Firing order, as the name suggests, is the order in which ignition for the cylinders take place. Firing order helps in regulating heat dissipation and vibrations. It also impacts smoothness in driving, engine balance and sound.
Generally firing order of 4 cylinder engine engines are kept as 1-3-4-2, 1-3-2-4 and 1-2-4-3. These sequences are designed using few simple equations that are discussed below. This article explains about the firing order by taking an example of four stroke four cylinder engine and discusses about various types of 4 cylinder engines as well as naming of engine cylinders.
Working of 4 stroke engine
A four stroke or four cylinder engine achieves one power cycle after every four strokes of piston. A stroke is completed when piston travels from top dead center to bottom dead center or vice versa.
A four stroke engine has following stages-
- Intake- It is also known as suction stroke. Air fuel mixture enters the cylinder during this stroke. The piston is at the top dead center initially and moves towards bottom dead center.
- Compression- Air fuel mixture that has entered the cylinder is compressed in this stroke. The piston is at the bottom dead center and moves towards top dead center.
- Combustion- This is also called ignition stroke. Second revolution of crank begins during this stroke. Fuel is ignited by a spark. The piston moves towards bottom dead center.
- Exhaust- The waste is spilled out of the cylinder through the exhaust valve in exhaust stroke. The piston returns back to top dead center.
Four cylinder engine four stroke engine
In a four cylinder four stroke engine, the cylinders work on four stroke cycle and has total four cylinders that perform each stage of cycle independently.
When first cylinder is in its suction stroke, second cylinder might be in exhaust stroke, third cylinder in ignition stroke and fourth cylinder in compression stroke. This way power is transmitted continuously in a four cylinder engine.
Arrangement of cylinders in 4 cylinder engine
There are many ways in which cylinders are arranged and numbered. Arrangement is important for engine sizing and numbering is important for finding the firing order.
The different types of arrangements in a four cylinder engine are as follows-
- Straight engine- Cylinders are placed in a single line and are numbered from #1 from front to rear.
- V engines- In this type of arrangement, engines are placed in an inclined position such that they make a V letter between them. Each cylinder is placed on the opposite of previous cylinder. Numbering is done from front to rear starting from #1.
Image credits- Wikipedia
Image credits- Wikipedia
How to determine firing order of four cylinder engine
Firing order of 4 cylinder engines is found by following a simple procedure. The parameters that are kept in mind while deciding the firing order are dampening of vibrations, low stresses on bearings and proper heat dissipation from the cylinders.
Following are the methods by which firing order is determined-
- Balancing- Balancing the primary forces, secondary forces and the moments is the most accurate way to find the firing order. This ensures that the there will be less heat dissipating problems and low vibrations.
Primary forces are found using following equation-
Secondary forces are found using following equation-
The crank angle is found using the relation-
Where n means number of cylinders.
For four cylinder engine, n=4
Crank angle represents the angle by which the crank has to rotate in order to fire one cylinder. So, in a four cylinder engine one cylinder fires after every 180 degrees rotation of crank.
For balancing, the conditions are that algebraic sum of all horizontal and vertical forces should be zero and sum of all moments should be zero. This means that the force polygon (for both primary and secondary forces) and couple polygon should form a closed figure.
By following this approach, common firing orders obtained are- 1-3-4-2 and 1-3-2-4.
- Approximating- It is clear that firing adjacent cylinders simultaneously will have heating problems and the force exerted on bearings will be more hence producing high vibrations. So we need to fire alternate cylinders which leaves us at firing orders of 1-3-4-2 which is most commonly used in four cylinder engines.
If one uses firing order as 1-2-3-4, then by using the method of balancing, it can be found that only primary and secondary forces are balanced but moments are not balanced i.e. couple polygon does not form a closed figure.
It is quite obvious that firing 2nd cylinder right after 1st cylinder will create heating problems and have more vibrations.
Meaning of 1-3-4-2
Firing order 1-3-4-2 depicts the sequence in which cylinders are fired. Spark takes place in first cylinder followed by third, fourth and second cylinder.
When the first cylinder is fired, third cylinder gets ready to be fired that means it will be in its compression stroke. In next 180 degrees of crankshaft rotation (crank angle 360 degrees) the third cylinder enters the power stroke. Meanwhile second cylinder is in the intake stroke and fourth cylinder gets ready for firing stroke.
In next 180 degrees rotation (crank angle 540 degrees), the fourth cylinders enters power stroke and second cylinder performs compression stroke. First cylinder is in its intake stroke and third cylinder in exhaust stroke.
In next 180 degrees rotation (crank angle 720 degrees), the second cylinders performs power stroke, fourth engine is in its exhaust stroke, third cylinder in intake stroke and first cylinder in compression stroke.
After completing 720 degrees of crank rotation, one power cycle is said to be completed.