Advanced Vehicle Technology Episode 1 Part 2 doc

1.2 Engine, transmission and body structure mountings 1.2.1 Inherent engine vibrations The vibrations originating within the engine are caused by both the cyclic acceleration of the reci

floor, a rule or tape is used to measure the distances

between centres both transversely and diagonally

These values are then chalked along their respective

lines Misalignment or error is observed when a

pair of transverse or diagonal dimensions differ

and further investigation will thus be necessary

Note that transverse and longitudinal

dimen-sions are normally available from the

manufac-turer's manual and differences between paired

diagonals indicates lozenging of the framework

due to some form of abnormal impact which has

previously occurred

1.2 Engine, transmission and body structure

mountings

1.2.1 Inherent engine vibrations

The vibrations originating within the engine are

caused by both the cyclic acceleration of the

reci-procating components and the rapidly changing

cylinder gas pressure which occurs throughout

each cycle of operation

Both the variations of inertia and gas pressure forces generate three kinds of vibrations which are transferred to the cylinder block:

1 Vertical and/or horizontal shake and rock

2 Fluctuating torque reaction

3 Torsional oscillation of the crankshaft 1.2.2 Reasons for flexible mountings

It is the objective of flexible mounting design to cope with the many requirements, some having conflicting constraints on each other A list of the duties of these mounts is as follows:

1 To prevent the fatigue failure of the engine and gearbox support points which would occur if they were rigidly attached to the chassis or body structure

2 To reduce the amplitude of any engine vibration which is being transmitted to the body structure

3 To reduce noise amplification which would occur

if engine vibration were allowed to be transferred directly to the body structure

Fig 1.9 Body underframe alignment checks

4 To reduce human discomfort and fatigue by

partially isolating the engine vibrations from

the body by means of an elastic media

5 To accommodate engine block misalignment

and to reduce residual stresses imposed on the

engine block and mounting brackets due to

chassis or body frame distortion

6 To prevent road wheel shocks when driving

over rough ground imparting excessive rebound

movement to the engine

7 To prevent large engine to body relative

move-ment due to torque reaction forces, particularly

in low gear, which would cause excessive

mis-alignment and strain on such components as

the exhaust pipe and silencer system

8 To restrict engine movement in the fore and aft

direction of the vehicle due to the inertia of the

engine acting in opposition to the accelerating

and braking forces

1.2.3 Rubber flexible mountings (Figs 1.10, 1.11

and 1.12)

A rectangular block bonded between two metal

plates may be loaded in compression by squeezing

the plates together or by applying parallel but

opposing forces to each metal plate On

compres-sion, the rubber tends to bulge out centrally from

the sides and in shear to form a parallelogram

(Fig 1.10(a))

To increase the compressive stiffness of the

rubber without greatly altering the shear stiffness,

an interleaf spacer plate may be bonded in between

the top and bottom plate (Fig 1.10(b)) This

inter-leaf plate prevents the internal outward collapse of

the rubber, shown by the large bulge around the

sides of the block, when no support is provided,

whereas with the interleaf a pair of much smaller

bulges are observed

When two rubber blocks are inclined to each other

to form a `V' mounting, see Fig 1.11, the rubber will

be loaded in both compression and shear shown by the triangle of forces The magnitude of compressive force will be given by Wcand the much smaller shear force by WS This produces a resultant reaction force

WR The larger the wedge angle , the greater the proportion of compressive load relative to the shear load the rubber block absorbs

The distorted rubber provides support under light vertical static loads approximately equal in both compression and shear modes, but with heavier loads the proportion of compressive stiffness Fig 1.10 (a and b) Modes of loading rubber blocks

Fig 1.11 `V' rubber block mounting

to that of shear stiffness increases at a much faster

rate (Fig 1.12) It should also be observed that the

combined compressive and shear loading of the

rubber increases in direct proportion to the static

deflection and hence produces a straight line graph

1.2.4 Axis of oscillation (Fig 1.13)

The engine and gearbox must be suspended so that

it permits the greatest degree of freedom when

oscillating around an imaginary centre of rotation

known as the principal axis This principal axis

produces the least resistance to engine and gearbox

sway due to their masses being uniformly

distrib-uted about this axis The engine can be considered

to oscillate around an axis which passes through

the centre of gravity of both the engine and gearbox

(Figs 1.13(a, b and c)) This normally produces an

axis of oscillation inclined at about 10±20 to the

crankshaft axis To obtain the greatest degree of

freedom, the mounts must be arranged so that they

offer the least resistance to shear within the rubber

mounting

1.2.5 Six modes of freedom of a suspended body

(Fig 1.14)

If the movement of a flexible mounted engine is

completely unrestricted it may have six modes of

vibration Any motion may be resolved into three

linear movements parallel to the axes which pass

through the centre of gravity of the engine but at

right angles to each other and three rotations about

these axes (Fig 1.14)

