# Sine wave

Discussion in 'The Projects Forum' started by Teena123, Apr 20, 2013.

1. ### Teena123 Thread Starter Member

Nov 22, 2012
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This is something i just thought of, how is it that an internal combustion engine produces a sine wave instead of a square wave??

2. ### atferrari AAC Fanatic!

Jan 6, 2004
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Measured where?

3. ### blah2222 Distinguished Member

May 3, 2010
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Though I do not much about engines, I believe that it works under the same principal as generators and pretty much anything used to create electricity from a kinetic energy.

If you look at how the generator parts in the engine are moving, you notice that they are rotating in a circular path. The movement is not an abrupt back and forth jerk where there is little time spent in transition between the two points.

If the electricity generated was in the form of a square wave, the latter jerky motion mentioned would need to be performed, which is not the case in these engines. They have a nice smooth rotation.

4. ### aws505 Member

Mar 11, 2013
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If you're using a motor to generate AC, you're usually circulating a conducting loop in a field generated from a permanent magnet. blah2222 is correct, here. For more details check out the following links:

Hyperphysics Explains how motors generate AC. To understand the previous link, you'll need to first have a healthy understanding of Lenz's Law and Faraday's Law.

5. ### mlog Member

Feb 11, 2012
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You need to explain yourself. I haven't a clue what you mean.

6. ### crutschow Expert

Mar 14, 2008
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A generator generates the desired sine-wave voltage because the windings are arranged to do that. It's not intrinsic to rotary motion. Some Stirling engine generators, designed to operate from a solar heat source, have a reciprocating AC sine-wave generator driven directly from the piston with no rotary generator motion at all.

An engine can have rotary motion but that has nothing particular to do with sine-waves.

7. ### vk6zgo Active Member

Jul 21, 2012
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But,in normal practice,a conventional generator (actually alternator) is designed to be rotated.

Luckily,this rotation is a byproduct of the engine's primary purpose which is to produce smooth rotary motion of its drive shaft.
The engine's flywheel smooths out the reciprocating mechanical energy supplied by the pistons.

OP,it may be good idea to Google for "Internal Combustion Engine",
"Otto Cycle",or similar subjects.

8. ### WBahn Moderator

Mar 31, 2012
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It is a natural consequence of harmonic motion, be it rotational or linear. To get anything other than a sine wave (intentionally) is more difficult.

9. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I assumed the OP was referring to the piston displacement [w.r.t time] in the standard IC engine as being sinusoidal in nature.

10. ### WBahn Moderator

Mar 31, 2012
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In that case, it's not sinusoidal. Even if we assume a uniformly rotating crankshaft. As the stroke increases the approximation to sinusoidal motion improves.

11. ### wayneh Expert

Sep 9, 2010
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You sure about that? How can it not be?

12. ### blah2222 Distinguished Member

May 3, 2010
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Referring back to my post, I mentioned rotational motion of a generator because it was a quick example to get the point across to the OP. Of course, a piston moving back and forth linearly makes the case as well.

The initial question was to distinguish between a sine and square wave. I think you guys are taking this a bit off topic.

13. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Not the stroke increasing, but the length of the connecting rod. As the connecting rod approaches infinity length, the piston motion approaches a pure sine.

14. ### wayneh Expert

Sep 9, 2010
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Oh, I think I see the point that WBahn was making. And yes, we've hijacked the OP's thread off course.

#12 likes this.
15. ### WBahn Moderator

Mar 31, 2012
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You are correct. I mispoke and meant the length of the connecting rod, not the stroke.

16. ### WBahn Moderator

Mar 31, 2012
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I don't know if we have or not because I still can't tell what the OP's actual question was about. Is WHAT as sine wave instead of a square wave?

17. ### BillB3857 AAC Fanatic!

Feb 28, 2009
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Wouldn't the pistons exhibit a linear, reciprocating motion with a velocity profile exhibiting a sinusoidal nature.

18. ### WBahn Moderator

Mar 31, 2012
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An approximation to a sinusoidal profile with the approximation imrpoving as the ratio of the connecting rod length to the stroke increases.

19. ### wayneh Expert

Sep 9, 2010
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The x-axis position of the crankshaft rod bearing is a sinusoid, since it's rotating evenly through a circle, but the rod is not horizontal unless it's infinitely long, so there is a sight deviation from sinusoid for the piston end of the rod.