Fuel Thermochemistry

Thermochemistry and One Ignition Cycle:

Thermochemistry deals with energy, heat and work. Much of what we study deals with static systems in which no change is taking place. But we are not content with an automobile or truck that just sits in the driveway, so this discussion deals with situations undergoing change.

The change in energy of a system that is going through any process is the sum of the heat added TO the system and the work done ON the system.

delta E = delta q + delta w

If heat flows from the system and work is done by the system as in the case of fuel combustion, then both delta q and delta e are negative and delta E is likewise negative. This situation expresses the conversion of the potential or stored energy of the fuel converted to heat and kinetic energy and transferred from the system to its surroundings.

Consider one cylinder of that Dodge RAM truck outside. In one ignition in which a small amount of fuel is injected into the cylinder, is compressed to a smaller volume and ignited with a spark in the presence of air, we see both the heat and work that results. The engine gets hot and the cylinder moves back down and the resulting work drives the truck. Heat is added to the system and work is done by the system - the energy of the system - the engine - changes.

Where did the energy come from? What was the origin of both heat and work. The ignition of the gasoline in the cylinder caused a rapid chemical reaction with the conversion of the hydrocarbon fuel in the presence of oxygen to carbon dioxide and water with the release of energy. The standard for the measurement of energy - different from the methods of using energy - is called standard enthalpy. Standard enthalpy is the heat given off or required for a chemical reaction carried out on one mole of the substance under standard conditions - that is 25 degrees Celsius and one atmosphere pressure.

Thus for octane - one component of the mixture of hydrocarbons called gasoline, the combustion of one mole - about 114g - follows the equation below and the standard enthalpy of combustion is -1307kcal/mol.

C₈H₁₈ + 12.5O₂ --> 8CO₂ + 9H₂O delta H_c = -1307kcal/mole

Please understand this standard enthalpy is a measurement of the energy available. No more no less. But this is confusing, isn't it? We said energy was the sum of heat given off and work done. How then can we measure energy by determining heat effects and ignoring the work. The reason is that we define work as a force acting over a distance.

·         Gas expanding in a cylinder, against a constant resisting pressure, is work.

·         A gas expanding into the atmosphere with no restraint is not work.

In the absence of work, all the energy appears as heat. We can measure heat rather well, so standard enthalpies are collections of heat measurements done under the conditions of no work. But they are measurements of energy none the less.

So thermochemistry is limiting. There is only so much energy available for each mole of combustible material. You might look up the standard enthalpy for hexane and find delta H_c = -997kcal/mol. Octane might look to more energy producing than hexane, but in practice, we use a fuel by weight or volume, not by mole. So what is important is the energy per gram:

·         Octane: delta H_c = -1307kcal/mole/114g/mol = 11.5kcal/g

·         Hexane: delta H_c = -997kcal/mole/86g/mol = 11.6kcal/g

Energy from fuels is limiting. We can obtain no more than about 11-12 kcal/g from fuel combustion. If we are heating a home with this fuel, we want all of that energy as heat, and as we have seen, burning in the atmosphere without capturing the exhaust gases does just that.

But in an auto we want to maximize the work, not the heat. We know that energy is released as heat AND work. We will define how we maximize work and why the early auto engineers wanted to make high compression engines in the section on Fuel Thermodynamics.

1. Thermochemistry is the relationship between chemical reactions and energy changes.

2. Thermochemical and other data on elements and compounds is catalogued and reported by the National Institute of Science and Technology (NIST).