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 
When not to change
Page 83
If you've noticed an error in this article please click here to report it so we can fix it.
NOT ONLY is a lubricating oil conmated with use, it also "wears out" through suffering from additive depletion, which is particularly serious in the case of a high-output engine.
A detergent additive prevents the small particles of carbon formed during combustion from coalescing and keeps the particles in suspension; because of their very small size the suspended particles can be allowed to circulate freely with the oil as they cannot cause damage to the working parts or block the oilways.
With depletion of the additive the particles tend to coalesce and to form a sludge which may block the oilways and reduce the lubricating properties of the oil. In this case, therefore, the contaminent is a product of additive depletion, other forms of contaminent, such as fine dust, being harmful to the system whether or not there is sufficient additive in the oil to suspend the carbon particles.
Because the properties of the oil circulating in the engine are dependent on a large number of variables, the oil-change period recommended by the engine maker or oil company is of necessity an arbitrary standard and in practice represents a "safe compromise".
If the change period were based on the quality of the oil in the sump it would vary from considerably less than the mileage recommended to several times the mileage, according to operating conditions, the state of the engine, the type of filtration system, the filter element employed and the element change frequency.
When an engine is operating at low loads and low coolant temperature acidic products of combustion are not evaporated and mix with the oil, while continuous near-peak loading tends to oxidize the oil, different types of additive being required to combat the effects of each condition.
The cost of an oil is mainly a function of its level of detergency and of the other types of additive with which it is treated, a Series 3 oil (which is normally required for turbocharged engines) being the most costly oil in common use. Continuous operation at high speed may also cause rapid wear of the valve gear unless the oil is treated with the appropriate additive.
Employing the "best possible" oil to meet all possible contingencies would add very substantially to the oil bill. But if a simple means were evolved of checking the quality of the sump oil this would enable the change period to be safely extended in a typical instance to a mileage well above the average existing mileage recommended by the engine maker.
The saving derived from extending the period would more than offset the extra cost of using the higher grade oil. And incorporating the "best possible" filtration system in the engine would also pay off handsomely in the long run.
Although analysing a complicated fluid such as a highly-additive-treated lubricating oil is a complicated laboratory exercise— and evolving a gauge that indicated when one or more of its properties was substandard might in practice be impossible— it should not be beyond the capabilities of oil technicians to produce a detergencylevel gauge.
If this were used in conjunction with a gauge showing the pressure drop across the oil filter, it should enable the oil-change period and the period between engine overhauls to be greatly extended.
The depletion of a detergency additive normally corresponds to the depletion of other additives. Producing a suitable gauge should not present insurmountable difficulties. Its availability would encourage the use of the highest grade oil compatible with the type of engine (and in some cases of .a higher grade oil than was strictly needed) and the "universal" use of a single grade high-quality oil by operators.
In a paper on filtration problems presented earlier this year by Mr. R. D. Owen of the Mobil Oil company, the author pointed out that dilution of lubricating oil, caused by blow-by, increased the rate at which harmful deposits were formed and that deterioration of engine condition shortened the period that oil could be used without "dump-out" of the insolubles.
Operator plays safe
Mr. Owen also emphasized that poor combustion in either petrol or diesel engines increased the rate at which insolubles were formed. He mentioned that a high oil consumption increased the frequency of replenishment and thus reduced the percentage level of insolubles.
These comments underline the impossibility of judging oil deterioration on mileage; the average operator wisely plays safe by changing the lubricating oil at a mileage that could in most instances be safely exceeded.
The efficiency of air filtration is also an important factor with regard to oil contamination on two counts: one—the extent to which contaminants are trapped by the filter element and two—the amount of soot created by poor combustion as a result of partial blockage of the filter.
Given that the onset of a dangerous deterioration of the lubricant's properties was not too rapid and depending on the type of service on which the vehicle was engaged, a quality gauge that gave warning of an impending critical condition some hundreds of miles before it materialized would provide ample latitude for arranging an oil change as and when required.
In a typical case the use of oil-quality gauges by a large operator running a number of different types of vehicles on a variety of routes would enable appropriate oil-change routines to be organized according to type and route. The gauges would provide a safeguard against premature deterioration.
News was given by Mr. B. G. Curtis of the Fram company some months ago that a nationalized bus undertaking was saving £96,000 a year by employing a Fram fineelement by-pass filter in addition to the standard full-flow filter (of a different make). This illustrates the value of a second filter of this type, since drastically reducing the pore size of a full-flow filter element can cause a large pressure drop across the filter.
Although the use of a by-pass filter in addition to a full-flow type represents an established practice—its advantages have long been recognized by the filter companies—related claims have been disregarded by operators generally because realistic proof has been lacking.
It is particularly noteworthy that in the case quoted the by-pass filters have enabled oil-change periods to be extended from three weeks to nine months but it is not known how the quality check is made on which the periods are based. As indicated earlier, ascertaining the detergency level is the most important consideration given that the oil is well filtered.
Before the advent of additive-treated oils when straight mineral oils were used for all types of power unit, laboratory and road tests had indicated that a used lubricating oil provided better resistance to high stresses under extreme conditions than a new oil.
Presumably repeat impact loading of the lubricant has a favourable effect on the molecular structure of an oil with regard to stress characteristics.
Nothing has been heard of this claim for upwards of 15 years, but according to recent news from America one or more companies there are reprocessing selected used oils and are marketing the lubricant at double the price of the new oil.
Commenting on this news, the spokesman of a filter company observed that the first 20 minutes after renewal of the lubricating oil was the "most dangerous time" in the life of the engine between changes.