(IVA
`
`ancc
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`hcte
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`V
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`BMW1047
`Page 1 of 35
`
`

`

`First published in Great Britain in 1995 by
`Arnold. a member of the Hodder Headline Group
`338 Euston Road. London NWl 38H
`
`Second impression 1997
`
`© 1995 Heinz Heisler
`
`All rights reserved. No part of this publication may be reproduced or
`transmitted in any form or by any means, electronically or mechanically,
`including photocopying, recording or any information storage or retrieval
`system, without either prior permission in writing from the publisher or a
`licence permitting restricted copying. In the United Kingdom such licences
`are issued by the Copyright Licensing Agency: 90 Tottenham Court Road,
`London WIP9HE.
`
`Library of Congress Cataloging-in-Publication Data
`Number: 95 74841
`
`ISBN 1 56091 734 2
`
`Printed and bound in Great Britain
`
`BMW1047
`Page 2 of 35
`
`

`

`Roots
`compressor
`
`~/
`-Q/
`Cb/
`o·;
`
`/ ....
`
`L----1
`
`<)<ti
`Cb
`/
`I o·
`
`~
`~ 60
`..c a
`(/)
`..c
`~ 40 35%
`(I)
`~
`0
`C.
`~ 20
`E
`0
`~ o~_._ _ __.__.__.__~___.-~
`2000
`4000
`6000
`0
`Rotational speed (rev/min)
`
`51%
`
`I
`
`G - Lader
`compressor
`
`~- 6.37 Comparative power absorbed by Root's-
`: 'G' -Lader-type superchargers relative to rotational
`~'='9d
`
`um boost pressure is limited to just under
`~ bar.
`The adiabatic compressor efficiency rises rapid(cid:173)
`with an increase in boost pressure, reaching a
`~ak of about 68%, and then gradually decreases
`-=ig. 6.24).
`It is claimed that there is very little temperature
`:-:se during the compression process so that with
`e low amount of power absorbed by the com(cid:173)
`-ressor and the relatively cool delivery of the
`)()St charge, it makes this type of supercharger
`-=ry efficient and extremely suitable for boosting
`~ all- to medium-sized car petrol engines.
`
`90 ,----,---.----,----.----.---,
`
`~ 85
`~
`G"
`C
`(I)
`'(3 80
`~
`(I)
`
`C.l ·c m
`§ 75 g
`
`70 L-__JL----'------'-- - - ' - - - - ' - -~
`6000
`4000
`2000
`0
`Eccentric shaft speed (rev/min)
`
`Fig. 6.38 Relationship between volumetric efficiency
`,nd eccentric shaft speed at constant boost pressure
`-:,r a 'G' -Lader supercharger
`
`6.7 Turbochargers
`
`6.7.1 Introduction
`
`A typical petrol engine may harness up to 30% of
`the energy contained in the fuel supplied to do
`useful work under optimum conditions but the
`reminaing 70% of this energy is lost in the follow(cid:173)
`mg way:
`
`7% heat energy to friction, pumping and dyna(cid:173)
`mic movement
`9% heat energy to surrounding air
`16% heat energy to engine's coolant system
`38% heat energy to outgoing exhaust gases
`
`Thus, the vast majority of energy, for design
`reasons, is allowed to escape to the atmosphere
`through the exhaust system.
`A turbocharger utilizes a portion of the energy
`contained in the exhaust gas- when it is released
`by the opening of the exhaust valve towards the
`end of the power stroke (something like 50°
`before BDC)-
`to drive a turbine wheel which
`simultaneously propels a centrifugal compressor
`wheel.
`The turbocharger relies solely on extracting up
`to a third of the wasted energy passing out from
`the engine's cylinders to impart power to the
`turbine wheel and compressor wheel assembly.
`However, this does produce a penalty by increas(cid:173)
`ing the manifold's back-pressure and so making it
`more difficult for each successive burnt charge to
`be expelled from the cylinders. It therefore im(cid:173)
`pedes the clearing process in the cylinders during
`the exhaust strokes.
`The ideal available energy which can be used to
`drive the turbocharger comes from the blow(cid:173)
`down energy transfer which takes place when the
`exhaust valve opens and the gas expands down to
`atmospheric pressure (Fig. 6.39). This blow-down
`energy is represented by the loop area 4, 5 and 6
`whereas the boost pressure energy used to fill the
`cylinder is represented by the rectangular area 0,
`1, 6 and 7.
`Turbocharged engines produce higher cylinder
`volumetric efficiencies compared with the nor(cid:173)
`mally aspirated induction systems. Therefore,
`there will be higher peak cylinder pressures which
`increase the mechanical loading of the engine
`components and could cause detonation in petrol
`engines. Therefore, it is usual to reduce the
`engine's compression ratio by a factor of one or
`
`315
`
`BMW1047
`Page 3 of 35
`
`

