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Old 01-18-2012, 05:17 AM   #52 (permalink)
mort
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Hi IamIan,
The chart is for wide open throttle. It appears almost identically here: Internal Combustion Engine Fundamentals by John Heywood but without the troubling "friction" line and with a km/hr scale. Chapter 12 in Heywood has a good discussion of heat balance, but it only discusses WOT conditions. The fomulae can be used for throttled situations. Another book I like is Engine Testing Theory and Practice by Anthony Martyr, Michael Alexander Plint

Quote:
Originally Posted by IamIan View Post
The Second thing I noticed is that ... that graph looks to contradict what mort was expecting ...
The situation I was referring to was an engine that was throttled, but producing substantial power, 36 hp in fact.

For WOT, using the graph as an example. The maximum efficiency occurs at a slightly higher rpm than shown. As rpm decreases, particularly below the rpm where the engine has the best breathing, power output falls at a faster rate than the rpm partly because the engine isn't getting the same amount of air-fuel charge as at the higher rpm. There is less charge in the cylinder, so the pre-combustion pressure & temperature are lower. That lowers thermal efficiency. The combustion gases expand into the full cylinder, lowering the exhaust temperature well below the full power temp. At the same time, as rpm drops the amount of time that the high cylinder temperatures of combustion has to dump into the cooling system increases - so the coolant takes away more heat and the exhaust less. Just as the graph shows.

Additionally, if you consider an engine at idle, throttled of course, the bhp is zero so all the heat in the fuel must come out in the coolant and exhaust (plus radiation...) At idle the coolant heat is infinity times the shaft power.

What I expect to happen in a throttled engine, well above idle, is that since the air-fuel charge is reduced, perhaps 1/4 of WOT, the pre-combustion pressure & temperature will also be reduced. Again the gasses expand into the full cylinder and the exhaust temperature is lowered. But the time that the coolant is exposed to the hot cylinders is the same as for WOT at the same rpm. However, the average temperature is reduced by the smaller charge and by having a full expansion stroke. Heat flow is proportional to temperature difference. So I would expect heat put into the coolant to be much less than 1/4 of coolant heat at WOT at that rpm.
So here is a graph I made up of what I think a constant rpm vs throttle heat balance would look like.

As I said before the worst case allowance calls for a radiator capable of dumping 100% of full shaft power. But at reduced power, particularly throttled down at cruise, the radiator only needs to handle a fraction of that. At wide open throttle as rpm drops the shaft power falls. And while the fraction of heat going into the cooling system rises it never gets larger than 100% of maximum output shaft power.
Also see the original WOT graph distorted to show energy balance at lower rpm compared to maximum power. The heat going into the cooling system is at a maximum at full power.
-mort
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