TRANSPORT and ROAD RESEARCH LABORATORY
Department of the Environment Department of Transport
SUPPLEMENTARY REPORT 636
COLD START FUEL CONSUMPTION OF A DIESEL AND A PETROL CAR
by
T C Pearce and M H L Waters
Any views expressed in this Report are not necessarily those of the Department of the Environment or of the Department of Transport
Assessment Division Transport Systems Department
Transport and Road Research Laboratory Crowthorne, Berksb_ire
1980 ISSN 0305-1315
Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.
This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.
Abstract
1.
2.
3.
CONTENTS
Introduction
Method of test
Presentation of results
3.1 Constant speed tests
3.2 Fuel consumption when idling
4. Discussion and conclusions
4.1 Constant speed tests
4.2 Fuel consumption at idling
4.3 General conclusions
5. Acknowledgements
6. References
7. Appendix 1 :
8. Appendix 2:
Comparison of constant speed tests with results from other work
The distribution of car journey lengths
8.1 Introduction
8.2 General travel
8.3 Car travel for different journey purposes
8.4 Concluding remarks
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(C) CROWN COPYRIGHT 1980 Extracts from the text may be reproduced, except for
commercial purposes, provided the source is acknowledged
COLD START FUEL CONSUMPTION OF A DIESEL AND A PETROL CAR
ABSTRACT
Measurements have been made of the fuel consumption of a petrol and a diesel car when starting from cold. The cars were the 1.1 litre petrol VW Golf and the 1.5 litre diesel version, which have the same passenger accommodation and nearly identical road performance.
It was found that the diesel car used less fuel in the 'warm-up' period than the petrol, both when being driven at constant speed on a test track and with the engine idling and the car stationary.
1. INTRODUCTION
It is well known that the fuel consumption of cars is higher during the first part of a journey when the engine and
vehicle are cold, than when the car is fully warmed up. Many tests in the past have shown the magnitude of the
'cold start' effect (see for example References 1,2 and 3), but most o f the tests have been with petrol engined cars,
though some results for a diesel and petrol car are given in Reference 4.
The Laboratory has been carrying out comparative tests of a small diesel car and its equivalent petrol
engined version to determine the relative fuel consumption when driven by different drivers, with different loads,
and under a variety of traffic conditions 5. These tests were intended to examine the fuel consumption of the
cars when fully warmed up. The objective of the work described in this report is to compare the fuel consumption
of a diesel and petrol car when they both start from cold. Measurements were also made of the variation of fuel
consumption with the engine idling, again starting from a cold engine condition.
A short series of tests were carried out early in 1980 on the Laboratory 's test track, and are reported now as
part of the continuing investigation into the relative performance of a small petrol and diesel car.
2. METHOD OF TEST
The two cars used for the tests were the Volkswagen Golf 1.5 litre diesel engined car, and the 1.1 litre petrol car.
They have the same body shells and passenger accommodation and have nearly identical road performance. They
are described more fully in References 4 and 5, and are illustrated in Hate 1.
The diesel and petrol cars use different methods of ensuring that the engines start reliably from cold. The
diesel engine employs electrically heated 'glow plugs' in the indirect chamber of the cylinder head so that fuel, on
injection, ignites at once even when the cylinder head is cold. When the engine is running, this assistance to
combustion is no longer needed, as the heat of compression is sufficient. The petrol engine has a fuel enrichment
device ('choke'), which alters the carburettor setting so that a fuel-rich mixture is provided for the cold engine.
The degree of fuel enrichment is thermostatically controlled, with a temperature sensor in the water jacket
which heats the inlet manifold. Normal fuel/air ratio is restored automatically once the temperature at the sensor
reaches the specified value (a coolant temperature of 30°C).
Before the tests were carried out, both cars were serviced and the engine conditions adjusted where necessary
to the manufacturer 's specification.
Two series o f tests were carried out. The ftrst was intended to show how fuel consumption varied as the cars
warmed up from a cold start. In order to minimise external factors such as traffic and driver effects, it was decided
that for this series the cars should be driven at constant speed throughout. The route used was a 7.55 km circuit
over the T R R L test track and is illustrated in Figure 1. Tests were carried out at steady speeds of 48, 64 and
80 km/h (30, 40 and 50 mile/h) and the route was covered four times for each speed - so that the total distance
driven was approximately 30 kin.
The tests were carried out during the winter of 1980 when air temperatures were between 0°C and 6°C.
Before each test, the cars were parked overnight in the open at the side of the track. For a test run at a particular
speed, the cars were started and driven off as soon as possible at the selected steady speed. Both cars were driven
round the track together, but sufficiently far apart to avoid any influence of air turbulence from the leading car on
the second vehicle.
Before starting each test the following parameters were measured: wet and dry bulb temperatures; wind
speed and direction; and air pressure. This supplementary information is shown in Table 1.
The circuit on the TRRL test track was divided into 14 sections with lengths varying from 0.5 km to 0.7 km,
and the fuel used by each car was measured at the end of each section during the test by the instrumentation
described in Reference 5. The measured fuel consumptions for each section, at each of the three steady speeds, are
shown in Table 2 expressed as litres/100 km. For both cars the measurements have been corrected to a fuel
temperature o f 15°C. Fuel temperatures were recorded at the end of each section for the diesel car, whilst ambient
temperature was used to correct the consumption for the petrol car to 15°C.