`

`3 Useful
`Required
`energy to
`energy
`devel_oped fill cylinder
`in cylinder under
`boost
`pressure
`Blow-down
`energy
`available
`to drive
`4 turbocharger
`
`TDC
`
`BOC
`Volume
`
`Fig. 6.39 Petrol engine cycle pressure-volume
`diagram showing available exhaust gas energy
`
`two. Thus, a compression ratio of 10: 1 for a
`normally aspirated engine would be derated to
`9: 1 for a low boost pressure or even reduced to
`8: 1 if a medium to high boost pressure is to be
`introduced. Similarly, for a direct injection diesel
`engine which might normally have a compression
`ratio of 16: 1, when lightly turbocharged the
`compression ratio may be lowered to 15: 1 and, if
`much higher supercharged inlet pressures are to
`be used, the compression ratio may have to be
`brought down to something like 14: 1.
`The compression of the charge entering the
`cells of the impellor depends upon the centrifugal
`force effect which increases with the square of the
`rotational speed of the impellor wheel. Conse(cid:173)
`quently, under light load and low engine speed
`conditions the energy released with the exhaust
`gases will be relatively small and is therefore
`insufficient to drive the turbine assembly at very
`high speeds. Correspondingly, there will be very
`little extra boost pressure to make any marked
`improvement to the engine's torque and power
`output in the low-speed range of the engine.
`Thus, in effect, the turbocharged engine will
`operate with almost no boost pressure and with a
`reduced compression ratio compared with the
`equivalent naturally aspirated engine. Hence, in
`the very low speed range, the turbocharged en(cid:173)
`gine may have torque and power outputs and fuel
`consumption values which are inferior to the
`unsupercharged engine.
`Another inherent undesirable characteristic of
`turbochargers is that when the engine is suddenly
`accelerated there will be a small time delay before
`the extra energy discharged into the turbine hous-
`
`316
`
`ing volute can speed up the turbine wheel. 'I:.
`during this transition period, there will be ,
`little improvement in the cylinder filling proc~
`and hence the rise in cylinder brake mean e::
`tive pressure will be rather sluggish.
`
`6.7.2 Altitude compensation
`(Fig. 6.40)
`
`Engine power outputs are tested and rated
`sea-level where the atmospheric air is most deru
`however as a vehicle climbs, its altitude is
`creased ~nd the air becomes thinner, that is. · ~
`dense. The consequence
`is a decrease
`volumetric efficiency as less air will be drawn iJ:
`the cylinders per cycle, with a correspon~
`reduction in engine power since power is direc:
`related to the actual mass of charge burnt in ·
`cylinder's every power stroke. A naturally asp
`rated engine will have its power output reduc=
`by approximately 13 % if it is operated apprc
`imately 1000 m above sea-level. Superchar~
`the cylinders enables the engine's rated po\\ -
`above sea-level to be maintained or even e
`ceeded.
`With a turbocharged engine there will still Y(cid:173)
`some power loss with the engine operating at hi::--(cid:173)
`altitudes, but the loss will be far less than if tb:
`engine breathing depended only on natun
`aspiration. As can be seen (Fig. 6.40) at 1000 ::::r
`the power loss is only 8% compared with th=
`naturally aspirated engine where the power de(cid:173)
`crease is roughly 13%. The reason for the tu:(cid:173)
`bocharger's ability to compensate by raising i(cid:173)
`boost pressure is that the turbine speed increa.sc-.
`
`100
`
`90
`
`80
`
`70
`
`Q)
`>
`~
`cb
`
`Q)
`Cf)
`
`Q)
`
`Q)
`
`cil ....
`::
`0 a.
`C ·o,
`C
`Q)
`X
`ro
`~
`~ 0
`
`-
`
`cu
`e.
`0.3 ~
`:::,
`Cf)
`0.2 fil
`C.
`
`0.1 -
`
`Cl)
`0
`0
`ID
`
`60
`0
`
`0
`3000
`2000
`1000
`Altitude above sea-level (m)
`Fig. 6.40 Effect of altitude on rated engine power at
`sea level for both naturally-aspirated and turbochargec
`engines
`
`BMW1047
`Page 4 of 35
`
`

