# Lesson 8 - Combustion engine or solar cell?

In the previous lesson you have examined several different types of fuel. In this lesson you will take a closer look at the process in which the chemical energy that is contained within the fuels, is converted into mechanical energy. This is the last part of the overall process, as shown in figure 35. In general two methods are available for this conversion process. These are the internal combustion engine and the fuel cell.

The internal combustion engine

The combustion engine can convert fuel into motion of the car. It does this by burning the fuel inside the engine, hence the internal combustion engine. Two types of engines can be distinguished: the diesel engine and the gasoline engine. The working principle of both types of engines is that a small amount of fuel is injected into a combustion chamber. After ignition of the fuel, combustion gasses that are formed, force the piston to move. The movement of this piston is converted into movement of the car.

The fuel cell

In a galvanic cell, chemical energy is converted into electrical energy. Both batteries and fuel cells are types of galvanic cells. The fuel cell differs from the battery, however, in that the fuel is continuously supplied. This creates an advantage over a battery as it does not have to be recharged but refuelled.

In lesson 5 you have seen that some substances can be decomposed into other substances by means of electrolysis. In the case of water, hydrogen and oxygen were produced by sending an electrical current through the water. Two half reactions occurred, one at each electrode. In the same chapter you have seen the reverse reaction, the combustion of hydrogen gas. Although it might not be obvious from the combustion reaction, an electrical current can be produced by this reaction as well. Just as in electrolysis, two half reactions occur in the combustion reaction. By physically separating these two half reactions the electrons that are released in one half reaction can be send through an external wire, thus creating an electrical current, to the other half reaction in which the electrons are absorbed. In figure 36 this process is schematically shown. The (half) reactions that take place in this fuel cell are:

The first half reaction will occur at the catalyst coated surface at the left side of the fuel cell, indicated by the left blue area. The created H+ ions (protons), indicated by + , migrate through the membrane to the other catalyst coated surface on the right side of the fuel cell. The electrons, however, indicated by -, are transported through the electrical wire on the top, where they can deliver electrical energy before arriving at the catalyst on the right. There, the oxygen molecules, hydrogen ions and electrons meet and are combined to form water.

The fuel cell in figure 36 is only one out of many types of fuel cells being developed and produced. All these types of fuel cells differ in catalyst, operating temperature, type of fuel required etc.

Ethanol can be used in both a gasoline engine and a fuel cell. In the first it is combusted directly, while in the fuel cell the half reactions are physically separated.An internal combustion engine costs about €20 - €30 per kW. The price of a fuel cell is much higher at the moment. For more information: www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_challenges.html.