COMBUSTION DURATION AND ENGINE PERFORMANCE

 

Designing an engine is like a work of rocket science. Regarded as the “Heart of a vehicle”, it’s revving generates all the power that eventually goes to the wheels and makes the vehicle move and the combustion chamber is where this power generation originates. To design this chamber, an engine designer needs to consider in mind a lot of vital factors one of which is the “combustion duration”. A perfect ignition timing, duration and the air-fuel ratio are a few factors that lead to proper combustion and power generation. So let’s delve further into the effects of engine parameters on combustion duration and vice-versa.

 

EFFECT OF ENGINE PARAMETERS ON COMBUSTION DURATION

ENGINE SPEED

Regulating the engine speed has a lot to do with turbulence,i.e., a sudden disturbance created in the air-fuel mixture. It’s similar to the atmospheric instability, such as violent, unpredictable air movements resulting from a storm. Increasing the engine RPM causes the piston to come up with great speed during the compression stroke and when this piston head strikes the air-fuel mixture, turbulence is created which increases the rate of heat transfer leading to an increase in flame speed and rapid burning of the air-fuel mixture.

COMPRESSION RATIO

In simple words, compression ratio is the ratio of the largest volume of the combustion chamber to it’s minimum volume. As the compression ratio increases, it leads to an increase in the end-of-compression temperature and pressure. Also, the mixture gets squeezed to a great extent which increases turbulence and hence, the flame speed. The mixture gets squeezed in a way that a major portion of it is exposed to the hottest area of the chamber in order to burn rapidly and avoid the detonation effect. This, in turn, favours a decrease in the combustion duration by burning the whole mixture in less time.

SPARK PLUG LOCATION

In an S.I. engine, a spark plug is what ignites the air-fuel mixture and starts the combustion process. Deciding the location of the spark plug poses two challenges: optimum combustion duration and suppressing detonation. As the spark plug is shifted from the peripheral position to the center, the combustion duration decreases. Too near the periphery, chances of detonation increase, too near the center, the cylinder pressure decreases. Positioning the spark plug near the periphery, the flame has to travel a long distance to reach the furthest point in the combustion chamber. The part of the mixture present at that point already gets heated high enough(due to compression) and combusts much before the actual flame reaches it. This creates shockwaves in the cylinder and hence, leads to detonation which adversely affects the cylinder and engine performance. So this demands an optimum location of the plug to reduce the detonation effect and at the same time, reduce the combustion duration by decreasing the flame traveling distance and increasing the combustion rate.

SPARK ADVANCE (link here)

Ignite the mixture too early, the slight cylinder pressure increase will tend to push back the piston downward leading to an ineffective compression. Ignite the mixture late, and the temperature and pressure won’t be high enough due to the decrease in flame development duration leading to an ineffective power stroke. Both these spark timings will lead to an increase in the combustion duration.

Hence, a proper spark timing is desired that is neither too early nor too late. This timing is called the Maximum Brake Torque(MBT) angle at which point, activating the spark plug results in the maximum end-of-compression temperature and pressure and also the maximum torque increase. The engine delivers it’s best output work at this angle and most importantly, this timing leads to the minimum combustion duration.

Now that we have seen how the above factors affect the combustion duration, an equally important factor plays a vital role in deciding this duration: the proper combination of air and fuel that burns in the combustion chamber. Let’s discuss below how this combination actually comes into play in the big picture.

EQUIVALENCE RATIO(∅) (link here)

This is the ratio of the mass of fuel to the mass of air in the combustion chamber and greatly influences the combustion rate and it’s duration. Lean mixtures(air>fuel) lead to an increase in the combustion duration whereas rich mixtures(fuel>air) decrease it. Combustion of lean mixtures generates less thermal energy due to little fuel available and this decreases the flame propagation speed on account of which it takes more time for the entire mixture to burn properly. Rich mixtures, in contrast, generate more thermal energy, the flame propagation speed is high and hence, the air-fuel mixture gets burnt in less time decreasing the combustion duration.

EFFECT OF COMBUSTION DURATION ON ENGINE PERFORMANCE

When linking combustion duration with engine performance, ignition timing and ignition delay are the important criteria to be delved upon. Ignition timing refers to the point before or after TDC when the spark plug ignites. Ignition delay, in the case of S.I. engines is the lapse of time from passage of the spark to a noticeable pressure rise in the cylinder caused by compression. Now, these two will be decided based on whether the mixture is lean or rich and this, in turn, will decide the combustion duration and performance.

If the mixture is lean(air>fuel), a short spark duration will be unfavourable due to a lower probability of the presence of flammable mixture at the point of ignition. An increase in the spark duration is desired in order to provide more time for reaction due to excess presence of oxygen in the mixture. Also, due to a short spark duration, the mixture inflammation might fail and the flame will die. The ignition delay will increase due to the spark terminating earlier than desired without a significant rise in cylinder pressure. In contrast, a longer spark duration will minimize the time lag between the termination of spark and the rise of cylinder pressure.

Now how does ignition timing advance affect all this? Prior to the point when the spark plug ignites, the mixture attains a certain temperature and pressure after which activating the spark plug ignites the mixture spontaneously. If the mixture is lean, an optimum temperature won’t be attained in order for proper combustion of the mixture. So the ignition timing advances,i.e., the spark ignites certain degrees before than in the case of a rich mixture so that the desired temperature is achieved for the inflammation of the mixture. Also, the spark duration is long due to the reason discussed earlier. A similar discussion can be carried out if the mixture is rich(fuel>air). Eventually, this increase(decrease) in combustion duration is desired for lean(rich) mixtures to obtain the optimum power output and better engine performance.

But yes, there are limitations to an increased combustion duration. The fuel consumption will decrease due to the fact that a leaner mixture will be desired. Thermal efficiency will improve only to a little extent. Therefore, it becomes clear that any attempt to increase the combustion duration either by reducing the compression ratio, locating the spark plug near the periphery or operating at leaner mixtures, though is going to improve the engine economy, will result in a reduced power output.

 

APPENDIX

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