Firing order - Definition

The firing order is the sequence of sparking of the spark plugs in a reciprocating engine, or the sequence of fuel injection in each cylinder in a Diesel engine.

In a straight engine the spark plugs (and cylinders) are numbered, starting with #1, from the front of the engine to the rear. In most cars the front of the engine also points to the front of the car, but some manufacturers (Saab, Citroën) in some models place the engine 'backwards', with #1 towards the firewall.

In a V engine the right bank is numbered first, followed by the left bank.

The numbers are usually cast on the cylinder head or the intake manifold or the valve cover(s).

In a conventional engine, the correct firing order is obtained by the correct placement of the spark plug wires on the distributor. In a modern engine with an engine management system and direct ignition, the Electronic Control Unit (ECU) takes care of the correct firing sequence.

These are some usual firing orders:

See also

• two stroke cycle
• four stroke cycle