Combustion Turbine-Heat Recovery

Combustion turbines generate a large volume of very hot air. The exhaust is also high in oxygen content as compared to other combustion exhaust streams, as only a small amount of oxygen is required by the combustor relative the total volume available.

Depending on how much thermal energy is required for the application, the turbine exhaust may be supplemented by a duct burner.

A duct burner is a direct fired gas burner located in the turbine exhaust stream. It has a very high efficiency due to the high inlet air temperature, and is used to boost the total available thermal energy. The turbine exhaust boosted by the duct burner is directed into the waste heat boiler, called the Heat Recovery Steam Generator, or HRSG commonly pronounced as 'HerSig'.

Turbine exhaust can also be ducted directly into hot air processes, such as kilns and material drying systems. This is the least costly first cost, as there is no boiler or steam system to purchase. Turbine exhaust can also be ducted directly into absorption chillers for large cooling loads.

The system will also include a diverter for times when waste heat is not needed. The diverter vents the turbine exhaust to atmosphere; this substantially reduces the system efficiency, as only the electric energy is being used. Single or Simple Cycle electric plants (typical of peaker plants) dump all of their turbine exhaust, as they have no thermal requirements. These plants generally use turbines with recuperators to maximize their electrical efficiency.

The higher the electrical efficiency of the turbine, the lower the available thermal energy in the exhaust. Newer turbines with recuperators, and larger sized turbines, tend to have higher efficiencies.

Combustion Turbine-Operations

A gas turbine has a compressor to draw in and compress air; a combustor (or burner) to add fuel to heat the compressed air; and a turbine to extract power from the hot air flow. The gas turbine is an internal combustion (IC) engine employing a continuous combustion process. This differs from the intermittent combustion occurring in diesel and automotive IC engines. About 2/3rds of the shaft power produced by the turbine is used to run the compressor, leaving about 1/3rd available to turn a genset to produce electrical power.

Gas Turbine Cycles
A cycle describes what happens to air as it passes into, through, and out of the gas turbine. The cycle usually describes the relationship between the space occupied by the air in the system (called volume, V) and the pressure (P) it is under. The Brayton cycle (1876), shown in graphic form as a pressure-volume diagram, is a representation of the properties of a fixed amount of air as it passes through a gas turbine in operation. These same points are also shown in the engine schematic above.

Air is compressed from point 1 to point 2. This increases the pressure as the volume of space occupied by the air is reduced.

The air is then heated at constant pressure from 2 to 3. This heat is added by injecting fuel into the combustor and igniting it on a continuous basis.

The hot compressed air at point 3 is then allowed to expand (from point 3 to 4) reducing the pressure and temperature and increasing its volume. In the engine, this represents flow through the turbine to point 3' and then flow through the power turbine to point 4 to turn a shaft or a ship’s propeller. The Brayton cycle is completed by a process in which the volume of the air is decreased (temperature decrease) as heat is absorbed into the atmosphere.

A gas turbine that is configured and operated to closely follow the Brayton cycle is called a simple cycle gas turbine. Most aircraft gas turbines operate in a simple configuration since attention must be paid to engine weight and frontal area. However, in land or marine applications, additional equipment can be added to the simple cycle gas turbine, leading to increases in efficiency and/or the output of a unit. Three such modifications are regeneration, intercooling and reheating.

Regeneration involves the installation of a heat exchanger (recuperator) through which the turbine exhaust gases pass. The compressed air is then heated in the exhaust gas heat exchanger, before the flow enters the combustor.

If the regenerator is well designed (i.e., the heat exchanger effectiveness is high and the pressure drops are small) the efficiency will be increased over the simple cycle value. However, the relatively high cost of such a regenerator must also be taken into account. Regenerators are being used in the gas turbine engines of the M1 Abrams main battle tank of Desert Storm fame, and in experimental gas turbine automobiles. Regenerated gas turbines increase efficiency 5-6% and are even more effective in improved part-load applications.

Intercooling also involves the use of a heat exchanger. An intercooler is a heat exchanger that cools compressor gas during the compression process. For instance, if the compressor consists of a high and a low pressure unit, the intercooler could be mounted between them to cool the flow and decrease the work necessary for compression in the high pressure compressor. The cooling fluid could be atmospheric air or water (e.g., sea water in the case of a marine gas turbine). It can be shown that the output of a gas turbine is increased with a well-designed intercooler.

Reheating occurs in the turbine and is a way to increase turbine work without changing compressor work or melting the materials from which the turbine is constructed. If a gas turbine has a high pressure and a low pressure turbine at the back end of the machine, a reheater (usually another combustor) can be used to "reheat" the flow between the two turbines. This can increase efficiency by 1-3%. Reheat in a jet engine is accomplished by adding an afterburner at the turbine exhaust, thereby increasing thrust, at the expense of a greatly increased fuel consumption rate.

