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Old January 14th, 2008, 06:48   #1
20vK
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Default CFM v's BHP?

Hi Guys,

With gas, you need around 150cfm of air to make each 100bhp, (this is very much a generalisation, so please don't flame me )

Is there a similar generalised formula for the PD diesel engine?

I'm looking at compressor maps and specs and trying to get my head around how to get a close guestimate on how much air / what size turbo I need to hit my targets.

I understand that compressor efficiency etc all has to be factored in, but I need somwhere to start from!

Many thanks,

Rich
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Old January 14th, 2008, 07:37   #2
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Rich take a look at / on www.kermatdi.com its where you will find a turbo or reference to one which should meet your target figure........or another option would be allards vt2 turbo plus manifold , either way i think you will be satisfied although kerma's hammerhead unit has slightly more power potential.
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Old January 14th, 2008, 07:40   #3
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No need for the scientific approach as all the guess work has been done for you , to make it a no brainer bolt on option.
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Old January 14th, 2008, 08:19   #4
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Well - that's my problem!

Bear in mind, my car is currently a 150PD and I'll be uprating injectors, intercooler, clutch & flywheel, diff, manifold (exhaust), turbo, cam, exhaust (no cat), egr delete and mapping.

- Kerma suggested their ST3 (not exactly sure what turbine and compressor wheel it has - $3k +import tax)
- Allard said I'd need something like their VT7 or something with a GT25 turbine wheel (which they said wouldn't give significant lag) - again no details on housing / compressor size
- You're running a GT25BB (I believe)
- and I've seen a dyno of a VNT22 hitting my target

My target is 280bhp (@ the fly) and 400 lb ft of torque (@ the fly), with a decent torque / boost curve.

Is it too much to want to know what I am buying?!I've been quoted $2200 for a VNT20 (from the UK), and I know I could have a hybrid made up for me to my spec, if I need to go bigger. I would then know what I have, and be able to make an informed decision if I wanted to go bigger or make changes at some point in the future.

With the tuner-shop turbos, I have no idea what I'm getting, and they are both modified to fit the manifolds. If I have a problem at all with a turbo, I will have to approach them to get a replacement, rather than having the choice to upgrade the spec on any future turbo I may buy, from any turbo supplier.

If I know what I'm getting, then I have a better chance of predicting how the egt/s boost / efficiency will change as I make adjustments

Does this make sense to anyone, or am I alone!

Last edited by 20vK; January 14th, 2008 at 08:30.
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Old January 14th, 2008, 09:14   #5
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Got you mixed up with someone else........but you have a pm now.
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Old January 14th, 2008, 09:48   #6
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Cracking!

Cheers fella!
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Old January 14th, 2008, 12:44   #7
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I have done a detailed analysis of the 1997 R-TDI, which produced 190 bhp from a VE-injection pump engine (does not matter in principle which injection system is used). The results fit the published data pretty well.

In that analysis, the engine had an airflow of 303 CFM to make 190 bhp rated power, meaning about 159 CFM per 100 HP, which makes a lot of sense: My calculation was based on the Diesel engine operating at an AFR of 19:1 (Lambda 1.3). A gasoline engine operating at maximum power requires that the AFR be enrichened substantially from stoichiometric. I have seen values down to 10:1, but more commonly it's around 12:1. The stoichiometric AFR of gasoline is actually nearly equal to that of Diesel, between 14.5-14.7:1.

The point is, the ratio of AFRs, 19/12 is exactly in the range of the ratio of the airflow requirements from your rule-of-thumb you gave for gasoline engine (159/100).

Short answer to your question is, consider about 150-160 CFM airflow requirement per 100 bhp in a Diesel engine. The R-TDI engine was a particularly clean-running engine and very-vell developed and engineered. Developments in fuel injection systems, as well as the fact that many tuners will not regard smoke to the same extent that VW Motorsport did/does, means that you could get away with running with a lower Lambda value, thereby reducing the specific airflow requirement. The current lower limit is a Lambda value of 1.2 (AFR=17.5), and allowing for a slightly worse BSFC, you could get away with a slightly lower airflow requirement, hence the 150 CFM/100 bhp lower limit.

The long(er) answer is here.

Dealing in lb/min is much better than CFM.
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Old January 14th, 2008, 13:38   #8
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Thank you for that - it really really helps me!

That could make sense, too as the energy rating of diesel is very similar to petrol / gasoline, although it is a touch higher, (and CR is higher). This, (almost too), conveniently ties in with the slightly higher cfm rating of 159! (slightly more energy, slightly more air)!

I can now go and stay up all night doing sums and calculations, to choose the best turbo for me, in terms of lag, Thermal Efficiency and projected bhp - so expect me to be exhausted tomorrow morning for work! ha ha!

