Terms Starting with H

Harmonics-When valve springs are opened and closed rapidly, they may vibrate.  Periods of vibration are called harmonics.

Header-Welded steel pipe used as an exhaust manifold.

Heat dam-The narrow groove cut into the top of the piston.  It restricts the flow of heat down into the piston.

Heater core-A small radiator-like heat exchanger.  Hot coolant from the engine flows through the heater core.  Airflow from a fan passes through the fins on the heater core, picking up heat to warm the passenger compartment.

Heat exchanger-A device used to transfer heat from one medium to another. The radiator is a liquid-to-air heat exchanger.

Heat stove-An enclosed area made of thin sheet metal around the exhaust manifold. It preheats air passing over the exhaust manifold before the air enters the air cleaner.

Heat transfer-The process of moving heat from a warmer object to a colder object.

Heat treated-A process in which a metal is heated to a high temperature, then quenched in a cool bath of water, oil, or brine (salt water).  This process hardens the metal.

HEI-High-energy ignition system:  A type of ignition system used on vehicles in which the coil and distributor are physically combined into one unit.

Hemispherical combustion chamber-A type of combustion chamber that is shaped like a half circle.  This combustion chamber has the valves on either side with the spark plug in the center.

High test-Gasoline that has an octane number of 90 to 95.

Horsepower-A measure of work being done per time unit.  One hp equals the work done when 33,000 pounds have been lifted 1 foot in 1 minute.

Hydraulic system-A brake system in which brakes are operated and controlled by hydraulic fluid under pressure.

Hydrocarbons-A term used to describe the chemical combinations of hydrogen and carbon.  Also, a type of automotive pollution produced by incomplete combustion.  Hydrocarbons are considered partly burned hydrogen and carbon molecules resulting from incomplete combustion.

Hydrometer-An instrument used to measure the specific gravity of a battery acid.

Hypoid gear oil-A thick oil used in differentials.


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