valve train for diesel engines

Understanding an automobile valvetrain

A valvetrain or valve train is part of an internal combustion engine that controls the operation of the intake and exhaust valves. There is an intake valve that allows the air-fuel mixture into the combustion. The exhaust valve allows the exhaust gases out of the combustion chamber once the process is completed.

complete setup of a valvetrain

Today we’ll be looking at the definition, layout, functions, working, and components of a valvetrain in an internal combustion engine.


Valvetrain definition

A valvetrain is a component that is designed to open and close the intake and exhaust valves so that air/fuel mixture can enter and leave the combustion chamber as gases. Nowadays, engines are designed with overhead cam assemblies which are known as overhead camshafts. It is located near the top of the engine.

Unlike the ones that locate the camshaft lower in the engine and use push rods to move the valve assemblies. This valvetrain layout is called Cam-in-block. Some valvetrain layout does not use camshaft. It uses technologies such as solenoids to control individual valves. it is known as Camless.

Valvetrain Functions and Working Principle

The primary functions of the valvetrain are to control the opening and closing of the valves. The valvetrain also controls the flow of air and fuel that enters the combustion chamber and leave as exhaust gases. In the working of a valvetrain, the camshaft plays a very important role as its rotational movement aids the opening and closing of the valves using the cam lobes. Although there are various parts that help the valve train to work.

In most heavy-duty diesel engines, four valves are used, that is, there are four valves in each cylinder. Two valves for the intake air/fuel and two for the exhaust gases. The intake valves have a larger diameter than the exhaust valves, which allows much airflow to the cylinder. Exhaust valves are designed to withstand higher temperatures of hot exhaust gas than the intake valves. This is because the fresh air that flows through the intake valves keeps it at a lower temperature.

In order to keep the valves intact, that’s from burning, the intake and exhaust valve transfers heat to the cylinder head else they get burnt. There is an injector in the middle of the valve which is pushed down to inject fuel into the cylinder. Well, the timing for the valve opening and closing is incredibly precise, making it’s working efficiently. In newer engines, electrical signals are used to nod the injector, instead of the mechanical valve train. This process is even more accurate.

Components of a valvetrain

Below are the different components that aid the working of a valve train:


The function of the camshaft is to control the timing and lift the profile of the valve opening. This is achieved with the cam lobe on the rotating shaft. A camshaft is driven by the crankshaft and it rotates at half the speed of the crankshaft in the case of a four-stroke engine. The crankshaft transfers motion to the camshaft using a metal timing chain or mostly a rubber timing belt. A set of gears can also be used.


A pushrod is a long, slender metal rod that is used in overhead valve engines. It’s also used to transfer motion from the camshaft in the engine block to the valves in the cylinder head. There is a lifter on the bottom end of a pushrod that gets in contact with the camshaft. The camshaft lobe moves the lifter upwards, which moves the pushrod. The top end of the lifter further pushes on the rocker arm, which opens the valve.

Rocker arm/bucket tappet:

The engine design determines the ways the valve will be actuated, either by a rocker arm, finger, or bucket tappet. In overhead camshaft engines, bucket tappets or fingers are used, upon which the cam lobes contact. Rocker arms are used in overhead valve engines, which are actuated by a pushrod and pivot on a shaft. It can also be pivoted on individual ball studs which will help to actuate the valves.


The valve used in most modern engines is called a poppet valve. Some valves like sleeve valves, slide valves, and rotary valves are also considered at times. Generally, poppet valves are opened by the camshaft lobe or rocker arm. It’s closed by a coiled spring called a valve spring.

Below is the diagram of a valvetrain:

valvetrain diagram

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That’s it for this article “Automobile Valvetrain”. I hope the knowledge is attained, if so, kindly comment, share, and recommend this site to other technical students. Thanks!


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