An internal combustion engine works on the principle of ideal gas law. The law states that any increase in the temperature of a gas would increase its pressure to result in expansion of the same gas. The gas-fuel or air/fuel mixture is introduced in a closed chamber known as cylinder where it is ignited by producing a spark in a 4-stroke engine. The mixture burns due to its flammable property and also due to the difference in the temperature. The piston that is making a linear motion inside the cylinder increases the temperature and pressure when it travels from bottom dead center  (BDC) to top dead center (TDC). The mixture gets ignited when the cylinder is sparked and by the ideal gas law principle it expands to push the piston down. This stroke is also known as power stroke as power is produced in this stroke. The explosion is so gigantic that if the piston is not made of a high strength material then it would create a hole inside the piston head. This is what goes on inside a single cylinder engine. The piston is connected to the crankshaft which is finally connected to the rear wheel. This linear motion is converted into rotation motion of the crankshaft which is transferred to the rear wheel to bring the bike in motion. This somewhat indicates that the wheel rotates only when the engine is going through the power stroke. The remaining strokes are not giving any push to the wheel. Then why does not the rider feel the bike slowing down when the engine is not giving any push to the wheel in the remaining strokes? The reason is that the whole process is so quick that the lag is not felt at all by the rider. So how fast is a piston moving or how much time is taken by a piston to complete one full cycle of 4 strokes? I have shown a small calculation to answer that and that is only for those who like to play with numbers but rest who think that “God made man but why man made math?” could just skip it and continue to read further.

The calculation shows how quick the cycle is and also shows that with increase in rpm there is decrease in cycle timing. If you ever want to feel how the bike would behave in power stroke and in rest strokes then recall that moment when your bike used to move on some last drops of fuel. You might have felt the bike moving with jerks. The bike used to accelerate only when those last drops were sucked in by the cylinder but due to fuel that was not enough for a smooth motion of the bike. The bike used to accelerate then stop again it accelerate and stop which kept repeating itself until the last drop was ignited. The same nature of the motion would be felt if you ride a bike with the time slowed down. You would feel the bike accelerating at the power stroke and stops in the remaining strokes. The bike would again accelerate in the next power stroke and the cycle keeps on repeating.

In petrol engine

Imagine a bike with a multi-cylinder engine say for e.g. 2009 BMW S1000RR which has an inline 4 cylinder 4-stroke engine then what impact would it make?

The ignition system has a distributor which is responsible to supply the necessary spark to the spark plugs. So by a general observation it could be said that the number of sparks produced by the distributor in one cycle is equal to the number of the cylinders. Firing order becomes very important in such engines.

Firing order is the sequence in which each cylinder gets sparked one by one. It depends on the number the cylinders which is four in this case. Engine has to be configured and arranged such that each cylinder is at a different instantaneous stroke to give a balance power output. This means that the timing of the spark should be different in each cylinder. Spark happens in power stroke which should be balanced in order to obtain a balanced movement of the crankshaft. This gives an uninterrupted power output. If all cylinders get the spark at the same time then the crankshaft would suffer from enormous amount of stress and would get imbalanced to result in damage. This is why firing order is calculated by the stress acting on the bearing corresponding to the cylinder that was sparked. The number given to each cylinder is done from left to right or front to rear starting from 1 to 4. Numbering is the first step of this system. If 2^nd cylinder is sparked right after the 1^st then it would increase the pressure on the bearing as cylinder 1 and 2 both are adjacent to each other and would induce a lot of vibration in the engine. The next problem faced by the engine would be of back pressure. The exhaust gases when removed out of the cylinder increase the pressure in the exhaust pipe. When both 1^st and 2^nd cylinder gets sparked back to back then they would also remove out exhaust gases in the same sequence. This would double the pressure in the exhaust pipe again due to the close location of both the cylinders. The next and last observed problem would be the enormous amount of heat produced that would heat up the first section of the engine make the cooling system to get active in this region developing load in this section. These all problems make firing order a crucial part in the working principle of an engine. The calculation reveals that instead of 2^nd cylinder if 3^rd cylinder is sparked that these problems would come to the lowest points. When 3^rd cylinder is sparked after 1^st then the stress developed by the spark would spread evenly on the crankshaft and balance it while it operates. The next problem also gets solved when 3^rd cylinder is selected for sparking as the exhaust gas removed from this cylinder would travel more to reduce the pressure in the exhaust pipe. When 3^rd cylinder is sparked subsequently after 1^st cylinder then the heat produced in the process heats up two different sections of the engine which does not affect the balance of the cooling system. So, the order of spark is 1-3-4-2 which is the most commonly used one. The experiment also gave some other orders for a safe and smooth working of the engine which are 1-3-2-4, 1-4-3-2 and 1-2-4-3. Here, the numbers state the specific cylinder in the engine. The design of the crankshaft becomes a crucial thing to set any of the above firing order. Different firing orders are chosen depending on different arrangements of cylinders in different engines. This helps the driver to make the car reach its optimum power and give the best performance output.  These orders help the bike crank out a continuous power output throughout entire range of rpms.