These modes of movement may be summarized

as follows:

Linear motions Rotational motions

longitudinal 5 Pitch

2 Horizontal lateral 6 Yaw

3 Vertical

1.2.6 Positioning of engine and gearbox mountings (Fig 1.15)

If the mountings are placed underneath the com-bined engine and gearbox unit, the centre of gravity

is well above the supports so that a lateral (side) force acting through its centre of gravity, such as experienced when driving round a corner, will cause the mass to roll (Fig 1.15(a)) This condition is undesirable and can be avoided by placing the mounts on brackets so that they are in the same plane as the centre of gravity (Fig 1.15(b)) Thus the mounts provide flexible opposition to any side force which might exist without creating a roll couple This is known as a decoupled condition

An alternative method of making the natural modes of oscillation independent or uncoupled is achieved by arranging the supports in an inclined

`V' position (Fig 1.15(c)) Ideally the aim is to make the compressive axes of the mountings meet

at the centre of gravity, but due to the weight of the power unit distorting the rubber springing the inter-section lines would meet slightly below this point Therefore, the mountings are tilted so that the compressive axes converge at some focal point above the centre of gravity so that the actual lines

of action of the mountings, that is, the direction

of the resultant forces they exert, converge on the centre of gravity (Fig 1.15(d))

The compressive stiffness of the inclined mounts can be increased by inserting interleafs between the rubber blocks and, as can be seen in Fig 1.15(e), the line of action of the mounts con-verges at a lower point than mounts which do not have interleaf support

Engine and gearbox mounting supports are normally of the three or four point configuration Petrol engines generally adopt the three point support layout which has two forward mounts (Fig 1.13(a and c)), one inclined on either side of the engine so that their line of action converges on the principal axis, while the rear mount is supported centrally at the rear of the gearbox in approximately the same plane as the principal axis Large diesel engines tend to prefer the four point support Fig 1.12 Load±deflection curves for rubber block

arrangement where there are two mounts either side

of the engine (Fig 1.13(b)) The two front mounts

are inclined so that their lines of action pass through

the principal axis, but the rear mounts which are

located either side of the clutch bell housing are not

inclined since they are already at principal axis level

1.2.7 Engine and transmission vibrations

Natural frequency of vibration (Fig 1.16) A sprung

body when deflected and released will bounce up and

down at a uniform rate The amplitude of this cyclic movement will progressively decrease and the num-ber of oscillations per minute of the rubnum-ber mounting

is known as its natural frequency of vibration There is a relationship between the static deflec-tion imposed on the rubber mount springing by the suspended mass and the rubber's natural frequency

of vibration, which may be given by

n0ˆp30 x Fig 1.13 Axis of oscillation and the positioning of the power unit flexible mounts

where n0 = natural frequency of vibration

(vib/min)

x = static deflection of the rubber (m)

This relationship between static deflection and

natural frequency may be seen in Fig 1.16

Resonance Resonance is the unwanted

synchron-ization of the disturbing force frequency imposed by

the engine out of balance forces and the fluctuating cylinder gas pressure and the natural frequency of oscillation of the elastic rubber support mounting, i.e resonance occurs when

n

n0ˆ 1 where n = disturbing frequency

n0 = natural frequency Transmissibility (Fig 1.17) When the designer selects the type of flexible mounting the Theory of Transmissibility can be used to estimate critical resonance conditions so that they can be either prevented or at least avoided

Transmissibility (T) may be defined as the ratio

of the transmitted force or amplitude which passes through the rubber mount to the chassis to that of the externally imposed force or amplitude generated

by the engine:

T ˆFt

Fdˆ 1

n0

 2

where Ft ˆ transmitted force or amplitude

Fd ˆ imposed disturbing force or

amplitude This relationship between transmissibility and the ratio of disturbing frequency and natural frequency may be seen in Fig 1.17

Fig 1.14 Six modes of freedom for a suspended block

Fig 1.16 Relationship of static deflection and natural

frequency

Fig 1.15 (a±e) Coupled and uncoupled mounting points

The transmissibility to frequency ratio graph

(Fig 1.17) can be considered in three parts as follows:

Range(I) Thisistheresonancerangeandshouldbe

avoided It occurs when the disturbing frequency

is very near to the natural frequency If steel mounts

are used, a critical vibration at resonance would go

to infinity, but natural rubber limits the

trans-missibility to around 10 If Butyl synthetic rubber is

adopted, its damping properties reduce the peak

transmissibility to about 21 ¤ 2 Unfortunately, high

damping rubber compounds such as Butyl rubber

are temperature sensitive to both damping and

dynamic stiffness so that during cold weather a

noticeably harsher suspension of the engine results

Damping of the engine suspension mounting is

necessary to reduce the excessive movement of a

flexible mounting when passing through resonance,

but at speeds above resonance more vibration is

transmitted to the chassis or body structure than

would occur if no damping was provided

Range (II) This is the recommended working

range where the ratio of the disturbing frequency

to that of the natural frequency of vibration of the

rubber mountings is greater than 11 ¤ 2and the trans-missibility is less than one Under these conditions off-peak partial resonance vibrations passing to the body structure will be minimized

