There are two types of heavy duty diesel engines, two stroke and four stroke. Both designs have been available for many years, and both are proven and well accepted in the diesel industry. To understand their difference, one must first be clear on the basic design characteristics of each.

Every diesel 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.

Advantages of the Two stroke Design

Zoche Radial Diesel Engine

Weight and Power
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.

Response and Acceleration
Since every cylinder of a two stroke engine produces a power stroke for every revolution, there is a quick response to load changes.

Durability
The two stroke "spreads the load", each cylinder is producing two lighter power impulses per two revolutions instead of the single heavy impulse of a four stroke engine. 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 diesel engines 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 engine 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.

Smoothness
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.

Lower Exhaust Temperatures
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.

Higher piston loads
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 with the piston pin. Some innovative design uses a sort of ball bearing in the piston with pressure lubrication, this design solves the problem with cooling and lubrication of the piston.

Indirect vs direct fuel injection
A DI (direct injection) engine has the diesel fuel injected straight into cylinder at the top of the compression stroke. In the old days this meant that it burned and expanded very quickly, making 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 of the engine, such as two stage injection, electronic control (FADEC), and acoustic shrouds and fancy engine mounts to mask the rattle.

Wilksch Airmotive Diesel Engine

For an aircraft to have a DI engine this means extra hardware (2 stage injectors, common rail etc), dual electronics for redundancy, weight and all of this complexity for about 10% more efficiency.

The IDI engine 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.

General Engine Parameters

As a diesel engine generally has more torque than a gasoline engine of the same horsepower rating. I have included an explanation of both and the results of torque on a propeller.

Horsepower versus Torque
Simply defined, power is the rate at which the torque is produced (pound-feet per second or Newton meters per second). If two objects produce the same torque, the faster rotating object has more power.

According to experts from SAE, one horsepower (1 HP) equals 550 ft-lb. per second or 33,000 ft-lb. per minute. Another familiar formula is the one which states RPM x Torque / 5252 = horsepower.
Torque is the measurement of the strength of the rotational movement and determines how fast a car, boat or airplane accelerates up to a required speed.

The diesel engine is better suited for producing torque than producing power. The force driving the engine's pistons downward is the expansion of the air/fuel mixture within the cylinders. A greater expansion means more force on the piston. Diesel fuel contains more BTU energy per volume than gasoline. The extra heat available allows a larger percentage of expansion than gasoline does. This greater expansion produces more force to push the piston downward (torque).

Diesel fuel does not burn as rapidly as the more volatile gasoline. The relatively slower expansion limits the maximum piston speed and this results in a lower maximum engine speed (rpm), increasing engine life.

Torque and Propellers
For aircraft useage, higher torque means that you can turn a larger propeller with wide turbo-prop like blades which produces higher thrust at a lower RPM with less noise than smaller propellers that turn at higher RPM. The larger propellers can give faster take-off and better climb performance as well as better cruise. Propellers are more efficient at lower rpm levels of 1600 - 2000.