What Does Engine Timing Mean?

Educational

Nov 29th, 2021

What Does Engine Timing Mean?

What is engine timing?

Before we answer that, let’s take notice of the car engine's many moving parts. Isn’t it fascinating how all these different parts get their acts together, moving with clockwork precision?

Well, they should. Otherwise, pistons would fire irregularly, the crankshaft would not rotate as desired. The camshafts, pistons, valves, and crankshaft need to move in a precise manner, and there’s no room for error.

The timely input of each part is critical for the collective output of the engine. So how do all the parts coordinate to work without colliding or causing delays?

You’re right, they all point to one crucial factor – engine timing.

Before delving into the ins-and-outs and the importance of engine timing, let's see first what occurs inside engine cylinders.

How an Engine Works

Four-stroke cycle

Let’s a standard four-stroke engine as an example.

  1. Intake – The first movement of the piston within that cylinder is downwards. This movement allows the air-and-fuel mixture to get into the cylinder via the inlet valve.
  2. Compression – The piston will start moving upwards but only after reaching the bottom point called the bottom dead centre (BDC). The inlet valve closes and compresses the 'trapped' mixture. 
  3. Power – The spark plugs generate a spark which then ignites the air-and-fuel mixture, resulting in combustion. 
  4. Exhaust – The resultant force of combustion pushes the piston back to the bottom dead centre, but this time, the exhaust valves open, expelling gas out of the cylinder to complete the cycle.

The cycle starts again, and every time, it lasts four strokes (intake, compression, power, and exhaust). 

The crankshaft rotates twice (720°) to complete the four-stroke cycle. The 360° crankshaft rotation allows the piston to move from the top dead centre to the bottom dead centre. The second 360° rotation enables the piston to move from the bottom dead centre to the top dead centre.

What Parts Are Responsible for Engine Timing

As you can imagine, the execution of these movements needs precise timing, and a timing belt or chain serves this particular purpose. The timing belt connects some parts of the engine, thereby allowing the series of events to be in sync.

Timing belt

You can spot the engine belt easily in your car. It runs around various engine components, in particular, the camshaft and crankshaft sprockets and water pump pulley. The timing belt is toothed and thick, thus suited for its function. It rotates in unison with the crankshaft.

Water pump

The water pump regulates the flow of the coolant depending on the engine speed, that is, how heavy or light the engine is working. If it is working hard, the water pump increases the rate of flow of the coolant to allow more of it to circulate the engine block. 

Timing belt tensioner

The timing belt tensioner maintains the timing belt at a constant tension to ensure that the belt does not slide or slip out. It keeps the teeth of the belt meshed with the teeth of the sprockets at all times. 

Timing marks

Timing marks on the valve cover and sprockets are used to harmonise the timing system with the ignition.

Timing chains

Today, timing chains have replaced timing belts in most modern cars because they are more durable. 

Timing belt vs Timing chain

  • Timing belts wear out after about 65,000 kilometre and thus need to be replaced. Also, the belt can lose tension or be overly tight, or its teeth could fall out when not working, causing damage to other parts.
  • Timing chains don’t require replacing but need to be lubricated to run without fail. Timing chains are costlier than timing belts, but since you don’t need to replace it, it comes out cheaper over time.

This is not to say that the chained timing system is flawless. If the timing chain tensioner reduces the force applied to the chain, the chain will make rattling sounds. 

Valve timing

Apart from the engine timing system, valve timing plays a crucial role, too. The inlet and exhaust valves must open at a specific time for a particular interval. 

Lobe profiles

Lobe profiles on the camshaft control valve timing.

  • The inlet valves open for a specific time interval. During this time, the air-and-fuel mixture gets into the cylinder. 
  • The exhaust valve also opens to allow the gases resulting from combustion to escape. 

The lobes make this possible by controlling the lift and timing of these valves.

Ignition timing

The ignition timing system also influences engine timing. As mentioned earlier, a spark in the engine ignites the air-and-fuel mixture. The ignition timing system determines the exact time for the spark to ignite the mixture, and it can delay or advance the spark timing to meet the required engine conditions. 

  • There is delayed ignition if the sparks occur after the piston gets to the top dead centre. Ignition delay (aka retardation) results in wastage of pressure created in the cylinder. 
  • On the other hand, it is advanced if the spark is pre-energised just before the piston reaches the top dead centre.

Ignition timing light

A timing light is used to monitor the ignition timing. 

As you can see, all these intricate systems contribute to engine timing, which rarely goes wrong.

That’s reassuring, of course, but you cannot take chances. A regular check of your timing chain or belt and all the other engine components, is necessary to ensure that all is well. 


By Sam O.