More turbo questions.....

[Biggy72]

So I've been reading through the forums, searching, trying to find anything on the volumetric efficiencies with a turbo in front of a 600f4i. I'm working on a couple different books and I've got a few equations that the prof that teaches internal combustion engines gave me. It seems that either the volumetric efficiency would dictate boost, or vice versa, but my equations are just going around in circles.

Also where do you use corrected flow compared to actual flow? I'm assuming the corrected flow has some sort of efficiency built in specific to the turbo itself, but I'm not sure, and the corrected flow should be used to actually calculate the VE.

Is the BSFC the only place where the fuel mass comes into play? It seems like once the fuel mass ignites the expansion coming out the turbo should play some sort of roll in spinning the turbo faster, then making more boost, but once again I could be wrong.

Now I'm trying to figure out how to calculate what the theoretical EGT would be. I haven't come across anything except for experimental numbers to shoot for. Is there any sort of equation to find this?

Our advisor is still trying to tell me that we will never choke our turbo, and the actual choke point will be at the intake valve. I don't agree with this, but I don't have anything to back it up besides dyno graphs from a stock bike, and from our restricted engine from last year. Hopefully she can give me a better explanation tomorrow, but I'm thinking running a moderate amount of boost we should choke the restrictor near 10,000 rpm, maybe a little below that.

I'm hitting a wall right now where I understand quite a bit of the theory behind what's going on, I'm just running out of equations to calculate those things. I've got a spreadsheet going currently, but it keeps changing and so do all of my numbers on it. Hopefully I can come up with something a little more solid within a couple days.

[Biggy72]

So I've been reading through the forums, searching, trying to find anything on the volumetric efficiencies with a turbo in front of a 600f4i. I'm working on a couple different books and I've got a few equations that the prof that teaches internal combustion engines gave me. It seems that either the volumetric efficiency would dictate boost, or vice versa, but my equations are just going around in circles.

Also where do you use corrected flow compared to actual flow? I'm assuming the corrected flow has some sort of efficiency built in specific to the turbo itself, but I'm not sure, and the corrected flow should be used to actually calculate the VE.

Is the BSFC the only place where the fuel mass comes into play? It seems like once the fuel mass ignites the expansion coming out the turbo should play some sort of roll in spinning the turbo faster, then making more boost, but once again I could be wrong.

Now I'm trying to figure out how to calculate what the theoretical EGT would be. I haven't come across anything except for experimental numbers to shoot for. Is there any sort of equation to find this?

Our advisor is still trying to tell me that we will never choke our turbo, and the actual choke point will be at the intake valve. I don't agree with this, but I don't have anything to back it up besides dyno graphs from a stock bike, and from our restricted engine from last year. Hopefully she can give me a better explanation tomorrow, but I'm thinking running a moderate amount of boost we should choke the restrictor near 10,000 rpm, maybe a little below that.

I'm hitting a wall right now where I understand quite a bit of the theory behind what's going on, I'm just running out of equations to calculate those things. I've got a spreadsheet going currently, but it keeps changing and so do all of my numbers on it. Hopefully I can come up with something a little more solid within a couple days.

[Pete M]

I sympathise, took me a bit to work it all out.

My advice would be to figure out what would happen without the restrictor first. Then think about what the restrictor does in terms of flow. If you're not sure, look up compressible flow and converging-diverging nozzles in a fluids textbook.

To answer some of your specific questions. Assume a VE relative to manifold conditions, an engine speed, a mass flow, and an intake temperature and a boost number will drop out of that (simple m = rho*v).

Corrected flow should only be used on a compressor or turbine map. Forget the term "corrected flow" and think of it as "map flow". You take your actual mass flow and convert it to map mass flow before plotting any points on the compressor map. Corrected flow exists to account for how the behaviour of the compressor changes with the speed of sound and inlet density. Never use the corrected flow outside the compressor as it doesn't correspond to anything real there.

Fuel mass, pretty much ignore. It'll take up some volume of the charge inducted, but you're assuming a VE anyway, it will be swallowed up in this. Later when your engine is running, you can calculate what your VE actually is and update your models accordingly. If you're looking at a turbine map though, take the intake mass air flow you calculated and add the fuel mass flow to it.

