What is the Combustion Formula for Lean C8H18 in Air?

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Discussion Overview

The discussion revolves around the combustion formula for lean C8H18 (octane) in air, specifically focusing on the products of combustion and the estimation of CO2 emissions. The context includes theoretical and experimental aspects related to combustion processes in engines operating under lean fuel conditions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant presents a proposed combustion reaction formula for lean C8H18, indicating specific lambda values and questioning the validity of their calculations.
  • Another participant highlights the complexity of combustion reactions, noting that even with lean burn conditions, various hydrocarbons, NO, NO2, and CO may still be present in the exhaust.
  • A later reply suggests assuming that a high percentage of carbon in the fuel converts to CO2, questioning the choice of octane as the fuel in the discussion.
  • Another participant recommends using a program like STANJAN to determine exhaust gas composition, implying that computational tools may provide more accurate results.

Areas of Agreement / Disagreement

Participants express differing views on the combustion process and the accuracy of the proposed formulas. There is no consensus on the exact combustion products or the validity of the initial calculations, indicating ongoing debate and uncertainty.

Contextual Notes

Participants acknowledge the presence of unburned fuel and other emissions, such as NOx and CO, which complicate the estimation of CO2 concentrations. The discussion reflects limitations in the proposed models and assumptions regarding combustion efficiency.

Who May Find This Useful

This discussion may be of interest to those studying combustion processes, engine performance, or emissions analysis, particularly in the context of lean fuel operations.

nordmoon
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I am doing a thesis work and I am trying to figure out the amount of C02 and the products in combustion of Air and C8H18. Its known that the engine is operating on lean fuel, but I am having trouble with the reaction formula. This is not my subject. The engine is operating at lambda 1.2 to 1.6. (Phi = 0.83 - 0.62).

Here is my try:

0.83 C8H18 + 12.5O2 + 94N2 <--> 6.64C02 + 7.47 H20 + 94 N2 + 2.12 O2 ?

0.62 C8H18 + 12.5O2 + 94N2 <--> 4.96C02 + 5.58 H20 + 94 N2 + 4.7 O2 ?

Is there something wrong here? Does this really happen as the fuel + air ignite?
 
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Unfortunately it's not that easy. There are many complex reactions taking place during the combustion process, and even with a lean burn engine, you'll still have a fair amount of various hydrocarbons in the exhaust. Depending on the conditions, you'll also produce a significant quantity of NO and NO2, and CO too.

Is this a degree thesis? What are you trying to achieve with this?
 
brewnog said:
Unfortunately it's not that easy. There are many complex reactions taking place during the combustion process, and even with a lean burn engine, you'll still have a fair amount of various hydrocarbons in the exhaust. Depending on the conditions, you'll also produce a significant quantity of NO and NO2, and CO too.

Is this a degree thesis? What are you trying to achieve with this?

Yes, its a little complicated to describe here. But in short I am trying to 'estimate' the C02 molefraction for an estimation of the width of the line profile in simulated spectra and a very approximate 'estimation' of the amount of C02 from total in a simulation for optical thickness of investigated bands. I know that this does not meet the reality and that you probably have unburned fuel and NOx, CO, internal ERG and so on. But I do not see how I can determine those fractions of NOx, C0 etc. The ultimate goal is to determine the C02 concentration in a measurement.
 
Why not assume that something like 99% of the carbon in the fuel ends up as CO2?

Why are you considering the fuel to be Octane?
 
Depending on what kind of accuracy is needed, a program like STANJAN (there may be newer/better programs; it's been a while since I did this sort of calc) could be used to determine exhaust gas composition.
 

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