Two Stroke Petrol Engine

Two Stroke Petrol Engine

Joseph Day & Son of Bath patented the first successful design of a three-port two-stroke engine in 1889. The underside of the piston was used in conjunction with a sealed crankcase to create a scavenge pump ('scavenging' refers to the pushing out of exhaust gas by the induction of fresh charge).

Fig. 1 Two-Stroke-Cycle Petrol Engine

In one crankshaft revolution or two complete piston strokes, this engine completes the cycle of events - induction, compression, power, and exhaust.

Crankcase-To-Cylinder Mixture Transfer

As the piston descends down the cylinder, the exhaust port (E) is first exposed, allowing the burnt exhaust gases to escape into the atmosphere. Simultaneously, the underside of the piston moves downward, compressing the previously filled mixture of air and atomised petrol in the crankcase.

The transfer port (T) will be revealed when the piston moves further outward, and the compressed mixture in the crankcase will be transferred to the combustion-chamber side of the cylinder. The situation in the cylinder will then be such that any leftover burnt products of combustion will be pushed out by the fresh charge entering the cylinder — this is known as cross-flow scavenging.

Cylinder Compression and Crankcase Induction

The piston is moved in the direction of the cylinder head while the crankshaft revolves. The piston seals off the transfer port first, then closes the exhaust port fully after a short time. The mixture of air and atomised petrol will be compressed to around one-seventh to one-eighth of its initial volume as the piston continues to move inward.

The inward movement of the piston increases the overall volume in the crank-case, creating a depression in this region, while the new charge is compressed between the combustion chamber and the piston head. The bottom part of the piston skirt will uncover the inlet port (I) about halfway through the cylinder stroke, allowing a fresh mixture of air and petrol produced by the carburettor to be induced into the crankcase chamber.

Cylinder Combustion and Crankcase Compression

A spark plug in the cylinder head's center will be timed to spark and ignite the dense mixture just before the piston reaches the peak of its stroke. Under full load, the charge's burning rate will rapidly raise the gas pressure to a maximum of about 50 bar. The burning mixture expands, driving the piston back along its stroke and lowering cylinder pressure in the process.

With the piston at the peak of its stroke, fresh mixture will have entered the crankcase through the inlet port, given the state beneath the piston in the crankcase. The piston skirt will cover the inlet port as the piston moves down its stroke, and any further downward movement will compress the mixture in the crankcase, preparing for the next charge transfer into the cylinder and combustion chamber space.

The combined cycle of events adapted to a threecylinder engine is shown in Figure below show the complete cycle in terms of opening and closing events and cylinder volume and pressure changes respectively.

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