle body injectors or manifold injectors at a ratio around 14:1.Leaner mixture,same power,same combustion tempratures,but better combustion efficiency.Now we have direct injection of the misted fuel directly entering the combustion chamber.Having no chance to seperate in the intake stream anymore,we now have fuel air mixtures in the range of 14.5:1 and 14.7:1.
The combustion efficiency has been improved again with very slight thermal increases and slightly more power.

As fuel delivery systems have improved the quality of the fuel entering the combustion chamber,combustion efficiency has been improved.I know there are a great many factors involved in the overall efficiency of an engine.But I am trying at the moment to point out the combustability of gasoline fuel within a given engine design.To increase gasoline fuel percentage utilized,is in this case nothing more then the percentage of fuel burned in the combustion chamber.To know the percentage of fuel combusted in the cylinders,you would need an exhaust gas analyzer.And the EGR valve would need to be disabled,as well as the air injection,and the catalytic converter removed.

The only thing that would be proven analizing a car with all emission controls intact,would be the combined combustion efficiency of the engine and emission controls togather.