The cycle of operations of four-stroke engines is completed in two turns of the crankshaft. With such an operating cycle, the crankshaft receives energy from the piston only during one half its turn when the piston moves on the power stroke. During the remaining three half turns, the crankshaft continues to revolve by inertia and, aided by the flywheel, it moves the piston on all its supplementary strokes - exhaust, intake, and compression. Therefore, the crankshaft of a single-cylinder engine operating on the four-stroke principle revolves nonuniformly: it accelerates on the power stroke and decelerates on the supplementary strokes of the piston. Furthermore, the single-cylinder engine usually produces little power and features excessive vibration. For this reason, modern tractors and automobiles are powered by multiple-cylinder engines.
For a multicylinder engine to run uniformly, the power strokes of its pistons must be spaced rotationally at one and the same crank angle (i.e., they must occur at regular intervals, called the firing intervals). To find this angle, the duration of the engine cycle, expressed in degrees of crankshaft rotation, is divided by the number of the engine cylinders. For example, in a four-cylinder four-stroke engine, the power stroke occurs every 180° (720°:4), i.e., every half turn of the crankshaft. The other strokes in this engine occur also every 180°. Therefore, the crankshaft throws (or crankthrows) of four-cylinder four-stroke engines are spaced at 180°, i.e., they lie in a single plane. The crankthrows of the first and fourth cylinders are arranged on one side of the crankshaft, and those of the second and third cylinders, on the opposite side. Such a shape of the crankshaft provides for even firing intervals and a good engine balance, since all the pistons simultaneously reach their extreme positions (two pistons reach their TDC at the same time as the other two reach BDC).
Fig.5 (a) Schematic diagram and (b) firing-order diagram of a four-cylinder four-stroke engine
The order in which like piston strokes occur in the engine cylinders is known as the firing order. The firing order of the four-cylinder tractor engines produced in this country is 1-3-4-2. This means that after the piston in the first cylinder has completed its power stroke, the next power stroke occurs in the third cylinder, then in the fourth cylinder, and finally, in the second cylinder (Fig. 5).
When selecting a firing order for a particular engine, the designers try to distribute the load on the crankshaft as uniformly as possible.
Multicylinder engines used on automobiles and tractors may have an in-line or a two-bank (V-type) cylinder arrangement. In an in-line cylinder engine (Fig. 6a), all the cylinders are arranged vertically in a straight line, while in a two-bank engine, the cylinders are arranged in two banks set at an angle to each other (Fig. 6b). V-type engines are more compact and less heavy than their in-line cylinder counterparts.
Fig. 6. Multicylinder engines
(a) in-line cylinder arrangement; arrangements in V-6 and V-8
type engines, respectively;
(b) V-type cylinder arrangement; 1 through 8 – serial numbers
(c) and (d) crankthrow of cylinders
In a six-cylinder four-stroke engine, like piston strokes occur at 120-degree intervals. Therefore, its crankthrows are spaced in pairs in three planes with an angle of 120° between them (Fig. 6c). In an eight-cylinder four-stroke engine, like piston strokes occur every 90°, and so the crankthrows are arranged crosswise with an angle of 90° between them (Fig. 6d). With an eight-cylinder four-stroke engine, eight power strokes occur for every two revolutions of the crankshaft, which makes for very smooth running of the engine. Modern six- and eight-cylinder automotive engines use V-type cylinder arrangements. The firing order of eight-cylinder four-stroke engines is 1-5-4-2-6-3-7-8 and that of six-cylinder ones, 1-4-2-5-3-6.
Knowing the firing order of an engine, one can correctly connect the ignition wires to the spark plugs, or the fuel pipes to the fuel injectors, and adjust the valves.