- #1
sofaspud
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Hi, first post newbie here, so go easy please
I'm trying to get a grip on the understanding of a car cooling system for a friends race car. It seems the more I look into it the more questions I'm left with. For this post I would like to clarify the coolant side, leaving the air side for another post.
Having trawled the net I see most opinion says the power output from the engine is around 33% of the fuel energy available, with the rest rejected by the engine to the cooling system & exhaust, i.e. roughly equal distribution of fuel energy between the 3, power = cooling = exhaust.
SAE J1349 (Jun 90) originally suggested 85% of power produced is delivered (after friction & ancillaries etc) to leave a nett available flywheel power of 28.3% which again is a figure I've found in my trawls.
Sorry if this is a long & rambling post, but the following questions leave me uncertain.
If it would be best to break these into separate posts let me know.
so:
1/
What I can't find is how this % distribution changes if a turbocharger is fitted. While some exhaust heat is used to drive the turbo to pump more air & hence produce more power, some is rejected into the cooling system by the water & oil used by the turbo.
I would guess its a relatively small change if indeed there is any, but if the turbo is extracting heat from the exhaust it must alter the distribution?
2/
If the engine is running non stoichometric (λ <1 typically) mixtures then surely not all the potential fuel energy is being released as heat?
The engine I'm looking at runs λ typically around 0.85 (AFR ~ 12.5:1) when under steady state full load, and AFR of ~11.1:1 during acceleration enrichment.
This being the case, how much does this alter(reduce) the heat?
Engines I have knowledge of all run 'rich' mixtures under full load. Has this been accounted for in the basic 'rule of thumb'?
if say for (an extreme) example the engine power produced was only 20% of the available fuel energy would the heat rejection (to cooling) still be equal to the produced power? or does the heat reduce in a non-linear fashion?
3/
Bear in mind as this is a race engine it has a separate large oil cooler;
If you use the 'std' 33% to determine the heat rejection into the cooling system, how would this typically be distributed amongst the various elements conduction, radiation, Water, Oil?
If I use the basic rule of thumb values above to rough out some figures for a 700bhp turbocharged petrol engine then I get:
available power: 700bhp = 522kW's
total mech power: 522/0.85 = 614kW's
target coolant temp = 90 deg C
Radiator dT = 15 deg C
(values for mass & Cp found from here:
http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html)
if this was rejected solely into the water cooling it would require a cooling system flow rate of approx 600 litres/min (134 gpm). This is a high rate so clearly some heat is dissipated by the other 3 elements?
4/
I found this SAE paper on the Ford GT cooling design:
http://www.roush.com/Portals/1/Downloads/Articles/2004-01-1257.pdf
In this they seem to be designing for a heat rejection to the cooling system of only around half the available output power of the engine rather than the nominal equal value total mechanical power produced. This is for a supercharged engine, but the figures they use are significantly different to the 'rule of thumb' figures, even more so as the oil cooler is an oil to water (rather than air) exchanger.
I found some 'finger in the wind' suggestions of 1litre/min/bhp which suggests this engine requires 700 litres/min!
Elsewhere I've seen figures for similar power engines using 100 gpm (~450 l/m) but also an oldish BMW F1 engine of circa 850 bhp using 450 l/m. These higher power (esp the F1) engines also use a much lower value of dT than I have in the approximation above(q3)
I realize the engine doesn't spend it life under full load condition, but any help in understanding these greatfully received.
Thanks if you got this far!
John
I'm trying to get a grip on the understanding of a car cooling system for a friends race car. It seems the more I look into it the more questions I'm left with. For this post I would like to clarify the coolant side, leaving the air side for another post.
Having trawled the net I see most opinion says the power output from the engine is around 33% of the fuel energy available, with the rest rejected by the engine to the cooling system & exhaust, i.e. roughly equal distribution of fuel energy between the 3, power = cooling = exhaust.
SAE J1349 (Jun 90) originally suggested 85% of power produced is delivered (after friction & ancillaries etc) to leave a nett available flywheel power of 28.3% which again is a figure I've found in my trawls.
Sorry if this is a long & rambling post, but the following questions leave me uncertain.
If it would be best to break these into separate posts let me know.
so:
1/
What I can't find is how this % distribution changes if a turbocharger is fitted. While some exhaust heat is used to drive the turbo to pump more air & hence produce more power, some is rejected into the cooling system by the water & oil used by the turbo.
I would guess its a relatively small change if indeed there is any, but if the turbo is extracting heat from the exhaust it must alter the distribution?
2/
If the engine is running non stoichometric (λ <1 typically) mixtures then surely not all the potential fuel energy is being released as heat?
The engine I'm looking at runs λ typically around 0.85 (AFR ~ 12.5:1) when under steady state full load, and AFR of ~11.1:1 during acceleration enrichment.
This being the case, how much does this alter(reduce) the heat?
Engines I have knowledge of all run 'rich' mixtures under full load. Has this been accounted for in the basic 'rule of thumb'?
if say for (an extreme) example the engine power produced was only 20% of the available fuel energy would the heat rejection (to cooling) still be equal to the produced power? or does the heat reduce in a non-linear fashion?
3/
Bear in mind as this is a race engine it has a separate large oil cooler;
If you use the 'std' 33% to determine the heat rejection into the cooling system, how would this typically be distributed amongst the various elements conduction, radiation, Water, Oil?
If I use the basic rule of thumb values above to rough out some figures for a 700bhp turbocharged petrol engine then I get:
available power: 700bhp = 522kW's
total mech power: 522/0.85 = 614kW's
target coolant temp = 90 deg C
Radiator dT = 15 deg C
(values for mass & Cp found from here:
http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html)
if this was rejected solely into the water cooling it would require a cooling system flow rate of approx 600 litres/min (134 gpm). This is a high rate so clearly some heat is dissipated by the other 3 elements?
4/
I found this SAE paper on the Ford GT cooling design:
http://www.roush.com/Portals/1/Downloads/Articles/2004-01-1257.pdf
In this they seem to be designing for a heat rejection to the cooling system of only around half the available output power of the engine rather than the nominal equal value total mechanical power produced. This is for a supercharged engine, but the figures they use are significantly different to the 'rule of thumb' figures, even more so as the oil cooler is an oil to water (rather than air) exchanger.
I found some 'finger in the wind' suggestions of 1litre/min/bhp which suggests this engine requires 700 litres/min!
Elsewhere I've seen figures for similar power engines using 100 gpm (~450 l/m) but also an oldish BMW F1 engine of circa 850 bhp using 450 l/m. These higher power (esp the F1) engines also use a much lower value of dT than I have in the approximation above(q3)
I realize the engine doesn't spend it life under full load condition, but any help in understanding these greatfully received.
Thanks if you got this far!
John