Fluid Mechanics, Thermodynamics of Turbomachinery

 Definition of a turbomachine

We classify as turbomachines all those devices in which energy is transferred either to, or from, a continuously flowing fluid by the dynamic action of one or more moving blade rows. The word turbo or turbinis is of Latin origin and implies that which spins or whirls around. Essentially, a rotating blade row, a rotor or an impeller changes the stagnation enthalpy of the fluid moving through it by either doing positive or negative work, depending upon the effect required of the machine. These enthalpy changes are intimately linked with the pressure changes occurring simultaneously in

the fluid. In the earlier editions of this book, open turbomachines, such as wind turbines, pro-pellers and unshrouded fans were deliberately excluded, primarily because of the conceptual difficulty of properly defining the mass flow that passes through the blades.

However, despite this apparent problem, the study of wind turbines has become an attractive and even an urgent task, not least because of the almost astonishing increase in their number. Wind turbines are becoming increasingly significant providers of electrical power and targets have even been set in some countries for at least 10% of power generation to be effected by this means by 2010. It is a matter of expediency to now include the aerodynamic theory of wind turbines in this book and so a new chapter has been added on the topic. It will be observed that the problem of dealing with the inde-terminate mass flow has been more or less resolved.Two main categories of turbomachine are identified: firstly, those that absorb powerto increase the fluid pressure or head (ducted fans, compressors and pumps); secondly,those that produce power by expanding fluid to a lower pressure or head (hydraulic,steam and gas turbines). Figure 1.1 shows, in a simple diagrammatic form, a selectionof the many different varieties of turbomachine encountered in practice. The reasonthat so many different types of either pump (compressor) or turbine are in use is becauseof the almost infinite range of service requirements. Generally speaking, for a given setof operating requirements one type of pump or turbine is best suited to provide optimum conditions of operation. This point is discussed more fully in the section of this chapter concerned with specific speed.

Turbomachines are further categorised according to the nature of the flow path through the passages of the rotor. When the path of the through-flow is wholly or mainly parallel to the axis of rotation, the device is termed an axial flow turbomachine (e.g. Figure 1.1(a) and (e)). When the path of the through-flow is wholly or mainly in a plane perpendicular to the rotation axis, the device is termed a radial flow turbomachine (e.g. Figure 1.1(c)). More detailed sketches of radial flow machines are given in Figures 7.1,7.2, 8.2 and 8.3. Mixed flow turbomachines are widely used. The term mixed flow in

 this context refers to the direction of the through-flow at rotor outlet when both radial and axial velocity components are present in significant amounts. Figure 1.1(b) shows a mixed flow pump and Figure 1.1(d) a mixed flow hydraulic turbine.

One further category should be mentioned. All turbomachines can be classified as

either impulse or reaction machines according to whether pressure changes are absent or present respectively in the flow through the rotor. In an impulse machine all the pressure change takes place in one or more nozzles, the fluid being directed onto the rotor.

The Pelton wheel, Figure 1.1(f), is an example of an impulse turbine.

The main purpose of this book is to examine, through the laws of fluid mechanics and thermodynamics, the means by which the energy transfer is achieved in the chief types of turbomachine, together with the differing behaviour of individual types in oper-

ation. Methods of analysing the flow processes differ depending upon the geometrical configuration of the machine, whether the fluid can be regarded as incompressible or not, and whether the machine absorbs or produces work. As far as possible, a unified treatment is adopted so that machines having similar configurations and function are considered together.

 

Units and dimensions

The International System of Units, SI (le Système International d’Unités) is a unified

self-consistent system of measurement units based on the MKS (metre–kilogram–

second) system. It is a simple, logical system based upon decimal relationships between units making it easy to use. The most recent detailed description of SI has been published in 1986 by HMSO. For an explanation of the relationship between, and use of, physical quantities, units and numerical values see Quantities, Units and Symbols (1975), published by The Royal Society or refer to ISO 31/0-1981.

