Landracing Forum
Tech Information => Technical Discussion => Topic started by: Ken Yooper on January 18, 2013, 07:19:19 PM
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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. Perhaps maximum torque is a product of volumetric efficiency as it relates to RPM?
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.
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 2500RPM and if it deteriorates to a 50% volumetric efficiency at 5000 RPM (25 CID per cylinder) would the engine torque be the same? Seems the airflow through the engine would be identical at both speeds. 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?
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, 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.
Quite positive the answer is out there so, chime in Salt Racers.
Thanks in advance -
KenB
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The book "Engine Airflow" by Harold Bettes ISBN 978-1-55788-737-1 has a lot of material on this subject matter. It is in print, easy to find, and does not cost much.
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Perhaps maximum torque is a product of volumetric efficiency as it relates to RPM?
Hey, Ken.
By the way, love the U.P. My wife and I honeymooned there, at Wells State Park.
Cam timing, ramming effect and scavenging effect determine peak VE, and they are all related to flow, which in a running engine is related to RPM. That's why an over cammed engine won't idle - too much overlap equals less VE - but "comes up on the cam" with increased RPM when intake charge trapping and scavenging comes into play. All of these variables are tunable, so yes, VE, and therefore torque, is related to RPM.
Let me turn the question around. Why would anything other than peak VE produce anything other than peak torque? Peak VE gives maximum pressure against the piston, and greatest force to the pushrod and crank, and therefore, the greatest number of ft lb's of force.
I think you're confusing horsepower with torque. Horsepower is a measured product of torque and RPM, or more specifically, time. Torque is not time related. While RPM affects torque in the sense that it affects the way the engine takes in a charge and expels exhaust related to volumetric efficiency, no further torque beyond peak can be gained simply by varying the speed at which it is produced.
Get Harold's book. He's very good at de-muddling the techno-babble.
:cheers:
Chris
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Some modern production engines spread their higher VE with switchable intake tuned length, and run longer length at lower rpm, shorter for faster rpm. They often use Helmholtz resonators to quiet the intake at wide open throttle when the intake tuning is working "just right". They are so well tuned, intake, that a broken clip on an air filter lid can reduce driveability!
They also have variable intake cam timing....advanced in the mid range, retard at high rpm and a little off full retard at idle and low rpm. When you split the controller, you find (for the average driver) most wear in the "about 20-30% from full retard" range.
Many are also using variable exhaust timing, advancing for better internal egr at light throttle cruise for example. Due to requirement for fast cat light off, the days of correctly tuned exhaust manifolds are pretty much gone. Some now use a simple thinwall stainless steel log type.
Somebody tell me, how did it come to be....that grandma needs a near 300hp, 6-speed Camry ......to go to her hairdresser? :?
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Ken,
My apologies for hacking up your post. I'm a slow & crappy typist. But on the upside, I have 40+ years building, engineering & dyno testing engines. I've attached some thoughts in blue......, IMHO......
I’ve been reading for a long time that volumetric efficiency is greatest when the engine is producing maximum torque. Yes, true Not totally convinced that is true. Perhaps maximum torque is a product of volumetric efficiency as it relates to RPM? Peak V/E is independent of rpm, notice that when the peak torque increases, its' rpm point can & does change, depending on the modification(s).
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. Yes, but "low" rpm varies for a number of reasons. V/E% = (measured cfm / potential cfm) * 100%
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. Yes.
If the engine experiences a volumetric efficiency of 100% (50 CID per cylinder) at 2500RPM and if it deteriorates to a 50% volumetric efficiency at 5000 RPM (25 CID per cylinder) would the engine torque be the same? No, due to other factors, such as parasitic drag. Seems the airflow through the engine would be identical at both speeds. The TOTAL SUM of the airflow at both speeds would be the same. Small, but important difference. 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 – Yes. would they cancel each other? They don't "cancel" each other out, they are the same because of the math. Using your example: (I apologise for the simplicity of the math) 1*100% = 100% 2*50% = 100%.....
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? At the point where the induction/exhaust systems do the best at filling the cylinders. And you are smart to think of the engine as a "pump", since that is what it is. Would maximum torque occur at the RPM where maximum CFM is happening? Not necessarily. You can have a higher cfm# at higher engine speeds, but the V/E percentage could be lower. This is why both torque & bhp fall after a certain point.
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. Well you can ignore friction & tuning for the purpose of discussion & beverage consumption, but ultimately racers have to deal with those realities.
Quite positive the answer is out there so, chime in Salt Racers.
