dchi_t

iTrader: (1)

wow

Resolute

iTrader: (2)

Mechanical Engineering, minor in Metallurgy

overZealous1

iTrader: (27)

my point was just the fact that in his comparison, that the only way the motor making power at a lower rpms will see more stresses on the motor is if it was n20 injected.

Enron Exec

I am tring to answer what i think was the original question of stress of a single individual part. You are answering his question in the manner of load on the entire engine. I think Sharif used the term "load/stress" and that encompassed our 2 differnt topics together inadvertently.

Because this is the FI section, i assumed Quamen wanted to know what parts where at the highest risk of failing rather then the load placed on the engine.

Not to nit pick but the bolded areas below are where i disagree:
"In your own example, the explosion that makes 400lb/ft at 3000rpm, will have the same energy in the explosion that generates 400lb/ft in the 5000rpm engine. How do we know this? because both engines generate a FORCE of 400lb/ft of TORQUE on the piston. "

This is not correct. The 400lb/ft at 3000 rpms is distributing that torque across 1500 combustion strokes while the 400lb/ft at 5000rpms is distributing that torque across 2500 combustion strokes. For each stroke, there is more stress put on the 3000 rpm motor.

Enron Exec

May I ask what semester are you in? Have you had a class along the lines of Statics and Structures? That should be a Sophomore or 3rd semester class imo.

Enron Exec

Just an after thought. It would be simpler to just use torque and at what rpms when talking technical stuff here. Because we should always mention rpm at whatever HP and Torque are made, its redundant to mention both HP and Toprque. They are the same thing; one just has a few missing units. Plus... torque wins races, horsepower sells cars. Thats muscle car talk but has alot of truth to it.

Oh yea... LOAD can be thought as the inverse of POWER. So the more power the car is making, the more load there is on the motor.

Resolute

iTrader: (2)

Junior. I have taken Statics, but this is an issue of Kinematics, not statics. I have taken that too. I don't have the time now, but will gladly explain this further later. For now, consider this: torque is a force, it can be measured and is applied independently of time. Independent is the key word. The force acting on the piston is an application of torque to the crankshaft. In the same way you apply a force of x lb/ft on a wrench to tighten a nut. It is not applied in lb/ft per sec, it is how many lbs applied on a 1ft lever arm. It is when you combine this force with the number of times applied that you have hp. I think people have misunderstood this. The fact that the torque is applied more often per minute is the reason you will have higher hp figures on the motor that applies it higher in the rpm range. The load is the same though in my example of two engines making 400 lb/ft of torque with all else being equal. The amount of work the engine performs has increased, the load on individual strokes is the same. As far as torque winning races, hp selling cars.. that's another thread all together. This is becoming a good discussion, I'll check in later... still looking for that article I copied.
Will

lucidazn

iTrader: (2)

Shouldn't it be directly proportional then? When it's inverse of each other, one gets smaller as the other gets higher.

What kind of units would you use for "load on individual strokes"?

Sharif@Forged

iTrader: (92)

Res, arent we also making the assumption that the interial load at 6100rpm is greater than the trq loads at 4000rpm?? Has this been established in automotive engineering circles?

Resolute

iTrader: (2)

What you have stated here is incorrect. now, I don't want this to turn into a pissing contest, so feel free to quote some source as to where I am wrong. When I have the time, maybe tomorrow night, I'll see if I can copy the equations on torque, mep, hp, and mechanical efficiency to a jpg and post it, and work through some calculations to help clear things up. For now, imagine cranking that wrench I mentioned earier with 50lbs of force. The wrench is one foot long exactly from the center of the nut you're turning to the center of your handle. Imagine that this wrench is a torque wrench and can measure the 50lbs of force you apply. Now, when you crank this wrench 360 degrees, you pull it off, replace it back at your original starting position and crank it another 360 degrees. You continue this process for one minute and after one minute have turned the nut 10 times, each time 360 degrees. You have just applied 50lb/ft of torque at a rate of 10 times a minute, or 50lb/ft at 10 rpm. Your amount of work is that 50lb/ft times the 10 revolutions you made in that minute, but your torque, as seen on the wrench, never rose over 50lbs of force. The next time you increase the number of rotations to 20 per minute. Your rpm has increased, your torque has not, but the net result is an increase in the amount of work performed at 20rpm vs 10 rpm. same thing if you apply more torque, the amount of work increases. but the torque is NEVER spread over the whole period of time you are working, or the rpm. It is ALWAYS an individual amount of force that is applied over and over again in that time.
Will

