Concepts In Depth: Heat Engines and Internal-Combustion Engines

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CONCEPTS IN DEPTH

Heat Engines A refrigerator uses mechanical work to create a difference in temperature and thus transfers energy as heat. A heat engine is a device that does the opposite. It uses heat to do mechanical work. One analogy for a heat engine is a water wheel. A water wheel uses the energy of water falling from one level above Earth’s surface to another. The change in potential energy increases the water’s kinetic energy so that the water can do work on one side of the wheel and thus turn it. The greater the difference between the initial and final values of the potential energy, the greater the amount of work that can be done. Instead of using the difference in potential energy to do work, heat engines do work by transferring energy from a high-temperature substance to a lowertemperature substance, as indicated for the steam engine shown in Figure 1. Figure 1: A heat engine is able to do work (b) by transferring energy from a high-temperature substance (the boiler) at Th (a) to a substance at a lower temperature (the air surrounding the engine) at Tc (c). (a)

Th

(a)

Work done Heat engine (b)

(b)

(c) (c)

Tc

For each complete cycle of the heat engine, the net work done will equal the difference between the energy transferred as heat from a high-temperature substance to the engine and the energy transferred as heat from the engine to a lower-temperature substance. The larger the difference between the amount of energy transferred as heat into the engine and out of the engine, the more work the engine can do. 1 of 3

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CONCEPTS IN DEPTH

Heat Engines (continued) Internal Combustion Engines Are Heat Engines The internal-combustion engine found in most vehicles is an example of a heat engine. Internal-combustion engines burn fuel within a closed chamber (the cylinder). The potential energy of the chemical bonds in the reactant gases is converted to kinetic energy of the particle products of the reaction. These gaseous products push against a piston and thus do work on the environment (in this case, a crankshaft that transforms the linear motion of the piston to the rotational motion of the axle and wheels). Although the basic operation of any internal-combustion engine resembles that of an ideal heat engine, certain steps do not fit the model. When gas is taken in or removed from the cylinder, matter enters or leaves the system so that matter in the system is not isolated. No heat engine operates perfectly. Only part of the available internal energy leaves the engine as work done on the environment; most of the energy is removed as heat. In Figure 2 on the following page, the steps in one cycle of operation for a gasoline engine (one type of internal-combustion engine) are shown. • During compression, shown in (a), work is done by the piston as it compresses the fuel-and-air mixture in the cylinder without transferring energy as heat. Once maximum compression of the gas is reached, combustion takes place. • The chemical potential energy released during combustion increases the internal energy of the gas, as shown in (b). • The hot, high-pressure gases from the combustion reaction expand in volume, which causes the piston to do work on the crankshaft, as shown in (c). • Once all of the work is done by the piston, energy is transferred as heat through the walls of the cylinder and by the physical transfer of the hot exhaust gases from the cylinder, as shown in (d). • A new fuel-air mixture is then drawn through the intake valve into the cylinder by the downward-moving piston, as shown in (e).

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Heat Engines (continued) Spark plug fires

Spark plug Intake valve closed

Exhaust valve closed

Fuelair mixture

Cylinder

Expanding combustionproduct gases

(b) Ignition

Piston Connecting rod Crankshaft (a) Compression

(c) Expansion

Intake valve open

Exhaust valve closed

Intake valve closed

Exhaust valve open

Combustionproduct gases

Fuelair mixture

(d) Exhaust

(e) Fuel intake

Figure 2: The steps above show one complete cycle of a gasoline engine.

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