-
I've been waiting a LONG time for the opportunity to present itself to open this can of worms. This is the grand daddy of all questions regarding engine performance, and I'm really curious as to who thinks what. What is it about an engine that makes it accelerate? Why does one engine accelerate better, or faster than another comparable engine from point A to point B? Lets face it...this is what wins races, not dyno numbers. So, is it horsepower? Torque? Both? Neither? :idea:
Let 'er rip. :D
-
To quote a wise man "Money$$$$$$$$$$$$$$$$$$$$$$$"
I'm sure this thread will take a few turns
-
-
have you ever searched Torque vs HP on this site?
LOL
So the engine accelerating or the vehicle?
-
Here is my engine accelerater
http://www.***boat.com/image_center/...BUTTON-med.JPG
Sorry its not a more complex answer
-
I see the answer being contained in Sir Isaac Newton's First Law of Motion: An object at rest tends to remain at rest, and an object in uniform motion tends to remain in uniform motion, absent the application of an unbalanced force.
-
have you ever searched Torque vs HP on this site?
LOL
So the engine accelerating or the vehicle?
What's the difference? A load is a load. What makes one engine accelerate a given "load" from point A to point B faster than another? If the rpm range you anticipate operating in is, for instance, from 3000 to 6000rpm, won't the combination that will pull the "known load" from the bottom to the top the quickest ALWAYS be the engine to run regardless of HP? So what makes the difference?
-
I see the answer being contained in Sir Isaac Newton's First Law of Motion: An object at rest tends to remain at rest, and an object in uniform motion tends to remain in uniform motion, absent the application of an unbalanced force.
Think acceleration rate...
-
So what makes the difference?
Traction!
-
ok ... wait a minute ... I have to warm up my fingers ...
-
Now you've gone and done it, there's a lot of variables here!
Bore, stroke, rotating mass, port velocities, compression, flame fronts, lobe designs, scavenging, on and on. :hammerhea :hammerhea :hammerhea
This is awful technical for me this time at night I'll have to think about this and revisit when I'm more awake. :eek: :messedup: :idea: :idea:
-
Well I started thinking about this the other day when posting on the other thread ... and I suppose I'll start with a brief brief on the relationship between a connecting rod and it's crankshaft because I think it's important to engine acceleration.
It seems simple at first to think that when the crankshaft is at 0 (top dead center) ... with the piston is all the way up ... and when the crankshaft is at 180 (bottom dead center) the piston is all the way down. That part's easy.
What happens when the crankshaft is at 90? You'd think that the piston is halfway down the bore, right? Well that's not exactly true. The angularity of the rod at that point brings the piston down a little bit further - the connecting rod length is the hypotenuse of a triangle where the short side is the sideways distance from the center of the cylinder to the center of the crankpin.
What does this mean to an engine tuner? Well ... there are several issues to discuss: ignition timing, combustion chamber growth rate, vacuum production, camshaft events, etc.
The first thing I'd like to address is the chamber growth rate. Because a piston in a 1.5:1 rod ratio motor moves down from TDC a little bit quicker in a given amount of crank rotation, a short rod combo will increase the chamber size quicker, creating more vacuum, bringing greater pull through the intake port, setting up a high velocity charge for later on in the stroke. Because a short rod combo pulls harder through the ports, it will develop torque at a lower RPM than a long rod motor ... all else being kept equal. At higher RPM, it will run out of breath sooner since it is pulling harder through the ports ... so a long rod combo should make more horsepower by making more torque higher up in the RPM range.
God I love this stuff ...
Another issue is that because the 1.5:1 motor's piston moves faster near TDC than does a 2:1 motor ... the intake valve opening should be given consideration. Because the 1.5 motor develops stronger vacuum ... the intake valve could be opened later to take advantage of the greater pull. Or, because the exhaust gasses are being forced out a little harder (piston approaching top of stroke quicker) one of two things could happen: Either there will be a higher residual pressure in the combustion chamber because there is a bottleneck downstream in the exhaust and the intake should be opened later to allow the pressure to equalize; or there will be a greater scavenging effect from the higher velocity exhaust gas in a header primary, allowing the intake to be opened earlier.
Generally ... I believe and could be wrong ... the short rod motor can deal with greater valve overlap than a long rod motor.
Because the long rod motor's piston moves away from BDC faster than does the short rod motor's piston, the intake valve MUST be closed sooner in a long rod motor ... or else you run the risk of pushing the fresh charge right back out the intake port and losing dynamic compression. The exhaust valve should also be opened later.
Chamber growth rate is something that should be very interesting to people looking at their ignition: the 1.5:1 motor's combustion chamber grows quicker leaving TDC than does a 2.0:1 motor, which allows more room for the expanding combustion gasses, which lowers the early cylinder pressure slightly. This means a short rod motor needs slightly more ignition timing to develop peak cylinder pressure earlier than in a long rod motor. Furthermore, because the piston is moving fastest when the rod and crank are at 90* from one another, and because that happens sooner with a short rod motor, higher cylinder pressure at that point will create the greatest leverage on the crankshaft.
