Module 6. I. C. engine

Lesson 16

TWO STROKE CYCLE ENGINE

16.1 Introduction

A 2-S cycle engine is one in which the complete cycle of engine comprising of all the six processes i.e. suction, compression, heat addition, expansion, heat rejection and exhaust take place within only 2 strokes of piston or one revolution of crankshaft. It was devised by Duglad Clark in 1880. Actually the construction of engine is such that the compression and suction of charge take place separately but simultaneously on the opposite side of piston in a single stroke and expansion and exhaust also take place in a single stroke each consuming a part of piston stroke. In this way power is produced in each revolution of crankshaft of engine unlike a 4-S engine in which power is produced in alternate revolution of engine. Due to this reason a 2-S engine produces more power as compared to a same size 4-S engine, which is beneficial in such application where lighter engine producing more power is required.

16.2 Two Stroke Cycle Engine

16.2.1 Construction

A 2-S cycle engine is simpler in construction than a 4-S engine due to absence of valves and valve operating mechanism. The construction of a 2-S engine as shown in Fig. 16.1 is basically same as that of a 4-S engine.

The major components are same i.e. cylinder or cylinder block, crank case, cylinder head, piston, connecting rod, crankshaft etc. The difference is that, in place of valves, only two ports are there in the wall of cylinder. One port is named as transfer port which is connected to the crankcase itself through a gallery in the cylinder block. Just opposite to transfer port but slightly above is another port named as exhaust port which is connected to exhaust pipe for exhaust of burnt gases. These ports are covered and uncovered by the movement of piston itself, thus there is no separate valve operating mechanism. Due to these ports and absence of valves, the construction of a 2-S engine is simple and so its initial cost is also less. Another change from a 4-S engine is that here in 2-S engine, crankcase is also used for suction of fresh charge. For that a self operating valve is there in the crankcase, which is operated by change in pressure within crankcase due to upward and downward movement of piston. One more slight difference is in the shape of piston that a hump is provided on its top face to facilitate or guide the movement of fresh charge coming from transfer port upto the top portion of cylinder and escaping of exhaust gases through exhaust port The importance of this hump in piston and upward direction of transfer port line or gallery as shown in the Fig. 16.1 is to facilitate minimum mixing of fresh charge with exhaust gases and maximum replacement of exhaust gases with fresh charge. It will be clear after understanding the working of a 2-S Engine.

16.2.2 Working of a 2-S engine

First of all, we must know that a 2-S Engine also may be classified as S.I. Engine and C.I. Engine and what are the differences between a 4-S Engine S.I. and 4-S C.I. Engine, same are here in 2-S S.I. Engine and 2-Stroke C.I. Engine. Let us discuss the working of a 2-S petrol engine. Let us start from the position of piston at TDC. Above the piston, fresh charge in compressed condition and occupying the clearance volume is present. As soon as the spark occurs at the points of spark plug, combustion of fresh charge starts and heat energy is released. These hot burning gases expand and push the piston downward and produces the work. This expansion/working stroke continues upto four fifth of the piston stroke until the piston uncovers the exhaust port. As soon as exhaust port is uncovered, burnt gases at some higher pressure than atmosphere find their way out through exhaust port. On the down side of piston, fresh charge is filled all over the cylinder and crankcase which was slightly compressed simultaneously by downward movement of piston during expansion stroke.

With further downward movement of piston, transfer port is also uncovered just after partial uncovering of exhaust port. As soon as the transfer port is uncovered, ‘slightly compressed fresh charge’ in the crank case rushes to the space in cylinder above the piston. As the gallery of transfer port is inclined upwards and also because of hump in piston, fresh charge goes towards the top space of cylinder and pushing directly the burnt gases out of exhaust port. This process of sweeping away of burnt gases directly by incoming fresh charge is called scavenging. This scavenging is actually the most inefficient part of a 2-S engine, because here fresh charge & burnt gases directly meet and some fresh charge may also escape through exhaust port while some burnt gases may remain inside the cylinder. The burnt gases remained with fresh charge in cylinder will dilute the charge and reduce the power output. Also any amount of fresh charge, if escapes with burnt gases through exhaust part without taking part in combustion & expansion in next cycle is a direct loss of fuel. Thus due to poor scavenging process in a 2-S engine, it is less fuel efficient than that a 4-S engine. Rather the efficiency of a 2-Stroke engine depends on the efficiency of scavenging process.

