I am learning the complex compromises of cooling a turbocharged piston engine. In a nutshell: the system needs to provide enough cooling to just cope with the maximum strain which happens in the last few thousand feet of the climb, where air is thin, at an airspeed lower than cruise and at higher power setting than cruise.
One ends up with a cooling system that is oversized for the cruise condition, causing unnecessary drag.
Solutions include cowl flaps and other clever setups that reduce cooling airflow based upon angle of incidence.
There is also this annoying detail that one needs more cooling on hot days while the efficiency of the cooling system is hampered by the hot air…
In my aircraft, it seems that both oil and coolant thermostats are inop because I frequently see oil T dropping to 160 F or even slightly below in cruise whereas coolant drops to 150F in the descent if I chop power. Both should be thermostat limited to 178F.
This proves that the engine is getting too much cooling in cruise. So here a crazy idea:
Taking it even further, one could use more water and reduce airflow even further so as to increase cruise speed.
5 to 10 Kg of water can easily be accommodated in the baggage compartment and one would locate the pressure pump in the engine compartment and run the high pressure water hose in an area that does not see freezing temps at altitude.
A specialized nozzle and 13 bar pressure pump would spray a very fine mist in the intake so as to maximize the effect.
Looks like an inexpensive and rather safe solution?
Thoughts ?
A classical solution used on aircraft around the WWII era was to inject water into the engine intake. This would provide a much more efficient cooling. It is still used in some turbine engines for hot-and-high operation.
Water spray in induction is used in highly tuned car engine to reduce EGT, apparently working for 50 to 100 °C reduction. Water spray on intercooler also works but much less as it must be very tuned so that droplets are not too big to stick to radiator. Did you thought about icing?
Thanks for the comments !
Yes indeed water or water and methanol (MW-50) were used directly on inlet air in WW2.
I would not dare suggest something so intrusive without serious design in this context…
On the other hand, merely spraying a radiator with vaporised water, while less efficient would be safe ( affecting the engine like rain I suppose).
The key question is: how much additional cooling to enable full climb power at ISA +25?
I am looking at negating the effect of a 15 degree OAT increase during 5 minutes. The aircraft climbs and cools fine at ISA+10 up to FL200, so this would really put southern Europe and north Africa with ground temps up to 40C within “easy” operating parameters.
Indeed icing is a key issue and my thoughts were:
locating the water tank and its ducting inside the cabin will keep them safe.
running the high pressure line in the warm areas of the engine compartment is easy.
the nozzle itself may however need an ice prevention solution . I am hoping that careful investigation and placement would probably have it heated “just enough “ by the engine’s radiated heat, without falling into the opposite evil…
I also thought about using demineralised water and a filter for obvious reasons…
Any other input most welcome thanks.
Flyingfish wrote:
On the other hand, merely spraying a radiator with vaporised water, while less efficient would be safe
How do you know it will even work ? A radiator is designed and sized for air to be “pumped” through it due to pressure difference in front and aft. This pressure differential is more or less constant for a given air speed. Injecting water onto it will most certainly reduce the flow of air. Water will cool, but more than the air that would otherwise flow there? Sounds like a cool experiment though 
While injecting water/alcohol in the induction is very effective through the vaporization of all water in the process, I doubt that will be the case with water spray on an air-oil- (even less -water) radiator because it is simply not designed for such.
I doubt your air-oil or air-water radiators will have enough cross-section or temperature to force significant water vaporization (simply not designed for that)
My impression is the heat exchange through the heating of liquid water will be much less effective than vaporization, but you can run the numbers and tell us what you find.
Some cars have water spray on the (outside of) air-to-air intercoolers (even road ones, I seem to recall Mitsubishi Evo’s) and the effect should be similar. On my aircraft I see max around 120C on the air-air intercooler induction at altitude (about 2.5 constant compressor pressure ratio in cruise). IN the Evo, I doubt PR is higher than 2:1 and then only for brief periods unless at full throttle on the motorway so the temperature is not going to be vaporization-high either…(+65C for a 2:1 PR) . I think on the car it is more relying on the transients (unlike the aircraft) and system thermal inertia where having a cold intercooler will give you a few seconds of extra cooling while it is time to lift the throttle for the next bend. Not thought for continuous state cooling (as a minimum you would need several continuous minutes at the top of a climb). I think it is only a fancy copy of rally car racing where PR’s are much higher and hence more effective cooling.
