Inlet and Exhaust Valves

engine valve stood on caliper
6mm diameter valve made from stainless steel

The main function of the engine inlet and exhaust valves is to let air in and out of the cylinders.

Air Intake Valve – controls the air and fuel flow into an internal combustion engines.

Bounce – If the speed of valve is closing too high, the valve can hit the seat so hard it bounces off again.

Camshaft – The shaft which carries the various cams required for the operation of inlet, exhaust, fuel, and starting-air valves.

Cooling – the intake valve will be cooled by the incoming air and fuel mixture. The exhaust valve will run at much higher peak temperatures as it sits in the exhaust gas stream.

A significant amount of cooling occurs when the valve is in contact with the valve seat and the heat is conducted away to the cylinder head. Some exhaust valves are filled with sodium to improve heat transfer.

Exhaust Valve – The valve through which the burnt gases are allowed to pass out to the exhaust manifold

Float – A high-rpm engine condition in which the valve lifters lose contact with the cam lobes because the valve springs are not strong enough to overcome the momentum of the various valvetrain components.

Lifter – The cylindrically shaped component that presses against the lobe of a camshaft and moves up and down as the cam lobe rotates.

Mass – Intake and exhaust valves must be optimised for mass to operate in high speed engines. The intake valve needs the most attention as it is the largest of the valves. In some engines the intake valves are made from titanium.

Material – the exhaust valve will see extremely high temperature and corrosive exhaust gas. For the intake and exhaust valves on model engines it’s common to make them from stainless steel with phosphor bronze valve guides.

Overhead Cam – A camshaft used for operating both valves and unit injectors, located on top of or within the cylinder head.

Overhead Valve – The camshaft is within the cylinder block and uses pushrods to actuate rocker arms above the cylinder head to actuate the valves.

Overlap – The amount of time that the inlet and exhaust valves are open at the same time, to allow momentum charging of the cylinder or to scavenge additional exhaust gas from the cylinder.

Piston to Valve Clearance – an intake or exhaust valve will not survive contact with the piston. Hence on a number of pistons you will see recesses on the top surface to allow slightly more space for the opening of the valves. This also makes it imperative to get the timing correct.

Poppet Valve – A valve opened by the action of a cam and closed by a spring.

Ports – The inlet or exhaust ports allow air to flow into or out of the cylinder head. Run from the manifold face to the valves.

rocker arm on an engine

Rocker Arm – A lever that transmits the action of the cam, usually by means of a push rod, to the stem of the intake or exhaust valve, sometimes also to the starting-air valve and fuel valve.

The rocker arm highlighted on a Bentley BR.1 Rotary engine.

Adjustable Rocker Arm – A type of rocker arm with an adjusting nut that can be tightened or loosened to adjust the valve to rocker arm gap.

Seat – That part of the valve mechanism upon which the valve face rests to close the port.

Sidevalve Engine – An internal combustion engine with valves placed in the engine block beside the piston. Also known as a “Flathead”.

Sleeve Valve Engine – In the sleeve valve engine the conventional valve train was replaced by a ported cylinder liner that was then rotated to align with ports in the block so as to achieve the timing for the intake and exhaust strokes for the four-stroke cycle.

Sodium Filled Exhaust Valves – The valve stem is drilled and partially filled with sodium to increase heat transfer to the valve guide and hence reduce the temperature of the exhaust valve.

Spring – The spring which is used to close a valve.

Spring Retainer – Holds the valve spring in a state of compression

Tappet Noise – Noise caused by the lash or clearance between the valve stem and rocker arm or between the valve stem and valve lifter.

Valvetrain – The collection of parts that make the valves operate in an internal combustion engine.

About Nigel 308 Articles
Have been making models since I was around 7 years old and using a lathe from the age of 11, a self taught engineer with a passion for making model engines.

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