4age (any model) loss of power from cooling

[punkture]

hey everyone, for uni were doing an assignment on cross flow heat exchangers (radiator) now weve pretty much finished, and the engine all the properties are based on is a 4age. We need to know now how much power from the output of the engine is lost from cooling, our lecturer reckons it is somewhere between 20-30%, any help much appreciated thanks

[banana_socks]

That is the most open ended question possible. It would vary by huge amounts depending on the operating conditions. The only possible way to calculate this would be to use temperature probes before and after the radiator whilst hooked up to a dyno.

20-30% is probably the correct order, but I don't know what you'd hope to do with a more specific answer.

[Hiro]

That is the most open ended question possible. It would vary by huge amounts depending on the operating conditions. The only possible way to calculate this would be to use temperature probes before and after the radiator whilst hooked up to a dyno.

You'd have to include the oil system in there too, as there is some heat absorbed by the oil (not as much as the coolant though)

[punkture]

That is the most open ended question possible. It would vary by huge amounts depending on the operating conditions. The only possible way to calculate this would be to use temperature probes before and after the radiator whilst hooked up to a dyno.

20-30% is probably the correct order, but I don't know what you'd hope to do with a more specific answer.

welocme to uni =( haha well we need it to evaluate whether or not our over heat loss rejected by the radiator is in the ball park, we are getting very low heat coefficients, especially for the coolant and it may be due to our mixture or our flow rate of water pump:

we used: 80ltr/min, which has velocity of .86m/s given 67 total tubes of 2x13mm dimensions with .2mm wall thickness;
our running temperature of coolant was 90* and assumed 25* rejected heat at outlet i.e 65* after cooling..

is their any ball park figures for the loss of power from the engine by cooling? would it just be the water pump bring driven or?

[Supra967]

The general rule we ended up with was a 40% split between exhaust and block. At low speed there was more time for the gases to heat the cylinder walls so it was about 30/10 block/exhaust, at high speed there was less time for heat transfer so most went out the exhaust for 10/30 block/exhaust.

If you look up the Crower engine, he introduces a steam cycle which absorbs heat from cylinder walls and exhaust to vapourise water (he has no block cooling). He's claiming 40% increase in fuel economy at all engine speeds, which gives some back-up to the 'general' rule.

I'd play around with your flow rate till you get the desired figure

[punkture]

The general rule we ended up with was a 40% split between exhaust and block. At low speed there was more time for the gases to heat the cylinder walls so it was about 30/10 block/exhaust, at high speed there was less time for heat transfer so most went out the exhaust for 10/30 block/exhaust.

If you look up the Crower engine, he introduces a steam cycle which absorbs heat from cylinder walls and exhaust to vapourise water (he has no block cooling). He's claiming 40% increase in fuel economy at all engine speeds, which gives some back-up to the 'general' rule.

I'd play around with your flow rate till you get the desired figure

cheers mate, ill assume probably 15%, so 13.2kw loss to cooling then, (block only) ill suss the other figures for the heat rejected and see if its in the ball park our heat coefficient was ~1250w/m2K

[banana_socks]

There would also be quite a delay between the heat loss within the block, and change in temperature of the coolant. If you're running at steady state it's not an issue, but for varying loads it will be.

If you're trying to work out the coefficient of the radiator, you also need to consider the convection of the air from the radiator. Will this be natural or forced? Air speed just from the fan, or a moving car? There are empirical relationships you can use for the heat exchanger as a whole based on a few parameters.

I'm still a little confused as to what you're trying to come up with. If you're just analysing a stock 4A cooling system, it shouldn't be more than simple convection. If you know the answer you want, then it's just a matter of fudging your assumptions till you get it. (At least that's what I learnt at uni)

[KENut]

Partially relevant, when I did a similar experiment with a single cyl 4 stroke we ended up with aprox 18% loss through cooling from fuel input

[KENut]

Wait, it seems like you're asking two questions.
Power taken from engine output for cooling system
And power dissapated through the cooling system, ie heat loss.

Are you asking both or?...

[Hiro]

Fairly sure the question is about energy loss through heat - ie percentage of the energy contained in the fuel that gets converted to heat (the rest will go to noise and expansion,out of which only the expansion will do any useful work in the engine)

[punkture]

yeh power lost through cooling as a percentage of whole. yeah were doing overall, we have included free convection too, as well as at idle, 20km, 60km and 100km, but my part is only the coolant.

[banana_socks]

Pardon if my terminology is slightly wrong, it's been a couple of years since I did heat transfer.

Assuming you have a constant flow rate through radiator, and both the inlet and outlet temperatures are known and at steady state. You can now work out the heat dissipated through the radiator:
change in temperature x heat capacity x mass flow rate = power
degrees x kJ/kg.deg x kg/s = kJ/s = kW

As this would be through convection, the heat loss is equal to the temperature differential times the convection coefficient. You can now work out this coefficient, and then compare it to empirical data to see if you're in the ballpark. Otherwise your assumptions on flow rate and temperature change are probably off.

At higher speeds your convection coefficient should increase, as should your flowrate. I'm not sure how this will affect the temperatures TBH.

All this is assuming steady state. If you're even considering dynamic heat transfer, just quit now.

[punkture]

yeah we have to do the heat loss (q) with inlet temp at 15* and speeds of 0,15,100km/h, and inlet at 40* speeds of 0,15,100. also have to do the overall heat coeffcicient (U) etc.. my part was the inner tube fluid i.e the coolant, and others are doing the free convection, forced convection etc.

Banana socks your fair on track their but you have to consider the Reynolds number and nussults as the fluid becomes turbulent and laminar at the start etc.

So assumiung the engine priduces 88kw from a factory rebuild, it spins upto 88kw at @6600rpm etc, what percentage of power from that figure is lost due to the cooling process

[KENut]

None, because that power is output power?

Also, I'd think the coolant is turbulent even at idle (or are you considering the air flow as laminar to begin with?)