`

`_ :-ectly with any increase in pressure difference
`ierween the exhaust gas entering the turbine and
`exit pressure, which is the ambient air press(cid:173)
`..:e. Therefore, as the altitude increases the air
`-:-.ecomes thinner and the pressure drops, but the
`-:-essure in the exhaust manifold which impinges
`:no the turbine wheel remains substantially the
`~e. The result is that the pressure differential
`-ross the turbine increases and therefore raises
`:1e turbine assembly's speed and, correspond(cid:173)
`-gly, the compressor's boost pressure.
`The necessary boost pressure required to main(cid:173)
`:..un the engine's sea-level power, the loss of
`~a-level power rating, and the turbocharger's
`_bility to compensate partially for the decrease in
`ill density with increasing altitude driving, is
`.:ompared in Fig. 6.40.
`
`,6.7.3 Description of turbocharger
`Fig. 6.41)
`
`c\ turbocharger comprises three major compo(cid:173)
`:ients, an exhaust-gas driven turbine and housing,
`..1 centrifugal compressor wheel and housing and
`..1n interconnecting support spindle mounted on a
`pair of fully floating plain bearings, which are
`themselves encased in a central bearing housing
`made from nickel cast iron (Fig. 6.41).
`
`Compressor
`.
`.
`Figs 6.41 and 6.42)
`The impellor compressor wheel (Fig. 6.42) 1s an
`aluminium alloy casting which takes the form of a
`disc mounted on a hub with radial blades project(cid:173)
`ing from one side. This causes the air surrounding
`the compressor wheel to be divided up into_ a
`number of cells (something like 12 blades). The
`hub of the compressor-wheel onto which the
`blades are attached is so curved that air enters the
`cells formed between adjacent pairs of blades
`axially from the centre. The enclosed air is then
`divided by the passageway formed between the
`hub and the compressor-wheel housing internal
`wall so that the fl.ow path moves through a
`right-angle causing the air to be expelled radially
`from the cells. Once the air reaches the periphery
`of the compressor-wheel it then passes to the
`parallel diffuser (gap) formed between the be_ar(cid:173)
`ing housing and the compressor-wheel housmg
`(Fig. 6.41). From the diffuser gap the ai~ fl~ws
`into a circular volute-shaped collector, which 1s a
`constant expansion passage from some starting
`point to its exit.
`
`Turbine
`(Figs 6.41 and 6.42)
`The exhaust gas temperature at the inlet to the
`turbine wheel, under light-load to full-load high(cid:173)
`speed operating conditions, may range between
`600°C to 900°C. Consequently, the turbine is
`usually made from a high-temperature heat(cid:173)
`resistant nickel-based alloy such as 'Inconel'.
`Ceramic materials such as sintered silicon nitride
`are being developed as an alternati've to nickel
`based alloys, and these materials have the advan(cid:173)
`tage of weighing less than half that of suitable
`metallic materials. These ceramic materials have
`a higher specific strength and a much lower
`coefficient of expansion than their counterparts,
`they are also capable of operating under the same
`full-load exhaust gas temperature conditions at
`similar maximum rotational speeds as the nickel(cid:173)
`based alloys.
`The turbine wheel (Fig. 6.42) takes the form of
`a hub supporting a disc at one end and a number
`of radial blades which project axially and radially
`from both the hub and disc respectively. The
`outer edges of the blades are curved backwards to
`trap the impinging exhaust gases .
`Exhaust gases from the manifold enter the
`spherical graphite cast-iron turbine housing flange
`entrance, the gases then flow around either a
`single or twin volute passageway surrounding the
`turbine wheel (Fig. 6.41). The gases are then
`forced tangentially inwards from the throat of the
`turbine housing so that they impinge onto the
`blade faces . The flow path then directs the gases
`gradually through a right-angle so that they come
`out axially from the centre of the turbine hub, and
`they are then expelled into the exhaust pipe
`system.
`In most turbocharger designs the turbine and
`spindle are joined together by some sort of weld(cid:173)
`ing process such as inert gas welding, resistance
`welding, or electron beam welding. The general
`trend these days is to attach the turbine to the
`spindle by the friction welding solid phase tech(cid:173)
`nique (Fig. 6.42). The turbine and spindle are
`brought together under load, with one part re(cid:173)
`volving against the other so that frictional heat is
`generated at the interface. When the joint area is
`sufficiently plastic as a result of the increase in
`temperature the rotation is stopped and the end
`force increased to forge and consolidate the
`metallic bonds. The surface films and inclusions
`that might interfere with the formation of these
`bonds are broken up by friction and removed
`from the weld area in a radial direction owing to
`
`317
`
`BMW1047
`Page 5 of 35
`
`

