Comprerssor Map vs EGT's

[NoSeeUm]

So maybe I had too much spare time on my hands today. But I got to looking at the compressor map at the Garret Website and I got curious. I (a friend and myself) immediately figured how to read the chart, but found it of no use. This is because I had no clue what CFM (expressed as lbs/min on the chart) the engine was pulling at a given RPM and boost pressure.



Surfed the web, discovered out how to calculate "equivalent" CFM at various boost pressures vs engine RPM. Added this conversion to the Garret chart and got amazed.



If the Garret sales people did not doctor this up too much from the engineering people it shows that stock turbo just about perfectly fitted to the engine. I assumed that the chart showed a HX35 SO (235 Hp) it plots out right in the middle of the chart where I would place the highest efficientcy island.



What I came up with is at 20 psi boost (with a 90% Volume Efficientcy):

1800 RPM = 398 CFM (60 MPH)

2000 RPM = 442 CFM

2200 RPM = 487 CFM (a little over 70 MPH)



These CFM numbers cross sit right in the middle of the compressor map chart. If you look at the stock torque curve this is where the engine is designed to build torque. Pretty smart, right?



So once again, I get myself confused. If one wants to add injectors, electronic boxes or what ever to increase stock Hp then one must also increase air flow. This means getting another turbo to control EGT's.



How would one go about doing that to match a turbo up to the Hp increase?



I kind of envisioned that it would require setting the waste gate at a pressure that crosses the compressor map at a spot where the turbo runs at the most efficient area and with the engine running at 1800 to 2200 RPM.



Other than trial and error, word of mouth how would you really know that this turbo over that turbo is the one you need?



And will this really ad up to controlling EGT with the Hp the engine is putting out.



Thanks for any replies;

Jim

 

[Alan Reagan]

Now that you know how to use the map:



Pick 4 different operating scenarios. Highway cruise (2000 rpm), part throttle acceleration (say 2500 rpm), full throttle acceleration at 3000 rpm, and full throttle acceleration at 3400 rpm.



Second, calculate the volumetric flow for each one of those cases. Then, making estimates of the intercooler outlet temperature , turbo discharge pressure, volumetric efficiency, etc. . calculate the mass air flow for each case. You may also want to check the difference between summer and winter, ie air temps at maybe 90 deg F and 40 deg F. This will affect the manifold temperature and so the air flow. Note that when cruising and at idle, you will probably never have zero pressure. The turbo has to pump up the air some, even if it is only to 0. 5 psig or so. Besides the mass air flow, calculate the Pout/Pin for each case.



Third find the compressor maps for the turbos you are interested in. Plot the points from the 4 cases on the compressor map.



Fourth, evaluate the proposed compressors performance. Are the idle/cruise operating points to the left of the surge line? Then this turbo will surge and isn't a good choice. Is the 3400 rpm point so far out to the right that it is off the map? Then this turbo doesn't flow enough for your application. You want all the operating points within the map, and preferably at as high an efficiency as you can get.



Maybe that will help decrease the trial and error some.

 

[NoSeeUm]

More head stratching

Thanks Allan, you seem a couple of light years ahead of me on this.



In my specific case I measure boost pressure at the manifold, not at the compressor discharge. So in considering this, can't I use my indicated boost pressure in calculating engine CFM, because it already takes into account my pressure losses due to flow? I realise that boost pressure at the compressor would be higher and is there a generally accepted constant for this?



Also I understand that the compressor MAP is calculated to STP. Would that mean that as ambient temperature and or barometric pressure changes this would effectively just shift the compressor envelope some to the right, left, up or down, but that the actually operating point would remain in the at the relative same spot? This would make sense to me, because from what I can interpret the stock turbo sits pretty much in the middle of the envelope so it has quite a bit of wiggle room as ambient conditions change.



