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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
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.
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
7. Appendix 1 :
8. Appendix 2:
Comparison of constant speed tests with results from other work
The distribution of car journey lengths
8.2 General travel
8.3 Car travel for different journey purposes
8.4 Concluding remarks
(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
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.
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
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.
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 cons