HA!! I have a 1 liter Boxer 1978 actually a little under a liter and yeah I get about 60mpg with it at 75HP from 2 cylinders and twin carbs.

One of the big problems with flat engine designs is the crank shaft split case design and oil leaking into the cylinder heads when it stops. Centerstands are preferred on the boxers and the K bikes with their inline 3 and 4 cylinders have to heads on the right "high" side with the side stand for that reason. Everyong should remember the oil leaks from the old VW air cooled engines part from the expansion of the cylinders to the heads causing the pushrod tube to leak and some from the fact that the oil was flowing horizontally across seals that leaked instead of straight down.

As far as single cylinder engines are concerned Forgetaboutit they shake too much to be reliable except at low rpm and you will never get one to run smooth enough and produce enough power to push a car.

Three cylinder engines do run very smooth without counter balance shafts - my Geo was unbelievably smooth. My 80 rabbit would buzz so bad between 50 and 60mph you could not see out the inside rear view mirror. After driving teh Rabbit for 14 years I got my Geo and actually had my hearing improve from the quietness and smoothness of the Geo engine compared to the VW 4.

Now if you want to talk efficient then check out the linear stroke crankless electric power generators from Australia.

https://www.freepistonpower.com/fp3.htm
-----------------------------------------
Salient features of a 100kW FP3 module:

Size: 660x280x280mm
Mass: 100kg
Two-stroke operation
Integral charge compressor
Power Density: 1kW/kg; 2 kW/litre
Fuelled by Gasoline, Diesel (Bio, JP8), LPG, Ethanol, Hydrogen, etc.
Direct, high pressure fuel injection
Variable exhaust valve lift and timing
Variable compression ratio and stroke
Mechanical simplicity (software ‘replaces’ conventional con-rods, cam and crankshaft)
Readily scalable from 25 to 500kW output power
Efficiency 50%

The next generation IC engines will revert to the design of the WW1 era rotaries, which were not reciprocating engines. To better understand them google "animated engines gnome" Matt Keveney has a nice animation, which clearly demonstrates the effectiveness of the original aircraft rotary.

Forget all the pollution and emissions issues involved in the design they are all solveable with modern technology.

Reciprocation losses are the largest percentage of pumping losses. The piston begins at TDC accelerates to 90 degrees, decelerates to 180 accelerates again to 270 then decelerates again to TDC, with two complete revolutions to complete one 4 cycle combustion event per cylinder.

Thats 8 voilations of newtons law of inertia for every power stroke. I understand that this is not the sole cause of energy losses in current IC recip engines, but consider this. The roatry aircraft engine was responsible for tripling the speed records of aircraft from 1909 to 1913. A 753 CI rotary engine in 1913 produced 165 HP. The same year Mercedes Grand Prix engine was 1400 CI and ran at 2200RPM versus 1300 for the Rotary. The MB engine burned a quart of oil every 20 miles.

The original roatry engine is the basis of my future engine concept with many significant modifications;

Supercharged two stroke operation just like the topic engine alos with 3 to 1 bore to stroke ratio, designed to operate at low speed
Diesel combustion
Fuel injection
Closed loop oil system that also serves as cooling system
Elimination of all throttle control
Single intake and exhaust ports on center journal feeding reversed cylinders with pistons directly connected to outer rotating rim of the engine with no connecting rods
Adjustable crankshaft journal independent of engine support bearing that allows you to completely eliminate stroke with zero journal offset relative to rotating engine mass, which transforms engine into a free spinning flywheel for energy storage.
The free piston engines are improvements but they are still recip engines with greater vibration issues than conventional IC recips.
Due to narrow operating speed range, this will be much more adaptable to HCCI (homogenous charge compression ignition) hich basically is compression ignition using gasoline with lean burn ratios like a diesel, also adaptable to other fuels.

Nmame and engine that can transform itself into a flywheel eliminating 98% of all pumping losses and pulse and glide itself while propelling a car at a constant speed using a CVT but preferrably an IVT (infinitely variable transmission) with computer controls that are constantly changing the effecitve "gear" ratios as the flywheel storage speeds declines.

