The last couple of years development in aircraft engines has been more or less concentrating on diesel engines. We have seen one off installations to fully developed production lines.
A number of companies are active on this market primarily due to major concern of long term availability and the relative high price of AVgas (Europe). Jet fuel is available worldwide, even in places where AVgas is not and has to be flown in.
Diesel engines use either the two or four stroke principle, these are fundamentally different and we will show some basic differences here.
A number of engines that are being developed are of the two stroke design. A two stroke diesel can not be compared to a two stroke gasoline engine, they use a mix of fuel and oil (1:50) to lube the engine by pressure in the crankcase (caused by the moving piston) and finally burning this mixture will result in very polluting exhaust gases (obviously blue smoke).
Two stroke diesels have more in common with their four stroke cousins, but the engine strokes occur in one revolution: intake, compression, power and exhaust. That seems like a challenge, and it is. Correct timing is crucial here. Some engine designs use intake ports and exhaust valves, where as others have ports only and the pistons acting toward each other in pairs in the same cylinder, like the German JUMO diesels from WW-II, see image.
Read this tutorial about the Junkers Jumo 205 for a complete explanation on how they operate (from enginehistory.org).
They must use some type of blower to start as their intake stroke is not capable of inhaling air (at this point the intake port is open but the piston is almost reaching or is at bottom dead point so there is no displacement of anything. The blower, usually a roots type, is engine driven and blows air under pressure into the cylinders. Most of these engines also have a turbocharger which takes it over from the roots blower when the engine is running, usually after reaching a certain RPM. And as RPM range is rather narrow (900 - 2700 RPM) the turbo can be perfectly matched to the engine requirements.
Engine design is simple: they have a low pressure fuel pump (some 300 bar, common rail pumps are around 1800 bar), no electronics, no gearbox and a very reliable turbo. Just add oil, fuel, connect the wires and start. The battery is only needed to activate the glow plugs and start the engine. They are also not really fussy about the fuel: JET A (A1), diesel, kerosene, lamp oil, vegetable or peanut oil will do.
NASA has published the "Lightweight two stroke cycle aircraft diesel engine technology enablement program" in August 1985, it is rather dated but still a very interesting document. Careful: this document is quite large, 116 pages (14 MB).
This type uses two revolutions to accomplish its task. Intake, compression in the first revolution and power and exhaust stroke in the second revolution, timing is less critical. Power is lower compared to its two stroke cousin, but that can be overcome by running it at a higher RPM and using a gearbox to reduce the RPM for the propeller.
Everything else being equal, compared with a two stroke of the same displacement and RPM, the four stroke type will be heavier and have less power for its weight. A number engines that are being developed are of four stroke design Centurion/Thielert and Austro (based on a Mercedes A type diesel and as of 2011 a Steyer Motor).
They must use a gear box (to obtain more power they need more RPM, two strokes are usually direct driven and have more power/torque at lower more efficient propeller RPMs), have electronic engine management (FADEC, which must be dual for redundancy, a dual electrical system is also needed) and use common rail for higher injection pressures and precise controlled injection timings.
These engines are modern but very complex compared with the two stroke type. Weight is higher too. Thielert has developed a 135/155 bhp diesel weighing almost 150 kg (no fluids) without propeller. It is an expensive modern engine which uses somewhat less fuel (10%-15%) than the two stroke diesel. These engine installations are also quite complex, heavy, need dual electrical systems and a lot of hoses and radiators for cooling and lubrication. Not exactly the right engine for the first time aircraft builder.