Damn I feel like Im back in college, quit makin me read so much!!!

 

Reading is a good thing dude - but will only get you so far - you have to understand theory and also have good practical hands on for it to be of any use, ie think before you wrench - trust me I used to just spin bolts and take stuff apart but the more you know about its design, the easier it is to fix something or modify it to make it better. Just some food for thought.

 

OK Fine but I still don't see how a modern diesel which uses a lot of engineering to prevent the piston from riding the cylinder wall and centers the force on the top center of the piston would thus create all of the massive amounts of torque it creates. They work at getting a piston to float on oil and only the rings to slide on the wall.

Also a single cylinder gas engine only applies power fo 10Ëš per 270Ëš of rotation. Thus choppy. A modern diesel is now thrusting upto 33Ëš of rotation per 270Ëš due to a much longer fireing time as they just spray more fuel.

What I don't get is why is all the engineering trying to force the piston down staright while the desired force is along a cylinder wall? Longevity sure but could you not design and make a reliable engine that develops force here?

What I don't get is that at a given 10Ëš of crank rotation would not the side thrust created buy the piston being foced downwards (Rotationg components free to move to a degree) and acting on a the pinton rod at an agle not be a loss. Because the face of the cylinder wall is not forcing the piston anywhere just the forces wedging themselve against the wall focing the piston downward? Thus in a moderen diesel where using a mexican hat design piston controlling the combustion gasses to force the cylinder straight downward onto the connecting rod piton piton and transmitting most of its force directly onto the connecting rod which due to its design and current connecting points vector said applied thrust onto the crank shaft connecting rod journals at an angle to the main bearings. Thus you have a lever, and applied force acting on the lever. Even if you do get a a thrust load angle to the crank it would still have to be applied back through the same connecting rod.

Heck maybe I am wrong but please do draw that picture? I just seems weird that on a V8 in which the determined thrust load would be applied due to rotation of the engine that the GRAVITY side generally wears the most(and that can't be greater than the forces we are taling about)???

 

Yeah...the samll hobby 2 stroke uses a "sleve" which acts like the rings with exhaust port holes machined in them....that is why they are so compact, yeah there is torque produced like you said....that is why they can get off the line so quick with their wieght to torque ratio.

Pretty cool info mechengineer201 gives us all something to think about!

 

See if this helps....

[IMG]local://upfiles/16149/6A3621F6F6354073A01651529DFB0D2C.jpg[/IMG]

[IMG]local://upfiles/16149/3A9F94D9C14B47D68E04076F2CCE3114.jpg[/IMG]

 

In your text the sum of the thrust applied by the block and and thrust applied to the piston are forced along the piston rod which in turn now has an angle to push on the lever which is the crank throw.

Well as I see what you are saying about the thrust load of the piston forcing against the side of the cylnder wall in reference to the load moment. This is to keep the reciprocating mass from becoming a rotating mass. Yes this would be a very necessary design element to make a block that is strong enough to sustain the necessary loads. As this situation would definitely indicate that for a given load lengthening the rod & or crank throw would thus create a longer lever arm for the block(due to the increase in piston pin to crank main centerlines) reducing the force necessary to hold the piston in place. So for given load you would actually need less pressure on the top of the piston.

Yes in basic theory this does indicate that the (thrust load*pin height from crank)=moment load. Neat coincidence but no torque is applied hear just the required resistance that allows the thrust of the piston to be vectored down the rod where it can act upon the crank.

Just look at a torque wrench, the applied torque is at the wrench head. and any restance to motion or thrust from my hand applies a force to a lever that is twisting around a point. The twisting point is the crank main journals. The lever is the crank throw. The thrust applied is via connecting rod. We just use the piston and block to direct the force created in combustion. Just so happens the resistance needed by the block integrity is directly proportional to engine torque and pinheight.

This does bring some intersting ideas of canted piston bores though minimzing thrust on cylinder walls during application of torque.

 

I mean yeah we could make it more complicated - actually my lil proof is completely wrong (everything is an appox but some are better than others), but a decent approximation for the simplest of piston geometry and layout (much like a 2-stroke hobby engine), and makes the point I was after - I'm not about to overcomplicate something and make it so no one even understands what the heck I'm talking about, the point was to understand something very very basic that is so often overlooked - any realistic model has to account for the dynamics, what I showed was merely a simple statics "snapshot" - I actually have some way better approximations I did a while back for investigation of exhaust gas acoustics because as you may know the gas velocity has to do with your piston kinematics and your cam function, as well as port and valve size and shape. The other thing you have to account for is inertial forces in the conn rod and the crank as common sense tells you makes these member want to fly apart (ie centirfugal force) when in reality they are not being accelerated (centripital acceleration) as much and in the direction they should be do to their inertia opposing changing motion. Another complication is primary and secondary forces - which is a big deal on inline four cylinders without balance shaft(s) because the secondaries are additive and put alot of load on the mains.

 

Yeah exactly - of course the force has to wind up acting on the crank, we could take the whole thing apart and look at the forces acting on every member seperately and you would be absolutely correct! you just cant forget the conn rods role in the whole process because it is the direct linkage between the piston and the crank and as far as stresses are concerned - quite high - and compressive during the power stroke, so the conn rod must be designed to resist buckling, which is the onset of bending in the first mode and its weight and rotational inertia, which I previously stated as being insignificant, become very significant at higher engine speeds.