Essay/Term paper: Internal combustion engines

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Internal Combustion Engines

Introduction

Internal Combustion Engine, a heat engine in which the fuel is burned (
that is, united with oxygen ) within the confining space of the engine itself.
This burning process releases large amounts of energy, which are transformed
into work through the mechanism of the engine. This type of engine different
from the steam engine, which process with an external combustion engine that
fuel burned apart from the engine. The principal types of internal combustion
engine are : reciprocating engine such as Otto-engine, and Diesel engines ; and
rotary engines, such as the Wankel engine and the Gas-turbine engine.
In general, the internal combustion engine has become the means of
propulsion in the transportation field, with the exception of large ships
requiring over 4,000 shaft horsepower ( hp).
In stationary applications, size of unit and local factor often
determine the choice between the use of steam and diesel engine. Diesel power
plants have a distinct economic advantage over steam engine when size of the
plant is under about 1,000 hp. However there are many diesel engine plants much
large than this. Internal combustion engines are particularly appropriate for
seasonal industries, because of the small standby losses with these engines
during the shutdown period.

History

The first experimental internal combustion engine was made by a Dutch
astronomer, Christian Huygens, who, in 1680, applied a principle advanced by
Jean de Hautefeuille in 1678 for drawing water. This principle was based on the
fact that the explosion of a small amount of gunpowder in a closed chamber
provided with escape valves would create a vacuum when the gases of combustion
cooled. Huygens, using a cylinder containing a piston, was able to move it in
this manner by the external atmospheric pressure.
The first commercially practical internal combustion engine was built by
a French engineer, ( Jean Joseph ) Etienne Lenoir, about 1859-1860. It used
illuminating gas as fuel. Two years later, Alphonse Beau de Rochas enunciated
the principles of the four-stroke cycle, but Nickolaus August Otto built the
first successful engine ( 1876 ) operating on this principle.

Reciprocating Engine

Components of Engines

The essential parts of Otto-cycle and diesel engines are the same. The
combustion chamber consists of a cylinder, usually fixed, which is closed at one
end and in which a close-fitting piston slides. The in-and-out motion of the
piston varies the volume of the chamber between the inner face of the piston and
the closed end of the cylinder. The outer face of the piston is attached to a
crankshaft by a connecting rod. The crankshaft transforms the reciprocating
motion of the piston into rotary motion. In multi-cylindered engines the
crankshaft has one offset portion, called a crankpin, for each connecting rod,
so that the power from each cylinder is applied to the crankshaft at the
appropriate point in its rotation. Crankshafts have heavy flywheels and
counterweights, which by their inertia minimize irregularity in the motion of
the shaft. An engine may have from 1 to as many as 28 cylinders.
The fuel supply system of an internal-combustion engine consists of a
tank, a fuel pump, and a device for vaporizing or atomizing the liquid fuel. In
Otto-cycle engines this device is a carburetor. The vaporized fuel in most
multi-cylindered engines is conveyed to the cylinders through a branched pipe
called the intake manifold and, in many engines, a similar exhaust manifold is
provided to carry off the gases produced by combustion. The fuel is admitted to
each cylinder and the waste gases exhausted through mechanically operated poppet
valves or sleeve valves. The valves are normally held closed by the pressure of
springs and are opened at the proper time during the operating cycle by cams on
a rotating camshaft that is geared to the crankshaft . By the 1980s more
sophisticated fuel-injection systems, also used in diesel engines, had largely
replaced this traditional method of supplying the proper mix of air and fuel;
computer-controlled monitoring systems improved fuel economy and reduced
pollution.

