Function

The two-stroke engine is to a lot of people directly associated with the legendary Trabant, but in this case it is an engine for flying. The engine represented in the picture is - quite unusual - a flat-four cylinder Boxer engine with injection and - probably due to security - with double ignition. It is an engine for miniature airplane flying, because the transmission is flanged on in front. Just the propeller in front is missing. The engine should not be referred to as aircraft engine, because for this there are strict rules and obviously priceless insurance implications.
The engine works according to the principle of the loop scavenging and does not have an oil sump, but for each cylinder separated crank cases. Its crankshaft has four cranks with each an angle of 90°.

How it works

Intake, compression, combustion and exhaust stroke are combined such that only two piston strokes (one turn of the crankshaft) are necessary. This in contrast to the four-stroke engine . With the two-stroke engine, the intake stroke and exhaust stroke run parallel. The compression takes place either by charging or in the crank case during the combustion stroke.

Advantages

The two-stroke engine as compared to the four-stroke engine is lighter because the capacity is almost halved, and it has fewer mobile parts, making it more reliable in principle. Less (movable) parts make the engine more compact, lighter, more stable in spite of high numbers of revolutions, increasing in essence its efficiency. Its principle is therefore especially suitable for small engines. In the case of a motorcycle engine, in spite that the respective pistons must steer the complete exchange of gases, it is amazingly short-stroke laid out. The production of highly stressed parts, like the cylinders, which have to bear temperature differences, and the roll-stored crankshaft, is difficult. Nevertheless, the production costs are lower compared to four-stroke engines.
The two-stroke engine becomes especially efficient if it is combined with a charger or a direct injection system. In this case scavenging losses and mixture-lubrication are omitted. As a result its weight per horsepower is excellent, and its emissions do not resemble those of the Trabant any more. For the future there will definitely be new developments around this engine.

Disadvantage

The scavenging is the weak point of the two-stroke engine. It can still not be calculated, and is therefore strongly dependent on the correct estimate of the engineer. In fact, what makes it especially tricky is the wide speed range of the two-stroke engine, as it has an ability for high numbers of revolution. It is (so far) just possible to optimize the scavenging for a much smaller speed range. The shape, arrangement and cross-sectional changes of all channels are relevant. In addition, the discharge opening and the tuning of the exhaust pipe should be taken into account.
Unfortunately, the advantage of fewer movable masses is more than compensated by the scavenging problems. Thus, harmful emissions (CH, CO) and a high fuel consumption remain. Two-stroke engines are somewhat inclined to operating uncertainties due to 'spark bridges'.

Application

The two-stroke engine is mainly used for two-wheelers and sometimes combined with a catalyst. It is frequent with large ship Diesels with charging and exhaust valves.