I have a question for everyone to think about - and if you build engines you should really know this -

What creates torque in a piston engine?

This seems like a silly question but its really one of those things where there is more to it than meets the eye....
For those of you who would answer expanding gasses forcing the piston down during the combustion process, I will tell you now that you are completely incorrect - heres why...

For torque to be present there must be a lever arm, there must be a force acting perpendicular to this lever arm creating a twist about the axis of rotation - a wrench or ratchet is a perfect example of this, you push or pull on the wrench perpendicular to its length, the length of the wrench giving you a lever arm for which to rotate the bolt, and as common sense would tell you, you would never push along the line of the lever arm, i.e. pushing the end of the wrench towards center the bolt - but this exactly what the force on your piston does - it all acts right through the axis of rotation of the crank, so there is no lever arm because the center of your piston is in-line with the axis your of the main bearings, which of course support the crank and allow it to rotate. Hmmm... you should now be thinking that you don't know as much about engines as you previously thought, so where does torque come from? We all know its there, just how? I'll give you a hint - whatever it is has to be perpendicular to the force from combustion and have some distance away from the crank axis and if you have ever rebuilt engines and noticed how cylinders wear, you might be able to muster something up here. Bragging rights are at stake - give it a shot, but i'll tell you its really easy once you know the answer but its one of those things where your intuition fails you. Good luck, I hope somebody surprises me and gets it right!!

 

no f'in clue, tell us?


heres a wild shot:
the twisting movement of the crankshaft (torque) comes from the piston being pushed down, just after TDC?


No? i didnt think so, if im right, GO ME!

 

Hmm this may be over most ppls heads -

I kinda already told you that wasnt it - force acting on the piston face cant cause torque, no matter where the piston is relative to TDC or BDC - thats y I wrote all that I did - at least you tried....

 

Isn't it the piston rings?

 

Nah - but your getting closer, the reason i'm shooting this one down is that small hobby 2-stroke engines without piston rings still create torque (although it is a pretty small amount)

 

You don't really have to have force applied perpendicular to the lever arm, but you do have to compensate for the applied angle if it isn't... Otherwise engines would only have torque at one particular point every two rotations per cylinder (assuming 4-stroke here).

For those still coming up with answers, remember that torque is static - there is no motion involved... Once an object is moving under torque, you have horsepower (HP=(torquexRPM)/5252).

Piston rods are not attached at the crankshaft center point.

Even though I see what you are saying, it still starts in the combustion chamber - conversion of chemical energy into thermal and pneumatic energy. The differences in rod length vs. stroke (and piston offset, like we SOHC's have) makes a big difference in torque output comparing two engines with identical displacement. That's one of the reasons why the phrase "There's no replacement for displacement" is so terribly inaccurate.

 

ok, new try, torque comes from the piston rings sealing off the combustion chamber, therefore causing the piston to want to move in the direction of the crankshaft rotation and not the other way???????

once again, if im wrong, i didnt think so, if im right (doubt it) go me!

 

Yes the force created is inline with the crank main bearings but the force is not applied there. It is applied to the to the connecting rod journals which have a crank throw. This offset allows the thrust created by the rapidly burning and thus greatly expanding gasses to apply said trust onto crank throw allowing torque to be created. And the folly of you have HP once in motion is laughable. You only apply torque as the measurement of HP is a rate of torque application.

For instance a steam engine like in a an old time locamotive makes the most torque when the engine is at maxmimum pressure and 0 RPM. however once in motion the engine still applies torque as the gasses are expanding faster than the cylinder travel. Thus you have a reduced force but still one acting on the lever arm of the piston rod journal to main journal centers. Thus you have a lever.

Simply and no more.

When ou have cylinder wear this is due to the imbalance and thrust loading of the piston face and wrist pin location. If we were to assume this is evedence of the torque being created you'd have to say a modern diesel which still has the cylinder honing clearly visible after about 250,000 miles is creating pitful amout of torque. HA!!! This is due to the careful control of the combustion gases and cylinder loading forcing the piston to travel straight down into the bore and not scuffing the cylinder walls. Also due to the length of the cylinder skirt. run slipper skirt you piston can rock more and you get more scuffing.

Rings have nothing to due with torque directly but to ensure proper chamber sealing allowing for high comustion pressure and more efficient burning of gasses. Thus with more efficietcy you creat more thrust (If you have more drag from to high ring pressure, to much piston rock you limit the thrust being applied to the crank arm by the rod via the piston) for the crank arm. More thrust on the and of a lever the more torque. That is why if you get a stroker crank with more throw you can generally get more torque.

Torque = force * distance applied (For rotational force)

Thus to get more torque, you can increae the force applied or the distace from center.

HP is work done. A direct conversion from TORQUE applied at a given RPM!!! If you no longer have torque you no longer have HP.

Have FUN

 

I don't see where the argument is. Saying the same thing two ways. The way I try to explain torque vs. HP is: hold a book out in front of you. The weight of the book (due to gravity) applied to the length of your arm is applying a certain amount of torque on the pivot point, which is your shoulder. That torque is there with your arm perfectly still, there doesn't have to be movement. Without movement, there is no power. Once you let your arm drop, you have movement under force, which is power. That's all. I wasn't saying torque disappeared once you have motion... sorry for the confusion.

