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Recently I read something about piston acceleration and I realised that I was previously wrong in my understanding. I also realised at this point that I had not really thought about it too much before and had just assumed.

So really, this post is just me trying to work out how much this is common knowledge. This forum is perfect, as we are all car nuts so understand what is going on in there, but we are not all engine builders who would (should) definitely know this. So this is just a round-up of what you all think (before checking Google).

So, who here knew that the highest acceleration of the piston is achieved at TDC? Also, who knew that the highest velocity of the piston is NOT at 90 degrees from TDC/BDC?

Tom...

Makes sense but I never really thought about it
Is it due to the reversal of direction occurring at TDC and BDC? I understand that's the most wear inducing point of action on the rings.

Nope, not BDC, ONLY TDC.

I forgot to mention that bit. It gets the confusion neurons flowing for a while.

OK. I'll bite. Where did you read this? I'd like to see their discussion further. Rate of accelleration being highest at the direction transition sounds resonable but I don't understand why it is not also at BDC. I gotta do some research.......

Yo, Tom-
Chuck beat me to the posting... I was wondering why BDC isn't a "fast point" as well. Interesting... I'd research it, but I'm kinda figuring the engineering discussion would make me go cross-eyed... Can somebody 'splain it in words even a geologist can understand???

I thought exactly the same about BDC. I also thought Vmax would have been at 90 degrees but it is not (for similar reasons that BDC is not also a peak velocity.). But then I spend half a day researching it and it all made sense.

I'll try to dig up the best description I can find of what is going on, but until then, stew on this accel/angle graph There is actually a bump at BDC where the accel is slightly higher either side of BDC! Not very clear in this pic though. It's all to do with stroke/rod length ratios and actually does make sense. With some stroke/rod length ratios, the bump is really noticeable.

OK. I got it now. It is certainly something I never considered. It is not the obvious as shown by some of the incorrect explanations out there in cyber-space.

Visually, the peak speed occurs when the tangent rod to the crank sweep occurs (in other words at the peak angle of the rod). This changes with rod length vs stroke length of different engines.

Thanks for the edumacation!

I think this is the one that explained it the best.

http://www.epi-eng.com/piston_engine_te ... basics.htm

Just remember, when the crank is at 90deg ATDC, the piston has moved FURTHER than half the stroke. This is because the conrod big-end has also moved horizontally by a distance of stroke/2, thus pulling the piston down further than just stroke/2. As such, from 90ATDC to BDC, there is less distance to travel and accelerations are lower.

Funky huh!

This stuff underlies why an I4 engine vibrates. The parts are balanced ( pistons, rods ) but since the acceleration is different at the top and bottom of the strokes the engine vibrates up and down. At the top of the stroke the pistons yank the whole engine upwards. So it vibrates at twice the engine speed.

If you imagine the crank swinging thru the bottom of the stroke, since the connecting rod is only twice as long or so you can see it won't move much. If the connecting rod were infinitely long, these things wouldn't happen. Steam engines sometimes had very long rods to help with this. It may have made more difference because they usually were 1 stroke engines. Steam was used on both the top and the bottom of the piston. A 2 cylinder steam engine has as many power strokes as a V8.

I think the issue with the wear from the piston rings may be different, a bit. The rings float on the oil film when they are at speed, the same way the crankshaft floats on oil film in it's bearings at speed. Since there is no speed involved at the ends of the piston stroke, the rings don't really float anymore. Which is sort of OK since they aren't moving anymore.

The TDC area though is exposed to the combustion and the BDC is not. That must make some difference, it's a much more harsh environment. In addition the BDC is exposed to oil in the crankcase for much of the time, it is well lubricated. The TDC just gets oil that's dragged up by the piston and then it has a high pressure flame every other cycle...

Sounds like a hard life being a TDC

The real problem boils down to the mass of the piston ring and the force/mementum in the changing of directions. That is why you see the real thin rings or Dyke rings used to reduce the mass to prevent ring flutter. A full race engine SBC at top engine RPM is seeing 180 cycles per second. F-1 and M/C engines are way up there. That is why ring land clearance is so critical.
Dave W

I remember doing this in mechanics (Physics). It's not a too complicated geometry problem. We had to find an equation to describe the motion based on lever(rod length) and crank pin distance( 1/2 stroke). And yes, you would get some odd forces. A larger rod ratio gives a more sinusoidal curve. We calculated forces based on no rod mass, it gets unsolvable VERY quickly and must be done computationally.

Now my head really hurts. Thanks a lot guys.

Tom

How can a piston be accelerating when it is stationary? Or doesn't it stop as it changes direction? Seems to me that at TDC and BDC piston acceleration is zero.

Bill

Bill- Hey, let "I ain't no engineer" answer that one! It seemed that way to me as well, at first glance. However, the velocity comes to zero for an instant at TDC and BDC, but the acceleration is the greatest, because it goes from that zero velocity to moving "full speed" in the next instant. I suspect these "instants" are nano-seconds or thereabouts.

(How'd I do, y'all?)

JDK