Design and Fabrication of Simple Turbo Alternator S.Arunkumar, A.Sridhar, S.Praveen vaitheeswaran, S.Sasikumar, Sefin Jose Department of mechanical engineering, Nandha College of technology, Erode. Abstract The aim of the project is to generate power using exhaust gas. In engine about 33 percent of energy wasted in exhaust gas and about 4 percent of engine power is utilized to run alternator. So we would like extract some energy from this energy. Turbo alternator is an Ac generator used to generate power required to electrical system of the vehicle using the exhaust gas waste energy. It will improve brake power of the engine. Here we fabricated simple model of turbo alternator. 1. Introduction A Turbo-Alternator is a machine which uses a Turbine to conserve exhaust energy and it will transfer this mechanical energy transferred to alternator to produce electrical energy. The recovered energy will be stored in battery. And this energy can be utilized in electrical systems. 2. Exhaust process The exhaust process consists of two steps: Blowdown and exhaust stroke. When the exhaust valve opens near the end of the expansion stroke (point 4 in Figure 1.1), the high-temperature gases are suddenly subjected to a pressure decrease as the resulting blowdown occurs. A large percentage of the gases leaves the combustion chamber during this blowdown process, driven by the pressure differential across the open exhaust valve. When the pressure across the exhaust valve is finally equalized, the cylinder is still filled with exhaust gases at the exhaust manifold pressure of about one atmosphere. These gases are then pushed out of the cylinder through the still open exhaust valve by the piston as it travels from BDC to TDC during the exhaust stroke. Temperature of the exhaust gases is cooled by expansion cooling when the pressure is suddenly reduced during blowdown. Although this expansion is not reversible, the ideal gas isentropic relationship between pressure and temperature serves as a good model to approximate exhaust temperature T7 in the hypothetical process 4-7 of Figure 1.1. 6
Figure 1.1, Air-standard Otto cycle for engine at Wide Open Throttle (WOT), showing process 4-7 experienced by exhaust during Blowdown. P7 is the pressure in the exhaust system and is almost always very close to one atmosphere in value. Gas leaving the combustion chamber during the blowdown process will also have kinetic energy due to high velocity flow through the exhaust valve. This kinetic energy will very quickly be dissipated in the exhaust manifold, and there will be a subsequent rise in enthalpy and temperature. The first elements of gas leaving the combustion chamber will have the highest velocity and will therefore reach the highest temperature when this velocity is dissipated (point 7a in Figure 1.1). Each subsequent element of gas will have less velocity and will thus experience less temperature rise (points 7b, 7c, etc.). The last elements of gas leaving the combustion chamber during blowdown and the gas pushed out during the exhaust stroke will have relatively low kinetic energy and will have a temperature very close to T7. Choked flow (sonic velocity) will be experienced across the exhaust valve at the start of blowdown, and this determines the resulting gas velocity and kinetic energy. If possible, it is desirable to mount the turbine very close to the exhaust manifold. This is done so that exhaust kinetic energy can be utilized in the turbine. 2.1 The energy in the exhaust system Figure 1.2 shows the energy potentially available in the exhaust system, with an ideal cycle. The exhaust valve opens at BDC, point 5, where the cylinder pressure is much greater than the ambient pressure at the end of the exhaust pipe. If the contents of cylinder at EVO were somehow allowed to expand isentropically and reversibly down to the ambient pressure (to point 6), then the work that could be done is represented by the cross-hatched area 5-6-1. This work could be recovered by allowing the piston to move further than normal as shown in Figure 1.2. However, this requires an engine with an exceptionally long stroke and in practice it is found that the additional piston friction offsets the work gained by an ultra long expansion stroke. The work represented by area 5-6-1 is therefore potentially available to a turbocharger turbine placed in the exhaust manifold. It is called the 'blow-down' energy, since it involves the combustion products being 'blown-down' from cylinder pressure at point 5 to atmospheric pressure at point 6, when the exhaust valve opens. Figure 1.2 represents a naturally aspirated engine. 2.2 Engine cycle and comparison The Air-Standard Otto Cycle approximates the spark-ignition combustion engine 1-2 Isentropic Compression 2-3 Constant Volume Heat Addition 3-4 Isentropic Expansion 4-1 Constant Volume Heat Rejection 7
