Someone asked me this and it made me wonder. When an engine is running,does the piston come to a complete stop for an instant when it changes its stroke? If so,why doesn’t it tear the engine apart? If not,how can it NOT stop before reversing? Then again,the crankshaft is rotating… Anyone have an idea? I have no clue,only guesses.
Yes and no
I remember an excellent musing into this very question by Dennis Simaniatis in the “Tech Tidbits” column in Road and Track five or six years ago. Basically, at the top and bottom of the stroke, the piston’s velocity is zero. However, it is only there for an infinitessimally small amount of time. Imagine it’s speed as a graph. The line crosses the axis (zero velocity) as it changes direction. At the intersection, it is “stopped”. The intersection is only a point, however.
Try to imagine this as more of a curve than it looks. Positive velocity means the piston is moving up, negative velocity means it is moving down.
.......................................
...........___.....................___.
........../...\.................../...\ positive
........./.....\................./..... velocity
......../.......\.............../......
......./.........\............./.......
------+-----------+-----------+-----------> time
...../.............\........./.........
..../...............\......./..........
.../.................\...../........... negative
__/...................\___/............ velocity
.......................................
When the piston is actually stopped, its acceleration is a maximum. That is, it will get moving again shortly, and the transition is smooth. There is a property called ‘jerk’ which is the rate of change of acceleration. It is appropriately named, because that is how you feel when a subway train lurches to a stop, or a car hits a curb. The piston has a jerk of zero at that moment, it is a very smooth acceleration. It is the jerk that would do damage, just as it is the jerk that gives you whiplash in an accident.
Stupid jerk!
If they didn’t stop, they’d fly right out the top of the cylinder, wouldn’t they?
Yes. It stops in the way a swimmer getting to the end of a pool has to stop before returning.
Let’s say when the piston goes up, it’s velocity is positive, i.e. +250m/s. And when it goes down the velocity is negative, i.e. +250m/s. Say the piston goes up and then down. At some point, velocity has to be 0m/s before it go the other way. It might be an instant, a microsecond, but it still has to be at 0m/s.
Sorry, meant to say when piston is going down it’s -250m/s.
I thought the driveshaft attached to the pistons always went the same way, and it was the gears that altered the subsequent shafts that turned the wheels, including reverse. Reverse, I thought, was a clever combination of cogs that caused a forward rotating camshaft translate into a reverse rotating shaft.
Am I totally wrong?
It also depends on what you mean by “stop”. If you’re just talking about vertical movement, then it definitely stops. However, that doesn’t necessarily mean that it is no longer moving relative to the cylinder walls (and thus potentially having to overcome a static coefficient of friction).
You can design a piston mechanism so that while it travels up and down, it is rotating in a sinusoidal pattern, such that when it’s at its maximum and minimum position vertically (and thus “stopped”), it’s actually in the center of its “twist” (and thus not stopped relative to the cylinder).
Try this experiment: wrap your thumb and forefinger around a soda can and move the can up and down like a piston. You may notice a “stick” at the top and bottom while it’s stopped. If you add a twisting cycle with the appropriate phase, the surfaces are always moving relative to one another, and you never get the “stick”. The way to synchronize the two motions (up-down and twist) is to picture a spot on the side of the can and try to make that spot go in a circle.
Of course, this may have absolutely no bearing on car engines. Given that they’re well-lubricated, they probably don’t have to worry about that problem with the static coefficient of friction at the extremities of the piston stroke.
As to the question about why, if the pistons are stopping and starting so quickly, the engine doesn’t explode under the tremendous forces, it’s because the appropriate tremendous forces are opposing those tremendous forces, balancing everything out. If you don’t believe that, think about this: when you’re driving 60mph, a spot on the edge of your tire goes from 0 to 120mph and back to 0 again about 1500 times per minute*, and it’s just made of rubber!
(*numbers based on a 27-inch outer diameter tire at rolling at 88 feet per second, which is about 60mph if I remember correctly).
The engine’s crankshaft and camshafts always turn in the same direction. Reverse is created inside the transmission, by putting an extra gear in the power flowpath so that the output (drive) shaft turns in the opposite direction of the input (engine) shaft.
Well, the whole concept of being stopped only really has meaning for an interval (regardless how small) of time. The cylinder, however, is only “stopped” for a single point in time. Between time A and time B, there is always movement. So, in that sense, it never stops.
This subject, BTW, will also lead to many knock-down drag-out arguments between power engineers, so be careful of spreading the word. It is a “controversial” question, with a heavily debated answer among some. I have personally seen people come to blows over this!
And yes, it does stop for an infinitely small amount of time. (bobbing and weaving)
Just wanted to thank everyone for their responses to my question. I know more now than I did! Thanks.
A piston’s path exactly follows the amplitude of the sine wave defined by the vertical component of the crank’s rotation. Since the derivative (speed) of the sine wave is the value of the cosine wave, the piston stops every time the cosine is zero, which is when the sine is at max or min (piston at top dead center and bottom dead center).
So out of curiosity, what is the controversy about?
I certainly don’t wish to come to blows over this (that’s Anthracite’s specialty, apparently), but I DO have to point out that the piston’s motion is NOT exactly a sinusoid. The crankshaft journal’s vertical motion is sinusoidal (neglecting RPM fluctuations due to the piston-firing loads being discrete rather than continuous). However, the connecting rod’s angle to the cylinder centerline is constantly changing as well, making the vertical distance from the crankshaft journal to any point on the piston a function of the cosine of the connecting rod angle. The resulting path of the piston’s motion looks approximately sinusoidal, but is mathematically more complex.
To answer the OP: Yes, the piston’s velocity is zero for an infinitesimal time at both top-dead-center and bottom-dead-center. Its acceleration (the rate at which its velocity is changing) is at a MAXIMUM at these times, though. The acceleration is at a minimum when the velocity is maximum, when the piston is halfway down and the crankshaft throw is perpendicular to the cylinder centerline.
I just saw an engineering reference to what I think you’re asking. Where yes, the piston has to stop. But with the steam engine, the intake valve begins to open before reaching tdc, which as I understand it has a cushioning or buffering affect in contrast to the abrubt stop. That’s a steam engine, but I think it’d apply to the gasoline engine as well.
I only have this phone so I can’t readily pull the article up and paste it. It was an engineering book describing the effects as a steam engine operates. Which there’s valve timing for gas.
I hope this is inline to the question you were asking. It gets old when others don’t listen to the question posed, though maybe with good intent, but frustration levels vary, lol.
That question is damn near 18 yrs old.
Given how zombies lurch, I’m thinking that the pistons in question probably don’t have the smoothest motion any more.
I seem to recall reading that some very high-end engines are designed with a slight twisting motion like galt described (lo, these many years ago), precisely to avoid the static friction issue.
no. the wrist pins prevent the pistons from doing what he describes. they simply cannot “twist” in the cylinder bore.