Combustion Turbine

1. Air Intake Section
2. Compression Section
3. Combustion Section
4. Turbine Section
5. Exhaust Section
6. Exhaust Diffuser

Combustion turbines have been used for power generation for decades and range in size from units starting at about 1 MW to over a 100 MW. Units from 1-15 MW are generally referred to as industrial turbines, a term which differentiates them from larger utility grade turbines and smaller microturbines. Combustion turbines have relatively low installation costs, low emissions, high heat recovery, infrequent maintenance requirements, but low electric efficiency. With these traits, combustion turbines are typically used for cogeneration, as peakers, and in combined cycle configurations.

The Pellistor Catalytic Gas Detector

Pellistors are miniature calorimeters used to measure the energy liberated by the burning of a combustible (flammable) gas or vapour. A pellistor consists of a coil of small- diameter platinum wire supported in a refractory bead on which is deposited a layer of catalytic material, on which the gas is burnt. The coil serves two purposes. Firstly, it is used to heat the bead electrically to its operating temperature, about 500°C, and secondly it is used to detect changes in temperature produced by the oxidation of the flammable gas. The earliest forms of catalytic gas sensors consisted solely of bare coils of platinum wire, operating at 800-1000°C. At such temperatures, platinum wire evaporates extremely quickly causing signal drifts resulting from a reduction in the wire diameter. The specification for such a sensor, which is still produced commercially, requires that the sensor has a life of 100 hours. The majority of present day devices, as stated earlier, have the coil cloaked in a porous ceramic onto which is deposited the precious metal catalyst. The enhanced catalytic activity resulting from the much larger surface area of catalyst available permits much lower operating temperatures of around 500°C, resulting in lower power drain and longer device lifetime.


The concept of the pellistor is based on the fact that the most foolproof way to determine whether a flammable gas is present in air is to test a sample by trying to burn it! A pellistor consists of a very fine coil of wire suspended between two posts. The coil is embedded in a pellet of a ceramic material, and on the surface of the pellet (or 'bead') there is a special catalyst layer.

In operation, a current is passed through the coil, which heats up the bead to a high temperature. When a flammable gas molecule comes into contact with the catalyst layer, the gas 'burns'. The reaction occurs without a flame since the level is below the Lower Explosive Limit (or LEL) of the gas. However, just as in a burning reaction, heat is released which increases the temperature of the bead. This rise in temperature causes the electrical resistance of the coil to rise. There is another bead in the circuit which is identical to the detector bead, but does not contain any catalyst

This bead will react to changes in humidity, ambient temperature etc, but will not react to flammable gas. All that is required is a comparison of the resistance of one bead against another in a Wheatstone bridge type circuit in order to obtain a meaningful signal.

gas sensor-application

(i) Gas Leak Protection: - These are areas of industry where the possibility of flammable gas build-up is small, but the consequences of a gas escape could be catastrophic. These tend to be industries which by their nature have large volumes of gases piped around the works:

• Engineering companies
• Metal working plants
• Research laboratories

(ii) Confined Space Entry

The most prominent application for portable gas detection instruments. The instrument is used to check the atmosphere of sewers, tanks and other vessels prior to entry for maintenance purposes. These instruments invariably are 'multi-gas'. They have 3 or even 4 sensors included in the package. Large volumes of these instruments are purchased by:

• Public utilities - especially water and telecoms
• Chemical and petrochemical - for entry into vessels
• Cabling contractors
• Piling contractors

• Tunnelling contractors
• Civil engineers
• Landfill operators

(iii) Hazardous Area: Working Areas of industry where the build-up of flammable gas or vapour is an ever present danger. These instruments are very often the same multi-gas instruments used for confined space entry, but there are areas where single gas monitors ('explosimeters') are used. Typical industrial sectors here are:

• Chemical and petrochemical industries
• Oil/gas exploration
• Mining

gas sensor-introduction

Gas sensor has recently attracted much attention due to increasing demand of environmental monitoring and other gas detecting applications. Among different types of gas sensor, thin film gas sensor has been much of interest because of microelectronic batch- fabricated compatibility, reproducibility, and ability to form multilayer device structures. In this work, thin film based gas sensing circuit is designed for immediate applications of CO detection for environmental monitoring. Ion assisted deposition (IAD) process offers several advantages for gas sensor fabrication, including reactive deposition for gas-sensitive metal- oxide material optimization and improved thin film adhesion for better device reliability. The metal oxide layer was deposited on alumina or glass substrates. The sensors were tested with reducing gases, in the temperature range between 200 C and 350 C and the electrical change in gas sensor is detected.

Gas sensors interact with a gas to initiate the measurement of its concentration. The gas sensor then provides output to a gas instrument to display the measurements. Common gases measured by gas sensors include ammonia, aerosols, arsine, bromine, carbon dioxide, carbon monoxide, chlorine, chlorine dioxide, Diborane, dust, fluorine, germane, halocarbons or refrigerants, hydrocarbons, hydrogen, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, hydrogen selenide, hydrogen sulfide, mercury vapor, nitrogen dioxide, nitrogen oxides, nitric oxide, organic solvents, oxygen, ozone, phosphine, silane, sulfur dioxide, and water vapor. Important measurement specifications to consider when looking for gas sensors include the response time, the distance, and the flow rate.

