Re: If you want to be invisible on a car lot...


Bill Baka wrote:
frkrygow@xxxxxxxxx wrote:

It's SIMPLE, all right, but it's wrong. At least, if you imagine it's
just piston ring friction at work.

Not friction, inertia. Lower RPM and more torque make better use of all
those cubic inches.

Inertia of the piston does not decrease engine efficiency.

Efficiency is decreased when energy is _lost_. That is, when it leaves
the system in an unproductive way. But the kinetic energy of a moving
piston doesn't leave the system. It just transfers to the next moving
piston, perhaps through the flywheel. When an engine runs at constant
speed, the total kinetic energy of its moving parts is almost exactly
constant.


The great bulk of the coast-down difference you described comes from
fighting the compression and the pumping losses of the engine, not just
the friction of the piston rings and other moving components.

No pumping losses when the throttle is closed, just piston losses.

Sorry, you're wrong. The throttle is never perfectly closed. There's
always some flow past it. Likewise, there's always some flow past the
valves. Anyone who's noticed exhaust gases escaping a decelerating car
should understand this.

Sure, it takes energy to accelerate a piston. That energy comes almost
entirely from the nearby piston which is decelerating. They are simply
trading kinetic energy back and forth.

Not so.

Well, depending on the engine design, the energy may pass through the
flywheel, which is just a temporary energy storage device. The
important point is, the kinetic energy of the piston is _not_ lost. It
does not leave the system.

Have you taken a thermo course? If so, imagine a control volume around
the engine. If your losses were just energy accelerating and
decelerating the piston, how would the energy leave the system?

Let me ask again: How would that energy leave the system?

You claim to be an electrical engineer. The electrical analog for
inertia (or mass) is inductance. In an oscillating LC circuit, does
the inductance lose energy? No, of course not. It merely stores
energy, transferring it back to the capacitance and vice versa. Damping
the oscillation to zero requires insertion of a resistance BECAUSE the
resistor is a device which converts energy to heat. This allows the
energy to leave the system.

If the energy doesn't leave the system, the system has the same amount
of energy.

Damn, It is NOT rocket science.

Right. You should be able to understand this. First law of
thermodynamics. Easy stuff.

- Frank Krygowski

.

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