An engine requiring one revolution to complete one working cycle.

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


The top of the piston in an internal combustion engine above the fire ring, exposed to direct flame impingement.


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.


Freezing of a piston ring in its groove in a piston engine or reciprocating compressor due to heavy deposits in the piston ring zone.

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