The response time is the amount of time required from the initial contact with the gas to the sensors processing of the signal. Distance is the maximum distance from the leak or gas source that the sensor can detect gases. The flow rate is the necessary flow rate of air or gas across the gas sensor to produce signal. Gas sensors can output a measurement of the gases detected in a number of ways. These include percent LEL, percent volume, trace, leakage, consumption, density, and signature or spectra. The lower explosive limit (LEL) or lower flammable limit (LFL) of a combustible gas is defined as the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and ignited. In gas-detection systems, the amount of gas present is specified in terms of % LEL: 0% LEL

being a combustible gas-free atmosphere and 100% LEL being an atmosphere in which the gas is at its lower flammable limit. The relationship between % LEL and % by volume differs from gas to gas. Also called volume percent or percent by volume, percent volume is typically only used for mixtures of liquids. Percent by volume is simply the volume of the solute divided by the sum of the volumes of the other components multiplied by 100%. Trace gas sensors given in units of concentration: ppm. Leakage is given as a flow rate like ml/min. Consumption may also be called respiration given in units of ml/L/hr. Density measurements are given in units of density: mg/m^3. A signature or spectra measurement is a spectral signature of the gases present; the output is often a chromatogram. Common outputs from gas sensors include analog voltage, pulse signals, analog currents and switch or relays. Operating parameters to consider for gas sensors include operating temperature and operating humidity.

Graphite and CFRP machining

High speed machining is common practice in many workshops generating electrodes through high speed, which are then used in SEDM. Cutting speeds are so as high as the machine can, using very high speed spindles (40.000 rpm) although with little power.

Graphite is very soft (he’s the pencil mogul) yet its character is abrasive thus we recommend using diamond coated tools achieving a very high duratioon. Another problem is the dust generated and must be aspirated effectively (specially problematic in CFRP, carbon fibber reinforced plastics), otherwise one can infilter CNC, and other electrical elements causing short-circuits. Plus he has an abrasive effect which can lead to damage of the machine carriage.

Fig.(Left.) High speed milling of a graphite electrode. (Right.) Image of same. Centre Detail of introduction in an electroerosion machine.

Machining of thermo-resistant alloys

Thermoresistant alloys are also called superalloys, two examples are nickel (Inconel 718) and cobalt (Haynes 25). Regarding their machining, they are even worse than titanium, A very high cutting machine speed for these is 80 m/min. Therefore, their machining is performed on conventional machines, and is by no means a high speed process. The term conventional does not mean a low quality machine, but rather it does not have spindle over 6000 rpm nor does it work with feeds over 4 m/min.

The added value of components manufactured in these alloys is very high and the machines used usually have a very high precision, therefore high range machines and a lot of precision. If you will allow a simile, i.e. if one could choose between a high speed car of “I want and can’t have’ or a high-end German one, which would prefer ?. Machining of superalloys was the purpose of the study using hybrid machining techniques, as shown in Fig. and will be commented in the corresponding.

Fig. Cobalt alloy machining at the Univeristy of the Basque County with plasma asistance.

Machining of titanium alloys

With coated hard metal tools machining at cutting speed around 200 m/min is currently common, against the 40 m/min applied 8 years ago. Thus, it is not truly a high speed machining but a much ‘quicker machining’. Diameters of tools used are between 4 and 20 mm, therefore the milling machines do not require spindles over 6000 rpm.

Titanium alloys present several problems:

- They have very low thermal conductivity, and therefore heat concentrates in the cutting area.

- High temperatures in the contact area between tool/chip and the high chemical reactivity of the titanium alloys with most tool materials, are the main causes for the rapid crater wear.

- The low elasticity module of these alloys causes flexions in the part, particularly on thin walled parts. This causes large inaccuracies on the finish and enables machining instability

Fig.11 Aeronautic engine components (photo courtesy of Volvo Aero).

Iron casting machining in the die stamping sector

In this sector HSM is located exclusively in the superfinish operation of the matrizes pursuing a specific object, i.e. reduction of maximum roughness ( Rt) of surfaces with values of 10 microns or less. As feeds can reach 5 to 10 times higher than conventional machining, it offers the possibility of increasing the number of passes to the same extent for the same finish time. The result is a better quality surface, reducing subsequent manual polishing tasks, which might imply almost 30% of the total mould manufactuer.

Machines used are gate type with 5 axes. However, they do not usually machine with the 5 axes simultaneously, but simply orientate headstock and machine after that. As they are superfinishing operations with ball-end tools and over-thicknesses of 0,2 mm we would be talking about a machining process similar to that of tempered steels regardig their physical nature. Superfinishing times with HSM are very long, 39 o 40 hours a big die. This is why it is important for the process to be highly reliable and the tool unlikely to break or wear during the operation. The iron castings used like the globular type GGG70, are easy to machine. Cutting speed reaches 400 m/min with coated hard metal tools and up to 1000 m/min with the PCBN. In Fig. 10 we can see a machine in progress and a partially mechanised boot.

Fig. Milling a die and aspect of this die prior to the operation.