I intend to set a soft redline at 2500 RPM, but I want to use a working RPM range of lets say 1600 RPM to 2300 RPM. Going mostly to 2500 RPM to make a 3rd to 4th shift, at infrequent times. I would not be concerned to go off the MAP at those short time periods. I don't need 3400 RPM data because I won't go there.



For my experience in frame style industrial gas turbines, EGT is a dynamically controlled temperature. You basically, have a fueling curve base upon turbine load and ambient temperature. To control EGT you increase or decrease engine air CFM by throttling at the compressor inlet. Once the inlet throttle value reaches minimum (maximum air), you control EGT using fuel flow rate.



So if I understand correctly, that to control EGT in a turbo charged diesel engine you must have some excess air. The amount of excess air is based upon the required amount of air to support combustion and then some extra to cool it. Once your run out of excess air the EGT will rise. I suppose you would need to throw out exhaust any restriction and or timing changes here to make things equal.



So... ...



In the case of a turbo diesel compressor MAP and increasing engine Hp you would need to maintain some percent of excessive air to be able to control EGT. The EGT actually, just seems to float freely. So by control I mean stay below a maximum temperature. This would be for a given fueling rate.



Can I assume that the way really to get excess air is to up boost pressure while keeping that boost (PR) vs CFM (lbs/min) value in the middle of the compressor envelope based upon engine RPM?



Would there be some thumb rule for saying like XX increase in Hp requires YY increase in boost cool pressure?



Thanks again;

Jim

 

[Hohn]

Great posts, fellas.


When I look at a compressor map, I try to conceptualize it, or put some application behind the axes of the graph.

For example, one Y axis is Pressure Ratio (Pin/Pout). You can conceptualize this as boost pressure at constant RPM.

The X axis is airflow in Lbm. This is actually air moving into the engine. I think it's appropriate to think of this in terms of both RPM and Boost.

Anyway, Alan's post about using maps to select a turbo a right on.


I'd also add that you want to strongly consider the surge line on the far left. This is the point on the map that's most significant, imo. You need to make sure that your vehicle cannot generate conditions that would cause the turbo to operate to the left of this line, where airflow is wildly unstable.

That said, the farther away from the surge line you go, the laggier the turbo is. So it's kind of like the "price is right"-- you want to get as close as you can WITHOUT GOING OVER.

Anyone can keep a turbo out of surge by making it a laggy dog.

The other complication is that as turbos get bigger (compressor size), the surge line moves farther to the right. That makes sense, since we know that bigger turbos have more lag.



Anyway, I'd follow Alan's methodology for picking points on the map, but I would use different data points. For example, I'd never pick 3400rpm, because I'm so rarely there that I don't feel it should be a consideration-- and I'm perfectly fine with being off the map for a couple seconds under those conditions.

Instead, I'd pick steady cruise at 1800rpm, WOT at 2600, WOT at 1500, and steady cruise at 2300.

At WOT, you're going to rev the engine higher than normal. So pick higher RPM points.

Also, consider picking the RPM in the MIDDLE of your intended operating range. So if you operate from 1500-3000rpm at WOT, then pick 2250 rpm. If your dead in the middle of the map at 2250rpm, then you're in pretty good shape overall, and it's a good compromise.

Anyway, that's my opinion, valid or not.

jh

 

[Hohn]

Thanks Allan, you seem a couple of light years ahead of me on this.

In my specific case I measure boost pressure at the manifold, not at the compressor discharge. So in considering this, can't I use my indicated boost pressure in calculating engine CFM, because it already takes into account my pressure losses due to flow? I realise that boost pressure at the compressor would be higher and is there a generally accepted constant for this?

Also I understand that the compressor MAP is calculated to STP. Would that mean that as ambient temperature and or barometric pressure changes this would effectively just shift the compressor envelope some to the right, left, up or down, but that the actually operating point would remain in the at the relative same spot? This would make sense to me, because from what I can interpret the stock turbo sits pretty much in the middle of the envelope so it has quite a bit of wiggle room as ambient conditions change.

