Stepping on the gas

Vehicle emissions legislation for passenger cars has been in place in Europe since the early 1970s. Initially, the legislation focussed on pollutants such as carbon monoxide from petrol vehicles that affect local air quality.

Since then the legislation has developed to encompass other air quality pollutants such as oxides of nitrogen and fine particulate matter, and has become more sophisticated in its approach to measuring and setting limits for vehicle emissions.

In the same period there has been increased recognition that vehicle emissions also contribute significantly to wider environmental problems such as acid deposition and regional-scale air pollution episodes (such as the ‘summer smogs’).

Today, the legislation has reached an important point in beginning to address emissions of importance to climate change and in particular emissions of carbon dioxide (CO2 ).

The recent publication of the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC) has underlined the importance of climate change and the benefits of early action to tackle it.

The road transport sector accounts for 23% of CO2 emissions in the UK and it is the only major sector where emissions of CO2 are projected to grow. In Europe as a whole, road transport CO2 emissions have increased from 21% of total emissions in 1990 to 28% in 2004.

So what are the factors that affect the quantity of CO2 emissions emitted by vehicles? One of the most important factors affecting vehicle CO2 emissions is vehicle weight. Broadly speaking, an increase in vehicle weight of 10% leads to an increase in fuel consumption and hence CO2 emissions of 5-7%. For example, 1.3 litre petrol Ford Ka which weighs 887 kg emits 147 g/km CO2 compared with a 2.0 litre Ford Mondeo, which weighs 1,378 kg and emits 187 g/km CO2 .

An important trend in recent years has been the increase in the weight of passenger cars. It is estimated that in Europe, average vehicle weights have increased by 30% in the last 30 years, a trend also mirrored in the US. This increase is due to many factors such as increased safety requirements for vehicles, higher-spec vehicles and even the fitting of emissions control equipment to control emissions of local air quality pollutants.

Even though average vehicle weights have increased in recent years, vehicle manufacturers have been able to produce more efficient vehicles through improved engine design and aerodynamics. Nevertheless, reductions in vehicle weight or a move towards the use of smaller, lighter vehicles remains a key factor affecting vehicle CO2 emissions.

Another important factor in recent years in the UK has been the switch from petrol to diesel. Diesel vehicles are inherently more efficient than petrol vehicles and typically have fuel economy benefits of about 25% and emit 15-20% less CO2 than comparative petrol vehicles.

The reason why the fuel efficiency advantage of diesel does not translate into an equivalent benefit for CO2 is because a litre of diesel contains about 10% more carbon than a litre of petrol. In 1990 only 6% of newly registered vehicles were diesel; in 2006 it reached 38% .

Compared with Europe, where diesel car penetration in 2006 was 51%, the UK has room for more growth in diesel car numbers. This change in the UK was credited as being due to two important changes to the taxation of vehicles. In March 2001 all vehicles were taxed according to their certified CO2 emissions and in April 2002 the company car benefit-in-kind tax was also changed to a system based on CO2.

Newer vehicle technologies also offer reduced CO2 emissions. One of the most promising technologies to emerge in recent years is the hybrid vehicle. These vehicles are configured in various ways but combine two power systems: a conventional internal combustion engine and an electric motor. Unlike conventional combustion engines, they consume no fuel while stationary and offer the greatest benefit to reducing CO2 (and other pollutants) under slower-speed urban-type driving conditions.

These vehicles have the potential to offer significant reductions in CO2 under these conditions. The petrol-engined Toyota Prius for example emits 40% less CO2 compared with comparable petrol cars and 17% less CO2 compared with diesel cars over the legislated emission tests cycle.

These vehicles are likely to offer a bridge between conventional fuels and longer-term aims of using technologies such as fuel cells.

In 1998 the European motor industry trade association, ACEA, on behalf of the European car manufacturers, negotiated a voluntary agreement with the Commission. The industry voluntarily agreed to reduce CO2 emissions from new cars to an average of 140 g/km. The agreement set two targets for new car fleet average CO2 emission: the first 165-170 g/km limit was achieved in 2003 and the second was to achieve 140 g/km in 2008.

Recent data from the Society of Motor Manufacturers and Traders (SMMT) for the UK indicates that in 2006 the average figure was 167.2 g/km, someway short of the 140 g/km target. However, progress has been made across the fleet as the average new car sold in 2006 emitted 11.9% less CO2 than the average in 1997, a reduction mostly the result of the switch from petrol to diesel.

An important issue here is the willingness of the public to purchase lower-emitting vehicles: they are manufactured but not yet bought in sufficient numbers. The lack of progress towards achieving reductions in average CO2 emissions from vehicles led the Commission to propose mandatory limits in January this year.

Under these proposals, vehicle manufacturers would be required to ensure that the average CO2 emitted across their range of vehicles is below 130 g/km by 2012; a less stringent limit than that originally proposed of 120 g/km. The details of this proposal have yet to be worked out, but if adopted could have a major impact on the motor industry.

The purchasing habits of individuals and organisations, or more widely the issue of behavioural change, are an important factor affecting the average CO2 emission from vehicles. Recently, there has been an increase in the purchase of small cars but also an increase in larger vehicles such as 4x4 and Sports Utility Vehicles.

The SMMT calculate that if the lowest emitting vehicles were bought in each vehicle class (e.g. mini, super-mini, sports), then a 30% reduction in vehicle CO2 emissions would be realised. These lower emitting vehicles tend to be dominated by petrol/electric hybrids and diesel vehicles.

At the city scale, research has shown that the introduction of the London congestion charging zone (CCZ) led to reductions in both pollutants of concern to public health and emissions of carbon dioxide.

In the zone itself, which was expanded again in mid-February, it was calculated that total vehicle emissions of CO2 reduced by almost 20%, which represents a considerable reduction. This reduction was due to two principal effects of almost equal importance.

First, the straightforward reduction in traffic flows and second, the reduction in congestion itself. The benefit elsewhere in the UK will depend on the extent to which traffic flows are reduced and the reduction in congestion; factors that will in part be determined by the charge levied at drivers. Late last year, Ken Livingstone also announced plans for basing the CCZ charge on emissions of CO2.

An aspect of growing importance will be full recognition of non-tailpipe emissions of greenhouse gases. In particular, emissions of CO2 and other greenhouse gases associated with fuel production, processing and transportation (sometimes referred to as fuel cycle or life-cycle emissions) can be important.

These emissions tend to be of greater importance for some of the emerging fuel options including biofuels and hydrogen. In this respect, the carbon advantage of biofuels warrants close scrutiny. These fuels require significant energy inputs to grow, harvest and process them and these processes are not necessarily carbon neutral.

Also important are other greenhouse gases that can be associated with some of these fuels; in some cases the production of biofuels result in the emission of nitrous oxide — a greenhouse gas with a warming effect over 300 times that of CO2. To ensure that fuels of the future actually result in reduced emissions of greenhouse gases therefore requires these non-exhaust emissions to be robustly quantified and accounted for.

Dr David Carslaw is a principal research fellow at the Institute for Transport Studies at the University of Leeds. He has been a member of the government’s Air Quality Expert Group (AQEG) since 2002.