A friend of mine Matt and I talk for days about this ... and it is the basis of the engine and it's acceleration IMO ... one thing to remember is that an engine is an air pump it sucks it in and spits it out ... it's all about how easy you make it happen.
My fingers are now tired.
-
- the connecting rod length is the hypotenuse of a triangle where the short side is the sideways distance from the center of the cylinder to the center of the crankpin. Not always, and in fact, only part of the time. The amount of time this is true is directly related to the rod/str. ratio.
-
oh so picky ... I told you ... my fingers are tired ...
-
Not always, and in fact, only part of the time. The amount of time this is true is directly related to the rod/str. ratio.
Wait a minute ... the amount of time (while related to the rod/str ratio) is meaningless ... the amount of time is a determined only by RPM ... how little of the time it is there has nothing to do with the degree/angle ...
-
Not always, and in fact, only part of the time. The amount of time this is true is directly related to the rod/str. ratio.
When the rod and crank throw are at a true 90deg. :rollside:
-
Wait a minute ... the amount of time (while related to the rod/str ratio) is meaningless ... the amount of time is a determined only by RPM ... how little of the time it is there has nothing to do with the degree/angle ...OK...sorry...I meant time in crank degrees of rotation. But you're right...I'm just bein picky. It's really irrelevant weather it's the long leg or short leg.
BTW...you're all f'd up with this short rod fantasy you have. :argue:
-
When the rod and crank throw are at a true 90deg. :rollside:Impossible for the rod to be the hypo. if the rod and throw are @ 90*.
-
BTW...you're all f'd up with this short rod fantasy you have. :argue:
Hey now ... I don't have any fantasy about your short rod! We're talkin' engines here ... not what your deficiencies are.
If you want a more rapid RPM ramp, you'll need a short stroke/short rod/big bore motor, period. It's been proven. You'll never change the facts ... including how short you might be.
-
-
Think acceleration rate...
Since acceleration itself is a rate , specifically the rate at which velocity changes versus time, I assume by this you are referring to the rate at which acceleration changes versus time.
Well I started thinking about this the other day when posting on the other thread ... ..........
.......................
......My fingers are now tired.
For years I've had a lot of similar stuff rattling around in my poor brain. I've always assumed that the engineers somewhere must have written the necessary formulas to relate the expansion rate of the burning gas to piston position and speed, to cylinder pressure, to crank angle, etc, etc. Got to be a big Kray somewhere capable of running simulations, right? :D
-
Impossible for the rod to be the hypo. if the rod and throw are @ 90*.
Correct. At this point the hypotenuse would be the line from crank centerline to piston pin centerline.
-
I've been waiting a LONG time for the opportunity to present itself to open this can of worms. This is the grand daddy of all questions regarding engine performance, and I'm really curious as to who thinks what. What is it about an engine that makes it accelerate? Why does one engine accelerate better, or faster than another comparable engine from point A to point B? Lets face it...this is what wins races, not dyno numbers. So, is it horsepower? Torque? Both? Neither? :idea:
Let 'er rip. :D
Steel;
Engine don't accelerate, vehicles do. But some do increase RPM faster than others..... :) :)
-
Since acceleration itself is a rate , specifically the rate at which velocity changes versus time, I assume by this you are referring to the rate at which acceleration changes versus time.
For years I've had a lot of similar stuff rattling around in my poor brain. I've always assumed that the engineers somewhere must have written the necessary formulas to relate the expansion rate of the burning gas to piston position and speed, to cylinder pressure, to crank angle, etc, etc. Got to be a big Kray somewhere capable of running simulations, right? :D
Old...yes, this is all about acceleration over time, since we're trying to get from point A to point B in the least amount of time as possible. Increasing velocity is increasing acceleration, etc, but that's symantics AFA my question.
What makes an engine accelerate? Your second paragraph touches on where I think this will eventually go.
Busby, in his not-so-eloquent terms says it's the short rod that helps. (I'll get back to you and your short jokes later, pal.) There's too many other things to consider to say that the rod ratio by itself is going to "make" an engine accelerate or not. But where's the motivation coming from? I'm thinking more in terms with what's going on in the combustion chamber. Good burn vs. better burn, vs. best burn. In this sense, Daryl Morgan may have a point.
-
Hey now ... I don't have any fantasy about your short rod! We're talkin' engines here ... not what your deficiencies are.
If you want a more rapid RPM ramp, you'll need a short stroke/short rod/big bore motor, period. It's been proven. You'll never change the facts ... including how short you might be.
I'll say this.
OK...No I won't.
However, in regards to your "short" comment, think about it this way, BIG guy. Put you in your boat, run it. Now put ME in your boat.
Advantage who? :yuk: :D
-
For an engine to accerate: Correct sequence of events....
-
Busby, in his not-so-eloquent terms says it's the short rod that helps. (I'll get back to you and your short jokes later, pal.) There's too many other things to consider to say that the rod ratio by itself is going to "make" an engine accelerate or not. But where's the motivation coming from? I'm thinking more in terms with what's going on in the combustion chamber. Good burn vs. better burn, vs. best burn.
