The following generalizations may be made relating to the relative merits of two-stroke and four-stroke petrol engines in their basic form:
1] The two-stroke engine performs twice as several power strokes per cylinder per revolution. In theory at least, this might be expected to make twice the performance of a four-stroke engine of equivalent size. Unfortunately, this is not realized in practice because of the difficulties encountered in successfully purging the exhaust gases from the cylinder and then filling it completely with a clean combustible charge. The scavenging efficiency of the basic two stroke petrol engine is therefore reduced.
2] In performing twice as many power strokes per revolution, the two-stroke engine be able to deliver a smoother flow of power, but this may be less true at low engine speeds when irregular firing or ‘four-stroking’ can result from poor scavenging.
3] An obvious practical advantage of the basic two-stroke engine is the mechanical simplicity conferred by its valve less construction, which contributes to a more dense and lighter engine that should be less expensive to make.
4] Reduced maintenance requirements might reasonably be expected with the basic two-stroke engine by virtue of point 3. There is, however, the well-known tendency for carbon formation to have a blocking effect on the exhaust
ports, which impairs engine performance by reducing scavenging efficiency.
5] The fuel consumption of the basic two-stroke engine is adversely affected by the poor cylinder scavenging, which allow part of the fresh charge of air and fuel to escape through the exhaust port before final compression of the charge takes place.
6] There is a greater danger of overheating and piston seizure with a two-stroke engine, which be able to set a limit on the maximum usable performance. It is more difficult to cool satisfactorily, because it does not have the benefit of the second revolution in the four-stroke cycle when no heat is being generated.
7] Lubrication of the two-stroke petrol engine is complicated by the need to bring in oil into the fuel supply to constitute what is generally termed a petroil mixture. The working parts of the engine are thus lubricated in aerosol fashion by oil in the air and fuel charge, and this tends to boost harmful exhaust emissions. It is for this reason that the basic two-stroke petrol engine is now obsolescent for cars.
Scavenging: further details
Frequent reference has been made to the naturally poor Scavenging efficiency of the basic two-stroke petrol engine. The word ‘basic’ has been used deliberately and is intended to relate to the Day type of early two-stroke engine, which had a deflector-head piston to promote a cross-scavenging effect on the burnt charge leaving the cylinder (Figure 1a).
This not entirely successful scheme persisted until the mid 1920s, when Dr E. Schnürle of Germany developed an substitute loop-scavenging system. In this the deflector on the piston head is omitted and two transfer ports with angled
passages are disposed on either side of, instead of opposite, the exhaust port (Figure 1b). The loop-scavenge effect produced is such that before the two streams of fresh charge intermingle, they converge upon the cylinder wall at a point furthest away from the exhaust port, so there is less chance of escape.
fig.1 Early three-port scavenging systems: (a) cross scavenging (b) loop-scavenging
The Day type of early two-stroke engine also used what would now be classified as a three-port system of scavenging.
This system comprises inlet, transfer and exhaust ports, all in the cylinder wall, and necessarily imposes a restriction on the period during which a fresh charge of air and fuel may enter the crankcase.
Fig.2 Later two-port scavenging systems: (a) reed valve (b) rotary valve (Yamaha)
To achieve more complete filling of the crankcase, the later two-port system of scavenging is now normally employed. In this system only the transfer and exhaust ports are in the cylinder wall, the inlet port being situated in the crankcase itself and controlled by moreover an automatic flexible reed valve (Figure 2a) or an engine-driven rotary disc valve (Figure 2b)
which improve the torque and power characteristics respectively of an engine. The two-port system of scavenging thus allows the fresh charge to continue entering the crankcase during the whole, instead of part, of the induction-compression stroke, albeit with a little additional mechanical complication.
Further development of the two-stroke petrol engine
Fig.3 Toyota S2 supercharged two-stroke engine.
The development objective of this engine is to deliver better power more smoothly than a conventional one, and also to confer very high torque at low engine speeds. A Roots-type supercharger or blower is used to accomplish positive scavenging of the exhaust gases. Unlike the two-stroke petrol engines so far described, the Toyota engine borrows a lot of features from established four-stroke practice. It has a four valve cylinder head with two valves for intake and two for exhaust, which do of course open and close twice as often in this two-stroke application. The engine is provided with electronically controlled ignition and fuel injection systems, with fuel being injected directly into the cylinders.