SI engine combustion. CO2 dissociation?

In summary, the extra power that is produced when using a rich mixture in an SI engine is due to the fact that there is more chance of all the oxygen reacting with the excess fuel.
  • #1
varnish
10
0
Hey guys,
Had this over in the mechanical engineering section but I think I might have more luck with getting an answer over here.

The question is, what is happening to make the extra power when using a rich mixture in an SI engine?

An air fuel ratio (AF:R) of 14.7:1 is stoichiometric for petrol (gas), but an AF:R of 12.6:1 makes more power.

One common theory I have heard is that not all the fuel and oxygen react in a stoichiometric mix, so by upping the fuel there is more chance of all the oxygen reacting with the excess of fuel.

For some reason I have got the notion that when the CO2 in the cylinder dissociates due to the heat of combustion, the liberated oxygen is free to react with the excess fuel.
But I can't find any sources that state this, they all talk about equilibrium reactions and NOx production.

The information in this link gives some insight:
http://www1.gantep.edu.tr/~ozcan/kit...n/7699X_13.pdf [Broken]

The author [above] states:
"If there is no dissociation then the peak pressure is always reached at the stoichiometric ratio. However, when dissociation occurs the [air fuel ratio] at which the peak pressure occurs is moved to the rich region. This is because dissociation tends to increase the amount of substance in the products..."

The author doesn't state what he means by "substance in the products".
Anybody have any idea what he might be talking about?

Gordon Blair (design and simulation of four stroke engines) states on the subject of CO2 dissociation that:
"at the height of combustion...the gas composition would now be 4.3% CO, 9.36% CO2, and 2.1% O2."

With 2.1% free oxygen in the cylinder, surely some would react with the excess fuel present in a rich mixture.

I'm not making 2 + 2 = 5 am I?
 
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  • #2
Would really appreciate some more insight on this topic. I'm sure I'm not the only one interested in this.
 

1. What is an SI engine combustion?

An SI (Spark Ignition) engine combustion is a type of internal combustion engine where the fuel is ignited by a spark plug. It is commonly used in gasoline-powered vehicles.

2. How does an SI engine combustion work?

An SI engine combustion works by combining air and fuel in a confined space, which is then ignited by a spark plug. The resulting explosion pushes the pistons, which in turn, powers the engine.

3. What is CO2 dissociation in the context of SI engine combustion?

CO2 dissociation in SI engine combustion refers to the process of carbon dioxide (CO2) breaking down into carbon monoxide (CO) and oxygen (O2) due to high temperatures in the engine. This process can occur during the combustion of fuel in an engine, and the resulting CO can contribute to air pollution.

4. How does CO2 dissociation affect the environment?

CO2 dissociation in SI engine combustion can contribute to air pollution, as carbon monoxide is a harmful gas that can negatively impact air quality and human health. Additionally, the production of CO2 from the burning of fossil fuels is a significant contributor to climate change.

5. How can CO2 dissociation in SI engine combustion be reduced?

CO2 dissociation in SI engine combustion can be reduced by using alternative fuels, such as biofuels, that produce less CO2 during combustion. Additionally, improving the efficiency of the engine and implementing emission control technologies can also help reduce CO2 dissociation and its impact on the environment.

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