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Graph of cylinder pressure vs crank angle, where???
by
greg123
on 12 Oct, 2007 15:36
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For a little project I need a graph of something very simple, cylinder pressure (inside cylinder) vs crank angle through the 4 strokes. So I can see the pressure at any given point, eg at 90 degrees in the power stroke, at the end of the exhaust stroke etc.
Does anyone have a link to such a resource? I know it will vary on intake and exhaust restriction/type of engine/diesel or petrol, however I want something that will give me a baseline.
Would appreciate any pointers!
Greg.
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#1
by
Mark(The Miser)UK
on 12 Oct, 2007 16:29
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It's impossible to determine.
Compression stroke is a bell curve because of non linear relation between piston translation and crank rotation...
Except that it isn't because of the fuel combustion....
[You'd have a slight vaccuum with no fuel injection because of the heat losses...]
Residual heat at bottom of stroke maintains 10's of psi pressure...
All vary according to individual engine; size; coolant type; thermostat; rpm;load;cam position etc.
I do have a generic picture but it can contain no numbers. Only way is to measure your particular engine at your particular required operating conditions.
I know this is not the answer that you want :cry: but unless a test has been done on your engine type and close to your setup I can't see a way of providing data with any meaning. There are formulae for compression stroke...
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#2
by
greg123
on 13 Oct, 2007 03:14
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I have seen engines at uni with variable compression and a sensor (pressure) installed in the cylinder head.
I'm happy with generic numbers because, for example, there will always be the lowest pressure part way through the induction stroke, highest pressure I'd imagine just after ignition, exhaust stroke starting off high and as the vale opens rapidly reduces and so forth.
Different engines will have variation in numbers but the pattern will be the same, eg you are not going to find one where a vacuum exists during the power stroke and the peak pressure is during induction!!! They will all follow the same sort of pattern, just as it's possible to point to an engine and guestimate it's EGT's roughly - my lawnmower won't be making 1500 but a race prepped car that has a glowing manifold might!
This rough data must exist somewhere. Or some sort of software to model it - I have demo engine analyser but I can't see an output to plot cylinder pressure.
Greg.
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#3
by
Mark(The Miser)UK
on 13 Oct, 2007 13:30
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just as it's possible to point to an engine and guestimate it's EGT's roughly - my lawnmower won't be making 1500 but a race prepped car that has a glowing manifold might!
Greg
Ha ha
You must have a faulty mower then :shock:
http://www.letsmow.com/faq.asp#q13
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#4
by
greg123
on 13 Oct, 2007 13:44
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Friggin Nora, that's a RACING mower!
I have to push mine....
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#5
by
Mark(The Miser)UK
on 13 Oct, 2007 14:47
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Did you see the bit about not removing the 'gov'ner' :shock:
Imagine tinkering with your mower and not telling your neighbour then letting him see you whizz across the lawn :wink:
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#6
by
greg123
on 13 Oct, 2007 15:20
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When I very first started with engines I bought a petrol rotovator/cultivator which wouldn't run. I had no concept of a crank but did roughly figure there was a piston and a spark plug - this is the level I was at. After playing with it for a few days and washing the spark plug in the sink... it finally fired up. With a MASSIVE plume of smoke and started to rev right up.
I had disconnected some bent bits of wire that appeared to be in the way of the throttle, which I assumed was holding it back for some reason. The noise the thing made and the smoke I ran for it, watching from a distance as about 15 seconds after it started there was a large bang and then it wound down very smoothly like an electric motor.
After stripping the engine and learning that it had snapped what I learned was called the con rod and later relating this to over-revving from the disconnected govener, I have to this day been unhappy revving engines - particularly if it's near their design limit ;-)
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#7
by
subsonic
on 13 Oct, 2007 19:36
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Mower racing is big in Maine. Friends son in law put a 1100cc ninja motor into his riding mower. Open class. Freaking nut's. What the hell would the obit say? Joe blow was killed Sunday afternoon in a high speed roll over while riding his Sears lawnmower. EST speed was 75mph.
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#8
by
bigblockchev
on 14 Oct, 2007 16:18
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Large diesel engines are sometimes tested with a cylinder "indicator" which if memory serves is exactly what you describe a graph of cylinder pressure versus crank angle. do a search for indicator diagram and you will find related stuff such as
http://www.hnsa.org/doc/fleetsub/diesel/chap9.htm maybe not exactly what you are seeking but close in any case. Cheers Dan
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#9
by
greg123
on 15 Oct, 2007 02:37
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That's the kind of thing I'm looking for! I will search more for that and hopeuflly stumble on a full graph.
Cheers!
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#10
by
xud9te
on 16 Oct, 2007 09:20
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Hows this?
Its a 4 cyl, 4 stroke, 1.4 litre petrol (k-series)
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#11
by
Mark(The Miser)UK
on 16 Oct, 2007 15:26
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That graph doesn't look real.
The start of combustion doesn't show. Is it an average of lots of cycles?
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#12
by
greg123
on 17 Oct, 2007 00:19
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xud9te, thanks for that. It exactly like what I want, however I have been head scratching to try and work it out. I don't see the compression rise and I can't see the 'vacuum' (below atmo) one would typically expect in part of the induction cycle - the graph is 4 cycles long but much of it shows an almost constant pressure, other than the spike.
Do you have any more info on this graph?
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#13
by
OM617
on 17 Oct, 2007 01:12
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#14
by
xud9te
on 17 Oct, 2007 08:35
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Mr the Miser:
It was real when I recorded it last week!
It is an average of 500 cycles, yes, so there is some smoothing. We take SOC from the spark and mass fraction burned.
Greg:
Maybe the P-V diagram will help you better?
Cheers
greg
