Engine Components and Terminology (Engine Parts)

Engine Parts: Introduction

This 3D model shows all major internal components associated with a typical four stroke combustion engine as well as reference marks indicating areas of interest. A summary list is given below, followed by a detailed description of each engine component.


Engine Components (Summary)

  • Rocker Arm
  • Tappet Clearance
  • Valve Spring
  • Fuel Injector
  • Fuel Inlet Pipe
  • Suction Valve
  • Exhaust Valve
  • Fuel Injector Nozzle
  • Clearance Volume
  • Cylinder Bore
  • Top Dead Centre (TDC)
  • Push Rod
  • Piston Crown
  • Piston Ring Grooves
  • Stroke
  • Piston
  • Piston Skirt
  • Piston Pin
  • Cylinder Wall
  • Camshaft
  • Cam
  • Cam Follower
  • Bottom Dead Centre (BDC)
  • Connecting Rod
  • Crank Web
  • Crankpin Bearing Journal
  • Crankshaft


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Engine Components (Detailed)

Rocker Arm
Lifting of the cam lobe causes the rocker arm to push down upon the valve stem. This movement opens the associated valve.

Tappet Clearance
The space between the valve stem and rocker arms. The tappet clearance allows for thermal expansion of the engine parts when warming up, this ensures the valves open and close correctly. Tappet clearance is also known as valve lash.

Valve Spring
Springs used for returning the valves back to the closed position and maintaining the valves in their closed position when force from the rocker arms is absent.

Fuel Injector
Fuel is pumped to the fuel injector and then through the injector nozzle into the engine cylinder.

The top of this injector connects to an electrical solenoid which is used for more accurate injection timing.

Fuel Inlet Pipe
Fuel is supplied to the injector through this pipe.

On this model there are two suction inlet valves and two exhaust discharge valves.The suction valves supply air whilst the discharge valves discharge the exhaust gas. This type of valve is often referred to as a poppet valve.

Fuel Injector Nozzle
Fuel is sprayed into the engine cylinder through the fuel injector nozzle. It is important that the fuel injection ports do not become blocked. Any blockage of the nozzle will change the injection spray pattern and reduce engine efficiency.

Clearance Volume
The clearance volume is the distance from the top dead centre of the stroke and the top of the cylinder liner.

Cylinder Bore
The cylinder bore represents the internal diameter of the cylinder liner. It is possible to calculate the cylinder displacement by calculating the cylinder bore and piston stroke.

Top Dead Centre (TDC)
TDC represents the maximum transit of the piston in the direction of the cylinder valves. Push Rod The push rod transfers radial movement from the cam lobe to the rocker arms.

Piston Crown
Due to its location, the piston crown encounters significant pressures and temperatures. The design of the crown varies considerably, as many crowns have a unique topography to distribute the exhaust gasses created by the combustion process.

Piston Ring Grooves
The piston rings are located in the piston ring grooves. Note that the piston rings for this model are not displayed.

The stroke represents a measurement of the total distance travelled by the piston (TDC to BDC). The reference point is measured from the top of the piston crown.

The force generated by combustion is transferred to the piston. The piston is split into many parts, this includes a piston skirt, piston crown, piston rod and piston pin. See our 3D piston model for more information.

Piston Skirt
The type of skirt shown here is a ‘full’ skirt.

Piston Pin / Gudgeon Pin
The pin joins the piston skirt to the piston rod.

Cylinder Wall
An arrow indicating the cylinder wall, also referred to as the ‘cylinder liner’. The cylinder liner forms the combustion chamber.

The camshaft is used to control the timing of the engine. This includes when fuel is injected and when the air inlet and exhaust valves open and close.

Cams are used to open and close the inlet and exhaust valves. On larger engines cams are also used to operate fuel pumps. A cam is also referred to as a ‘cam lobe’.

Cam Follower
The cam follower is lowered and raised by the cam lobe. The follower transfers movement from the camshaft to the load (valve, pump etc.).

Bottom Dead Centre (BDC)
BDC represents the piston’s point of maximum transit in the direction of the cylinder base. In other words, the piston will not travel further towards the cylinder base than the BDC reference point.

Connecting Rod
Note that the term ‘connecting rod’ and ‘piston rod’ are sometimes used intermittently. Large two stroke engines have both, with the piston rod always being the rod between the crosshead and piston pin.

Reciprocating Motion
Rotary motion is converted by the engine design into a reciprocating linear motion. This conversion allows the piston skirt to move up and down the cylinder rather than rotating.

Crank Web
The crankshaft is connected to the piston rod via the crank webs and crank pins. Crank webs allow the reciprocating piston motion to be converted to a rotary motion.

Crankpin Journal Bearing
The crankpin journal bearing is installed between the connecting rod and crankpin. Bearing materials vary although softer metals such as white metal (babbit metal) are often used.

Main Journal Bearing

Main journal bearings are plain metal bearing that seat upon the crankshaft. Unlike crankpin journal bearings, main journal bearings align with the crankshaft centre axis of rotation.

The crankshaft is not a single straight shaft as it is separated at intervals by the crank webs. The alignment of the crankshaft though is constant throughout the engine.

Rotary Motion
This engine is an internal combustion engine. The force created by the combustion process is transferred to the piston and then the crankshaft. The process causes the crankshaft to rotate and the piston to reciprocate linearly.

The flange often connects to a flywheel although it can connect to any load requiring rotary motion.


Additional Resources