Road Load Determination - Vehicle Preparation

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Road Load Determination - Vehicle preparation

Authors:
Peter Smeds – Swedish Transport Administration

Iddo Riemersma – Transport & Environment

DTP Subgroup
Lab Process ICE
Stephan Redmann
Copy to:
Werner Kummer
Nikolaus Steininger
1(7)
TDOK 2010:237 Mall_Brev engelska v.1.0 (Form ID)
Document Date: 2010-11-19
Item Number: 1.0
Project Number:
1(7)
TDOK 2010:237 Mall_Brev engelska v.1.0 (Form ID)
Document Date: 2010-11-19
Item Number: 1.0
Project Number:

Background

At this moment a world-harmonized light-duty test procedure (abbreviated to WLTP) is being developed. This effort involves to review the current test procedures that are being used in different regions (mainly Europe, USA and Japan), and to come up with an improved draft test procedure (DTP). At the same time a test cycle will be developed that is a good reflection of driving behavior in real-life. The DTP workgroup is split in 5 subgroups, that focus on specific parts of the test procedure. Within the subgroup Lab-processes for internal combustion engines (LabProc_ICE) a further subdivision into interest groups is made. This paper specifically focuses on one of these interest groups called ‘Road Load Determination’ (RLD), which involves all of the technical requirements for preparation of the vehicle before the road load tests take place. The result from RLD is then used for adjustment of the chassis dynamometer during the process of homologation of the vehicle. During the last expert meeting in Brussels on 5-6 October 2010 it became apparent that the current road load test procedure as laid down in ISO 10521 - 1 & 2 has a number of omissions that will cause to influence the test results in a positive sense, from the manufacturer’s point of view. As a consequence, the road load of production vehicles will be higher in comparison to the carefully prepared road-load test vehicle. This will also affect the fuel consumption and CO2 emissions. Some first exploratory tests have shown that CO2 figures may be some 10 – 15% too optimistic. This is one of the reasons that an officially declared fuel consumption by the manufacturer does not match the customer’s experience.The situation is even accelerated by the global strive for energy conservation and reduced CO2 emissions which leads to development of new fuel saving technologies and more competition between vehicle manufactures. To avoid penalty fees, reduce vehicle taxation and improve product competiveness vehicle manufactures put a large effort in achieving the best possible coast down figures within current regulations.

During the expert-meeting this issue was first raised by the Swedish Transport Administration and was backed up by Transport & Environment. The RLD interest group decided that STA should undertake the task to investigate this, and to produce some written input for the discussion. This investigation should sum up the ‘flexibilities’ in the ISO 10521 test procedure as well as the tolerances that are stretched to the most favorable end. Also the options available to address these issues properly in the DTP should be identified. This paper will present the results of the investigation and the options for the DTP, aiming at a RLD procedure which is representative for production vehicles under real-life conditions while at the same time being repeatable and reproducible. In the current outline this paper is intended to provide input to the discussions in the DTP expert meetings; the defining of a legislative text proposal will be part of the next phase, when agreement has been achieved on which issues to address and the specific option to fix it. The Swedish Transport Administration and Transport & Environment have not conducted any tests or measurements; this document is based on earlier experience and discussions with engineers within the field of the subject.

Preparation for road test – Vehicle preparation and conditioning

Before the road test is performed vehicle manufacturers may carefully adjust the vehicle. According to ISO 10521-1, these adjustments should fulfill the requirement “as specified by the manufacturer” (5.2.1.1). This kind of terminology allows vehicle manufactures to “specify” vehicle settings with wide tolerances. In this way, one end of the tolerance band may be used for production vehicles and the other end may be used for a smooth coast down, while both settings are withinthe manufacturers specification.

Option for solving this in the Draft Test Procedure:

Making an inventory of vehicle adjustments that may influence the road load of the vehicle, and to specify a fixed tolerance band (absolute or relative)

Wheel alignment

Wheel alignment is an important parameter for the vehicle´s ability to coast. Wheel alignment is a tradeoff between good handling and the ability to coast. During mass production in the factory it is possible to adjust wheel alignment rather tight to the specifications. However, if a wide tolerance band is specified by the manufacturer, the wheel alignment could be changed within the tolerances to a setting for better coast down results. Both the alignment on front and rear axles can be adjusted.

Option for solving this in the Draft Test Procedure:

Specify a fixed tolerance band for wheel alignment, either absolute or relative.

