What is 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. 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.
Firing order affects the vibration, sound, and evenness of power output from the engine. The firing order heavily influences crankshaft design.
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.