Manufacture of forge dies and recovery thereof

This sector has traditionally used shinking electrodischarge (SEDM) for the manufacture of forging dies in treated steels of approximately 40 HRC of hardness. Nevertheless, the simplicity of the cavities, wide tolerances and roughness requirements to be reached have made this one of the high speed milling star applications.

In the last eight years there has been a real migration from (SEDM) to HSM. In this sector, development times of new series are critical given the high competition between forgings, thus the reduced HSM process times is a highly important factor. In Fig. we can see two die cases, one medium-sized and the other small. Both are high speed machined in under 2 hours from treated steel (already hardened) to over 40 HRC. High speed milling also allows dies to be remachined for their recovery. Once used in the forging process some parts of the die are worn out, so welded material is added. After this process it is remilled at high speed, in this case a somewhat uneven material which co-exists with the original, i.e. additions and even over-tempered areas. The final result is a die ready for forging again.

Fig.9 Forging die of securing element and a suspension bearing.

Machining treated or tempered steel in mould sector

The entry of high speed in this sector has perhaps been a decisive factor in the rapid spread of this technology, since number of users is high yet company size small. I.e. there is a very varied demand which for machine tool manufacturers represents a clear target sector and numerous. If HSM had continued being exclusively for the aeronautical sector it would have had its heyday in the second half of the 1990s. The die and mould sector means talking about numerous plastic injection companies of parts of all sizes and applications, companies dedicated to aluminium and zamac injection; and finally those dedicated to forging. There seems to have been a reciprocal effect between cutting process development and machine tool performance.

In the 1990s tools were developed enabling tempered steel cutting conditions to be increased (@ 50HRC) beyond those considered conventional. These tools were and are of submicrograin hard metal, coated in TiAlN, undoubtedly the kings of machining today, or the PCBN tools PCBN (Polycrystalline Cubic Boron Nitride). Using these tools cutting speeds can be increased 4 and 5 times. This cutting speed obliges the machine to have a spindle capable of spinning at high speed (> 15000 rpm). This spindle rotation speed, together with feeds per tooth recommended for tools implies the machine must maintain working feedrates higher than usual, i.e. greater than 5 metres/minute. Moreover Numerical Control must control axes which are interpolated with sufficient precision. Therefore a machine with very high performances is required, called “ high speed machine ”. This machine has a high speed spindle, a CNC capable of governing spatial movements at high feeds and be very rigid to achieve good precision.

Fig. Small plastic injection mould machined in HSM.

Growing industrial demand for these machines has led to rapid development of different machinery aspects and subsystems like electro-spindles, the axis drives new structures equipped with greater robustness, etc. Thus, machines with superior technology to the conventional have appeared. The new machines also open new application possibilities and substantial improvements in the process, like greater cutting stability, greater contour precision, possibility of machining on 5-axis simultaneously, etc.

Fig. Moulds machine on 3 and 5 axes machines.

n conclusion, if the ‘egg’ came first, the possible new cutting speeds in the ‘proces’, subsequently was the ‘chicken’ i.e. the ‘high speed milling machine’ equipped with features highly superior to the conventional. And at the same time, these new features enable greater process performance, opening new perspectives. Therefore, we find ourselves in a spiralling improvement process aimed at seeking ‘global machining solutions’ with greater productivity and precision, not to mention capable of generating greater added-value for the user.

The high speed milling of tempered steel moulds is centred on the finishing operation with a ball-end mills. In this phase excess material of 0.2 or 0.3 mm is eliminated. Cutting geometry is reflected in Fig. 8. Due to the complex geometry of the cavities ball-end mills whose diameter should not exceed 20 mm., must be used. If one bears in mind the slopes of the shapes to be generated vary between 0 and 90º inclination, one can conclude reaching an effective cutting speed of 300 or 400 m/min (at point A of the figure) requires head rotation speed to exceed 15,000 rpm. However, effective cutting speeds have a value of 200 to 400 m/min, not 4000 m/min as Solomon claimed (see Fig.). Therefore, the chipping process at these speeds is similar to conventional without variation of basic phenomena.

Fig. Cutting speed ratio with axial depth and slope.

In conclusion, high speed milling of hardened steels is ‘a conventional process’ from a thermophysical viewpoint, but performed on a ‘high speed machine’, which machines small chip thicknesses much quicker than the conventional one. Nevertheless, in this case we should forget numerous theories (like Solomon’s) which are found in many informative articles which may lead to confusion.

Another aspect to highlight is that it is currently becoming difficult to clearly separate a high speed machine from a conventional one for the mould world. Some industrial solutions have even appeared like spindle machines with direct coupling of motor and spindle which reach 12,000 rpm., being a cheaper and more robust option for multiple sector applications.

Machining magnesium alloys

These alloys are even softer than the aluminium ones and so easier to machine. The most widely known is AZ91, which is cast and given its lightweight is used in manufacturing parts previously made from aluminium.

The main problem posed is the inflammability of the chips and problems of possible explosion of stored chips, therefore it is a question of safety. Cutting speed may be higher than for aluminium.

