What is an Engine firing order?
The firing order of an internal combustion engine is the sequence of ignition for the cylinders. In a spark-ignition (e.g. gasoline/petrol) engine, the firing order corresponds to the order in which the spark plugs are operated. Firing order affects the vibration, sound, and evenness of power output from the engine.
In engines, cylinders don’t fire in the sequence of 1-2-3-4-5-6 and so on as it could cause the crankshaft to deform or break. The order or sequence in which the engine cylinders fire or generate & deliver power is called the engine firing order.
Related: What is an Internal Combustion engine?
The firing order heavily influences crankshaft design. In a Diesel engine, the firing order corresponds to the order in which fuel is injected into each cylinder. Four-stroke engines must also time the valve openings relative to the firing order, as the valves do not open and close on every stroke.
Related: What is Crankshaft?
Common firing orders
Common firing orders are listed below. For V engines and flat engines, the numbering system is L1 for the front cylinder of the left bank, R1 for the front cylinder of the right bank, etc.
- In two-cylinder engines, the cylinders can either fire simultaneously (such as in a flat-twin engine) or one after the other (such as in a straight-twin engine).
- In straight-three engines, there is no effective difference between the possible firing orders of 1-2-3 and 1-3-2.
- Straight-four engines typically use a firing order of 1-3-4-2, however some British engines used a firing order of 1-2-4-3.
- Flat-four engines typically use a firing order of R1-R2-L1-L2.
- Straight-five engines typically use a firing order of 1-2-4-5-3, in order to minimise the primary vibration from the rocking couple.
- Straight-six engines typically use a firing order of 1-5-3-6-2-4, which results in perfect primary and secondary balance.
- V6 engines with an angle of 90 degrees between the cylinder banks have used a firing orders of R1-L2-R2-L3-L1-R3 or R1-L3-R3-L2-R2-L1. Several V6 engines with an angle of 60 degrees have used a firing order of R1-L1-R2-L2-R3-L3.
- Flat-six engines have used firing orders of R1-L2-R3-L1-R2-L3 or R1-L3-R2-L1-R3-L2.
- V8 engines use various different firing orders, even using different firing orders between engines from the same manufacturer.
- V10 engines used firing orders of either R1-L5-R5-L2-R2-L3-R3-L4-R4-L1 or R1-L1-R5-L5-R2-L2-R3-L3-R4-L4.
- V12 engines use various different firing orders.
In a radial engine, there is always an odd number of cylinders in each bank, as this allows for a constant alternate cylinder firing order: for example, with a single bank of 7 cylinders, the order would be 1-3-5-7-2-4-6.
Moreover, unless there is an odd number of cylinders, the ring cam around the nose of the engine would be unable to provide the inlet valve open – exhaust valve open sequence required by the four-stroke cycle.
How to determine the firing order of the engine?
Firing Order is Determined by the Number of Cylinders contained within that engine & Crankshaft Alignment/Offset of each Crank-Journal, during the Design/Manufacturing Process.
The firing order is determined when the engine is DESIGNED, so as to make it run as efficiently and as smoothly as possible. The forces and loads exerted by pistons on the crankshaft are calculated. The required counterweight is calculated. Plugging all these into the dynamic balancing equations, the firing order is determined such that, minimal vibrations are produced.
Designing Parameters of Firing Order:
- Number of Cylinders,
- Torsional vibrations,
- Heat distribution,
- arrangement of cylinders,
- Crankshaft Alignment/Offset of each
Why The Firing Order Is Important
The correct firing order is very important because mixing up the spark plug wires may prevent the engine from starting, cause it to backfire, and run very poorly if at all.
NOTE: On engines where two adjacent spark plugs fire right after each other, it is important to make sure the spark plug wires are not routed right next to each other for a long distance.
Related: What are Spark plugs?
This can cause a crossfire between the plugs because the magnetic field created by the spark going to one plug may fire the next plug prematurely, causing the engine to run rough and misfire. To prevent this from happening, crisscross the two plug adjacent plug wires to cancel out the magnetic induction.
On engines with distributor-less ignition systems or coil-on-plug ignition systems, the firing order is controlled by the ignition module or engine computer.
The computer receives an input signal from the crankshaft position sensor (and camshaft position sensor on some engines) to determine which piston is coming up to the top dead center on its compression stroke. It then fires that spark plug, and the next, and so on in the firing sequence.