Great Britain was the first of the English-speaking countries to begin, in the 1960s,

the long process of abandoning the old Imperial System of Units in favour of the

International System of Units, and was soon followed by Canada, Australia, New

Zealand and South Africa. In the USA a ten year voluntary plan of conversion to SI

units was commenced in 1971. In 1975 US President Ford signed the Metric Conversion

Act which coordinated the metrication of units, but did so without specifying a schedule of conversion. Industries heavily involved in international trade (cars, aircraft, food and drink) have, however, been quick to change to SI for obvious economic reasons, but others have been reluctant to change.

SI has now become established as the only system of units used for teaching 

engineering in colleges, schools and universities in most industrialised countries throughout the world. The Imperial System was derived arbitrarily and has no consistent numerical base, making it confusing and difficult to learn. In this book all numerical problems involving units are performed in metric units as this is more convenient

than attempting to use a mixture of the two systems. However, it is recognised that some problems exist as a result of the conversion to SI units. One of these is that many valuable papers and texts written prior to 1969 contain data in the old system of units and would need converting to SI units. 

Some SI units

The SI basic units used in fluid mechanics and thermodynamics are the metre (m),

kilogram (kg), second (s) and thermodynamic temperature (K). All the other units used in this book are derived from these basic units. The unit of force is the newton (N), defined as that force which, when applied to a mass of 1 kilogram, gives an acceleration to the mass of 1 m/s2

. The recommended unit of pressure is the pascal (Pa) which is the pressure produced by a force of 1 newton uniformly distributed over an area of 1 square metre. Several other units of pressure are in widespread use, however, foremost of these being the bar. Much basic data concerning properties of substances (steam and gas tables, charts, etc.) have been prepared in SI units with pres-

sure given in bars and it is acknowledged that this alternative unit of pressure will continue to be used for some time as a matter of expediency. It is noted that 1 bar equals105 Pa (i.e. 105 N/m2), roughly the pressure of the atmosphere at sea level, and is perhaps an inconveniently large unit for pressure in the field of turbomachinery

anyway! In this book the convenient size of the kilopascal (kPa) is found to be the most useful multiple of the recommended unit and is extensively used in most calculations and examples.

In SI the units of all forms of energy are the same as for work. The unit of energy

is the joule (J) which is the work done when a force of 1 newton is displaced through a distance of 1 metre in the direction of the force, e.g. kinetic energy (1/2 mc2) has the dimensions kg ¥ m2/s2

; however, 1 kg = 1 Ns2/m from the definition of the newton given above. Hence, the units of kinetic energy must be Nm = J upon substituting dimensions.

The watt (W) is the unit of power; when 1 watt is applied for 1 second to a system

the input of energy to that system is 1 joule (i.e. 1 J).The hertz (Hz) is the number of repetitions of a regular occurrence in 1 second. Instead of writing c/s for cycles/sec, Hz is used. The unit of thermodynamic temperature is the kelvin (K), written without the ° sign, and is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The degree celsius (°C) is equal to the unit kelvin. Zero on the celsius scale is

the temperature of the ice point (273.15 K). Specific heat capacity, or simply specific heat, is expressed as J/kg K or as J/kg°C. Dynamic viscosity, dimensions ML-1 T-1 , has the SI units of pascal seconds, i.e. Hydraulic engineers find it convenient to express pressure in terms of head of a liquid. The static pressure at any point in a liquid at rest is, relative to the pressure acting on the free surface, proportional to the vertical distance of the free surface above that point. The head H is simply the height of a column of the liquid which can be supported by this pressure. If r is the mass density (kg/m3

) and g the local gravitational acceleration (m/s2), then the static pressure p (relative to atmospheric pressure) is p = rgH, where H is in metres and p is in pascals (or N/m2

). This is left for the student to verify as a simple exercise.

