CNG-Air Flame speed in IC engines 2

[amirfarzad]

Dears

I have heared many explanations for flame speed comparison of different fuels in internal combustion engine. But I have never found any reference to make a sharp conclution to say CNG falme speed is lower than gasoline in the same condition. Or any diagram, test results or also calculation to investigate the subject!
Thanks

 

[patprimmer]

A rough test is to set optimum ignition timing for each fuel. The greater the ignition lead required for maximum power, the slower the flame.

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[amirfarzad]

patprimmer

As you know there are two aspects affect ignition timing, ignition delay and flame speed. I am sure that delay is longer for CNG. But for flame speed, it is more complicated!

 

[patprimmer]

Another not quite so rough test would be to measure rate of pressure rise.

Regards

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[j2bprometheus]

amirfarzad -

The laminar flame speed of natural gas is lower than that of gasoline.

Natural gas is typically > 90 % methane, and methane has a relatively low laminar flame speed compared to other hydrocarbons.

"Gasoline" is a very complex mixture of thousands of compounds, so it is rare to see laminar flame speed data for gasoline since it is difficult to completely describe the gasoline that was tested. You are more likely to find flame speed data on iso-octane.

The flame speed in the engine is a function of the turbulent flame speed. The turbulent flame speed is modeled as a function of the square root of the laminar flame speed ( see

http://www.ent.ohiou.edu/~juwt/HTMLS/fluent/fluent6/ug/chp15.pdf)

, so changing from gasoline to natural gas should have a relatively weak effect on turbulent flame speed.

Looking at some laminar flame speed data for various fuels suggests (but does not prove) that the TURBULENT flame speed for methane should be about 90 % of that for gasoline. So combustion duration should be about 10 % longer for natural gas than for gasoline at stoichiometric.

There is some data of laminar flame speeds for iso-octane & methane published in a paper by Metghalchi & Keck. Let me know if you want the exact reference.


j2bprometheus

 

[amirfarzad]

j2bprometheus

Thank you for the references. I have seen the following paper, would be helpful also:

LAMINAR BURNING VELOCITY OF
METHANE-AIR-DILUENT MIXTURES
Matthew Ulinski, Patrice Moore, Mimmo Elia, Mohamad Metghalchi
Department of Mechanical, Industrial, and Manufacturing Engineering
Northeastern University
Boston, MA

using Metghalchi & Keck calculations I found that, in the condition of higher temperatures and rich mixtures iso-octan laminar flame speed would be higher than CNG. but in low temperatures and lean mixtures laminare flame speed are comparable.

but steel I am searching for an experimental measurement of turbulent flame speed, to know what happens in the real engine!

 

[dcasto]

This organization has been studing natural gas engines forever

http://www.gmrc.org/