Green Chemistry and the Ten Commandments

Spark plug

Exhaust

valve

Intake

valve

Piston

Crankshaft

Figure 6.8. An internal combustion piston engine in which a very rapidly burning mixture of air and
fuel drives a piston downward during the power stroke and this motion is converted to rotary mechanical
motion by the crankshaft.

The efficiency of an internal combustion engine in converting heat from burning
fuel into mechanical motion is described by the Carnot equation, where T 1 is the peak
temperature of the highly compressed combustion gas at the beginning of the power
stroke and T 2 is the lower temperature at the bottom of the stroke. Overall, a typical
gasoline engine converts about 25% of the energy in the gasoline to mechanical energy.
You may know that diesel engines get much better mileage than gasoline engines, and
U.S. tourists are thankful for the efficient diesel automobiles that they can rent for
European travel after seeing the exorbitant prices of fuel in Europe. The reason that
the diesel engine is about 50% more efficient than a comparable gasoline engine is that
a diesel engine compresses the air in the cylinder to a very small volume; the diesel
engine has a compression ratio of about 16:1. Fuel is injected into the cylinder at the top
of the compression stroke, and the compressed gases are so hot that they ignite without
requiring a spark plug for ignition. This extreme compression results in a very high peak
temperature, the engine follows Carnot’s equation, and the fuel efficiency is relatively
much higher than that of a gasoline engine.

6.6. Green Engineering and Energy Conversion Efficiency

All agree that increased efficiency of energy conversion and utilization is desirable.
During the last century enormous advances have been made in the efficiency of energy
utilization. The early fossil-fueled electrical power generating plants from around
1900 were only about 4% efficient in converting chemical energy to electrical energy;
modern ones exceed 40%. The change from picturesque steam locomotives to bland, but
efficient diesel locomotives that took place during the 1940s and 1950s resulted in an
approximately four-fold increase in the energy efficiency of rail transport.

Chap. 6. Energy Relationships 147