Machining aluminium alloys

Undoubtedly this kind of machining is close to the physical high speed concept, since cutting speeds can reach a value of 2000 m/min, or higher when using milling plates whose diameters exceed 50 mm. At this speed chip generation is different from conventional, mainly because almost all the heat generated by the deformation energy inherent in the chipping process is evacuated with the same, which is highly positive for both tool and part.

We could say lightweight alloys is the most traditional field of high speed machining, known since the 1970s and applied to the fuselage component machining sector. In fact the first systematic HSM studies were in the aeronautical field.In the case of aluminium alloys, there are two different cases as per alloy type:

- Aeronautical alloys, particularly the 2000 or 7000 series, called malleable or wrougth. Easily machinable, used in component construction obtained by eliminating a large amount of chips from an initial prismatic block. A small component example is shown in Fig. 5 (left).

- Cast alloys (series 3xx), used mainly in car engines (blocks and pistons), are highly abrasive because they contain silica. The typical operation is planing and finish of a cast part very similar to the final one, with little chip volume. An example is the block shown in Fig.

Airframe component. Aluminium Aluminium engine block.

High speed machining concepts for different materials

The first question is what does one understand by high speed machining. In innumerable articles reference is made to historical theories regarding temperature reduction with high speed cutting as described by Carl Salomon, who patented the process in the 1920s ( German patent #523594).

It is also common to find ballistic references consisting of firing projectiles against materials to simulate material deformation at high cutting speeds like those of Kronenberg in the 1960s.

There are also complex discussions related to chipping under different thermophysical conditions depending on the cutting speed magnitude. Each definition has its own nuances and motivation, valid within a certain application range. In almost all high speed reports, the same generalities are repeated (copying or ‘inspiration’ among authors is evident), many of which use the Solomon curves. You have to remember Solomon put forward his idea almost 15 years prior to modern cutting models, thus it was a highly intuitive invention. He performed milling experiments at speeds over 15,000 m/min. To sum up this patent put forward: “tool temperature and wear increase with cutting speed until reaching a maximum value called critical speed, from they decrease with speed. Critical speed depends on material, as shown in Fig. Thus results will be optimum when machining above this value”.

Today we know temperature and wear always increase, although they tend to stabilise when cutting speed is high enough. The McGee (1979) curve is considered more appropriate for aluminium, although it is just one more obtained by different researchers.

Solomon Curves.

Regarding other materials, it is difficult to verify whether machining steels withstands speeds of 1700 m/min well, because prior to reaching these values, the tools break, As we see in the figure, the HSS rapid steel only withstands up to 650ºC, and the hard metal tool approximately 850ºC. Forgetting the generalities which are always repeated and whose repetition sometimes only manages to this kind of ‘high speed’ term magic halo, cutting values understood today as high speed are reflected in Fig. 4, with orientative cutting speed in each case. To reach these speeds machines capable of doing so are required, since they may be limited by their construction and above all by main motion (spindle). As can be seen there is an area (marked in red) where the machine to be used is high speed, coinciding with today’s industrial offer of this type of machine. Another area (in blue) requires a very high speed machine which although extant today is still in the prototype phase.

High speed machining

This machining type is contrary to the aforementioned traditional concept. Chip section is maintained, i.e. feed per tooth and immersion conditions (radial and axial depths of cut) and to a great extent are even reduced, however, cutting speed is higher than usual. How much is it increased? There is no real academic response, however, it is understood to be higher than ‘traditional’ (x10,x20,...) cutting speed. From the academic viewpoint one could say v _c is increased to the point where the thermophysics of the chipping process varies considerably in relation to that of the conventional process.

This definition implies one or several of these aspects:

- The shearing process deformation speed occurring in the primary shearing area, exceeds 10 ⁵ s ^-1.

- Almost all the heat is evacuated with the chip, the process being close to adiabatic conditions regarding the material. Almost no heat is transmited to the tool, which is optimum in preventing its degradation.

- The effect of chip direction change, which is the material kinetic momentum change, is appreciable and should be considered in the global energy balance.

With this definition we would find almost none of today’s machining processes are high speed, when almost all machine offerers claim that what their milling machines allow is. Why the paradox? Its explanation can be found in the following section. We can advance that only in easily machinable lightweight alloys of magnesium or aluminium, the chipping process differs from that of conventional speeds. However, for this fact to be evident, cutting speeds must be much higher than those applied in today’s industrial HSM processes, which must exceed a cutting speed of 2500 m/min. In the case of steels, castings, difficult-to-machine alloys, titanium, etc., even with today’s cutting speeds being considerably higher than conventional, there is no great variation in intrinsic mechanisms (viscoplastic, thermal, etc.) associated with chipping.High Speed Machining will be abbreviated here to HSM. It should be remembered HSM is recognised worldwide even as a verb (its participle is HSM’ed)

High performance machining, current sense

Today the HPM (High Performance Machining) term is much more general referring to all kinds of cutting technology, substantially improving two aspects of the process:

- Productivity measured as material removal rate, i.e. the amount of material eliminated in a time unit. One should also ensure machining is done under conditions which do not lead to excessive tool wear and tear.

