ForcedAeroMotive: SuperCharged DA40

[Antoine]

I have had an email exchange with Rod, the designer about this back in December, 2010, here are his words:

"These are low boost systems and do not warrant an intercooler. Induction temperatures will rise 20 to 70 degrees F over ambient."

While I trust Rod's word, I am a bit puzzled because the theoretical calculation does not come anywhere close this very good number:

Assuming that MP is boosted from 20" to 30" (roughly) thats a 50% increase in pressure. If I remember correctly my college physics (pV=NkT someone?) that converts into a proportional increase of the gas temperature in K.

if the OAT is 0 C ( 273 K) and we apply the above mentionned boost, we should see a 50% increase in temperature to roughly 400 K which is 135 C or 260 F.

Thats well over 200 F and very hot indeed... so what gives?

[Chris]

Assuming that MP is boosted from 20" to 30" (roughly) thats a 50% increase in pressure. If I remember correctly my college physics (pV=NkT someone?) that converts into a proportional increase of the gas temperature in K.

if the OAT is 0 C ( 273 K) and we apply the above mentionned boost, we should see a 50% increase in temperature to roughly 400 K which is 135 C or 260 F.

Thats well over 200 F and very hot indeed... so what gives?

Maybe it is because you are also increasing the mass of air flowing through the engine (N), which is really the whole point, so not all of the increase goes into T.

[Antoine]

Assuming that MP is boosted from 20" to 30" (roughly) thats a 50% increase in pressure. If I remember correctly my college physics (pV=NkT someone?) that converts into a proportional increase of the gas temperature in K.

if the OAT is 0 C ( 273 K) and we apply the above mentionned boost, we should see a 50% increase in temperature to roughly 400 K which is 135 C or 260 F.

Thats well over 200 F and very hot indeed... so what gives?

Maybe it is because you are also increasing the mass of air flowing through the engine (N), which is really the whole point, so not all of the increase goes into T.

More head scratching... Do we really have a mass increase? We are compressing a certain volume and mass of air into a smaller volume... I would feeel comfortable saying that the mass is unchanged?
Maybe it's the wrong physical model?

[Chris]

More head scratching... Do we really have a mass increase? We are compressing a certain volume and mass of air into a smaller volume... I would feel comfortable saying that the mass is unchanged?
Maybe it's the wrong physical model?

But the point of increasing pressure is to get more oxygen into the cylinders, so I would say that there is a definite mass increase per cycle. Increasing pressure without moving more air through the engine wouldn't improve performance.

The compressor draws in ambient air and pumps it into the intake manifold at increased pressure, resulting in a greater mass of air entering the cylinders on each intake stroke.

Another way to think about it is that there are three independent ways to increase pressure: raise temperature, decrease volume, or increase air mass. The volume of the induction system is constant. The supercharger is injecting more air mass and the temperature and pressure increases are side effects.

[Chris]

More head scratching... Do we really have a mass increase? We are compressing a certain volume and mass of air into a smaller volume... I would feel comfortable saying that the mass is unchanged?
Maybe it's the wrong physical model?

But the point of increasing pressure is to get more oxygen into the cylinders, so I would say that there is a definite mass increase per cycle. Increasing pressure without moving more air through the engine wouldn't improve performance.

Hmmm, I see how I got myself confused. I was comparing non- vs. charged and you are talking about just the mass going through the single charged system. You are right, of course,... that mass stays constant while it passes through the charger and gets compressed on its way to the engine.

[Chris]

Assuming that MP is boosted from 20" to 30" (roughly) thats a 50% increase in pressure. If I remember correctly my college physics (pV=NkT someone?) that converts into a proportional increase of the gas temperature in K.

Perhaps it can only boost from 20" to 23" at 0 C, which would give about a 70 F increase in temperature if I'm doing the math correctly?

[Antoine]

Lol Chris, looks like my confusion is contagious! Sorry for ruining your day!

