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

GEARBOX

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.

PISTON VALVE

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

PISTON ENGINE

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

PERFORMANCE NUMBER

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.

OVERRUNNING CLUTCH

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.

OVERSQUARE ENGINE

An engine with bore diameter greater than the stroke length.

OVERDRIVE

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

OVERHEAD-VALVE ENGINE

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.

OVERGEAR

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.

OPTICAL INDICATOR

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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