I intend to set a soft redline at 2500 RPM, but I want to use a working RPM range of lets say 1600 RPM to 2300 RPM. Going mostly to 2500 RPM to make a 3rd to 4th shift, at infrequent times. I would not be concerned to go off the MAP at those short time periods. I don't need 3400 RPM data because I won't go there.

For my experience in frame style industrial gas turbines, EGT is a dynamically controlled temperature. You basically, have a fueling curve base upon turbine load and ambient temperature. To control EGT you increase or decrease engine air CFM by throttling at the compressor inlet. Once the inlet throttle value reaches minimum (maximum air), you control EGT using fuel flow rate.

So if I understand correctly, that to control EGT in a turbo charged diesel engine you must have some excess air. The amount of excess air is based upon the required amount of air to support combustion and then some extra to cool it. Once your run out of excess air the EGT will rise. I suppose you would need to throw out exhaust any restriction and or timing changes here to make things equal.

So... ...

In the case of a turbo diesel compressor MAP and increasing engine Hp you would need to maintain some percent of excessive air to be able to control EGT. The EGT actually, just seems to float freely. So by control I mean stay below a maximum temperature. This would be for a given fueling rate.

Can I assume that the way really to get excess air is to up boost pressure while keeping that boost (PR) vs CFM (lbs/min) value in the middle of the compressor envelope based upon engine RPM?

Would there be some thumb rule for saying like XX increase in Hp requires YY increase in boost cool pressure?

Thanks again;
Jim

Since the compressor map is calibrated to STP, it *is* somewhat self-correcting. I wouldn't worry about that aspect.

Your diesel needs excess air for sure. Without it, EGT will be super high, since diesel fuel burning at stoich ratio produces EGTs of about 1500° (IIRC). On your engine, EGT is the best indicator of the QUANTITY of excess air-- or more accurately, the RATIO of air to fuel.

As for the boost vs RPM, I believe you are correct in your thinking. Since the engine's RPM is constant ,and the displacement is constant, the only way to deliver more air (lbm) is to increase boost pressure. Being in the middle of the map is VERY good.

As for a rule of thumb, you can't really use one. Some diesel guys will say "1 psi for every ten HP" or something like that. But it's terribly inaccurate and imprecise.

That's because the turbo is NOT a linear device. Your engine can make HP with ZERO boost.

Also, boost levels are not linear-- you can't just apply Ideal Gas Law. That's why there are efficiency islands. As you move around on the map, the efficiency changes, and you'll go from being very close (say 85%) of the value predicted by IGL to not even near (50% or worse).


The other consideration is that high EGT may be cause by more than just not enough excess air. If the engine's pressure ratios (boost vs drive pressure) are grossly imbalanced, then the engine will try to suck in exhaust and this really hurts burn rate, causing very high EGTs-- even if there would otherwise be plenty of air available.

So, EGT may be misleading under certain circumstances. IOW, high EGT might not just be an air fuel ratio thing, but rather a consequence of improper engine operating conditions.

h

 

[Alan Reagan]

Remember, when you are doing your calculations, add 14. 7 PSI to the manifold pressure when making the calculations for lbs/min.



I don't disagree with the above post on 3400 rpms. I put that there in case you do occasionally get that high. I rarely do, BUT on occasion, when my auto downshifts (as in a passing situation), the RPMs to zoom up and you need to make sure the turbo is good in that range or there will be a lag and loss of power.



When I have more time, I'll look at your questions and see if I can answer more.



One thing, the excess air you are talking about... ... . are you referring also to volumetric efficiency a little in that all the air/heat is not cleared from the cylinder on each exhaust stroke? Or are you thinking that all the air is not used in the combustion and what is left over is cooling air? Not clear there.



Finally, EGTs may be lowered by a more efficient intercooler. The more boost you have, the more compression you have by the turbine which in itself builds more heat which needs to be "shed" before the air gets to the intake manifold.

