EGT equation

[mangel83]

I was searching some info on EGT calculation and I found this equation in a spanish book:

egt = ( iamf / egmf ) * ( IT + ( LHV / (4 * afr * lambda * cp ) ) ) where:

iamf: intake air mass flow (kg/s)

egmf: exhaust gas mass flow ( iamf + fuel mass flow ) (kg/s)

IT: intake temp (ºC)

LHV: fuel low heat value (kJ/kg)

afr: air fuel ratio

cp: average air specific heat between egt and IT (kJ/kg*ºC)

According to this equation egt is proportionally inverse to afr & lambda, what as I recall it's not true, the lower afr/lambda values, the lower the egts.

I'm thinking there must be something wrong with this equation, any comments on this??, does anyone know a different way to theoretically approximate EGTs? maybe recommend some books or something. thanks

[pengulns2001]

your not going to find afr and lambda in the same equation since one is a function of the other, i would say you have something wrong there

i also dont see anything about pressures which raises a flag in my book, although i have never tried to calculate egt values

[Kirk Feldkamp]

Was there any context to this equation, or was it just stuck in the text with no explanation? Is it derived? It probably carries a lot of qualifiers.

Ignition timing will have a dramatic effect on EGT... so much so that I wouldn't trust an equation like this without understanding the assumptions behind it.

-Kirk

[Homemade WRX]

Originally posted by turbotwig:
Ignition timing will have a dramatic effect on EGT
that was my first thought and first variable I looked for in his listing...
there has to be more explanation or constants/assumed constant variable that go along with this equation

[BeaverGuy]

I think that the lambda in that equation is most likely not the lambda associated with AFR. In order for that equation to be correct lambda would need to be egmf/iamf. Also, there are three major assumptions that I can see from the equation.

One is 25% of the fuel energy remains in the exhaust as heat(that is where the 4 comes from).
The second is that there will be complete combustion of the fuel. This is from the fact that no combustion efficiency factor is used. Roughly trueas it seems to stay above 95% for most situations.
The third is that the specific heat of the exhaust gas will not be appreciably affected by the combustion products. While true for a jet turbine which runs AFR far leaner than an reciprocating internal combustion engine it is not likely true for typical gasoline or diesel engines.

Not including pressure directly in the equation is of no major consequence because of the specific heat, it can be included there because it is an average of the values between the two state points of intake and exhaust. However, the spedific heat will be heavily dependent on the averaging method of those values so the method you use will impact the results from the equation.