Engine Cylinder Design and Function - Part 8

Stroke
The route of all power


So we’ve looked at many of the different aspects of the cylinder but one thing that we’ve overlooked is the stroke. I’ve previously mentioned the importance of the bore and the stroke. The ‘bore’ is the internal diameter of the cylinder, however the stroke is NOT the length of the cylinder. The stroke is defined by the ‘throw’ of the crankshaft, and is the distance the piston travels inside the cylinder.

Below shows the bore (1) and the stroke (2). Notice how the cylinder is absent from this diagram. Cylinders are always larger than the stroke to guide the piston as it travels. The piston travels to the top of the cylinder, but does not travel to the bottom of the cylinder.

The image below illustrates how the cylinder guides the piston due to the fact that it is longer than the stroke.

  

In some engines, especially single cylinder engines the cylinder protrudes further into the engine casing than the body of the barrel, this is to retain a suitably long cylinder but reduce the overall height of the engine.

Stroke as stated before is determined by the throw of the crankshaft and we’ll go more into this subject in the crankshaft section, its just important to point out at the length of the cylinder isn’t the stroke length.





Porting the cylinder
Cylinder valves 


2 stroke engines differ from there counterparts (4 strokes) due to 2 differences, the cycle of the system and the use of the cylinder to control the fluids of the system. This might sound like technical bumf, and…….. well it is. What is meant by this is that the 2 stroke complete 2 strokes (once up and down) and the whole cycle starts over again. The fluids of an engine mean both fuel and air, either as a mixture or not.

2 stroke engine have port cut into the sides of the cylinder that allow air/fuel to pass into the cylinder and back out again at the appropriate time. For now we’re looking at cylinders so we’ll skip all the port timing nonsense until later. What is important is that the cylinder plays a major role in the 2 stroke design unlike the 4 stroke were it simply functions as a hole.

Ports in a 2 stroke petrol cylinder.

There are 2 major types of cylinder porting used in combustion engines. These are single flow diesel 2 strokes and multiple porting used on most petrol 2 strokes. This porting arrangement isn’t so much to do with the difference between the fuels themselves but the speed at which the engine operates.

  

2 stroke Diesel

 The massive Wärtsilä-Sulzer RTA96-C its 89 feet long and 44 feet wide, 

1,820 liters per cylinder and produces around 107,000 horsepower.

2 stroke diesel engines are usually reserved for heavy machinery and ship engines. The largest engine in the world is a 2 stroke diesel pictured above. These engines also have poppet valves like 4 stroke engine and have ports at the bottom of the cylinder just above the piston at BDC (bottom dead centre).

The function of these port is to fill the cylinder with fresh air when the piston reaches BDC, this is controlled by the piston which uncovers the ports at it bottoms out. The exhaust gases escape though a poppet valve which resides in the cylinder head. These engine are very efficient, hence there main application is to power large ship that run for weeks. 

Above is a liner from a 2 stroke diesel that has been cut away, the piston is also present to demonstrate the uncovering of the ports. This system work very well as the diesel fuel is more of an oil than a solvent and lubricates the cylinder unlike 2 stroke petrol that require an oil to be added to the fuel. Unlike petrol engines only air flows through these ports, as the diesel is directly injected into the cylinder. More on diesels later.

Ported Petrol

Nearly all 2 stroke petrol engines use both exhaust ports and transfer ports to move fluids in and out of the cylinder. Some of these ports have quite complicated paths that direct the flow in and out more efficiently. The number of ports change from engine to engine and as research into 2 stroke engines progresses these numbers seem to keep on increasing.

It is becoming more common for engines to have Aluminium barrels with Nikasil coated cylinders. This combination is to give good thermal transfer and a hard wearing bearing surface.

Small radio controlled aircraft that usually run on petrol or methanol have cylinder liners that are ported. They are usually made from brass with a chrome plated cylinder wall for extra flight hours. This chrome surface is cheaper than Nikasil and will easily outlive the engine.