`

`Clamp
`set bolt
`
`Compressor
`involute
`cover
`
`Heat
`shroud
`
`Bearing Turbine
`housing sealing
`ring
`
`Oil
`inlet
`
`Turbine
`twin involute
`housing
`
`Turbine
`wheel
`
`... Q) -Q)
`E
`C\1 =a
`....
`
`Q)
`(.)
`:::,
`-0
`C
`
`Air
`inlet
`
`... Q)
`+-
`Q)
`E
`
`C\1 =a ... Q)
`
`(.)
`:::::,
`-0
`X w
`
`Turbine
`housin;
`flange
`
`Compressor
`sealing
`ring
`
`Oil
`deflector Oil
`exit
`funnel
`
`Air
`insulation
`space
`
`Parallel
`diffuser
`
`Exhaust gas entry
`
`Fig. 6.41 Turbocharger construction
`
`marked plastic deformation on the surfaces. The
`burrs surrounding the joint are then ground away
`leaving a high-quality joint.
`
`Spindle and bearing assembly
`(Figs 6.41 and 6.42)
`The turbine wheel and steel spindle can be
`welded together in a vacuum by an electron
`beam. This joining process produces a neat joint
`in which the heat flow areas are held to a mini(cid:173)
`mum so that there is very little distortion and
`machining is kept to the minimum. The medium
`carbon steel spindle is induction hardened where
`
`the bearings contact the spindle. The holk
`space between the turbine wheel and the spin
`(Fig. 6.42 sectioned-view) prevents heat be·
`transferred through the centre of the spinrue
`Thus, heat is carried away along the outer secti
`of the spindle, which can readily be cooled by
`lubricating oil. The spindle is supported on a pill!'
`of free-floating phosphorus bronze plain beari..Ja
`and around the outside of each bearing shell
`six radial holes and, depending upon des _
`there may be a circumferential groove machias
`to distribute the lubrication oil. The rotatior.
`the spindle assembly in conjunction with the
`the engine's lubrication sys-
`supply from
`
`318
`
`BMW1047
`Page 6 of 35
`
`

`

`Burr
`to be
`removed
`
`Turbine
`
`Spindle
`
`After friction welding
`
`Before friction welding
`
`Turbine and
`spindle
`assembly
`
`Spindle
`
`Compressor
`wheel
`Fig. 6.42 Compressor and turbine wheel assembly
`
`causes the viscous drag to rotate these bearings
`at approximately one-third of the spindle's rota(cid:173)
`tional speed.
`The large-diameter shouldered section next to
`the turbine-wheel is grooved to house two piston
`rings and a similar single piston ring is positioned
`on a grooved collar at the opposite end of the
`spindle next to the compressor wheel (Fig. 6.41).
`These piston-type rings remain stationary when
`the spindle rotates, their function being to pre(cid:173)
`vent exhaust gas or compressed air entering the
`bearing housing chamber. Oil should be pre(cid:173)
`vented from reaching the piston rings as any film ,
`foam or splash entering the seal region will leak
`
`out. The spindle bearing at the turbine wheel end
`is separated from the large diameter shouldered
`section of the spindle by a cast-in-oil drain slot
`cavity so that oil, spurting from the end of the
`bearing, spills into this cavity and then drains
`down to the oil exit funnel. In contrast, at the
`compressor wheel end, an oil deflector, fixed to
`the thrust bearing, protects the piston ring seal
`from contamination by shielding the bearing oil
`spray from the piston rings, the oil is then permit(cid:173)
`ted to drain down to the bearing housing exit.
`During operation, the exhaust gas impinging
`onto the turbine-wheel, and the compressed air
`reaction on the compressor wheel under varying
`
`319
`
`BMW1047
`Page 7 of 35
`
`