Adjustment of brakes

The space between a brake pad and brake disc is very small. There is a risk of contact between the pad and the disk, even when the brake pedal is not pressed. This will cause some extra friction. To avoid this, the pistons inside the brake caliper are pushed back by a technician in the preparation shop prior to the tests. This will eliminate friction and improve the vehicle’s ability to coast. In ISO 10521-1 , 5.2.1.1, this is referred to as “parasitic drag”. However, if it is normal for a production vehicle to have some parasitic drag, it should not be allowed to adjust the brakes. However, , it is of the interest by manufacturer to minimize the risk for contact by “push back” of the pads, thereby improve the repeatability of the tests.

Option for solving this in the Draft Test Procedure:

The test procedure could require that as a part of the preparations of the coast down test the driver should press the brakes from a certain speed to stand still (possibly also specifying a minimum deceleration). The resulting parasitic loss (if any) will lead to a measured road load that is more representative for real life situations. However, the problem with this kind of preconditioning is that this may conflict with the repeatability of the coast down test results, since the contact between pads and discs might vary depending upon driving conditions, temperature etc. This may need some more discussion to come to a suitable solution that will do justice to the representativity of the road load test results, while the repeatability of the test can still be guaranteed. At this moment there is no data available that shows the influence of this phenomenon, but physically it is expected to be rather marginal. Before putting a lot of effort into this matter it could be sensible to first prove that the effect is substantial.

Ambient conditions

South of Spain, southern Italy and Arizona (USA), are some popular test tracks for coast down testing. It is presumably no coincident that these tracks are located in areas with a warm climate. One of the reasons could be that the weather conditions in those areas are more stable, but another good reason may be that the friction of bearings and other moving parts are in favor of the test results. The correction factors only take into account the effect on air density associated with a temperature, humidity, altitude and air pressure differing from standard conditions.

Option for solving this in the Draft Test Procedure:

• ISO 10521 (5.1.1.2) requires that the temperature during Road Load Determination should be between 274-308K (0 – 35° C). For testing on the chassis dynamometer the current proposal of the WLTP suggests 25°C ± 5°C. To better reflect test cell conditions the Road Load Determination temperature regulation adjusted to 25°C ± 5°C as well.
• The correction factors may be expanded in such a way that they also account for other temperature related influences on the road load. This may also be beneficial from a competition point of view for test tracks in areas with a colder climate. On the other hand, it will be difficult to derive a generally valid correction formula for all vehicles, since the influence may vary for different vehicle makes and models. Also there is probably no data available to calculate such a correction.

Tires – tire wear, pressure, type and rolling resistance

There are a number of issues related to the tires and the tire choice that need to be addressed.

Tire wear

The RLD regulations allow tires to be worn to 50% between a new and a bald tire. (ISO 15021-1, 5.2.1.1). The resulting effect will be that the rolling resistance is lower. New tires are grinded in a machine and then broken in by mileage collection down to the lowest limit.

The tires have wear indicator strips running across or parallel to the tread. The letters TWI are printed on the side of the tire. When approximately 1.6 – 2.2 mm is left on the tread, these strips will be level with the tire surface, that the tire should be replaced. A new tire has some 8-9mm of thread dept. 50% of this means 4-4.5mm.Since vehicle users normally will replace tires before they are completely bald (typically between 2 and 3 mm), the mentioned 50% is representative for an average tire wear.

Option for solving this in the Draft Test Procedure:

To better reflect average tire wear and the associated rolling resistance, a wear value of 30-40% from a new to a bald tire should be considered.

TirePressure

The tire pressure is set to the maximum pressure that is mechanically allowed for the particular tire. Vehicle manufacturer may in the instructions manual also specify a “ECO pressure” that is higher than regular recommended pressure. During coast down “ECO pressure” is used. The obvious reason for that is to reduce rolling resistance.

Option for solving this in the Draft Test Procedure:

A requirement should be included to specify a tire pressure that is similar (within a certain tolerance) to the recommended tire pressure of the production vehicle’s tires. Since the recommended tire pressure is also related to the tire make and dimensions, a supplementary requirement to cover that will be needed. See also the next point.

Tire Choice

Vehicle models are in most cases offered with various tire specifications. The basic model of a vehicle platform might be equipped with tires with a high profile and smaller dimension (diameter) of the rim, while the more “sporty version” usually is equipped with low profile tires on a wider rim. Since the coast down tests are performed well in advance of the production of a new vehicle type,manufacturers claim that the final choice of tire brand, model and dimensions is not decided yet at the time of the coast down. This allows them to select a small tire with low rolling resistance and air drag to be used for the coast down tests. There is no legislative mechanism to force a manufacturer to deliver the production vehicle with the same tires.

