3.4. Vehicle test cycles
The new emission standards of the EU and the U.S.A. demand more sophisticated optimisation of all the compounds that contribute to the performance of the emission control system. In order to control the vehicle’s emissions, several test procedures have been developed and standardized. The most common test procedures are the European Driving Cycle (EC2000) and FTP (Federal Test Procedure) established by the EU and the U.S. Environmental Protection Agency (EPA) respectively. The test procedure is always a compromise between real driving conditions and the repeated measurements in the laboratory scale, and it has to, in a given accuracy, represent real driving style and conditions. (Heck & Farrauto 1996, Sideris 1998)
Test cycles are performed on a dynamometer. The emissions are sampled during the cycle by a constant volume sampler (CVS) and they are expressed in g/km. The test methods and conditions are standardized. The European test driving cycle is based on Euro III and it is presented in Fig. 10. The driving cycle consists of two parts, ECE15 and EUDC, that correspond to urban and highway (extra-urban) driving conditions in that order. ECE15 test cycle simulates a 4.052 km urban trip at an average speed of 18.7 km/h and at a maximum speed of 50 km/h. Its duration is 780 seconds. The same part of the ECE15 driving cycle is repeated four times to obtain an adequate driving distance and temperature. The EUDC cycle instead illustrates the aggressive, high speed driving at a maximum speed of 120 km/h. Its duration is 400 seconds and 6.955 km at an average speed of 62.6 km/h. Since the year 2000, the 40 seconds idle period at the beginning of the European driving cycle (EC2000) was omitted. Thus, the emissions during the cold start are included in the driving cycle and this makes EC2000 a more realistic test procedure. (Heck & Farrauto 1997, Koltsakis & Stamatelos 1997)
FTP test cycle, as shown in Fig. 11, corresponds to the European test driving cycle. It illustrates real driving time and conditions by measuring the speed of a vehicle as a function of time. FTP test cycle simulates the 17.7 kilometres (11.4 miles) of driving cycle through Los Angeles at an average speed of 34.1 km/h (corresponding to 21.2 miles per hour). The test measures the emissions of CO, HC and NOx, and involves a cold start after the engine’s idle period for eight hours, a hot start, and a combination of urban and highway conditions (Heck & Farrauto 1996, Sideris 1998). A change that is being assessed is to expand the FTP to include the conditions that involve aggressive driving behaviour at high speed and acceleration, and rapid fluctuation in speed and start-up after the engine is turned off for intermediate periods. This will result in higher concentrations of pollutants and greater demand on the catalyst. (Heck & Farrauto 1996, Sideris 1998)
The comparison of these two test cycles, EC2000 and FTP, is rather difficult because both the speed and time profiles of the cycles are quite different. At the beginning of the ECE15 cycle, the driving speed is slow and the load of an engine is lower compared to the FTP cycle. Thus, in the European test cycle, both the engine and catalytic converter warm up slowly. This is also observed by looking at the emissions of CO and HC and catalyst light-off values that are much smaller in the case of the FTP cycle. In this case, the European test cycle is more reliable, because the emissions formed during the cold start are taken into account. On the other hand, the FTP test cycle is a transient test cycle with a highly dynamic nature, whereas the European test cycle is a more static one. Therefore, the FTP test cycle corresponds better to emissions during real driving, where the temperature of exhaust gases increases gradually. (Laurikko 1994, Lox & Engler 1997, Sideris 1998, Laurikko 2001)