Introduction:
The spaces where the pistons move up and down are called cylinders. The inside of the cylinder is not completely smooth, as this would allow the oil to easily travel past the piston from the crankcase to the combustion chamber. Therefore, honing grooves are applied. The principle of honing is described later on this page.
The size of the cylinders determines the engine displacement. Each type of engine uses a displacement like 1.6, 2.0, or sometimes even 6.0 liters. This means (in a 1.6) that there is space for 1.6 liters of air in all the cylinder spaces combined. The space is measured between the piston at BDC (bottom dead center, when it is at the bottom starting the compression or exhaust stroke) and the cylinder head. The larger the displacement, the taller/wider the cylinders are. This also depends on the bore (cylinder diameter) and stroke (cylinder height).
The displacement can be calculated using bore x stroke volume data. More information about displacement and bore x stroke volume can be found on the page Calculating Displacement.
More information about the 4-stroke process can be found on the pages Gasoline Engine and Diesel Engine.
Types of Cylinder Liners:
The cylinders can be directly bored into the engine block, cast as a complete engine block, or they can consist of separate inserted liners. Bored or monolithic cast cylinders are called dry liners, while separate cylinder liners are often referred to as wet liners. Wet liners can be individually replaced and manually placed. They come into direct contact with the coolant and have thicker walls than dry liners, as they must be sufficiently strong on their own. Extra gaskets are used to prevent fluids from entering the oil pan along the wet liners.
In air-cooled engines, the cylinders form a separate unit, equipped with cooling fins, and are mounted on the crankcase. The surface area of the fins depends on the extent of required cooling.
When a cylinder head is removed from an engine block with separate cylinder liners, special care must be taken to ensure the liners have not shifted upwards. Replacing the cylinder head otherwise might cause problems.
To check this, a long straightedge or a perfectly straight metal piece should be placed across the full width of the engine block. Any protruding liner will be immediately noticed, and the liner can then be gently (carefully!) tapped back down.
The right image shows an engine with wet liners, and the below image shows an engine block with cast or bored cylinders.
A head gasket is fitted between the engine block and the cylinder head. The head gasket ensures sealing between the cylinders and between the oil and coolant channels.
Honing:
The inner surface of a cylinder wall is not perfectly smooth. If it were smooth and the piston moved up and down inside it, a quantity of oil would always travel past the piston into the combustion chamber at the top. This is not desirable. Additionally, if the engine stops briefly, no motor oil would remain between the piston and the cylinder wall. The engine would then run “dry” for a moment until motor oil again rises past the piston. To prevent this, small honing grooves are applied to the cylinder wall. (Grooves are also to the side of the piston, but that’s discussed later). Honing grooves are simply tiny scratches made at a certain angle in the cylinder wall, which partially trap the oil.
Honing grooves are usually applied at an angle of 47 degrees, or sometimes 90 degrees from each other with special honing tools attached to a drill. The images below show special honing tools.
The honing of the cylinder must be performed very carefully. Too few honing grooves result in increased oil consumption, while too many cause damage to the protective layer in the cylinder wall.
During engine overhauls, the cylinder is sometimes bored and an oversized piston fitted. This increases the total displacement, necessitating re-honing. Engines experiencing excessive oil consumption, where grooves have become polished, or those with light scratches in the cylinder wall can be restored using honing tools. If there is a deep scratch in the cylinder wall, for example from an object entering the combustion chamber, it may be so deep that honing is ineffective. In such cases, only boring the cylinder for an oversized piston is viable, or otherwise, a new engine must be installed.
The pistons must also have light grooves on the sides. These serve to retain a small amount of oil for lubrication. If pistons are smooth and the honing structure has disappeared, oil consumption may increase. Replacing the pistons and inspecting the honing grooves in the cylinder wall is the best measure.
Honing grooves can wear faster when driving conditions are not optimal, such as:
- Driving too hard with a cold engine: the piston is forced hard against the cylinder wall by inertia while the engine is cold, and the piston is not properly expanded by heat. More information on piston expansion can be found on the page Piston.
- Lack of lubrication or running too long with old (thick) oil, causing lubrication issues.
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 aligned vertically in a row, known as an inline engine.
Cylinders can also be arranged in a V-shape at 60 or 90 degrees. These are V engines. If the cylinders are placed horizontally on either side of the crankshaft, it is a boxer engine.
The more cylinders an engine has, the smoother it runs, and the more consistent the torque delivered. There are more power strokes distributed across two rotations or 720 degrees of the crankshaft. Additionally, the flywheel can be lighter with a greater number of cylinders. Balance shafts necessary for two and three-cylinder engines to dampen the vibrations from power strokes are not needed in an eight-cylinder engine.
- Inline Engine: Cylinders are vertically aligned in a row. This configuration is the most common. Modern inline engines usually have four cylinders, although fuel-efficient, environmentally friendly three-cylinders are now found in, for example, the VW Polo, and Fiat’s two cylinders. BMW always places its six cylinders inline, never in a V-shape.
- V Engine: Here, cylinders are arranged at an angle of 60 or 90 degrees. The most common are the V6 and V8 engines.
There are also V5, V10, and V12 engines. In a V12 engine, six cylinders are on one side of the V-shape and six are on the other. - VR Engine: A combination of inline and V engine, mainly used by Volkswagen known for VR5 and VR6 engines. In a Golf R32, the cylinders are set at a 15-degree angle to each other. This configuration combines the advantages of inline and V engines. In an inline engine, the advantage is a single cylinder head, while in a V engine, the forces from the piston/connecting rod can be transferred to the crankshaft at larger angles relative to each other.
- W Engine: The cylinders are arranged in a W shape. This was applied in W12 engines of the VW Touareg, Phaeton, Audi, and Q7.
This engine configuration can actually be seen as two V engines mounted to a single crankshaft. The advantage of a W engine is that, considering the number of cylinders, the block length is reduced compared to a V engine. There is slightly more space between the radiator and the bulkhead. However, this reduces the space on either side of the engine block between the cross members.
Repair and maintenance (such as spark plug replacement) is more challenging. Removing the cylinder heads requires the complete engine to be removed from the vehicle.
- Boxer Engine: The cylinders are horizontally placed at an angle of 180 degrees.a0
The cylinders are placed horizontally opposite each other at an angle of 180 degrees. The advantage of this flat engine is that the car’s center of gravity is significantly lowered. The engine also experiences fewer vibrations because the piston vibrations cancel each other out. As a result, the engine is much better balanced and doesn’t need separate balance shafts. Boxer engines are used in both passenger cars and motorcycles. Subaru is known for using boxers, as well as the Citroën 2CV and the old VW Beetle.
Firing Order:
The firing order is the sequence in which the mixture is ignited in the cylinders one after the other. The firing order depends on the engine’s construction and the distribution of load over the crankshaft. The table shows the common firing orders.
| Engine Type: | Number of Cylinders: | Firing Order: |
| Inline Engine: | 3 | 1-3-2 |
| a0 | 4 | 1-3-4-2 or 1-2-4-3 |
| a0 | 5 | 1-2-4-5-3 |
| a0 | 6 a0 | 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 |
| a0 | 8 a0 | 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 |
| a0 | 6 a0 | 1-2-5-6-4-3 or 1-4-5-6-2-3 |
| a0 | 8 a0 | 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 |
| a0 | 10 | 1-6-5-10-2-7-3-8-4-9 |
| a0 | 12 a0 | 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 |
| a0 | 6 | 1-6-2-4-3-5 |