You generally can't calculate EGT. Guesses based on other people's empirical data are all you've got until you get an engine running. If you assume 800-850 degrees, you won't be wildly wrong.

Whether you choke your turbo or not is largely a factor of the turbo you're running. Mainly, if you're running at high enough rpm, at high enough boost to choke, you'll either choke your restrictor or your compressor inducer. And unless you have a very small compressor, it'll be the restrictor that chokes. Whether your intake valves choke or not is fairly irrelevant. Fundamentally, the restrictor choking is the only thing that matters, because the engine is downstream of the turbocharger. If the restrictor isn't choked, run more boost until it does. The higher manifold pressure will mean you'll get more air into the engine regardless. Stuff doesn't choke in the sense of having no way to get more air into it if you can vary the inlet pressure. You may see mach 1, and it may be choked flow, but changes in boost pressure changes the choked mass flow.

This is a dodgy explanation, but it's as good as i'm going to do at 3 AM.

[VFR750R]

Assume VE for a naturally aspirated motor which will give you a volume flow rate. This won't change with a turbo. The turbo will effect the density of that volume flow rate and thus the mass flow rate. Lots of people talk about turbos increasing the VE but that is volume flow rate through the engine at STP. The turbo must consume more volume at STP to provide the mass flow required to move the same volume of air at a higher density.

The restrictor chokes first...and will be the only thing that chokes. I only know of one turbo on the planet that will choke before a 19mm restrictor and it isn't a GT12.

Your prof is thinking of a Mach index of.5 choking which is a 'soft' limit. All that means is that a one point in the intake stroke the flow is choked and i will garuntee you mach index of .5 is higher then the rpm you choke a 19 or 20mm restrictor.

[Biggy72]

Ok I started out figuring a n/a ve and then making the density adjust according to that, but then my prof told me that with a turbo the ve will always be above 100%. I sent an email to Cam at Honeywell and he said I was going in the right direction, but once I started trying to make the ve above 100% with a normal density everything started going funky on my graph.

Cam wants me to estimate a EGT and then I can figure out where on the turbine map I'll be. I haven't seen any equations for this sort of calc, but I need to research more. I was just going on the assumption that the mass of air through pumping would be the mass and pressure on the compressor map. Can anyone elaborate on what I should do with the EGT?

Thanks for all of your help.

[Pete M]

VE relative to ambient conditions is the most useless number (yay, we run a VE of 200%, how good are we!). Use a number relative to the manifold, an approximate VE for an NA engine. A turbo engine isn't really any different, it will just be breathing pressurised air. The turbo will affect VE somewhat, due to the change in pressure ratio across the engine, either in a good way or bad way depending on what EMAP you end up with. But assuming an NA VE is a good first approximation.

The rest, basically what VFR said. Was mostly what i was trying to say without just coming out and saying it. If you're running the stock ports, the valves may choke for part of the intake stroke, especially at high rpm, but that doesn't really affect you in any way.

EGT, as i said, assume one. Estimate doesn't necessarily mean calculate. There are models that will give you a number that may be more or less accurate, but it depends on what you want to do here. If you just assume one based on what other teams tell you, you'll be able to plot on your turbine map to within the accuracy of everything else you're assuming (like VE). That'll be enough to select a turbo and see how it will roughly behave. That's all you really need until you get the thing on a dyno and get some more precise data.

[VFR750R]

Like Pete said, Ricardo or GT power would have a hard time calculating EGT's closer then 50 deg without a team of engineers so trying to calculate yourself with paper or even excel or matlab would be wasted time. I would assume 1550ish (F)for gas and 1450ish for alcohol. The actual value is effected by so many factors, we could be here for days and not get any closer then that. Even where the egt tip is across the pipe diameter has measurable effects.

[Pete M]

Oh right, Fahrenheit and all that jazz... My 800-850 number was in Celsius, which works out to be about what VFR said.

[Kirk Feldkamp]

This was a while ago when I was still working at Honeywell:

http://fsae.com/eve/forums/a/tpc/f/125607348/m/77610337...10447431#92910447431 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/77610337431?r=92910447431#92910447431)

-Kirk

[Biggy72]

So I'm still working on the EGT, I'm going to assume 850ºC, but I'm trying to figure out where I put that number into my calcs. The only thing I'm coming up with right now is that it will change the mass flow on the compressor map and the only equation I'm finding is:

m_dot corrected= m_dot(To,in/Tref)^(1/2)*(pref/po,in)

Where To,in is the EGT temp, Tref is the ambient temp, pref is the ambient pressure, and po is the exhaust back pressure?