In diesel engine

Diesel engine has no spark plugs to ignite the mixture but instead has a fuel injector which is sprayed when the piston is about to complete the compression stroke. The inlet port only supplies air into the cylinder which gets compressed at the time of compression stroke. The pressure and temperature of the air increases when compressed as the piston reaches TDC. The fuel injector which is responsible for spraying the fuel sprays the fuel in such a way that it atomizes into small droplets later vaporizes and finally gets mixed with the compressed hot air which was already present in the cylinder. The piston on reaching TDC forces this mixture to reach ignition temperature and ultimately the mixture burns. So, here you might have observed that instead of supplying spark to the cylinder, diesel engine has a fuel injector which sprays fuel inside the cylinder so that it ignites on getting mixed with the air. Firing order in a multi-cylinder engine is very important due to similar reasons mentioned in the former section. Firing order does not depend on the fuel on which an engine is running because basically all engines work on the same principle to complete one and only task of bringing a vehicle in motion. The firing order remains the similar to a petrol engine.

In Cylinder Cut-Off

A car that is climbing a slope requires hell of a fuel to produce sufficient amount of power to climb that slope. More fuel is required when the car is carrying huge amount of load. Whereas, the same car would not require the same amount of fuel when it is coasting or cruising on a highway with one or few passengers. All cylinders run in any situation whether the car is climbing a slope or cruising on a level road. This means that the car is using more than enough amount of fuel when all cylinders are running even if fraction of the total power is required in doing so. A new system was introduced in 1981 used by a U.S based automaker Cadillac by the name cylinder cut-off or cylinder deactivation and strictly used in V6 or V8 where the effect could be clearly observed. Some cylinders are deactivated in this technology in order to save fuel. This reduces the size and power of the engine and uses only the amount of fuel that is necessary for the vehicle in that specific situation. This is done by temporary shutdown of the valves on a specific cylinder which then does not allow any further fuel or air to enter into the cylinder. The same cylinder is not sparked as well. Thanks to the computer or ECU present in the car that makes cylinder deactivation function safe and precise for optimum results. ECU detects whether the car is cruising or climbing a slope and also the load carried by the car. The system activates or deactivates on the basis of input data read by the ECU. So a V6 engine after deactivation runs on either 3 or 4 cylinders. The result is less fuel usage and reduction in emissions. The studies show that cylinder deactivation can save fuel and reduce emission by 6-10 percent. The throttle pedal also has to be kept open wider than in the previous case as in the later case; less number of cylinders are in use. This gives the air a much uninterrupted path to enter the cylinder which is not in observed when the system inactive. In that case the throttle is relatively less opened for the air to enter the cylinder and inherit more pumping losses. This system gives best output in a high capacity engine like six or eight cylinder engine or even more than that. Cylinder deactivation cannot be felt by the driver in any situation but could be known by a sign displayed on the instrument cluster. The point that is hidden inside this section is that if a cylinder is deactivated then of course the remaining cylinders must keep on running in order to continue the motion of the car. And as I have mentioned earlier that in a multi-cylinder vehicle, firing order is an important parameter to be considered. So what would be the firing order now? First let us know which cylinders are deactivated.

In an inline-n cylinder engine

(Inline- four)

When the cylinder cut-off is activated then cylinder 2 and 3 are deactivated which makes the car run on only 2 cylinders. So the firing order becomes the basic 1-2 or 2-1. Cut-off could be taken to a next level by deactivating all the cylinders except one to extract the best mileage out of the engine. However by deactivation one thing should be understood that power output is affected to a great extent.

(Inline-six)

When the cylinder cut-off is activated then cylinder 3 and 4 are deactivated to make the engine downsize itself to four. So the firing order becomes similar to a conventional four cylinder engine i.e. 1-3-4-2, 1-3-2-4, 1-4-3-2 and 1-2-4-3.

In a V-n arrangement engine

(V-4)

The diagram shown above is only to let you know how the one side of pistons are angled with the pistons on the opposite side. The pistons on left side move parallel to each other rather than making an angle as shown. The diagram shows that there is one more piston behind the front one. Same is true for the piston on right hand side. The two sets of cylinders make an angle of 45 or 60 or more or less with each in a V type engine arrangement. One set has two cylinders on one side and two on the other side. When the cylinder cut-off is activated, the two cylinders on one side are deactivated. This means that this set of cylinders would not get any spark. The pistons which all remain in action are painted with green in the diagram and red painted pistons have been deactivated by ECU. The firing order occurs in only two possible ways i.e. 1-2, 2-1.

(V-6)

The two sets of cylinders make some angle with each other. One set contains 3 cylinders and the other set also contains same number of cylinders. The same technique is used when cylinder cut-off is activated. One full set of cylinders is deactivated making only the remaining set in operation. The set in operation is painted with green and indicated by a tick mark. The set adapts the firing order of a three cylinder engine i.e. 1-3-2. The choice of which cylinders are to be cut-off completely depends on the type and arrangement of the engine. Some cars have engine assembled longitudinally or transversely which is considered by the developers in installing and setting the cut-off system. However, driver sometimes feel the vibration developed after cylinder deactivation because the forces acting on the cylinder are reduced and limited on only the active cylinders. Some brands fit the engine on improved mountings which absorb unbalanced forces produced by the engine in cylinder cut-off situation and reduce the vibration from travelling across the whole body of the car. Well if there is any other sequence of firing order that I have missed then do let me know in the comment section below. Thank you!!   

  by AutoVogue