Thanks in advance -
KenB
If I could make one suggestion to you, I would say start doing some analysis of some real dyno data, ie, do some calculations of things like V/E% and BMEP (in psi), if they are not on the dyno sheets. And do some graphing of that data. Here is an easy way to get started with some practical application of engine engineering, download the .pdf spreadsheet and my comments of 38flattie's blown FlatCad engine, found in his build diary, about page 100/101. I've done some of the calculations there as an example. Note that the peak bmep blown is different from peak bmep with the boost subtracted. When conditions change as a result of modifications, the results typically change, and not always the way you expected/predicted..... Note also that a lot of engineering is speculative when without hard data......
If you really want to know a lot about this subject here is a suggested reading list: (Be forewarned, engine engineering is math & physics intensive..... it's, um, complicated......)
For the non-engineer;
"Engine Airflow" By Harold Bettes as suggested by M/M
"Performance Automotive Engine Math" by John Baechtel
both books are recently published and up to date.
For the engineer or those who want a deeper understanding of the theories & process;
A, the classics,
"The High-Speed Internal-Combustion Engine" by Sir Harry Ricardo
"The Design and Tuning of Competition Engines" by Philip Hubert Smith
"Scientific Design of Exhaust and Intake Systems" by Philip H Smith and John C Morrison
B, Collegiate Texts,
"The Internal Combustion Engine in Theory and Practice:" Vols. 1 & 2, by Charles Fayette Taylor
"Internal Combustion Engine Fundamentals" by John Heywood
and there are others, but this is a pretty stout (& expensive) list......
:cheers:
Fordboy
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Thanks for all the replys - good information for sure.
"The High-Speed Internal-Combustion Engine" by Sir Harry Ricardo was required reading when in college in the early 60's. Think I have a copy back in MI, try and find it after the winter subsides a little when I return to da Yooper.
The Philip Hubert Smith compositions are new to me - guess I'd better visit the local library or Amazon.
Thanks again for provoking the thought processes in this old mind!
Rest assured - no one hacked or hijacked the thread. Hell, it's winter and if you can't race, well, we can still talk. That's what benches are for - LOL
KenB
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Our puny brains are best at analyzing static events. We look at the performance of an engine at a particular rpm or a particular set of events.
The full scope of what is happening isn't static, it's a very dynamic event where all of the functions come in to play. If you externally rotate an engine with no combustion taking place you have a vastly different set of circumstances.
There are lots of other things that have to happen to make a race engine, but air flow is everything. Without peak airflow the rest of it doesn't matter. Flow benches, porting, polishing, valves, cams . . . All things to make the air flow better. In a running engine this isn't a smooth event. There are definite peaks and valleys in the flow and pressure curve that have to be taken advantage of. Tuned exhaust headers exist for a reason. Don't think so? Fab up a set of short zoomies and see how much horsepower you are losing. Intake tuning also comes in to play. Both only happen at a fixed rpm based on the speed of sound. You get to pick the rpm to maximize the effect.
Torque, horsepower and rpm all go hand in hand. Torque by definition isn't rpm related. Maximum torque as it relates to a running engine is related to rpm. Looking at it from a dynamic point of view, maximum torque happens when the peak pressure is delivered averaged over the rpm. That view is a series of power pulses that peak on the power stroke and taper to nothing at the bottom of the stroke. The exhaust stroke, intake stroke and compression stroke are just causing drag until the next power pulse.
Class, if you want to go fast, you really need to buy the books on the reading list. Library? No way, they want the book back. If your copy isn't dog eared then you haven't read it enough times.
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The book "Engine Airflow" by Harold Bettes ISBN 978-1-55788-737-1 has a lot of material on this subject matter. It is in print, easy to find, and does not cost much.
Harold use to post here and in the Tuesday Chat, have not seen him for a while. Is he still around?
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The book "Engine Airflow" by Harold Bettes ISBN 978-1-55788-737-1 has a lot of material on this subject matter. It is in print, easy to find, and does not cost much.
Harold use to post here and in the Tuesday Chat, have not seen him for a while. Is he still around?
I've PM'd him a few times and have received response, but it has been a while. He's a pretty gracious guy, and his writing demystifies a lot of the dynamic issues Dean points out.
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I frequent Jack Kane's site below and have posed the same question(s) to them. I'll post their response as soon as I get it.
http://www.epi-eng.com/index.html
Again, thanks for the input.
KenB
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There might be an advantage to "roughing in" the tuning as best as you can where you live, and doing the final work on a dyno in SLC.
