Diesel Math

[Bobbie Martin]

Dr. Lee,
There was a thread about CFM on the 6.5 Turbo forum. I wondered if you might be able to provide some info for the CFM calculator. Any idea what the approximate VE would be on a NA and turbo 6.5? I have read when you turbo or supercharge an internal combustion engine you don't raise the VE but actually increase the displacement. For example, if you turboed a 400 CID engine to 1.5 bar, you have actually increased the displacement to 600 CID. Would you concur? Any idea what the air fuel ratio would be at idle & full power?

One other question, I remember reading somewhere that diesels have relatively mild cam timing, specifically closing the intake valve very early (compared to a spark engine) to keep the combustion pressures high. Would it work to grind a cam that closed the intake valve a little later to lower the combustion pressure - effectively getting 18:1 compression pressures with the stock 21:1 pistons?

[britannic]

There's not much latitude to play with cam timing on diesels, however, Dr. Lee reported on Crane cams recently and they have done some research on better profiles for our diesels. However, less overlap is better with forced induction, so that more air pressure can be retained in the cylinder (a little overlap is needed to help the exhaust gases speed their way out of the cylinder).

Volumetric efficiency will affect how much air can be stuffed into a cylinder over a given time, so true displacement is actually less than expected. Any existing obstructions/restrictions in the intake, cylinder head and exhaust will be magnified as the forced induction ratio is increased.

If you want to read an excellent book on this subject, check out A. Graham Bell's "Forced Induction Performance Tuning", although it's aimed at gasoline motors, it's really illuminating and the math is relevant for diesel.

Compression ratio changes are best handled by changing the pistons or increasing head gasket thickness (I prefer the former method). As regards air/fuel ratios, in a diesel the only ratio that's important is having enough air to burn all of the fuel injected. Diesels can't run lean and more air is always better - they can however, have too much fuel injected for the amount of fuel available.

[ 01-09-2004, 09:49 AM: Message edited by: britannic ]

[Uncle Wally]

Hot Rod Magazine just did a couple of feature articles on determining how to size turbo's for gasoline engines. I took an interest in these articles as I am looking into building a twin turbo 6.5 for a first gen Blazer. The purpose being little spool up with more boost and flow at the top end.

This equation is for cfm requirements of an NA engine.

CFM=(displacement x rpm x VE)/3456

For a boosted engine.

Pressure Ratio=(14.7+boost)/14.7

CFM boosted = CFM unboosted x Pressure Ratio

A 6.5 is around 396 CID. 3800 - 4000 rpm is about as high as I would consider (3200-3600 is the sweet spot on my current truck). 15 psi is easily attainable for a constant boost level.

(396 x 3900 x 1)/3456 = 446.875 cfm NA

446.875 x 2 = 893.75 cfm Boosted

Here where the tricky part comes in. All the compressor maps that I have looked at have units of flow in lb/min. So you'll need to convert cfm to lb/min. A safe thumbrule for sea level is cfm x .07 = lb/min.

That puts the boosted engine requirement at 63 lb/min of air flow.

Now the trick is discussing which compressor/turbine combination will work best for you. I would suggest talking to someone from the Turbo manufacturer for that. I am still trying to get someone from Turbonetics to call me back so I can figure out how to do the twin turbo stuff. Common sense is that you use half the lb/min of airflow with the same desired boost level. But none of the compressor maps that I looked at would work in that ball park.

I am sure that there is a ton more to this subject, so anyone that is educated, the extra info would be appreciated. I still have a long way to go to tie all the loose ends up on my system.

Waldo

[jjbouwens]

I tried to follow your calculation but got lost in your conversion from cubic inches.Anyhow I arrived at approx. half the amount.This I believe you overlooked the fact that a four stroke engine needs 2 revolutions for one inlet stroke.
--------------
96 Brave motorhome with 6.5 TD abt 50 tkm.

[Peter J. Bierman]

I did some very rough theoretical airflow calculations on my Twin Turbo Diesel engine,
( see the boost on a butged article)
This is not science but it works.
The turbo's were selected on the rough airflow and exhaust flow and on availebility.
The results are promising for the moment.
Going to the two smaller turbos will couse less turbo lag but I am not sure about more boost then a single set up.
You probably need waste gate turbos to aviod stalling on a low rpm/high boost turbo.
My waste gates are set at 17psi = 15 psi MAP, and spool up fast and can deliver all the boost I need.

It is however interesting to see where the calculations will take you.

Good luck on your project.

Peter

[Uncle Wally]

CID=#cyl x stroke x 2 x 3.1614 (pie) x 1/2 bore

The GM manual has everything in metric, so I made a gross conversion using the 348 CID = 5.7L and applied that to a 6.5 L engine. It should come in just a tad smaller than a 400CID engine.

I'll look at my numbers again, but half as much seams like I missed something.

Waldo

[moondoggie]

Good Day!

1L = 61.0238 in cubed
6.2L = 378.3 in cubed
6.5L = 396.7 in cubed

Yes, I'm way too picky, but mom still likes me. smile.gif

Blessings!

Brian Johnson, #5044

[grape]

very simple formula.......10 cubic feet per minute for every 100 horsepower. so 300 horse takes 30 pounds for minute.

