Two Stroke Engine

You would be familiar with the two common internal combustion engines, S.I and C.I. There is one more type of engine known as the two stroke engine that is commonly used for low power applications such as dirt bikes, mopeds, lawn movers, jet skis, etc.

Let's look at some of the advantages of two stroke over four stroke engines. The name suggests that we get power for every two strokes of an engine i.e. for every one revolution of crankshaft, two strokes fire once. Whereas in four stroke engines, the engine fires only once for two revolutions of the crankshaft. Two stroke engines do not have valves, so it doesn't require camshaft thereby reducing the overall weight of the engine. The construction is simpler in two strokes. Two strokes can produce twice the amount of power for a single revolution when compared to four strokes, therefore it has a greater power to weight ratio.



WORKING:

There is an inlet port and an exhaust port. When the piston moves up from b.d.c to t.d.c, the inlet port is uncovered and it allows the air-fuel mixture to enter the crankcase. Simultaneously, the air-fuel charge present in the combustion chamber is compressed and ignited by means of a spark plug.

In the next stroke where piston moves from t.d.c to b.d.c, there is an expansion due to combustion and a lot of heat energy is released. The inlet port is covered and the air-fuel mixture trapped at the bottom in the crankcase is transferred to the upper part of the cylinder through a transfer port. While the piston moves to the bottom, it uncovers the exhaust port through which the exhaust gases escape to the atmosphere. The compressed air-fuel charge pushes the exhaust gases out. A cross-flow design will ensure that a minimal amount of fresh charge could only flow out with the exhaust gases.

The crankshaft starts gaining momentum and the power cycle repeats itself. You can see that there are various simultaneous processes occurring in every stroke.

• When the piston moves up, it compresses the air-fuel charge in the combustion chamber and also captures the heat energy released when the charge is ignited. On the other hand, it also creates a vacuum at the bottom so that a fresh air-fuel charge is sucked inside the crankcase via the inlet port.


• When the piston moves down, it first covers the inlet port and pushes the fresh air-fuel charge from crankcase to the combustion chamber. Then it uncovers the exhaust port and the exhaust gases escape out to the atmosphere.

WHY ENGINE OIL IS MIXED WITH THE FUEL:

You would have noticed in four stroke engines that the engine oil is poured in the crankcase of the engine. But it is not so in the case of two stroke engines. The engine oil is rather mixed with the fuel and stored in the fuel tank. Since the crankcase is not separate in two strokes, it is used to pressurize and send the charge to the combustion chamber. So the crankcase cannot hold the lubrication oils. The engine oil is mixed with the fuel to lubricate the engine components as and when the charge is sucked in.

DISADVANTAGES COMPARED TO FOUR STROKE ENGINES:

• Two strokes are not fuel efficient. During expansion stroke, few amount of fresh charge escapes with the exhaust without getting burnt.


• The rate of pollution is higher. Unburnt hydrocarbons pose a serious threat to the environment.


• The engine parts wear out faster due to lack of an individual lubrication system.