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A valve is an apparatus that functions to open or close an entrance, i.e. an inlet or pipe, to allow or prevent the flow of liquid, air or gas, in a vehicle that operates with an internal combustion engine. The car's ECU (Engine Control Unit) relies on the use of valves to make vital adjustments to a whole range of processes to ensure the engine is running properly.  The ECU adjusts the vehicle performance by controlling the operation of a series of valves, switches, actuators, sensors and motors. 

A valve typically consists of an inlet and a corresponding outlet with which it engages/makes contact, to either open or close the entrance to accurately control the flow of the substance. The inlets and outlets are commonly known as ports. Different valves are actuated differently, and will operate either mechanically or electronically/automatically, or a combination of both, in conjunction with/powered by other components, directed and managed by the car's ECU or operated by the flow of the substance itself. As is the case with the simple, yet effective, check valve which is forced open by the substance moving in the other direction. An example of a check valve is the air injection valve, which stops the flow of gas in one direction, preventing exhaust fumes from re-entering the car's air intake system, which helps to prevent damage to the engine.

The most obvious examples of valves in an internal combustion engine are the poppet valves, which comprise of the intake and exhaust valves. The cylinder head is comprised of a series of passages that carry the air and fuel combination needed for combustion to the cylinders; the liquid  coolant, needed to keep the engine cool; and harmful fumes (produced in the engine in the process of combustion) from the exhaust  valves to the exhaust manifold and away from the vehicle. The flow of these substances is controlled by the valves, either opening or closing to allow or block the flow.