A potential analogy could be what happens when we fly our air-cooled engines through rain: I never saw a significant CHT variation, and then our cyl heads are way hotter than any air-oil or air-water radiator so some vaporization is to be expected. I have not run rainy-wx surveys on my air-oil or air-air intercooler, but the former would be biased by the vernatherm.
Spraying water on the rad isn’t such an outlandish idea – P51 racers like Strega do it.
You’ll see a lot more success though in ensuring the air has a laminar flow in the intake duct and properly wets the whole radiator face. How much you can modify the duct without falling foul of STC requirements is debatable :-).
Thanks again guys, your input is much appreciated.
@T28: I totally agree with you. Here some food for thought: there are three radiators in my aircraft that would be candidates:
1. The coolant radiator which faces forward and receives the airflow at the optimum 90 degree angle.
One would think this is an ideal situation, but the inlet duct is too short and the resulting fast expansion is causing a lot of pressure loss and maybe blanking of 40% of the radiator. I believe this because I could see damage distribution on the forward face. Only the central portion is showing bent fins, implying that the remaining 40% are “second class citizens “
During the relevant part of the climb, this radiator is flowing coolant at 100C or more
2. The oil cooler, also forward facing is fed by a NACA inlet – according to NACA this is not an adequate application of this design.
Adding insult to injury, there is a 2 inch duct stealing some of the forward pressure for other uses. Fin damage is homogeneous.
At least this cooler has a proper plenum in the back, and dedicated louvers that seem to work. Oil temp is also above 100C in the relevant condition.
3. The third is the turbo intercooler . It is mounted parallel to the airstream, attached to a plenum which is fed by another NACA inlet. Comparing the cooling efficiency of this with data from a PA46 shared by @denopa, I conclude that there is significant room for improvement. Air temperature entering this radiator is even hotter than the oil and coolant, consistent with @Antonio’s data…
In other words all three radiators would be showing surface temperatures above the boiling point of water, especially so at altitude where the lower pressure will make water boil at lower temperature. Maybe I’m being naïve, but from the videos I’ve seen on YouTube, it seems that the key is to deliver very finely vaporized water, and then let the hot radiator tun this to vapor, a process which sheds considerable heat…
@LeSving – Will my idea work? I honestly do not know… So far I’ve had several disappointments and just enough success to keep me going. I think it is fair to say without exaggeration that my aircraft has the best performance of the fleet, especially its ability to climb to cruise, up to a moderately warm days. This is precisely what I would like to take one step forward, by making Galatea capable of coping with hot temperatures and escaping to altitude where ISA+20 is no problem in cruise
An optimum diffuser curve radius is in the single digits, so achieving that on the Extra water rad is not really feasible I think. Next best option is increasing the frontal intake duct area, which will penalise somewhat the cruise speed, combined with diffuser vanes in the duct to smooth the flow. Not easy but worth the effort.
A myth that is well entrenched is that liquid cooling is inefficient and cumbersome but a well set-up system can deliver exhaust air at a velocity in excess of plane TAS.
“An optimum diffuser curve radius is in the single digits, so achieving that on the Extra water rad is not really feasible I think. “
I have actually tried: MT propeller kindly shared the pressure distribution data of my prop and I made an oval inlet located much further forward and lower with a carefully shaped slow expansion duct.
When the mockup cowl was installed I realised that the lateral prop blast would cause lots of drag and so I stopped there.
Getting this approved only to find out that it makes the aircraft x knots slower was not a motivating prospect … But the project was a nice challenge…
“Next best option is increasing the frontal intake duct area, which will penalise somewhat the cruise speed, combined with diffuser vanes in the duct to smooth the flow. Not easy but worth the effort.“
I once took my aircraft to the Extra factory and they changed the shape of the exhaust side of this radiator’s ducting, to make it consistent stent with the final design ( mine is one of the early production planes and its cowling is different in many respects). This apparently minor change really helped lower temps, but it may have cost 1 knot…
“A myth that is well entrenched is that liquid cooling is inefficient and cumbersome but a well set-up system can deliver exhaust air at a velocity in excess of plane TAS.”
After 600 hours with water cooling, I am convinced that it is optimal for turbo SEPs