Option for solving this in the Draft Test Procedure:

A requirement could be included to demand that the same tire make and model as fitted to the production vehicle will be used for the road load tests. However, it needs to be recognized that even the same vehicle model may be factory delivered with different tire types and dimensions. At the same time, tire development as well as tire costs are not stationary, so manufacturers would appreciate some flexibility. As an alternative, the manufacturer could be required to only fit tires on the production vehicle that have the same or higher rolling resistance, and have the same or larger dimensions as the tires used during the coast down tests. If necessary, he can choose to perform different tests for different tire types, as long as the rolling resistance and dimension criteria are met. It is recommended that tire dimensions and relevant properties such as rolling resistance are mentioned in the test report.

Test track

The design of the test track can also influence the results of the RLD. In the US legislation on road load determination it is required to only have commercially available asphalt on the test track, to prevent that special low friction asphalt is used which is not applicable for public roads.

Option for solving this in the Draft Test Procedure:

The same requirement as applied in the US legislation could be copied into the WLTP.

Vehicle Weight

During the coast down test, the vehicle is equipped with measurement equipment. To compensate for this added weight some irrelevant parts of the interior of the vehicle may be removed, e.g. the back seat. Manufactures might be tempted to remove more weight than was added. At the same time fluid levels in the vehicle may be reduced to a minimum, and standard accessories like a spare wheel and a jack are left out of the vehicle.

Option for solving this in the Draft Test Procedure:

Similar to the tire choice issue, a requirement could be included to demand that the mass of the vehicle used for the coast down test is the same or higher than that of the production vehicle.

Vehicle Body

Concerning the vehicle body, different tactics are used to reduce drag, such as taping the gaps between the body panels, and/or demounting roofrails and mirrors. Also the ground clearance could be lowered, which will also have a positive effect on the aerodynamic drag.

Option for solving this in the Draft Test Procedure:

Similar to the tire choice issue, a requirement could be included to demand that the aerodynamic drag of the vehicle used for the coast down test is the same or higher than that of the production vehicle. This does not necessarily have to be actually measured, but the TAA auditor witnessing the homologation process should carefully monitor differences in the bodywork and the ground clearance. It is recommended to include relevant parameters and remarks in the test report such as fitted exterior bodyparts, ground clearance, and maybe more.

Transmission

While the engine is decoupled from the wheels during coast down, some parts of the transmission (including the differential) are rotating with the wheels. Replacing the normally applied transmission oil by a low-viscous low-friction alternative could potentially give a slightly lower road load.

Option for solving this in the Draft Test Procedure:

A simple statement to use a transmission oil with similar or worse lubrication characteristics as for the production vehicle will be sufficient. For that purpose some of the oil characteristics will need to be specified in the test report.

Correction factors

P.M.

Other issues

Other issues, with possibly only a small effect are:

• Applying oil to wheel bearings for lower friction
• …

Alternative approach

So far, a number of issues have been identified that could be solved to some extend in the DTP by adding a number of extra requirements. This could be referred to as a feed-forward approach: the requirements are aimed at the vehicle for coast-down testing, for the purpose of ensuring that the future production vehicle will match the measured road as close as possible. Please note that there is no legislative mechanism in this feed-forward approach to actually check if the road load of the production vehicle is actually the same.

At the introduction of this document it was stated that the objective is to develop “a RLD procedure which is representative for production vehicles under real-life conditions while at the same time being repeatable and reproducible”. An alternative method to achieve this is to use a feed-back approach: the production vehicle will also be subjected to a road load test, and results will be compared. The only thing necessary for the test procedure is to require a repetition of the emission tests when it is observed that the road load of the production vehicle is higher than that of the prototype vehicle tested earlier (maybe a small threshold can be allowed). In this case, road load testing would have to be part of the CoP procedure, or even the in-use compliance checking. Whether this is a feasible option needs to be discussed, but from a scientific point of view this is more preferable than the feed-forward approach.

Conclusions

• The results from a Road Load Determination reflect the best possible climate and best possible vehicle setup.
• ISO 15021-1 part 5.1.1.2 may need adjustments to better reflect the temperature in the emission test facility.
• ISO 15021-1 part 5.2.1.1 may need adjustments to better reflect the set up of a vehicle delivered to end users.
• Further discussions are necessary to determine how to correct this.

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