Energy Content

• Energy density measures how much energy a fuel stores per unit volume. If we compare two fuels, for example, we could ask how much energy we obtain by burning a gallon each of them. Gases like hydrogen have variable density, because the mass per unit volume varies widely with temperature and pressure; consequently, another way to compare energy content is in terms of specific energy, or energy per unit mass. A good fuel has a high energy content both in terms of specific energy and energy density. This makes it easier to transport energy in the form of the fuel.
  
  

Octane Rating

• Internal combustion engines can exhibit undesirable behavior called "knocking," where pockets of the gas-air mixture in the cylinder do not ignite at the right time. Some fuels are more prone to this behavior than others. Consequently, fuel is often characterized in terms of the octane rating, where its propensity to cause knocking is compared to a mixture of heptane and iso-octane. (A chemist would call this second molecule 2,2,4-trimethyloctane). A fuel that exhibits the same anti-knocking capability as a mixture of 50 percent iso-octane and 50 percent heptane would have a 50 octane rating. Good fuels have higher octane ratings.
  
  

Volatility

• Volatility measures a liquid's tendency to evaporate. Another similar measurement is vapor pressure, which is the partial pressure of the vapor inside a closed container of the liquid at room-temperature equilibrium. A highly volatile liquid, for example, evaporates readily and hence has a higher vapor pressure at room temperature. It is desirable that liquid fuels be somewhat volatile so they can mix with air when sprayed into the intake manifold, but not so volatile that they exist almost entirely in the gas phase, since this makes them more difficult to store.
  
  

Corrosion

• Another consideration is corrosion and/or other effects the fuel might have on the system. Alcohols, for example, mix well with water. At high concentrations, some of the water may separate from the alcohol and pool in the tank or elsewhere in the engine, accelerating rusting. Ethanol can also increase risk of degradation of rubber or aluminum surfaces in the fuel engine, while gasoline can be corrosive to some plastics. Ideally, good chemical fuels do not corrode or damage fuel systems.
  
  

Speed of Reaction

• Some reactions take place very slowly -- like rust forming on exposed iron. Others take place very rapidly -- like the detonation of TNT. Chemical reactions involve breaking bonds between atoms in existing molecules and formation of new bonds between atoms. The first step, breaking bonds, always requires energy, while the second step releases energy. If more energy is released through formation of new bonds than is consumed in breaking the old bonds, the result is a net release of energy. This is what happens when fuels burn in air. It still takes energy to make the initial steps of the reaction happen, however, and this initial energy barrier or activation barrier explains why you need to supply heat or a spark to make wood or gasoline burn.
  
  

Activation Energy

• If the activation barrier is too high, we have to supply a huge amount of energy to make the reaction take place, so this is undesirable. If the activation barrier is too low, however, the fuel detonates before we want it to burn, or it's unstable and dangerously explosive, so this isn't desirable either. We want an activation barrier that's in between these two extremes.