Well ... I agree 100% ... but when we're speaking about an internal combustion engine, we're speaking about volumetric effiency, right? And how does the stuff get into the chamber in the first place? By the pistons downward motion creating a vacuum. If the rate of the downward motion directly effects the VE, then the rod ratio it very important in equating what your goals are when building an engine. In a not so eloquent term ... I will explain to you Scott that this is the first stroke ... the intake stroke ... the beginning of how we make an engine run.
Once you grasp this idea ... we'll work on the next stroke ... compressing what we've filled the cylinder with ... it's called the "compression stroke", let me know when you're ready Scott ... in your post you were moving onto things way too quick, kinda prematurely, but you might be used to that ...
-
For an engine to accerate: Correct sequence of events....
hey ... I didn't get to page two yet ... I see you have already started speaking about what I am getting to ...
-
When a swimmer finds his stroke and kick rythum he or she is fast.....
When a cyclist finds his pedeling rythum he or she is fast. . .
When a runner finds his stride he or she is fast. . .
When an engine has a tuned induction and exhaust path, it is fast. . .
-
When a swimmer finds his stroke and kick rythum he or she is fast.....
When a cyclist finds his pedeling rythum he or she is fast. . .
When a runner finds his stride he or she is fast. . .
When an engine has a tuned induction and exhaust path, it is fast. . .
well put ... timing is everything ...
-
Impossible for the rod to be the hypo. if the rod and throw are @ 90*.
What I was getting at was,
The piston would be moving at it's fastest speed at this point at a given rpm. :boxed: And by that time the BANG has already done most of it's work.
-
I hate to have to quote myself ...
What happens when the crankshaft is at 90? You'd think that the piston is halfway down the bore, right? Well that's not exactly true. The angularity of the rod at that point brings the piston down a little bit further - the connecting rod length is the hypotenuse of a triangle where the short side is the sideways distance from the center of the cylinder to the center of the crankpin.
Pythagoras was a Greek mathematician who lived about 2500 years ago, and who developed the most famous formula in geometry, possibly in all of mathematics! He proved that, for a right triangle, the sum of the squares of the two sides that join at a right angle equals the square of the third side. The third side--the side opposite the right angle--is called the hypotenuse of the right triangle. The two shorter sides are usually called "legs."
This formula is called the Pythagorean Theorem in honor of Pythagoras. It is usually written as the equation below, where a and b are the measures of the legs of the triangle and c is the measure of the hypotenuse.
http://www.***boat.com/image_center/...358formula.gif
So with my original post of the crank being at 90* (from TDC) ... the hypotenuse would have to be the rod angle :squiggle:
http://www.***boat.com/image_center/...hypotenuse.gif
-
So with my original post of the crank being at 90* (from TDC) ... the hypotenuse would have to be the rod angle :squiggle:
http://www.***boat.com/image_center/...hypotenuse.gif
Ummm, don't you mean rod length? :)
-
Picture this,
http://www.***boat.com/image_center/...ypotenuse1.gif
Large black circle is crank journal.
Red circle is rod journal.
Yellow circle is piston pin.
Piston speed should be at it's max in this position. Yes/no. Or is it at 90deg to the bore? :boxed:
-
Picture this,
http://www.***boat.com/image_center/...ypotenuse1.gif
Large black circle is crank journal.
Red circle is rod journal.
Yellow circle is piston pin.
Piston speed should be at it's max in this position. Yes/no. Or is it at 90deg to the bore? :boxed:
Mike;
Piston speed is Zero at TDC
Maxium piston speed should be at two points relative to crankshaft rotation about 75 degrees and about 285 degrees of crankshaft rotation
Maximum piston acceleration will be about 10 degrees before and after top dead center.
Bob
-
Mike;
Piston speed is Zero at TDC
Of course. :rollside:
-
I've been waiting a LONG time for the opportunity to present itself to open this can of worms. This is the grand daddy of all questions regarding engine performance, and I'm really curious as to who thinks what. What is it about an engine that makes it accelerate? Why does one engine accelerate better, or faster than another comparable engine from point A to point B? Lets face it...this is what wins races, not dyno numbers. So, is it horsepower? Torque? Both? Neither? :idea:
Let 'er rip. :D
Throttle position!!
LOL. :D
-
Ummm, don't you mean rod length? :)
No ... I meant angle, refering to the diagram the length of the rod was irrelevant ... just trying to make sure that we were all speaking of the same angle/segment of the triangle ...
-
What happened to Scott? I'm ready to talk about the next stroke! Compressing what we brought in ... and what effects heat plays ...
-
No ... I meant angle, refering to the diagram the length of the rod was irrelevant ... just trying to make sure that we were all speaking of the same angle/segment of the triangle ...
Yeah, I know what you were getting at, just busting your butt about calling a straight line(the hyp) an angle. :)
BTW, as interesting as I find this, it's giving me flashbacks to my physics, geometry, trigonometry and, worst of all, calculus courses taken 30-something years ago. :D