Combustion temps and pressures

[Diesel Freak]

I got bored today and wanted to see what was going on inside the combustion chambers on my truck.



Assumptions:

k=1. 4

Cutoff Ratio (Cr): 2

Compression Ratio (r) 17:1

Constant Specific heats

Gas constant for air: . 06855 Btu/(lbm*R)

Exhaust Manifold pressure @30 psi boost: 42. 7 PSIA

EGT in Manifold: 1610R





I came up with a max combustion temp of 2268R (1809F)

and a max combustion chamber pressure of 857. 5 PSIA (843psi)



pure aluminum melts at 1220F... ... I hope the forged pistons I have contain a high ammount of silicon





does anyone know the exact Cr of the ISB at WOT and 1950 RPM?

 

[Lil' Dog]

Just curious as to what fuel flow you estimated. I don't have a good grip on how to get that #... Or did you assume an arbitrary A/F ratio??



J-eh



(edit) The pressure you got is BMEP?? A peak pressure should be around 1000 to 1200 Psi for a diesel. . Just my experience. .

 

[Champane Flight]

good thing

Its a good thing your pistons have three strokes to cool off!

 

[Diesel Freak]

Originally posted by Lil' Dog

Just curious as to what fuel flow you estimated. I don't have a good grip on how to get that #... Or did you assume an arbitrary A/F ratio??



Did not assume a fuel flow... . I worked backwards from point 4 to point 3 on an IDEAL P-v Diesel Cycle



J-eh



(edit) The pressure you got is BMEP?? A peak pressure should be around 1000 to 1200 Psi for a diesel. . Just my experience. .



No I did not get BMEP. the MEP for a 245 HP ISB is



505ft*lbf/359 in^3



or... 16. 88 psi



MEP is a ficticious pressure that if acted on a piston during the entire power stroke, would produce the same ammount of net work as that produced during the actual cycle.

 

[Lil' Dog]

I understand BMEP, but most of the natural gas engines I analyse have BMEPS in the order of 80 up to 130 Psi, so I haven't pulled out the calculator for the Diesel, but expected about 160 to 180, even higher for the high Hp of the ISB for the small displacement.



Just curious about the math man, no biggie.



J-eh

 

[Diesel Freak]

not a problem.



Do the natural gas engines have a higher specific output?



This also brings to light why Propane works the way it does on a diesel, and also why head gaskets can blow.



The addition of propane turns the diesel cycle into a combined cycle. (otto/diesel)



The propane ignites at once when the diesel is first admitted (constant volume heat addition where chamber pressures skyrocket)



then the rest of the diesel is burned during the injection duration (constant pressure heat addition where cylinder presure remains the same while the piston moves downward)





keep in mind that I am talking about Ideal processes

 

[Lil' Dog]

Ahh yes. The Peak pressures are more even in the diesel and thusly the lower BMEP... Gottcha. I remember using a pressure rise analysis technique where you are worried about Psi/deg crank rotation instead of the absolute peak pressure. You plot the first derivative of the pressure-time wave.





I believe that the NG engines may have a higher specific output. But these are large industrial units, slow speed 300 - 400 Rpm bangers with bores 14 - 20", strokes up to 22".



All makes sense. I don't get to analyse alot of diesels, so this is good noggin' knockin' material.





Jason

 

[Diesel Freak]

looking at the numbers, I think I need to cool the engine off a little before I add more fuel. I dont want to get too low, 60F out of the intercooler would be perfect.

 

[RustyJC]

BMEP of the ETH at peak BHP of 245 @ 2700 RPM = 199. 96 PSI



This is calculated using the following formula:



BHP = (PxLxAxN)/33,000



Where:

P = Brake Mean Effective Pressure (PSI)

L = Length of Stroke (feet)

A = Piston Area (square inches)

N = Number of Power Strokes/minute



Solving for P



245 = (Px0. 3933ftx12. 692sqinx[1350x6])/33,000



245 = 40,433xP/33,000



8,085,000 = 40,433xP



199. 96 PSI = P



This is comparable to large 4-cycle power generation diesels such as the Cooper-Bessemer LSVB-20-T (municipal power generation) with a BMEP of 200 PSI and the KSV-20-T (emergency nuclear standby power generation) with a BMEP of 250 PSI.



A typical spark ignited, natural gas fueled pure turbocharged 2-cycle integral engine/compressor (the Cooper-Bessemer GMVH-12) has a BMEP of 125. 3 PSI - half as much power per power stroke, but twice as many power strokes as a 4-cycle!



Rusty

 

[Lil' Dog]

Heh, RustyJC



That was what I was after... Thanks. . I don't think DF and I were on the same page. Those are the numbers I was expecting.



Hmmm, too much reading and not enough thought on my part, should have just pulled out the calculator like you did . Boy do I feel silly #ad




Funny, I was just working on 2 LSV-16-SG generators last week. That's what got me goin on this.



What is your specialty????



J-eh

 

[Diesel Freak]

Originally posted by Diesel Freak

No I did not get BMEP. the MEP for a 245 HP ISB is



505ft*lbf/359 in^3



or... 16. 88 psi



MEP is a ficticious pressure that if acted on a piston during the entire power stroke, would produce the same ammount of net work as that produced during the actual cycle.

GFCE on my part..... now on the same page



(note to self the ISB has six cylinders)

 

[Lil' Dog]

Now I feel like a newbie... . GFCE?? BTW sorry about the confusion.

 

[Diesel Freak]

GFCE= Gross Fu#%^$@ Conceptual Error

 

[Lil' Dog]

Too funny!!!!! LMAO... . #ad

 

[Diesel Freak]

hehe, thought you would like that

 

[Diesel Freak]

well, does anyone have a clue what the Cut Off ratio of the ISB is? (and I do know this changes once it is BOMBed)

 

[C Schomer]

You guys lost me way back there! I was told once that a gas engine, lugging and working to the max hits over 3000psi at ignition. Craig

 

[Diesel Freak]

Originally posted by C Schomer

You guys lost me way back there! I was told once that a gas engine, lugging and working to the max hits over 3000psi at ignition. Craig

Possible, but probably only from pre-ignition and the collision of two flame fronts... . this is the knocking you hear while lugging a gas engine..... bad juju