Resolute

iTrader: (2)

That is not an assumption. This is a very common dynamics problem, and has been proven. i can also post the equations for this and the amount of stress for our pistons at 6100rpm is not comparatively bad, but if you have never seen what accelerative forces can do at that speed, it will probably blow your hair back. Or not, i guess.. I find it absolutely stunning anyway.
Will
EDIT: Incase EnronExec wants to know, Dynamics is a Junior level course, and I've studied it too. I also think this might get more in-depth than the original poster wanted, although it all has delt with stress on the engine and when and how that stress occurs, I think it might be better to move it over to the thread I started in the Engin/Drivetrain forum since he is now interested in whether his engine is tuned properly and if it will hold togethger at his boost levels, and probably not so interested as to which of his hypothetical engines has more stress and why. I figure it's up to him, but this way the posts might focus more on his last question. Also, I apologize for any delayed responses, I don't have internet connection at my house and when I can get online varies with where I have to be, so be patient for responses. thanks.

overZealous1

iTrader: (27)

i am also pretty fascinated by all the forces generated inside your motor. ofcourse it is engineered to stay together, but if most people knew exactly what was going on in their motors, they would never want to start them up!!!!!! hahahaha

Enron Exec

Your right, i foo'ked up there. They should be proportional.
Im glad there are actually ppl reading this to make sure no one is just spewing out BS.

Id use lbs or Newtons. To get the force exerted on each piston top, id take the torque at the crank and divide it by 6 ( because we have 6 cylinders ), then divide the result by 2 ( because the crank turns 2 times or 720 degrees to complete the 4 strokes ). The result would be the torque each cylinder delivers to the crank. To convert torque to linear force, id have to find the rod length. That should result in the force exerted on top of each piston during the combustion stroke. I think there are a few other things to consider, like the size of the lobs on the crank but im not sure. Maybe there is an equation floating around where we can plug-n-chug.

Enron Exec

No pissing contest intended. Im stuck at a relative's house 2000 miles away from my PC and my APS built motor Z; im franticly looking for something to do. Im so bored, ive resorted to posting away my day. So if anything, I thank you Will for keeping me sane.

About your post. Yes, this is all straight forward and correct. But a wrench turning a nut differs from 6 connecting rods turning a crank in that the force applied to the nut is continuously 50ftlb across how ever many revolutions while the force turning the crank is 6 seperate instances for every 2 revolutions. You can actually feel the force generated by each piston at very low RPMs if you have a very light flywheel. Well, at least i can. The very reason many cars come with heavy flywheels is to smooth the torque delivery to the transmission and therefor a smooth ride for their occupants. If your exmaple of a wrench and nut was actually how modern day engines delivered torque, we wouldnt need a flywheel at all because we would always get a consistent smooth torque delivery... kinda like some DC motors!

Anyways, if you have a nice professor, TA, or an engineering dorm buddy that could put their 2 cents in. Id be very interested.

amolaver

iTrader: (1)

Not that this is significant in this context, but this statement isn't true. If it was, why would every high power internal combustion engine design be reaching for RPM's? F1, bikes (witness the '06 Yammy R6 - redlines at 17.5K, power peak @ 14.5K), even the mighty 7 liter LS7 z06 redlines at 7K rpm (via Ti conrods) and makes peak power @ 6300rpm (source: Road & Track Dec '05). Winning races requires work be done. More work gets done at higher RPM's per unit time.

ahm

overZealous1

iTrader: (27)

would be really interesting to see this equation, as your torque arm is essentually changing length converting the linear force to rotational.

Enron Exec

Hmmm no.

Im not even going to.