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Ken,
Toured Jack Kane's website. And then I remembered who he is, from his AETC presentation. He is a very knowledgeable and competent guy, the information on his site is "AAAA". He too, has a suggested reading reference list, larger than my list, but, interstingly enough, his list contains most of the books I recommend. Reference Library, Section 5, Engines & Turbomachinery.
By all means, use Jack Kane's list. But start with the recently published books by Bettes & Baechtel, also very knowledgeable guys. Then move on to the classics, and finish with the collegiate level texts. Yes, I know it's a lot of reading & comprehension. It's the price you pay for entry.........
Class, if you want to go fast, you really need to buy the books on the reading list. Library? No way, they want the book back. If your copy isn't dog eared then you haven't read it enough times.
I could not have said it better Dean.
A note on libraries: My local library (in Podunkville, south of the "Cheddar Curtain" :-D) has NONE of the books on my list. My sister-in-law, Marion the Librarian at a larger, regional library, searched the whole regional library system for the list. Only found 2 titles in reference only sections, which means those books can't be loaned out, only read at that library. The collegiate level texts MAY be available for loan at a collegiate library in a college town somewhere, but don't count on it. Unfortunately, this is pretty esoteric subject matter.
If you have chosen some "oddball" powerplant to be the centerpiece of your LSR project, then you have to accept that almost everybody else will not care about your trials & tribulations. You have to chose to either be able to do the "cipherin'" yourself or have someone like Jack Kane do it for you. And that's BEFORE you start whittlin' out any specialized parts you need..........
If you chose to race something common, such as the ubiquitous small block Chevy, you still need to understand the concepts and be able to do the math. And that's just to be able to sort through the GIANT amounts of parts available, ALL for differing applications............
OR, you can purchase a complete powerplant from a proven competent builder/tuner, that works also. I guess it depends on whether you fancy yourself as an engine guy or a chassis guy.
Last thought. There are some very knowledgeable people on this forum, who are typically willing to lend a hand, if asked. I'm too new to know everyone, but there are people who have been around a long time who can point you in the right direction.
:cheers:
Fordboy
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When I tested a 2000 LS1 engine (350HP, 347ci, 2v pushrod V8, digital engine), I noticed that the torque "curve" was nearly flat until peak HP, then dropped fast.
The HP curve was a diagonal line.
What that means, is the VE is being maintained across the RPM range of the engine.
Race engines are different though. No interest in power under 3000 rpm, so the torque curve is a hump.
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JustaRacer,
Sorry to hack up your posting, but I'm still a slow & crappy typist.....
When I tested a 2000 LS1 engine (350HP, 347ci, 2v pushrod V8, digital engine), I noticed that the torque "curve" was nearly flat (how flat & over what rpm range?) until peak HP, then dropped fast.
The HP curve was a diagonal line. See formula & example below.
What that means, is the VE is being maintained across the RPM range of the engine. Maintained by +/- what factor or tolerance?
Race engines are different though. No interest in power under 3000 rpm, so the torque curve is a hump. Some racing engines have no interest in power under 15,000rpm.
IMHO.........
It would be interesting to know the numbers (TQ, HP, rpm, V/E, bsfc, bsac, bmep, etc, etc,) for your example above. If you can post them up, we could have an idea of the rpm range, torque 'flatness', etc. It is impossible to comment insightfully about these issues without the numbers. For instance, is the rpm range small or the 'flatness' range wide? Etc, etc.
HP = TQ*rpm/5252 For example:
HP @ 2626rpm = TQ*(.5)
HP @ 5252rpm = TQ*(1)
HP @ 10504rpm = TQ*(2)
HP @ 15756rpm = TQ*(3)
HP @ 21008rpm = TQ*(4) This explains why F/1 engines operate in this rpm range.........
HP becomes a linear function of rpm if TQ is a constant, hence the "diagonal line".
I'm not sure of the exact quote, but it goes something like this: "Rpm's, by their very nature are free, and he who can modify his engine to take advantage thereof, will prosper." Sir Harry Ricardo, circa 1920 (?)
Let's talk theory for a moment. For the purposes of these examples, we are going to ignore blow-by cfm & overlap cfm, which are portions of measured cfm.
Volumetric Efficiency % = (measured engine cfm/potential engine cfm)* 100%
Potential cfm is defined as the theoretical air capacity of an engine. For 4 strokes: Air Capacity in cfm = (displacement in cu. in. * rpm)/3,456
So then, if V/E% remains a constant (ie: absolutely flat) and potential cfm increases with rpm, then to maintain that constant V/E%, something must be increasing at the same incremental rate as potential cfm. That "thing" is the torque. (This presumes other factors, such as bsfc, remain constant as well..... Remember, "it's complicated.......")
