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.
The numbering system for cylinders is generally based on the cylinder numbers increasing from the front to the rear of an engine (See engine orientation below). However, there are differences between manufacturers in how this is applied; some commonly used systems are as listed below.
Cylinders numbered from front (#1 cylinder) to rear.
The frontmost cylinder is usually #1, however there are two common approaches:
The selection of whether the #1 cylinder is on the left bank or right bank usually depends on which bank is closer to the front of the crankshaft. However, the Ford Flathead V8 and Pontiac V8 engine actually have the #1 cylinder behind the cylinder from the opposite bank. This was done so that all Ford engines would have cylinder #1 on the right bank and all Pontiac engines would have cylinder #1 on the left bank, to simplify the process of identifying the cylinders.
The cylinders are numbered around the circle, in clockwise direction with the #1 cylinder at the top.
The simplest situation is a longitudinal engine located at the front of the car, which means the engine's orientation is the same as the car's. This illustrates that the rear of the engine is the end that connects to the transmission, while the front end often has the drive belt for accessories (such as the alternator and water pump). The left bank of the engine is on the left side of the car (when looking from behind the car), and vice versa for the right bank of the engine.
For a transverse engine located at the front of the car, whether the front of the engine is at the left-hand or right-hand side of the car is best determined based on the side of the car where the transmission is located (which corresponds to the rear of the engine). Most transverse engine front-wheel drive models have the front of the engine at the right-hand side of the car (except for many Honda cars). As a consequence, the left bank of a transversely V engine is usually closest to the front of the car.
For cars where the engine is installed 'backwards' (i.e. the transmission is closer to the front of the car than the engine), cylinder #1 is located towards the rear of the car. This is the case for the Citroën Traction Avant and many rear-engine cars.
Contrary to most car engines, a ship's engines are often numbered starting from the end of the engine with the power output. Large diesel truck and locomotive engines, particularly of European manufacture, may also be numbered this way.
Cylinders on V engines often include a letter representing the cylinder bank. For example, a V6 engine could have cylinders A1-A2-A3-B1-B2-B3, with cylinders A1 and B1 located at the power output end of the engine.
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 a radial engine, there are 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.
To minimise vibrations, most engines use an evenly spaced firing interval. This means that the timing of the power stroke is evenly spaced between cylinders. For a four-stroke engine, this requires a firing interval of 720° divided by the number of cylinders, for example a six-cylinder engine would have a firing interval of 120°. On the other hand, a six-cylinder engine with an uneven firing interval could have intervals of 90° and 150°.
Engines with an even firing interval will sound smoother, have less vibration and provide more even pressure pulses in the exhaust gas to the turbocharger. Engines with an uneven firing interval usually have a burble or a throaty, growling engine sound and more vibrations.
The main application of uneven firing intervals is motorcycle engines, such as big-bang firing order engines. Examples of odd-firing engines are most four-stroke V-twin engines, 1961-1977 Buick V6 engine, 1985-present Yamaha VMAX, 1986–present Honda VFR 750/800, 1992-2017 Dodge Viper V10, 2008-present Audi/Lamborghini 5.2 V10 40v FSI, Aston Martin 5.9 V12 1999-2018 and the 2009-2020 Yamaha R1 (inline four engine with a crossplane crankshaft).
The following table is a running list of firing orders by application: (Please expand on the list if you see something missing.)
|Number of Cylinders||Firing Order||Application|
|1||1||Curved Dash Oldsmobile|
|2||1-2||Buick Model B, C, F, G, 14, James and Browne|
|3||1-2-3||Saab two-stroke, Perodua Kancil engine|
|1-3-2||BMW K75 engine, Subaru Justy engine|
|4||1-3-4-2||Most straight-4s, Ford Taunus V4 engine|
|1-2-4-3||Some British Ford and Riley engines, Ford Kent engine, Riley Nine|
|1-3-2-4||Subaru 4-cylinder engines, Yamaha R1 crossplane|
|1-4-3-2||Volkswagen air-cooled engine|
|5||1-2-4-5-3||Straight-five engine, Volvo 850, Audi 100|
|1-3-5-4-2||GM Atlas engine|
|6||1-5-3-6-2-4||AMC straight-6 engine, Chrysler Slant-6 engine, Mercedes-Benz M104 engine, Maserati 3500 GT I6, Volkswagen VR6 engine, Opel Omega A, Nissan L Engine|
|1-4-3-6-2-5||Mercedes-Benz M272 engine, Volkswagen V6's (both engines are 90-degree V6's)|
|1-6-5-4-3-2||GM 3800 engine, Rover KV6 engine|
|1-2-3-4-5-6||General Motors 60° V6 engine, Mazda JE 3.0 litre 60-degree V6 engine, Chrysler Pentastar engine|
|1-4-2-5-3-6||Ford Cologne V6 engine, Ford Essex V6 engine (UK)|
|1-6-3-2-5-4||Subaru Alcyone/XT-6/Vortex ER-27 Flat-6|
|1-6-2-4-3-5||Porsche Boxster Flat-6|
|1-6-2-5-3-4||Maserati Quattroporte IV V6-4AC-24|
|1-4-2-6-3-5||Toyota HZ engine|
|7||1-3-5-7-2-4-6||7-cylinder single row radial engine|
|8||1-8-7-3-6-5-4-2||Nissan VK engine|
|1-8-7-2-6-5-4-3||GM LS engine|
|1-8-4-3-6-5-7-2||Chrysler and Dodge V8s, Chevrolet Small-Block engine, Pontiac, Rover V8, Toyota UZ engine|
|1-5-6-3-4-2-7-8||Cadillac 368, 425, 472, and 500 V8 engines|
|1-5-4-8-6-3-7-2||Ford Modular 5.0, Ford Flathead, Bentley L410 V8 (from 1959 to 1986)|
|1-5-4-8-3-7-2-6||5.2 L "Voodoo" V8|
|1-5-4-2-6-3-7-8||Ford Small Block 221/255/260/289/302, FE Series 352/360/390/406/410/427/428, Ford 385 series Big Block 429/460|
|1-5-3-7-4-8-2-6||Ferrari V8's, (all are flat-plane crank)|
|1-3-7-2-6-5-4-8||Porsche 928, Ford Modular V-8, 351 Windsor, 5.0 H.O., 335 series (351C/351M/400M), Bentley L410 V8 (from 1987 to present)|
|1-3-6-8-4-2-7-5||Alfa Romeo 8C|
|1-2-7-3-4-5-6-8||Cadillac Northstar Engine|
|9||1-3-5-7-9-2-4-6-8||9-cylinder single row radial engine Lycoming R-680-13 engine]|
|10||1-10-9-4-3-6-5-8-7-2||Dodge Viper V10|
|1-6-5-10-2-7-3-8-4-9||BMW S85, Ford V10|
|12||1-7-5-11-3-9-6-12-2-8-4-10||2001 Ferrari 456M GT V12|
|1-7-4-10-2-8-6-12-3-9-5-11||1997 Lamborghini Diablo VT|
|1-4-9-8-5-2-11-10-3-6-7-12||Caterpillar 3412, Toyota GZ engine|
|1-12-5-8-3-10-6-7-2-11-4-9||Audi VW Bentley W12 engine|
|1-12-4-9-2-11-6-7-3-10-5-8||2011 Lamborghini Aventador|
|14||1L-1R-2L-2R-4L-4R-6L-6R-7L-7R-5L-5R-3L-3R||(Wärtsilä)-Sulzer 14ZV40/48 V14 marine diesel|
|16||1-12-8-11-7-14-5-16-4-15-3-10-6-9-2-13||2003 Cadillac V16 engine|