Internal Combustion Engines
An internal-combustion engine is a heat engine that burns fuel and air
inside a combustion chamber located within the engine proper. Simply stated, a
heat engine is an engine that converts heat energy to mechanical energy. The
internal- combustion engine should be distinguished from the external-
combustion engine, for example, the steam engine and the Stirling engine, which
burns fuel outside the prime mover, that is, the device that actually produces
mechanical motion. Both basic types produce hot, expanding gases, which may then
be employed to move pistons, turn turbine rotors, or cause locomotion through
the reaction principle as they escape through the nozzle.
Most people are familiar with the internal-combustion reciprocating engine,
which is used to power most automobiles, boats, lawn mowers, and home generators.
Based on the means of ignition, two types of internal-combustion reciprocating
engines can be distinguished: spark-ignition engines and compression-ignition
engines. In the former, a spark ignites a combustible mixture of air and fuel;
in the latter, high compression raises the temperature of the air in the chamber
and ignites the injected fuel without a spark. The diesel engine is a
compression-ignition engine. This article emphasizes the spark-ignition engine.
The invention and early development of internal-combustion engines are
usually credited to three Germans. Nikolaus Otto patented and built (1876) the
first such engine; Karl Benz built the first automobile to be powered by such an
engine (1885); and Gottlieb Daimler designed the first high-speed internal-
combustion engine (1885) and carburetor. Rudolf Diesel invented a successful
compression-ignition engine (the diesel engine) in 1892.
The operation of the internal-combustion reciprocating engine employs
either a four-stroke cycle or a two-stroke cycle. A stroke is one continuous
movement of the piston within the cylinder.
In the four-stroke cycle, also known as the Otto cycle, the downward
movement of a piston located within a cylinder creates a partial vacuum. Valves
located inside the combustion chamber are controlled by the motion of a camshaft
connected to the crankshaft. The four strokes are called, in order of sequence,
intake, compression, power, and exhaust. On the first stroke the intake valve is
opened while the exhaust valve is closed; atmospheric pressure forces a mixture
of gas and air to fill the chamber. On the second stroke the intake and exhaust
valves are both closed as the piston starts upward. The mixture is compressed
from normal atmospheric pressure (1 kg/sq cm, or 14.7 lb/sq in) to between 4.9
and 8.8 kg/sq cm (70 and 125 lb/sq in). During the third stroke the compressed
mixture is ignited–either by compression ignition or by spark ignition. The
heat produced by the combustion causes the gases to expand within the cylinder,
thus forcing the piston downward. The piston’s connecting rod transmits the
power from the piston to the crankshaft. This assembly changes reciprocating–in
other words, up-and-down or back-and-forth motion–to rotary motion. On the
fourth stroke the exhaust valve is opened so that the burned gases can escape as
the piston moves upward; this prepares the cylinder for another cycle.
Internal-combustion spark-ignition engines having a two-stroke cycle combine
intake and compression in a single first stroke and power and exhaust in a
The internal-combustion reciprocating engine contains several subsystems:
ignition, fuel, cooling, and exhaust systems.
The ignition system of a spark-ignition engine consists of the sparking
device (the spark plug); the connecting wire from the plug to the distributor;
and the distributor, which distributes the spark to the proper cylinder at the
proper time. The distributor receives a high-energy spark from a coil, or
magneto, that converts low-voltage energy to high-voltage energy. Some ignition
systems employ transistorized circuitry, which is generally more efficient and
less troublesome than the mechanical breaker-point system used in the past. Most
ignition systems require an external electrical energy source in the form of a
battery or a magneto.
Spark-ignition engines require a means for mixing fuel and air. This may be
either a carburetor or fuel injection. A carburetor atomizes the fuel into the
engine’s incoming air supply. The mixture is then vaporized in the intake
manifold on its way to the combustion chamber. fuel injection sprays a
controlled mist of fuel into the airstream, either in the intake manifold or
just before the intake valve or valves of each cylinder. Both carburetors and
fuel injectors maintain the correct fuel- to-air ratio, about one part fuel to
fifteen parts air, over a wide range of air temperatures, engine speeds, and
loads. Fuel injection can compensate for changes in altitude as well.
Internal-combustion engines require some type of starting system. Small
engines are generally started by pulling a starting rope or kicking a lever.
Larger engines may use compressed air or an electric starting system. The latter
includes a starter–a high-torque electric motor–to turn the crankshaft until
the engine starts. Starting motors are extremely powerful for their size and are
designed to utilize high currents (200 to 300 amperes). The large starting
currents can cause a battery to drain rapidly; for this reason a heavy- duty
battery is usually used. Interrupting this connection is an electrical switch
called a solenoid, which is activated by the low- voltage starting switch. In
this way the ignition switch can be located away from the starter and yet still
turn the starter on and off.
The cooling system is important because internal-combustion engines operate
at high temperatures of combustion–spark- ignition engines at approximately
2,760 degrees C (5,000 degrees F) and diesel engines at even higher temperatures.
If it were not for the cooling system, these high temperatures would damage and
melt many parts of the engine. The cooling system essentially dissipates the
heat of combustion in metal, water, or air and automatically regulates the
temperature so that the engine can operate at its optimum temperature–about 93
degrees C (200 degrees F).
Air-cooled engines, popularly used to power small lawn mowers, chain saws,
power generators, and motorcycles, as well as small cars and airplanes, often
require no moving parts, and therefore little or no maintenance, for the cooling
system. The head, or uppermost part, of the cylinder and the cylinder block have
fins cast into them; these fins increase the surface exposed to the surrounding
air, allowing more heat to be radiated. Usually a cover or shroud channels the
air flow over the fins. A fan is sometimes included if the engine is located
away from a stream of fast-moving air.
Water-cooled engines have water jackets built into the engine block. These
jackets surround the cylinders. Usually a centrifugal water pump is used to
circulate the water continuously through the water jackets. In this way the high
heat of combustion is drawn off the cylinder wall into the circulating water.
The water must then be cooled in a radiator that transfers the heat energy of
the water to the radiator’s cooler surrounding fluid. The surrounding fluid can
be air or water, depending on the application of the engine.
Internal-combustion engines include an exhaust system, which allows the hot
exhaust gases to escape efficiently from the engine. In some small engines the
exhaust gases can exit directly into the atmosphere. Larger engines are noisier
and require some type of muffler or sound deadener, usually a canister with an
inner shell that breaks up the sound waves, dissipating their energy within the
muffler before the exhaust gases are permitted to escape.
The power capacity of an engine depends on a number of characteristics,
including the volume of the combustion chamber. The volume can be increased by
increasing the size of the piston and cylinder and by increasing the number of
cylinders. The cylinder configuration, or arrangement of cylinders, can be
straight, or in-line (one cylinder located behind the other); radial (cylinders
located around a circle); in a V (cylinders located in a V configuration); or
opposed (cylinders located opposite each other). Another type of internal-
combustion engine, the Wankel engine, has no cylinders; instead, it has a rotor
that moves through a combustion chamber.
An internal-combustion engine must also have some kind of transmission
system to control and direct the mechanical energy where it is needed; for
example, in an automobile the energy must be directed to the driving wheels.
Since these engines are not able to start under a load, a transmission system
must be used to “disengage” the engine from the load during starting and then to
apply the load when the engine reaches its operating speed.