 

I never made that claim - obviously conn rods are not coincident to the cranks rotational axis - mechanically the conn rod rotates about the instanteneous center of zero velocity (point where 2 lines perpendicular to the velocity of each end of the link intersect, not a constant point), and in no way can the ic and the crank axis coincide for this type of linkage. The conn rod, despite this, is in pure rotation at two points - TDC and BDC. The piston must also have zero velocity at TDC and BDC - unless your engine is literally flying apart - and as such the conn rod is pure rotation about the piston pin. The connecting rod also translates in the axial direction of the cylinder bore and at 90 and 270 degrees of crank rotation from TDC it is in pure translation - the ic is at infinity because the velocity of the small end is parallel to that larger...

and this is true

but....

offseting the piston pin axis from the crank axis will create very little torque (its lever arm always being that distance) and engine without it will still create torque - think about an engine whose piston center is colinear to the crank axis, as I initially stated.

in this case -

You are correct on the point that forces are vectors and that if a force is applied in any other way than axial or tangential to the axis of "impending" motion (impending because in "statics" there may be no significant motion other than that what HAS TO occur in reality to cause deflections and vibrations, or having motion which does not change with time) that it will be the component perpendicular to this axis.

They are MECHANICALLY THE SAME SYSTEMS - - this has absolutely nothing to with why your engine creates torque - mechanically it doesnt matter what fuels your fire - its still a reciprocating piston system and dynamically the same. You need to recognize that they all have the same layout: crank axis - contrained in space, rod bearing axis - constrained to rotate about the crank axis, piston pin axis - constrained to vertical translation. For simplicity, think about the outcome with the piston pin axis colinear with the crank axis. Its a simple 3-point linkage, a triangle right? So the reason a steam engine creates torque is the same as why a gas or a diesel engine does.

Torque causes rotary motion and statics involve motions of constant velocity as well as zero veloctiy! - NET torque about an axis will cause an acceleration about that axis according to mr newtons 2nd law in rotational terms, in that angular acceleration is inversely proportional to net torque about that axis. This proportionality constant is called moment of inertia (also mass moment, 2nd moment, polar moment, etc) and is the rotational analogy of a linear mass in dynamic terms. Power is a derived term, it is the instantaneous rate of change of work with respect to time - its a measure of how fast somthing can do work. Work is the component of force parallel to the displacement summed over the total displacement - ie the old force x distance - which is highly simplified as compared to the true definition being the integral over the distance of the scalar product of the force vector and the infinatesimal displacement vector. You can think about the engine doing working a few ways - force on the piston acting thru its stroke, torque on the crank acting thru the amount of angle of rotation, pressure acting thru the changing cylinder volume, or the amount of chemical energy in fuel that is converted to mechanical work. The first two are work in mechanical terms, the third is a thermodynamic approach, and the last is overall efficiency which is sometimes inverted and called specific fuel consumption.

You still dont see how you dont have a lever arm here, should I make a diagram? "Thrust" will not cause torque

HA - you just answered the question but you dont realize that your piston ALWAYS must ride cylinder wall - this is what creates torque - and it has nothing to do with the control of the combustion process (this is actually important for other reasons) - the fact of the matter is that the conn. rod must for all intensive purpose transmit force to the crank, and it being essentially a two force member all this force must lie along its centerline (except its weight which is fairly small in comparison to the loads it transmits), so this means if you are just past tdc your conn rod will not be vertical, so a component of the force it transmits will be acting on the cylinder wall. This force on the cylinder wall does have a significant moment (lever) arm!!! - and just past tdc it is slightly less than the sum of the crank throw (half the stroke) and the conn rod length. Thats what creates torque and thats why high torque engines will have skirts to better handle there loads - the longers the skirt - the lesser the force required to keep the piston from rocking - they always rock to some degree unless your engine is siezed!! Now there is a difference between the piston skirt sliding along cylinder wall and rocking and wedging on the cylinder walls - sliding is desireable, it is essentially a lubricated linear bearing between the piston and the cylinder - wedging is undesireable but is going to happen because there has to be some clearance between the piston diameter and the bore. So if you have been paying attention it is the sliding contact which creates the majority of your torque output - also not that the lever arm decrases and you approach bdc as does your cylinder pressure and the angle the conn rod makes with the cylinder wall changes - this changes the magnitude of the force on the wall (common sense says it will be more the closer it is to being pependicular), therefore torque is not even close to constant throughout the stroke. It is quite choppy as you may know if you have ever used a single cylinder engine at low rpm (the flywheel tends to smooth this out by storing some mechanical energy in its inertia). ewetho was the closest although was slightly off track - Oh and the reason for careful combustion control is to create the maximun cylinder pressure at the most optimal point of the combination of crank angle and conn rod position so as to create the best torque by best making use of the dynamic characteristics of this type of linkage.