The ideal dual combustion cycle of a diesel engine in naturally aspirated and turbocharged form. Since the inlet and exhaust pressures are above ambient, and more fuel is burnt in the engine, the cylinder pressure throughout the cycle, and particularly during combustion, is substantially higher for the turbocharged cycle. The compression ratio of the engine must be reduced to prevent an excessive maximum cylinder pressure being reached. 3. Main components and its description: 3.1 The main components in our model are: Turbine Turbine housing Bearing Alternator Shaft Nut and bolts 3.2 Turbine: Radial inflow turbines are universally used, usually friction or electron beam welded to the shaft. The turbine wheel must sustain the same high rotational speed of the compressor and operate at gas temperatures up to 900 K. The turbines are investment cast in high temperature creep resistant steels, such as 713 C Inconel. Its properties exceed the requirements but it is a readily available material, from the gas turbine industry. 3.3 Turbine Housing The turbine housing must stand the same high gas temperature but is not subject to rotating stress. However it should be strong enough to sustain a rotor burst. Sand or shell cast SG iron (spheroidal graphite nodular) is used in most applications, and is free of scaling at temperatures up to 900 K. Ni-resist is used for higher temperatures but is more expensive, and prone to cracking. 3.4 Bearing: 8
For low cost, simplicity and ease of maintenance, the bearing system must be designed to use the lubricating oil of the engine. Two fully floating sleeve bearings are used, free to rotate at a speed less than the rotor speed, and determined by bearing clearances. The outer oil film imparts an additional degree of damping into the bearing system, but both the shaft to sleeve to housing clearances are large for improved stability. Typical clearances are 0.02 to 0.05 mm between shaft and sleeve and 0.07 to 0.1 mm between sleeve and housing. These large clearances mean that oil filtration down to about 20 /im only is required; easily met by full flow paper engine oil filters. A large oil flow rate is required, due to large clearances and the need to cool the turbine end bearing. Bearing material is typically leaded bronze with added tin flashing. 3.5 Alternator: Alternator is an AC power generator which converts turbine mechanical energy into electrical energy. Alternator has electromagnet poles to generate magnetic flux. When alternator rotor rotates it cuts magnetic flux to generate AC power. The diode circuits were used in alternator control unit to convert AC power into DC power. 3.6 Shaft: A drive shaft, driving shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement between them. Drive shafts are carriers of torque: they are subject to torsion and shear stress, equivalent to the difference between the input torque and the load. They must therefore be strong enough to bear the stress, whilst avoiding too much additional weight as that would in turn increase their inertia. 4. Working principle A turbo-alternator has a turbine which rotates by using exhaust dynamic energy of an engine. A turboalternator consists of a turbine and an alternator on a shared shaft. The turbine converts dynamic energy to rotational force, which is in turn used to drive the alternator. The alternator produces electrical energy which can be stored in battery. The objective of a turbo-alternator is to improve the engine's efficiency. A naturally aspirated automobile engine uses only the downward stroke of a piston to create rotation movement and there too some wastage while blowdown. This energy will be utilized to run the turbine. 4. Application 4.1 Naturally Aspired Engines: In Naturally Aspired Engine, the amount of exhaust gas flow is less than the amount of gas flow in Turbocharged / Supercharged Engine. So the amount of energy recovered from exhaust is less when compared to Turbocharged / Supercharged Engine. 4.2 Turbocharged / Supercharged Engine: The alternator may best suitable for supercharged engine as there is no energy recovery system for exhaust. In Turbocharged Engine, the exhaust energy highly utilized to run compressor. So the alternator can be placed after turbocharger to recover some more energy from exhaust. 9
4.3 Hybrid Vehicles: In recent years automobile manufactures try to increase efficiency by providing hybrid system. And government also provides tax subsidy for hybrid vehicles. This alternator can be used for hybrid system. 5. Merits Small size High engine specific power High power to weight ratio Improvement in efficiency Save fuel 6. Demerits Inappropriate turbine may cause negative impact on the power generation and engine power output too. High cost 7. Conclusion In designing and fabricating this project, we gained invaluable technical knowledge, regarding material solution, planning the project, group efforts in achieving the target, cost estimation and also confidence in doing works. However, in completing the project we felt that aims of the implementation of project work in our courage is completely fulfilled. References Books: 1. Engineering Fundamentals of the Internal Combustion Engine, by Willard W. Pulkrabek 10
2. The theory of Turbocharging Website: a. www.google.co.in b. http://www.fueleconomy.gov/feg/atv.shtml 11