The second series of tests was concerned with the variation of idling fuel consumption as the engines warmed
up. Again both cars were parked overnight in the open at the side of the track so that the cars and engines were at
a suitably low temperature. The car engines were started and the fuel used was sampled with the instrumentation
after every minute for 30 minutes. For the diesel car, fuel temperatures were again sampled in order to correct the
data to a fuel temperature of 15°C. The corrected results are shown in Table 3 with the fuel consumption measured
in cubic centimetres per minute. The weather conditions during the test are given in Table 1.
In the next Section the results from the tests are presented, and some of the difficulties encountered are
discussed.
3. PRESENTATION OF RESULTS
3.1 Constant speed tests
The basic measurements o f fuel consumption show considerable variation between sections even on the final
circuit o f the route when the effect of cold starting is small. This variability is illustrated in Figure 2 for some of
the tests o f bo th cars, and shows that variation of the order of plus and minus 20 per cent is being experienced.
2
This is almost certainly caused by changes in the track gradient round the circuit, and Figure 2 shows the track
height above a datum level plotted on the same distance scale as the fuel consumption. The similarity between the
two graphs is quite striking, and tends to support the connection between fuel consumption and track gradient.
As a first step in reducing the variability, fuel consumption as a function o f distance from start has been
plotted on the basis of a three point moving average. Figure 3 gives an example for the petrol and diesel cars at a
steady speed of 80 km/h. It can be seen that the diesel car reaches a fully warmed up condition rather more
quickly than the petrol car, though there is an obvious difficulty in defining precisely when the 'fully warmed up '
condition is reached. This point will be returned to later.
However, it appears that by the time the fourth circuit is reached, both cars can be considered to be warmed
up. Table 4 then shows the measurements of fuel consumption on each of the four circuits, and the ratio of this
consumption to that on the fourth circuit when the cars are assumed to be fully warmed up. Thus for a trip of
7fi km (circuit 1) at 80 km/h, the petrol Golf uses 2¼ times the fully warmed up fuel, while the diesel Golf uses
about 11/3. The values in Table 4 exclude fuel used in the initial section of the first circuit because this includes
fuel used to accelerate the vehicles to cruising speed, a smaU amount of idling time necessary before the cars could
be driven off before stalling, and (for the petrol Golf) the fuel used to fill the carburettor float chamber.
One apparent anomaly in Table 4 is that the fully warmed up fuel consumption at 48 km/h is higher for both
cars than at 64 km/h and 80 km/h. This is unlike tests reported in the motoring press, and by the Laboratory with
the same cars 5, and some further measurements were made to try and resolve the difference. These are described
in Appendix 1, but it must be admitted that some uncertainty remains. It seems most likely that the
effect of the track gradients at the low speed made driving at a constant 48 km/h difficult with small throttle
openings, and that the higher fuel consumption may be a result of variability in speed induced by the track gradients.
Certainly the variation in the section fuel consumption as shown in Figure 4 is greater for the petrol car than at the
other higher speeds*.
Presenting the results of the constant speed tests in a more general way, two concepts are useful. The first
is the distance required from starting the journey for the engine to become fully warmed up: the second is the
concept of the extra fuel used because the journey started with a cold vehicle.
It has already been noted that it is difficult to define precisely when the car can be said to have reached a
'fully warmed up' condition. The procedure that is adopted here is to plot the fuel consumption on each section
of the circuit (as a three point moving average to reduce the variability) and to note the distance from the start
when the fuel consumption is reduced to 10 per cent above the average fuel consumption of the last circuit. The
procedure is illustrated in Figure 3 and Table 5 gives the results for the three constant speed tests. No great
accuracy can be claimed for these measurements, but they can be taken as indicating the order of distance which
is needed at the different speeds before the car can be considered to be in a 'wanned up' condition and therefore
running at its steady state fuel consumption. It can be seen that the diesel car requires around 5 km to reach this
state while the petrol car needs between 10 and 20 kin.
* The standard deviation of the fuel consumption is 1.3 litres/100 km at 48 km/h compared with 0.8 to 0.9 for the higher speeds.
The extra fuel used in starting from cold can be expressed in many ways. One way is to measure the
cumulative fuel consumption f rom the start o f the journey up to a distance S from the start, and compare it with
the fuel consumption which would be expected if the car were fully wanned up*. In the present tests this value is
taken to be the average of the t'mal circuit ( f rom 22.65 to 30.20 km from the starting point). These normalised
cumulative fuel consumption curves are plotted in Figures 5, 6 and 7 for the three constant speeds, and the
advantage of the diesel car over the petrol car is substantial in both cases. For example on a 10 km journey, at
64 kin/h, the diesel would use about 1.4 times the fuel of the fully warmed up car, while the petrol car would
require 2.1 times the fuel.
The results in Figures 5, 6 and 7 can be expressed another way in terms of the Fuel Equivalent Distance of
a trip starting with a cold engine. The Fuel Equivalent Distance is defmed as that trip length at the fully warmed up
fuel consumption which would use the same fuel as the actual cold start trip. The results plotted in Figure 8 are
easily derived f rom Figures 5, 6 and 7 and, for each car, roughly fall in a broad band not strongly dependent on
speed. Thus for the petrol car, a 5 km trip with a cold engine is equivalent to about a 13 km trip with the
engine fully warmed: for the diesel car it represents about an 8 km trip.
For longer journeys, o f 20 krn and more, the Fuel Equivalent Distance for the petrol car is about 11 km more
than the actual distance travelled, and for the diesel car is about 3 km more. This excess distance represents the
'fixed penal ty ' due to a cold start. It can also be expressed roughly as a quantity of fuel: for the petrol car it
represents about three-quarters o f a litre; for the diesel car about one-seventh of a litre.
