Theoretical Combustion Pressure Calculations 1

[mountaininventor]

Well here I am again trying to calculate something in a field where I have no experience.

I know there are a lot of variables but I have been trying to determine roughly the amount of pressure in a normally aspirated gasoline engine that is generated at the moment of combustion. I just want something typical. I bought the books by Charles Taylor "The Internal Combustion Engine in Theory and Practice" but it is way beyond my needs and doesn't include simplified analyses.

So I thought I would ask here and make up a simplified model. A piston in a 200 ml cylinder (closed chamber); air/gasoline mixture; compressed to 25 ml; Piston doesn't move from combustion so that the chamber volume remains constant.

What would be the pressure after combustion? How would I calculate it? and/or where can I look this data up for varying fuels mixtures and oxygen sources?

 

[nagy]

If you send me your email I can send a program to do this calculations.

Nagy

 

[ivymike]

How would I calculate it?

For your simplified system, try this approach:

Based on the desired air-fuel ratio,
calculate the mass of air in the chamber before compression
calculate the mass of fuel...

M.total = P1*V1/(R * T1) where R.air ~ 287 m^2/(s^2 * K)
M.air = M.total * AFR / (AFR+1)
M.fuel = M.total / (AFR+1)

Assume a combustion efficiency.

Using the lower heating value of the fuel, calculate the energy released by combustion of all the fuel, then multiply by your efficiency.

for gasoline, Q.HV ~ 43,000 kJ/kg
Q.in = M.fuel * Q.HV * n.comb

Using the gas properties of air (mixture properties should be fairly close to air), calculate the temperature of the mixture after combustion

Q.in = (M.fuel + M.air) * c.v * (T2 - T1)
c.v ~ 821 m^2 / (s^2 * K)
T2 = [Q.in + (M.fuel + M.air) * c.v * T1] /
(M.fuel + M.air) * c.v

Calculate the pressure after combustion using the ideal gas law:

P2 = P1 * (T2/T1)

 

[mountaininventor]

Thanks
My e-mail is mrgl.llc@verizon.net

ivymike
I assume that an ideal theoretical AFR would be based on the typical % O2 in Air and the reaction equation for the fuel. ie set the molar ratio between oxygen in air and fuel such that (2n)O2 + CH(2n+2) -> CO2 + (n+1)H2O. Or is this being too nieve?

For R.air ~ 287 m^2/(s^2 * K is the K a constant or degrees Kelvin?

To find theoretical Max, I suppose that the combustion efficiency could be assumed at 1 with the further assumption that all reactants are equal, the chamber is static, the reaction goes to completion, there is no heat loss from the chamber and the world is perfect? But just out of cureosity, what would be a typical combustion efficiency for a gasoline engine? Like one that runs within EPA emmition standards.

 

[ivymike]

for gasoline, stoichiometric AFR (mass basis) is about 14.7:1 to 14.9:1. I'm sure you could get there via your approach, but shortcuts are nice. There are tables out there (Taylor probably has some in the back of his book) that will give you stoich AFR for a long list of hydrocarbons.

yep, K is Kelvin

I think it's common to have combustion efficiencies in excess of 98%. I'm not so much of a combustion/emissions guy, though, so perhaps someone else can better answer that.