We all know the basics of the internal combustion engine ie the piston is attached to the con.rod which in turn is attached to the crankshaft, now the piston changes direction at TDC and BDC but does it stop?.
After getting a few views on this in your local try them with this if we are taught that a point has position but no size, does the centre of the crankshaft revolve?.
ndbw
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Have you tried Bank's?
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The piston in a engine only stops when you interupt the fuel supply i.e. switch of the engine. The piston never stops when you are driving your car about.
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I know nothing about it but surely if a piston is moving vertically up a cylinder, reaches the top of its travel before moving vertically down it must be stationary for a fraction of a millisecond at the moment it changes direction? Same at the bottom of its travel. If it moved in an arc (ie like throwing a ball in the air away from you) then it would not be stationary.
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A coconut for PhilW. It stops at the point where it changes direction when, if I recall correctly, although it has 0 velocity is undergoes the maximum acceleration. I studied all this in Mechanics of Machines classes a long time ago.
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So does the con rod, just under the gudgeon pin, stop?
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So does the con rod, just under the gudgeon pin, stop?
It stops going up and down, but it is rotating at that point.
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As someone who has been involved in engine development I can tell you that the piston does indeed stop - twice per crank revolution - once at the top and once at the bottom.
At high RPM the accelerative forces are quite tremendous - in fact a on a Ford Zetec engine you are looking at +/- 2500g at 6500rpm. This is an enormous acceleration. One of the reasons that recent engine designs tend to use 'shallow' pistons is to reduce the piston mass and therefore the forces within the engine.
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2500g at 6500rpm. I'll take your word for it, but what are the g forces in a Grand Prix engine. You know, the ones that get to 18,000rpm...
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On a different but similar theme , do you think that in the top 180 degrees of crankshaft rotation, the piston's vertical travel is the same as in the bottom 180 degrees?
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Yes............I think so!
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On a different but similar theme , do you think that in the top 180 degrees of crankshaft rotation, the piston's vertical travel is the same as in the bottom 180 degrees?
It depends on whether the compressive elasticity of the conrod assembly and crankshaft, etc is the same as the stretching elasticity. For an extreme example, imagine the conrod is replaced with a 'slinky'. Extreme stretch at the top of the stroke, almost no compression at the bottom.
V
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And as to the original question, yes it does stop. If you graph the speed of the piston, there is a point where the velocity must pass zero to go from +ve to -ve, or vice-versa (quantum effects notwithstanding)
And the "point" question, no, the centre of the crankshaft doesn't rotate. As you so elegantly describe, it's a point, not an object, so there is no contradiction. Once you start talking about objects (such as a particular molecule at the centre of the crankshaft) then it does rotate.
V
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Actually, its not as subtle as that, but you are correct in that it does indeed travel further in the top half of the stroke.
The action of the big end coming from the 90 degree point up to fall in line with the axis between the main bearing and the small end, has the effect of giving the piston a further push up the bore. And not just a few microns either, more like a few millimeters IIRC.
For a simple test, place a pen at a roughly 60 degree angle from the edge of the computer desk, with a finger tip resting alongside the other end. Now push the pen end along the desk edge to a right-angle position and see 'vertical' travel relative to the resting finger tip.
Of course, it has the reverse effect on the bottom half of the stroke as the action is reversed.
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It depends on whether the compressive elasticity of the conrod assembly and crankshaft, etc is the same as the stretching elasticity. For an extreme example, imagine the conrod is replaced with a 'slinky'. Extreme stretch at the top of the stroke, almost no compression at the bottom.
Will that vary between compression stroke and exhaust? In one the piston is compressing the air/petrol mixture, in the other just pushing the burnt gas out through the exhaust valve. Or does the inertia stretching outweigh that?
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On a different but similar theme , do you think that in the top 180 degrees of crankshaft rotation, the piston's vertical travel is the same as in the bottom 180 degrees?
Only if the axis of motion of the piston passes through the axis of rotation of the crank & gudgeon pin and the rods are straight and 'non-handed'. Neither is a prerequisite of engine design, though most common engines will meet these criteria.
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yes
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I assume you wrote this thread ndbw after you had clearly spent far too much time in your local.
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Hi Peter D
I must confess to all who replied on this thread that originally this question was posed in the magazine Motorcycling in around 1945,the replies then as now were many and varied from the highly technical to who's taking the mickey,in the end the editor said enough,apologies to all.
ndbw
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Yep according to the law of physics and engineering mechanics the piston does indeed stop, nomatter however momentary the pause in travel is.
The other reason is that crankshafts have something called an inffective crank angle that basically means that with a fraction of a degree of rotation (Seconds and minutes of a degree) the piston no longer moves and therefore can be said to be without velocity...hope it helps...
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On a similar theme...
Does the bottom of a road wheel/tyre travel backwards on a forward going vehicle...?
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Spanner,
No. Unless grip has been lost and the wheel is spinning.
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But isnt there in any movement of a treaded tyre some form of slip in relation to the surface it is rotating over??
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Ben, Does that mean that the driven wheels will be going backwards and the non-driven will be going forwards (drag)?
My brain hurts now - I'm off to bed
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Even if all grip is lost the motion of all wheels is the same.while in contact with the road.on a non limited slip differential if you raise one road wheel from the ground it will travel in the opposite direction while the wheel on the ground won`t move
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