1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164 
Keep it quiet!
Page 89
Page 90
If you've noticed an error in this article please click here to report it so we can fix it.
I put these points to Otto Holtermann, Ford's truck development engineer, and he agreed that because of the twin pressures of environmental legislation and driver comfort noise reduction would require a continued engineering effort in the years to come.
External noise
External vehicle noise has been controlled to stated levels in Britain since the first piece of legislation (largely unenforced) was introduced on July 1 1968. In this type of check, microphones are positioned at set distances from the path of the vehicle while it is driven past at set speeds. The worst noise condition in this drive-past type of test is when the vehicle is accelerating in a low gear. In this condition the engine itself produces the largest contribution to the total noise and has as a consequence received most attention from the engineers. However, as the research went deeper engineers needed to define the contribution to total noise that each part of the vehicle made. It soon became obvious that a more closely controlled engine was needed and Ford, together with other engine manufacturers through a sub-committee at MIRA, set up a special test cell to isolate the basic engine noise.
This work will culminate in the adoption of a new British Standard on engine noise. Having defined the noise level of the bare engine a proper order of priority can be given to other extraneous noises such as those produced by the cooling fan, the air intake manifold and, of course, the exhaust manifold.
Early work showed that vibration of the exhaust pipe especially between the engine and expansion chamber, was a leading noise source and so this received much attention. As with all engineering problems there is nearly always a way round if money is no object, but once price is compounded into the problem an answer is that much harder to find. Ford engineers chose what has been a successful and reasonably cheap solution: badly resonating pipes are made double skinned. While the pipe is still straight a second tube which is just a sliding fit is slid over the first. The two pipes are then bent to the required shape and because of the small amounts of distortion that take place the two pipes are locked together.
The effect of this simple idea is to make the single pipe, which rings like a bell when struck, sound completely dead and thus reduce the amount of noise it can emit. This type of construction is used on the 0-series trucks to advantage. The exhaust note itself is subdued by using an expansion box followed by a large silencer.
The larger the engine the larger this silencer needs to be and if Ford breaks into the 200 bhp engine and above class this factor alone will cause a few headaches.
With big trucks it's not sufficient merely to quieten the engine, for other factors such as transmission noise and tyre noise add considerably to the total noise level. Tests carried out by the Road Research Laboratory indicate that at 38 tons gvw tyre noise alone will bring a vehicle very close to the legal limit — in wet conditions it could well be in excess of it. Road surface conditions and the tyre tread pattern generate the noise and it is interesting that smooth roads generally produce more noise than do rough ones; research has shown that tyre /road noise in decibels (sound pressure level) can vary on the same truck by some 20 dBA. This being the case, the drive-past legislation may well have to be rewritten to include a standard tyre ."road combination, For some years now Ford has been sponsoring further research work at Southampton University where several attempts have been made to reduce the engine noise level at its source. Early attempts concentrated on encasing the engine in a noise insulating shield and were encouragingly successful. This method is expensive, however, and really only economic in the most troublesome areas of the engine; these proved to be the sump and bearing region of the crankcase. The nature of the insulation and its application is all-important; further work in this field involves the continual evaluation of material combinations to achieve the best cost-benefit result.
As a general result of this type of insulation, accessibility to the engine suffers and a considerably larger cooling system is needed as a result of the reduced radiating area of block and sump. Even this basic approach to the problem does reduce the noise level but only at considerable extra cost. Current work at Southampton University is aimed at reducing the metallic ring of the block on the lines used with the exhaust pipe mentioned earlier. To achieve this an engine block is cut into many small parts and "stucktogether again. The results so far are encouraging but obviously still a long way removed from an engineering reality.
The net result of all the work done so far and it is considerable — is that improvements can be made, but only at significant extra cost, and the diesel engine will be on the noisy side for a good few years yet.
Driver comfort
As I said earlier, the driver immediately benefits from any improvement in engine noise level made to comply with the external noise regulations, but further refinements are making the driver's environment even more acceptable.
Every Ford commercial vehicle built in the past five years or so has been subjected to vigorous noise insulation evaluation tests. The basic vehicle, be it a van or a truck, is naturally noisy but before the level of insulation is decided upon a thorough cost-effectiveness exercise is carried out. Noise is an assembly of a whole range of discrete frequencies and one of the first jobs of the engineer is to break down the jumble of noise into its component frequencies in order to identify the predominant ones. Usually as a first step these peak frequencies can be removed effectively and cheaply with a correspondingly big reduction in the noise level. Once all the peaks have been reduced to the general level, however, any further noise reduction becomes more difficult and more expensive.
The agreed measurement scale for noise, ie dBA is logarithmic and the normal rules for summing effects do not apply. All other scales are linear — two lengths 80ft plus 801t, for example, equal 160ft but two noise levels 80 dBA plus 80 dBA equal 83 dBA. In reverse, as an example of the problem. if one noise source is reduced to zero the sum is only 77 dBA — not 40 dBA as might be expected.
By now it will be appreciated that noise is not a simple subject and its suppression not a simple problem. The only certainty is that all noise reduction costs money and progressively more so as the noise level is reduced.
An ever-recurring problem is holes in the dash or floor which are inadequately sealed and therefore allow noise to enter the cab. For this reason Ford argues that tilt-cabs on trucks offer a definite advantage in this respect as the floor can be made in one piece with the minimum number of holes.
An undoubted aspect of driver appeal in the current Transit range is the low noise level. Sound insulation on these vans includes a thick under-body coating, a thick felt /rubber sandwich on the dash and floor and an absorbent wool material under the head lining, also the large interior door panels are fully trimmed. These items are not extras, but part of the standard specification, and are included in the price of the cheapest model. The extra cost involved is obviously considerable — Ford would not say how much -but the marketing people realize that noise level is an important selling point and are prepared to sanction the extra cost. It is interesting that Ford always develops its products so that the inherently least quiet combination of gearboxes and rear axle ratios meets the required noise level, so that the majority of vehicles — those with higher rear axles — are quieter than engineering and legal requirements.
Pollution
As a footnote on an associated problem — pollution — just about a month ago the DoE announced that all diesel engines manufactured after October 1 1972 and used on or after April 1 1973, will have to comply with the exhaust opacity limits as laid down in BS AU 141a: 1971. Ford has been working on the assumption that such legislation would be passed sooner or later and although most effort has been put in on petrol engines, diesel engines have not been forgotten.
The new 2.4-litre diesel engine to be announced shortly, for example, has been designed to comply with any foreseeable legislation from the outset. A precombustion chamber design was chosen as this is known to give good smoke-free performance at high speed.
In addition, though a precombustion system has been shown to give optimum efficiency with greater smoothness of running and better overall power characteristics in small diesels, with good fuel economy, and it has the added advantage of reducing exhaust emissions, especially the oxides of nitrogen, an area in which direct-injection diesels have not shown up too well.