| INTERNAL COMBUSTION ENGINES | ||
| The internal combustion engine is a variety of heat engine, ie. a machine for converting heat energy into useful mechanical work through the compression and expansion of a so-called working fluid. It is characterized by the fact that it obtains heat from the burning of fuel in the same gas that it uses for the energy conversion process. In this way it differs from, for example, the steam engine, where the heat source is external to the working fluid. Two main types of internal combustion engine are prevalent nowadays: piston engines and gas turbines, the latter being typified by the turbojet or jet engine. | The diesel is an internal combustion piston engine distingushed from the petrol (or gasoline) engine in that combustion ignition in the cylinders is caused by heat generated during compression of the air, rather than by an electric spark. Hence diesels are technically referred to as 'compression-ignition', as opposed to 'spark-ignition', engines. The fact that the fuel used is normally a slightly heavier oil fraction than petrol is not distinctive; compression-ignition engines can be adapted to run on a wide variety of other fuels including liquified gases, alcohols, vegetable oil, animal fats or even powdered coal. |
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The
engine is named after its inventor,
Rudolph Diesel, the Parisian-born
Bavarian engineer who first proposed the
principle of compression-ignition in his
patent application of 1892. This was
actually 16 years after the introduction
of the first practical spark-ignition
engine by Nicklaus Otto, and reflects the
greater technical difficulties that
compression ignition posed. 
The first
diesel prototype, built by Maschinefabrik
of Augsburg, ran for one minute on the
17th of February 1894. The following
year, after much development, it achieved
a power output of 17 kW (23 hp) with a
then-impressive thermal efficiency of
16.6%, thus securing the patent. Over the
following ten years Diesel prospered on
the sale of manufacturing rights, but
then lost his fortune through legal
disputes and the engine's lack of
substantial market success. Sadly, it was
not until after his premature death in
1913 that Diesel's invention started to
achieve widespread popularity.| APPLICATIONS | ||
| Today, due to its
superior fuel efficiency and durability, the
diesel is used for the majority of combustion
engine applications. This success may not seem
obvious to most people who are familiar with the
more numerous petrol engines employed in small
road vehicles and garden machinery, applications
where lightness, responsiveness and low-cost are
at a premium. The table below gives a clearer
view of the main fields of combustion engine use.
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Modern diesels range in
size over eight orders of magnitude, from 0.1cc
single-cylinder model aircraft engines, such as
the 50 gram Nano on the left, which produces
about 15 W (0.02 hp) at 25,000 rpm; to giant
marine power plants like the 22,000L Sulzer
RTA96C on the right. At over 2000 tonnes the
14-cylinder version of this diesel is the largest
internal combustion engine in the world. It
develops around 80 MW (108,000 hp) at its maximum
speed of 100 rpm, with a thermal efficiency of
over 50%.
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| PRINCIPLE OF OPERATION | ||||||
| The greater efficiency of
the diesel, compared with petrol engines, stems
from its higher compression ratio and lack of
throttling. In a petrol engine, the proportion of
air to fuel in the cylinder must approximate an
ideal value, known as the stiochiometric ratio,
in order for the spark to initiate combustion. In
most designs, this entails creating a homogenous
mixture prior to its intake into the cylinder;
consequently the degree to which this charge can
then be compressed is limited to ratios less than
13:1, by the need to avoid spontaneous ignition.
If uncontrolled combustion starts as the piston
is still moving up (known as pre-ignition) or if
it occurs in the unconsumed charge whilst the
sprark-ignited flame is still progressing (known
as knock, pinking or detonation) the engine can
suffer considerable damage. In the diesel, fuel mixing takes place in the combustion chamber, at a point when ignition is required. This means that air alone is in the cylinder during the compression stroke and the main limit on the compression ratio, which can be as much as 25:1, is the strength of engine components. Hence, for the same displacement, a diesel is typically heavier and more durable than a petrol engine. The higher pressure tolerance also allows the diesel to take full advantage of turbocharging, where energy is recovered from the exhaust stream and used to do some of the compression work. Since a piston engine is essentially a constant-displacement pump, the most practical way of modulating the torque produced by the petrol engine is to throttle the air intake at part-loads. This reduces the intake pressure and hence the density of the charge and the mass of air/fuel mixture in the cylinder. Unfortunately, the throttling process involves some waste of energy; the engine effectively has to suck harder. A few modern petrol engines attempt to avoid this problem by using direct injection into the combustion chamber, creating a stratified charge at part-loads with stoichiometric conditions only around the spark plug. In contrast, the diesel's torque is controlled by altering the amount of fuel injected into the combustion chamber. At part-loads the air/fuel ratio is very high, ensuring that the fuel is completely burnt in the excess of air. However, it is the fuel injection process that poses the biggest technical problems in diesel engineering. In order to get the fuel to mix efficiently in the dense air of the combustion chamber, in a period which can be less than one millisecond, very fine atomization and hence high injection pressures are required. Even then, it is impossible completely to mix fuel with all the air in the cylinder; hence, for the same displacement, a diesel typically develops lower maximum power than a petrol engine. |
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| FUEL INJECTION SYSTEMS | ||
| In early diesel engines,
fuel was vaporized and blasted into the
combustion chamber using a separate compressed
air supply. This rather unsuccessful method was
soon superceded by high-pressure fuel pumps that
force compressed fuel through very small injector
nozzle holes to achieve the required atomization.