 Dimensional analysis and performance laws

The widest comprehension of the general behaviour of all turbomachines is, without doubt, obtained from dimensional analysis. This is the formal procedure whereby the group of variables representing some physical situation is reduced into a smaller number of dimensionless groups. When the number of independent variables is not too great, dimensional analysis enables experimental relations between variables to be found with the greatest economy of effort. Dimensional analysis applied to turbomachines has two further important uses: (a) prediction of a prototype’s performance from tests conducted on a scale model (similitude); (b) determination of the most suitable type of machine, on the basis of maximum efficiency, for a specified range of head, speed and flow rate. Several methods of constructing non-dimensional groups have been described by Douglas et al. (1995) and by Shames (1992) among other authors.

The subject of dimensional analysis was made simple and much more interesting by Edward Taylor (1974) in his comprehensive account of the subject. It is assumed here that the basic techniques of forming non-dimensional groups have already been acquired by the student.

Adopting the simple approach of elementary thermodynamics, an imaginary envelope (called a control surface) of fixed shape, position and orientation is drawn around the turbomachine (Figure 1.2). Across this boundary, fluid flows steadily, entering at station 1 and leaving at station 2. As well as the flow of fluid there is a flow of work across the control surface, transmitted by the shaft either to, or from, the machine. For the present all details of the flow within the machine can be ignored and only externally observed features such as shaft speed, flow rate, torque and change in fluid properties across the machine need be considered. To be specific, let the turbomachine be a pump (although the analysis could apply to other classes of turbomachine) driven by an electric motor. The speed of rotation N, can be adjusted by altering the current to the motor; the volume flow rate Q, can be independently adjusted by means of a throttle valve. For fixed values of the set Q and N, all other variables such as torque t, head H, are thereby established. The choice of Q and N as control variables is clearly arbitrary and any other pair of independent variables such as t and H could equally well

have been chosen. The important point to recognise is that there are for this pump, two control variables. If the fluid flowing is changed for another of different density r, and viscosity m, the performance of the machine will be affected. Note, also, that for a turbomachine handling compressible fluids, other fluid properties are important and are discussed later.

So far we have considered only one particular turbomachine, namely a pump of a given size. To extend the range of this discussion, the effect of the geometric variables on the performance must now be included. The size of machine is characterised by the impeller diameter D, and the shape can be expressed by a number of length ratios, l1/D, l2/D, etc.

Incompressible fluid analysis

The performance of a turbomachine can now be expressed in terms of the control variables, geometric variables and fluid properties. For the hydraulic pump it is convenient to regard the net energy transfer gH, the efficiency h, and power supplied P, as dependent variables and to write the three functional relationships as

By the procedure of dimensional analysis using the three primary dimensions, mass, length and time, or alternatively, using three of the independent variables we can form the dimensionless groups. The latter, more direct procedure requires that the variables selected, r, N, D, do not of themselves form a dimensionless group. The selection of r, N, D as common factors avoids the appearance of special fluid terms (e.g. mℋℋℋℳℳ, Q) in more than one group and allows gH, h and P to be made explicit. Hence the three relationships reduce to the following easily verified forms.

Energy transfer coefficient, sometimes called head coefficient

which is frequently used is the velocity (or flow) coefficient f = cx /U where U is blade tip speed and cx the average axial velocity. Since

Because of the large number of independent groups of variables on the right-hand side of eqns. (1.2), those relationships are virtually worthless unless certain terms can be discarded. In a family of geometrically similar machines l1/D, l2/D are constant and may be eliminated forthwith. The kinematic viscosity, - ℌ = m/r is very small in turbomachines handling water and, although speed, expressed by ND, is low the Reynolds number is correspondingly high. Experiments confirm that effects of Reynolds number on the performance are small and may be ignored in a first approximation. The functional relationships for geometrically similar hydraulic turbomachines are then,

This is as far as the reasoning of dimensional analysis alone can be taken; the actual form of the functions f4, f5 and f6 must be ascertained by experiment.

