2nd class motor questions

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Ratherbeonvacation
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2nd class motor questions

Postby Ratherbeonvacation » Sat Oct 19, 2013 11:12 am

Here trying to study and came across this question

How is the angular spacing of a crank shaft determined? Also looking for a sketch of a crank shaft showing the angles. Anyone know where I can find this? books, manuals, internet? etc

Thanks

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JollyJack
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Re: 2nd class motor questions

Postby JollyJack » Mon Oct 21, 2013 6:53 am

That's an interesting question, I wonder where it came from? If my memory serves, I covered this in the second year of my apprenticeship as an Engine Fitter in UK. The purpose of angular spacing, indeed, of crankshaft construction, is to distribute the shocks of power strokes from each cylinder in such a way that the torsional oscilation induced by them is minimized. It follows, therefore, that impulsed transmitted to the crankshaft are evenly spaced throughout it's rotation and along it's length.

Consider a 4 stroke, single acting 4 cylinder engine, firing order 1342. Crank pins are 180 degrees apart. The four power strokes are 360 degrees, or one rotation, apart, and spread as much as possible. A 6 cylinder engine, firing order 153624, has crankpins spaced 120 degrees apart. Impulses are more evenly spaced along the length.

There's quite a good explanation in Sothern's Marine Diesel Oil Engine, page E 42 et seq. (I have the 10th edition)
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Ratherbeonvacation
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Re: 2nd class motor questions

Postby Ratherbeonvacation » Mon Oct 21, 2013 10:00 am

Thanks

So pretty much as simple as it seems. Back to the basics.

4 stroke 720 degs/# cylinders

2 Stroke 360 degs/# cylinders

And then of course the firing order is considered to determine which crank is angled when.

thought there might have been some top secret underground concept that TC would be looking for.

Am I correct in saying this Jack? Or has all this looking at books have me totally in space.

Thanks again.

jimmys
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Re: 2nd class motor questions

Postby jimmys » Tue Oct 22, 2013 4:43 am

I would be very careful generalising into formulae, a V6 engine can have the cranks at 90 degrees, sometimes, and a flat crankshaft straight 6 can have the cranks at 180 degrees, 3 out to the left and three out to the right.
There is a large number of configurations now, be aware of these configurations, not in the books. Better timing helps and a flat crankshaft is cheap to manufacture. Always pick a medium speed and a large bore, one of each to study.

Ratherbeonvacation
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Re: 2nd class motor questions

Postby Ratherbeonvacation » Tue Oct 22, 2013 8:41 am

Thanks for the reply

Thats why the question threw me off. Is there a General rule when determining this. The question says "how are they calculated" not how are they calculated for "____ type of engine"

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JollyJack
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Re: 2nd class motor questions

Postby JollyJack » Tue Oct 22, 2013 7:27 pm

There are two basic principles when approaching Examinations and can be summed up by two simple acronyms.

RTFQ (Read The F***#@g Question)

KISS (Keep It Simple, Stupid)

Understand what the question asks, it's usually how well you can interpret English, and answer what is asked for. If the question is about pistons, don't talk about crankshafts.

Regard the Examiner as a first trip cadet or oiler. Do NOT assume he or she "knows what you mean". If you don't say it, it will be assumed that you don't know it and the question will be marked accordingly.

To illustrate, I was asked a question, 2EKG, to sketch and describe a coil ignition circuit for a gasoline motor. Well, I had a motorbike with a battery, primary coil, points, condenser, cam, secondary coils and spark plug. QED, a gift, full points for that question and it wasn't even mentioned in Orals.

RTFQ, KISS

Incidentally, I couldn't find that question about calculating crankshaft angles in the question bank. The questions are usually about operation, maintenance and repair, not design.
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jimmys
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Re: 2nd class motor questions

Postby jimmys » Wed Oct 23, 2013 8:40 am

There is a general rule and it concerns static balance. To check this we set the crankshaft up on v blocks on the surface table, oil the v blocks and spin the crankshaft. The angles selected must give static balance so the crankshaft must not stop in the same position all the time. So for a six cylinder angle will be 360/6 = 60 degrees, we can balance in groups of 2 at 120 degrees or flat crankshaft, 3 out left and three out right.
You need to remember it is very difficult to dynamically balance a machine that is not in static balance. I think this may be the rule your friendly TC examiner is looking for.

regards

Ratherbeonvacation
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Re: 2nd class motor questions

Postby Ratherbeonvacation » Wed Oct 23, 2013 10:50 am

Thanks for all the feedback. If you know of something else dont be afraid to share. I am sure I will have a few more questions over the upcoming weeks. Thanks again


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