In Two-Stroke Diesel Engine that air only is introduced into the engine cylinder prior to the injection of fuel oil, an additional departure from two-stroke petrol engine practice is that, instead of using crankcase compression, a rotary blower is utilized to charge the cylinder with low-pressure air.



This type of blower is sometimes also used for supercharging four-stroke cycle engines. The distinction that should be made here, however, is that whereas a supercharger is used just to raise power output of a four-stroke engine, a similar blower is important for a two-stroke diesel in order that it shall work at all.


Fig.1 Cross-section of a blower-charged two-stroke diesel engine (General Motors)

Furthermore, a couple of exhaust valves is located in the cylinder head to provide a uniflow system of scavenging. This means that there is no modify in direction for the cylinder air stream, which is in contrast with the loop system of scavenging. The two-stroke diesel engine is therefore mechanically further complicated.

In the basic two-stroke diesel cycle, the following cycle of events is continuously repeated all the time the engine is running and while the rotary lower is providing air to the inlet ports of the cylinder (Figure 2):

1] The induction-exhaust event:

Air only is allowed to the cylinder during the period the inlet ports are uncovered by the piston, which occur towards the last quarter of the power-exhaust stroke and about the first quarter of the induction-compression stroke. During this part of the cycle, the exhaust valves are opened immediately before the cylinder inlet ports are uncovered and then closed just before the ports are covered again. This series of exhaust valve events not only ensures that the exhaust gas pressure falls lower than that of the scavenging air supply, and thus prevents any return flow of exhaust gases, but also leaves the charge in the cylinder somewhat pressurized prior to final compression. Hence, the combination of uncovered inlet ports and open exhaust valves allows air to be blown throughout the cylinder, which removes the remaining exhaust gases and, similarly, fills it with a new charge of air. Since neither the air nor the exhaust gases change direction in passing through the cylinder, the term uniflow scavenging can justifiably be applied.


Fig.2 The two-stroke diesel engine cycle: (a)/(b) induction-compression (c)/(d) power-exhaust (General Motors)

2] The compression-power event.

The remaining three-quarter portions of the induction-compression and power-exhaust strokes occur in a very related manner to that of the four stroke diesel engine; that is, the advancing piston compress the air into the smaller volume of the combustion chamber and raises its temperature high enough to make sure self-ignition of the fuel charge. This is injected into the combustion chamber immediately before the piston begins to retreat on its power-exhaust stroke. It should be noted that the operating cycle of the two-stroke diesel engine has been described in terms of events rather than strokes in order to aid understanding.

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