Excuse me if I’m stupid but why does a low temperature differential imply the need for a larger surface to dissipate heat? If the engine is cooler isn’t there less heat to dissipate to begin with?
Leaving alone the minor considerations like the ease of engineering airflow through a 90deg bend and aircooling fins vs engineering laminar airflow through a duct and core, and the heat dissipation efficiency difference between cylinder fins and radiator fins.
European tax Euros at work:
Shorrick_Mk2 wrote:
Why does a low temperature differential imply the need for a larger surface to dissipate heat?
Physics. All other things being equal, heat is transferred in proportion to the temperature difference between the two masses (metal and air). A water cooled engine normally has the coolant at 80 degrees Celsius, so the “radiator” surface temperature is lower than that. Air cooled engines are running at twice the temperature so, in principle, need half the surface area. That is at air at zero Celsius, on a hot summer day, the water cooled engine needs three times the area.
This isn’t the whole story, though – some of the heat is removed via the oil cooler, which is somewhere between these two extremes.
If the engine is cooler isn’t there less heat to dissipate to begin with?
But not half as much. Let’s assume the Diesel turns 60% of fuel into heat (optimistic), and the AVGAS burner 75% (pessimistic) – so heat generated goes fpdown by 15/75 = 20 percent. Not 50 percent.
However, all in all, i don’t think any of this is really relevant in the grand scheme of things. Nobody would call a DA40 or DA42 draggy, the airframe overall makes more difference than the cooling drag of the engine(s).
This isn’t the whole story, though – some of the heat is removed via the oil cooler, which is somewhere between these two extremes.
Which is a very good point, and also the reason for concepts using high boiling temperature cooling liquids.
Those concepts aside, removing some of the heat load with the lubricating oil allows you to spray liquid directly on hot spots, bottoms of pistons etc. which is quite effective.
Of the energy generated by combustion, around 44% goes out of the exhaust, 8% is lost via the oil cooler, and 12% is lost directly from the cylinders to the airflow. The rest is going usefully into the propeller. To put it in perspective, a 250HP engine during climb is trying to lose about 40kW to the airflow which is the maximum output of a large domestic boiler. Reference here
Conduction is proportional to the delta-T. Radiation is proportional to delta-T ^ 3. Convection, not sure, but it probably isn’t relevant with air cooled engines.
This is all great technical stuff.
The sr22 today already hosts both the eps And continental engine. As far as I can tell within the existing cowling. So the drag issue is theoretical?
I believe that the days for the old lyco and contis are over .. Escpecially if we want to have a future for ga. 100ll should have been replaced ages ago. In the end it will be replaced but it has taken far to long.
Diesels have taken far to long as well but the Diamonds have great succes allthough their engines are relatively small.
Eps, conti and sma promise proper engines..
For some reason sma wants to keep everything very restricted and in the meantime is very unsuccesful.
I believe and hope that conti and eps will take a different approach .. Making their engine widely available .. Conversions would then cost approx 60-80k including stc’s etc. Overhauling my current engine is maybe 40-45k.
Eps is an entirely new 8 cyl engine build as an aircraft engine. Conti is a converted merc car engine. Which one would you rely on?
“Even the most recent attempts at FADEC on diesel engines have proved unreliable”. Is that so?
@Michael, I got the answer about cooling drag, thanks, but could you also answer my other questions?
Serious vibration that literally destroys props;
I’ve never heard about this before. Could you explain?
Weight increase due to prop dampening, cooling system and the higher density of “heavy” fuels.
How is the fuel density important? The fuel mass should be what matters and since jet/diesel fuel has (slightly) higher energy density than petrol, you can get the same amount of energy for less weight with jet/diesel fuel (assuming the same thermal efficiency of the engine).
I’ve never heard about this before. Could you explain?
I don’t know if anyone will be able to find references because obviously no aircraft was actually delivered with such an issue, but it was found that some/most traditional aluminium propellers were not able to cope with the high torque pulses which diesel engines deliver, as a result of their high compression ratio.
The stress on a propeller is mainly tension (the centrifugal force – according to Hartzell the max rpm tensile stress is 25% of the limiting yield stress of the material) but there is a significant component resulting from the “whole picture” of the combustion events producing sharp torque pulses (3 per revolution on a 6-cyl 4-stroke) coupled to the flexibility of the propeller, causing the propeller blades to oscillate around their steady state positions. Hence diesels have the clutches (usually); it isn’t done for the benefit of the engine.
How is the fuel density important? The fuel mass should be what matters and since jet/diesel fuel has (slightly) higher energy density than petrol, you can get the same amount of energy for less weight with jet/diesel fuel (assuming the same thermal efficiency of the engine).
I don’t have the numbers (they were posted in past threads) and it could well be that there is no range loss for a piston diesel. There is of course a big range loss for a turboprop conversion but that’s because the SFC of a TP engine is so much worse. The Jetprop deals with this better than most conversions but is still around 1/3 worse than the piston PA46 flown on best-economy (not really comparing same speeds of course 
).
Peter wrote:
Hence diesels have the clutches (usually); it isn’t done for the benefit of the engine.
One diesel I saw at Friedrichshafen this year (can’t recall which one) instead of a clutch had a flexible coupling between the crankshaft and the propeller which allowed a bit of rotational “slack”. Wouldn’t that reduce the torque pulses at the propeller?
