Engine Principles : 
What is ignition and what is meant by advance/retard?

 
Once the piston has drawn in the air/fuel mixture (charge) on its induction stroke, the piston begins to ascend and compress the charge. In an ideal world we would wait until the piston had reached TDC (Top Dead Centre) and the charge was fully compressed, and then as the piston passed TDC ignite the charge to generate an expanding explosion and generate downward force on the piston. However in reality the laws of physics do not allow this to happen. The issue comes about by the fact that from the moment the spark plug ignites, there is a time delay as the flame front advances in the combustion chamber in order to burn enough charge to generate a downward force on the piston. If we waited until the piston had reached TDC before igniting the compressed charge, the piston would already have started its descent down the cylinder before we applied any significant force to the piston. This would result in a power loss, although the engine would run quite happily.

In order then to create the most power from the engine, we must revise the point at which we ignite the compressed charge so that we apply a downward force to the piston just as it passes TDC. We term this IGNITION ADVANCE, and is specified as a number of crankshaft degrees before piston top dead centre. This is abbreviated to BTDC. (e.g. 12 Deg BTDC indicates the spark plug fires when the crankshaft is at a position 12 degrees before the piston will be at top dead centre). Now this is where we must introduce another factor. Engine RPM.

As engine rpm rises the time we have to ignite the charge so that it is generating a downward force just as the piston passes TDC is effectively shorter. The burn rate of the compressed charge remains relatively constant throughout the rpm range and I do not wish to discuss how the burn rate varies with rpm as it is linked to many factors beyond the scope of the average reader. Since we can assume the burn rate stays constant, but rpm is increasing, it goes hand in hand that we must continue to advance the ignition timing in order to generate the downward piston force at the optimum time. A typical full rpm advance is in the region of 35 crankshaft degrees for a normally aspirated engine such as a 2.0 Pinto.

Now we move onto the issue of forced induction engines or to simplify matters turbocharged engines. The burn rate time that directly determines the ignition advance is determined by several factors. One of the most significant is how close to self ignition the compressed charge is. All flammable mixtures have a flash point where by if you compress (or heat up) the mixture enough if will self ignite. (For simplicity picture a diesel engine). In our turbocharged engine, we have an rpm range where the turbo does not generate any positive pressure above atmospheric. This is usually up to 2500rpm. Up to this point the ignition curve of the engine will be fairly typical, in that it will be gradually advancing with engine rpm.

Now if we consider at 2500rpm the turbo begins to create an air intake pressure above atmospheric, it also becomes clear that the cylinder charge volume (or pressure) will increase. As the piston rises and compresses this charge the temperature rises. The more we force air into the cylinder (i.e. as boost pressure rises) the greater the heat generated and similarly as there is a greater density or air in the cylinder, once it is compressed the pressure and hence heat is even higher. Due to this greater compressed pressure and heat, the charge mixture is closer to its flash point (self ignition), and hence the time required to burn the charge to generate a downward force is less. We term this issue IGNITION RETARD and is a reduction in crankshaft degrees before piston TDC when the spark plug is fired. (e.g. 20deg BTDC to 15 deg BTDC is known as 5 degrees ignition RETARD).

Now finally to explain why ignition RETARD is necessary lets look at what would happen if we didn’t. If the piston was approaching TDC and the charge mixture had been ignited at a certain advance that resulted in the charge mixture generating a significant downward force on the piston before it had passed TDC, we run into a condition where by the piston is still traveling up the cylinder and we are trying to force it back down. This condition results in a situation that is potentially disastrous and the likely outcome is a damaged piston and bent con-rod. In serious instances crank shaft damage can also occur. So next time you fancy a little twiddle with your distributor in the hope of extracting a few extra bhp, think carefully as it may be the last time it generates any bhp at all!!


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