I'm not sure if this is right or not, but our advisor seemed to be making it more complicated then it needed to be and she wasn't sure what to do with it either. If the EGT comes in for something else give me some sort of hint as to where to go from there.

Thanks

[VFR750R]

I'm not sure, but EGT shouldn't increase your mass flow into the compressor unless it speeds up your turbine and hence compressor.

Obviously there must be a mass balance into the engine and out, therefore at steady state the compressor (plus fuel) must flow the same mass as the turbine. Maybe that helps.

A higher EGT will mean the same mass flow takes up more volume which should spin the turbine faster because an exhaust manifold has a fixed volume and if you heat a fixed volume you build more pressure. Same reason turbo manifolds are usually thick walled and small, lost heat is lost pressure is lost energy.

[Pete M]

The corrections use the inlet conditions of the compressor or turbine you're analysing. If you're looking at a compressor map, then you're interested in the temperature of the air entering the compressor. The EGT is only relevant if you're looking at a turbine map.

Don't forget that inlet temperature also goes into calculating corrected speed, not just corrected flow. This is less important with a compressor map but is very important if you don't want to be totally confused when trying to figure out a turbine map.

[Biggy72]

I wish I had found this sooner.... it would have helped me quite a bit. Oh well.
http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech103.html

So I understand how to calc the mass flow in the turbine side, but now I'm trying to figure out how the turbine speed affects everything. It seems like it would have to mess with the pressure, but I haven't found any equations for this sort of thing.

[Pete M]

Find the turbo speed you need to run from the compressor map, and then look at the turbine map and see if it's achievable. If your exhaust mass flow is higher than the map's mass flow for that speed, then you'll be able to sustain that speed (the excess mass flow goes through the wastegate). If the calculated exhaust mass flow is below the map line, tough, you can't make that boost at that rpm.

Once you've found the point on the turbine map that you're operating at, the back pressure can be read off the expansion ratio axis. Although this needs a bit of iteration as back pressure affects the corrected mass flow.

Unless of course you're talking about a variable vane turbine map... they're not quite as simple. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Biggy72]

My biggest problem with everything is understanding what exactly everything on the graphs are. So I take it the topographic type lines on the compressor map are the lines of efficiency, and the sort of horizontal lines that slope off to the right are the turbine rpm lines. I've found quite a bit of info on this stuff so I'm pretty sure all of that is right.

On the turbine map though there are a series of different symbols on lines going horizontally. I haven't found much information to actually explain what all of this stuff is. Also is the pressure ratio the pressure in the header over the atmospheric pressure?

I found from an old spreadsheet posted on here that the exhaust manifold pressure is 1.5*the intake pressure, but I think that's a pretty bold assumption for all areas of the map.

I'd like to stay away from the variable vane setup if at all possible, so I'm not worried about that stuff too much.

[Pete M]

The lines of symbols you see are speed lines. If you know what speed you want, you can look it up and get a mass flow and expansion ratio. Expansion ratio is inlet pressure over outlet pressure. Inlet pressure is essentially EMAP, outlet pressure is essentially atmospheric pressure (unless you have a restrictive muffler.

I definitely wouldn't assume that the exhaust manifold pressure is 1.5 times the intake manifold pressure. There's no need for such an assumption anyway, you can calculate this with the maps provided.

[Biggy72]

I thought that's what the lines were on the turbine map, but the numbers only go up to 120208 at the most, where the compressor map has all the way up to 240000. Shouldn't everything match up? The compressor can't turn faster than the turbine. If I solely go off of the turbine map I barely hit the compressor map.

[Pete M]

The speeds on both maps are corrected speeds, as i mentioned a few posts above (totally confusing isn't it? http://fsae.com/groupee_common/emoticons/icon_smile.gif ). You need to correct the turbine map speeds based on the temperature of the exhaust entering it. It's the formula on the garrett maps with N in it. Once you do this, you'll find the two speed ranges correspond.

[Biggy72]

Sorry about that. I was going about the turbo speed stuff backward and putting the Nphy where Nt or Nc should have been. Thanks for all of your help.