`

`speed and load conditions, produces a certain
`amount of end thrust as the spindle will want to
`move axially first in one direction and then the
`other.
`Provision for end float or thrust control is
`provided by a thrust spacer (Fig. 6.41) which has
`a deep central groove. This thrust spacer is sand(cid:173)
`wiched between the stepped spindle shoulder and
`the piston-ring spacer. A phosphorus bronze
`thrust bearing plate, which is attached to the
`bearing housing, slips into the central groove.
`Consequently, the walls on each side of the
`thrust-plate become thrust-rings, whilst the re(cid:173)
`duced diameter portion between them forms a
`spacer sleeve, its width being critical to control
`any end movement of the compressor-wheel and
`the turbine wheel.
`To minimize the heat transference from the
`turbine-wheel exhaust gas flow path to the bear(cid:173)
`ing housing, a space is created between the tur(cid:173)
`bine-wheel and the bearing housing. This air
`space is enclosed by a stainless-steel heat shroud
`pressing, shaped in the form of a cup, which is
`located immediately behind the turbine-wheel.
`This relatively large air gap provides an effective
`heat barrier, and therefore insulates the bearing
`housing from the hot turbine assembly.
`Both radial plain bearings and the axial end
`thrust bearing are supplied with ample oil from
`the engine's lubrication system via drillings made
`in the bearing housing and in the bearings them(cid:173)
`selves (Fig. 6.41). The oil supply has two major
`functions: firstly, to lubricate the bearings so that
`a hydrodynamic oil film can be established so
`that, in effect, the shaft and bearings are floating
`on oil; and secondly, to remove excess heat from
`the bearing assembly. Thus, it is just as important
`to be able to return the circulating oil to the
`engine's sump as it is to flood the bearings in the
`first place with lubricant.
`In some turbochargers, the bearing housing
`incorporates a
`liquid coolant jacket through
`which coolant from the engine's cooling system is
`made to circulate via a pair of flexible inlet and
`outlet pipes.
`
`6.7.4 The operating principles of
`compressor and turbine
`
`The operating principle of the compressor
`(Fig. 6.43)
`With the spindle assembly rotating, the air cells
`formed between adjacent blades sweep the en-
`
`trapped air around the compressor housin=(cid:173)
`curved wall: the air mass is therefore subjected t
`centrifugal force.
`This force produces a radial outward motion r
`the air, with its velocity and, to some extent, ili
`pressure becoming greater the further the al!'
`moves out from the centre of rotation (Fig. 6.43
`The air thus moves through the diverging pas(cid:173)
`sages of the cells to the periphery where it is fl~
`out with a high velocity. More air will, at the sa~
`time, be drawn into the inducer due to th
`forward curved blades at the entrance, and t~
`tends to generate a slight depression. Hence.
`encourages a continuous supply of fresh charge t
`enter the eye of the impellor.
`Once leaving the outer rim of the impellor the
`tangential air movement relative to the impel!
`has its maximum kinetic energy, but, as it
`pressure energy that is required, the air is e
`panded in the parallel diffuser so that its veloci
`sharply falls while, simultaneously, its pressure
`rises. In other words, the kinetic energy at the
`entrance to the parallel diffuser is partially co
`verted into pressure energy by the time it arri\
`at the outer edge of the parallel annular-shaped
`passageway.
`The air then leaving the diffuser is pr
`gressively collected in the volute, from some
`starting point where the circular passageway is
`its smallest, to its exit where the passage is at ·
`largest cross-section. The volute therefore pr
`vents the air discharged from the diffuser becom(cid:173)
`ing congested and, at the same time, it continu
`the diffusion process further; that is, the
`movement is slowed down even more whereas
`pressure still rises.
`
`The operating principle of the turbine
`(Fig. 6.43)
`Exhaust gas from the engine's cylinders is ex
`led via the exhaust manifold into the turb·
`volute circular decreasing cross-section passa"'
`way, at a very high velocity, where it is direct
`tangentially inwards through the throat of
`turbine housing. The released gas kinetic ener
`impinges on the turbine-blades, thereby imp
`ing energy to the turbine-wheel as it pas
`through the cells formed between adjacent bla
`with a corresponding decrease in both gas velo ·
`and pressure (Fig. 6.43). The exhaust gas with
`rapidly decreasing energy moves radially inwar
`and, at the same time, its flow path mo\
`through a right-angle so that it passes axialk(cid:173)
`along the hub before leaving the turbine housin~
`
`320
`
`-
`
`BMW1047
`Page 8 of 35
`
`

