Putting some numbers on the diesel engine

Diesel engines convert the chemical energy in fuel to mechanical energy. Energy is released in a series of combustions as fuel reacts with oxygen from the air. The chemical equation of diesel fuel combustion is 4 C12H23 +71 O2 –> 48 CO2 + 46 H­2O. Combustion reactions are spontaneous yielding a -∆G. The reaction goes from 71 moles of O2 gas to 48 moles of CO2 yielding a -∆S. Combustion reactions break bonds between the molecules signaling an exothermic reaction or -∆H.

To find the value of ∆G, we will use the values 50.1 kJ/mol for C12H23, 0 kJ/mol for O2, -393.5 kJ/mol for CO2, and -229 kJ/mol for H­2O. To solve for ∆G, we calculate 46(-229) + 48(-394) – 4(50.1) which equates to -29622 kJ, a -∆G for a spontaneous reaction. To find the value of ∆S, we can use the equation ∆G = ∆H – T∆S. Using the -56,000 kJ for  ∆H  from our prior blog post,-29622 kJ for ∆G, and the 483 K ignition temperature of diesel fuel we can calculate the ∆S. This equates to -54 kJ, a -∆S.

Steve Lynch


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