02GF74
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| posted on 11/6/08 at 03:43 PM |
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stupid quetion - cam totation
can't get my head round this without drawing and thinking it but but does the camshaft(s) need to rotate in same direction as crank?
I reckon not although not come across an engine that does not have them like that.
saw this: and those circles are presumably for gears to take the drive to the cams?
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bilbo
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| posted on 11/6/08 at 03:47 PM |
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Quite a few engines (such as the earlier ford cologne V6s) drove the cam through gears instead of belt or chain, so they would rotate opposite to the
crank.
---------------------------------------
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BenB
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| posted on 11/6/08 at 03:49 PM |
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They usually rotate in the same direction but there's no reason why they couldn't be "cut" in reverse.
It's also normal for them to rotate at half the speed of the crank.
You could have them going the opposite direction if you cut them reverse and you could have them rotating at crank speed but the profile would be
harsh and there would be large empty spaces on the cam (not ideal)...
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02GF74
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| posted on 11/6/08 at 03:57 PM |
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quote:
Originally posted by BenB
You could have them going the opposite direction if you cut them reverse and you could have them rotating at crank speed but the profile would be
harsh and there would be large empty spaces on the cam (not ideal)...
are you sure? I thought the 1/2 speed was due to 4 strokes or 2 revoultion per cycle, now this is really hurting my brain .....
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MikeRJ
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| posted on 11/6/08 at 04:04 PM |
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quote:
Originally posted by BenB
you could have them rotating at crank speed but the profile would be harsh and there would be large empty spaces on the cam (not ideal)...
Nope, this wouldn't work. If the cam was running at the same speed as the crank, it would have to open and close both valves in one crank
revolution.
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clairetoo
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| posted on 11/6/08 at 04:23 PM |
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The cams in my engine are - one on each head is belt driven from the crank as normal , then the other on each head is geared from the belt driven one
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The strange thing's are , the cambelt drives the inlet cam on one head and the exhaust cam on the other , and the gears mean the inlet and
exhaust cams rotate in opposite directions...
All of which made it a bit of a challenge to work out the fireing order for the EDIS and coilpack
Its cuz I is blond , innit
Claire xx
Will weld for food......
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02GF74
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| posted on 11/6/08 at 04:36 PM |
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quote:
Originally posted by MikeRJ
quote:
Originally posted by BenB
you could have them rotating at crank speed but the profile would be harsh and there would be large empty spaces on the cam (not ideal)...
Nope, this wouldn't work. If the cam was running at the same speed as the crank, it would have to open and close both valves in one crank
revolution.
yep I agree. for 4 stroke, in one revoution the exhaust is moved without moving the inlet then somehow cam on the next crank revolution opens the
exahaust without touching the inlet. I cannot see how it can be done with a solid cam.
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Mark G
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| posted on 11/6/08 at 06:43 PM |
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quote:
Originally posted by 02GF74
yep I agree. for 4 stroke, in one revoution the exhaust is moved without moving the inlet then somehow cam on the next crank revolution opens the
exahaust without touching the inlet. I cannot see how it can be done with a solid cam.
Because it uses a different cam lobe for the inlet to the exhuast.
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DIY Si
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| posted on 11/6/08 at 07:00 PM |
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But both would move in that one revolution, so it wouldn't work. They could move at different times, but they would still have to move. They
need to open the valve once for every two rotations of the crank, so it's not possible with 1:1 speed ratios.
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Liam
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| posted on 11/6/08 at 07:55 PM |
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As above - cant have a cam spinning at crank speed on a 4 stroke! Valve cycle has to last 2 revolutions of the crank so to be implemented with a
camshaft it must spin at half crank speed. If a camshaft span at engine speed, whatever it does during one crank revolution would just be repeated in
the next.
Liam
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martyn_16v
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| posted on 11/6/08 at 08:45 PM |
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The crank can run either way round, it really doesn't make any difference, the pistons still just go up and down. The cam(s) has to run in the
direction it's been designed for though, the open and close ramps are profiled differently. That and on a single cam the lobes will be in wrong
place.
EDIT: that's assuming it's a 4 cyl with 1342 firing order. Odd fire v6 won't work backwards, the firing order will go all wrong.
Need to have a think about v8's, I reckon some will be alright...
[Edited on 11/6/08 by martyn_16v]
[Edited on 11/6/08 by martyn_16v]
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BenB
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| posted on 12/6/08 at 09:58 AM |
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Actually, it's true- I'm talking ballcocks.
If you had the cam spinning at crankshaft speed you'ld need half a cycle per rotation which clearly isn't possible....
It's a good thing I'm not an engineer
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