Ignition

In all engines some means of igniting the fuel in the cylinder must be
provided. For example, the ignition system of Otto-cycle engines , the mixture
of air and gasoline vapor delivered to the cylinder from the carburetor and next
operation is that of igniting the charge by causing a spark to jump the gap
between the electrodes of a spark plug, which projects through the walls of the
cylinder. One electrode is insulated by porcelain or mica; the other is grounded
through the metal of the plug, and both form the part of the secondary circuit
of an induction system.
The principal type of high-tension ignition now commonly used is the
battery-and-coil system. The current from the battery flows through the low-
tension coil and magnetizes the iron core. When this circuit is opened at the
distributor points by the interrupter cam, a transient high-frequency current is
produced in the primary coil with the assistance of the condenser. This induces
a transient, high-frequency, high-voltage current in the secondary winding. This
secondary high voltage is needed to cause the spark to jump the gap in the spark
plug. The spark is directed to the proper cylinder to be fired by the
distributor, which connects the secondary coil to the spark plugs in the several
cylinders in their proper firing sequence. The interrupter cam and distributor
are driven from the same shaft, the number of breaking points on the interrupter
cam being the same as the number of cylinders.

Cooling System

Because of the heat of combustion, all engines must be equipped with
some type of cooling system. Some aircraft and automobile engines, small
stationary engines, and outboard motors for boats are cooled by air. In this
system the outside surfaces of the cylinder are shaped in a series of radiating
fins with a large area of metal to radiate heat from the cylinder. Other engines
are water-cooled and have their cylinders enclosed in an external water jacket.
In automobiles, water is circulated through the jacket by means of a water pump
and cooled by passing through the finned coils of a radiator. Some automobile
engines are also air-cooled, and in marine engines sea water is used for cooling.


Starter

Unlike steam engines and turbines, internal-combustion engines develop
no torque when starting, and therefore provision must be made for turning the
crankshaft so that the cycle of operation can begin. Automobile engines are
normally started by means of an electric motor or starter that is geared to the
crankshaft with a clutch that automatically disengages the motor after the
engine has started. Small engines are sometimes started manually by turning the
crankshaft with a crank or by pulling a rope wound several times around the
flywheel. Methods of starting large engines include the inertia starter, which
consists of a flywheel that is rotated by hand or by means of an electric motor
until its kinetic energy is sufficient to turn the crankshaft, and the explosive
starter, which employs the explosion of a blank cartridge to drive a turbine
wheel that is coupled to the engine. The inertia and explosive starters are
chiefly used to start airplane engines.

Otto-Cycle Engines

The ordinary Otto-cycle engine is a four-stroke engine; that is, its
pistons make four strokes, two toward the head (closed head) of the cylinder and
two away from the head, in a complete power cycle. During the first stroke of
the cycle, the piston moves away from the cylinder head while simultaneously the
intake valve is opened. The motion of the piston during this stroke sucks a
quantity of a fuel and air mixture into the combustion chamber. During the next
stroke the piston moves toward the cylinder head and compresses the fuel mixture
in the combustion chamber. At the moment when the piston reaches the end of this
stroke and the volume of the combustion chamber is at a minimum, the fuel
mixture is ignited by the spark plug and burns, expanding and exerting a
pressure on the piston, which is then driven away from the cylinder head in the
third stroke. At the end of the power stroke the pressure of the burned gases in
the cylinder is 2.8 to 3.5 kg/sq. cm (40 to 50 lb./sq. in). During the final
stroke, the exhaust valve is opened and the piston moves toward the cylinder
head, driving the exhaust gases out of the combustion chamber and leaving the
cylinder ready to repeat the cycle.
The efficiency of a modern Otto-cycle engine is limited by a number of
factors, including losses by cooling and by friction. In general the efficiency
of such engines is determined by the compression ratio of the engine. The
compression ratio (the ratio between the maximum and minimum volumes of the
combustion chamber) is usually about 8 to 1 or 10 to 1 in most modern Otto-cycle
engines. Higher compression ratios, up to about 12 to 1, with a resulting
increase of efficiency, are possible with the use of high-octane antiknock fuels.
The efficiencies of good modern Otto-cycle engines range between 20 and 25
percent (in other words, only this percentage of the heat energy of the fuel is
transformed into mechanical energy).