Resolute

iTrader: (2)

LOL, that's cool. I know the feeling. I'm at work, and would love to just put everything aside and hack this out.
Dorm buddy! I have a wife and a house man.
Now here's the kicker, I'm finishing up my ME, but am planning on going to CSU for my masters asap in Motorsports Engineering. Right now, I get to hang out and help with their prototype racer and facilities on the weekends. Besides being hella fun, I get to schmooze the three profs into liking me enough to enter the program. Now, the facilities they have are pretty awesome. Besides a typical chassis dyno, they have a brake friction dyno, chassis flex dyno, shock dyno, all kinds of geeky metallurgy stuff, and most important for our discussion, an engine brake dyno.
Now, I would love to use some of this stuff on the VQ, and maybe I'll have a chance one day, like hooking up a pressure indicator to the head would be of enormous value, but for now I have to settle for playing with a one liter Yamaha. Now, this is important because of some things you said.
First, why isn't the torque applied over the course of all the revolutions at a certain rpm? In our case, lets imgain the engine with a power peak at 6100rpm makes its peak torque at 4000rpm. Now, if I understood you correctly, you are saying that at 4000rpm, the 400lb/ft of torque is pread over 2000 power strokes. If the max torque occured at say 2000rpm, it would be spread over 1000 power strokes, and therefore more stress is on the engine making torque at the lower rpm. Here is where an engine brake dyno comes in handy, and you pretty much alluded to the same in your last post. The dyno could give a crap about rpm, in fact it doesn't even measure rpm. We only need to moniter rpm to calculate hp. What it does measure is the amount of torque applied on EACH INDIVIDUAL POWER STROKE. Ours actually measures this twofold, by measureing the amount of pressure placed on the mounts and with a brake on the flywheel. Think about it like this, Even with our V6, no two pistons are firing at the same time, and if max torque is applied on every piston when the crank angle is at 74 degrees (it is on the VQ, just trust me on that one cause I couldn't even begin to type out the math) then every time the piston on any cylinder is on its power stroke and the crank angle for that piston is 74.23 degrees, then the max torque is being applied to the crankshaft centerline. Even with 6 cylinders, there are seperate, individual power strokes and seperate individual "surges" of max torque. As you stated, you can feel this at low rpm without that dual-mass flywheel to dampen them out. The dyno records the max amount of torque applied at any one time, regardless of rpm. It just happens that at 4000rpm in our engine, the max torque developed from any one of our six "surges" is 400lb/ft of torque. Now, as far as we are concerned, the rpm is useless, we might as well just say that the engine develops its peak torque, or even better, the individual cylinders each develop their peak torque when the piston speed is 17.7 meters per second. Because that's how fast our piston is moving when the engine is at 4000rpm. The only reason we care about 4000rpm is to calculate the hp. The reality is, if we were measuring the torque over a one minute period, then the max torque developed over the whole minute would be divided out among all the seperate strokes, but we aren't because that's not how torque is measured, that's how horsepower (work) is calculated, the engine at any one instant of time while spinning at 4000rpm has a max torque applied to the crank of 400 lb/ft. If we plot this out with torque as a function of engine speed and time, and kept our engine speed constant at 4000rpm, you would see that the engine has a "push" developing 400lb/ft of torque every .0075 seconds. Every .0075 seconds, another power stroke would occur on the piston and produce 400lb/ft of torque, and this max torque occurs with a piston speed of 17.7 m/s, or when the engine is at 4000rpm. The amount of work the engine does is a function of this torque occuring on each power stroke, and the number of times it occurs in that one minute time period.
I think this is confusing because people see that hp and torque are the same, with a few constants changed around. They are not. They are so different its crazy. As Dave Coleman once said, asking what's the difference between hp and torque is like asking what's the difference between orange and soft, or so he said something to that extent. If we deal with this in terms of Kilowatt output as a function of Newton/meters of torque applied over time, then I think this would be way easier to grasp, especially since the term horsepower is a bit decieving as a term for power.
It's been three hours since I started typing, I keep getting interrupted to actually do some work, and then come back and type a little more while I can. so i hope this is somewhat coherent.
Will

overZealous1

iTrader: (27)

i would of thought the lever arm would change effective length as the mixture doesn't fire all in one instant, but for a duration of the stroke, and changes the leverage as it travels down. so that 74 degrees of crank angle is where you just get the highest point of power and leverage, with everything past that trailing off?
these are very good write ups and i am learning a few things from them. most all my experience comes from many books and building many motors and hanging out in machine shops though, and not from a classroom invironment, where some of these theories were not covered, but had to figure out on my own, lol. sounds like you have alot of cool tools to play with there though.

Sharif@Forged

iTrader: (92)

This gets my vote for the nerdiest thread ever. I love it...keep it up! :S