`

`Compressor
`
`Compressed
`air
`discharge
`
`Flanged
`mounting
`
`~
`~
`·5
`·5
`0
`0
`~
`~
`'O
`'O
`C
`C
`(\'j
`(\'j
`~1---------l.-L.-~ ~i-~-~~~
`::J
`::J
`(/)
`(/)
`(/)---~- ~ (/)-- - - -~
`.. I £ --D~is~ta~n_ce_~
`£ I... Distance
`Compressor
`
`Turbine
`
`fig. 6.43 Turbocharger principle
`
`-:::ie expansion of the gas ejected from the tur(cid:173)
`~:ne-wheel then produces a sudden drop in its
`d ocity and pressure as it enters the silencer pipe
`-:-stem. Turbine speed and boost pressure are
`..1rgely dependent upon the amount of energy
`.:ontained in the hot, highly mobile exhaust gases
`.md on the rate of energy transference from the
`gas to the turbine-blades. Thus, at idle speed very
`i ttle fuel is supplied to the engine, and therefore
`:he energy content in the outgoing exhaust gas
`will also be very low whereas, with increased
`engine speed and load conditions, considerably
`more fuel is consumed by the engine, which in
`rum releases proportionally more energy to the
`escaping exhaust gases. Hence, at light load and
`low speed, the turbine assembly speed can be
`something like 30000 to 50000 rev/min, whereas
`at high speed and high load operating conditions
`the spindle and wheel assembly can revolve at
`speeds up to 120000 to 150000 rev/min, depend(cid:173)
`ing upon design and application.
`
`6.7.5 Compressor impellor and
`housing design
`
`Compressor and housing arrangements
`Closed or shrouded impellor with scroll diffuser
`(Fig. 6.44(a) ). The impellor may be closed or
`shrouded; that is, the impellor is cast so that the
`cells or channels are completely enclosed. This
`construction eliminates direct leakage as the in(cid:173)
`duced air is flung radially outwards in the cells.
`However, it is difficult to cast radial cells so that
`they curve backwards and also provide an axial
`angled inlet at the eye of the impellor. Other
`important disadvantages which must be consi(cid:173)
`dered are that the mass of the shroud is supported
`by the blades, such that at high rotational speeds
`the blades are subjected to severe centrifugal
`stresses. In addition, the shroud which is away
`from the central hub raises the impellor wheel's
`moment of inertia and thus impedes its ability to
`accelerate or decelerate rapidly. This design has
`been succeeded by the open-cell type impellor.
`
`321
`
`BMW1047
`Page 9 of 35
`
`

