Determining combustion burn time

In summary: I don't remember the equations off the top of my head. :( In summary, Frank is developing an Excel Spreadsheet tool to demonstrate the effects of changing specifications on power generation of automobile engines. He is unsure how to calculate the mass burned fraction and needs help from someone more experienced.
  • #1
fkatzenb
5
0
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
 
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  • #2
fkatzenb said:
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:

[tex] Y\sim (1+e^{a\alpha})^m[/tex] 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 [tex]\alpha[/tex] 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.
 
  • #3
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
 
  • #4
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
 
  • #5
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?
 
  • #6
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
 

1. How is the combustion burn time determined?

The combustion burn time is determined by measuring the time it takes for a substance to completely burn in a controlled environment. This is typically done by using a stopwatch or timer and recording the time it takes for the substance to burn completely.

2. What factors can affect the combustion burn time?

There are several factors that can affect the combustion burn time, including the type and amount of fuel, the presence of oxygen, temperature, and pressure. Other variables such as the shape and size of the substance, as well as external factors like wind and humidity, can also impact the burn time.

3. Why is it important to determine combustion burn time?

Determining the combustion burn time is important in understanding the behavior and properties of different substances when they are burned. This information can be used to improve safety measures and develop more efficient combustion processes. It can also help in predicting the environmental impact of burning certain substances.

4. How is the combustion burn time used in research and development?

In research and development, the combustion burn time is often used to compare the burn characteristics of different fuels or substances. It can also be used to study the effects of different conditions on combustion, such as varying oxygen levels or fuel mixtures. This information can be used to improve combustion processes and develop new technologies.

5. Can the combustion burn time be accurately predicted?

While the combustion burn time can be estimated based on certain factors, it is difficult to predict it with complete accuracy. This is because there are many variables that can affect the burn time, and they may not always be consistent or controllable. However, by conducting careful experiments and collecting data, scientists can make more accurate estimations of combustion burn time.

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