Well, let's look at it by the numbers.

Let's assume aluminum heads which will allow a higher compression ratio without detonation. Toss in pistons that have "mystery plasma coating X" up top to keep the heat in the combustion chamber instead of soaking down through the body of the piston. Ditto on coating the combustion chambers in the heads. Our combustion chambers have the perfect shape to squish the fuel - air mixture to give us a perfect flame path. We are going to need gapless rings to eliminate all blowby or leakage down in to the crankcase. We will need to do everything possible to eliminate parasitic loss of power to the reciprocating assembly and valvetrain. Roller everything, as light as possible. Wasted power means less efficiency... If we are using a wet fuel, we need to maintain perfect suspension of the fuel as the smallest possible particles in the incoming air charge. A gaseous fuel would mix more completely, but something like propane only has about half the BTU's of regular gasoline. Since a turbo uses "wasted energy" getting blown out the exhaust to give forced induction, we may as well find the perfect turbocharger to increase the overall airflow through our hypothetical gas miser, while still maintaining minimal fuel used to power the car down the road. If we apply Indy car thinking to our engine and crank this sucker up to around 70 pounds of boost and make our operating range of the engine 9000-12000 rpm, our engine won't have time during a revolution to allow detonation to occur regardless of how lean the mixture is. Pre-heat our fuel to around 180f-200f so we get our perfect atomization through our injectors that create a perfect fog dispersal in to the air charge.

Can it be done? maybe... Can the engine be built in a cost efficient manner that the average Joe can afford?I really don't see how.