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An engine that requires two revolutions of the crankshaft to complete one working cycle.

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.

Related Terms


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Oil-insoluble materials that result from oxidation and decomposition of lube oil and contamination from external sources and engine blow-by. These can settle out on machine or engine parts. Examples are sludge, varnish, lacquer and carbon.


The ratio of actual work output of a heat engine to the ideal output.


Gasoline and diesel engine oil quality levels established jointly by API, SAE, and ASTM, and sometimes called SAE or API/SAE categories; formerly called API Engine Service Classifications.


Excessive smoothing of the surface finish of the cylinder bore or cylinder liner in an engine to a mirror-like appearance, resulting in depreciation of ring sealing and oil consumption performance.


A chemical compound whose molecules exhibit electrically positive characteristics at one extremity and negative characteristics at the other. Polar compounds are used as additives in many petroleum products. Polarity gives certain molecules a strong affinity for solid surfaces; as lubricant additives (oiliness agents), such molecules plate out to form a tenacious, friction- reducing film. Some polar molecules are oil-soluble at one end and water-soluble at the other end; in lubricants, they act as emulsifiers, helping to form stable oil-water emulsions. Such lubricants are said to have good metal-wetting properties. Polar compounds with a strong attraction for solid contaminants act as detergents in engine oils by keeping contaminants finely dispersed.


A filter located in a line conducting working fluid to a working device or devices.


The cuttings, and grinding fines that result from metal working operations.


Any of a number of systems which characterize lubricants according to viscosity for particular applications, such as industrial oils, gear oils, automotive engine oils, automotive gear oils, and aircraft piston engine oils.


Leak proof seal between crankshaft and compressor body in open type compressors.
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