- Quality with regard to greater dimensional precision and less surface roughness. Figure 2 shows a precision map for a test part machined at high speed, which served to detect a non-admissible error in machining.In the last 10 years there has been a minor revolution regarding improvement in processes and an increase in knowledge thereof.

The reasons may be several:

- Materials constituting manufactured components have higher mechanical features, leading almost always to lower machinability. A good example is the growing use of titanium, nickel and cobalt alloys (heat resistant), Csi infiltrated composites, etc.

- Cutting tools have greatly improved in the last 10 years, with the appearance of new hard metal grades (sintered carbide), extrahard materials have been perfected (PCBN, and PCD). Tools tend to become ever more specialised in one application, abandoning their purpose of being applicable to many material groups.

- Tool performance improvement has contributed decisively to new coating developments, ever harder and more resistant to high temperatures. TiAIN coating applied by PVD technology ( Physical Vapour Deposition) has been extremely important.

- Detailed knowledge of processes and their modelisation has been proven to contribute to earning money, i.e. value is obtained from knowledge albeit it still limited. An example is preduction of milling stability conditions ( chatter study); after 30 years and numerous articles on chatter prediction, today there are companies and consultancies earning money with it.

- In Europe,Asia, USA and of course Spain, the machine tool and manufacture by machining sector are very important, thus research resources have been assigned by companies and administrations. Furthermore, improvement in machinery and its process concerns both machine tool builders and suppliers likewise users thereof. Western industry tends to produce components with greater added value more and more; the term high performance refers to everything which contributes to increasing this value, either because it reduces production times and costs increases manufactured quality.

Thus, today High Performance Machining is understood as everything which incorporates notable improvement with respect to traditional machining, increasing process added value both in productivity and quality. The machining term includes chipping process with defined cutting tool (milling, turning, drilling, sawing) with non-defined edge or abrasives (grinding), and even non-conventional processes (electrodischarge, ultrasounics, etc).

High Performance Machining(HPM)-concepts

These two terms are not exactly synonymous and in the last decade have been taken and affected by different nuances. The exact meaning of the same is set out below, and which at times complies simply with the usual inflation of terms derived from their commercial use.

High performance machining, a traditional or classical concept

'High performance machining’ traditionally referred to that simultaneously applying great feeds and cutting depths, while maintaining cutting speeds considered conventional. Under these operating conditions, large chips arise as shown in Fig.This process should be carried out on machine tools with very powerful rigid headstocks. it is called High Removal Rate Machining, which is a more specific descriptive term than that understood by the vaguer ‘high performance’. Higher chip thicknesses than usual are generated by applying large feeds and great cutting depths. Cutting forces grow almost in direct proportion to the chipping section. Thus with this hypothesis we approach two kinds of problems: possible catastrophic tool failure ( breakage) due to high cutting forces or edge breakages at multiple points ( chipping).

Therefore, this kind of machining is only applicable to soft materials as in the case of lightweight alloys, low resistance steels or steels prior to hardening through tempering. A typical case is the turning of large crankshafts and casting rollers on large powerful heavy-duty lathes. In this case large tool inserts are used as shown in Fig.

(Left) Large turning tool inserts for large chip sections.

(Right) Large chip in turning.

pollution control systems in vehicles

Controlling the menace:

There is now a growing realization in the society at large about the need to curb the automobile related pollution and though catalytic converter has captured the popular imagination as the panacea for controlling pollution, in reality there are several methods and devices that help control pollution in the automobile. Some of these act independently while others function in tandem with other systems.

Proper maintenance of the vehicle does not mean regularly topping the air, oil and water coolant levels alone. It means getting to understand the function of various components and ensuring they perform the intended purpose. Many a times inexperienced and unqualified mechanics simply disconnect the system or component without realizing its function as it does not interfere with the normal running of the vehicle thereby exposing it to slow but sure damage in the long run. Most of the devices are meant to ensure the correct air/fuel mixture and controlled exhaust emissions in the fall in this category.

In view of this it is pertinent to know the various devices and systems that are incorporated in the automobile and the role they perform. Maintaining them in working condition will help ensure that the vehicle does not exceed its emission limits

Evaporative Emission Control System:

This system is to control non-exhaust pollution originating from evaporation from the fuel tank and the float chamber of the carburetor. Evaporative emissions from the carburetor are higher due to its proximity to heat generated by the engine. The system includes a positive seal fuel tank cap, vapour vent line, a canister containing, canister purge line, vacuum single line and a purge control valve. Fuel vapours from the fuel tank and carburetor float bowl are absorbed in the carbon in the canister. When there is high vacuum in inlet manifold, vacuum single line forces purge-control valve to open and admit fuel vapours from the canister into the intake manifold, to be burnt with incoming charge.

Crankcase ventilation system:

In older model engines, vapour from the crankcase were allowed to escape to the atmosphere through a venting pipe installed on the tappet/rocker-cover/engine-block/crankcase. On all new generation engines a close crank-case ventilation system is adopted. Vapours from the crankcase are routed to the air intake hose through a metering positive crankcase ventilation valve installed on the hose to be burned along with air/fuel mixture. This PCB valve requires replacement at regular intervals of 40-50000 kms.