One relation between y, f, h and Pˆ may be immediately stated. For a pump the net hydraulic power, PN equals rQgH which is the minimum shaft power required in the absence of all losses. No real process of power conversion is free of losses and the actual shaft power P must be larger than PN. We define pump efficiency (more precise definitions of efficiency are stated in Chapter 2) h = PN/P = rQgH/P. Therefore

 Thus f6 may be derived from f4 and f5 since Pˆ = fy/h. For a turbine the net hydraulic power PN supplied is greater than the actual shaft power delivered by the machine andthe efficiency h = P/PN. This can be rewritten as Pˆ = hfy by reasoning similar to theabove considerations

Work and potential energy

 Work and potential energy

Potential energy is closely linked with forces. If the work done by a force on a body that moves from A to B does not depend on the path between these points, then the work of this force measured from A assigns a scalar value to every other point in space and defines a scalar potential field. In this case, the force can be defined as the negative of thevector gradient of the potential field.

If the work for an applied force is independent of the path, then the work done by the force is evaluated at the start and end of the trajectory of the point of application. This means that there is a function U (x), called a "potential," that can be evaluated at the two points x_A and x_B to obtain the work over any trajectory between these two points. It is tradition to define this function with a negative sign so that positive work is a reduction in the potential, that is

where C is the trajectory taken from A to B. Because the work done is independent of the path taken, then this expression is true for any trajectory, C, from A to B.

The function U(x) is called the potential energy associated with the applied force. Examples of forces that have potential energies are gravity and spring forces.

Compressed air energy storage

 is a way to store energy generated at one time for use at another time using compressed air. At utility scale, energy generated during periods of low energy demand (off-peak) can be released to meet higher demand (peak load) periods. Small scale systems have long been used in such applications as propulsion of mine locomotives. Large scale applications must conserve the heat energy associated with compressing air; dissipating heat lowers the energy efficiency of the storage system.

,

where , and so, . Here,  is the absolute pressure,  is the volume of the vessel,  is the amount of substance of gas (mol) and  is theideal gas constant

Potential energy for a linear spring

A horizontal spring exerts a force F = (−kx, 0, 0) that is proportional to its deflection in the x direction. The work of this spring on a body moving along the space curve s(t) = (x(t), y(t), z(t)), is calculated using its velocity, v = (v_x, v_y, v_z), to obtain

For convenience, consider contact with the spring occurs at t = 0, then the integral of the product of the distance x and the x-velocity, xv_x, is x²/2.

The function

 

electrical-energy storage

1-Mechanical storage systems

The most common mechanical storage systems are pumped hydroelectric power plants (pumped hydro storage, PHS), compressed air energy storage (CAES) and fl ywheel energy storage (FES).

Pumped hydro storage (PHS)

With over 120 GW, pumped hydro storage power plants represent nearly 99 % of world-wide installed electrical storage capacity [doe07], which is about 3 % of global generation Capacity

 

Compressed air energy storage

(CAES)

Compressed air (compressed gas) energy storage is a technology known and used since the 19th century for different industrial applications including mobile ones.

 

2-Electrochemical storage systems

 

In this section various types of batteries are described. Most of them are technologically mature for practical use. First, six secondary battery types are listed: lead acid, NiCd/NiMH Li-ion, metal air, sodium sulphur and sodium nicke chloride; then follow two sorts of fl ow battery.

 

Waste Heat Recovery

 

 

Waste heat losses arise both from equipment inefficiencies and from thermodynamic limitations on equipment and processes. For example, consider reverberatory furnaces frequently used in aluminum melting operations. Exhaust gases immediately leaving the furnace can have temperatures as high as 2,2002,400°F [1,2001,300°C]. Consequently, these gases have highheat content, carrying away as much as 60% of furnace energy inputs. Efforts can be made to design more energyefficient reverberatory furnaces with better heat transfer and lower exhaust temperatures; however, the laws of thermodynamics place a lower limit on the temperature of exhaust gases. Since heat exchange involves energy transfer from a hightemperature source to a lowertemperature sink, the combustion gas temperature must always exceed the molten aluminum temperature in order to facilitate aluminum melting. The gas temperature in the furnace will never decrease below the temperature of the molten aluminum, since this would violate the second law of thermodynamics. Therefore, the minimum possible temperature of combustion gases immediately exiting an aluminum reverberatory furnace corresponds to the aluminum pouring point temperature 1,2001,380°F [650750°C]. In this scenario, at least 40% of the energy input to the furnace is still lost as waste heat (Appendix A: Documentation of Waste Heat Estimates)