3.2 Fuel consumption when idling
The basic results o f the idling tests have been presented in Table 3. The variation in fuel consumption of the
two cars while idling is also shown in Table 6 which aggregates the data into five minute intervals, but excludes
the first minute, when running (particularly with petrol car) was erratic. The results of Table 6, together with the
graph (Figure 9) show that the petrol car used fuel while idling at about five times the fully warmed up rate in the
first few minutes, compared with the diesel car's increase of less than 50 per cent. The car is assumed to be
warmed up when the idling consumption first drops to within 10 per cent of the average consumption of the last
five minutes o f the test. On this basis, the petrol car took about 20 minutes to reach its fully warmed up idling
consumption, compared with 10 minutes for the diesel car. It is interesting to note that, although the petrol car
uses far more fuel in the first few minutes, the idling consumption when fully warmed up is only about 15 per cent
higher than that o f the diesel car. This is also illustrated in Figure 10, which shows the cumulative fuel used while
idling, and the similar rates o f fuel used once the engines are warmed up.
4. DISCUSSION AND CONCLUSIONS
4.1 Constant speed tests
Although the tests at constant speed had some shortcomings, particularly at 48 km/h, the general conclusions
from the results as presented in Figure 8 are believed to be valid. These are that the diesel car has an appreciable .
7.55 ]~ S(Fuel used) * Normalised Cumulative Fuel Consumption = ~ 30.20
S ]~ 22.65 (Fuel used)
4
advantage in warming up more quickly than the petrol car, and in suffering a penalty of only 3 km excess distance
compared with about 11 km for the petrol car on the longer trips. For the short journeys (5 km and less) both cars
use much more fuel from a cold start than when fully warmed up, but the diesel car still has an advantage.
The distance required for the petrol cars to be fully warmed up (Table 5) is of the same order as that found
for a larger petrol car (a Rover 2000) in Reference 1. The excess distance, however, is much greater than that
measured for the Rover 2000 - about 11 km for the Golf petrol car compared with 1.5 km to 6 km for the Rover.
The reason for this is not certain, but it may be associated with the particular way the Rover was started in the
tests (with use of a manual choke). It is also possible that the automatic choke fitted to the Golf tends to enrich
the mixture for longer than is absolutely necessary. Further tests with a Golf with a manually operated choke
would be needed to investigate this point.
The importance of cold-start journeys on overall fuel consumption must be assessed in terms of the number
of trips that are made with a cold engine. This is not known at present, but it is known that a great many car
journeys are very short (80 per cent are under 16 km [10 miles] in length). Information on car journey length
from the 1975/76 National Travel Survey 6 is summarised in Appendix 2", and histograms of journey length
distribution for car travel are shown in Figure 11 but without knowing which car journeys are made from cold,
it is not possible to do more than make an illustrative calculation of the excess fuel used (as was done in
Reference 1, for example).
Suppose that half the car journeys (for all purposes) are made with a cold engine. (This is an obvious over-
simplification of a situation where perhaps 90 per cent of journeys to and from work start from cold, but possibly
only 10 per cent of journeys in the course of work.) Then using the proportion o f driver journeys of different
lengths from Table 8, and combining these values with the Fuel Equivalent Distance results o f Figure 8 leads to
the following results. The petrol car would use 1.37 times the fuel which would be needed if all trips were with a
fully warmed up engine: the diesel car would use 1.11 times the warmed-up fuel. (As an indication of the
sensitivity of these factors to the assumptions, the corresponding values if only one-quarter of the journeys started
from cold would be 1.19 for the petrol and 1.06 for the diesel car.) The diesel's warmed-up fuel consumption is,
of course, much lower than that of the petrol car because of its higher thermal efficiency. In previous tests 5 the
diesel car used 70 per cent of the fuel of the petrol car for the same journeys when fully warmed up. If half the
journeys were cold starts, as in the example above, the petrol car would use 1.37 times its datum (warmed-up fuel)
and the diesel car would use 0.78 times the petrol car's datum fuel - or only 57 per cent of the petrol car with
cold starts.
As has been explained, these are illustrative calculations only, but they do show the advantage of the diesel
car in reducing excess fuel used in cold starting with the pattern of car travel which exists at present. Further
analysis would be justified when more is known about the distribution of car journeys which start from cold.
4.2 Fuel consumption at idling
The fuel used at idling is clearly less important than the general use of fuel when the engine is cold, although
there are one or two significant points from the tests that have been carried out. The most interesting is perhaps
* The National Travel Survey results are quoted in the original units, miles, and not kilometres.
that the petrol car idling consumption was only 15 per cent higher than the diesel car when both were fully warmed
up. Thus in the normal conditions of engine idling in traffic, the diesel car has some advantage, but not a great one.
On the other hand, the amount of fuel used when idling in traffic should not be underestimated. Reference 7
reported tests with an instrumented car in Glasgow where nearly 40 per cent of the time was spent stationary with
the engine idling, and about 20 per cent o f the fuel used in trips through central Glasgow was spent with the engine
in the idling condition.
Clearly the high idling fuel consumption o f the petrol engine when cold (see Figure 9) is a disadvantage
compared to the diesel engine, but it should be put in perspective. It would be unusual to idle the engine from
cold for more than 5 minutes - and probably not good practice for fuel consumption anyway. If both engines
were idled for five minutes from cold, the petrol engine would use about 250 cc of fuel which would represent
about 2.2 km of urban driving: the diesel car, idled for 5 minutes from cold, would use about 70 cc of fuel which
would represent about 1 km o f urban driving when both cars were fully warmed up. The diesel car does therefore
have an advantage when starting from cold, but the advantage can be considerably reduced by driving both cars
away once the engine is running. I f the engines are idled for long periods, Figure 10 shows that the petrol car will
suffer an extra fuel usage of about 450 cc compared with the diesel car.