Exhaust Gas Re-circulation System:

Conditions of high-pressure and high-temperature exist in an engine which are conductive for the formation of oxides of nitrogen. Reduction is one of the conditions i.e. high-pressure or high-temperature, reduces formation of oxides of nitrogen. Exhaust gas recirculation helps reduction in combustion temperature in an engine. Exhaust gas are drawn into intake manifold from the exhaust manifold through an EGR valve. This valve remains closed during idle , as exhaust gases cause rough idling. At full throttle also the valve remains closed as exhaust gas recirculation is not required. Sometimes the device known as back-pressure transducer (BPT valve) is also incorporated in the system. All valves are basically to regulate the EGR to match the varying operating conditions besides bringing down the temperature to reduce the formation of oxides of nitrogen, hydro-carbons which escape through exhaust gases are also burnt with EGR

Spark Timings Control System:

The system is designed to retard full spark-advance except when the car is in high gear. In conventional engines the vacuum in the inlet manifold through a hose is utilised to operate the mechanism in the distributor for the purpose. However, lately the system has been improved to match load conditions as well as the operating temperature, for better results.

Full spark timing is retarded except when the car is in high gear and the engine is at normal operating temperatures. At all other times, the spark advance is retarded to one degree or another. This is achieved by a thermal vacuum valve, a high gear detecting switch and number of hoses. This system is called ' Transmission Controlled System ' (TCS)

In some later model cars, a system which works solely from engine coolant temperature changes has been incorporated. System includes a thermal vacuum valve, a vacuum delay valve and attendant hoses. This system is called 'Spark Timing Control System' (STC). This system performs the same function as TCS i.e., to retard full advance at times when high levels of pollutants would otherwise escape into exhaust gases.

Generally both the systems are trouble free unless wiring or hoses are mishandled or disconnected.

Catalytic Converters:

Catalytic converters are being widely used the world over This system is designed for the oxidation of pollutant gases escaping primary combustion in the engine within the exhaust system. The catalytic converter is a muffler shaped device installed into the exhaust system. The temperature for bulk gas oxidation and reduction of hydro0-carbon gases, carbon mono-oxide and nitric oxide gas is about 600 to 700 ° C. The temperature of exhaust gases in the exhaust system is lower. The catalytic converter oxidizes and reduces all the three pollutant gases at lower temperature because of catalytic chemical reaction. Catalytic converter becomes effective in the temperature range between 250 to 300 ° C. The use of catalytic converter has become wide spread as it is an effective means of controlling pollution.

The converter is filled with a monolithic substrate coated with small amounts of platinum and palladium through a catalytic action, a chemical change converts carbon monoxide and hydrocarbon into carbon monoxide and water. Such a converter is called a two-way catalytic converter.

A three-way catalytic converter is installed on cars to check pollution. Such a converter uses thin coating of platinum, palladium and rhodium over a support metal and acts on all three major constituents of exhaust pollution: hydro-carbon, carbon monoxide and oxides of nitrogen, oxidising these to water, carbon-dioxide and free hydrogen and nitrogen respectively.

Necessary oxygen required for catalytic reaction is provide to the converter by air induction system. Air is lead into the through an air-induction pump or by pulsations in the exhaust which cause air section. No regular maintenance is required for the catalytic converter system, except for periodic replacement of the air filters of induction system, is provided. Catalytic converters may also require replacements at about 80,000 kms or more. The catalysts durability is affected by engine durability. Any engine malfunction that will expose the catalysts to excessive amounts of unburnt fuel will severely overheat the catalyst and impair its efficiency and its life.

Water jet based tooling strategies for microproduction

The main objective of this contribution is to present a new tooling strategy based on the application of WJ machining, which would allow relatively quick and cost effective production of prototype micro components. In the first step the tool for MEDM is produced in copper by WJ technology. Then the copper tool is used by MEDM technology to produce the final tool in tool steel, which may be further used for replication processes such as hot embossing, pressure molding and others. The complete process chain is shown in Figure.

The proposed tooling strategy offers high flexibility and cost effectiveness. Additionally, it provides more freedom and testing opportunities during the development of new micro devices. The most common used tooling strategy is direct manufacturing of the tool by micro milling. When the features of the tool are rather ribs then grooves, the tooling strategy proposed in Figure has a great advantage over micro milling tool manufacturing, which is the most common used tooling strategy. In the latter case, an end-mill with a relatively small diameter has to remove relatively big volume of the tool which is time consuming and not cost effective.

The main application field of the proposed tooling strategy is the design and development of micro-fluidic devices. Typically, these devices require a well-controlled geometry and surface roughness. With this technology these devices can be manufactured relatively fast and in a cost effective way. Therefore many new concepts and designs can be experimentally validated during the development phase in order to improve the performance of the final product. In the actual context of R&D, flexibility in the manufacturing process enables variety and innovation in the design. The proposed tooling strategy consumes most of its machining time in MEDM machining while WJ machining accounts just for a small portion of the total machining time. However, facing this sequence of different processes, WJ machining of the MEDM tool has an important influence on the final result.

