# RE: interference fit

```glen powell wrote:

> At the instant of transition from upward deceleration to downward
acceleration
> and while the piston has no velocity at the instant of time that it is at
TDC
> how can this instant be the point of maximum acceleration......?

> zero acceleration occurs when change of velocity over time is zero.

Yes, but we're not talking about a single-cylinder engine here.  The
accelerative (RPM-increasing, crankshaft angular-velocity-increasing) forces
in an engine happen because in one cylinder or another, the fuel/air mixture
is being combusted and forcing one of the pistons down.  The decelerative
(RPM-decreasing, crankshaft angular-velocity-decreasing) forces happen due to
compression, exhaust expulsion, and frictional forces.  Since all cylinders
are connected to the crankshaft, they're _all_ going to move and be
accelerated by both sets of forces, even if it's at the other end of the
engine.  If you look at the crankshaft throws for an I-5 engine, you will
probably see that while one cylinder is closing in on TDC on the exhaust
stroke, another cylinder is firing, which will _increase_ the (RPM-increasing)
acceleration felt by that piston, even though it is just about to change from
going up to going down.  The short answer is that delta v is _never_ zero (or,
more accurately, it passes through zero instantaneously at a moment which
doesn't necessarily correspond to TDC of any given piston), each piston is
always experiencing it.  A better way to look at it would be to view the
crankshaft's rotation head on and measure _its_ angular velocity very
carefully.  Even at "idle", it's going to be constantly changing.  The more
cylinders you have, the "smoother" it will be.

I'm not a powerplant engineer, so I can't really talk about crankshaft throws
much beyond this.  But think of it this way - given your druthers, you could
arrange to have all 5 pistons at TDC simultaneously.  Why wouldn't you want to
do this?

Ground all of the spark plug wires on your engine except one.  Watch what
happens to the angular velocity of the crankshaft as a result.  Graph the
velocity of each piston with respect to time as the engine fires on one
cylinder.  You'll see what I mean.

Best Wishes,
Alex

```