Using the flywheel for high initial acceleration combined with the engines power you now have a "hybrid" that needs no electric motor or batteries, with a system life expectancy greater than the best current non hybrid designs.

Mileage could exceed even current P&G levels because P&G incurs much higher aero drap at higher pulse max speeds.

Stephen Marius Balzer was a rotary engine poineer who worked with Langley on his failed aircraft attempt. He built a car with a rotary and donated it to the Smithsonian in 1899. A motorcycle called the Megola was built in Germany with a rotary engine inside the front wheel, a front wheel drive motorcycle no less.

Even if you removed the cylinder head and pushed the car to 60 MPH, staying in gear, the reciprocation losses would be significant, with no airflow through the engine whatsoever. These forces are generally lumped into the "pumping losses" description when they are actually separate losses due to reciprocation not airflow. The reason I say this can only be understood when you realize that there was an engine design, long forgotten, that pumped air through itself without reciprocation. If you have ever seen one of the rotaries stop running the prop will freewheel for a lot longer than your IC recip engine will continue to rotate when you shut if off, even though it is still pumping air.

regards
Gary

Every time I hear somebody claiming efficiency in excess of 50% I wonder what rube did the performance calculations. According to the second Law of Thermodynamics the most efficient and engine can get (Carnot cycle) is 50%. Somewhere, somebody is claiming some waste heat utilization as shaft power and that is a lie.

Sludgy is right in theory. A single is always more efficient than a multi-cylinder engine. It may shake itself and the passengers and the rest of the vehicle to bits, but by George it is more efficient than a multi of equal parameters.

The surface-to-volume ratio is why gas turbine (Brayton-cycle) engines get more and efficient as they get larger. A PT6 is not very efficient but an LM2500 is as efficient as a diesel. Big utility gas turbines are 45% efficient simple cycle and about 48% compound cycle. Size matters.

As for three cylinder two-stroke engines, I remember my old Kawasaki H2 Widowmaker. After a hundred miles your hands were numb from the vibration. Suzuki water buffaloes, with their more upright inline triples, were a bit smoother.

The wwI era rotary block / fixed crank engine design might not be suited for automotive use for several reasons: weight, physical size, torque, ground clearance, center of gravity...
Go to Rhinebeck NY to see these in operation in the air craft in which they were for a short while the best of a limited choice of powerplants. Listen to the pilots that fly these beasts tell of the shortcomings that R.I.D.E. counts as benefits, especially the problems inherent in the narrow operating range and difficulties in maintaining control with the "pulse and glide" technique needed to regulate air speed.
The drawbacks were why these rotary case/fixed piston engines were replaced by radial piston engines and were then themselves replaced by reciprocating piston engines. Each was an improvement over the best of the prior ones.

The animated engines gnome is interesting but it effectivly looks like a wankel engine - rotating combustion chambers.

Straight six is the king of all engines. It might not be the most efficient, but it beats the pants of most V-6's and V-8's. My old Jeep had a 4.0 that was running with only 2 cylinders firing for almost a year (I was young and stupid). This summer, it's getting a 4.7 stroker to make some more heat and compression. They say that if I stay out of the throttle, I can get an easy 25mpg on 35" tires. Add a few tricks here and there and who knows?

The maximum theoretical efficiency of a Carnot cycle engine is limited by the upper and lower temperature excursions within the engine itself.. I've seen stirling engines that will work on the heat produced by the palm of your hand. The efficiency upper limit is mostly based on what temperature the hot side of the engine will bear before melting or weakening to the point it breaks.

https://www.sherline.com/images/9th01a.jpg

I had a 1982 Honda 500 Ascot single street bike.. With twin balancer shafts it was by no means an uncomfortable machine to ride and the fuel efficiency was excellent. I put almost 50K on that bike without ever even changing the chain.

https://bake.smugmug.com/photos/46199462-M.jpg