Diesel Engines

Theoretically the diesel cycle differs from the Otto cycle in that
combustion takes place at constant volume rather than at constant pressure. Most
diesels are also four-stroke engines, but operate differently than the four-
stroke Otto-cycle engines. The first or suction stroke draws air, but no fuel,
into the combustion chamber through an intake valve. On the second or
compression stroke the air is compressed to a small fraction of its former
volume and is heated to approximately 440° C (approximately 820° F) by this
compression. At the end of the compression stroke vaporized fuel is injected
into the combustion chamber and burns instantly because of the high temperature
of the air in the chamber. Some diesels have auxiliary electrical ignition
systems to ignite the fuel when the engine starts, and until it warms up. This
combustion drives the piston back on the third or power stroke of the cycle. The
fourth stroke, as in the Otto-cycle engine, is an exhaust stroke.
The efficiency of the diesel engine, which is in general governed by the
same factors that control the efficiency of Otto-cycle engines, is inherently
greater than that of any Otto-cycle engine and in actual engines today is
slightly over 40 percent. Diesels are in general slow-speed engines with
crankshaft speeds of 100 to 750 revolutions per minute (rpm) as compared to 2500
to 5000 rpm for typical Otto-cycle engines. Some types of diesel, however, have
speeds up to 2000 rpm. Because diesels use compression ratios of 14 or more to 1,
they are generally more heavily built than Otto-cycle engines, but this
disadvantage is counterbalanced by their greater efficiency and the fact that
they can be operated on less expensive fuel oils.

Two-Stroke Engines

By suitable design it is possible to operate an Otto-cycle or diesel as
a two-stroke or two-cycle engine with a power stroke every other stroke of the
piston instead of once every four strokes. The efficiency of such engines is
less than that of four-stroke engines, and therefore the power of a two-stroke
engine is always less then half that of a four-stroke engine of comparable size.
The general principle of the two-stroke engine is to shorten the periods
in which fuel is introduced to the combustion chamber and in which the spent
gases are exhausted to a small fraction of the duration of a stroke instead of
allowing each of these operations to occupy a full stroke. In the simplest type
of two-stroke engine, the poppet valves are replaced by sleeve valves or ports
(openings in the cylinder wall that are uncovered by the piston at the end of
its outward travel). In the two-stroke cycle the fuel mixture or air is
introduced through the intake port when the piston is fully withdrawn from the
cylinder. The compression stroke follows and the charge is ignited when the
piston reaches the end of this stroke. The piston then moves outward on the
power stroke, uncovering the exhaust port and permitting the gases to escape
from the combustion chamber.

Rotary Engine

Wankel Engines

In the 1950s the German engineer Felix Wankel developed his concept of
an internal-combustion engine of a radically new design, in which the piston and
cylinder were replaced by a three-cornered rotor turning in a roughly oval
chamber. The fuel-air mixture is drawn in through an intake port and trapped
between one face of the turning rotor and the wall of the oval chamber. The
turning of the rotor compresses the mixture, which is ignited by a spark plug.
The exhaust gases are then expelled through an exhaust port through the action
of the turning rotor. The cycle takes place alternately at each face of the
rotor, giving three power strokes for each turn of the rotor. The Wankel
engine's compact size and consequent lesser weight as compared with the piston
engine gave it increasing value and importance with the rise in gasoline prices
of the 1970s and '80s. In addition, it offers practically vibration-free
operation, and its mechanical simplicity provides low manufacturing costs.
Cooling requirement s are low, and its low center of gravity contributes to
driving safety.

Gas Turbine

Also called as combustion turbine, engine that employs gas flow as the
working medium by which heat energy is transformed into mechanical energy. Gas
is produced in the engine by the combustion of certain fuels. Stationary nozzles
discharge jets of this gas against the blades of a turbine wheel. The impulse
force of the jets causes the shaft to turn. A simple-cycle gas turbine includes
a compressor that pumps compressed air into a combustion chamber. Fuel in
gaseous or liquid-spray form is also injected into this chamber, and combustion
takes place there. The combustion products pass from the chamber through the
nozzles to the turbine wheel. The spinning wheel drives the compressor and the
external load, such as an electrical generator.
In a turbine or compressor, a row of fixed blades and a corresponding
row of moving blades attached to a rotor is called a stage. Large machines
employ multistage axial-flow compressors and turbines. In multi-shaft
arrangements, the initial turbine stage (or stages) powers the compressor on one
shaft while the later turbine stage (or stages) powers the external load on a
separate shaft.
The efficiency of the gas-turbine cycle is limited by the need for
continuous operation at high temperatures in the combustion chamber and early
turbine stages. A small, simple-cycle gas turbine may have a relatively low
thermodynamic efficiency, comparable to a conventional gasoline engine. Advances
in heat-resistant materials, protective coatings, and cooling arrangements have
made possible large units with simple-cycle efficiencies of 34 percent or higher.

