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A layer of ash on the heat-exchange surfaces of a combustion chamber, resulting from the burning of a fuel.

Related Terms

CROWN

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

CAM

Eccentric shaft used in most internal combustion engines to open and close valves.

ALKALI

Any substance having basic (as opposed to acidic) properties. In a restricted sense it is applied to the hydroxides of ammonium, lithium, potassium and sodium. Alkaline materials in lubricating oils neutralize acids to prevent acidic and corrosive wear in internal combustion engines.

LAMINAR FLOW

A flow situation in which fluid moves in parallel lamina or layers.

BLOW-BY

Passage of unburned fuel and combustion gases past the piston rings of internal combustion engines, resulting in fuel dilution and contamination of the crankcase oil.

POISE

A measure of viscosity (absolute viscosity) numerically equal to the force required to move a plane surface of one square centimeter per second when the surfaces are separated by a layer of fluid one centimeter in thickness. It is the ratio of the shearing stress to the shear rate of a fluid and is expressed in dyne seconds per square centimeter (DYNE SEC/CM2); 1 centipoise equals .01 poise.

SIMPLE CYCLE

Referring to the gas turbine cycle consisting only of compression, combustion and expansion.

NITRATION

Nitration products are formed during the fuel combustion process in internal combustion engines. Most nitration products are formed when an excess of oxygen is present. These products are highly acidic, form deposits in combustion areas and rapidly accelerate oxidation.

POSITIVE CRANKCASE VENTILATION (PCV)

System for removing blow-by gases from the crankcase and returning them through the intake manifold to the combustion chamber where the recirculated hydrocarbons are burned. A PCV valve controls the flow of gases from the crankcase to reduce hydrocarbon emissions.

SHEAR STRESS

Frictional force overcome in sliding one layer of fluid along another, as in any fluid flow. The shear stress of a petroleum oil or other Newtonian fluid at a given temperature varies directly with shear rate (velocity). The ratio between shear stress and shear rate is constant; this ratio is termed viscosity of a Newtonian fluid, the greater the shear stress as a function of rate of shear. In a non-Newtonian fluid, such as a grease or a polymer-containing oil (e.g. multi-grade oil) - shear stress is not proportional to the rate of shear. A non-Newtonian fluid may be said to have an apparent viscosity, a viscosity that holds only for the shear rate (and temperature) at which the viscosity is determined.
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