Cycles of Operation
Before explaining about engine operating cycles, it is necessary to first define a few terms we will be using.
‘CYCLE’ refers to one complete sequence of operations required to produce power in an engine. The cycle continuously repeats whilst the engine is running. Diesel engines can be designed to complete a cycle once during each revolution of the crankshaft or once during every two revs of the crankshaft.
A ‘STROKE’ is the movement of a piston either up OR down within the cylinder. The extremities of a stroke are known as Top and Bottom Dead Centres when the piston changes direction in its movement. Each piston completes two strokes for each revolution of the crankshaft.
‘TOP DEAD CENTRE’, or TDC, is the point at which the piston stops travelling up in the cylinder and starts moving back down again. Conversely, ‘BOTTOM DEAD CENTRE’, or BDC, is the point at which the piston stops moving downwards and starts travelling back up again.
The Four Stroke Cycle
As the name implies, this cycle of operation is completed once every four strokes of a piston, or once every two revolutions of the crankshaft.
Both spark ignition (petrol) engines and compression ignition (diesel) engines can be designed to operate on this cycle. The difference between these engines is the manner in which the fuel is ignited. In a petrol engine (spark ignition) the petrol/air mixture is ignited by a high voltage arc of electricity across the terminals of a spark plug. In a diesel engine (compression ignition) an atomised form of diesel is injected into the hot, compressed air of the cylinder, and auto ignition takes place.
Each of the four strokes of the piston plays a part in the engine operation, and each part of the operation must occur in turn and at the correct moment for the engine to function.
The four piston strokes are called ‘Induction’, ‘Compression’, ‘Power’ and ‘Exhaust’. These are rather informally sometimes referred to as ‘suck’, ‘squeeze’, ‘bang’ and ‘blow’.
Induction:
The cycle begins with the piston at Top Dead Centre in the cylinder. The inlet valve is open as the piston moves down within the cylinder. This downwards movement causes air to be drawn in through the open inlet valve.
Compression:
As the piston reaches the bottom of its stroke (bottom dead centre) the inlet valve is closed. As the exhaust valve is also closed, the volume of air above the piston is now trapped between the piston crown, the cylinder wall and the cylinder head. As the crankshaft continues to rotate, the piston is forced back up the cylinder, compressing the trapped air. The work done on the air in compressing it causes it to be heated up.
Power:
Just before top dead centre, an atomised mist of diesel is injected into the cylinder through the fuel injector in the cylinder head. After a short delay (known as ignition lag) this fuel will auto ignite in the hot compressed air. The burning of this fuel causes a rapid and large rise in pressure within the cylinder, and this pressure acts downwards on the piston crown. In this way, the piston is driven towards bottom dead centre, rotating the crankshaft.
Exhaust:
Around bottom dead centre of the power stroke, the exhaust valve will be opened, and as the piston is moved up within the cylinder, all the products of combustion and exhaust gases are driven out of the unit. Once the piston reaches top dead centre, the exhaust valve closes and the inlet valve opens, ready for the cycle to repeat.
As can be seen from the preceding diagrams, in the four stroke cycle, each unit only develops power once every four strokes. Three quarters of the time, the movement of the piston is having to be driven by the rotating crankshaft. Engines with fewer than four cylinders operating on the four stroke cycle have large flywheels on the crankshaft – the energy ‘stored’ in the inertia of a rotating flywheel is enough to keep the engine turning even when there is no unit on a Power stroke. Engines with four or more units are arranged so there is always a unit on a Power stroke.
Two Stroke Cycle
As the name implies, the two stroke cycle is completed in two strokes of the piston, ie one revolution of the crankshaft. The two strokes are known as the ‘Compression’ stroke, and the ‘Expansion’ stroke.
Slow speed diesels all operate on the two stroke cycle, a few medium and high speed diesels operate on the two stroke cycle.
Slow speed diesel engines’ liners generally have ‘scavenge ports’ cut into the bottom of the liner to allow the air for combustion to enter the cylinder. Some designs (mainly older engines) also have ‘exhaust ports’ above the scavenge ports. More modern two strokes have an exhaust valve on the cylinder head. Medium and high speed two strokes generally have conventional inlet and exhaust valves on the cylinder head.
Due to the small relative movement of the piston during scavenging in a two stroke engine, scavenge air must be at a higher pressure than atmospheric, so that when the inlet valve is opened or the scavenge ports unblocked, air will flow into the cylinder – the piston has little effect in drawing in scavenge air.
The sketches that follow are for a slow speed two stroke engine with both scavenge and exhaust ports, however, the process is the same for engines with scavenge ports and a single exhaust valve, and also for engines with both inlet and exhaust valves. The diagrams illustrate the relative timings of the scavenge air and exhaust blow down of the unit. Older slow speed engines have both scavenge and exhaust ports in the liners. The engine ‘timing’ in these engines is governed by the piston covering and uncovering these ports during the engine cycle. Newer, more powerful slow speeds generally have a single exhaust valve within the cylinder head, operated by hydraulics driven from the camshaft. Scavenging of these engines is still by ports at the bottom of the liner being covered and uncovered by the piston.
Compression Stroke:
During the compression stroke of a two stroke engine, the piston starts at the bottom of the cylinder liner, and begins moving upwards. The piston first covers the scavenge ports (or the air inlet valve, if fitted, is closed) preventing any more scavenge air from entering the unit. The exhaust port is at this stage still uncovered (or the exhaust valve, if fitted, is still open). As the piston continues to move upwards, the exhaust ports are blocked (or the exhaust valve is closed) and compression of the trapped air begins.
Once the exhaust port is blocked, or the exhaust valve shut, the piston continues to move up within the cylinder liner, and the trapped air is compressed. The work done in compressing the air causes it to be heated up to such a temperature that when the atomised fuel is injected into it (typically 5 degrees before top dead centre) the fuel will, after a short delay known as ignition lag, auto ignite in the hot, compressed air. The rapid expansion caused by the burning of the fuel acts down upon the piston crown, and the piston is driven down within the liner.
Expansion Stroke:
As the fuel ignites in the hot, compressed air, the pressure above the piston rapidly increases. There is nowhere for this pressure to be relieved, and so the piston is forced back downwards in the cylinder liner. This downward movement of the piston rotates the crankshaft, producing useful work at the engine output shaft. The piston moves downwards within the liner during the ‘expansion stroke’ until the exhaust port in the liner is uncovered (or the exhaust valve, if fitted, opens). At this point, any pressure still within the cylinder is exhausted, and once the scavenge air inlet port is uncovered, fresh air for the next cycle is admitted to the cylinder. As the piston reaches bottom dead centre, the cycle of operation begins again.
Cam/Crankshaft Rotational Relationship
In a four stroke engine, the crankshaft rotates twice every time the cycle of operations is completed. During the cycle, each operation must be completed once, ie the inlet valve opens and closes once, the fuel is injected once and the exhaust valve opens and closes once. As each of these operations is controlled by the camshaft, it can be seen that the camshaft has to rotate once for each cycle.
In a two stroke engine, the crankshaft rotates once every time the cycle of operations is completed. During the cycle, as in the four stroke engine, each operation is completed once, and it can be seen that the camshaft has to rotate once to achieve this.
In summary, in a four stroke engine, the crankshaft rotates TWICE in a cycle whilst the camshaft rotates ONCE. The camshaft rotates at half of the crankshaft speed.
In a two stroke engine, the crankshaft and the camshaft rotate at the same speed, ie both complete one revolution every time the cycle is completed.
