From: viking@iastate.edu (Dan Sorenson)
Subject: Re: Engine Theory 101
Organization: Iowa State University, Ames IA
Date: Thu, 7 Jan 1993 06:46:32 GMT

jfriedl@omron.co.jp (Jeffrey Friedl) writes:

>Being rather ignorant of the way of the engine ("enjin-dou" :-),

>About the clutch, I assume one has a the power of a rotating shaft from
>the engine transmitted (via the transimission, of all things) to the gear
>which drives the chain, and how this power is applied is related to
>the "gear". The idea of smaller gears providing more torque is easy enough
>to understand... what I'd like to know is the mechanics of what the
>clutch does in all this.

	A clutch is a brake, for all intents.  OK, think of it this way.
Let's say you have something spinning, and something next to it that is
not spinning.  Say, your front wheel is spinning and you want the rest
of the bike to loop about too.  Simply squeeze on that brake handle.
The friction of the pad against the disk will impart some force to the
pad, and whatever it in turn is connected to.  Eventually, the force of
friction will be great enough that the pad will not move on the disk,
and since the wheel is still turning so is the bike.  Just what you
wanted!  In a clutch, this same friction is used to cause one shaft to
drive another shaft, or to not drive it.  As for "burning out" a clutch,
have you ever felt your brakes fade on you?  It was due to heat from
over-use.  The same will happen to a clutch.  Eventually, you run out
of clutch plate (ie: brake pad) and no more clutch.


>Secondly, when I start in the morning I put the choke to about half and
>hit the switch.  The engine fires up, but sounds very rough. After a few
>seconds it settles into a less-rough but certainly not smooth grumble.
>After a minute or two, it suddenly reves up to a smooth (but fast,
>because of the choke) idle. I kill the choke and I'm on my way.

	The combustion chamber is, initially, at outside temperature.
The burning is ragged as the cylinder is very cool.  In only a few
firings the cylinder heats up enough to aid in vaporizing the fuel,
though the thing is by no means hot.  At this time, things smooth
out a bit.  As things warm up, the fuel is burning better, and the
motor runs faster (a choke limits air, hence increasing fuel in the
air/fuel mix; it won't burn as clean, but it will burn).  When it's
fully warmed up, things are at optimum and you no longer need the
choke.  That knocking sound you hear initially is not only motor
parts without enough lubrication, but may also be the result of an
uneven burn rate in the cylinder.  This is the same thing as the
knocking you might get if you lug that motor around on cheap gas,
but not as harsh.  Same principle, different degree of seriousness.

	As for running a motor up and killing the ignition, it's a
useless gesture these days.  At one time it served to get unburnt fuel
in the cylinder, which was seen as an aid to starting.  There is no
need for this on a virtually any motor still in use today.

< Dan Sorenson, DoD #1066 z1dan@exnet.iastate.edu viking@iastate.edu >
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