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
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188 
FUEL MARKS
Page 51
Page 52
Page 53
If you've noticed an error in this article please click here to report it so we can fix it.
You may know that temperature, and other factors, affect fuel consumption. But what can you do? David Wilcox finds out.
As we enter Keats' beloved season of "mists and mellow fruitfulness", we know the cold, dark days of winter cannot be far behind. Just as certain is the fact that vehicle fuel consumption will take a turn for the worse. In fact, it may have already started to suffer, although it can be difficult to detect the change quite so soon.
Some operators never spot it at all.When industry fuel expert Michael Coyle was working towards his doctorate on truck fleet fuel management in 1999 he conducted a survey of 82 fleets that had tried a variety of fuel-saving products and techniques. Even among these economy-conscious operators, Dr Coyle found some who were not aware of the effect of seasonality on their fuel figures.
Hopefully, things have improved since then. When so many more trucks come equipped with on-board computers that record and display fuel data there are fewer excuses for not getting to grips with the subject, And then there are TransportEnergy Best Practice Guides, plus free seminars and low-cost consultation from the Fuel Economy Advisors scheme, financed by the Road Haulage Modernisation Fund.
Lack of awareness of this seasonal pattern
Even among economy-conscious
in fuel consumption means it is all too easy to make flawed purchasing decisions. For example, a three-month trial of low-viscosity synthetic oil may well lead to some incorrect conclusions if the fuel figures are compared with those from the preceding three months.
A summary of three years' fleet average fuel consumption figures, part of Dr Coyle's research at the University of Huddersfield, shows a cycle peaking in July and usually troughi rig in December, which according to Dr Coyle is typical of many fleets.The summer miles per gallon (mpg) figure is 4-5% better than the year's average, while the winter figure is perhaps not surprisingly 4-5% worse.
Seasonal factors
Identifying this variation is the first step towards preventing dodgy decisions and spotting specious fuel-saving claims.The next is pinning down the reasons for the variation and seeing if any can be alleviated. In reality, there are a host of seasonal factors at work but most are small and difficult to measure in isolation. It just so happens that most of the good ones occur in summer and the expensive ones in the winter, so their cumulative effect becomes measurable.The most obvi ous seasonal variation is the ambient temperature.A rule-of-thumb is that an engine's fuel consumption suffers by about 1% for every 6°C it is below its optimum operating temperature.
Low winter temperatures lengthen the engine's warm-up period, soil spends more time in this sub-optimal zone. During that time, much of the energy in the fuel is being used to heat the engine and coolant, so fuel consumption increases. Losses within the engine are also higher during the cold start because the oil is more viscous and slow to circulate, demanding more pumping effort and less able to get to work to reduce friction. Low-viscosity engine oils like OW-30 or 5W-40 grades come to the fore under these conditions.
The link between temperature and oil viscosity is one of the reasons why operators' opinions differ about the fuel-saving credentials of low-viscosity oils. Double or even treble-shifting keeps engines close to their optimum temperature, reducing the opportunity for low-viscosity oil to play its trump card. Single-shifted trucks have more cold starts and are likely to see a bigger gap between their summer and winter mpg figures, so they will more readily identify fuel-savings with low-viscosity oils, Cab warm-up periods Winter fuel consumption is also high because engines are left to idle longer, defrosting windscreens and warming cabs.A typical tractor engine consumes about two litres of diesel an hour when idling —more during a cold start.
In contrast, a diesel-fuelled `night' heater consumes 0.2-0.5 litres per hour, depending on variables such as the ambient temperature and size of cab.
Refmements including 24-hour and sevenday timers, or even remote actuation via radio control or a mobile phone, allow cab heaters to be turned on in advance, eliminating extended engine idling It may be worth fitting the type of cab heater that is plumbed into the vehicle's coolant system so it warms the engine as well as the cab.
At about one litre per hour, the fuel consumption of these water heaters is higher than conventional cab air heaters but shortening the engine warm-up period should pay dividends.
