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Old 03-15-2013, 01:28 PM   #11 (permalink)
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Originally Posted by mechman600 View Post
Originally Posted by mwebb
peak combustion pressure must happen at about 14 degrees after top dead center

Is this true will all engines? Four stroke/two stroke? Gas/Diesel? etc...
Just curious. I have never heard the 14 degree ATDC figure before.
Peak pressure can happen anywhere in the expansion stroke, but 14 degrees is best for converting burning fuel into motion.
It's true for all piston engines where a reciprocating piston is connected to a crank via a connecting rod. 14 degrees ATDC is where the piston-rod-crank leverage is optimum. It is just geometry so: Otto, Diesel; 6, 4, or 2 stroke; any fuel... doesn't matter.

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Old 03-15-2013, 01:51 PM   #12 (permalink)
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Engine Compression Ratio - Tech - Popular Hot Rodding Magazine

The article is on compression ratio, but has a good section devoted to timing.
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Old 03-15-2013, 02:12 PM   #13 (permalink)
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Here is a youtube video talking about diesel combustion pressure.

Gasoline engines use constant volume heat addition, meaning idealy your fuel is to be burned, all your heat added when the piston reaches top dead center. Hence the volume of your working fluid remains constant as heat is added.
Since this can not happen for a number of reasons we use spark advance.

A diesel engine uses constant pressure heat addition. Which means as the volume of the working fluid changes the pressure remains during heat addition. The fuel usually does not ignite in a diesel engine until right around TDC, the fuel injects, lights off and the constant pressure of fuel being injected/burned ATDC drives the piston down the bore.
The constant pressure thing is what limits the diesel engines speed and at the same time gives the diesel its legendary efficiency.

Well that video wont load for me with my slow internet, but I believe they say that pressure peaks some where between 10* and 20* ATDC.
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Old 03-15-2013, 05:03 PM   #14 (permalink)
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Centroid of heat release is a better metric to use for combustion phasing than the crank angle of peak pressure. This is particularly true for common rail diesels were you can have multiple injection events in the same cycle (3-4 is not uncommon).

Consider the traces in the middle left of this graphic:

FYI, "AHRR" stands for "Apparent Heat Release Rate".

You can see that there's two "humps"--one from the compression and one from the combustion. A slight variation will make one or the other the higher peak and alter teh crank angle of peak pressure by 20 deg or so, even though the combustion hasn't really changed much at all. Infact, the irony is that very late combustion phasing (as in the middle right plot) cause the angle of peak pressure to occur earlier (at TDC from compression).

Also keep in mind that with modern common rail diesels, the combustion process is "shaped" via many injection events in the same cycle (3-4 is common, although I've seen more). It's not just a matter of squirt-it-in-let-it-go-boom anymore.
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Last edited by Diesel_Dave; 03-15-2013 at 05:08 PM..
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Old 03-16-2013, 08:46 PM   #15 (permalink)
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quit while you are behind

you can not use sound to figure out / measure weather or not your ignition timing is incorrectly advanced

as you have proved
stop doing what you have already proved does not work
stop referring to what you have proved does not work as if there is any value at all connected to it

learn from your mistakes
learn from your pain

if you have a scope you can make a circuit to measure exactly when peak compression pressure happens
with out that info you are guessing , and
your ability to SWAG the correct value is less than good

you do not have a diesel engine .
you do not have a OBD2 system and if your car even has OBD1 it will not have the ability to provide the data graphed in the above post
you will need to get the data
with a hi resolution scope
like Pico 6 or similar

do you have a scope ?
if yes you can make a circuit which will generate this waveform from which you can MEASURE actual AVERAGE Peak combustion pressure
adjust your IGN timing so that
peak combustion pressure AVERAGE is stable around
14 degrees ATDC

if nothing else you will learn to use a scope and learn to make simple circuits
work smarter

the blue trace is combustion chamber pressure as measured in real time with a pressure transducer threaded into spark plug hole
720 degrees between peaks as this is a 4 stroke engine
engine running BUT that cylinder does NOT fire or contribute
Green Trace is IGT ignition timing command from ECM spark begins at the end of the IGT square wave

you can measure that value with ANNOwave from AESwave
or measure the distance between peaks and divide by 720 that is the value you need to adjust

that is a common rail gasoline engine with pizeo injectors
direct injection
see last page

Last edited by mwebb; 03-18-2013 at 12:32 AM.. Reason: DSO scope hi resolution , link to circuit
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