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There are a number of emission models available and all are slightly different. According to Montazeri-Gh & Naghizadeh (2003), there are a number of different driving cycles developed to represent different types of vehicles and driving behaviour. Faiz et al. (1996) suggests that the most commonly used are the US Federal, the United Nations Economic Commission for Europe (ECE) and the Japanese test procedure.

2.1.6.1 USA driving cycle

In the USA, a driving cycle has been developed called the FTP-75, which is a transient test cycle used for emission certification testing of cars and light duty vehicles (Montazeri-Gh & Naghizadeh 2003). In this test procedure a light vehicle, typically a car or light goods vehicle is driven on a chassis dynamometer on a predetermined driving cycle. Samples of the exhaust emissions are collected throughout the procedure in a constant volume sampling system. According to Faiz et al. (1996) the driving cycle which lasts for 2,475 seconds is meant to reflect the varying nature of urban vehicle operation, with a average driving speed of 31.4 km/hr. The test is carried out in three phases, cold start, hot stabilised and hot start. The procedure starts with a 12-hour vehicle soak followed by a cold start where temperatures range from 200 to 300C. The emission results are calculated as weighted average of emissions measured for each of the three phases.

Experts maintain that one of the main disadvantages of this test procedure is the narrow speed range and the fact that the procedure does not represent realistic driving conditions. Faiz et al.

(1996) points out that the test procedure does not cover the full range of speed and acceleration conditions experienced by vehicles operating in the real world. Experts also maintain that FTP-75 provides a poor simulation of air conditioner operations. The model has been improved by adding supplementary models such as the Supplemental Federal Test Procedure (SFTP), which is used to represent the engine load and emissions associated with vehicles possessing air conditioning units. There is another supplementary test procedure called the US06 Supplemental Federal Test Procedure which was designed to address and improve the FTP-75 test cycle in the representation of “aggressive, high speed and/or high acceleration driving behaviour, rapid speed fluctuations, and driving behaviour following start up” (Faiz et al. 1996).

Emission testing for light duty vehicles is carried out using the "IM-240" test based on a chassis dynamometer schedule and used in a number of States for inspection & maintenance programmes. Some of the national driving cycles have been developed further to represent driving styles for individual states. For example "LA92" is a dynamometer driving schedule for light duty vehicles developed by the California Air Resources Board, represents a more aggressive driving cycle with higher speeds and rates of acceleration, with less stopping and less idles time than allowed for in the federal FTP-75.

2.1.6.2 European driving cycles

Driving cycles used for certification of emissions from light duty vehicles in Europe include the Economic Commission of Europe (ECE) and the New European Driving Cycle (NEDC). These driving cycles are based on a chassis dynamometer (Montazeri-Gh & Naghizadeh 2003). The European procedure consists of three tests, lasting 780 seconds and covering 4.052 km with an average speed of 18.7km/hr. The first test measures the exhaust emissions produced in a driving cycle on a chassis dynamometer, where the driving cycle is based on a typical urban area in Europe with 15 linked driving modes. The procedure has a maximum allowable speed of 50km/hr. The test starts with the vehicle soaking for a minimum of six hours at a temperature in the range of 200 to 300C. The engine is then started and left to idle for 40 seconds, followed by four test cycles without interruption. The second test takes samples of carbon monoxide concentrations from the exhaust emissions immediately after the last cycle of the first test. Crankcase emissions are measured in the third test.

The European test procedure has been criticised for being even less realistic than FTP-75 because the results are based on low rates of acceleration. The test uses an acceleration rate of 3.75km/hr/sec sustained for 4 seconds during the first peak, 2.61 km/hr/sec in the second peak and 1.92km/hr/sec in the last peak, which equates to less than one-fifth the rate observed in actual driving conditions. The European test procedure includes an extra-urban driving cycle (EUDC) which lasts for 400 seconds with an average speed of 62.6km/hr and a maximum speed of 120km/hr (Faiz et al. 1996). Barlow et al. (2009) points out that the New European Driving Cycle which is used to approve light vehicles for use in Europe is based on unrealistic steady state conditions. The test uses a cycle where there are periods of constant acceleration, deceleration and speed, which Barlow et al. (2009) argue does not reflect real driving operations, given that real world driving patterns are transient. Japan uses the MVEG- A drive cycle for emission certification and fuel economy of light duty vehicles (Montazeri- Gh & Naghizadeh 2003).

Faiz et al. (1996) maintains that the American and European test procedures are commonly used in other countries and that these test procedures have many commonalities. For example the volumes of emissions are measured by operating a test vehicle on a chassis dynamometer while collecting the exhaust emissions in a constant volume sampling system. The principal difference in the testing procedures is the driving cycle for the light duty vehicles. The US test involves transient variations in speed and load, which are considered very similar to actual driving, whereas the European and Japanese procedures are based on a “series of steady state operating conditions” (Faiz et al. 1996). The European test is simpler than the US test procedure and the maximum acceleration rate at 3.75 km/hr/sec being significantly less than the US test. However it is suggested by Faiz et al. (1996) and Barlow et al. (2009) that none of these test procedures accurately reflects real driving patterns and this has led to other driving cycles being developed. Barlow et al. (2009) suggests that these driving cycles’ model emissions based on varying the average speed of a vehicle trip, expressing emissions as grams of pollutant per vehicle kilometre. Barlow et al. (2009) suggests that there are limitations to this method of analysis based on average speed models, since it is difficult to assess ranges of vehicle operation and emissions behaviour in average speed scenarios. A typical example of this is vehicle emissions from a vehicle with a catalytic converter, in this type of vehicle the greatest output of pollutant is in the short sharp peaks which occur during gear changes and acceleration.