`

`(a) shrouded
`impeller
`
`(b) scroll diffuser
`
`(c) parallel wall
`diffuser
`
`(d) parallel tongue
`diffuser
`
`Fig. 6.44 Vaneless diffuser compressors
`
`scroll diffuser
`impellor with
`Open
`(Fig.
`6.44(b)). With open impellor and scroll diffus(cid:173)
`er, the impeller is cast with blades forming the
`walls of the cells, these blades can be shaped so as
`to provide the best inducement for the incoming
`air and the radial flow path can be curved back(cid:173)
`wards to optimize the flow discharge at high
`rotational speeds. However, there is a clearance
`between the outer edges of the blades and the
`internal curved walls of the housing which en(cid:173)
`closes the rotating cells. This gap, small as it is,
`will be responsible for leakage losses under high
`boost pressure operating conditions. With this
`arrangement, the kinetic energy of the air at the
`blade tips is converted into pressure energy by
`directly entering into the relatively large scroll
`volume. In other words, the air flung out at the
`rim of the impellor enters the scroll where it is
`diffused by the relatively large mass of air already
`occupying the circular passageway. Thus, the
`intermingling of the discharged air causes its
`velocity to decrease and its pressure to increase,
`which goes someway towards producing the de(cid:173)
`sired flow conditions for the charge.
`
`Open impellor with parallel wall diffuser (Fig.
`6.44(c)).
`If a more positive method of convert(cid:173)
`ing the kinetic energy to pressure energy is re(cid:173)
`quired, a parallel annular space between the
`impellor and the volute or scroll will enlarge the
`circular passage from the entrance at the impellor
`rim to where it merges with the discharge volute.
`Thus, as the air moves outwards in a semi-spiral
`and radial direction the air will expand, thus
`causmg its speed to reduce while its pressure
`rises.
`
`Open impellor with parallel tongue diffuser
`6.44( d)). To reduce the maximum di ..... · ~ (cid:173)
`the volute or scroll housing, the circu
`passageway can be cast to one side oft:- -
`diffuser with a much reduced diamere:(cid:173)
`at the sectional view of the compresso:
`between the diffuser and volute resen:
`gue; hence its name-parallel tong, ... ::
`This, in effect, produces a right-angle-.::
`similar to the normal parallel wall di:~
`but the overall dimensions of the he
`more compact:
`
`Compressor diffusers
`(Figs 6.45 and 6.46)
`The object of a compressor diffuse(cid:173)
`the air's kinetic energy produced a:
`impellor, to pressure energy by e,:-;::;.mlll
`that its velocity falls , thereby cau: --
`to rise.
`The are three types of diffuser:
`a)
`the scroll diffuser
`b) the vane ring diffuser
`the vaneless parallel wall di::
`c)
`
`The scroll-type diffuser (Fig
`scroll or volute is the circu:...:
`varying cross-sectional area ";\ -
`rounds the impellor rim. T.:
`flowing through the impellor :--
`the periphery of the blades i.
`very high velocity. The air \\.
`spiral flow path directly im.J
`much larger circular passage
`air so that it reduces speed .. -
`raises pressure. This form e>:"
`
`322
`
`BMW1047
`Page 10 of 35
`
`

`

`Vane
`diffuser
`
`Vane
`diffuser ---\-~~
`ring
`Volute ----....
`collector
`(scroll)
`
`Fixed
`vane
`blade
`
`. ane-diffuser compressor
`
`into pressure energy but at a
`low rate, therefore more effective
`_ .'
`:cs of diffusing the air charge are normally
`
`type diffuser (Fig. 6.45). This diffuser
`-~~ of an annular ring with vanes positioned
`_ -tially around one side. The diffuser ring
`.ts diverging multi-passageways joins the
`~or cell periphery outlets to the circular,
`"'le cross-section, volute collector. The vanes
`so positioned that they guide the air discharge
`the impellor rim to the volute in a tangential
`=CTion through passages of increasing cross(cid:173)
`-::on. Thus, since the energy contained by the
`zannot be destroyed, the effect of the expand(cid:173)
`: passages will be to slow down the air move(cid:173)
`nt; therefore, if energy is to be retained the air
`essure will rise.
`
`type diffuser
`Vaneless parallel-wall
`(Fig.
`6.46). Vaneless diffusers are parallel annular(cid:173)
`shaped passageways which connect the impeller
`cell rim exits to the circular snail-shell shaped
`volute outer passageway.
`The air enters the diffuser at radius R 1 , through
`a relatively small cross-sectional area A 1 , and is
`discharged at radius R2 through a proportionally
`larger cross-section A 2 • Thus, by similar triangles
`A1 A2
`- - -
`R1
`R2
`
`therefore
`
`R1
`A1 =A2-
`R2
`
`Hence, if R 1 is half that of R2 , then A 1 will be
`half the cross-sectional area of A 2 and vice versa.
`
`Parallel -
`diffuser
`Volute - - ----.
`collector
`(scroll)
`Fig. 6.46 Vaneless diffuser compressor w ith parallel w all diffuser illustrating expansion of flow area from inlet to
`exit
`
`--1----<1
`
`Parallel
`wall
`diffuser
`
`323
`
`BMW1047
`Page 11 of 35
`
`

