A diesel engine may be designed to work on the two-stroke or on the four-stroke cycle. Both of them are explained below.

The Four-Stroke Cycle

Proceeding clockwise round the diagram, both inlet (or suction) and exhaust valves are initially open. (ALL modern four-stroke engines have poppet valves.). If the engine is naturally aspirated, or is a small high-speed type with a centrifugal turbocharger, the period of valve overlap, i.e. when both valves are open, will be short, and the exhaust valve will close some 10deg after top dead center (ATDC).
Propulsion engine and the vast majority of auxiliary generator engines running at speeds below 1,000 r/min will almost certainly be turbocharged and will be designed to allow a generous through flow of scavenge air at this point in order to control the turbine blade temperature. In this case the exhaust valve will remain open until exhaust valve closure (EVC) at 50-60 deg ATDC. As the piston descends to outer or bottom dead center (BDC) on the suction stroke, it will inhale a fresh charge of air. To maximize this, balancing the reduced opening as the valve seats against the slight ram or inertia effect as the incoming charge, the inlet (suction) valve will normally be held open until about 25-35 deg ACBT (145-155 deg BTDC). This event is called inlet valve closure (IVC). The charge is then compressed by the rising piston until it has attained a temperature of some
550℃. At about 10-20 deg BTDC (firing), depending on the type and speed of the engine, the injector admits finely atomized fuel which ignites within 2-7 deg (depending on the type again) and the fuel burns over a period of 30-50 deg while the piston begins to descend on the expansion stroke, the piston movement usually helping to induce air movement to assist combustion.
At about 120-150 deg ATDC the exhaust valve opens (EVO), the timing being chosen to promote a very rapid blow-down of the cylinder gases to exhaust. This is done: (a) to preserve as much energy as is practicable to drive the turbocharger, and (b) to reduce the cylinder pressure to a minimum by BDC to reduce pumping work on the ‘exhaust’ stroke. The rising piston expels the remaining exhaust gas and at about 70-80 deg BTDC the inlet valve opens (IVO) so that the inertia of the out-flowing gas, plus the positive pressure difference, which usually exists across the cylinder by now, produces a through flow of air to the exhaust to ‘scavenge’ the cylinder.
If the engine is naturally aspirated the IVO is about 10 deg BTDC. The cycle now repeats.

The Two-Stroke Cycle

The two-stroke cycle, which, as the name implies, is accomplished in one complete revolution of the crank. Two-stroke engines invariably have ports to admit air when uncovered by the descending piston. The exhaust may be via ports adjacent to the air ports and controlled by the same piston (loop scavenge) or via poppet exhaust valves at the other end of the cylinder (uniflow scavenge). Starting at TDC combustion is already under the way and the exhaust opens (EO) at 110-120 deg ATDC to promote a rapid blow-down before the inlet opens (IO) about 20-30 deg later (130-150 deg ATDC). In this way the inertia of the exhaust gases - moving at about the speed of sound – is contrived to encourage the incoming air to flow quickly through the cylinder with a minimum of mixing, because any un-expelled exhaust gas detracts from the weight of air entrained for the next stroke.
The exhaust should close before the inlet on the compression stroke to maximize the charge, but the geometry of the engine may prevent this if the two events are piston controlled. It can be done in an engine with exhaust valves.
At all events the inlet ports will be closed as many degrees ABDC as opened before it (i.e. again 130-150 deg BTDC) and the exhaust in the same region.
Injection commences at about 10-20 deg BTDC depending on speed and combustion lasts 30-50 deg, as with the four-stroke.

Related Terms


A system of gears which alter the ratio between the revolution of the engine and the propeller shaft so the propeller operates in a relatively efficient speed range. By using a gearbox the engine and the propeller shaft will revolve at different speeds.


A cylindrical type of steam engine slide valve for admission and exhaust of steam.


A type of engine characterized by reciprocating motion of pistons in a cylinder. Also known as displacement engine; reciprocating engine.


One of a series of numbers (constituting the PN, or performancenumber, scale) used to convert fuel antiknock values in terms of a reference fuel into an index which is an indication of relative engine performance; used mostly to rate aviation gasolines with octane values greater than 100.


A clutch that allows the driven shaft to turn freely only under certain conditions; for example, a clutch in an engine starter that allows the crank to turn freely when the engine attempts to run.


An engine with bore diameter greater than the stroke length.


An automobile engine device that lowers the gear ratio, thereby reducing fuel consumption.


A fourstroke-cycle internal combustion engine having its valves located in the cylinder head, operated by pushrods that actuate rocker arms. Abbreviated OHV engine. Also known as valve-in-head engine.


A gear train in which the angular velocity ratio of the driven shaft to driving shaft is greater than unity, as when the propelling shaft of an automobile revolves faster than the engine shaft.


An instrument which makes a plot of pressure in the cylinder of an engine as a function of piston (or volume) displacement, making use of magnification by optical systems and photographic recording; for example, the small motion of a pressure diaphragm may be transmitted to a mirror to deflect a beam of light.

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