Figure: The process chain of micro-fluidic channel production

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On-line selection of the rough machining parameters upon the eroding surface size

The material removal rate and the surface roughness increase with increased power in the gap. In this way, rough and fine machining is distinguished. When rough machining is performed, the material removal rate should be as high as possible, while the achieved surface roughness does not play an important role.

The EDM process stability is determined by the proportion of harmful discharges in the gap between a workpiece and an electrode, i.e. arc and short-circuit discharges, which not only lower the material removal rate, but also increase the electrode wear. The process is more stable in the case of lower proportion of the harmful discharges. The main cause for unstable EDM process is the contamination of the gap with discharge products. But the surface power density in the gap also affects the process stability. To achieve the highest material removal rate, the roughing setup parameters should be tuned to the eroding surface size. The eroding surface is a projection of the engaged surface of the electrode to the plane perpendicular to the machining direction as shown in Figure. This was analyticaly prooved in the In general, the engaged surface is not plane and the eroding surface size changes with the depth of machining. To select the appropriate roughing setup parameters at any machining depth, the eroding surface size has to be determined on-line.

Figure: The eroding surface is a projection of the engaged surface of the electrode to the plane perpendicular to the machining direction.

Voltage and current in the gap define electric power in the gap (P=UI). There exists the optimal set of the setup parameters' values to obtain the certain power in the gap and the discharge voltage is nearly constant at all machining regimes, thus the power in the gap depends only on the current in the gap. In the literature, the boundary surface current density is given rather then boundary surface power density and it is stated that stable EDM process is achieved when the surface current density is less than 0.1 A. The relation between the surface current density and the material removal rate Vw is presented in Figure. At constant eroding surface size A1, the material removal rate increases with increased surface current density until the boundary surface current density is reached. Higher surface current density causes unstable machining process and the material removal rate decreases. When greater eroding surface is employed (A2), the higher current is needed to reach the boundary surface current density, thus the material removal rate is higher compared to the material removal rate at eroding surface A1.

To select the appropriate roughing setup parameters when eroding surface size varies during the machining, the eroding surface size has to be determined on-line.

Figure: Material removal rate Vw versus the surface current density


For on-line detection of the eroding surface size, it is necessary to monitor the appropriate process quantities z. Proper evaluation of the process quantities is the key to gain suitable process attributes x for the determination of the eroding surface size. The process attributes are the inputs into the model for the selection of the optimal rough machining parameters(see figure)

Figure: On-line selection of the roughing setup parameters of the EDM process

Design adaptation system for machining by EDM process

The EDM is still very often used, specially in tool engineering where tools for mass production are produced. The shape of the tool is a negative image of the product and it has to be easily and cheaply made. To adapt tool design, and at the same time also product design, for easier manufacture of the tool we distinguish two levels: design and manufacturing level. On the manufacturing level a manufacturing technology for tool machining is determined. There is always feedback information from manufacturing to design level to change the tool and the product design according to easier (cheaper) manufacture of the tool. The designer considers the suggestions of the technologist and together they find the best design by taking into account also the demands for the product and the tool.

A system for segmentation and determination of a proper machining process for machining each segment of the tool separately, has already been developed at the Faculty of mechanical engineering. A high speed milling (HSM) and the EDM process are considered as two machining processes for making each segment of the tool. In our work the system for adaptation of the product to easier tool manufacture with EDM process was developed. It is designed for designers to establish critical parts of the product from the point of view of machining the tool with EDM process. With these information the designer can adapt the critical parts of the product design without the tool engineer. By using the system it is possible to reduce number of information from manufacturing to design level and to reduce the time necessary to manufacture tools. In reality it is impossible to eliminate all information from manufacturing to design level or to replace the tool engineer with an expert system.

Figure: Scheme of the design adaptation system for EDM.

EDM controller

EDM process is very unstable, especially when working in fine regime. Often surface damage occurs by arc discharging. Unstable working is unavoidable, so the process is forced to work in liable, but effective region of working. The process is run by operator who overlook it and make a feedback control. Automation of process is the final goal of our research.

A strategy of controlling is a very important part of stable EDM action. It is complex and operator can master it after a long period of learning and collecting his own experience. Operator's knowledge and technological knowledge together with technological receipts can be added to adaptive controller which must be able to accept both of them. We are working on development of such a controller (Fig.). All basic function of controller (identification, reasoning and control) are computer made. Identification of process is well done by computer. The same is with reasoning, based on qualitative and probability assessment, which is usually reserved for human. Operator-to-computer communication is possible and controller can be improved. While the split between human way of reasoning and reasoning algorithms, a strategy of controller is developed by artificial intelligence (learning by example). Method FORS (First Order Regression System) is used. Rules IF-THEN-ELSE can be obtain by human demonstration of process leading. By this method, operator knowledge is transformed to algorithm used by computer. Rules IF-THEN-ELSE are human understandable, too.

Figure: Adaptive control system for EDM process control