Introduction:
The spaces in which the pistons move up and down are called cylinders. The inside of the cylinder is not completely smooth, because then the oil could easily move along the piston from the crankcase to the combustion chamber. That is why honing grooves have been made. The principle of honing is described further down on this page.
The size of the cylinders determines the engine displacement. For each type of engine, a displacement is used of e.g. 1.6, 2.0 or sometimes even 6.0 litres. This means (for a 1.6) that all the cylinder spaces together have room for 1.6 litres of air. The volume is measured between the piston in BDC (so when it is at the bottom and starts the compression or exhaust stroke) and the cylinder head. The larger the displacement, the higher / wider the cylinders are. This also depends on the bore (the diameter of the cylinder) and the stroke (the height of the cylinder).
The displacement can be calculated using the bore x stroke volume data. More information about displacement and the bore x stroke volume can be found on the page Calculating engine displacement.
More information about the 4‑stroke process can be found on the pages Petrol engine and Diesel engine.
Types of cylinder liners:
The cylinders can be directly bored into the engine block, cast as one complete engine block, or they can consist of separate inserted liners. Bored or one-piece cast cylinders are called dry liners and the separate liners are also known as wet liners. Wet liners can be replaced individually and installed by hand. They come into direct contact with the coolant and have a thicker wall than dry liners. They must be strong enough by themselves, after all. To prevent fluid from getting into the oil sump along the wet liners, extra gaskets are used.
In air‑cooled engines, the cylinders form a separate unit, are fitted with cooling fins and are mounted on the crankcase. The surface area of the fins depends on how much cooling is required.
When a cylinder head is removed from an engine block with separate cylinder liners, great care must be taken to ensure that the liners have not shifted upwards. Otherwise, problems can arise when refitting the cylinder head.
To be able to check this, a kind of large straightedge or a perfectly straight piece of iron must be placed over the entire width of the engine block. If a liner protrudes, this will be noticed immediately. The liner can then be gently tapped down again with light force (carefully!).
The image on the right shows an engine with wet liners and the image below an engine block with cast or bored cylinders.
Between the engine block and the cylinder head is the head gasket. The head gasket provides the sealing between the cylinders and between the oil and coolant passages.
Honing:
The inside of a cylinder wall is not nicely smooth. If it were smooth and the piston moved up and down in it, a certain amount of lubricating oil would always pass from the piston along the cylinder wall into the combustion chamber above the piston. And that is precisely not the intention. Also, when the engine has been stationary for a while, there would no longer be any oil left between the piston and the cylinder wall. The engine would then run “dry” for a short time before engine oil climbs back up along the piston. To prevent this, small honing grooves are made in the cylinder wall (also on the side of the piston, but that will be discussed later). Honing grooves are nothing more than small scratches made in the cylinder wall at a certain angle, in which the oil partly remains.
The honing grooves are usually made at an angle of 47 degrees, or sometimes 90 degrees from each other, using special honing tools fitted to a drill. The images below show special honing tools.
Honing the cylinder must be carried out very carefully. Too few honing grooves lead to higher oil consumption and too many damage the protective layer in the cylinder wall.
During engine overhauls, the cylinder is sometimes rebored and an oversize piston fitted. The total displacement then becomes larger and the cylinder must be honed again. Engines that suffer from high oil consumption, where the grooves have become smooth, or engines with a light scratch in the cylinder wall can also be restored with honing tools. If there is a deep scratch in the cylinder wall, for example from an object that has entered the combustion chamber, the scratch may be so deep that honing is no longer worthwhile. Only reboring the cylinder with an oversize piston still makes sense then, otherwise the engine must be replaced.
The piston must also have light grooves on the side. These also serve to retain a little oil for lubrication. When the pistons are smooth and the honing pattern has disappeared, oil consumption can increase. The best measure in that case is to replace the pistons and check the honing grooves in the cylinder wall.
The honing grooves can wear (faster) when the driving conditions are not optimal, such as:
- Driving too hard with a cold engine: the piston is pressed hard against the cylinder wall by the side thrust force, while the engine is not yet up to temperature and the piston has not yet expanded properly due to the temperature. More information about piston expansion can be found on the page piston.
- Lack of lubrication, or driving far too long with old (thick) oil and thus also a lack of lubrication.
Cylinder configurations:
There are two‑, three‑, four‑, five‑, six‑, eight‑, ten‑ and twelve‑cylinder engines. Bugatti even has a sixteen‑cylinder in the Veyron. The cylinders can be arranged vertically in line. This is then called an inline engine.