Range (III) This is known as the shock reduction range and only occurs when the disturbing frequency is lower than the natural frequency Generally it is only experienced with very soft rubber mounts and when the engine is initially cranked for starting purposes and so quickly passes through this frequency ratio region

Example An engine oscillates vertically on its flexible rubber mountings with a frequency of 800 vibrations per minute (vpm) With the information provided answer the following questions:

a) From the static deflection±frequency graph, Fig 1.16, or by formula, determine the natural fre-quency of vibration when the static deflection of the engine is 2 mm and then find the disturbing to naturalfrequencyratio.Commentontheseresults b) If the disturbing to natural frequency ratio is increased to 2.5 determine the natural frequency

Fig 1.17 Relationship of transmissibility and the ratio of disturbing and natural frequencies for natural rubber, Butyl rubber and steel

of vibration and the new static deflection of the

engine Comment of these conditions

a) n0ˆp30xˆp0:00230

ˆ0:0447230 ˆ 670:84 vib/min

n0ˆ 800

670:84ˆ 1:193

The ratio 1.193 is very near to the resonance

condition and should be avoided by using softer

mounts

b) nn

0ˆ800n

0 ˆ 2:5

; n0ˆ800

2:5 ˆ 320 vib/min

Now n0ˆp30

x thuspx ˆ30n

0

; x ˆ 30n

0

 2

ˆ 32030

 2

ˆ 0:008789 m or 8:789 mm

A low natural frequency of 320 vib/min is well

within the insulation range, therefore from either

the deflection±frequency graph or by formula

the corresponding rubber deflection necessary is

8.789 mm when the engine's static weight bears

down on the mounts

1.2.8 Engine to body/chassis mountings

Engine mountings are normally arranged to

provide a degree of flexibility in the horizontal

longitudinal, horizontal lateral and vertical axis of

rotation At the same time they must have

suffi-cient stiffness to provide stability under shock

loads which may come from the vehicle travelling

over rough roads Rubber sprung mountings

suitably positioned fulfil the following functions:

1 Rotational flexibility around the horizontal

longitudinal axis which is necessary to allow the

impulsive inertia and gas pressure components

of the engine torque to be absorbed by rolling of

the engine about the centre of gravity

2 Rotational flexibility around both the horizontal

lateral and the vertical axis to accommodate any

horizontal and vertical shake and rock caused by

unbalanced reciprocating forces and couples

1.2.9 Subframe to body mountings (Figs 1.6 and 1.19)

One of many problems with integral body design is the prevention of vibrations induced by the engine, transmission and road wheels from being transmitted through the structure Some manufacturers adopt a subframe (Fig 1.6(a, b and c)) attached by resilient mountings (Fig 1.19(a and b)) to the body to which the suspension assemblies, and in some instances the engine and transmission, are attached The mass

of the subframes alone helps to damp vibrations

It also simplifies production on the assembly line, and facilitates subsequent overhaul or repairs

In general, the mountings are positioned so that they allow strictly limited movement of the subframe in some directions but provide greater freedom in others For instance, too much lateral freedom of a subframe for a front suspension assembly would introduce a degree of instability into the steering, whereas some freedom in vertical and longitudinal directions would improve the quality of a ride

1.2.10 Types of rubber flexible mountings

A survey of typical rubber mountings used for power units, transmissions, cabs and subframes are described and illustrated as follows:

Double shear paired sandwich mounting (Fig 1.18(a)) Rubber blocks are bonded between the jaws of a `U' shaped steel plate and a flat interleaf plate so that a double shear elastic reaction takes place when the mount is subjected to vertical load-ing This type of shear mounting provides a large degree of flexibility in the upright direction and thus rotational freedom for the engine unit about its principal axis It has been adopted for both engine and transmission suspension mounting points for medium-sized diesel engines

Double inclined wedge mounting (Fig 1.18(b)) The inclined wedge angle pushes the bonded rubber blocks downwards and outwards against the bent-up sides of the lower steel plate when loaded

in the vertical plane The rubber blocks are subjected

to both shear and compressive loads and the propor-tion of compressive to shear load becomes greater with vertical deflection This form of mounting is suitable for single point gearbox supports

Inclined interleaf rectangular sandwich mounting (Fig 1.18(c)) These rectangular blocks are

Fig 1.18 (a±h) Types of rubber flexible mountings

Fig 1.18 contd