[grape]

Sorry about the above post please disregard. My units of measure are off......10 POUNDS of air per minute per every hundred horsepower. 300 pounds of air per minute for a 300 horse engine.

[grape]

one of these days i'm gonna get it right.....30 pounds of air per minute for a 300 horse engine.

[gmctd]

FYI Grape
The Letter\Pen icon, next to the "quotes" icon at the top of your post, will let you reword your entry at any time, or delete it if necessary.

You may have noticed many such "edits" in my posts. :cool:

edited

[Bobbie Martin]

Thanks to those that replied. What I was really trying to find out is if anyone has the approximate Volumetric Efficiency of the atmospheric (NA) and turbo diesels. If anyone knows, please post. Grape, is that formula for gasoline? I would think different fuels would have different air requirements.

[EWC]

Gasoline powered engines have around 80% - 85% VE . It is possible to have more with a high RPM race engine with manifold and exhaust tuning . Some of the NHRA engines are supposed to be in the 105% VE range . I would think that the NA diesel would be around the 80% - 85% range . The turbo should help with VE , obviously , but the boost should not be the yardstick for performance .

Holley has an interesting article on superchargers , in the tech section , and they talk about boost as being a restriction to the air flow . If you think about how the blowers are designed , roots , screw and centrifugal , you will see that they accomplish boost in different ways . You could almost throw the turbo in with the centrifugal blower if not for the type of drive .

One point is how much flow do you have with 15 lbs boost compared to 10 lbs boost with ported heads , cam and intercooler . That was one of my reasons that I bought a Crane Cams regrind a few years ago . I have a Banks kit for my 86 and thought that I don't have an intercooler or the electronics so I have to cheat the engine into thinking that it is bigger than it is . I'd like to see how my cam compares to the cam Dr. Lee talks about .

As far as lower compression , the NHRA Top Fuelers run a lower compression ratio that puts the piston further down and away from the head so that more molecules can be shoved into that area . This is done with more fuel and boost .

If you want to see the cam specs , just let me know .

[Bobbie Martin]

I would like to know the specs on the cam if you can post them or e-mail them to me.

I'm sure top fuel motors do have low compression - they run a zillion pounds of boost burning nitromethene so they really don't need a lot of static compression. By the way, while most are amazed at the specific output of these engine, I feel they are outdated and really pretty pathetic. They make some 5000 HP from around 500 CI, but run unlimited boost and basically unlimited fuel type and the service life is measured in seconds. When you factor in the displacement of the supercharger, the specific output is really not that great. Compare that to a Formula 1 engine. 3 litres (180 CID), normally aspirated, 19,000 RPM, 850 HP running on pump gasoline and they (usually) last for hours. Almost 5 BHP per cubic inch on pump gas isn't too bad!

[grape]

the formula above is for gasoline engines with a/f ratios in the mid 11's. From what i've read diesels run at the leanest in the high teens...ie 19 or 18-1 for a/f ratio at full throttle. So overshooting the air needs of a diesel isnt' going to hurt.....lower egt's and cleaner exhaust. This is where i'm going to start anyway with my compressor needs.

[EWC]

Crane Cams regrind 01112 and numbers @ .050 tappet lift : intake opens 6 ATDC and closes 24 ABDC , max lift 105 ATDC duration 198 and exhaust opens 36 BBDC and closes 14 BTDC , max lift 115 BTDC duration 202 . Lift on intake is 2934 and the exhaust is 3000 . With 1.5 rockers this gives you a little more lift than the stock cam . The duration is about 20 more than stock so this might help with more air flow and helping to cool things down a bit .

I was told this was ground for a guy with a boat engine but he was not able to be found to even say if the cam worked or not . I was the second person to get this grind and I have not run it yet to even see if it works .

The fuelers run about 7 or 8 : 1 ratio and the blowers are probably high helix 14-71's or some derivative of the Roots design , not the PSI which is a screw type and more efficient . 500 cubes , 90 % nitro and somewhere around 7000 HP , no one really knows or is saying . There is nothing around that will attack your senses like a fueler and if you haven't seen one you should . Just for the shock factor alone .

The Formula 1 cars are awesome with the advances in technology that they bring such as pneumatic valve springs . While I have never see one in person , the noise at 20,000 RPM has got to be ear splitting !

I would agree with Grape , get as much air in as possible . That's the way I'm going as well .

[Bobbie Martin]

I grew up in Southern Calif. and during my mis-spent youth I hung around OCIR so I know all about fuelers - nothing like seeing one at night.


While I have never see one in person , the noise at 20,000 RPM has got to be ear splitting ! It cannot be described. There is nothing in the world that has the ear piercing shriek of a F1 car at full throttle. We attend the F1 races at Indy. The first year I watched the race without earplugs - I can tell you I haven't made that mistake again!

[Uncle Wally]

VE for a turbocharged engine should be above 100% by a fair margin. You can get more air into the air pump when there is a positive pressure on the intake. Then just add more fuel - and walla - more power. As to how much effeciency exactly, I don't know how you could quantify that.

Personally - I prefer hangin on the fence when a Winston Cup car goes buy at full song. Next would be sitting in the bleachers at Bristol and hearing all of em at once, for a full 500 laps!

Waldo