You raise a valid point, my initial assumption was probably exagerated in terms of pressure increase.
I understand the system should restore sea level pressure (say 29 inches) at 8000 ft (around 22 inches?) that's a 30% increase. Using the friggin formula, we should expect the temperature to rise by 30% of 273 K which is about 82 K.

That's a 150 F increase... still twice the upper limit Rod stated...

[BRS]

I asked Rod to answer some of these questions for us. Here is what he has to say.

Brock,

The "average" turbo system will heat the induction air 200-220 degrees F and installing an intercooler will extract 100 degrees from this. That's a lot of heat removed but the remaining heat is why turbos perform so poorly at lower altitudes compared with their normally aspirated brothers. For instance, a new Cirrus TAT turbo and NA SR22 will get to 13000 at exactly the same time, 13.5 minutes. The turbo of course has been catching up for the last 6000 feet. A supercharged SR22 will get to 13000’ a full minute before either of them.

That brings us to your question on the heat of compression temperature rise in the DA-40. I don’t know what it is on the DA-40 because I haven’t measured it, but I can tell you this. All of our systems to date raise the temperature about 60 degrees on average. This is not enough to meet induction deicing regulations (carb heat} so heated inlet air must be retained. Put another way, alternate air on your DA-40 heats the intake charge more then the supercharger. It is also not enough heat to justify an intercooler. The heat exchange at these low temps would not be
efficient.

After you have flown for awhile and have [provided] some performance numbers...[and if there is enough] interest for us to pursue an STC. These temperatures will be measured during that certification process.

Hope this helps.

About heat generation: The turbochargers are driven by hot exhaust gasses and lubricated by hot engine oil. Thus the Turbocharger is hot even before the air is compressed. In the past, FAT's superchargers were lubricated by hot engine oil, it is from these that the data comes from which Rod has. On the DA40 we have gone to a self contained lubrication system. The engine oil is not heating up the SuperCharger. The only heat comes from lower deck cowl and some from compression. -brs

[BRS]

Ok Tommy, you've got my attention. I've been wanting to post an update for sometime but life has gotten in the way. Ill spare you the details.

Finally, the annual is done. Despite my best efforts to prevent a parts order delay we had one anyway. I ordered the wrong fuel probe for a chronic pesky issue. Then the one the shop ordered was bad so they had to do it again. I'm thinking a sight glad or tube is really solid engineering.

After the annual it was the paint for the cowl that held us up. Now the hold up is that Rod had to leave town this week. So the plane is done but untested. All this should be taken care of next week at erich point it will have to wait for my schedule an appropriate wx.

So really i've not much to report except that this project is alive and well and I sill miss my plane. The god side in all this is that in its absence i've been working on my cfi and just have the check ride to compleat.i suspect french my plan back will be just in time to reward all my studying.

Hopefully, more next week.

[BRS]

Thanks for the update, Brock. Once it is back in the Pacific NW, let's meet up somewhere and race our XL against your Super40 for a few laps around Mt St Helens.

Your airport or mine? St Helens would be a good test though to be fair to the modification we should make it up to 12,000' or more.

Is the plane back home? I'd pick KHIO or KVUO over Goheen as a starting point just for my comfort level. I haven't taken the DA40 into grass yet, and I'm not sure the partners would approve. Up to 12k or higher sounds fine with me. It would be interesting to compare the G1000 logs from both aircraft after flying the same profile.

No the plane is in limbo this week as Rod is out of town. It will be ready for pickup next week but I may have scheduling issues for going to go get it. If its not one thing its....

Don't want to fly into Goheen? Wimp (just kidding). You should have been there the day I took delivery. It took three attempts for the guy from Galvin to put it down. I've learned to have it down in half the field length but it can be tricky if from the North. Been wanting to make a video of slipping it between the trees on the north end to give to the folks who own the trees in the way of the final approach. We have been trying to get those cut down for several years.