WLTP-DTP-LabProcICE-053 Proposal for tire pressure during coast down testing Author: Iddo Riemersma (Transport & Environment) Version/date: 1.0 / 14 3 2011 Subject: Proposal for tire pressure Introduction In the current ISO 10521 1 standard the vehicle preparation paragraph for coast down testing only briefly addresses the tire pressure. The target pressure is defined as the manufacturer s recommended inflation pressure. If there is a difference between soak and ambient temperature of more than 5 K, the standard provides for a pressure correction function. Since the manufacturer may indicate this recommended pressure, there is a possibility that the tires are overinflated on the test vehicle to reduce rolling resistance. According to some very old formulas from the Michelin Handbook and Reimpell Fahrwerktechnik a tire that is recommended to be inflated at 1.9 bar but is overpressurized to 2.2 bar will have a 6 8% lower rolling resistance. As there are no requirements to ensure that the manufacturer s recommendation corresponds to the tire pressure of the production vehicle, a clear need exists for a better and well defined tire pressure for road load determination in the GTR. Rationale A recommendation for tire pressure will be closely linked to the tire make, model and dimensions but also the weight of the vehicle. At the time of coast down testing, the test vehicle will normally be a prototype or pre production vehicle. Until the production phase commences, the choice for the tires may change for reasons of marketing and/or procurement. Therefore, the manufacturer s recommended inflation pressure is not to be considered a fixed parameter, especially as it is beneficial to choose a higher value for it. These considerations clearly identify the need for a better definition of tire pressure. It is recognized that the definition of the tire pressure will have to cover the following aspects: 1. Ideally the tire pressure should be representative for the most common vehicle use; 2. The vehicle manufacturer should have no possibility to artificially raise the tire pressure in any way; 3. The tire pressure value should be well and indisputably defined; 4. The time difference between homologation and production requires a verification mechanism to check if tires and pressure on production vehicles are the same as applied on the coast down vehicle.
At first glance it seems not so straightforward to harmonize these requirements into one proposal for the GTR, so they will first be shortly discussed. Representativity The WLTP DTP aims to develop a testprocedure for homologation of all cars sold around the world, and which is representative for real life conditions. According to that objective, the world average vehicle use (number of passengers, average passenger weight and average vehicle load) should be established. To find a well substantiated figure for that (e.g. as a percentage of the vehicle weight in running order) would require an exhaustive study. An extra complicating factor is that car owners normally drive vehicles with a lower tire pressure as is recommended because they forget to check the pressure regularly 1. It seems that due to the lack of relevant information, concessions need to be made towards the representativity criterion. Furthermore, the current tight time schedule for the WLTP will not allow to put sufficient effort in developing an overall representative tire pressure definition. Independency The positive influence of tire pressure on rolling resistance can be an incentive for manufacturers to recommend a higher tire pressure than necessary from the technical viewpoint. As a consequence, there is a need for an independent advice on tire pressure. The only objective recommendation on tire pressure could be given by the tire manufacturer, but complete independency cannot be guaranteed. Furthermore, the tire pressure is one of the key parameters in the well balanced design of the vehicle dynamics so the car manufacturer may choose a different tire pressure from the advised pressure for a good reason. Rather than searching for an independent and objective tire pressure recommendation, the alternative approach could be to verify that the recommended tire pressure on the production vehicle corresponds to that of the earlier tested coast down vehicle. Surely, the tire pressure in the owner s manual will be recommended on the basis of good dynamic performance. And even if this pressure is somewhat high, it is still the reference for the owner to set the tire pressure and as a result it will also be higher on that vehicle in practice. Definition In order to obtain a well defined and indisputable tire pressure for the testprocedure there are generally three possible approaches: Develop an empirical formula that establishes the tire pressure based on relevant characteristics such as tire dimensions and vehicle weight. This certainly brings a welldefined and independent tire pressure for testing, but on the down side the representativity 1 It should be noted here that ever more new vehicles are equipped with a tire pressure monitoring system, which could improve the gap between actual and recommended pressure.