Most diesels use direct injection into the
combustion chamber, however many passenger car
applications employ indirect injection, where
fuel is atomized in a small pre-combustion
chamber which then shoots the expanding, burning
mixture into the main chamber. This method
necessitates lower compression ratios and hence
is not so fuel-efficient, however it does allow a
lighter, cheaper engine structure. In most fields of engineering, liquids are assumed to be incompressible; but at the pressures needed for diesel fuel injection, which range from 400 to over 2000 times atmospheric (40-200 MPa) the effects of compressibility become significant. At these pressures, equivalent to that at the tip of a nail punch when hammered, all conventional sealing materials will fail. Leakage in fuel pumps and injectors can only be prevented by using steel parts with extremely close-fitting hardened surfaces. Typically the clearance between the piston and barrel of a pumping element is around 0.5 mm. Manufacturing to these tolerances requires a high degree of precision and cleanliness, and is only carried out by a handful of companies worldwide. By contrast, petrol injection systems normally use pressures of less than 4 atmospheres and can be supplied by relatively simple electric pumps. Apart from injecting fuel according to the operator's demand for torque, the diesel fuel system must perform a secondary function, that of governing the engine. The pumping inefficiencies in a petrol engine increase with speed, imposing an upper limit on the latter and stabilizing idling. If the idle speed drops the pumping torque reduces, causing the engine to speed up again, and vice versa. Because this effect is not nearly so pronounced in the diesel, it has a natural tendancy to stall at low speeds and, if no load is applied when fuel is injected, to accelerate up to speeds which can destroy the engine. Thus, instruments known as governors are attached to the fuel control device in pumps or injectors, in order to achieve a stable idle speed by adjustment of the fuelling level, and to cut off fuelling over the maximum safe speed. Traditionally, governors consisted of mechanisms using fly-weights, levers and springs or pneumatic or hydraulic valves. However, the low cost and enormous flexibility of digital electronics combined with increasingly demanding emissions limits, fuel economy and drivability targets, mean that electronic control is now predominating. |
To generate the required
pumping forces, diesel fuel injection systems are
all driven off cams geared directly to the engine
crankshaft; however they vary in the arrangement
of the cams, pumping elements and injectors,
according to their different applications, as
follows: Distributor Pumps: One or more pumping elements are driven from a single, multi-lobed cam and the high-pressure fuel is then fed to the injector for each cylinder in turn, via a rotatary hydraulic distributor, and a set of high-pressure pipes (injector lines). This system is relatively cheap and compact but rather limited in the maximum pressure that can be delivered. It is mainly used on small engines, ie. below about 1L/cylinder displacement. In-Line Pumps: A number of pumping elements, one for each cylinder, are driven off a camshaft, with each element supplying a single injector via a line. This system is mainly used for medium-sized engines up to 5L/cylinder displacement. Unit Pumps: Each cylinder has a separate pump with a single element, injector and line, driven off a camshaft in the engine. This system is mainly used on large engines, ie. over about 3L/cylinder displacement, although electronically-controlled versions have been used in smaller applications. Unit Injectors: Each cylinder has a separate pumping element and injector integrated into a single unit mounted in the cylinder head and driven off a camshaft. This system allows very high pressures to be generated and is used in small to meduim-sized applications, particularly when electronically controlled. Common-Rail Systems: A high-pressure pump with one or more pumping elements driven from a single, multi-lobed cam, feeds an accumulator (common rail), from which fuel is supplied to the injector for each cylinder via a line. Electronic control is essential to this system and makes it very flexible. For this reason it is becoming increasingly popular on small to medium-sized road-going applications. |
| BIBLIOGRAPHY AND LINKS |
| "The
Internal Combustion Engine in Theory and
Practice" (Vol. 1 - 2nd ed.) by Charles Fayette
Taylor, MIT Press 1985, ISBN:
0-2627-0026-3 "Diesel Engine Reference Book" (2nd ed.) edited by Rodica Baranescu and Bernard Challen, SAE 1999, ISBN: 0-7680-0403-9 "Bosch Diesel-engine Management" (2nd ed.), SAE 1999, ISBN: 0-7680-0509-4 Institution of Mechanical Engineers, Professional Engineering Publications "Biography of Rudolph Diesel" by Martin Leduc "Rudolf Diesel and the Second Law of Thermodynamics" by Walter Kaiser, German News June/July 1997 "First Diesel Engine", exhibited in the Power Machinery department of the Deutsches-Museum, Munich The diesel engine, from "Start Your Engines", Thinkquest Library Publicity from a consortium of American diesel manufacturers: Diesel Technology Forum |