On Sat, 29 Oct 2011 18:20:28 -0700, engineman wrote:

John,

Here is a question for you and your metalhead group.

Q: Say you had a 2 CYCLE motorcycle and you wanted to push start
it. Suppose that you got on it and turned on the ignition but didn't
hit the start button. Instead you depressed the clutch lever and put it
in 1st gear. Then four of your not so bright friends, two on each side,
pushed you and the bike BACKWARDS!! Remember there are two each side so
they should be able to keep you from falling over. After the bike gains
some momentum you pop the clutch. Now, the question is: WILL THE BIKE
START WITH THE ENGINE RUNNINGBACKWARDS??!? If not, why not? The reason
that it comes up is that I read about a new class of ULCCs
(UltraLargeCrudeCarriers -- Way big mutha oil tankers) that have 2
cycle diesel engines with NO transmission, clutch or reduction gears of
any sort. The drive shaft comes out the back of the engine and goes out
the back of the hull and the propeller is bolted directly to the end of
it No gears, no clutch, no nothing. The way that they reverse it is by
shutting of the engine and then restarting it BACKWARDS!!?! It is
designed to do this, I'm wondering if all 2 cycle engines will run
backwards or not? Inquiring minds want to know.

Bill

From Engineman

It depends on the engine. Most model airplane two strokes (which is what
I'm mostly familiar with) have crank-timed crankcase induction. So while
the exhaust and transfer (from crankcase to cylinder) work the same
forward and back, the intake into the crankcase is "one way" (some hot
motors will run -- poorly -- backwards).

There are some itty-bitty reed valve engines that run quite happily in
either direction -- in fact, a Cox 049 reedie will generally start the
opposite of the direction you flip it, when you're trying to flip it
forward. Flipping it backward, it generally starts in the same direction
as the flip. Go figure.

Note that model airplane engines are mostly glow ignition -- that's a
"sorta diesel" cycle where the ignition timing doesn't care about crank
rotation.

Most chainsaw and weed-whacker engines, on the other hand, have a piston-
timed crankcase induction (the piston uncovers a port between carburetor
and case). These have less ultimate power potential than a crank-timed
engine, but they're simpler to make. As mentioned elsewhere, even though
the induction, transfer, and exhaust timing is the same forward and
reverse, the spark will be off.

Scooter engines are probably mostly like chainsaw engines (if not
entirely so). Large motorcycle two strokes often use reed valve
crankcase induction, on the same principal as the little reedies for
model airplanes, but considerably different mechanics. They would still
run crappily, if at all, in reverse, because of spark timing.

Bigger engines start doing wacky (to me) things like running two pistons
in one cylinder with offset cranks (Junkers Jumo), or running ports in
the cylinder and valves in the head (I think it's usually transfer ports
and exhaust valves), or other creative means of improving gas flow. Most
of these are also going to run considerably less well in the reverse
direction.

But my entire experience with bigger engines than weed whacker engines
runs to reading Wikipedia pages.

So, the bottom line is: maybe it will, maybe it won't, and either way a
motorcycle is a stable system moving forward but not back -- so you'd
just fall off anyway.

--
www.wescottdesign.com