As promised, below is my total conversation with Jack Kane from EPI - he is in agreement with my posting this. The link to his site was posted earlier - here it is again if you missed it.
http://www.epi-eng.com/index.htmlAs always, this guy made a lot of sense to me - was positive he was correct but like I voiced earlier, it took a little thinking for it to get past the cobwebs-
Enjoy jack's thoughts and knowledge.
KenB
Hi Ken.
Glad to hear it made sense. (Sunday is typically the only day I have a chance to answer such inquiries).
I have no objection with you posting my replies. I would ask for attribution, however. I would imagine it is OK to keep the imbedded links to my site in the blog posting? (Would you send me a link to the posting?)
Thanks for your concern.
(I would love to go to BV sometime, bujt that timeframe is right in the highly hectic season just before the Reno Air Races, and the list of equipment I have racing there continues to grow.)
Sincerely,
JK
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On Mon, Jan 21, 2013 at 8:57 AM, <saltflatstealth@aol.com> wrote:
Hi Jack -
Thanks ever so much for your response, on a Sunday no less.
I would like to post your response in the Bonneville form which I visit but certainly not going to do so without you permission.
I don't know what happened to my thought process regarding this but after reading your comments and drawing a few curves and penciling in many equations, all of it came back. Very suddenly too. Somewhat embarrassed to admit that it had even been forgotten in the couple of decades since I was last involved in this.
You've probably been to Bonneville prior but if you've got nothing better to do why not stop by again this coming August or September - some pretty neat racers there.
Thanks again -
KenB
Hello, Ken.
Thanks for the kind words.
Due to laziness, I have imbedded my replies in your text.
(Caps are for visual separation, not loudness.)
Hope this helps.
Sincerely;
Jack Kane
EPI, Inc.
EPI - Greetings -
I have enjoyed your site for many years -thanks for that effort.
Below is a question I posted in a Bonneville racing blog.
I guess what I'm trying to determine is - does peak torque occur at the RPM where mass air flow is greatest as measured in CFM? NO, IT OCCURS WHERE BMEP IS THE MAXIMUM , WHICH TYPICALLY COINCIDES WITH MINIMUM BSFC. If we have an engine which has a relatively flat torque curve over a few thousand RPM, then actual cylinder filling (as a percent VE) must be less as the engine speed increases? Am I correct that torque is a product of BMEP and RPM - with a constant BMEP then torque would increase as RPM increased? NO! BMEP IS (TORQUE / DISPLACEMENT) TIMES A SCALING CONSTANT (150.
. THEREFORE, YOUR TORQUE CURVE AND BMEP CURVE HAVE EXACTLY THE SAME SHAPE, JUST DIFFERENT VALUES ON THE Y AXIS. Seems to me that with identical cylinder combustion pressures, but with more of them at higher engine speeds, should yield greater torque. YOU ARE CONFUSING TORQUE AND POWER. PLEASE SEE THIS ARTICLE . IF YOU HAD IDENTICAL CYLINDER PRESSURE TRACES AT TWO DIFFERENT RPM, THE LOWER RPM WOULD PRODUCE THE MOST TORQUE (SINCE INERTIA AND FRICTIONAL LOSSES INCREASE WITH RPM) BUT DEPENDING ON THE DETAILS, THE ONE AT THE HIGHER RPM WOULD LIKELY PRODUCE MORE POWER.
Please examine the question regarding motoring the engine and the resulting CFM of swept volume as it relates to torque.
Realizing that you're busy but perhaps you could dissect my thoughts and provide me with some sound engineering principals (PRINCIPLES) to clarify my thinking processes?
I'm an old ME long retired from the car business and will be the first to admit that my problem solving abilities are certainly not what they used to be.
Thanks ever so much.
Ken Betz
N/A Volumetric efficiency, cylinder filling, torque - at sea level and on the salt
I've been reading for a long time that volumetric efficiency is greatest when the engine is producing maximum torque. Not totally convinced that is true. IT IS TRUE IN ALMOST EVERY CASE, AND WHERE IT IS NOT TRUE, IT IS DUE TO SOME FORM OF ANOMALY. Perhaps maximum torque is a product of volumetric efficiency as it relates to RPM? NO, IT OCCURS WHERE BMEP IS THE MAXIMUM , WHICH TYPICALLY COINCIDES WITH MINIMUM BSFC.
