Ignition timing

Ignition timing

For reciprocating engines, the point in the cycle at which the fuel-oxidizer mixture is ignited has a direct effect on the efficiency and output of the ICE. The thermodynamics of the idealized Carnot heat engine tells us that an ICE is most efficient if most of the burning takes place at a high temperature, resulting from compression—near top dead center. The speed of the flame front is directly affected by the compression ratio, fuel mixture temperature, and Octane rating or cetane number of the fuel. Leaner mixtures and lower mixture pressures burn more slowly requiring more advanced ignition timing. It is important to have combustion spread by a thermal flame front (deflagration), not by a shock wave. Combustion propagation by a shock wave is called detonation and, in engines, is also known as pinging or Engine knocking.

So at least in gasoline-burning engines, ignition timing is largely a compromise between an earlier "advanced" spark—which gives greater efficiency with high octane fuel—and a later "retarded" spark that avoids detonation with the fuel used. For this reason, high-performance diesel automobile proponents such as, Gale Banks, believe that

There’s only so far you can go with an air-throttled engine on 91-octane gasoline. In other words, it is the fuel, gasoline, that has become the limiting factor. ... While turbocharging has been applied to both gasoline and diesel engines, only limited boost can be added to a gasoline engine before the fuel octane level again becomes a problem. With a diesel, boost pressure is essentially unlimited. It is literally possible to run as much boost as the engine will physically stand before breaking apart. Consequently, engine designers have come to realize that diesels are capable of substantially more power and torque than any comparably sized gasoline engine.