actual cylinder pressure in the otto cycle engine

240SicknessX

240SicknessX

forgot how to drive
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So, it has been brough to my attention that different types of dynos are inaccurate when comparing data. It would be more accurate to compare how much cylinder pressure is in the combustion chamber which could lead to engine failure. So how 'bout it? Has anyone read a book or a internet artical that explaines how to calculate cylinder pressure. NA compression, compressor effeciencyand flow rates, air temprature, possibly throw in intercoolers if it dosent get too confusing. All of these things, who knows a formula to toss all of that togeather into a number that we can use as cylinder pressure?

maybe start out with a simplified version of just a compressor flow rate at a specific PR and VE, a constant IAT, and a constant na cylinder pressure.


lets start throwing around ideas w\ backed up information, see if i left anything out, discuss.
 
extreme cylinder pressure usually doesn't blow engines. most failures are from detonation, improper tuning, crappy gas. Plus, depending on how large of an area your combustion chamber is, will effect power. Having a 84 mm bore at 180psi and having a 87mm bore at 180 psi are similar only in pressure. like with turbos- the ammount of air and the air pressure are 2 different things although related. I dont think you can measure the pressure directly (which is the point of ditching the dyno). I think there's too much crap to take into consideration too.

hp= (moving a known weight a certain distance)/(a period of time) I dont know why this is a problem to calculate through a dyno. Of course the tranny, weight of the flywheel, pulleys, ac...etc effect hp. They slow down the ammount of time it takes to move a known weight (increasing the denomonator-spelling???). I think the problem is with the regulation. When comparing 2 exact engine 1992 240's with ka-t's and one has 14 inch gram lights and the other has 22" 50lb diamond rims they will both blow at the same engine output, but it will seem, according to a wheel dyno, like the one with 22's had less...
... ok you have a good idea, I understand now-- sorry
 
green_machine said:
When comparing 2 exact engine 1992 240's with ka-t's and one has 14 inch gram lights and the other has 22" 50lb diamond rims they will both blow at the same engine output, but it will seem, according to a wheel dyno, like the one with 22's had less...
... ok you have a good idea, I understand now-- sorry
Click to expand...

That's why they have dynos that test the engine outside of the car to test it without those factors. if you buy a aftermarket hotrod engine they will give you a dyno figure of HP output not hooked upto a car.
 
i knew there were going to be alot of things to consider, and im leaning towards the "too much crap to figure on" statement. Ive over boosted my car once and blew the tires off the ground in second gear. There was very little load on the engine because of the lack of tracksion. If the same boost spike were to happen in 3rd or 4th it could have been a little more detremental. I guess you would have to take into consideration the load that the engine is also trying to propel. To figure actual load corrosponding to cylinder pressure is out of my range of knowledge to determine where a particular engine were to fail.

well to an extent, cylinder pressure is what kills an engine. Detonation raises the cylinder pressure exponetonally i belive. Detontion raises cylinder pressure beyond what the engine internals are able to withstand. Its kinda hard to belive, someone with a 180hp engine can generate more cylinder pressure then a 400hp engine just by a pre iginition, although its true and the exact reason why you can blow up a ka @ 180 hp or 400hp.

with in reason, providing the same type of dyno was used and was properly calibrated you could use similar dyno data to compare power. dynojet vs dynojet or which ever you choose. really cant compare dynojet vs a mustang dyno because the mustang subtracts other varibles when producing a number on paper. any one else support this?


edit note- with some fast frame photography scientist were able to measure pre igintion flame fronts traveling at 6500-6800 ft per second. (5x10 to the -5 sec, or less). 1320 ft in a mile. http://naca.larc.nasa.gov/reports/1942/naca-report-727/naca-report-727.pdf
 
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good artical, i need to reread it again once i am a little less tired. but i did find this: Under the conditions that generally exist inside a cylinder, the flame front velocity is usually around 90 feet per second, or 60 mph.

just to put what i said about a pre ignited flame front traveling at 6500 ft per second in perspective.
 
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