What Does Engine Timing Mean?

Educational

Nov 22nd, 2019

What Does Engine Timing Mean?

An average car engine has many moving parts. Isn’t it truly fascinating how all these parts work together?

The engine of any car is made up of intricate components. These parts relate differently to each other to ensure that the engine runs as it should. Precise timing is necessary so that each of these parts works without collision or delays.

The individual working of the engine components is crucial for the collective output of the engine. The camshafts, pistons, valves, and crankshaft need to move precisely at the specified time and range in terms of distance. Calculations are done to ensure there is no margin for error.

Engine timing is, therefore, a crucial factor. Otherwise, pistons would fire irregularly, or the crankshaft would not rotate as desired. Before delving into the ins and outs and the importance of engine timing, let's see first what occurs inside engine cylinders.

Working of the Engine

For a standard four-stroke engine, the first movement of the piston within that cylinder is downwards. This movement allows an air-and-fuel mixture to get into the cylinder via the inlet valve.

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. The spark plugs generate a spark which then ignites the air-and-fuel mixture. 

Combustion takes place and resultantly pushes the piston back to the bottom dead centre. This time the exhaust valves open. The gaseous product of combustion gets out of the cylinder to complete the cycle.

The cycle starts again, and every time it lasts four strokes, i.e. 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.

Timing Belt

As you can picture, 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.

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.

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 pumps increase the rate of flow of the coolant to allow more of it to circulate the engine block. 

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 on the valve cover and sprockets are used to harmonize the timing system with the ignition.

Timing Chains

Today, timing chains have replaced timing belts. Most modern cars make use of timing chains.

Timing chains have the edge over timing belts in terms of durability. Timing chains last much longer, keeping the engine in time. Timing belts wear out after about 40,000 miles 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.

Unlike timing belts, timing chains don’t require replacement but needs to be lubricated. Oil ensures the timing chain stays lubricated and runs well. Timing chains are costlier than timing belts, but repeatedly replacing belts is even more expensive.

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 & Ignition Timing

Apart from the timing system, the valve timing and ignition timing play a crucial role, too. Both of these timing systems influence the engine timing. The inlet and exhaust valves have to open at a specific time for a particular interval. Perfect timing ensures the engines run efficiently, producing the needed output. 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.

As mentioned earlier, a spark in the engine ignites the air-and-fuel mixture, which is where ignition timing comes in. Ignition timing system determines the exact time for the spark to ignite the mixture. The ignition timing 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 (retardation) results in wastage of pressure created in the cylinder. On the other hand, it is advanced if the spark is pre-energized just before the piston reaches the top dead centre.

A timing light is used to monitor the ignition timing. Engine timing rarely goes wrong, but you cannot take chances. A regular check of your timing chain or belt, as well as other engine components, is necessary to ensure all is well. 

Author:

Sam O.