Saturday, 9 May 2015

Engine Cylinder Design and Function - Part 6

Cylinder Sleeves
Replaceable cylinders 




Some engines are fitted with sleeves also called cylinder liners. These are usually machined from cast iron and are replaceable. Cast iron is a very good bearing surface, this is why a lot of piston rings are made from cast iron. Once the cylinder has become worn the sleeve is removed and a new one is fitted. Piston rings and pistons are usually replaced at the same time to increase the life of the engine. 






Set of 4 liners, machined from cast iron.


The sleeves are machined to be slightly larger than the bored hole for it to fit into. The engine block is then heated so the hole expands. The sleeve is then pressed into the hole. 


 A 2 stroke liner being pressed into the cylinder.

Sleeves are usually fitted with ‘o’ rings to seal them in the block. Sleeves were usually reserved for 4 stroke engines but some 2 stroke engines also have sleeves. These are a bit more complicated as the ports have to line up with the sleeve.


2 stroke sleeve, with machined ports clearly visible 

 
We’ll quickly brush over the process for replacing the liners in a 2 stroke engine as it is quite a delicate and unique process. Firstly a billet of centrifugally spun cast iron is machined to the correct dimensions and bored to receive the piston. The old liner is pressed out of the cylinder and discarded. 

 Cast iron blank ready to be machined , and the cylinder after the removed liner has be discarded.

The ‘new’ liner is pressed in either by cooling the liner or heating the cylinder.


This new liner isn’t ported yet. Each engine is slightly different, so the port layout needs to be transferred to the liner. This is done by pouring acid into each port. The acid will etch the cast iron, leaving a witness mark. The liner is then pressed out again ready for machining. 

Close-up of the etched liner.


The new liner is placed in the milling machine and the ports are machined into the liner wall. Great care must be taken to match up the angle of the ports to maintain the blend between port and liner.

 Machining of the liner.


The finished liner is then pressed into the cylinder, making sure that the ports line up perfectly during installation. Most liners have a flange machined at the top to retain the liners when in service. 




The installed liner with the flange clearly visable.





Wet or Dry
Cylinder cooling


Due to the nature of cylinder liners, some designers have gone step further and designed the liner to be the entire cylinder itself. This is called a ‘wet’ liner, as the block only provides the outside casing element. The liner is the entire cylinder, this increases the thermal conductivity of the sleeve as it comes in direct contact with the water jacket. A water jacket is basically the void in which the coolant flows. One disadvantage of using wet liners is the structural rigidity of the entire block.
Above is a 4 cylinder block which clearly shows the 'wet' liners installed.


The liners are fitted with either a flange seal, or ‘o’ ring. The liners have to be machined to either and some examples have both. The pictures below shows how these are fitted.




Dry liners just fit into a bored cylinder like the 2 stroke example and transfer their heat throught this material into the water jacket.

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