Question

Consider an internal combustion engine like that used in automobiles. The most common fuel used here is gasoline which is a mixture of hydrocarbons, and an octane rating is given for standardization. Therefore, we will approximate gasoline as being composed of the hydro- carbon, octane, C8H18. Another fuel that is common in racing applications is nitromethane, CH3NO2, used in top fuel dragsters.

Let’s consider the combustion of each fuel with oxygen in an internal combustion engine. Let the 8-cylinder engine displacement be 429 in3 total.

GOAL: Determine and compare the maximum enthalpy of reaction, ∆Hrxn, for the combustion of both octane and nitromethane in a single cylinder of the engine. This means determine the maximum amount of fuel that can burn in the cylinder filled with 1 atm of air and the amount of heat that reaction would generate.

Here are some suggested steps to guide you along. You will have to look up some data in your book or on the internet.

1. Write a balanced chemical equation for the reaction of octane and nitromethane with O2 gas, assuming complete combustion. For octane, the products would be CO2 and H2O. For nitromethane, the products would be CO2, H2O, and N2.

2. Determine ∆Hrxn, at 298 K, for both in kJ/mol fuel. Which is greater?

3. Determine the amount of O2 gas available for the reactions in a single cylinder. Assume T = 298 K.

4. Determine how much fuel can be reacted with the amount of O2 in the cylinder.

5. Determine the heat released, ∆H at 298 K, for burning that much fuel for both octane and nitromethane.

Which fuel gives the highest total heat output and therefore greatest engine perfomance?

Answer 1

1)    The balanced chemical equations:

2) 2. Determine ∆Hrxn, at 298 K, for both in kJ/mol fuel. Which is greater?

Enthalpy of octane =

Enthalpy of nitromethane=

The enthalpy of nitromethane is greater

3) Determine the amount of O2 gas available for the reactions in a single cylinder. Assume T = 298 K.

429 In3 = 7.03005 lts, PV=nRT

4. Determine how much fuel can be reacted with the amount of O2 in the cylinder.

0.08% of octane can react with O2

1.3% of nitromethane can react with O2

5. Determine the heat released, ∆H at 298 K, for burning that much fuel for both octane and nitromethane.

So octane can give highest total heat output and therefore greatest engine performance