Ignition advance is the number of degrees before top-dead-centre (TDC) that a spark occurs. The reason for ignition advance is that the spark to combust the fuel/air mixture needs to be timed so that the point of peak combustion pressure is when the piston is just beyond TDC. If the point of peak combustion pressure is too early and before TDC the pressure wave will slow down the speed of the piston travelling up towards it, and may cause detonation (knocking) which is very damaging to the engine. If the point of peak combustion pressure is too late, the pressure wave will ‘chase' the piston as it travels back down the cylinder in the combustion stroke and most of the energy will be lost.
As the speed of the engine rises, the ignition advance angle needs to increase. This is because the time to combust an unchanging air/fuel mixture is approximately constant. If the ignition advance angle were kept the same, the point of peak combustion pressure would move further and further into the combustion stroke losing more and more power. Therefore the ignition advance needs to be increased to bring the point of peak combustion to just beyond TDC.
Basic mapping varies the amount of ignition advance in relation to engine speed, this is called speed mapping and can be shown on a graph as below.
The optimum amount of ignition advance varies from engine to engine and through different fuel types, timing maps of different engines using different fuels will be different. It is not possible to calculate the best map for your engine; you need to test the engine on appropriate test equipment to generate the engine maps.
Emissions can be controlled via the use of ignition advance in addition to controlling the air/fuel mixture. Large ignition advances will promote the formation of oxides in the exhaust gases and increase engine power (to an extent), but will also decrease the engines fuel consumption.
If an engine is set up for maximum power the resultant carbon monoxide levels will be too high to meet current emissions legislation, so it is common practice to adjust the ignition timing at different load levels to suit (see Load Mapping). Generally the ignition timing is retarded somewhat to reduce CO and NOX emissions.
A trade off between exhaust emissions, fuel consumption and engine power has to be taken by the application engineer during testing.
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ignition@gill.co.uk
Ignition Advance & Speed Mapping