`

`Q)
`
`:3
`en
`en
`...
`Q)
`a.
`en m
`Cl
`iii
`:::, m Pb
`.c
`X
`Q) Pe
`"C
`C m
`cii
`0
`0
`Cl)
`
`Po
`
`Exhaust
`pressure
`pulse
`
`BDC1
`
`BDC2
`
`BDC3
`
`B0C1
`
`120 180
`
`pressure o
`I
`<Pol
`8 DC1
`
`Cyl.1
`
`420 480 540 600 660 720
`I
`8DC1
`
`8DC2
`
`TDC2
`
`Induction
`period
`
`BDC3
`
`TDC3
`
`Cyl.3
`
`Pb
`Pe
`
`Po
`
`Scavenging
`period
`
`Fig. 6.47 Exhaust gas pressure variation in activated six-cylinder turbocharged engine manifold
`
`Accordingly, the air leaving the impellor and
`passing through the parailel diffuser will reduce
`its speed in proportion to the increase in the
`annular passageway cross-sectional area. In con(cid:173)
`trast, the air discharge pressure rises.
`Parallel-wall diffusers have a broad operating
`speed range over which a moderate compressor
`efficiency is maintained, whereas the vane ring
`diffuser operates the compressor at a fairly high
`efficiency but over a much narrower speed range.
`
`6.7.6 Exhaust gas control and turbine
`housing design
`Pulsed exhaust discharge
`(Fig. 6.47)
`It is important for effective cylinder scavenging
`that pulsed exhaust gas energy is introduced to
`the turbine wheel, in contrast to a damped steady
`flow of exhaust gas. With a four-cylinder engine,
`single-exhaust manifold, this is possible as there is
`an exhaust discharge every 180° so that there is
`very little exhaust gas interference between cylin(cid:173)
`ders.
`
`However, with more than four cylinders, ex(cid:173)
`haust gas will discharge at shorter intervals tha12
`the 180°; that is, for 5, 6 and 8-cylinder engin~
`the intervals between exhaust discharges will be
`144°, 120° and 90° respectively. To overco~
`exhaust gas interference in the manifold, man(cid:173)
`ifolds are sub-divided so that, in the case of aa
`in-line six-cylinder engine, cylinders 1, 2 and 3 ar;
`grouped together and, similarly, cylinders 4. : \
`and 6 are grouped together so that there is now ;n
`extensive exhaust interval between sub-divide
`manifolds of 240°. The exhaust discharge fro
`each half-manifold is then fed to the turbine(cid:173)
`wheel through two separate passageways. If the
`exhaust gas in the branch pipes is permitted
`discharge in the form of a pulse (Fig. 6.47) uie
`initial blow-down from the open exhaust val
`port will produce a rapid pressure rise until
`peaks. The exhaust pressure then quickly de(cid:173)
`creases to a minimum value before the ne
`cylinder, sharing the same common man if
`gallery, discharges another lot of exhaust g
`This cycle of events will therefore be continuous.:
`repeating. By reducing or even eliminating inte-r
`cylinder exhaust gas interference, by sub-divicfu>I
`
`324
`
`BMW1047
`Page 12 of 35
`
`

`

`.e manifold if need be, the exhaust pressure in
`c manifold will fall towards the end of the
`'.laust stroke to a value below the mean com(cid:173)
`;;ssor pressure (Fig. 6.47). Thus, during the
`ve overlap near the end of the exhaust period
`d at the beginning of the inlet period; a positive
`;;ssure difference will exist between the cylinder
`rake and the cylinder exit, which will cause a
`ow-through of fresh charge from the intake
`anifold to the exhaust manifold. If there is
`fficient pressure difference the fresh charge will
`:ish into the cylinder and push out the residual
`rllaust gases still remaining in the unswept com(cid:173)
`ustion chamber space. The effectiveness of this
`""'--avenging action will also depend on the engine
`~ ed and the actual valve opening area during
`:ie time of valve overlap.
`
`Jivided turbine housing passageways
`The turbine housing for a four-cylinder engine
`~ormally has a single volute circular passageway
`nto which the four merged branch pipes dis(cid:173)
`.:harge their individual exhaust gas pulses at inter-
`als of 180° through a 360° throat entrance to the
`turbine wheel.
`When there are more than four cylinders, bet(cid:173)
`:er turbine response is obtained by dividing the
`exhaust manifold into two halves. The discharge
`from each half manifold is then fed to the turbine
`~·heel through two separate passageways formed
`:.n the turbine housing. Alternating exhaust gas
`pulses from each group of branches discharge at
`relatively prolonged intervals through the throat
`of the turbine housing in a pulsed jet stream
`against the turbine-blades. This pulse impinge(cid