The engine firing order is an important topic to discuss. In this article, I will discuss the engine firing order according to the engine cylinder.
Engine Firing Order
Firing order is the sequence in which the power impulses occur in an engine. When the cylinders are in line, the cylinder nearest to the radiator is designated as number 1. The one directly behind it is number two and so on. In a V-type engine, the numbering of cylinders is not uniform.
Though generally, an automotive engine has four or more cylinders to understand the engine balancing properly knowledge of balancing one and two cylinders engine is important.
In the case of a single-cylinder engine, there is one power stroke for every two revolutions of the crankshaft, and thus in spite of having a large flywheel, it may not run smoothly and quietly. Due to the relatively large cylinder size and large time between power strokes, the parts must be made large and heavy to withstand the resultant rough operation.
If there are two, four, six, or eight cylinders in the engine there will be 2, 4, 6, or 8 power strokes for two revolutions of the crankshaft meaning thereby uniform application of torque and thus smoother operation.
In a four-cylinder engine, there is no time period when a cylinder is not delivering power during the cycle. In an engine with more than four cylinders, there will be a time period during which more than one cylinder is delivering the power simultaneously.
If the power strokes are spaced equally more cylinders mean less vibration and less work has to be done by the flywheel to store and release the energy. Hence flywheel could be lighter in the case of engines with a higher number of cylinders. For the same power require the number of cylinders can be increased by making their sizes smaller.
Engine Firing Order of Different Cylinders
In the arrangements of the engine cylinder, there are various types of engine cylinders are available. In the engine firing order system, the firing of the spark plug depends on the distributor or the ignition coil.
In an engine, the firing order depends on the crankshaft position. This crankshaft position helps to tell us which piston is at TDC or which is in BDC.
According to the engine cylinder arrangement the engine firing order is set. The types are –
- One-cylinder engines
- Two-cylinder engines.
- Four-cylinder engines.
- Six-cylinder engines.
- Eight-cylinder engines.
- V-type eight-cylinder engines.
I will discuss each of these cylinder engine firing orders below.
One Cylinder Engines
Here because of one power stroke during two crankshaft revolutions, there are uneven distributions of power. Since there is one piston and connecting rod that reciprocates with no working parts to counterbalance their weight, a single-cylinder engine can not be balanced mechanically. This diagram represents a single-cylinder engine with a chart showing the occurrence of different processes in the cycle.
Balancing up to some extent is possible by using counterweights attached to the crankshaft and also by using a flywheel heavy enough to produce momentum. Fluctuations in the speed of the spark will cause vibrations, even in the best-designed single-cylinder engines, making these undesirable for use in motor vehicles.
This diagram shows the two-cylinder vertical engine with a 180° crankshaft. Here, the pistons move in opposite directions. Hence the engine is well balanced as far as primary inertia forces are concerned.
The power balance is shown in the chart. In (A) both power strokes occur in 1st revolution of the crankshaft while there are no power strokes during the second revolution.
In (B) power stroke occurs·at the beginning of the 1st revolution and then at the end of the 2nd revolution. In both cases, there is an irregular production of power which sets up vibrations and causes the engine to run unevenly. A two-cylinder engine has a better balance if the cylinders are horizontal and arranged on the opposite sides of the crankshaft.
This diagram represents a four-cylinder engine with piston positions and charts displaying the processes occurring in different cylinders during the two revolutions of the cycle.
In a cylinder engine, the pistons are positioned at opposite ends. If in cylinder number 1 and 4 the pistons are positioned at the top dead center in cylinder number 2 and 3 these are at the bottom dead center. The pistons move in opposite directions.
These will move downwards in cylinders number 1 and 4 and upwards in cylinders number 2 and 3. This arrangement tends to neutralize primary inertia forces because piston subassemblies, i.e., piston, piston pin, and small end of the connecting rod, are of the same weight. This provides good primary mechanical balance.
As far as secondary inertia forces are concerned in a four-cylinder engine, these are not balanced. An example given below will demonstrate it. The pistons in cylinders numbers 1 and 4 are at the top dead center (T.D.C.) and the pistons in cylinders numbers 2 and 3 are at the bottom dead center (B.D.C. ).
If the stroke is 100 mm and connecting rod is 200 mm long, two pistons (2 and 3) are 150 mm above the crankshaft center and two pistons (1 and 4) are 250 mm above, the crankshaft centre. In view on the right-hand side, when crank arms are horizontal with all four pistons near (but not exactly at), the middle of the stroke. The connecting rod forms a hypotenuse of 200 mm and a horizontal side of 50 mm in the right-angle triangle. Computing the third side, it comes out to 193 mm.
