issues not addressed by wltp

though significant improvements are achieved in wltp, there are also issues that for practical reasons are not dealt with. A good example is the ambient temperature during vehicle testing. this is still under discussion but likely to be harmonized at a set point of 23°c. this is much higher than the representative european average temperature. since beyond the eu, for example, in countries like india, average temperatures are higher, and since it is easier and more cost-effective to heat a laboratory than to cool it, the temperature for the world-harmonized test procedure is purposely kept at this high level. within the eu, a discussion is ongoing as to whether it is possible to correct the fuel consumption rate to a more representative temperature, for example, by using the results of the -7°c temperature test. however, such a correction would be outside the scope of wltp and would be part of regional certification requirements.

there are also issues that cannot be completely solved. the test procedure has to allow cer- tain tolerances, otherwise, it would be impossible to comply with the procedure’s prescrip- tions. for test laboratories that have equipment capable of meeting tolerances more tightly than required, this may offer the manufacturer a possibility to exploit this to its own benefit. for instance, the test procedure is offering a tolerance on tracing the speed profile set by the test cycle, as it is impossible to follow the respective target speeds precisely during the test. the current proposed tolerance window is +/-2 km/h and +/-1 second. for an experi- enced lab driver or a robot it is possible to use some of this tolerance window to reduce the workload of the vehicle, thereby lowering its fuel consumption. Because the wltp test cycle is much more dynamic than the current nedc, it will be more challenging to stay within this tolerance, so room for optimizing is expected to be less. on the other hand, some of the more dynamic peaks in the cycle may be ‘flattened’ to reduce fuel consumption.

some other examples of issues that are not or are only partially addressed by wltp are listed below:

»

As previously indicated, a type-approval test cycle needs to be appropriate for every

55  most of the earlier issues mentioned for conventional internal combustion engine vehicles may also apply to electrified vehicles.

vehicle. this means that, for example, even underpowered vehicles have to be able to perform the accelerations of the driving cycle. even though a classification scheme is introduced in wltp with dedicated test cycles for low-powered vehicles or those with low maximum speed, the accelerations will still have to be suitable for vehicles at the lower end of the power-to-mass ratio range. for cars with relatively high power- to-mass ratios, such gentle accelerations will be far from real-life driving behavior.

»

the road-load determination is determined on a test track, so the ambient conditions such as air temperature, pressure, and humidity may vary from day to day. in effect, the measured road load will be affected. this has been resolved already in current legislation by adjusting the measurements to standard conditions. there are, however, indications that high temperatures still alter the road-load determination, which means that the correction formulas are not fully adequate. within wltp there are no resources to check and improve upon these corrections.

»

it is acknowledged that the smoothness of the test track will influence the rolling resistance of a vehicle for which the road load is determined. so far it has proved difficult to find a suitable indicator for the roughness of actual road surfaces. even if such an indicator were found, it would be a difficult political decision to require a less smooth surface if this would render some of the test tracks inadequate.

»

An increasing number of on-board systems are using energy from the engine, such as the air conditioner, lights, electric window heating, seat heaters, etc. these will negatively impact the fuel consumption rate when switched on. however, not every vehicle has the same kind of options installed, and to what extent they are used will depend on the circumstances and customer habits. for some of these amenities (e.g., the air conditioner) the amount of energy used is linked to the external conditions such as outside temperature and solar radiation. in order to keep the type-approval process manageable, on-board energy consumption has not been incorporated in the test procedure. for air conditioners in passenger cars, the european commission is currently working on a separate test procedure, the mobile Air-conditioning test procedure (mActp).

»

manufacturers are increasingly offering options that change how the engine and transmission operate. this is especially a problem for automatic transmissions, where users can often select “economy” or “sport” modes. procedures are needed to ensure that manufacturers do not test their vehicles in economy mode only, at least without demonstrating that such modes will be routinely used in practice. this issue is being discussed in the context of wltp and is likely to be dealt with at least to some extent. possible solutions considered for wltp include a) defining a default/predominant mode that will determine in which mode the vehicle should be tested or b) averaging best- and worst-case modes, in case no agreement on a robust definition of a default/predominant mode can be found.

currently, discussions are taking place in wltp to correct systematic deviations from the target value in what is called a ‘normalization’ procedure. this could be done by post- processing of the test results toward normal conditions. it has not yet been decided if this approach will be applied and, if so, if it would be added to the wltp test procedure itself or implemented at the regional level. for a more detailed discussion on options for reducing test cycle flexibilities, see smokers, Kadijk, and dekker (2012).