Band with of sun radiation

In a point at the top of Earth’s atmosphere, the beam of nearly parallel incident

sunrays is referred to as extraterrestrial radiation (ETR). ETR fluctuates about

6.9 % during a year (from 1412.0 Wm-2 in January to 1321.0 Wm-2 in July) due to the Earth’s varying distance from the Sun

                 

Wind turbine

A wind turbine is a device that converts kinetic energy from the wind into electrical power. The term appears to have migrated from parallel hydroelectric technology (rotary propeller). The technical description for this type of machine is an aerofoil-powered generator.

The result of over a millennium of windmill development and modern engineering, today's wind turbines are manufactured in a wide range of vertical and horizontal axis types. The smallest turbines are used for applications such as battery charging for auxiliary power for boats or caravans or to power traffic warning signs. Slightly larger turbines can be used for making contributions to a domestic power supply while selling unused power back to the utility supplier via the electrical grid. Arrays of large turbines, known as wind farms, are becoming an increasingly important source of renewable energy and are used by many countries as part of a strategy to reduce their reliance on fossil fuels.

Types

Horizontal axis

Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator

Vertical axis design

Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically. One advantage of this arrangement is that the turbine does not need to be pointed into the wind to be effective, which is an advantage on a site where the wind direction is highly variable. It is also an advantage when the turbine is integrated into a building because it is inherently less steerable. Also, the generator and gearbox can be placed near the ground, using a direct drive from the rotor assembly to the ground-based gearbox, improving accessibility for maintenance.

 

Breeder reactors

²³⁸U is not usable directly as nuclear fuel, though it can produce energy via "fast" fission. In this process, a neutron that has a kinetic energy in excess of 1 MeV can cause the nucleus of ²³⁸U to split in two. Depending on design, this process can contribute some one to ten percent of all fission reactions in a reactor, but too few of the about 1.7 neutrons produced in each fission have enough speed to continue a chain reaction.

²³⁸U can be used as a source material for creating plutonium-239, which can in turn be used as nuclear fuel. Breeder reactors carry out such a process of transmutation to convert the fertile isotope ²³⁸U into fissile Pu-239. It has been estimated that there is anywhere from 10,000 to five billion years worth of ²³⁸U for use in these power plants. Breeder technology has been used in several experimental nuclear reactors.

By December 2005, the only breeder reactor producing power was the 600-megawatt BN-600 reactor at the Beloyarsk Nuclear Power Station in Russia. Russia has planned to build another unit, BN-800, at the Beloyarsk nuclear power plant. Also, Japan's Monju breeder reactor is planned to be started, having been shut down since 1995, and both China and India have announced plans to build nuclear breeder reactors.

The breeder reactor as its name implies creates even larger quantities of Pu-239 than the fission nuclear reactor.

The Clean And Environmentally Safe Advanced Reactor (CAESAR), a nuclear reactor concept that would use steam as a moderator to control delayed neutrons, will potentially be able to burn ²³⁸U as fuel once the reactor is started with LEU fuel. This design is still in the early stages of development

 

Coal world consumption per day

According to a Greenpeace analysis, between the months of January and September 2015 coal use around the world was down by at least 2.3% and by as much as 4.6% versus the same period last year.

 

This historic fall was caused by a ‘perfect storm’ of circumstance: dire fossil fuel economics, rising renewable energy uptake, slowing global energy demand, and China’s crackdown on air pollution.

Because this fall doesn’t happen without China, by far the largest coal consuming country in the world.

After its rapid coal growth made a dramatic u-turn in 2014, China has seen a massive decline in its use so far this year.