There are a couple of other temperaturerelated factors to throw into the mix. First, cold air is denser than warm air, and because engine power is limited by inlet air density — hence charge-cooling — this means a cool day should be good news.
But by the same token, higher air density will count against the truck because it is one of the factors in assessing aerodynamic losses. And second, diesel fuel density changes with temperature. Diesel at 3°C occupies 2.25% less volume than when it is at 30°C. Since we normally measure fuel by volume, this change in density affects the calculations. But these are fine details;there are other seasonal factors that are likely to be more discernable.
Dr Coyle suspects the seasonal difference in average wind speed has a considerable influence on fuel consumption. He bases that belief on the fact that aerodynamic drag increases with the square of vehicle speed, so a change in wind speed has an equally pronounced effect on drag. UK weather records show that the highest mean wind speeds occur in December, January and February, and the lowest speeds are in June,..Tuly and August. Consequently, wind speed variations overlay the temperature differences. Some operations are bound to be more sensitive to wind than others.
The prevailing wind direction in the UK is from the south-west, so changes in wind strength are more telling on routes along that axis. Similarly, trucks with poor aerodynamics will suffer more in high winds, so the usual rules about roof-spoilers,fairings and cab gaps apply. Double-deck trailers deserve special attention because standard cab-roof spoilers are too low.A convex fibreglass moulding for the exposed top section of the trailer's front bulkhead will work in combination with the tractor's standard spoiler.
Wet weather changes The action of a tyre displacing water on a wet road consumes energy. It takes more energy to eject water than air from the tread, and standing water on a road is a barrier for the tyres to cut through, so fuel consumption rises during wet weather. Unsurprisingly, that is another black mark against the winter: UK weather records reveal that December, January and November (in that order) are the wettest months and May, July and April are the driest.There is a heavy geographical bias too. Not only does Scotland have twice as much rain as the South of England, but also the difference between the dry and wet months in Scotland is much more marked, heightening the effect.
The British Standard for diesel demands a lower -cold filter plugging point" temperature in winter than in summer. so that paraffin hydrocarbons in diesel do not begin to crystallise into wax crystals that can block the vehicle's fuel system. The Standard's definition of winter is 22 October to 15 March , but those dates relate to diesel at a retail pump.
Most refiners will start producing their winter grade diesel several weeks before that to ensure that all summer product is through the supply chain before the October date. Much of the seasonal deterioration in mpg figures is often laid at the door of this specification change, but fuel companies do not agree. They claim that the proportion of "wax anti-settling aids" is far too small, and say they can sometimes achieve the required low-temperature performance in ultra-low-sulphur diesel by subtle blending, without using the additive at all.
Aerodynamic demands
Air-conditioning is an engine's largest single auxiliary demand with a clear seasonal bias, so this increasingly popular option has potential to harm fuel consumption in summer rather than winter.The extra belt drive from an engine to run the air-conditioning compressor needs about Shp (6kW), which is roughly equivalent to bolting-on a second altemator.A test run by Exel at this June's British Transport Advisory Committee/Institute of Road Transport Engineers technical trials at MIRA attempted to see if this extra demand led to higher fuel consumption. The short answer is no; on the Daf XF95.430 there was no real difference when the air conditioning was running. But it is logical to suppose that a less powerful engine would not take the extra power demand quite so easily.
Car fuel consumption in northern Europe is reckoned to suffer by 5-10% when the airconditioning is operating, but there are a huge number of variables to consider, such as ambient temperatures, engine size and road speeds. And the alternative— opening the windows —is not cost-free either. Another result from the BTAC/IR1E event found that a tractor's fuel consumption deteriorated by 7% during constant speed lapping of MIRA with cab windows open,spoiling the aerodynamics. Unsurprisingly, there was little effect when the rig was running at low speed on the test's stop/start section.
One final factor that has a loose seasonal connection could be one of the most important ones.That is traffic congestion.
Morning peak traffic in urban areas declines by around 20% during school holidays, so if fuel consumption is already starting to deteriorate in your fleet now. it may have little to do with the weather — yet. •