4.3 General conclusions
The tests were not exhaustive enough to cover all situations of interest, but the results indicate the order of
magnitude o f the penalties o f cold starts for the two particular cars which were tested. The diesel car had a
substantial advantage in lower fuel consumption during the constant speed tests from cold, and an advantage,
though a lesser one, in idling fuel consumption. (More detailed graphs of the results are presented in Reference 8.)
I f further tests were carried out, it would be useful to extend the range of ambient temperatures that were
covered to see whether either type o f car was particularly sensitive to low temperature. The present measurements
were made when the air temperature ranged f rom 0°C to 6°C.
It would also be useful to see whether the automatic choke on the petrol car was unnecessarily increasing
cold start fuel consumption. Finally, it would be valuable to analyse the effect o f cold starting on the fuel
consumpt ion of the whole car population when additional data on car trips becomes available.
5. ACKNOWLEDGEMENTS
The work described in this report was carried out in the Assessment Division of the Transport Systems Department
of T R R L .
6. REFERENCES
. EVERALL, P F and J NORTHROP. The excess fuel consumed by cars when staging from cold. Ministry
o f Transport, RRL Report LR 315. Crowthorne, 1970 (Road Research Laboratory).
. BLACKMORE, D R and A THOMAS (editors). Fuel economy of the gasoline engine. London, 1977
(The Macmillan Press Limited).
. WATERS, M H L and I B LAKER. Research on fuel conservation for cars. Department of the Environment
Department of Transport, TRRL Report LR 921. Crowthome, 1980 (Transport and Road Research
Laboratory).
4. HOFBAUER, R P and K SATOR. Advanced automotive car systems Part II: a diesel for a sub compact car.
SAE Paper 770113, Detroit, 1977 (Society of Automotive Engineers).
5. WEEKS, R. The fuel consumption of a petrol and a diesel car. Department of the Environment Department
of Transport, TRRL Report LR 964. (In preparation).
. DEPARTMENT OF TRANSPORT. National Travel Survey: 1975/6 Report. Published by Government
Statistical Service. London, 1979 (H M Stationery Office).
7. EASINGWOOD-WlLSON, D, P NOWOTTNY and T C PEARCE. An instrumented car to analyse energy
consumption on the road. Department of the Environment Department of Transport, TRRL Report
LR 787. Crowthome, 1977 (Transport and Road Research Laboratory).
. PEARCE, T C. Cold start fuel consumption of a diesel and a petrol car. Assessment Division Working
Paper (80) 6. (This is an unpublished document available on direct personal request.)
TABLE 1
VW Golf cold start tests - supplementary information
Test 48 km/h 64 km/h 80 km/h Idling
Date
Air temperature t~c)
Humidity (%)
Barometric pressure (rob)
Wind
Track surface
Vehicle load
2612/80
6.3
95
993
Zero
Dry
~ o
8/1/80
3.6
95
994
Zero
Dry
Driver + one
11/1/80
0.0
100
993
Zero
Dry
18/3/80
2.0
100
985
15 kin/hE
N.A.
N.A.
8
TABLE 2
VW Golf cold start fuel consumption (L/100 km) by section
Circuit number
1.
.
Sample point
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Distance from start (km)
0.50
1.00
1.50
2.00
2.50
3.06
3.62
4.18
4.69
5.19
5.59
6.19
6.84
7.55
8.05
8.55
9.05
9.55
10.05
10.61
11.17
11.73
12.24
12.74
13.24
13.74
14.39
15.10
48 km/h
30.51
19.67
16.20
14.29
15.50
15.46
11.13
20.95
10.31
9.56
12.45
11.15
8.89
11.19
10.53
11.72
9.05
9.32
9.64
9.79
9.26
10.85
10.10
9.04
11.06
11.54
8.14
11.60
Petrol Golf
64 km/h
41.23
17.97
17.91
15.62
17.31
17.23
12.56
10.84
10.64
8.44
10.77
10.05
7.59
7.19
6.43
7.90
6.73
5.07
6.94
7.61
6.02
5.68
6.62
5.04
7.49
6.80
5.18
5.94
i 80 km/h
35.63
19.83
18.00
15.68
16.70
16.29
15.18
13.23
16.08
11.33
14.62
13.30
10.72
10.81
11.49
11.25
9.52
7.98
9.22
9.07
6.24
5.86
5.06
5.45
7.56
7.64
5.51
5.91
48 km/h
12.85
7.43
5.52
5.04
5.95
6.83
4.52
4.43
5.79
3.68
5.30
5.60
3.76
4.39
4.45
5.75
3.75
3.60
5.33
5.67
3.96
4.19
4.69
3.02
5.33
5.00
3.19
4.27
Diesel Golf
64 kmfh
16.86
6.61
7.75
6.53
3.95
6.74
4.92
4.26
5.51
3.31
5.24
5.41
3.48
3.87
4.21
4.99
4.58
3.40
4.60
5.44
3.44
3.44
4.37
2.61
5.09
5.01
2.70
3.76
80 km/h
15.58
11.49
8.90
6.94
7.59
7.98
5.77
5.56
6.49
4.16
6.82
6.41
4.53
4.99
5.68
7.05
5.59
3.61
5.61
6.34
4.87
5.06
5.30
4.04
6.03
5.87
3.89
4.90
TABLE 2 (continued)
Circuit number
.