Conversely, if the torque remains "absolutely flat" over an rpm range, then V/E% must be declining as the rpm increases, although the rate of decline will depend on the rpm range in question. A small rpm range can allow for a small % decline in V/E%, which may appear to be 'flat', depending on the graph ranges.
Because of the wide range of engines discussed on this site, it is important to be somewhat specific. What a small block Chevy uses will be different from a 4cyl, 250cc Honda. Not trying to bust you, just want to avoid generalities.
When engines are modified for higher outputs per displacement, the operating rpm range is often narrowed, hence the "humped" shape of the torque curve. This is a result of the fact that "enhancements" in one area tend to "compromise" other areas.
Running engines, whether they are racing engines, aircraft engines or street engines, are complex mechanisms filled with complex and interdependent variables. "It's complicated." To get the result you want, may require a compromise somewhere else in the mechanism or the output......
Reference the reading list in reply #4 above if you need more detailed explanations........
:cheers: :cheers: :cheers:
Fordboy
edits were for spelling & grammar.... sorry....
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Let me criticize my own thought processes. I dunno if I had a huge brain fart or was severely constipated – not that it matters. LOL. Been tossing this garbage around for a few days and have at least fifteen pages of calculations and graphs (all dog-eared too!) and nothing was adding up until a ton of bricks hit me square between the eyes at 4:00 AM this morning.
Torque has nothing to do with time or RPM but rather it has to do with power strokes per revolution. In my example there are four power strokes per revolution of the crank – makes no difference if the engine speed is 2500 RPM or 5000 RPM – still have the same number of power pulses per rev. To get equal torque, there must be very similar things happening on top of the piston – VE, BMEP are extremely close here regardless of engine speed – the small differences can be attributed to parasitic losses.
In my erroneous thoughts I had equal torque at vastly different engine speeds which means the engine is producing more power at the elevated speeds – but I had the engine consuming the same amount of fuel/air. Equal torque means twice the HP at 5000 RPM as at 2500 RPM with the same amount of fuel/air? Hmm, that’s not going to happen folks – sort of in conflict with the Conservation of Energy along with similar wars with the Laws of Thermodynamics - duh -LOL.
I’m old and haven’t been involved with this stuff for a couple of decades - completely understood the process long ago but somehow got confused as of late. That’s what I get while walking the dog while simultaneously thinking about Bonneville. Never could multi task!
Anyway, to achieve some amount of forgiveness, I guess I’m buying the beer for the salt next summer. Thinking a few 30-packs are in order. Perhaps Seldom Seen Slim of the Yooper will be gracious enough to let me drop them at his rig for all to enjoy – even though I have yet to ask him. Hey, all the posters to this thread can even help themselves to more than one!
I knew the clarification was out there so thanks again for the guidance and even more for the patience. It’s tough getting old, even more difficult to admit it sometimes.
Peace –
KenB
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"It's complicated." Of course it's complicated. If it were easy everyone would have the same horsepower.
The fast guys know all of the above and spend a large amount of the budget experimenting on the dyno.
That LS1 engine is a street "performance" engine. It's still only a fraction of full race horsepower.
When you hear someone talking about getting "up on the cam", or "it's peaky", or for a two-stroke "up on the pipe", all talking about tuning elements designed to produce peak hp at a certain rpm.
The cam opens the valves the same at all speeds (ignoring variable timing.) The piston is capable of pulling in the volume of the cylinder. How the tuning pulls together at peak hp is what matters. 21,000 rpm. That's just sick. Wouldn't you rather have 21,000 power pulses in the same amount of time instead of the 8,000 you are getting now?
In 1990 the F1 Renault RS2 3.5 liter for Williams had 660 hp at 12,800. BMW's P83 3.0 liter V10 engine used in 2003 season managed an impressive 19,200 rpm and 900 bhp.
It's been downhill ever since. The engine costs were truly astronomical and F1 has been cutting power ever since.
The biggest break through was pneumatic valve springs. Coiled springs are the single biggest thing preventing you from higher rpm's. Oh, yeah there are many others relating to accelerating the mass of components. What was it somebody said? Oh yeah, "it's complicated".
Pop quiz. There are no restrictions in most LSR engines. How many of you are running pneumatic valve springs?
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Ken, bring all the beer - and whatever other potables - you want to the SSS pit. It's safe from me, and maybe if everyone that stops by for refreshments - gets Nancy to share one then, too - - well, I betcha some memorable fun will ensue. Just let me know so I can have the video recording system ready. There might be value in the replays.
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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.html
As 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 (150.8). 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