The cylinders can also be at an angle of 60 or 90 degrees in a V‑shape: that is called a V‑engine (for example a V6 or V8). If the cylinders are mounted horizontally to the left and right of the crankshaft, this is a boxer engine.
The more cylinders an engine has, the more smoothly it runs and the more even the torque delivery is. After all, there are more power strokes distributed over two crankshaft revolutions or 720 degrees of the crankshaft. The flywheel of an engine with a higher number of cylinders can also be made lighter. Balance shafts, which are necessary in 2‑ and 3‑cylinder engines to dampen engine vibrations originating from the power strokes, are not needed on an 8‑cylinder engine.
- Inline engine: The cylinders are arranged vertically in line. This configuration is the most common. Modern inline engines usually have 4 cylinders, but nowadays economical, environmentally friendly 3‑cylinder engines are also found, e.g. in the VW Polo, and 2‑cylinders at Fiat. BMW always places its 6 cylinders in line, never in V‑form.
- V‑engine: Here the cylinders are at an angle of 60 or 90 degrees. The most common engines are the V6 and V8 engines.
There are also V5, V10 and V12 engines. In a V12 engine, 6 cylinders are on one side of the V‑shape and the other 6 on the other side. - VR‑engine: A combination of an inline and a V‑engine. This is mainly used by Volkswagen, which is known for the VR5 and VR6 engines. In a Golf R32 the cylinders are arranged at an angle of 15 degrees to each other. This combines the advantages of the inline and V‑engine. The advantage of an inline engine is that it can be equipped with a single cylinder head, while in a V‑engine the forces from the piston / connecting rod can be transmitted to the crankshaft at a greater angle to each other.
- W‑engine: The cylinders are arranged in a W‑shape. This was used in the W12 engines of the VW Touareg, the Phaeton, the Audi and the Q7.
You can actually see this engine layout as two V‑engines mounted on one crankshaft. The advantage of a W‑engine is that, given the number of cylinders, the length of the block is reduced compared with the V‑engine. There will be a bit more space between the radiator and the bulkhead panel.
However, this means that the spaces at the sides of the engine block between the cross members are reduced.
Repair work and maintenance (such as replacing the spark plugs) do not become any easier. To remove the cylinder heads it will be necessary to remove the complete engine from the car.
- Boxer engine: The cylinders are placed horizontally at an angle of 180 degrees.
The cylinders are placed horizontally opposite each other at an angle of 180 degrees. The advantages of this flat engine are that the centre of gravity of the car is immediately much lower. The engine will also suffer less from vibrations, because the piston vibrations cancel each other out. The engine is therefore much better balanced and does not need separate balance shafts. Boxer engines are used in both passenger cars and motorcycles. Subaru is known for its use of boxer engines, as are the Citroën 2CV and the classic VW Beetle.
Firing order:
The firing order is the sequence in which the mixture in the cylinders is ignited one after another. The firing order depends on the design of the engine and the way the load is distributed over the crankshaft. The table shows the usual firing orders.
| Engine type: | Number of cylinders: | Firing order: |
| Inline engine: | 3 | 1-3-2 |
| 4 | 1-3-4-2 or 1-2-4-3 | |
| 5 | 1-2-4-5-3 | |
| 6 | 1-5-3-6-2-4 or 1-5-4-6-2-3 or 1-2-4-6-5-3 or 1-4-2-6-3-5 or 1-4-5-6-3-2 | |
| 8 | 1-6-2-5-8-3-7-4 or 1-3-6-8-4-2-7-5 or 1-4-7-3-8-5-2-6 or 1-3-2-5-8-6-7-4 | |
| V‑engine: | 4 | 1-3-2-4 |
| 6 | 1-2-5-6-4-3 or 1-4-5-6-2-3 | |
| 8 | 1-6-3-5-4-7-2-8 or 1-6-2-8-3-7-4-5 or 1-3-7-2-6-5-4-8 or 1-5-4-8-6-3-7-2 or 1-8-3-6-4-5-2-7 | |
| 10 | 1-6-5-10-2-7-3-8-4-9 | |
| 12 | 1-7-5-11-3-9-6-12-2-8-4-10 or 1-12-5-8-3-10-6-7-2-11-4-9 | |
| Boxer engine: | 4 | 1-4-3-2 |
| 6 | 1-6-2-4-3-5 |