.
Sample point
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Distance from start (kin)
15.60
16.10
16.60
17.10
17.60
18.16
18.72
19.28
19.79
20.29
20.79
21.29
21.94
22.65
23.15
23.65
24.15
24.65
25.15
25.71
26.27
26.83
27.34
27.84
28.34
28.84
29.49
30.20
48 km/h
I0.10
10.10
7.15
7.62
8.95
10.41
7.79
8.79
9.26
5.32
11.04
7.36
5.58
6.06
8.38
8.66
7.15
4.86
6.43
6.84
5.68
6.55
7.58
4.96
9.14
6.73
6.78
7.10
Petrol Golf
64 km/h
6.06
7.12
6.28
5.09
6.71
7.06
5.68
5.46
6.45
4.64
7.12
6.98
5.18
5.61
5.96
7.33
6.10
4.87
6.51
7.06
5.67
5.63
6.01
4.65
7.13
6.77
5.02
5.74
80 km/h
7.00
7.36
7.07
5.57
7.12
7.43
6.28
5.83
7.24
5.04
7.59
7.23
5.67
6.18
6.58
7.39
7.28
5.52
6.97
7.41
6.22
6.05
7.62
4.88
7.58
7.01
5.50
5.91
48 krnfh.
4.68
5.59
3.54
3.42
4.86
5.31
3.13
4.19
4.60
2.84
5.12
4.89
3.34
4.14
4.35
5.21
3.75
3.23
4.86
5.35
3.33
3.91
4.29
2.84
4.63
5.00
3.33
3.84
Diesel Golf
64 km/h
4.03
5.06
3.85
3.40
4.42
4.91
3.28
3.30
4.19
2.60
4.47
4.89
2.84
3.36
4.65
4.45
4.00
• 3.02
4.54
4.95
3.63
3.75
3.81
2.50
5.18
4.50
2.86
4.17
i
80 krn~
5.07
6.82
4.98
3.43
5.82
6.27
4.19
5.21
4.97
4.18
5.43
5.85
3.76
4.47
5.01
6.67
4.99
4.19
5.78
5.79
4.35
5.04
4.93
3.57
5.84
5.44
3.76
4.44
10
TABLE 3
VW Golf cold start idling fuel consumption (cc/min)
Time from start (min)
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
Petrol Golf
47.7
39.6
50.7
64.9
50.7
50.7
50.7
59.9
36.5
20.3
21.3
19.3
20.3
24.4
22.3
14.2
16.2
15.2
13.2
10.1
9.1
10.1
10.1
10.1
9.1
10.1
9.1
10.1
10.1
9.1
Diesel Golf
18.3
12.3
12.4
12.5
10.5
10.6
10.7
9.8
9.9
8.9
10.0
8.0
9.1
9.2
8.2
9.3
8.3
8.4
9.5
7.5
8.6
8.6
7.6
8.7
7.7
8.8
7.8
8.9
7.9
7.9
11
TABLE 4
Variation in fuel consumption by circuit (excluding initial section)
Distance from start (km)
0-- 7.55 7.55--15.10
15.10--22.65 22.65--30.20
0-- 7.55 7.55--15.10
15.10--22.65 22.65--30.20
0 - 7.55 7 .55-15 .10
15.10-22.65 22 .65-30 .20
Circuit number
1 2 3 4
1 2 3 4
Speed (km/h)
48
64
80
Petrol Golf
L/IO0 krn
13.52 10.07
8.11 6.84
12.34 6.32 6.05 5.97
14.51 7.75 6.55 6.49
Ratio to Circuit 4
1.98 1.47 1.19 1.00
2.07 1.06 1.01 1.00
2.24 1.19 1.01 1.00
Diesel Golf
L/IO0 km
5.20 4.39 4.20 4.10
5.08 4.08 3.86 4.04
6.65 5.23 4.99 4.94
Ratio to Circuit 4
1.27 1.07 1.02 1.00
1.26 1.01 0.96 1.00
1.35 1.06 1.01 1.00
TABLE 5
Distance required for the cars to be warmed up*
Speed (krn/h)
48 64 80
Petrol Golf (V~n)
21.2 9.0
11.0
Diesel Golf - (km)
6.6 4.7 4.7
* DeFined as first reaching within 10 per cent of the average fuel consumption measured on the last circuit. (See Figure 3)
TABLE 6
Variation in fuel consumption while idling by five minute intervals (excluding initial minute)
Petrol Golf Diesel Golf Period
Ratio to cc/Min Ratio to (mins) cc/Min last interval last interval
2 - - 5 6 - 1 0
11--15 16-20 21--25 26--30
51.48 43.62 21.52 13.78 9.70 9.70
5.31 4.50 2.22 1.42 1.00 1.00
11.93 9.98 8.90 8.60 8.24 8.30
1.44 1.20 1.07 1.04 0.99 1.00
12
6(12.8)
13(2.7) 12(12.8)
5(2.7)
7(10.3)
4(0.0)
14, start(0.9) 8
3(6.4)
10
2 1(0.0) / f ~ 9 ( 1 0 . 9 )
0 300m I 1 I I
Fig.1 TRRL TEST TRACK SHOWING CIRCUIT AND MEASUREMENT POINTS AT THE END OF EACH SECTION (figures in brackets are the relative heights of the track in metres)
4
10
2
12
-o 8
o
~ 4
t-
O
PETROL GOLF (last circuit)
"
~ _ ~ o
" ~ . / \
\ / ° ~ °
i ~ , ~
• I i / - '~ ' , , ," \ // \ ' , 80kin/.