The country’s 3-5% fall in the first half of 2015 (43-69Mtce) accounts for more than half of the world’s coal consumption reduction.

And yes, this analysis uses the coal stats recently reported on by the New York Times (but covered by us six months ago).

Things are changing elsewhere as well; coal use in the US, for instance, is way down in 2015, with a string of coal plant closures and mine retirements pushing production to its lowest point in three decades.

These shifts have more than offset coal gains in other countries around the world, most notably India, where consumption has grown by around 5% (8-13Mtce)

What’s also remarkable is that coal has continued to trend downwardsdespite global benchmarks falling to record lows.

With the UN climate summit in Paris to kick off at the end of November, you’d imagine all this might come up

Exothermic process

In thermodynamics, the term exothermic process (exo- : "outside") describes a process or reaction that releases energy from the system, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e.g. a battery), or sound (e.g. explosion heard when burning hydrogen). Its etymology stems from the Greek prefix έξω (exō, which means "outwards") and the Greek word θερμικός(thermikόs, which means "thermal").The term exothermic was first coined by Marcellin Berthelot. The opposite of an exothermic process is an endothermic process, one that absorbs energy in the form of heat.

The concept is frequently applied in the physical sciences to chemical reactions, where as in chemical bond energy that will be converted tothermal energy (heat).

Exothermic (and endothermic) describe two types of chemical reactions or systems found in nature, as follows.

Simply stated, after an exothermic reaction, more energy has been released to the surroundings than was absorbed to initiate and maintain the reaction. An example would be the burning of a candle, wherein the sum of calories produced by combustion (found by looking at radiant heating of the surroundings and visible light produced, including increase in temperature of the fuel (wax) itself, which with oxygen, have become hot CO₂ and water vapor,) exceeds the number of calories absorbed initially in lighting the flame and in the flame maintaining itself. (i.e. some energy produced by combustion is reabsorbed and used in melting, then vaporizing the wax, etc. but is (far) outstripped by the energy produced in breaking carbon-hydrogen bonds and combination of oxygen with the resulting carbon and hydrogen).

On the other hand, in an endothermic reaction or system, energy is taken from the surroundings in the course of the reaction. An example of an endothermic reaction is a first aid cold pack, in which the reaction of two chemicals, or dissolving of one in another, requires calories from the surroundings, and the reaction cools the pouch and surroundings by absorbing heat from them. An endothermic system is seen in the production of wood: trees absorb radiant energy, from the sun, use it in endothermic reactions such as taking apart CO₂ and H₂O and combining the carbon and hydrogen generated to produce cellulose and other organic chemicals. These products, in the form of wood, say, may later be burned in a fireplace, exothermically, producing CO₂ and water, and releasing energy in the form of heat and light to their surroundings, e.g., to a home's interior and chimney gasses.

Endothermic process

In thermodynamics, the term endothermic process describes a process or reaction in which the system absorbs energyfrom its surroundings; usually, but not always, in the form of heat. The term was coined by Marcellin Berthelot from the Greek roots endo-, derived from the word "endon" (ἔνδον) meaning "within" and the root "therm" (θερμ-) meaning "hot." The intended sense is that of a reaction that depends on absorbing heat if it is to proceed. The opposite of an endothermic process is an exothermic process, one that releases, "gives out" energy in the form of (usually, but not always) heat. Thus in each term (endothermic & exothermic) the prefix refers to where heat goes as the reaction occurs, though in reality it only refers to where the energy goes, without necessarily being in the form of heat.

The concept is frequently applied in physical sciences to, for example, chemical reactions, where thermal energy (heat) is converted to chemical bond energy.

