Active cylinder?

Tuesday, August 20, 2013

Tags :  engine-structure

In this issue’s article on Engine Structure I want to look more closely at the cylinder liner and its ability to play a significant role in the overall cooling system in race engines. Despite its passive role historically, this rather simple component has grown in significance in the engine’s cooling system – especially the liner’s exterior, rather than its interior, since guiding the piston requires a more or less round shape there.

Where the influence on the cooling system can be seen is on the outside of the liner or sleeve. We are talking wet liner design here, where the coolant is in direct contact with the liner. The interaction between cooling system and dry liner, where the liner is pressed into the block material, is less significant.

So what can we see when looking at a given wet liner’s outer shape? The liner’s outside diameter does not need to be a clean cylindrical shape; it can be specified in numerous different ways to be of influence on the cooling capability. Let us discuss some of these in more detail.

Outer diameter

The outer diameter of the liner can be used to increase or decrease coolant flow velocity around the liner by increasing or decreasing the outer diameter of the liner. Increasing the diameter makes the gap between block and liner smaller, which increases the velocity, and vice versa of course.

This also gives us the freedom to specify the outer diameter in different sizes at different liner heights, which will influence the coolant flow velocity in the coolant jacket around the cylinder. Of course, it is not simply a case of modifying this gap, the coolant system as a whole needs to take account of the overall pressure drop across the coolant jacket. Also, the location of the coolant feed and return ports need to be chosen in such a way that they provide sufficient flow and pressure levels. 

Dynamic seal

One of the more significant cooling optimisations with a liner’s outer diameter is the creation of a so-called dynamic seal, which is when, at a given height of the liner, the liner diameter is chosen such that it almost touches the wall of the block. In a sense this gives two coolant chambers – one above this very small local gap and one below.

This can be very useful for increasing velocity near the top of the liner, cooling the combustion as much as possible. In the lower region of the coolant jacket (lower piston positions) the flow does not have to be so high.

Of course, this type of design will not work with a single simple coolant feed port, since the coolant flow would take the path of lowest pressure drop, leading to almost no flow in the upper, high velocity, region. To enable coolant flow in that region, a supply from the highest pressure (just after the coolant pump) is required, and the return should be connected near the lowest coolant pressure (as close to the pump suction side as possible). Naturally, coolant temperature also remains a very important factor, it needing to be as cool (relatively speaking) as possible. 

Local geometries on the liner outer surface

More often one can see local geometries on the liner outside surface, which are specified in order to guide of steer the coolant flow and doing so, influencing the cooling capacity by increasing flow and/or pressure locally. In issue 72 (June/July 2013) of Race Engine Technology magazine there are some very good visuals of how this could look.

One interesting idea is where the top flange provides openings (often drillings) where the coolant can pass to the top deck and into the cylinder head. This is mostly used with dry-to-wet liner transitions, still enabling coolant flow from block to head, through the liner flange. Another advantage is that the coolant flow can be optimised by modifying the dimensions and location of these ports. 

Material

Although this article has looked specifically at geometrical possibilities, one should of course always link geometry to material. Given a certain combustion load (maximum firing pressure) the liner must be able to withstand these loads mechanically and thermally. There will always be a balance or optimum between strength and thermal conductivity. For additional information on the materials though, I recommend that you read the article on Liners in the above issue of RET.

As I mentioned at the start, liners did not at first really have any other role than to guide the piston up and down, and keep the coolant on the outside and the gases on the inside. Since then, however, the liner has taken on a more active role in the overall coolant system of race engines, just by being not too simple and not too perfectly round on the outside. Cool, isn't it? 

Fig. 1 - ‘Open deck’ configuration with external features for heat transfer 

Written by Dieter van der Put

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