 

[NoSeeUm]

OK I'm about a light year behind Hohn also, that Stoich and IGF stuff has mountains of brain dust on it.



But, see if I have this right... .



I'm looking at the MAP for the Dodgezilla (click the Technical Info tab)



Lets say I set the waste gate at 35 psi and assume a 90% VE.



RPM PR Boost CFM Mode

1800 1. 27 4 215 (60 MPH cruise empty - using HX35)

2100 1. 41 6 277 (70 MPH cruise empty - using HX35)



1500 3. 40 35 475 (bottom of the usable RPM loaded)

1800 3. 40 35 570 (60 MPH pulling a load loaded)

2200 3. 40 35 697 (58 MPH in 4th gear loaded)

2500 3. 40 35 792 (top of the usable RPM loaded)



2700 1. 54 8 391 (floating my valves empty - estimated)

2700 3. 40 35 856 (floating my valves loaded)



Am I seeing this as sitting pretty well at a good operating spot on the MAP? My thinking is that this MAX operating parameter and you can slide downward on the chart and be fine, just not left or right.



Now I need to figure out what Hp would push the turbo to achieve this?

{snip}

One thing, the excess air you are talking about... ... . are you referring also to volumetric efficiency a little in that all the air/heat is not cleared from the cylinder on each exhaust stroke? Or are you thinking that all the air is not used in the combustion and what is left over is cooling air? Not clear there.

{snip}

I guess I was talking about excess air in the terms of just have more air present than what is actually needed to satisfy combustion. So you would spread the BTU's out over a larger amount of air so you would end up with less BTU/lbs or a lower temperature increase. IE one component of cooling air and another component of combustion air.



One more question, I assume that the horizontal lines that taper downward from left to right are RPM lines?



Jim

 

[Hohn]

Correct. Those lines are RPM (shaft speed).

You are also correct on the downward slide vs. left/right.

I'm impressed how intuitive this seems to be for you, Jim. Lots of folks don't understand a map even after it's explained. You seem to understand it (and the implications) even before an explanation was offered.

jh

 

[NoSeeUm]

Thanks Hohn, I had never seen a turbo compressor MAP before, but I did have a pretty good idea of what they would look like. But I have seen many other charts that look allot alike that apply to a very wide range of applications. So it was fairly easy for me to relate what I already knew to the compressor map curve.



I did have help with the lbs/min to CFM conversion from a friend. Now that guy is smart, basically he did the whole calculation in his head from memory and just gave me a constant to apply. (I know he did, because he is the type who talks to himself allot when he is thinking. )



Jim

 

[Hohn]

Alexander Hamilton talked to himself all the time.

He was amazing.

 

[NoSeeUm]

Argh!!!!



I kept wondering about the turbine performance and now I have an idea as too why there is not much talk about turbine sizing to match the compressor map.



I got to reading this and more or less got my answer.

Operating Range; A/R Ratio

Low-end; 0. 58

Midrange; 0. 69-0. 81

High-rpm; 0. 96



{snip}



Given an equivalent turbine trim and A/R ratio, as engine displacement increases, the operating rpm range characteristics of the turbine decrease. Then there's also the heat the unit will see from the engine and exhaust gases, which change the unit's efficiency curve. Wastegate location and design also affects the turbine's performance. The interrelationship of all these factors is extremely complex, so there are no simple selection maps for turbines like those available for compressors. Even for experienced turbo installers, it often boils down to trial-and-error--kind of like trying several different size carbs on a normally aspirated motor. About the best advice we can give is that once you've settled on the compressor, consult your favorite turbo dealer for advice on mating it to a turbine housing that's best suited for your application's needs.

I just could not get past the part of "How to size the turbine correctly?" After reading this part it had allot of value for me. It does not really explain engine Hp increases nor diesel applications that well, but the point is pretty well stated. I suppose you could some how qualify the comparison between displacement size increases to Hp increases.



Jim