Two-Stroke Diesel Engine Cycle

Two-Stroke Diesel Engine Cycle

Large diesel engines are very often two-stroke types. Note that all four operating processes are executed in one, single engine revolution (induction, compression, expansion

Inlet ports open and…

and exhaust). The diesel engine requires a charge of air that is compressed to raise its temperature above the self-ignition point of the fuel. This air charge is supplied by an air pump or pressure charging device (turbo or supercharger). The pressurized air from this device passes into the combustion chamber via ports in the cylinder wall. The exhaust gases leave the combustion chamber via cam operated poppet valves. The incoming charge forces the exhaust gases out via these valves and this provides the cylinder scavenging process.

Down stroke The two stroke diesel engine cycle is illustrated in the diagrams. During the downwards movement of the piston, the hot expanding gases are forcing the piston down the bore, producing

Closed

torque at the crankshaft, this is the expansion process. As the piston approaches BDC, the exhaust valve opens and the remaining pressure in the exhaust gas starts the evacuation of the gases in the cylinder via the open valves. As the piston moves further down to BDC, inlet ports are exposed around the bottom part of the cylinder bore, these allow the pressurised, fresh air charge from the air pump (or turbocharger) to fill the cylinder, evacuating the remaining exhaust gas via the valves and completing the exhaust and induction cycles

Upstroke At BDC, the cylinder contains a fresh air charge and the piston then begins to move up the cylinder bore. The inlet ports are closed off by the piston movement and the air charge is trapped and

Diesel two-stroke compressed due to the deceasing

cycle volume in the cylinder. At a few

degrees before TDC, the air temperature has risen due to the compression process and fuel is injected directly into the combustion chamber, into the hot air charge, where it vaporises, burns and generates thermal and pressure energy. This energy is converted to torque at the crankshaft via the piston, connecting rod and crankshaft during the down stroke.

Rotary or Wankel Engine The rotary engine has been used in a limited number of passenger car applications. The engine uses a complex

Rotary engine geometric rotor that moves within a specially

shaped housing. The rotor is connected to the engine crank shaft and turns within the housing to create working chambers. These are exposed to inlet and exhaust ports to allow a fuel/ air charge in, compress it and expand it (thus extracting work), then evacuate the waste gases and restart the cycle. The rotor has special tips to provide a gas tight seal between the working chambers (similar function to piston rings)

Summary ฀฀This animation shows a view from above of the four-stroke cycle operating in a four cylinder engine.

Note the firing order of 1-3- 4-2 and how each cylinder runs through the four- strokes: induction, compression, power and

exhaust – or, suck, squeeze, bang, blow!

 Explain what is meant by ‘internal combustion’.

 Explain the four-stroke cycle.

 Look back over the previous section and write out a list of the key bullet points here:

ENGINE COMPONENTS

Crankshaft and Camshaft ฀฀ The camshaft rotates once for the two revolutions of the crankshaft during the four stroke cycle. The drive from the crankshaft to the camshaft has a 2:1 ratio produced by the numbers of teeth on the driven and driver gears. Rotational data for the camshaft is usually given as degrees of crankshaft rotation and this needs to be considered in relation to the four-stroke cycle. The four- stroke cycle occurring over two full revolutions of the crankshaft has a 720° rotational movement.

Valve Timing Diagram ฀฀ Looking at the four-stroke cycle and the relationships of the crankshaft rotation, the piston position in the cylinder and the opening and closing of the valves is best observed by looking at a valve timing diagram. This diagram is one method of providing data for valve opening and closing positions.

Valve Timing Data Valve timing data is given in engine workshop manuals as degrees of crankshaft revolution. This can be as written data or by means of valve timing diagrams. In the most

popular valve timing diagram two Circular valve timing diagram circles, one inside the other, are used

to represent the 720 degrees of crankshaft rotation through which the crankshaft moves for a complete cycle. Each stroke is represented by an arc of 180 degrees with induction and compression on the outer circle and combustion and exhaust on the inner circle. The valve opening and closing positions are marked and the duration of crankshaft rotation displayed by a thicker line.