If volumetric efficiency is defined as a percent of the swept volume of a cylinder, then the cylinder would be filled most completely at some low rpm after which it would drop off as the engine speed increases. NOT QUITE. TYPICALLY, AN ENGINE CAN REACH ALMOST 100%-VE ON THE STARTER WITH WOT. AS IT BEGINS TO RUN, THE EARLY-OPENING AND LATE-CLOSING OF THE INTAKE WORK TO REDUCE THE AVAILABLE VE UNTIL THE SPEED INCREASES TO WHERE THE CAM IS OPTIMIZED AND THE TUNING (IF ANY) OF THE INTAKE AND EXHAUST RUNNERS WORK TO MAXIMIZE VE. WITHOUT RAM AND PULSE TUNING, AN ENGINE WON'T ACHIEVE 100% VE (IN FACT, FEW DO), HOWEVER, A PROPERLY TUNED SYSTEM CAN ACHIEVE UP TOWARDS 110%... BEYOND PEAK VE, THE TORQUE BEGINS TO FALL OFF, BUT IF IT DOESN'T DROP PRECIPITOUSLY, POWER WILL INCREASE WITH RPM UNTIL THE TORQUE DROPS OFF BADLY. (THESE CONCEPTS ARE EXPLAINED IN GREAT DETAIL HERE AND HERE AND HERE. AN ENGINE WHICH HAS TWO EQUAL-VALUED TORQUE PEAKS, ONE AT A HIGHER RPM THAN THE OTHER, OBVIOUSLY HAS BETTER VE AT THE HIGHER RPM PEAK, BECAUSE AT HIGHER RPM, THERE ARE GREATER FRICTIONAL AND INERTIA LOSSES TO OVERCOME, SO MORE IMNDICATED TORQUE MUST BE AVAILABLE TO OVERCOME THOSE LOSSES AND YIELD THE SAME BRAKE TORQUE..
Here’s my question. Assume; 400 CID eight cylinder engine – fifty CID per cylinder – 200 CID per revolution of the crankshaft. For sake of discussion let’s assume the engine produces 400 lb./ft of torque at 2500 RPM and it produces 400 lb./ft of torque once again at 5000 PRM with perhaps with a higher number somewhere in between. At 2500 RPM we have 10,000 power strokes per minute while at 5000 RPM we will have 20,000 power strokes per minute.
If the engine experiences a volumetric efficiency of 100% (50 CID per cylinder) at 2500 RPM and if it deteriorates to a 50% volumetric efficiency at 5000 RPM (25 CID per cylinder) would the engine torque be the same? IF YOU ARE ASSUMING THAT TORQUE IS EXACTLY PROPORTIONAL TO AIRFLOW, THEN NO, IT WOULD BE LESS THAN HALF FOR THE REASONS EXPLAINED ABOVE. IF THAT PROPORTIONALITY WAS TRUE, AND THERE WERE NO FRICTION OR INERTIA LOSSES, THEN THE POWER WOULD BE THE SAME AT BOTH POINTS. Seems the airflow through the engine would be identical at both speeds (HENCE THE THEORETICALLY IDENTICAL POWER). The engine would experience only half the cylinder filling but twice the number of power strokes at the higher engine speed. Less force for each stroke but more strokes – would they cancel each other? AGAIN, YOU ARE FAILING TO UNDERSTAND THE DIFFERENCE BETWEEN POWER (THE RATE OF DOING WORK) AND TORQUE (A TWISTING FORCE THAT HAS ABSOLUTELY NO RELATIONSHIP TO RATE OR SPEED; YOU CAN PRODUCE 500 LB-FT OF TORQUE WITH A LONG ENOUGH WRENCH AND YET PRODUCE NO MOTION WHATSOEVER). PLEASE SEE THE REFERENCES LISTED ABOVE.
Another related thought: If we motor the WOT engine with an external source, at what speed would the engine exhibit maximum volumetric efficiency per revolution and at what RPM would the engine pump the most cubic feet per minute of air? Would maximum torque occur at the RPM where maximum CFM is happening? OBVIOUSLY NOT. LOOK AT A CUP ENGINE AS AN EXAMPLE. PEAK-VE OCCURS AT ROUGHLY 7500 RPM (530 LB-FT, OVER 225 PSI BMEP, BUT ONLY ABOUT 750 HP). PEAK POWER (ROUGHLY 850 HP) OCCURS AT AROUND 9000 RPM (DEPENDING ON THE SETUP). TO PRODUCE MORE POWER MEANS THE ENGINE MUST BURN MORE FUEL PER UNIT TIME AT THE OPTIMAL POWER MIXTURE, WHICH CLEARLY MEANS IT MUST FLOW MORE AIR AT THE HIGHER RPM.
Obviously, let’s ignore friction, as well as tuned induction and exhaust since motoring the engine is not exactly going to be producing much exhaust heat or noise. Just examine or think volumetric efficiency as much as practical. SORRY, BUT I AM COMPLETELY MYSTIFIED AS TO HOW YOU CAN IGNORE REAL ENTITIES THAT AFFECT THE ENGINE, REGARDLESS OF WHETHER IT IS BEING DRIVEN, OR IS OPERATING.
Quite positive the answer is out there so, chime in Salt Racers.
KenB