Endothermic (and exothermic) analysis only accounts for the enthalpy change (∆H) of a reaction. The full energy analysis of a reaction is the Gibbs free energy (∆G), which includes an entropy (∆S) and temperature term in addition to the enthalpy. A reaction will be a spontaneous process at a certain temperature if the products have a lower Gibbs free energy (anexergonic reaction) even if the enthalpy of the products is higher. Entropy and enthalpy are different terms, so the change in entropic energy can overcome an opposite change in enthalpic energy and make an endothermic reaction favorable.

electromagnetic waves

Electromagnetic radiation (EM radiation or EMR) is the radiant energy released by certain electromagneticprocesses. Visible light is one type of electromagnetic radiation; other familiar forms are invisible electromagnetic radiations, such as radio waves, infrared light and X rays.

Classically, electromagnetic radiation consists ofelectromagnetic waves, which are synchronizedoscillations of electric and magnetic fields that propagate at the speed of light through a vacuum. The oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy andwave propagation, forming a transverse wave. Electromagnetic waves can be characterized by either the frequency or wavelength of their oscillations to form the electromagnetic spectrum, which includes, in order of increasing frequency and decreasing wavelength: radio waves, microwaves, infrared radiation,visible light, ultraviolet radiation, X-rays and gamma rays.

Electromagnetic waves are produced whenever charged particles areaccelerated, and these waves can subsequently interact with any charged particles. EM waves carry energy, momentum and angular momentum away from their source particle and can impart those quantities to matter with which they interact. Quanta of EM waves are called photons, which are massless, but they are still affected by gravity. Electromagnetic radiation is associated with those EM waves that are free to propagate themselves ("radiate") without the continuing influence of the moving charges that produced them, because they have achieved sufficient distance from those charges. Thus, EMR is sometimes referred to as the far field. In this language, the near field refers to EM fields near the charges and current that directly produced them, specifically,electromagnetic induction and electrostatic induction phenomena.

In the quantum theory of electromagnetism, EMR consists of photons, theelementary particles responsible for all electromagnetic interactions. Quantum effects provide additional sources of EMR, such as the transition of electronsto lower energy levels in an atom and black-body radiation. The energy of an individual photon is quantized and is greater for photons of higher frequency. This relationship is given by Planck's equation E=hν, where E is the energy per photon, ν is the frequency of the photon, and h is Planck's constant. A single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light.

The effects of EMR upon biological systems (and also to many other chemical systems, under standard conditions) depend both upon the radiation's pow٢دer and its frequency. For EMR of visible frequencies or lower (i.e., radio, microwave, infrared), the damage done to cells and other materials is determined mainly by power and caused primarily by heating effects from the combined energy transfer of many photons. By contrast, for ultraviolet and higher frequencies (i.e., X-rays and gamma rays), chemical materials and living cells can be further damaged beyond that done by simple heating, since individual photons of such high frequency have enough energy to cause direct molecular damage.

Programmed ignition

 Programmed ignition

Programmed ignition is the term used by some

manufacturers; others call it electronic spark

advance (ESA). Constant energy electronic igni-

tion was a major step forwards and is still used

on countless applications. However, its limita-

tions lay in still having to rely upon mechanical

components for speed and load advance characteristics. In many cases these did not match ideally the requirements of the engine.

Programmed ignition systems have a major

difference compared to earlier systems in that

they operate digitally. Information about the operating requirements of a particular engine is programmed in to memory inside the ECU. The datafor storage in ROM is obtained from rigorous testing on an engine dynamometer and further development work in the vehicle under variousoperating conditions. Programmed ignition hasseveral advantages.

● The ignition timing can be accurately matched to the individual application under a range of operating conditions.

● Other control input can be utilised such as coolant temperature and ambient air

temperature.

● Starting is improved, fuel consumption is

reduced as are emissions and idle control is

better.

● Other inputs can be taken into account such as engine knock.

● The number of wearing components in the

ignition system is considerably reduced.

Programmed ignition or ESA can be a separate

system or included as part of the fuel control system. In order for the ECU to calculate suitable timing and dwell outputs, certain input information is required.