\ ' , !/_ ~ / ' / " N . v 64km/h
i " \ . / \ ' - " " . . . . X i ; X /
\ i \ / • 64km/h \-
DIESEL GOLF (last circuit)
0 I I I I I I I
23 24 25 26 27 28 29 30
Distance from start (km)
A
E 0 0
e-" 0
E t- O U
23 24 25 26 27 28 29 30
Distance from start (kin)
Fig.2 FUEL CONSUMPTION A N D T R A C K G R A D I E N T COMPARED
0
O0 0~
U- ...J
0
. J 0 r r I-- LU Q.
/
/ EL.
0
LU OrJ LU
I I I I I I ~" 0 CO 0~ GO ~ " O,J O,I ,--
E O
E
O0
L",I
__ 0
_ GO
"- E
-
E 2
- ~ ~ C
. - - 121
O
- - C O
O
O
A GP
aP
C ° ~
o E
R
r -
E
0 CO
I -
Z 0 I - tJJ C~
>-
Z 0 I - O.
Z 0 f,.) . .J UJ
U.
._= U.
(LU)IO 0 L/SaJJ.!I) UO!~.dLUnSUO0 l e n l
28
24
A
E o 20 0
" 16 .o
E ¢-
O
u 12 .-,i
8
4
I
!
I I I
I _1
I
PETROL GOLF
' , " V J! " , ' l / , , . , - , /,, ; ' , , . ,,,, ,., , . , ,.,,,; ' , , . - , , . . , , , , . . , , . , , , . , , tJ l~ I E I . I . • t I t E ~ • t
DIESEL GOLF
I I I I 1 1
4 8 12 16 20 24
Distance from start (km)
t " S~L
I S . t I
II
I 28
' , I
32
Fig.4 FUEL CONSUMPTION BY SECTION AT 48km/h
Q
(.0
I L~
,,/ /
0 (D ,,,.,J O
I - - i i i P,
j l
I I I
uo!~.dwnsuoo lan~ paz!leWJO N
u_ ._1 0 cD . - I W CD W
I
GO
0
O0
E
E o
4--
t~
. - - tn
O
oO
c.O
0 0
E ,=~
I - -
z 0 I-- Q=
z 0
,=.,I I.LI
M=
I.LI
I -
. - I
0 M,I N , - I
01: 0 z
I ~0
I UD
I I
uop, dwnsuoo lan~ paz!leLuJo N
I.L . . J 0
0 I - -
O .
I
/
- J 0
- J U.l
a
I 0
0
¢.D
0
O0
" - E
E 2
¢ -
° _ a
0
E
I - <
z 0 F- O.
(n 2: 0 (J
W
U.
ILl > ) - < ..J
u
Q UJ N , . I < :E r r 0 Z (D m
U.
p~
8 g:,
--.I
0
...1
U J
I I I I 1 I Q
0
0
O0
E v
E 8
e..
a
0
E 0 ¢0
Z 0
a.
Z o U . J W
L .
.J
U r~ W N
-J
0 Z
IJL
UO!),dLUnSUO0 lanJ paz!leuJJo N
45
A E
v
@ 25 C t1=
O l
O l
g 20
U -
4 0 - -
3 5 - -
30 -
15 -
10 --
/ Diesel Petrol ,J
• 80km/h O oOA
• 64km/h 0 00~7
I o°~o ° I
PETROL GOLF O ~ 0 o i E=ess O(~-Uo I distance 0_~
_9,,~ o / e e = = l oO~ o° / .,'..-." I
oO O i - : , . - 'A O AO O ~ i &== / 1
° o ~o •'~r'• / o o _•==:P /
o - - o - ' = ='I . / 0 0 J O; • • I t /
O O "~ - • 1 • / o _o ~ •U_l A = /
~.~ U ,_,0 ~ O n I • /
O u A • 0 ~ • . , - O n 0 . ~ e l I A • / /
0 0 ~ ~ • 1 1 1 • • / DIESEL GOLF
vO ~ • l • / _on ~ e@ • /
u ,~ .lll==/~=• // o z~ . I I • X . , ,
z3 ~ . • • / - ~ No 'cold start effect
o8 ,, = l l • • / / oO/ ~ _117- /
o o ~ " _ I • • / I 1 • / o O ~
slIA• / o 0 ~ I • /
o• A ee~A• • //
o ° " e&• / o~e /
0 ~/~/// t 0 5
/
I I I I
10 15 20 25 30
Distance from 'cold start" (km)
Fig.8 T H E "FUEL E Q U I V A L E N T D I S T A N C E ' FOR T H E P E T R O L A N D DIESEL CAR
0
0
._1 U.
._1
n-" ~, I - - U. ILl a. ~ E:
0 ~E E ~
/ I I I 1 1 I
Q 0 0 Q 0 O
(u!w/oo) ~o1~ lan::l
0
CO
0
00
° ~
E CD v
E
E I - -
Q
GO
0
A G , ) !
('~ v
~D Z
1.1.1 , - I
o ...I I.I.
UJ
I.I. O'J
° ~
0 LO
e-
E
c~ LL
0 ~9 ..J
0 re-"
LU O.
I I I I I 0 0 0 0 0 0 0 0 0 0
(oo) pasn lan:l
0 Q
r-
E
co
0~ U_
0
-J UJ O0 LU
r~
0 0
CO
~0
0
0
O0
E 0
E
C~
0 Vl
O0
~0
0 ~0
0
z .J r~
W .J
UJ
.d UJ
M.
