Car Fuel Problem, calorific values.

  • Thread starter Miller11
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In summary, the problem at hand is to find the output power in kW of an engine using a fuel mix of 90% Nitromethane and 10% Octane, with calorific values of 11.3 MJ/kg and 43.7 MJ/kg respectively. The engine in question can produce 100 kW when running on pure 100% octane with 20% excess air, and the volumetric, thermal, and mechanical efficiencies remain the same. Using the equation for power, it is determined that the mole per second (mol/sec) for both engines is 0.02. However, when attempting to find the power for the second fuel mix, a lower value is obtained, which is not expected. Further
  • #1
Miller11
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1) I need to find the output power in kW of an engine when running a mix of 90% Nitromethane
(CH3NO2), and 10% Octane (C8H18), their calorific values are 11.3 MJ/kg and 43.7 MJ/kg respectively.
what i know:
An engine that runs pure 100% octane with 20% excess air produces 100kW, and the volumetric, thermal and mechanical efficiency's remain the same.
I need to find out how much power the same engine using a 10/90 fuel mix would produce.2)Im using this equation:

Power (kW)=Power (kJ/sec)= [calorific value fuel(kJ/kg)] X [molar mass of fuel(kg/mol)] X
[mol/sec]

Im assuming since the volumetric efficeincy will be the same then the [mol/sec] i get for the first engine will be the same as the second fuel mix.

100kW = [(43.7 x 10^3)kJ/kg] X [(114.224)kg/mol] X mol/sec
hence,
mol/sec for both engines = 0.02.

But I am stuck from there and don't know how to continue, as when i try subbing this value into the same equation with the second fuels mix, i get a lower power which it shouldn't be.

Any help would be greatly appreciated.

Cheers Miller11
 
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  • #2
Is this problem from a thermodynamics course?
 
  • #3
To work this problem you will have to write the chemical equations for each fuel, balance them both, and determine how much fuel is being burned given the 20% excess air. In doing so you will find that you can burn more nitromethane fuel and therefore produce more power because nitromethane contains oxygen.
 

1. What is the definition of calorific value?

Calorific value, also known as heating value, is a measure of the energy content of a fuel. It represents the amount of heat released when a specific amount of fuel is burned completely.

2. How is the calorific value of a fuel determined?

The calorific value of a fuel is determined by measuring the amount of heat released during combustion in a controlled environment. This can be done through various methods such as bomb calorimetry or adiabatic flame temperature measurement.

3. What factors affect the calorific value of a fuel?

The calorific value of a fuel can be affected by various factors such as the type and composition of the fuel, the presence of impurities, and the combustion process. Moisture content, ash content, and sulfur content can also impact the calorific value of a fuel.

4. Why is the calorific value of a fuel important?

The calorific value of a fuel is important because it determines the energy output of the fuel. This is crucial in industries such as transportation and power generation, where the efficiency of the fuel directly impacts the overall efficiency and cost-effectiveness of the process.

5. How does the calorific value of different fuels compare?

The calorific value of fuels can vary significantly depending on the type of fuel. Generally, fossil fuels such as coal and natural gas have higher calorific values than renewable fuels such as biomass. However, the calorific value of a fuel is not the only factor to consider when determining its viability for a specific application.

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