There are two species of these heavy duty engines, two stroke and four stroke. Both designs have been available for many years, and both are proven and well accepted in the industry. Until now two stroke diesels are mainly used in the heavy shipping industry, Wärtsiliä-Sulzer is manufacturing one of the largest two stroke marine engines in the world: the RTA96-C. To understand their difference with four stroke diesels, one must first be clear on the basic design characteristics of each type.
Every engine has a combustion cycle of intake, compression, power and exhaust for each power impulse. Some engines, identified as four stroke or four cycle, use four piston strokes and two crankshaft revolutions to accomplish these functions.
A two stroke or two cycle engine performs intake, compression, power and exhaust in two piston strokes and one crankshaft revolution, twice as often as a four stroke engine. And this big difference gives the two stroke diesel engine its remarkable advantages.
The two stroke diesel engine has some advantages not found in the four stroke types which are of huge advantage to its application as an aero engine.
With twice as many power strokes per engine revolution, a diesel two stroke engine produces more power than a four stroke diesel of the same displacement. A two stroke engine of the same power as a four stroke, will not have the same weight and is usually heavier.
Since every cylinder of a two stroke engine produces a power stroke for every revolution, there is a quick response to load changes. This becomes an advantage if the engine is equipped with a constant speed propeller where its RPM control will be quicker to react.
The two stroke "spreads the load", each piston is producing two lighter power impulses per two revolutions instead of the single heavy impulse of a four stroke. At normal loads and speeds, there is no load reversal on pistons, rods, and bearings; this continuous downward loading reduces impact load effects. Lighter loading permits two stroke diesels to use more compact structural and load bearing parts without over stressing. The lighter power impulses are produced by smaller displacement cylinders, which means smaller pistons and shorter connecting rods for comparable performance. Shorter stroke lowers piston speed, a major factor in cylinder kit life.
All of these weight and size advantages are achieved without sacrificing engine life.
Two stroke engines run smoother than four stroke engines. This is because two stroke engines have twice as many power impulses at the same RPM. The lighter, more frequent power impulses mean less damping is required from the flywheel, hence smaller, lighter flywheels can be used. This permits more rapid acceleration and unsurpassed transient load response.
More air goes through a two stroke engine than a four stroke for the same amount of fuel consumed. This results in lower exhaust temperatures for two stroke diesels and in longer valve and turbo life.
The piston loads in a two stroke diesel are higher than in a four stroke, mainly because there is always a downward pressure on the piston either by the combustion or by pushing out the exhaust gasses. There is no load reversal to build up the oil film on the piston pin. An innovative manufacturer uses a sort of ball bearing in the piston with pressure lubrication, this design also solves the problem with piston cooling and lubrication. Time will tell if this will work and be durable over thousands of engine hours.
A DI (direct injection) engine has the diesel fuel injected straight into cylinder almost at the top of the compression stroke. In the old days this meant that it burned and expanded very quickly, making it a noisy, rattly engine. This is why most diesel cars were IDI (indirect injection); the rough behaviour was fixed by injecting the fuel into a small pre-combustion chamber which is connected to the cylinder by a narrow passage.
This slows down the combustion as the gasses have to escape through the narrow passage into the cylinder. This gives a softer bang and a smoother engine, but the gasses have to work harder, which spoils the efficiency a little. However, the newer breed of DI engines use other techniques to tame the behaviour: such as two stage injection, electronic control (FADEC), and acoustic shrouds and fancy engine mounts to mask the rattle.
For an aircraft to have a DI engine this means extra hardware (two-stage injectors, common rail etc), dual electronics for redundancy, weight and all of this complexity for about 10 % more efficiency.
The IDI does not have all of these but in return is simpler and more reliable, which definitely is what we are looking for in an aircraft engine.
A two stroke diesel is able to run without a turbocharger just by using a roots blower to scavenge the engine from exhaust gasses. This roots blower is not a supercharger but supplies the engine with enough air to clean the cylinders for the next fuel injection and power stroke. This is seen as a naturally aspirated engine.
Adding a turbocharger would give the advantage of more air thus more fuel which can be injected and the engine would able to sustain its rated power to a much higher altitude. More air means also better scavenging and cooler exhaust valves translating to a longer life and more reliable engine.