Two Stroke

The biggest diesel engine in operation, work on two stroke principle. It is called two stroke because it takes two strokes of piston to complete the processes of conversion of energy in fuel to work.

Because the piston is reciprocating, it takes two strokes of the piston and the

crankshaft must revolve once.

The crankshaft is revolving in the clockwise direction and piston is moving up the cylinder and compressing the charge air.

Energy is transferred to the air and its pressure and

temperature increases. By the time the piston approaches

TDC its pressure is over 100 bar and temperature is over

500 degree C.

Just before TDC, fuel is injected into the cylinder by fuel injector. The fuel is atomized into small droplets. Because they are very small, these droplets are heated very quickly and their temperature is above its self ignition temperature. The fuel starts to burn as the piston is passing over TDC. This raises the pressure and temperature of the air above the piston. This forces the piston down the cylinder and forces the crankshaft to rotate. It is during this stroke that work energy is being given to the engine; during the upward stroke, the engine has to do the work.

As the piston moves downwards, the useful energy of the burning fuel is expended. At about 110 degrees after TDC, the exhaust valve opens or the piston uncovers the exhaust port and the hot gases (mainly consisting of nitrogen, carbon di-oxide, water vapour and unused oxygen) begin to leave the cylinder and go into the exhaust manifold.

At about 140 degrees after TDC, the piston uncovers a set of ports known as scavenge ports. Pressurized air enters the cylinder via these ports and pushes out the remaining exhaust gas. This process is known as “Scavenging”. The piston goes past the BDC and starts moving up the cylinder and after sometimes closes the scavenge port. This enables a fresh charge of air on top of the piston. After this the exhaust valve closes or the exhaust port is covered by the piston. Then the compression of the air commences as piston starts traveling upwards.

In the two stroke trunk piston engine, the side thrust caused by the angularity of the connecting rod, is transmitted to the liner by the piston trunk or skirt. The skirt of the piston also acts to seal the scavenge air ports when the piston is at TDC. This prevents the scavenge air from pressuring the crankcase. Herein lies the disadvantage of this type of engine. Although it has low overall height, the splash of luboil from the bottom end bearing in the crankcase to lubricate the liner can find its way into the scavenge space, causing fouling and risk of scavenge fire. The crankcase oil must have additives to cope up with products of combustion and the acid formed due combustion of sulphur in the fuel. This design of trunk type two stroke diesel engine is used in only small powered diesel engines.

Two stroke timing diagram

In the two stroke trunk piston engine, the side thrust caused by the angularity of the connecting rod, is transmitted to the liner by the piston trunk or skirt. The skirt of the piston also acts to seal the scavenge air ports when the piston is at TDC. This prevents the scavenge air from pressuring the crankcase. Herein lies the disadvantage of this type of engine. Although it has low overall height, the splash of luboil from the bottom end bearing in the crankcase to lubricate the liner can find its way into the scavenge space, causing fouling and risk of scavenge fire. The crankcase oil must have additives to cope up with products of combustion and the acid formed due combustion of sulphur in the fuel. This design of trunk type two stroke diesel engine is used in only small powered diesel engines.