My friend asks a question

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
Reply to
engineman
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Most can. The ignition timing will be off -- the spark will be late -- and the engine will not develop much power.
____________________________________________________________________ Gardner Buchanan gbuchana(a)teksavvy(dot)com FreeBSD: Where you want to go. Today.
Reply to
Gardner
Ship board diesels are commression ignition (just like other diesels) and hence do not rely on ignition timing. (I believe)
Reply to
John G
John G brought next idea :
I will have to get spellchecker working??
Reply to
John G
The motorcycle hypothetical caused me not to reply initially, because there are two issues there: the ignition, as previously mentioned, and the type of intake valve used.
Battery and spark-coil ignition, as the previous poster said, will be off on timing but may allow it to run. Electronic ignition may be closer. Magneto ignition probably won't allow it to run backwards at all because the pulse to create the spark won't occur until well after TDC.
On the valves, reed-type or piston-port, yes. Rotary-valve, probably not.
I started a lot of model airplane engines backwards when I was a kid, unintentionally. They had reed valves (my little Cox engines) or rotary type (McCoy Red Head). My recollection of the McCoy, though, is that it would start backwards, so it may have been fairly symmetrical. The Coxes did it all the time, if you weren't careful.
Those engines were glow-plug ignition and, like compression ignition engines, didn't care which way they were running from an ignition standpoint.
Reply to
Ed Huntress
The reason that it comes up is that I read about a new class of
Actually, they probably need to do some stuff to make it run backwards properly, but it might not be a huge deal on a large engine like that. It likely has a blower of some sort that needs to run the right direction all the time. If it uses valves, then the cam timing needs to be switched. And, likely at least part of the injection pump needs to run the right direction. Some of that stuff may be purely electrical, so no need for reversing gears on the pumps.
Simple 2-strokes of the motorcycle, chain saw, model aircraft sort will run quite well backwards. It used to be great fun to run the early Saabs backwards and then switch off to an unsuspecting driver. Great hilarity!
My old Chevy Vega would Diesel backwards and blow all the hoses off the intake manifold after shutting it off hot. So, I learned to shut it off, wait for it to hit zero speed and then pop the clutch to prevent it from starting up backwards. It would run for an amazing time on fumes in the exhaust.
Jon
Reply to
Jon Elson
I imagine it will depend on the specific engine, but I seem to recall an old outboard that had no reverse. You just started it in reverse or something like that.
Reply to
Bob La Londe
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Push it backwards!
Reply to
Tom Gardner
In my limited knowledge the timing is "done" by when the fuel is injected by the fuel pump. I dont know if ship sized engines are the same as truck / car diesel engines.
Reply to
Dennis
I'm not familiar with the more modern large diesel, but older ships engines had individual injection pumps which ran off a cam shaft seperate from valve cam shaft. They were started by using compressed air on an additional valve on one of the cylinders. The injection cam shaft would be in either the forward or reverse position. Incidently,each injector pump drive could be lifted off the cam by using a special spanner. If you weren't getting full power out of it, you could isolate which cylinder was the culprit by lifting each injector individually. When you lifted one and found that it didn't cause the injector racks to open up more, you had found the faulty cylinder. Oil drilling rig diesels had butterfly valves on the air intakes as natual gas getting into the deisels could cause them to " run away", and they couldn't be stopped by just shutting down the throttle. Closing the valves on the air intakes stopped them dead
Reply to
Grumpy
Backwards starting is even more likely on 4-stroke model glow engines. Amazingly, even the throttle works. I asked my fellow modelers how that is even possible, but nobody knew and nobody cared. The explanation is quite simple: Since there is no fuel pump, model engines feed pressure from the exhaust into the tank to help fuel feed. When the engine starts backwards, fuel is slurped from the tank through the pressure hose into the exhaust. The carburetor valve now acts as an exhaust restriction valve to control engine power.
Some rotary valve engines had the option to rotate the front part of the crank case 90 degrees to enable backwards running. I think some Enya engines have this feature. Here you can see how the crank case is made from two parts screwed together:
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I seem to remember the Coxes had a spring system on the prop shaft that would automatically stop and restart the engine the right way. Here's a picture where the system can be seen:
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The primary use of this spring is to start the engine. The reverse protection is a nice bonus.
