Why Does Calculating Ethanol's AFR Yield Different Results from Standard Values?

In summary, the calculation of ethanol's air-fuel ratio (AFR) can yield different results from standard values due to factors such as variations in ethanol's composition, measurement techniques, and the specific conditions under which combustion occurs. These discrepancies arise from differences in temperature, pressure, and the presence of other components in the fuel mixture, which can affect the combustion efficiency and the actual AFR necessary for optimal engine performance. Understanding these variables is crucial for accurate tuning and performance optimization in engines using ethanol as fuel.
  • #1
D19A99G
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TL;DR Summary
Calculation of the theoretical value of AFR of ethanol
First I have seen a few forums dotted about covering this subject already, however I couldn't find exactly what I was looking for so I apologise if its somewhere on here already.

I am trying to calculate the AFR value for pure ethanol.
I am only including oxygen in the category of "air" due to other elements being inert.

The balanced equation is C2H5OH + 3O2 = 3H2O + 2CO2.

When I then calculate the AFR while also considering that oxygen only makes up 21% of air, the value I get is approximately 9.9. Which is different to the widely used value of 9. Is there something I'm doing wrong or has the value of 9 only being achieved through practical tests which cannot be replicated via theory?

Ethanol mass = 46, Oxygen mass = 96
Air mass = Oxygen*4.76 = 456.96
AFR = 456.96/46 = 9.93
When; C=12amu, H=1amu, O=16amu
 
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  • #2
You are assuming that oxygen makes up 21% of a gas of uniform molecular mass 32 amu. You have to account for the fact that nitrogen is lighter.
 
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  • #3
Yeah ok, I can see what you mean. If I neglect the other gases which constitute the make up of "air" and assume oxygen = 21% and nitrogen = 79%. Then 3*(32+(3.76*28)) = 411.84. This would give me a AFR of 8.95 which is much closer.
Then considering Argon as well would give me a value of 9.01.

That makes sense thanks
 
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  • #4
Sadly, you can't generally assume whether a generic 'percentage' is volume, mass, or whatever. Units matter. I use '%wt' and %vol' to prevent confusion (and collisions with Mars).
 
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FAQ: Why Does Calculating Ethanol's AFR Yield Different Results from Standard Values?

1. What is the air-fuel ratio (AFR) for ethanol?

The air-fuel ratio (AFR) for ethanol is typically around 9:1 by mass, which is different from gasoline's AFR of approximately 14.7:1. This means that ethanol requires less air to combust completely compared to gasoline, which is crucial for optimizing engine performance and emissions.

2. Why do calculated AFR values for ethanol differ from standard values?

Calculated AFR values for ethanol can differ from standard values due to variations in the ethanol blend, measurement techniques, and assumptions made during calculations. Factors such as temperature, pressure, and the presence of other components in fuel mixtures can also affect the actual AFR achieved during combustion.

3. How does temperature affect the calculated AFR for ethanol?

Temperature can significantly influence the density and vaporization characteristics of ethanol. As temperature increases, the density decreases, which can lead to a higher mass of ethanol being consumed relative to the volume of air. This change can skew calculated AFR values, making them appear different from standard values.

4. What role does engine tuning play in determining ethanol's AFR?

Engine tuning is crucial for achieving the optimal AFR for ethanol. Different engines may require specific adjustments to the fuel injection timing and quantity to achieve the desired combustion efficiency. If the engine is not properly tuned for ethanol, the calculated AFR may not align with standard values, leading to inefficiencies and increased emissions.

5. Can the presence of additives in ethanol affect AFR calculations?

Yes, the presence of additives in ethanol can significantly affect AFR calculations. Additives can change the combustion properties of the fuel, altering the stoichiometry required for complete combustion. This can lead to deviations from standard AFR values, making it essential to account for these additives when calculating the actual AFR for a specific ethanol blend.

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