Cylinder Layout & Firing Order

Over the years, there have been several engine designs including different layouts for the cylinders with the most common being of the in-line variety. However the cylinders may also be opposed either in a Vee, W or horizontally.

The pistons of any engine must have their power strokes in succession, this is referred to as the firing order of the engine, which is determined by two main factors including the crankshaft design, which will determine all the possible firing orders and the webs, which are designed in such a way as to provide the best possible balance and to ensure that regular firing strokes occur. The cams on the camshaft must also be arranged in such a way as to adhere to one of the possible firing orders.

Power strokes in an in-line four cylinder engine occur at 180° intervals and the pistons move in pairs with one and four forming one pair and two and three forming the other. For instance,in a four cylinder in-line engine with a firing order of 1,3,4,2, if piston two is on induction stroke, piston three will be on its power stroke, and pistons one and four on their compression and exhaust strokes respectively.

The power strokes of an engine must be spaced at uniform intervals with every interval being equal to the number of degrees per engine cycle divided by the number of cylinders. For example, the calculation used to determine the number of degrees between strokes on a four cylinder is as follows;

Number of degrees per engine cycle/number of cylinders

(720°/4) = 180°

On an in-line four-cylinder engine, the two possible firing orders are 1,3,4,2 or 1,2,4,3 as found on some engines. If the firing order were to be 1,2,3,4, the crankshaft and engine mountings would be subject to such high levels of vibration and stress that it would be unbearable to the vehicle occupants and the engine components would very soon wear out under such high levels of fatigue.
The firing order can be found in workshop manuals and various manufacturers literature. It may also be marked in a prominent place on the engine itself. This my also be defined by turning the engine in its normal direction of rotation with the rocker/camshaft cover removed and watching the order of which either the inlet or exhaust valves operate or to note the order by which the cylinders create pressure on compression stroke by means of turning the engine with the spark plug holes either covered by thumb or finger or appropriately plugged. This test can also be performed using a cylinder pressure gauge.

The cylinders of a straight engine are formed in a straight line parallel to one another and may be opposed either vertically  or slanted at an angle such as those found in early Saabs, the Hillman Imp and certain Triumph models. Owing to the reasonably spaced power strokes, this type of engine is relatively smooth in operation and any vibration or harshness is largely unnoticed by the vehicle occupants as rubber engine mountings damp out much of this.

In the most common cylinder layout found in the motor vehicle, the aforementioned in-line, the number of cylinders varies from design to design with four being the most popular however many manufacturers are now moving to three-cylinder engines for reasons of reduced emissions and compaction. In the past however, some designs have used as little as two cylinders and as many as eight as the straight eight engine was much cheaper to manufacture than its V8 counterpart. But this design was superseded many years ago. Other numbers in common use are five and six cylinders but again, the straight six engine has largely given way to the V6 although it is still in use by certain car makers.