may be compromised and the formula might have to be reviewed as car and tire technology progress. Also the dynamic design of the vehicle is not taken into consideration. Use the recommended pressure from the car manufacturer (listed in the owner s manual). This approach is far more easy as the first, but clearly compromises the independency criterion. Use of the recommended pressure from the tire manufacturer. As indicated earlier, this may improve the independency of the tire pressure figure for testing, but it does not consider the relation with vehicle dynamics and can therefore result in an unrepresentative value. Verification The time difference between homologation procedure and production phase requires that some sort of verification is necessary to ensure that the tire type, dimensions and pressures are the same, or at least that the characteristics are similar. The aspects described above illustrate that the need for verification increases even further, since this will bring improvement on the aspects of representativity, independence, and definition. It should be flagged at this point that as an alternative to the current road load determination procedure Transport & Environment together with the Swedish Transport Administration introduced the so called feedback approach. The basic idea for that alternative is to verify if the road load on in use vehicles is (within a certain tolerance) identical to the road load determined during homologation. This approach would solve a lot of issues concerning the road load, and in this case would simplify the tire pressure issue to a high degree. The verification of road load on in use vehicles requires manufacturers to apply tires and pressures during coast down tests that are (almost) identical to those on production vehicles. If this feedback approach is not accepted for the WLTP, the need for a better definition of tire pressure remains strong. However, the discussion above on important aspects for such a definition indicate that this is not straightforward and some sort of compromise will be needed. The following views and considerations have been used as input for the proposal. As there is no good alternative available for definition of an objective tire pressure, the only workable solution will be to use the manufacturer s recommendation. Normally, a minimum and maximum pressure is specified, ranging from a vehicle in running order (one or two drivers with no or only little luggage) to a fully loaded vehicle. Other tire pressure specifications, such as eco pressure will be ignored, as this opens the way for unrepresentative and/or unrealistically high tire pressures. Considering that: the average number of passengers and load will be higher than the situation for which the lower tire pressure is recommended, but will be much closer to the minimum required tire pressure than the maximum, and the actual tire pressure of vehicles will normally be lower than is recommended,
it seems justified to use the lowest recommended tire pressure as the closest to representative value for the coast down test. A verification mechanism will be necessary, at least to give the type approval authority the possibility to demand retesting should the tires and pressures on production vehicles be different from those during coast down tests. As there is a strong dependency between tire pressure and vehicle weight, a common approach for these parameters is recommended. Proposal for GTR The following text is proposed to be added to Annex 9, Paragraph 4.1 Vehicle Condition: The front and rear tires shall be inflated to the lower limit of the tire pressure range specified by the vehicle manufacturer. When the vehicle is taken into production, the type approval authority may verify if the applied tire types, dimensions and pressures as well as the weight of the production vehicle are in accordance with those of the vehicle submitted for the road load test. If any differences are found, it is upon decision of the type approval authority to require the road load to be reestablished. Open issues Do manufacturers use the same assumptions for defining the lower recommended tire pressure? Should the vehicle weight requirement be connected to the tire pressure (as proposed) or be a separate requirement?
Proposal for GTR Since vehicle owners tend to drive mostly alone in their vehicle, and the lower recommended vehicle pressure is for a car in running order including the weight of the driver, and the extra load may be compensated by a lower tire pressure in real life.. The following text is proposed to add to Paragraph 3.1 Definitions: vehicle coast down mode means a special mode of operation for which transmission components are mechanically and/or electrically decoupled from the wheels for the purpose of road load determination. Vehicle coast down mode is engaged mechanically or electronically without removing any parts from the transmission system. No propelling energy may be transmitted to the wheels in vehicle coast down mode. The following text is proposed to be added to Annex 9, Paragraph 4.1 Vehicle Condition: If components in the transmission system generate non negligible parasitic losses, the vehicle may be operated in vehicle coast down mode. Open issues Denk aan de tire choice, die ook op de een of andere manier vastgelegd moet worden. Het verband tussen gewicht en rolweerstand: het kan niet worden toegestaan dat een coast down test voertuig een lager gewicht heeft dan de tire pressure die daarbij hoort. The following issues could be taken up for discussion regarding the proposed GTR text: For some types of transmissions the vehicle coast down mode can possibly only be realized by physically detaching components. In the current proposal this is not allowed, because the vehicle would not be able to drive itself for the coast down tests. Also it would introduce more complexity for independent verification purposes. Should it be allowed to remove components?
Should a more strict definition be given for which types of transmissions vehicle coast down mode may be used (this brings the risk of not being technology neutral)? Is it necessary for verification purposes that the manufacturer reports how vehicle coast down mode is engaged? (possibly a certification issue) Is the proposal sufficient for windtunnel testing?