Block/head engineering question

[80083r]

I'm not an engineer... I can't afford to go to school, and I'm not driven enough to eat ramen for 4 years and do it anyway (besides, I'm 40 - I'm too old for ramen).

That said - I'm smart, technical guy, and I can't find an answer to this question. I'm hoping that some of the more experienced members can answer this:

Why does the block have a water jacket rather than simply bores with water flowing? I have half-assed idea why, but I would like confirmation. You would need a large number of bores, I think to achieve the same heat transfer - has it ever been tried that way? Is it just simply cheaper and easier to do it with a hollow around the cylinder walls? Is it a weight issue?

 

[IndigoMKII]

I'm no expert but I think there would be adverse affects with uneven cooling on the cylinder walls.

 

[hvyman]

Im confused. There is no seperate water jacket around each bore if thats what your thinking. water flows around both sides of all the bores and the front and back. The bores are too close together for the water to flow inbetween each bore.

 

[DeMoN2318]

Pics might help...

 

[80083r]

Pics did help.

Top is standard water jacket - hatched area is water.
Bottom is my other thought - hatched area is water.

Two factors come into play here - volume of water, and speed of flow. The water jacket has a HUGE volume of water to sink heat into.

My thought was that if you had bores (like the lower image) where water was pumped in from the bottom and flowed out the top, it would carry more heat away. I think that in order to balance flow to carry heat away (fast) vs flow to absorb heat in the block (slow) the bores would be much less efficient than having a large volume of water that flows moderately as is the case with actual blocks (?).

Crazy question make sense now? The answer I think is obvious now.

 

[toyotanos]

My take on the situation, mostly opinions and first thought stuff here:

The problem is going to be heat transfer and thermal load across the block. The reason most water jackets are so big is so the block warms up and cools down as evenly as possible. If you have hot/cool spots on the block, those area will be expanding at different rates than the rest of the block, and you can have stress build up (usually resulting in cracks). You can also have a scrubbing action that takes place between the block and attached components (head/timing cover) which can damage the gaskets between them. Think of it like this- how quickly can heat move through the water (which is already in motion) and how even if that column of water going to be heated as it moves through the block? Compare that to how quickly (or relatively slowly) heat moves through the iron of the block.

Another problem is when fluid moves quickly though a small space, there is the possibility of a condition called 'laminar flow' where the water itself doesn't actually absorb any of the heat because there is a small boundary layer of turbulence insulating the inner flow from making good contact. A good example if this is intake manifolds- polished manifold runners usually look good, but they flow slightly less than a slightly more rough one- the reason is the same laminar flow. That boundary layer of turbulent air actually 'lubricates' the airstream, allowing the slightly reduced diameter to move the air with less drag against the runner walls, resulting in a small increase of flow. Radiators can have the same situation, but its less common. It can be a good thing, and a bad thing, depending on the situation!

Ninja edit: Last thing: Some racing engine are completely solid and devoid of water altogether, either machined from a billet that way or 'cemented' by filling any existing water jackets with a filler. They are very strong, but cannot be run for long in most cases because of the heat buildup. Drag racing is usually where these engines are used, since the parent material (the block) can absorb the burst of heat from 1 run without much damage, before being shut down and towed back to the pits to cool off. This is a method I have little experience with, but have seen it a couple times when racers blow their engines.

/thoughts as of now

 

[IJ.]

Surface area...

 

[suprarx7nut]

Surface area...

Came here to sway just that. The mechanics of flow don't have much of an affect compared to the difference in surface area in the above situations.

I think the idea with engine cooling jacket design is to get the most surface area of water jacket possible as close to the heat source as possible while maintaining a sound structure in the block.

The small bores may move a smaller volume with more velocity, but they won't take away as much heat (which is the whole point of a water jacket).

Also keep in mind the idea is not to move water as fast as possible. The water needs time to absorb the heat in the engine and then time in the radiator to effectively cool that water back down. This is a part of what the thermostat does.

 

[hvyman]

Only thing is where are you going to pump that fluid fast? If it has no where to go its just going to get recirculated around a get hotter and hotter. Cant just keep cycling it through the radiator or else its going to be like running with no t stat and overheat.

More coolant there is more heat can be transferred as well.

 

[MightyAl]

Why don't they get rid of the water jackets entirely and put cooling fins on the outside to carry away heat. Wait what?

Just curious the thoughts on how that dovetails into this. Isn't that the terminus we are spiraling towards in this discussion.

 

[hvyman]

Mazworks makes a billet block with no cooling.

 

[80083r]

The problem is going to be heat transfer and thermal load across the block. The reason most water jackets are so big is so the block warms up and cools down as evenly as possible. <snip lots of good details>

Surface area...

Yes - these and volume of water is exactly what drawing a picture drove home for me.

Thanks for the responses everyone, I'm learning as I go.

 

[DeMoN2318]

Why don't they get rid of the water jackets entirely and put cooling fins on the outside to carry away heat. Wait what?

Just curious the thoughts on how that dovetails into this. Isn't that the terminus we are spiraling towards in this discussion.

This would magnify the uneven heating and cooling issue...the cylinder walls would buckle

 

[80083r]

This would magnify the uneven heating and cooling issue...the cylinder walls would buckle

What about early Porsche 911s, was that an issue?

 

[DeMoN2318]

What about early Porsche 911s, was that an issue?

Not sure on specifics...but i would assume it was since they no longer do it that way