I know it is really sad, but I am so stoked that you were able to show me these figures! It really has made my day. I'll have a look at the full article tomorrow, (during work)!

Once I've read the full article, it would be good to discuss any trends / anomolies with a fellow petrol/diesel head and number cruncher!

Many thanks,

Rich
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Old January 14th, 2008, 13:48   #9
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p.s. is lb/min a mass airflow, and therefore measured and corrected for a certain temperature, whilst cfm could be at any temperature, and therefore any density? Just a wild guess, as I know nothing about lb/min, but as you say it's better, that's the only reason I can think of!

Cheers,

Rich
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Old January 14th, 2008, 14:13   #10
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It is precisely the opposite. Volume depends on temperature and pressure, whereas mass is an intrinsic property; it doesn't depend on anything else, but is a basic property of how much "stuff" there is of something.

Also, as far as the engine making power is concerned, it couldn't care less about the volume flow rate. You could have tons of CFM of hot air, and the more important mass flow rate will be low. That's why I hate using CFM for turbocharger calculations. Calculations of volumetric efficiency are more meaningful using mass flow; the term volumetric efficiency itself is a misnomer -- by definition is a determination of the mass of air that an engine can take in relative to the swept volume.
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Old January 14th, 2008, 14:32   #11
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Yes - that would be down to one too many Grolsch's! I knew what I meant - but you actually said it!

As far as CFM goes, I only refer to it, as that is what the majority of supercharger manufacturers quote, (combined with volumetric / thermal / etc / efficiencies). I've been researching a supercharged engine & car I'm building for around 4 years or so, and turbos & diesels are new to me as from 1 month ago!

Mass flow makes so much more sense, as it actually describes what you are getting and would make calculations a whole lot easier. Naturally, we all try to influence density and cfm with intercooling!

If you don't mind, I've got a question for you with respect to my supercharged project. I'll pm you tomorrow, and if you have time, I would very much appreciate your thoughts on the matter.

Cheers,

Rich

Last edited by 20vK; January 14th, 2008 at 14:42.
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Old January 14th, 2008, 14:57   #12
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By the way, reading your original post, I misread your rule-of-thumb (100 instead of 150 CFM / 100 bhp).

After doing a little bit of research, the methodology I used of comparing the ratios of AFR to that of CFM is invalid, because I did not take into account the widely different BSFC of a gasoline engine compared to that of a Diesel. When doing so, using an AFR of 11.5:1 and a BSFC of 350 g/kWh (0.58 lb/HPhr), which corresponds to a brake thermal efficiency of 24% and is a typical result of a tuned gasoline engine at the rated power point, I get numbers that agree with your rule-of-thumb figure.

Here, the error was not due to one beer too many on my part, but a tired and fried brain working all day doing engine simulations at work.
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Last edited by TDIMeister; January 14th, 2008 at 15:21.
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Old January 14th, 2008, 15:40   #13
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You just cracked me up with that last sentence! ha ha!

I really envy your job - I find people jobs and try to fit in as much engine simulation, (read development), as possible and you do it for a living! I would love to work for a motorsport company doing development, but I took the wrong degree (business studies)!!

Hence I find myself in sales!

A few years back, I designed a stand-alone haldex controller. Was completely automated and variable, (as oe), but monitored and reacted (on paper) a lot quicker. Never got round to prototyping it on circuit boards, but the formula and theory was solid.

Yes, agree that petrol is rarely run stoichiometrically at rated power in the real world, but since we know this, we can make assumptions on how it will behave and I hoped that an oil-burner would behave in a similar or relative fashion!
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Old January 15th, 2008, 00:27   #14
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Yes, there are some rules that are applicable to both Diesel and petrol engines.

The air mass flow requirement for a given power output can be determined by multiplying the fuel flow rate times the AFR:
air mass flow rate = fuel flow rate * AFR

Fuel flow can be directly calculated from the BSFC and the brake power output at a point of interest (this is what I neglected in my calculation yesterday):
fuel flow rate = BSFC * Power

air volume flow rate = air mass flow rate / density

These formulae apply regardless of the fuel or engine design used. However, this is a classic example of the result being only as good as the input information (garbage in, garbage out). Most people do not have an intuitive feel of BSFC and what are typical values for petrol or Diesel engines at different operating points.
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Old January 15th, 2008, 05:33   #15
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TDIMeister
Thanks for your link that answers my "charging supercharging" question a week or so ago.

Your terms used and defined are spot on for my understanding.

http://forums.tdiclub.com/showthread.php?t=148730
"I have debated about the use of massively-sized turbos which can flow many times that capacity. I have explained that volumetric efficiency by definition is independent of boost pressure, and that it is instead a function of engine geometry and design, and has nothing to do with the turbocharger."

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