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#1 malbear

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Posted 17 April 2012 - 07:49

Consider a petrol engine wet liner cast iron. Would 4mm be thick enough or would 6mm be better. Also how much void space is necessary for the water coolant i.e. a depth of 3mm to 6mm from the sleeve to the inner block wall? I propose a slope starting at 3mm at the bottom and 6mm at the head. Opinions please?

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#2 Lee Nicolle

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Posted 17 April 2012 - 10:32

It seems that most liners are about 4mm thick, as are sleeves. That does seem to be enough. 6mm will be stronger but also harder to cool. It seems that most production car engines have more coolant area than you envisage. Though volume is probably not the problem but getting proper circulation/flow is the biggest drama. That is why so many engines have problems with bore wear, valves nipping up, heads cracking because of poor coolant flow with hot and cold spots in the engine.

Read the Smokey Yunick book, mainly Chevs but it makes you think about water flow, oiling etc. All the non glamorous or shiny bits!

Though I guess you are playing with bike engines, that is my brothers area of expertise. And a small bore bike engine should be plenty thick with a 4mm liner

#3 bigleagueslider

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Posted 19 April 2012 - 02:34

Consider a petrol engine wet liner cast iron. Would 4mm be thick enough or would 6mm be better. Also how much void space is necessary for the water coolant i.e. a depth of 3mm to 6mm from the sleeve to the inner block wall? I propose a slope starting at 3mm at the bottom and 6mm at the head. Opinions please?


malbear,

The liner thickness depends mostly on structural requirements like stiffness. You need to look at all of the combined loads from combustion pressure, piston skirt thrust, etc. and see if the resulting deflected liner shape is acceptable for your chosen wall thickness. Heat transfer across the liner wall is fairly critical around the upper part of the liner, so there is a limit to how thick you can go.

As far as the cross section dimension of the coolant jacket, with your wet liner design you have lots of freedom. Since convective heat transfer between the liner OD and coolant flow only takes place at the boundary layer, the most efficient approach is to keep the jacket cross section small and the flow velocity relatively high where cooling is needed most. The upper portion of the liner requires a greater amount of heat rejection than the lower portion. Your proposal to increase the jacket cross section from bottom to top would have the opposite effect, since it would reduce the coolant mass flow rate across the upper liner surface. If possible you would want to constrict all of the coolant flow into a thin, turbulent, high-velocity stream right against the upper liner surface. Unfortunately, other packaging considerations usually make this difficult to achieve in practice.

Lastly, just remember to follow good design practice, minimize rapid changes in flow passage cross section and eliminate any cavities than can trap air.

Good luck.
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#4 malbear

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Posted 20 April 2012 - 21:52

malbear,

The liner thickness depends mostly on structural requirements like stiffness. You need to look at all of the combined loads from combustion pressure, piston skirt thrust, etc. and see if the resulting deflected liner shape is acceptable for your chosen wall thickness. Heat transfer across the liner wall is fairly critical around the upper part of the liner, so there is a limit to how thick you can go.

As far as the cross section dimension of the coolant jacket, with your wet liner design you have lots of freedom. Since convective heat transfer between the liner OD and coolant flow only takes place at the boundary layer, the most efficient approach is to keep the jacket cross section small and the flow velocity relatively high where cooling is needed most. The upper portion of the liner requires a greater amount of heat rejection than the lower portion. Your proposal to increase the jacket cross section from bottom to top would have the opposite effect, since it would reduce the coolant mass flow rate across the upper liner surface. If possible you would want to constrict all of the coolant flow into a thin, turbulent, high-velocity stream right against the upper liner surface. Unfortunately, other packaging considerations usually make this difficult to achieve in practice.

Lastly, just remember to follow good design practice, minimize rapid changes in flow passage cross section and eliminate any cavities than can trap air.

Good luck.
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