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Thoughts about the intercooler for 914

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frans(at)privatepilots.nl
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Posted: Thu Jun 10, 2010 3:39 am Post subject: Thoughts about the intercooler for 914

The more I think about it, the more I wonder whether I should have an

intercooler at all.

(Apart from the fact that I think that the commonly used Bell

intercooler behind the naca inlet isn't doing much at all). Let's

suppose we have a proper setup, and the intercooler functions as intended.



There are three reasons for having an intercooler:

1) It reduces thermal strain on the engine by cooling the inlet air.

2) It allows the engine to produce a bit more power, as cooled air is

denser and thus contains more oxygen per volume. Cooling the air lowers

the pressure as well, so the TCU will boost up the pressure a bit more

until it reaches the target pressure in the airbox again. Thus, you will

end up with more dense air containing more oxygen per volume (and at the

same time of a lower temperature). The carbs will automatically squirt

more fuel in this denser air. You will end up with more power, but of

course to make this power you need more fuel as well. These HP's are not

for free.

3) If the OAT is extremely high, it could be possible that the heat

generated by boosing the pressure causes the inlet temperature to reach

the limit programmed into the TCU. The TCU will then reduce boost

pressure to stay below this temperature limit. This is 72C in older

TCU's, 88C in newer TCU's. The intercooler will prevent this

TCU-intervention from happening.



The downside of the intercooler:

1) Weight. This one is obvious. But of course in HP per weight these are

still "light-weight" HP's.

2) Drag. Intercoolers need loads of air to work well. The flow of air

the engine needs is high, and the temperature difference is small. So

you need to cool a lot of stuff that is just above ambient temperature.

This needs a lot of cooling air. Dragging air through a radiator is a

costly process. It might well be that the intercooler will cost you a

few knots cruise speed.



Do we actually need an intercooler?



No doubt, a properly setup intercooler will gain you a few HP's. The

commonly used Bell intercooler is worth 5 to 7 HP's (provided the setup

allows sufficient cooling air through it).



But, we use these HP's only during the take off run, when we run the

engine at 115%. It doesn't make sense to make more HP's when the

throttle is partially open: the throttle is partially open with a

reason, and if the intercooler increases the power of the engine, you

have to throttle back a bit more to stay at your intended fuel

consumption, MAP, speed and altitude.



Apart from this, the intercooler only works if the turbo is actually

boosting the pressure. Without boost, the air won't heat up so there is

nothing to cool at all.

During the cruise at moderate altitudes, as long as MAP is lower than

the ambient pressure, the turbo is not doing much, so the intercooler is

useless here. Even if MAP is slightly higher, the heat produced by

boosting up the pressure this tiny amount doesn't produce enough heat to

justify even thinking about it.



At high altitudes, let's say 10.000 feet or higher, the turbo is

probably working quite a bit during the cruise, for any higher MAP than

the ambient pressure can provide. Still, if you don't fly with wide open

throttle, the HP gain itself is useless. We then of course still have

the advantage of cooling the air, reducing thermal stress of the engine.

But how warm is the air actually at 10.000+ feet to begin with? Does it

actually make sense to cool it? Maybe the increase of temperature is

even beneficial, as this reduces the chances of icing and promotes

better fuel atomization.



The 5 to 7 HP gain allow you to take off in a bit shorter distance. This

would be great, but the fact that an Europa with a 914 can take off

already at a shorter distance than it needs to land, so this makes this

a theoretical advantage only. To practically benefit from the ability to

take off from an extremely short runway, you first have to land there,

which is impossible.



Then the final advantage is that it could be theoretically possible to

reach the warning limit of the TCU, 88 Celcius of the inlet temperature,

and if that happens the TCU will reduce boost and hence power. It must

be awfully warm if this happens, and I wonder if anyone has ever

experienced this loss of power in extremely hot OAT's.



So, to summarize:

1) During the cruise at low altitude the turbo isn't heating up the air,

so the intercooler is worthless.

2) During the cruise at high altitude the turbo is heating up the air,

but at that altitude the air is not very warm to begin with so it is not

a stress for the engine anyway.

3) During all this cruising we are carrying the weight of the

intercooler, and dragging loads of air through it. The intercooler is

costly in terms of cooling drag.

4) During take off we have 5 to 7 more HP's available, and cooler air

for the engine. But it doesn't allow us to take off from shorter runways

as the landing distance is already larger than the take off distance.



So.... I'm in serious doubt whether I should take out the intercooler.



Another option is to have a flap on it, to close off the useless air

flow through it during the cruise. An interesting way to achieve this is

to operate this flap by a piston, fed by the manifold pressure. As soon

as the manifold pressure is higher than the ambient pressure, it pushes

the flap open. So the intercooler is only producing drag when it

actually makes sense do to so. We could also achieve this with a servo

operated flap, controlled by a differential pressure sensor (combined

with a lower temperature limit). All this of course is quite a fuss for

a small gain in HP's, only used (not needed) during a fraction of the trip.



Opinions?



Frans

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rparigoris

Joined: 24 Nov 2009
Posts: 808

Posted: Thu Jun 10, 2010 7:04 am Post subject: Re: Thoughts about the intercooler for 914

Hi Frans



I agree that measuring temperature of air is not an easy thing to do without being influenced by mount.



My bottom line question to install intercooler or not was based on function of Bell intercooler.  If it is not doing job or extremely limited job, then may not be worth it.



It seems to me you are in the perfect situation to measure actual performance difference. Perhaps if you don't mind you can perform a few tests to measure performance now:

**In climb have intercooler flap opened and set up climb using 115% power, try maintaining climb to let things normalize, then set prop manually to hold RPM, note MP, then close intercooler flap. If the intercooler is doing anything then you should see a drop in RPM as you are now dumping warmer air into engine. Leave flap closed long enough to normalize, then open flap again and note if there is a RPM increase.

**Perform same test at 10K with full throttle in cruise.



If you do see a change then there is a good chance your air box sender is speaking with a forked tongue.



One major reason for not dumping air that is too warm through a 914 manifold, is that for a given octane fuel, detonation will occur a lot sooner with warmer air than with cooler air. That I believe is primary reason for TCU intervention.



Ron Parigoris

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frans(at)privatepilots.nl
Guest

Posted: Thu Jun 10, 2010 7:23 am Post subject: Thoughts about the intercooler for 914

On 06/10/2010 05:04 PM, rparigoris wrote:



 	  Quote: 	
		   It seems to me you are in the perfect situation to measure actual

 performance difference. Perhaps if you don't mind you can perform a

 few tests to measure performance now: **In climb have intercooler

 flap opened and set up climb using 115% power, try maintaining climb

 to let things normalize, then set prop manually to hold RPM, note MP,

 then close intercooler flap.


	


I will do so as soon as the cloud base will rise enough. 



 	  Quote: 	
		   One major reason for not dumping air that is too warm through a 914

 is for a given octane fuel detonation will occur a lot sooner than

 with cooler air. That I believe is primary reason for TCU

 intervention.


	


That's right. But I might assume that Rotax has set this temperature

limit to a specific value, and below this limit there is no risk of

detonation. I think it is quite a challenge to get the inlet temperature

up to the limit of 88C...



Frans

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