General Turbo Questions

[13brv3]

Greetings,

Since I ordered the Turbo engine for my Onex project, you can expect to see me start a few turbo discussions. That could be good or bad of course :)

First, I would really like to run auto fuel, and it's not recommended at this point. It sounds like it isn't approved because they haven't tested it, rather than because there would be a problem. Hopefully, they'll get to test it before I need to find out if it works.

As for the wastegate, I've read that they're using the lightest spring available, which is in the 42-44 in hg range, but they want to limit boost to 40 in hg so they recommend manually limiting boost via the throttle. Is this correct?

Finally, the 40 in hg limit doesn't specify compression ratio. The manual for the engine, and the turbo presentation I watched both say you can use 7 or 8 to 1 CR. From the charts I've found, it would appear that 1 point in CR is equal to about 4 PSI, which is about 8 in hg. If that's all true, then a 7:1 engine should be able to run 8 in hg higher boost than the 8:1 engine for the same cylinder pressure and power. Is my understanding correct on that?

If we can assume that the 40 in hg limit is for an 8:1 engine, then shouldn't we be able to set the CR to 7.5:1, then just let the 44 in hg wastegate control the boost automatically rather than trying to manually do it via the throttle? Also hopefully we can use 7:1 and run premium auto fuel.

I'm definitely NOT a turbo expert, so I'm just throwing this out for discussion. Stirring up trouble as usual :lol:

Cheers,
Rusty

[Rynoth]

As for the wastegate, I've read that they're using the lightest spring available, which is in the 42-44 in hg range, but they want to limit boost to 40 in hg so they recommend manually limiting boost via the throttle. Is this correct?

Finally, the 40 in hg limit doesn't specify compression ratio. The manual for the engine, and the turbo presentation I watched both say you can use 7 or 8 to 1 CR. From the charts I've found, it would appear that 1 point in CR is equal to about 4 PSI, which is about 8 in hg. If that's all true, then a 7:1 engine should be able to run 8 in hg higher boost than the 8:1 engine for the same cylinder pressure and power. Is my understanding correct on that?

If we can assume that the 40 in hg limit is for an 8:1 engine, then shouldn't we be able to set the CR to 7.5:1, then just let the 44 in hg wastegate control the boost automatically rather than trying to manually do it via the throttle? Also hopefully we can use 7:1 and run premium auto fuel.

I'm definitely NOT a turbo expert, so I'm just throwing this out for discussion. Stirring up trouble as usual :lol:

Cheers,
Rusty

The answer to your first question is yes, it's up to the pilot to keep the manifold pressure under 40in. Reports I've seen indicates that pressure will build as airspeed increases, so on takeoff roll you may actually have to decrease the throttle to stay under 40.

As for compression ratio vs manifold pressure, I'm no engineer and don't know if the math is that simple. With auto gas you're more likely to experience detonation if compression is too high, so I'd think the 40in max would still be appropriate with the 7:1 ratio. I.e. overall compression needs to be less without 100ll. Just my hunch.

[MichaelFarley56]

Rusty,

I believe you're correct; at this point, the factory hasn't been able to finish testing the turbo with auto fuel and as such, they can't recommend it at this point. That being said, I do know of at least one turbo owner currently flying behind, and having no problems with, ethanol free auto fuel. For me personally, I'd consider testing auto fuel but around my home airport it's nearly impossible to find ethanol free auto fuel so I don't mess with it. I'm fine sticking with 100LL!

As for your compression ratio questions, you're probably correct in your calculations but there's more to it than simple detonation worry. While that is a concern, when the factory chose 40" MAP as redline they also took reliability and heat issues into effect. The AeroVee itself can handle a lot higher boost levels than 40" but what happens over a long time when you run the engine that hard? You're also talking about a lot of heat when running the engine that hard, hence the 2 minute limit when running your engine above 35" MAP.

At this point, if you want to consider running auto fuel in the future my recommendation is to build your engine at the 7.0 to 1 compression (better margins for detonation issues anyway) and then stay tuned. I'm sure more people will test auto fuel in the near future.

[13brv3]

Mike,

As I understand it, an 8:1 engine at 40" should be making the same power as a 7:1 engine at 48". If (BIG if) I'm correct about that, then they should also be producing the same heat, and should be equally reliable.

I did read some other articles that said heat removal is the limiting factor for VW type engines, and the article actually predicted that any turbo application would be time limited, or used only as turbo normalization. They seem to think it would take a lot of money and modification to get more heat out.

The wastegate should limit the boost to some level, but if it has a 42-44 spring, I don't know what level it would eventually hold at. I know in the rotary engine world, we found that most internal wastegates were too small to really completely control the boost. It would be nice to have the boost controlled automatically, but it might require an external wastgate, and more cost/time/effort than it's worth.

Rusty

[MichaelFarley56]

I'm sure you know more about this than I do Rusty! Personally, I find it hard to believe that this small compression change would make that much of a difference to total power output, even if both engines are at the 40" max MAP.

Yes it would be nice to have some sort of automatic manifold pressure control but like you said it may quickly become way too complex. Personally, I'm fine with being the manifold pressure controller using throttle alone. Just like the PT6A's on a KingAir!

[mike20sm]

Here's a general turbo question I had recently. I read this article concerning water cooling http://www.turbobygarrett.com/turbobygarrett/sites/default/files/Garrett_White_Paper_01_Water_Cooling.pdf I like to run things as cool as possible so I was wondering if the factory chose not to run a water cooler cause generally Turbos go in cars which are much more demanding and found the water not needed for an airplane application?

[13brv3]

I've got to believe the overwhelming reason for not running water cooled turbos is the fact that we don't have water cooled engines. It would be reasonable in other auto conversions, but there's no way it would be feasible to add a radiator and pump. These are going to be relatively low hour engines, and they'll be much better cared for than the average car engine, so I'm happy with oil cooled.

As for the power difference, I'm still looking into that. The charts I've found show "effective compression ratio", and it's really more like 3psi or 6 in hg difference typically. I'm still not quite convinced that you get the same power at a particular effective compression ratio. For example, a 7:1 engine at 5psi is about the same ECR as an 8:1 engine at 8psi. More research seems to be in order.

Cheers,
Rusty

[Bryan Cotton]

One thing about airplanes - you have a long period of descent and taxi prior to shutdown. I bet the turbo is relatively cool after that. We will see if the early adopters have coking problems or not.

[13brv3]

OK, if anyone is having trouble sleeping (<g>), here's a web site that appears to be pretty good:
http://www.xcceleration.com/cr-boost%20101.htm

One excerpt from the page would seem to indicate that keeping the effective compression ratio (ECR) the same does not come close to keeping the power the same. From this example, it sounds like maybe you could run an additional 2 in hg when using 7:1 instead of 8:1.

So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential). Suddenly it's quite obvious why top fuel is running 5:1 compression, that's where all the power is!!

The one thing I can't figure out in his article is how he's getting the percentage of extra oxygen that he quotes. In one place, he says one extra psi gives you 3.4% more oxygen, and in the example above, it's 8.3 psi equals 28%. Those are consistent, but I can't see where he gets 3.4% per psi from. My defense is that I'm an electronics guy :)

One thing is for sure, I'm going to enjoy playing with the turbo, measuring temps, pressures, and anything else I can think of. It should keep the phase 1 time exciting, but hopefully not too exciting.

Cheers,
Rusty