,11 >
<
0
U.
i ,/ J
I
oO
co (O rr
d
Z
t~ DC
--J UJ
UJ
Lt)
a z
..J
0 0c
Q.
o~
v--
U. --J
0
-r
to
7. APPENDIX 1
Comparison of constant speed tests with results from other work
It was observed during the cold start tests that the measured fuel consumption (fully warmed up) of both cars was
higher at 48 km/h than at 64 km/h and 80 km/h. This trend had not been observed before in other tests with the
cars, nor was it apparent in other published results. Some additional tests were therefore undertaken in May 1980
both on the same test track route as used for the cold start tests, and on a length of public road which had been
used before for constant speed tests 5. The results of the cold start tests from Table 4, together with the new
check tests on the test track and public road are presented in Table 7, where the earlier measurements on the
public road, and published figures from the motoring press are included.
For the Golf petrol car, the scatter in fuel consumption measurements at 48 km/h is much larger than at the
higher speeds, but there is no consistent tendency for different results on the test track compared with those on the
public road. For the Golf diesel car, the scatter in fuel consumption measurements at 48 km/h is much less than
the petrol car, but again there is no consistent tendency for results on the test track to be different from those on
the public road.
No firm conclusion can be drawn from the figures in Table 7, except that the measurements of fuel
consumption on the petrol car have a tendency to be fairly widely scattered (+ 15 per cent) at a constant speed of
48 km/h. The same tendency has been observed by the manufacturer (Figure 61 in Reference 4) so that the present
results are not inconsistent with the manufacturer's values. It seems likely that the difficulty in obtaining closely
repeatable results with petrol car at 48 km[h is associated with changes in gradient of the track or the public road,
and the difficulty o f keeping the instantaneous speed constant while driving at low throttle openings.
While the comparison with results o f steady speed fuel consumption from other sources is inconclusive, the
repeat results and the comparison with the manufacturels tests do not suggest that either car behaved abnormally
on the cold start tests. TABLE 7
Steady speed fuel consumption of VW Golf petrol and diesel cars from different sources (litres/100 km)
Car type
Speed (km/h)
TESTS
TRRL test track, Jan/Feb 1980
TRRL test track, May 1980
Bagshot Road A322, May 1980
Bagshot Road A322, 1978/79
Published Figures
NOTES
1
2
3
4
5
Golfpetrol
48 64 ]
6.84
6.33 5.96
5.77
5.20
5.09
5.97
5.78 5.64
5.55
5.50
6.14
80
6.49
6.34 6.56
6.44
6.40
6.77
48
4.10
3.34 3.42
3.57
3.35
3.32
Golf diesel
64
4.04
3.65 3.75
3.76
3.65
3.72
80
4.94
4.63 4.57
4.52
4.30
4.41
NOTES:
24
1. Cold start tests: mean value of fourth circuit. 2. Repeat tests: cars driven for 30 krn, before fuel consumption measured on two circuits. 3. Repeat o f constant speed tests of Reference 5. 4. Measurements given in Reference 5. 5. Petrol car figures from 'Autocar ' 11/1/75: Diesel car values are manufacturers figures quoted in
'Autocar ' 29/4/78. (Converted from mph.)
8. APPENDIX 2
The distribution of car journey lengths
8.1 Introduction
The best available published source of information on the length of car journeys is the National Travel Survey,
1975/76 Report 6. Like most reports summarizing a complicated subject on the basis of a large survey, the published
Tables do not always cover every case of specific interest, and some re-casting is needed. This Appendix is coricerned
with the distribution of car journey lengths for different types of journey, and the NTS Tables have been put in a
more convenient form for this purpose. While the object was to obtain information about the use of cars, most of
the Tables relate to person-journeys (ie including car passengers) and also include private travel in vans and lorries,
so that the results can only be regarded as approximate.
There are other refinements in the NTS which have been glossed over to give a broad view of car travel. Most
data relate to journeys which may have several stages (by different mode, or change of ticket). With car travel,
most journeys are of one stage only so that the distinction is not important, but it does mean that some of the
sample totals for the other modes are inconsistent. If the reader wishes to have a more precise set of data, he should
refer to the source document 6.
8.2 General travel
Table 8 gives the percentage of journey stages of different lengths of car (or van) made by a driver, and by
driver and passenger combined. The similarity of the distributions gives some confidence for the use later of
combined driver and passenger distributions for different journey purposes.
The driver percentages can be taken to represent the use of the car. This gives an average stage length for the
car of 7.5 miles*: 79 per cent of car stages are less than 10 miles in length.
Under 1 mile
Driver 6
Driver and passenger 6 combined
Source:
TABLE 8
General travel: journey stages (by car or van): percentages
1 and under
2 miles
17
18
2 and under
3 miles
15
15
3 and under 5 miles
20
19
5 and under
10 miles
23
22
10 and under
15 miles
15 and under
25 miles
6
25 miles and
over
All stage
lengths
100
100
No. of stages
in sample (000)
(139)
(229)
Reference 6, Table 4.2
* NTS data distance data is recorded in miles rather than kilometres.
25
8.3 Car travel for different journey purposes
Table 9 gives the appropriate percentages of car travel as a function of journey purpose, and also (for
comparison) the percentage o f travel by all modes. The final column shows the proportion of car travel for each
journey purpose. Details of the length distribution of car travel are available in Reference 6 for all journey purposes
except travel to and from places of education, and for shopping journeys. Examples are given in Table 10 for the
following journey purposes:
(a) Journeys to and from work
(b) Journeys for personal business
(c) Social journeys (eg visiting friends and relatives)
(d) Journeys in the course of work
(e) Holiday journeys (over 25 miles)
The holiday journeys are only those over 25 miles, and the sample is small (1,600) so that the results have
to be treated with caution.