Hence the center of gravity of the body comprising of 4 pistons, piston pins, and smaller ends of connecting rods is about 7 mm lower when the crank becomes horizontal from the verticle. The total weight of these parts (approximately 6 kg) must be moved up 7 mm and down 7 mm during each 112 crankshaft revolution. The vibration has a frequency twice the engine speed. Hence a four-cylinder engine is subjected to undesirable vibration. In six or eight-cylinder inline engines, there is no such problem.
Charts show various firing orders for power balance. With either arrangement, power strokes are evenly distributed. Table A gives firing order 1-3-4-2 while Table B gives firing order 1-2-4-3. American four-cylinder cars have adopted 1-3-4-2 as the standard firing order.
Six cylinders inline engines are built with 3 pairs of crankpins 120° apart as shown in Fig. The arrangement is such that crank throws of numbers 1 and 6, 2 and 5 and 3 and 4 are in the same radial plane. The angle between the planes is 120°.
Hence there are six power strokes during two revolutions (i.e., 720°) of crankshaft or one power stroke every 120° of crankshaft rotation. Because of this, the engine with proper firing order has primary as well as secondary inertia forces perfectly balanced.
Viewing from the front away from the flywheel, crankpins 3 and 4 is to the left of 1 and 6, and as 1 and 6 start to move downward 2 and 5 are completing downward strokes. Crankpins 3 and 4 are on their upward stroke. With this arrangement, four different firing orders with good engine balance are possible. However, 1-4-2-6-3-5 is the firing order used with this arrangement.
In another arrangement, crankpins 3 and 4 are to the right of 1 and 6 when viewed from the front [Fig. 4.9 (b)]. Which is standard for all American in-line 6-cylinder passenger cars. Here 3 and 4 are finishing their downward stroke; 1 and 6 are starting downwards and 2 and 5 are on their upward stroke. Although four different firing orders are possible, the standard one is 1-5-3-6-2-4 which.is adopted.
The V-6 engines have a firing order 1-6-5-4-3-2 where the left bank is numbered 1, 3, 5, and the right bank is numbered 2, 4, 6 from the front of the engine. The passenger car has a 90° angle between banks whereas in heavy vehicles it is 60°. The opposed 6-cylinder engine (with 180° between the banks) uses six-throw crankshaft. The firing order 1-4-5-2-3-6 is adopted where cylinders 1, 3, and 5 are on the right bank and 2, 4, and 6, are on the left bank.
These are not popular these days as automobiles have engines with relatively less power sufficient to run them. An eight-cylinder engine may be used for large power output, if needed, in an automobile. These engines can be V-type or in-line. The details of both these types are as follows:
V-types eight-cylinder engines
This type of eight-cylinder engine has two banks known as left and right banks (Fig. 4.10). Each bank has four cylinders numbered 1, 2, 3, and 4. It is a combination of two 4-cylinder engines operating from a single crankshaft. The cylinders are so arranged that their center lines make an angle of 90° forming a V with crankshaft at the junction point.
As shown the connecting rods for the cylinder on the right-hand side operate on the same crankpins as corresponding connecting rods on the left. These connecting rods operate independently of each other.
The operation of the cylinder shown on the right is always ahead of the cylinders on the left by the angle equal to the angle of V. Since the V angle is 90 when piston cylinder number 1 on the left is at the top dead center.
The piston number 1 on the right has completed half-stroke downwards. Although primary inertia forces are balanced in V-8 engines, the secondary forces are out of balance and induced, thereby horizontal vibration which is neutralized by using friction dampers generally. Various patterns of designating the cylinders are used. American V-8 passenger car engines employ three patterns of numbering.
A – General Motors and Chrysler Products adopt this pattern. The standard firing order adopted is 1-8-4-3-6-5-7-2. Another manufacturer olds mobile also adopts the same pattern with standard firing order as 1-8-7-3-6-5-4-2.
B – Ford, Mercury, or Lincoln V-8 adopt this pattern. The firing order which has been adopted as standard is 1-5-4-8-6-3-7-2. Alternatively, another firing order 1-5-4-2-6-3- 7-8 has also been adopted as standard.
C – Buick has adopted this pattern of designating the cylinders. Firing order 1-2-7-8-4- 5-6-3 has been adopted as standard in this case.
Straight-eight engines with cylinders in line use a crankshaft with throws set at 90° from each other. The crank pins for cylinders 1, 8; 2, 7; 3, 6; and 4, 5 are in the same radial plane. There are two possible arrangements. 1, 8 may follow 3, 6 as shown in (a) or 4, 5 as shown in.
In all the 8-cylinder engines there is a power stroke for every 90° of crankshaft movement. The standard firing order in American straight eight is 1-6-2-5-8-3-7-4. These engines are more compact in width than V-8’s and their pistons do not bear on one side of the cylinder walls due to angularity of the cylinder. An 8-cylinder engine is inherently a perfectly balanced engine, both the primary and inertia forces are balanced.
In this article, I have discussed the engine firing order and other types of cylinders according to the arrangement of the spark plug.