The crankshaft sensor consists of a permanent

magnet, a winding and a soft iron core. It is

mounted in proximity to a reluctor disc. The disc

has 34 teeth spaced at 10° intervals around to

periphery. It has two teeth missing 180° apart, at

a known position BTDC. Many manufacturers

use this technique with minor differences. As a

tooth from the reluctor disc passes the core of the densor the reluctance of the magnetic circuit is changed. This induces a voltage in the winding, the frequency of the waveform being proportional to the engine speed. The missing tooth causes a 'missed’ output wave and hence engine position can be determined.

Engine load is proportional to manifold pres-

sure in that high load conditions produce high

pressure and lower load conditions, such as cruise, produce lower pressure. Load sensors are there fore pressure transducers. They are either mounted in the ECU or as a separate unit and are condected to the inlet manifold with a pipe. The pipe often incorporates a restriction to damp out fluctuations and a vapour trap to prevent petrol fumes reaching the sensor.

Coolant temperature measurement is carried

out by a simple thermistor. In many cases the

same sensor is used for the operation of the temperature gauge and to provide information to the fuel control system. A separate memory map is used to correct the basic timing settings. Timing may be retarded when the engine is cold to assist in more rapid warm up.

Combustion knock can cause serious damage to an engine if sustained for long periods. This knock or detonation is caused by over advanced ignition timing. At variance with this is that an engine in general will run at its most efficient when the timing is advanced as far as possible. To achieve this the data stored in the basic timing map will be as close to the knock limit of the engine as possible.

The knock sensor provides a margin for error. The sensor itself is an accelerometer often of the piezoelectric type. It is fitted in the engine block between cylinders two and three on in-line four cylinder engines. Vee engine’s require two sensors, one on each side. The ECU responds to signals from the knock sensor in the engine’s knock window for each cylinder; this is often just a few degrees each side of TDC. This prevents clatter from the valve mechanism being interpreted as knock. The signal from the sensor is also filtered in the ECU to remove unwanted noise. If detonation is detected the ignition timing is retarded on the fourth ignition pulse after detection (four cylinder engine), in steps until knock is no longer detected. The steps vary between manufacturers but about 2° is typical. The timing is then advanced slowly in steps of say 1° over a number of engine revolutions, until the advance required by memory is restored. This fine control allows the engine to be run very close to the knock limit without risk of engine damage.

Correction to dwell settings is required if the battery voltage falls, as a lower voltage supply to the coil will require a slightly larger dwell figure.

This information is often stored in the form of a

dwell correction map.

As the sophistication of systems has increased

the information held in the memory chips of the

ECU has also increased. The earlier versions of

programmed ignition system produced by Rover

achieved accuracy in ignition timing of
1.8°

whereas a conventional distributor is
8°. The

information, which is derived from dynamom-

eter tests as well as running tests in the vehicle, is stored in ROM. The basic timing map consists of the correct ignition advance for 16 engine speeds and 16 engine load conditions.

A separate three-dimensional map is used

which has eight speed and eight temperature

sites. This is used to add corrections for engine

coolant temperature to the basic timing settings.

This improves driveability and can be used to

decrease the warm-up time of the engine. The

data is also subjected to an additional load cor-

rection below 70°C. Figure 7.14 shows a flow

chart representing the logical selection of the

optimum ignition setting. Note that the ECU will

also make corrections to the dwell angle, both as a function of engine speed to provide constant energy output and due to changes in battery voltage. A lower battery voltage will require a slightly longer dwell and a higher voltage a slightly shorter dwell. A Windows® shareware program that simulates the ignition system (as well as many other systems) is available for download from my web site.

The output of a system such as this programmed ignition is very simple. The output stage, in common with most electronic ignition, consists of aheavy-duty transistor which forms part of, or is driven by, a Darlington pair. This is simply to allow the high ignition primary current to be controlled. The switch off point of the coil will control ignition timing and the switch on point will control the dwell period.

The high tension distribution is similar to a

more conventional system. The rotor arm, how-

ever, is mounted on the end of the camshaft with the distributor cap positioned over the top. 

7.15 shows a programmed ignition system.