What would happen when more fuel is added during a combustion process? Will the time of combustion decrease or increase or stay the same?
The short answer is "it depends."
Gasoline engines would ideally run with a stoichiometric mixture – just the right amount of fuel and air to produce complete combustion. Now in reality you don't necessarily get to do that. For example some aircraft engines are designed to run with an excess of fuel at high power in order to provide additional cooling, the "cool with fuel." Other factors such as exhaust gas temperature and emissions considerations will also impinge on the "perfect" mixture. As you add more fuel, the mixture becomes richer, within limits this will increase power and lower temperatures – at the expense of efficiency.
I think the burn time in a gasoline engine is related to the speed with which the flame front advances in the cylinder. I would guess that this is relatively constant and thus adding fuel would not produce much of a change in the burn time of a gas engine.
Diesel engines run with an excess of air, so in a diesel you can add fuel and increase power without loss in efficiency that would occur in a gasoline engine.
In a diesel there are two ways that you can add more fuel: you can use bigger injectors and pump the fuel in faster, or you can increase the injection time. If you do the former you'd expect the burn time to be about the same, if you extend the injection time then the burn time would also be extended.
Extending the injection time means that more of the more of the stroke is used for combustion and there are limits to the benefit that you can achieve here. If you advance ignition too far the combustion starts to resist the piston as it moves towards TDC, if you extend the burn for too long then you may be unable to extract all of the energy from the fuel.
Keep in mind that engines are designed to have the combustion chamber closed tight during combustion. If we relax that constraint then I'd say it depends on the timing of introduction for that additional fuel.
If combustion is already occurring then the fuel catches it and creates a second explosion that either
- Assists the explosion to make it more forceful or of a longer duration.
- Does nothing because the cylinder is as the bottom of the power stroke so there’s nothing else to receive the new power.
- Does nothing because a valve is open and the force of gas expansion leaks out leaving no pressure to be harnessed.
- Does little because the fuel is partially combusted, maybe even leaving fuel for the next cycle.
Viewed in a bigger picture I'd say that it would complicate matters and break the critical timing that an engine is designed to have. If you're designing an engine you need some simple discrete model of a combustion event, not a complicated chain of combustions that conspire in some complex hard to model fashion which introduces the need for additional timings that need to be coordinated and managed. I’d be curious of the effects on the valves in having a combustion where the wrong valves are open at the wrong time, should make a condition where you have no explosive pressure and things just don’t work. I’d wonder too if the fuel injectors could be at risk of catching fire too with the valves open when combustion occurs. The engine would probably just be destroyed firing at random, as if the timing belt were broke or something.
We might get some insight too by comparing to systems which have fuel on fuel on fuel combustion, like a rocket or something, which I don't know much about. All and all we have to keep in mind that combustion engines don't want combustion for combustion sake but rather seek to harness that chemical energy in gas and convert it into mechanical energy via rapidly expanding gases via the explosion from the spark.
Gas engines actually run with a variety of mixtures, stoch. is only one possible mixture. For max power, the engine will run rich; this is partially to enhance power and partially to protect the engine from knock during high load events. On the highway, the engine can run lean. Some Honda's will run as lean as 17:1 or even 20:1 on the highway cruising. There is also deceleration fuel cutoff; when you lift off the gas at a high rpm, the engine will cutoff fuel largely and the engine will go into a lean condition as well. At idle, the computer will target stoch. and run in closed loop via the O2 sensor(s). When you are blasting around at WOT, the engine most often runs in open loop and uses look-up tables to compute the fueling and the MAF or MAP senors to judge how rich to run.