8.4 Concluding remarks
Histograms o f the car journey length distributions for two journey purposes are plotted in Figure 11 from
Table 10. The histogram of general travel for car drivers from Table 8 is also shown for comparison.
The histograms of general travel and for journeys to work are similar, with mean journey (stage) lengths of
about 7 miles (11 km). Car journeys in the course of work tend to be longer, as the histogram shows, and the mean
length is about 17 miles (27 km).
26
TABLE 9
Travel by journey purpose
Journey purpose*
To and from work
In the course of work
Education
Shopping
Personal business
Eating and drinking
Escort
Social
Entertainment
Outdoor recreation
Holidays (over 25 miles only)
Journeys by car/van/lorry
Journeys by all modes
No. of journeys % in sample
(000)
55.0 25
13.5 6
6.2 3
32.5 15
20.6 9
7.2 3
17.4 8
43.2 20
10.I 5
11.6 5
1.6 1
218.9 100
No. of journeys in sample
(000)
107.7
17.1
48.4
94.0
45.3
15.1
25.8
75.1
19.9
33.3
2.0
%
22
4
10
19
9
3
5
16
4
7
(0.4)
Journeys by car/van/lorry
All modes
0.51
0.79
0.13
0.35
0.45
0.48
0.67
0.58
0.51
0.35
0.8
Totals i 483.7 100 0.45
* For full def'mition, see Reference 6.
Source: Reference 6. Tables 5.12, 6.2, 7.2, 8.2, 8.14, 8.21, 8.27(a), 9.3, 9.9, 9.15, 9.22.
27
TABLE 10
Percentages of journeys Of different lengths by car/van/lorry for five journey purposes
(a) Journeys to and from work
Employed men and women
Under 1 mile
1 and under 2 miles
15
2 and under 3 miles
14
3 and under 5 miles
22
5 and under
10 miles
26
10 and under
15 miles
10
15 miles and
o v e r
I No. of All journeys
journeys ~in sample (000)
lOO (55.0)
Source: Reference 6. Table 5.12
Co) Journeys for personal business
Journeys I 9
Source: Reference 6. Table 8.14
(c) Social journeys
,6 1,9 1 9 I 6 1 7 i lOO I (~06~
Jou~oys I ~ i l , ~4 119 1~4 1 9 1,4 i ~001(43~ Source: Reference 6. Table 9.3
(d) Journeys in the course of work
Journeys
1 and 2 and Under
under under 1 m i
2mi 3 mi
4 8 9
3 and i under
5rni
15
Source: Reference 6. Table 6.2
5 and under 10mi
23
10 and under 25 mi
24
25 and under 50 mi
11
50 and under 100 mi
100 and under 200 mi
200 mi and
o v e r
(0.4)
f No. of All [ journey,'
journeys in samph (000)
100 (13.5)
(e) Holiday journeys over 25 miles
25 and under
50 miles
50 and under
100 miles
100 and under
200 miles
200 miles and
OVer
All journeys
No. of journeys in sample
(000)
Journeys 31 l 31 26 12 100 ( 1.6)
Source: Reference 6. Table 9.3
28
g_
30 ,---
2 0 ' ~
10
0
0
m
m
Mean
I
10
I 15 20
Stage length (miles)
GENERAL TRAVEL -- A L L PURPOSES (car or van driver)
I 25 30
¢-
O °4
t -
D_
30
20
10
0
m m
Mean
I t
I
10 15
JOURNEYS TO AND FROM WORK (employed men and women)
I I I 20 25 30
Journey length (miles)
e- L
O
"5 O}
t -
t~
30
20
10
l
Mean
25
JOURNEYS IN THE COURSE OF WORK
50 75 Journey length (miles)
I 100 125
Fig.11 JOURNEY LENGTH DISTRIBUTION FOR A L L CAR TRAVEL AND TWO JOURNEY PURPOSES (see annexe B)
( 2 3 3 6 ) D d 0 5 3 6 3 8 0 l , S 0 0 1 2 / 8 0 H P L t d S o ' t o n G l 9 1 5
P R I N T E D I N E N G L A N D
ABSTRACT
COLD START FUEL CONSUMPTION OF A DIESEL AND A PETROL CAR: T CPearce a n d M H L Waters: Department of the Environment Department of Transport, TRRL Supplementary Report 636: Crowthorne, 1980 (Transport and Road Research Laboratory). Measurements have been made o f the fuel consumption o f a petrol and a diesel car when starting from cold. The cars were the 1.1 litre petrol VW Golf and the 1.5 litre diesel version, which have the same passenger accommodation and nearly identical road performance.
It was found that the diesel car used less fuel in the 'warm-up' period than the petrol, both when being driven at constant speed on a test track and with the engine idling and the car stationary.
ISSN 0305-1315
ABSTRACT
COLD START FUEL CONSUMPTION OF A DIESEL AND A PETROL CAR: T CPearce a n d M H L Waters: Department of the Environment Department of Transport, TRRL Supplementary Report 636: Crow'thorne, 1980 (Transport and Road Research Laboratory). Measurements have been made o f the fuel consumption o f a petrol and a diesel car when starting from cold. The cars were the 1.1 litre petrol VW Golf and the 1.5 litre diesel version, which have the same passenger accommodation and nearly identical road performance.
It was found that the diesel car used less fuel in the 'warm-up' period than the petrol, both when being driven at constant speed on a test track and with the engine idling and the car stationary.
ISSN 0305-1315
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