The scavenging takes place during the last fifth part of down stroke and first fifth part of upstroke of piston during which transfer port remains uncovered by piston. It means after the expansion stroke and uncovering the transfer port, piston reaches at BDC and again moves towards TDC. As the ports are at fixed position, so again after covering approximately one fifth of stroke from BDC to TDC, Piston covers or closes the transfer port and then the exhaust port. Now the space above the piston becomes a closed system and the fresh charge trapped inside is compressed by the piston during remaining four fifth of stroke. As the piston reach TDC, charge is fully compressed and is again ignited by the spark plug to start next working cycle.

Simultaneously, during the compression process above the piston, reverse is happening below the piston. By the upward movement of piston from BDC the space below the piston in the cylinder and crankcase increases due to which pressure decreases and the self operating valve in the crankcase opens. The fresh charge from the carburetor enters in the crankcase through this valve due to low pressure created by piston. Thus the charge above the piston is compressed and below it fresh charge is sucked in the crankcase from carburetor.

Ill effect of poor scavenging is however reduced in case of a 2-S Diesel engine because here exhaust gases are to be replaced by pure air only and there is no loss of fuel which is injected only afterwards scavenging.

In this way we have seen that there are no separate strokes for suction and exhaust processes, but these happen simultaneously with the help of ports by taking last and first one fifth part expansion and compression stroke. It became possible by using crankcase as an intermediate chamber and bottom side of piston as a reciprocating pump to shift the fresh charge from carburetor to cylinder of engine.

In a 2-stroke engine, as each alternate stroke is power/working stroke, the power output of the engine increases very much. Theoretically, power produced by a 2-stroke engine is double than that of a same size 4-stroke engine, but due to losses and poor scavenging efficiency, it is only 30% more in actual. Due to poor scavenging efficiency, fuel efficiency also decreases, but that is bearable in small applications where small size or weight of engine is more important.

Unlike 4-S Engine, in a 2-stroke engine, there is no change in valve operating timing as the ports are fixed. Thus the theoretical or actual valve timing diagrams are same. Port timing diagram of a typical 2-S engine is shown in Fig. 16.2.

The actual working cycle of 2-stroke engine on P-V diagram is also shown in fig 16.3.

Fig. 16.3 Typical P-V cycle diagram of a 2-stroke engine

Table 16.1 Comparison of two stroke cycle and four stroke cycle engines

Four Stroke Cycle Engine	Two-Stroke Cycle Engine
1) Working cycle of engine completes in four strokes of piston or two revolutions of engine.	1) Working cycle completes in only two strokes of piston or one revolution of engine.
2) One Stroke is working and next three strokes are idle. So torque output is not so uniform hence heavy flywheel will be needed to smooth the pulsating torque.	2) Each alternate stroke is power/working stroke. So more uniform torque is obtained hence lighter flywheel is sufficient.
3) Due to only one working stroke in two revolutions of engine, power output will be less.	3) Due to one working stroke for each revolution of engine, power output will be more. Theoretically, it should be double than that of a same size of 4-S Engine but in actual it is about 1.3 times of a 4-S engine
4) For a given power, the weight of engine is more because of valve operating mechanism and also because of less power output as discussed in previous point of comparison.	4) For a given power, the engine is light and compact due to absence of valve operating mechanism and also due to more power output of engine as discussed in previous point of comparison.
5) Because of the above mentioned facts, the cooling and lubrication requirements are less. Life is more because of less wear and tear.	5) Because of above mentioned facts, the cooling and lubricating requirements are more. Life is less because of more wear and tear.
6) High Initial Cost.	6) Low Initial Cost.
7) High Thermal Efficiency.	7) Low Thermal Efficiency due to poor scavenging.
8) Part load efficiency better than 2-S Engine.	8) Part-load efficiency is poor than a 4-S Engine.
9) It is used where thermal efficiency and long life is more important.	9) It is used where compactness and light weight of engine is more important than to be fuel efficient.