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Trend of fuel efficiency of cars
2.1. Trend of fuel efficiency of car fleets
Rice, 1982, Rice, 1984 and Rice, 1985 and Sorrell (1992) estimated the fuel efficiency trend of cars in Great Britain for the period 1973-1990. In the following, these two previous estimations of fleet fuel efficiency are summarized and compared with the result of Kwon (2004), on which this paper is based.
Rice (1984) estimated the fleet fuel efficiency of cars and light vans for the period of 1973-1982. According to his estimation, car and light van fuel efficiency improved from 10.33 l/100 km in 1973 to 9.57 l/100 km in 1982–up by 7.4% (0.85% per annum). In particular, the improvement was discernable after the 1979 fuel crisis. The annual change in fuel efficiency improvement for the three years after the 1979 fuel crisis amounted to 2.5% in Rices estimation. Rice (1985) revised the estimation because of the 1984 revisions by the Department for Transport (DfT) on the historic data-base of GreatBritain vehicle-km. The major effect of the revision was to reduce the absolute amount of car travel by about 10%, which led to a proportional deterioration of the calculated fuel efficiency.
Sorrell (1992) analysed the trend of fleet average fuel efficiency for the period 1983-1990. Due to the inaccuracy of data, he provided lower and upper bounds of the results according to different assumptions of mileage distribution between petrol and diesel cars. He found a fuel economy improvement of between 6.93% and 9.37%, corresponding to average annual improvements of 0.96-1.29%. This rate is slightly higher than the rate for the period between 1973 and 1982 estimated by Rice (1985). Further, according to the estimation of Sorrell (1992), almost static efficiency was recorded for the period 1984-1986, while there was a decline in efficiency in the period 1989-1990. He attributed the decline of fleet fuel efficiency into the penetration of cars with three-way catalytic converters.
Kwon (2004) also estimated the trend of fleet fuel efficiency in Great Britain over the period of 1970-2000 by dividing total fuel (petrol and diesel) consumption by total car mileage. Transport Statistics Great Britain (DfT, each year) provides total fuel consumption data as well as the share of fuel use for car and taxi. DfT data is based on the total consumption in the UK and is measured in tonnes. It is converted to the Great Britain basis by subtracting the consumption in Northern Ireland, and converted to litres at the annual declared density in the Digest of UK Energy Statistics. However, the share of fuel use for car and taxi suggested by the DfT does not seem to be precise to a detailed extent.
Another factor difficult to estimate in calculating fleet fuel efficiency is to distribute total car mileage between petrol and diesel cars. This study distributed total car km into petrol and diesel cars on the basis of total fuel consumption data and a supposed fuel efficiency of diesel cars compared to petrol cars. Diesel fuel is generally more fuel efficient than petrol by about 20-50%, according to the calculation of average fuel consumption rate of each engine class for new car models (New car fuel consumption and emissions figure July 2001, Vehicle Certification Agency). However, as the average engine capacity of diesel cars is generally larger than that of petrol cars, the difference of fleet fuel efficiency between diesel and petrol cars would be less significant than the difference in the same engine size class. Fig. 1 shows the average engine size of newly registered cars in Great Britain each year. As shown in the figure, the engine size of diesel cars among newly registered cars was much larger than that of petrol cars in the early 1980s and since then the gap of about 20% remains continuously. Therefore, Kwon (2004) assumed that the ratio of fleet fuel efficiency of diesel cars to that of petrol cars has been growing gradually from 1 in 1970s up to 1.3 (30% more fuel efficiencnt) in the 1990s.
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Fig. 1. Average engine size of newly registered cars in Great Britain.
Fig. 2 shows the trend of fleet fuel consumption rates of petrol cars in Great Britain estimated by Kwon (2004) along with the estimation of Rice, 1984 and Rice, 1985 and Sorrell (1992). The estimation of Kwon (2004) is similar to Rice (1984) and higher than Rice (1985) or Sorrell (1992). The difference in fuel efficiency estimation is due to different scopes of the estimation: cars only (Kwon, 2004), and cars and light van (Sorrell, 1992; Rice, 1984 and Rice, 1985). However, the difference of estimation method regarding mileage distribution between diesel and petrol cars as will be explained below may have a little effect on the result.1 Nevertheless, all studies show similar trends overall.
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Fig. 2. Trends of fleet fuel consumption rate of petrol cars.
The basic method of calculating fleet fuel efficiency in all these three studies is to divide total fuel consumption of car by travelled distance. However, there is some difference between calculation methods of each study. For example, Rice (1984) did not consider a separate figure for diesel cars because there was only very small consumption of diesel fuel for cars in that period. However, as Schipper et al. (1993) emphasized, the amount of petrol use can not represent fuel use for cars, especially after 1980s because the share of diesel cars increased rapidly since 1980s in Great Britain. Sorrell (1992) distributed car km between petrol and diesel cars on the basis of two different assumptions; first, that diesel cars have the same annual mileage as petrol