Cylinder arrangements and firing orders

D. Newbold, A. Bonnick, “Automobiles”, N3, 2012

 

There are three arrangements which may be used for an engine.

In-line engine: The cylinders are arranged in a single row, one behind the other. They may be vertical, as in most modern light vehicles, horizontal as used in coaches where the engine is positioned under the floor, or inclined at an angle to allow for a lower bonnet line.

Vee engine: The cylinders are arranged in two rows at an angle to one another. The angle for two-, four- and eight-cylinder engines is usually 90°. For six- and twelve-cylinder engines the angle is usually 60°.

 Opposed piston or cylinder engine: This is where the cylinders are at an angle of 180° apart and usually positioned horizontally.

Firing orders

When considering multi-cylinder engines and firing orders, the power strokes should be spaced at equal intervals to give the smoothest possible running of the engine. Each interval is equal to the number of degrees per cycle of operation. This will be 720° for a four-stroke engine. This is then divided by the number of cylinders, e.g. 720=4 x180 °.

Therefore the firing interval for a four cylinder in-line engine will be 180° and that for a six-cylinder in-line engine will be 720=6 x 120°. The firing order is determined by two things.

1. The position of the cylinders and the cranks on the crankshaft (this determines the possible firing orders).

2. The arrangement of the cams on the camshaft (this must be in accordance with one of the possible firing orders).

The arrangements on the crankshaft are such that the pistons on a four-cylinder in-line engine are moved in pairs, e.g. numbers 1 and 4 form one pair and 2 and 3 form the other pair. This means that when number 1 is moving down, on its power stroke, number 4 will also be moving down, but on its induction stroke. Depending on the firing order, when number 2 piston moves upwards it will either be on its exhaust or compression stroke, number 3 will be on its compression or exhaust stroke. From this then we can see that there are two possible firing orders for a four-cylinder in-line engine. These are 1342 or 1243, both of which are in common use today.

The reasons for using more than one cylinder are very complex but in simple terms they are as follows.

A multi-cylinder engine has a higher power­-to-weight ratio than a single-cylinder engine. With multi-cylinder engines there are more power strokes for the same number of engine revolutions. This gives fewer fluctuations in torque and a smoother power output.

A better acceleration is achieved due to smaller moving parts and more firing impulses.

The crankshaft is balanced better; the crankshaft of a single-cylinder engine cannot be perfectly balanced. Very good balance is obtained with six or more cylinders.

The piston crown cannot be adequately cooled on large single-cylinder engines; as the piston gets larger the centre of the crown becomes more difficult to keep cool.

Вариант 3

Combustion in a petrol engine

D. Newbold, A. Bonnick, “Automobiles”, N3, 2012

 

As the piston reaches TDC the fuel is ignited by the spark at the spark plug and the burning process of the mixture begins. As the gases rapidly expand the piston is forced down the cylinder on the power stroke. The speed of the flame front must not exceed the speed of the power stoke.

Combustion in a petrol engine originates (begins) at the spark plug and then progresses across the combustion chamber in a controlled manner. In the case of the diesel engine, combustion of the fuel is initiated (started) by the heat of the air in the chamber. As the droplets of fuel pass through the air they absorb the heat, and, if the temperature is high enough, the fuel will vaporize and ignite. Wide distribution of the fuel during the heating phase means that the burning process (combustion) starts at many different points in the chamber.

In direct injection systems, once ignition has started, most of the burning will tend to concentrate in zones fairly close to the injector. These zones must be fed with air in order to sweep away the burnt gases and supply the oxygen necessary for complete combustion. Any lack of oxygen in the combustion region will lead to black smoke in the exhaust.

A common and essential objective of all CI or diesel engine combustion systems is to achieve the maximum degree of mixing of the fuel, in the form of very fine droplets, with the air. This happens during injection of the fuel into the combustion chamber.

There are two main methods of introducing fuel into the combustion chamber: direct and indirect injection. In the ‘direct injection’ system the fuel is injected directly into the combustion chamber which is formed in the piston crown. The air is made to rotate in this cavity at 90° to the incoming swirl by the squeezing out of the air from between the cylinder head face and the piston crown as the piston approaches the end of its compression stroke. This rapid movement of the air is called turbulence.

Because of the low surface area/volume ratio giving low heat losses, a characteristic of the system is a considerable saving in fuel giving good fuel consumption results. A disadvantage is a reduction in volumetric efficiency, due to the necessity of giving the incoming air the swirling movement as it passes into the cylinder. This effect can be largely overcome by the use of the ‘indirect injection’ system.

In the ‘indirect injection’ system the required movement of the air is made by transferring it, towards the end of the compression stroke, from the cylinder space into a small chamber (usually located in the cylinder head) via a restricting throat, arranged so as to give rapid rotation of the air in the chamber. The fuel is injected into the chamber at a point where the passage of air past the tip of the injector will give the maximum degree of mixing.

 

Вариант 4