Determining combustion burn time

[fkatzenb]

This is a pretty cool site... I decided to register and ask my very first question...


I am developing an Excel Spreadsheet tool for learning purposes which demonstrates effects on power generation of automobile engines by changing various specifications like charge temperature, rpm, bore, stroke, displacement, manifold pressure, volumetric efficency, etc.

The part that I don't have figured out is the part I am least familiar with... combustion characteristics and burn time. I know that rate of change effects, mixture, temperature, pressure, etc effects burn time... but how. I would like to have a magical equation that takes in account mixtures (of Air, N2O, Methanol, Ethanol, & Propane), my changing volume over time (and rate, of course temp and compression pressure, etc.

This is purely a free learning tool and will never be used for money... hopefully that helps someone promote some work for me. I appreciate any and all help. Thanks!


Frank Katzenberger

 

[Clausius2]

This is a pretty cool site... I decided to register and ask my very first question...


I am developing an Excel Spreadsheet tool for learning purposes which demonstrates effects on power generation of automobile engines by changing various specifications like charge temperature, rpm, bore, stroke, displacement, manifold pressure, volumetric efficency, etc.

The part that I don't have figured out is the part I am least familiar with... combustion characteristics and burn time. I know that rate of change effects, mixture, temperature, pressure, etc effects burn time... but how. I would like to have a magical equation that takes in account mixtures (of Air, N2O, Methanol, Ethanol, & Propane), my changing volume over time (and rate, of course temp and compression pressure, etc.

This is purely a free learning tool and will never be used for money... hopefully that helps someone promote some work for me. I appreciate any and all help. Thanks!


Frank Katzenberger

The question you are asking is a very non trivial one. The best advice I can tell you is try consult the book "Internal Combustion Engines Fundamentals" of John Heywood. There you will find simple and advance mathematical modelation. Surely that equation you mention will exist, but it would be for me too difficult to develop it here.

As simple combustion models are concerned for heat engines, I recall just now the Wiebe model, which gives you the amount of heat released as a function of the crank angle:

$$Y\sim (1+e^{a\alpha})^m$$ where Y is the burned mass fraction and "m" and "a" are two coefficients experimentally fixed after doing a diagnosis experiment of the engine. This diagnosis experiment consists in measuring the indicated cycle (P-V) and deriving from it the amount of heat released in the combustion process. The crank angle $$\alpha$$ can be coupled with the piston movement and also with pressure by means of energy conservation considerations.


I encourage you to take a look at that book. It's a great job of Prof. Heywood.

 

[fkatzenb]

Bah! I actually own that book, however it is at my friend's house some 8 hours away, and he hasnt mailed it back. I love that book and has given me insight on what I have been doing wrong in my thought process of engines. In addition, it has given me a second wind on this spreadsheet on some formulas for completing this spreadsheet.

I guess once I get my book back, I can do some more investigation on exactly what I need. Then this thread should be interesting!

Frank

 

[fkatzenb]

Well I finally got my book and Chapter 9 seems to be the area for which I am most concerned about. In there we are a few approaches including the Wiebe function. Unfortunetly, it is based off of coefficents that need real life data to shape the % burned curve (Xb). These coefficents (a & m) naturally take into account compression ratio, volume change rate, and mixture ratio.

That means I need to use the approach of burned and unburned mixture states that is discussed in 9.2.1. Here is what I know:

• I plan on using the previous states to determine the new states.
• I know volume at each crank angle.
• I know pressure and temperature at each pre-ignition crank angle.
• I believe I understand how to determine my pressure and temperature from previous states and mass burned fraction.

Now for the more difficult stuff:

How do I determine the mass fraction burned from theoretical methods? Being able to determine that is key because it naturally includes the rate to 100%. A good example of this is in Figure 9-5 and 9-8.

-Frank

 

[Clausius2]

Now we have a problem: I don't have that book nearby me. I used it for studying heat engines in a course I took in first quarter of this year. I would need more information about what model do you want to code. Are you assuming an instantaneous change of composition after the compression stage to model combustion?

 

[fkatzenb]

You mean the ideal cycle? Nope. If I am reading this right, it is the equilibrium model for burned gases and specific heat models. I am also assuming a fully mixed, no heat loss model.

Frank