Realització de l’obra final

The drive-cycles used during type-approval tests are intended to represent how the ‘average’

driver uses a vehicle. This means the speeds, accelerations and operating conditions of the vehicle during testing ought to be representative of ‘average’ driving, to give consumers an adequate indication of their expected fuel costs.

In the real-world, drivers all use their vehicles differently, due to behavioural differences between drivers (high speed aggressive drivers vs. calm low speed drivers) and geographical differences (which impact the types of vehicles driven and the share of urban to rural driving). There will always be differences between real-world driving by individuals and the standardised drive-cycles used for type-approval testing (Greene et al., 2017). However, type-approval drive cycles ought to reflect average real-world driving conditions as closely as possible. There is increasing evidence that this is no longer the case in many vehicle markets (IEA and ICCT, 2019).

Several studies have attempted to compare estimates of the ‘real-world’ fuel consumption of several vehicles (also known as ‘on-road’ fuel consumption), to their type-approval values.

In 1994, Schipper and Tax (1994) reviewed evidence across several countries and estimated that type-approval values underestimated real-world fuel consumption by 15-25%. At the time (before 1997), type-approval fuel consumption in Europe was measured using an urban drive-cycle followed by two constant-speed driving periods at 90 kph and 120kph respectively. To better reflect real-world driving, the constant-speed sections of the drive-cycle were replaced from the year 1997 with the introduction of the New European Drive

Cycle (NEDC) (Bonilla and Foxon, 2009). However, recent studies have shown that the NEDC cycle still largely underestimates driving in the real-world (Fontaras et al., 2017).

There are several different drive-cycles used globally for vehicle type-approval testing.

The NEDC (figure 2.3, left) has been used in the European Union since 1997. During this time it was also adopted in other large vehicle markets outside of Europe, notably India, China, Australia, and Brazil. The USA uses a different drive-cycle for its CAFE standards, which was also adopted by Canada and South Korea. Japan uses a cycle known as the JC08 cycle (IEA and ICCT, 2019).

Each drive-cycle has different ranges of operation, speeds and accelerations. They also differ in their abilities to reflect real-world reported fuel consumption (IEA and ICCT, 2019).

The CAFE cycles currently in use in the USA for example, have been found to be more representative of real-world driving than the NEDC and the JC08. Type-approval drive-cycles have been updated over time to better reflect real-world driving. In 2008, the American CAFE cycles were changed to a 5-cycle test including periods with air conditioning use, aggressive driving and cold temperature driving. In 2018, the NEDC was replaced in Europe by the Worldwide Harmonised Light-Duty Test Procedure (WLTP) and its associated drive-cycle, the WLTC (figure 2.3, right).

NEDC WLTP

0 500 1000 1500 0 500 1000 1500

0 25 50 75 100 125

Time (s)

Speed (km/h)

Fig. 2.3 Speed traces of the New European Drive Cycle (NEDC) and the Worldwide Har-monised Light-Duty Test Cycle (WLTC)

Data on vehicles’ real-world fuel consumption has often been challenging to acquire.

Early studies relied on survey estimates and reports by motor enthusiasts (Hughes, 1992;

Schipper and Tax, 1994; Watson, 1989). This has continued in more recent literature, though an increasing availability of crowd-sourced data has allowed for more detailed estimates of the gap between type-approval and real-world values (Kühlwein, 2016; Ligterink and Smokers, 2016; Ligterink et al., 2016; Martin et al., 2015; Mellios et al., 2011; Mock et al.,

2.1 Trends in energy efficiency and technical improvements 19

2013; Ntziachristos et al., 2014; Pavlovic et al., 2016; Pelkmans and Debal, 2006; Tietge et al., 2019, 2017a; Wali et al., 2018a).

Perhaps the most comprehensive study to date for the European vehicle market is that of Tietge et al. (2019) in which the authors source driver-reported estimates of vehicle fuel consumption from 1.3 million different vehicles, 15 different publicly available sources and 8 different countries in the EU. The authors found that the average real-world fuel consumption of vehicles available in 2001 was 8% higher than manufacturer reported, type-approval values and by 2017 this gap had risen to 39%. The analysis by Tietge et al. (2019) is based on a large number of individual driver-reported entries of average mpg and fuel use. Depending on the composition of the vehicles reported in the sample, this may differ from the sales-weighted average fuel intensity of new vehicles in a country. There is yet to be an attempt to weight driver-reported entries by vehicle sales figures.

Fontaras et al. (2017) summarise the multitude of factors that affect the differences between type-approval and real-world fuel consumption estimates. These include:

• the degree to which the speeds and accelerations of vehicles on the tested drive-cycle resemble average driving (i.e. how representative type-approval drive-cycles are of real-world driving).

• simplifications to testing procedures intended to reduce the testing burden and avoid testing every permutation of vehicle and optional extra. These ‘flexibilities’ include testing ‘reference’ vehicles in inertia classes, on a single set of tyres. There is evidence to suggest that this has led to manufacturers (completely legally) testing vehicles with lower weight and rolling resistance than the equivalent vehicles sold on the road (Kadijk et al., 2012). Stewart et al. (2015) suggest the majority of the growing divergence between type-approval and real-world fuel consumption is due to an increasing degree of exploitation of test ‘flexibilities’ by manufacturers.

• the real-world energy use of vehicle auxiliaries (air conditioning, infotainment sys-tems, electric seats), which are not included in type-approval testing. The increasing penetration of auxiliaries in vehicles is one reason explaining the growing divergence between type-approval and real-world fuel consumption (Stewart et al., 2015).

• technologies which have a greater impact on fuel consumption in testing than in real-world conditions such a stop-start ignition (Fontaras et al., 2017).

Discrepancies between type-approval and real-world fuel efficiency undermines the two aims of vehicle efficiency regulation: to track and reduce emissions, and to accurately inform consumers. Past authors (Hu and Chen, 2016; Matas et al., 2017) who used type-approval data

to quantify incremental technical efficiency improvements (ITEI) noted that their findings may be biased due to the increasing divergence with real-world data, but there is yet to be a concerted effort to quantify ITEI using real-world fuel consumption data.