William Kovarik
Radford University
and
Matthew E. Hermes
Kennesaw State University

Fuels and Society: c. Fuel Thermodynamics

Back to b. Octane and Cracking

Chemical Principles

Back to Concept Map

What Chemistry Teaches Us About High Compression Engines

We are trying to identify the chemistry behind the engineering of effficient, powerful automobile engines. In actuality, the early automotive engineers such as Charles Kettering, knew from their first trials that power and efficiency in an engine improved as the gas-air mixture was compressed to higher pressures at ignition. But as the engineers built higher compression ratios they began to hear unwelcome "knocking" caused by uneven gas-air combustion. The knocking reduced the power of the explosion and could damage the engine itself. Kettering and others, including Thomas Midgley considered alcohol as a complete replacement fuel for gasoline because it combusted at high compression without the knocking. But they also sought a low concentration fuel "antiknock" additive for gasoline.

Well, fundamental chemical principles do guide the designers. These principles are both limiting and enabling. Thermochemistry limits by showing that there is only so much energy is available for moving the car down the road. Thermodynamics enables by showing us how to maximize the energy available for doing the work of propulsion.

Thermodynamics and the Compression Ratio:Thermodynamics deals with the relationship of heat and work. In this unit we remind ourselves that fundamental considerations of the Second Law of Thermodynamics lead us to a concept called Free Energy And Free Energy leads to one understanding why high vs. low compression in an automobile engine results in higher engine efficiency.

What is the Second Law of Thermodynamics? In any spontaneous process there is always an increase of entropy in the universe (system plus surroundings). A process is spontaneous if the result is an increase in disorder. Cigarette smoke, from the tip of a lit cigarette can be detected from room to room, across the restaurant, in seconds. This is because the odiferous combustion molecules spontaneously distribute themselves to a state of disorder. You cannot spontaneously reverse the process and force them all back into the cigarette. The process goes only one way.

What is entropy? Entropy is the property that measures this disorder. It cannot be observed or measured as a physical property such as temperature or pressure or mass. But it can be determined for system and surrounding.

In the surroundings, changes in entropy are governed by the heat flow into or out of the surroundings and by the temperature at which this energy flows. For a chemical reaction at constant temperature;

delta S_surroundings = -delta H_system/T^oK

For an exothermic reaction, delta H is negative, so delta S will be positive. But the higher the temperature of the process, the lower the impact of the heat flow in changing the entropy. Systems already at a high temperature are less affected by increase in heat energy than those at low temperature.

In the system, the change in entropy depends on increase or decrease of disorder in the process. Chemical processes that produce more moles of gas than exist as starting materials will show marked increase in entropy. Those that remain as liquids or solids will show a much smaller effect since the state of randomness for gas is so much higher than that for liquid or gas.

The change in entropy for the system cannot be be expressed the same way as that for the surroundings because of the involvement of changes in molecular disorder. For a system:

delta S_system is not equal to delta H_system/T^oK

as it is for the surroundings. To further complicate, system entropy changes cannot be calculated directly.

What is Free Energy? J. Willard Gibbs understood entropy and that entropy changes and enthalpy were related in a system undergoing chemical reactions. He reasoned that in the chemical process, some of the energy from the system represented the energy free to do work, the rest of the energy was due simply to the heat required or generated. Gibbs postulated a new state function, Free Energy (G), the energy available to do work, as:

G = H - TS

And for a process undergoing change,

delta G = delta H - Tdelta S

The convention is that whan delta G is negative, the process is spontaneous and can do work. The Gibbs equation implies that if we want a lot of work, we look for a high negative delta H and a decrease in entropy, or as small an increase as possible.

What do we mean when we say a higher efficiency? It means different things in different situations. If we are seeking to heat our home, we want all the energy from the burning of the heating fuel - gas, oil, coal, wood, to be converted to heat. And we want that heat to be delivered to the rooms of the house and not up the chimney. The first desire is a thermodynamics problem.. If we do no work, then all the energy will appear as heat.

But in an automobile engine we want as little heat as possible; we want the potential energy to be converted to work. After a long drive, when we lift the hood of the car and feel the heat rising into our face, we should recognize we are observing the inefficient application of energy for work. We will see we can improve this situation, but not completely alleviate it. There will always be a large of loss of potential energy to heat.

Why is a high compression engine more efficient? Because it does more work for each molecule of fuel burned. Look at the chemical reaction for the oxidation or combustion of octane:

C₈H_18(g) + 12.5O_2(g) --> 8CO₂(g) + 9H₂O(g) delta H_c = -1307kcal/mole

In an ideal case in an automobile cylinder, the fuel is vaporized to a gas and ignited in the presence of air. Air consists of 20% oxygen, 79% nitrogen and 1% argon. The nitrogen and argon do not react to any great extent.

Let us ignore the temperature in the cylinder for a moment. The gases are injected, the cylinder compresses them and they ignite to form products as shown releasing 1307kcal/mole of energy. There is an increases in the number of moles of gas from 13.5 to 17 from the reaction. (Remember when we brought in 12.5 moles of oxygen we also brought in about 50 moles of nitrogen and argon for a total of 63.5 moles in the initial state). Thus, from this alone we can judge the entropy or level of disorder increases in this chemical reaction. If the compression ratio was, let us say, 5 to 1, the pressure was 5 atmospheres before the detonation; after the detonation we have 67 moles of gas, the pressure has increased somewhat from the production of new gas molecules. Disorder increases, entropy increases , delta S is positive, from the new moles of gas. And so the Free Energy (G) available to do work is reduced by the value of T delta S.

But in an alternate case, if the compression ratio is 8:1, the pressure is higher initially and increases with new moles of gas, but the actual increase in entropy is less because there is less disorder at 8 atmospheres than at 5 atmospheres. The molecules have less freedom, the increase of T delta S is less than for the first case. The value of delta G is more negative, the system can do more work.

High Compression Engines: Engines are complex systems that never approach chemical or thermal equilibrium. Automotive engineers make difficult calculations in which the heat and temperature and compression and cooling effects cannot be so readily simplified. The above explanation is a gross simplification of one aspect of the role of chemical principles in engine design. You may well look at the above explanation and say it doesn't look as if the slight decrease in entropy change will make much difference in the work available. But consider this. At best, gasoline engines will have efficiencies of conversionj of energy to work of less than 30%. So any advantage taken is a true gain.

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