ProZac Posted June 14, 2012 Share Posted June 14, 2012 I'd like to know what the pressure inside my combustion chamber is, at the time of ignition. The reason is so I can load up some sparkplugs in a pressure vessel for testing, but I'd like the pressures I'm testing at to be vaguely accurate, or on the high side if anything. My current thoughts are this: Assume volumetric efficiency is 100%, and the cylinder has a compression ratio of 9.0:1, then if we talk in absolute pressure scales, at the bottom of the piston's stroke, the cylinder is completely full or air, thus at ~14.7 psi. Assuming the spark happens at exactly TDC (also an assumption for ease or use, the real angle at which this happens is pretty engine specific, and can be worked back later if the geometry of the crank and rods are known) then the pressure will be 9*14.7psi = 132.3psi? This pressure is pretty easily attainable for testing purposes, which is nice. If I was now thinking of a turbo engine, and we make the same assumptions of 100% VE, 9.0:1 CR, and 14.7psi of 'boost' then at BTD the cylinder will have 29.4psi of pressure inside (absolute scale), and at TDC we're now talking 264.6psi? This pressure (or anywhere near it) is a little more difficult to attain with the equipment I've got knocking around at home... But I might be able to rig something up. Does this sound about right to all you peeps out there in internets land? I'm writing/demonstrating a proof on concept thing (thus the gross assumtpions, real data can be gathered later) for one of the professors at uni so I can get a sweet sweet honours project for next year... Would like to have my i's dotted, etc... Just seems like my figuring is too simple, and I must be missing something. Currently I'm ignoring temperature effects, but I'm aware they have a role to play. Does anyone have any real world measurements / numbers / experience with this? Also, any thoughts as to how the introduction of fuel vapour into the mix would affect the load on the sparkplug? Chur. Quote Link to comment Share on other sites More sharing options...
Testament Posted June 14, 2012 Share Posted June 14, 2012 264psi or so is about right, althought if this is for uni shouldn't you be working in KPa or Bar really? regardless, 1 bar boost, 9:1 compression = 2 bar absolute x 9 = 18bar absolute/17 bar gauge this would be close to the cylinder pressure just before ignition occurs (ignoring differences due to ignition timing), but of course not the max pressure that occurs after ignition. do you have access to a lathe? make some kind of cylinder piston arrangement and use a porta power/ screw arrangement or something to exert pressure on it? not sure what you mean by "load" on the spark plug though? air only might presesnt different conductivity to air fuel mix - if its by a significant or negligible amount I'm not sure. Quote Link to comment Share on other sites More sharing options...
Unclejake Posted June 14, 2012 Share Posted June 14, 2012 N/A cylinder pressure is easily measured with a compression gauge, and for a petrol motor can be anywhere from 140-225 psi in my limited experience. I don't know about turbos (or turbo diesels which might be mega psi?........ but then again they don't have sparkplugs) Quote Link to comment Share on other sites More sharing options...
ProZac Posted June 14, 2012 Author Share Posted June 14, 2012 264psi or so is about right, althought if this is for uni shouldn't you be working in KPa or Bar really?regardless, 1 bar boost, 9:1 compression = 2 bar absolute x 9 = 18bar absolute/17 bar gauge this would be close to the cylinder pressure just before ignition occurs (ignoring differences due to ignition timing), but of course not the max pressure that occurs after ignition. do you have access to a lathe? make some kind of cylinder piston arrangement and use a porta power/ screw arrangement or something to exert pressure on it? not sure what you mean by "load" on the spark plug though? air only might presesnt different conductivity to air fuel mix - if its by a significant or negligible amount I'm not sure. Eh, PSI was what my brain was set to at the time of writing . Good to hear I'm in the ball-park and making some sort of sense. By load I mean it takes a substantial amount more ignition energy to get an acceptable spark when the cylinder is under pressure, than it does at just atmospheric pressure. Thus the reason for needing higher and higher performance coil / cdi / more dwell / destroying your nissan COP units these days . Quote Link to comment Share on other sites More sharing options...
Beaver Posted June 14, 2012 Share Posted June 14, 2012 N/A cylinder pressure is easily measured with a compression gauge, and for a petrol motor can be anywhere from 140-225 psi in my limited experience. this, and then put in all you other knowns and then you can probably solve any unkowns, allowing you to find the relationship between theoretical pressure (along the lines of what you have done already) and actual pressure. could then probably add boost and calculate actual pressure under boost. makes sense in my head/its likely way wrong Quote Link to comment Share on other sites More sharing options...
Roman Posted June 14, 2012 Share Posted June 14, 2012 Hey there's a late model 'lean burn' 4AFE (or 7AFE?) engine which has a sensor in the combustion chamber that directly measures combustion pressure. Used it to safely run the engine really lean under some conditions, as the title would suggest Quote Link to comment Share on other sites More sharing options...
ProZac Posted June 14, 2012 Author Share Posted June 14, 2012 Oooooooh, im going to go googling for that, as that shit is FASCINATING. Quote Link to comment Share on other sites More sharing options...
Llama Posted June 14, 2012 Share Posted June 14, 2012 Your initial maths is pretty much right. But yeah if you include temperature it gets rather complicated. It's pretty easy to work out if you idealise the cycle though, just assume once you hit max compression that the gas is ignited and all of it gets turned into heat energy instantaneously. Quote Link to comment Share on other sites More sharing options...
ProZac Posted June 14, 2012 Author Share Posted June 14, 2012 Not to interested in what happens after the ignition event, just the event itself. Just as long as I know I'm testing under some-what real world conditions . Quote Link to comment Share on other sites More sharing options...
Testament Posted June 14, 2012 Share Posted June 14, 2012 actually now that UJ mentions actual compression test measurements it reminds me it is neccesary to take into account the temperature rise due to compression as this will increase the pressure/hot gas likes to take up more space so pressure will be higher. you can use ideal gas law to work that out and get closer to reality (although still not that accurate without taking into account leakage and heat loss) http://en.wikipedia.org/wiki/Ideal_gas_law Quote Link to comment Share on other sites More sharing options...
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