The efficiency of gas-turbine cycles can be enhanced by the use of
auxiliary equipment such as inter-coolers, regenerators, and reheaters. These
devices are expensive, however, and economic considerations usually preclude
their use.
In a combined-cycle power plant, the considerable heat remaining in the
gas turbine exhaust is directed to a boiler called a heat-recovery steam
generator. The heat so recovered is used to raise steam for an associated steam
turbine. The combined output is approximately 50 percent greater than that of
the gas turbine alone. Combined cycles with thermal efficiency of 52 percent and
higher are being put into service. Gas turbines have been applied to the
propulsion of ships and railroad locomotives. A modified form of gas turbine,
the turbojet, is used for airplane propulsion. Heavy-duty gas turbines in both
simple and combined cycles have become important for large-scale generation of
electricity. Unit ratings in excess of 200 megawatts (MW) are available. The
combined-cycle output can exceed 300 MW.
The usual fuels used in gas turbines are natural gas and liquids such as
kerosene and diesel oil. Coal can be used after conversion to gas in a separate
gasifier.

Internal-Combustion Engines and Air Pollution

Air pollution from automobile engines ( smog ) was first detected about
1942 in Los Angeles, CA. Smog arises from sunlight-induced photochemical
reactions between nitrogen dioxide and the several hundred hydrocarbons in the
atmosphere. Undesirable products of the reactions include ozone, aldehydes, and
peroxyacylnitrates ( PAN ). These are highly oxidizing in nature and cause eye
and throat irritation. Visibility-decreasing nitrogen dioxide and aerosols are
also formed.
Five categories of air pollutants and percent contribution from all
transportation source and the highway vehicle subset are show in Table -1.
Virtually all of the transportation CO, about half the hydrocarbons, and about
one-third of the nitrogen oxides come from gasoline engines. Diesel engines
account for the particulate.
Emissions from internal-combustion engines include those from blowby,
evaporation, and exhaust. These can vary considerably in amount and composition
depending on engine type, design, and condition, fuel-system type, fuel
volatility, and engine operating point. For an automobile without emission
control it is estimated that of the hydrocarbon emission, 20 to 25 percent arise
from blowby, 60 percent from the exhaust, and the balance from evaporative
losses primarily from the fuel tank and to a lesser extent from the carburetor.
All other non-hydrocarbon emissions emanate from the exhaust.
At least 200 hydrocarbon (HC) compounds have been identified in exhaust.
Some such as the olefin compounds react products. These are termed reactive
hydrocarbons. Others such as the paraffin are virtually unreactive.

Special Developments

The Stratified-Charge Engine a modification of the conventional spark-
ignition piston engine, the stratified charge engine is designed to reduce
emissions without the need for an exhaust-gas recirculation system or catalytic
converter. Its key feature is a dual combustion chamber for each cylinder, with
a prechamber that receives a rich fuel-air mixture while the main chamber is
charged with a very lean mixture. The spark ignites the rich mixture that in
turn ignites the lean main mixture. The resulting peak temperature is low enough
to inhibit the formation of nitrogen oxides, and the mean temperature is
sufficiently high to limit emissions of carbon monoxide and hydrocarbon.
Two rather distinct means for accomplishing the stratified charge
condition are under consideration :
1. A single combustion chamber with a well-controlled rotating air
motion. This arrangement is illustrated (Fig.6) by the Texaco Combustion Process
(TCP), patented in 1949.
2. A prechamber or two-chamber system. This is illustrated by Fig.7,
which shows the general arrangement of the Honda Compound-vortex controlled-
combustion (CVCC) system.
For both systems, very careful development has proved to be necessary to
obtain complete combustion of the fuel under the wide range of speed and load
conditions required of an automotive engine.