I am not very familiar with diesels, but on normal car engines, I am quite sure the ignition timing is controlled by the fuel injectors. The engine compresses only air, and the fuel is injected into the hot air at just the right time. Since there is no risk if early ignition or detonation (only air is compressed), a turbo is an attractive option for a diesel.
Reply to
Robert Roland
Common glow plug aircraft engines will. Easily because their intake, exhaust, and ignition timing are symetrical about TDC.
Two-stroke spark ignition engines will probably not, the spark is advanced; extremely unlikely if they are also assymetric breathers.
Larger diesel engines can/will have assymetric intake, exhaust, and injection timing. To run backwards you just change that timing so it works the other way.
Reply to
xpzzzz
The large marine engines is usually reversed by shutting down and turning the engine in reverse accompanied with changing the injection pump(s) timing. Although the engine might well run in reverse without changing the injection timing it won't produce rated power.
-- John B.
Reply to
John B.
On Sat, 29 Oct 2011 23:54:47 -0400, Gardner
wrote:
The timing is one of those things they thought of when making those huge marine engines - the stroke is symmetrical so it's timed right (a camshaft to trigger the fuel shot) running forward or backward.
And the marine engines use a sliding crosshead between the piston connecting rod and the crosshead connecting rod to the crankshaft, to keep the offset thrust loads of the crankshaft away from the pistons.
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Reply to
Bruce L. Bergman (munged human
Some old *inboards*, too, like the Acadia make-and-break engines used on fishing boats in the Canadian Maritimes. You reversed them by waiting for the open flywheel to stop and then kicking it in reverse with your foot.
"Simplify, simplify. We fritter our minds away with detail."
Reply to
Ed Huntress
Jeez. And I'm listening to this, thinking about all of the engines that were SUPPOSED to start during my lifetime, but wouldn't, and here's one that shouldn't, but does. d8-)
Interesting.
I had one or two of those, but mostly I just had the plain (.049) engines that you propped with your finger. My finger was bloody or sore half the time when I was around 10 - 12 years old.
The spring start was Ok except that the engine would try to fire when you were winding the spring back. It was rough on the engine but those little Coxes were hard to destroy. They were a very clever piece of design, and we cited them as an example of "design for manufacture" back in the '70s at _American Machinist_. The crankcase was an extrusion.
Right. And if you really want to confuse yourself, look into the model airplane "diesels," which aren't really diesels at all, since they don't have injection. They're Homogeneous-Charge, Compression-Ignition engines -- paradoxically, one of the types being heavily researched right now for automotive applications.
Reply to
Ed Huntress
Polaris & Ski-Doo (maybe others now) both have "electronic" reverse on their snowmobiles and possibly ATV's, too. You can see here that it stops the engine briefly and instantly re-fires it in reverse:
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Reply to
David Courtney
Wrong. They still have timing, just no ignition.
They have to time the squirt of fuel into the combustion chamber at the right moment (5 or 10 degrees before TDC) so the main flame front goes off right after TDC. If you squirt the fuel in way early and it's hot enough to light off, the engine tries to run backwards.
Which isn't going to be pretty if it was full speed ahead (lots of inertia built up) and one cylinder goes way early.
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Reply to
Bruce L. Bergman (munged human
I first saw this on an old Mercury snowmobile, around the early 70's:
"In 1969 the Model 220 was unveiled, with the 250 debuting shortly thereafter. Both were based on the 150 cowl style, with the belly pan thankfully being a separate unit, allowing easier hood removal. Comparing these to the 150, they look a lot like the 150 on steroids. Both models were available with electric start (220E, 250E) and reverse (220ER, 250ER). The reversing mechanism was simple, the driven clutch was designed to operate in both directions, and the engines had two separate starters mounted over the toothed flywheel. To put the units in reverse, one would shut the engine off, flip a toggle switch and start the motor in reverse! This design was borrowed from their early "dock banger" outboards, which had no neutral gear but were started in forward or reverse from the control console."
=46rom not quite halfway down this page:
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I was pretty impressed at the time. My buddy didn't have a clue how it worked but I pretty much surmised how while watching him shut it off, flip a switch and start it up again. I recall that it ran "rough" but it did indeed go backwards...
--=20 Leon Fisk Grand Rapids MI/Zone 5b Remove no.spam for email
Reply to
Leon Fisk
No, the depend on INJECTION timing, which, from what I remember, is even MORE critical
Reply to
clare

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