Powered Two-Wheelers and Air Quality in London

Published on 2 April 2002 by Gill

This paper was written as a result of a London Motorcycle Working Group meeting where the Greater London Authority official responsible for air quality had some misconceptions about the implications of European Community emissions limits for powered two-wheelers and the capability of retrofitting three-way catalytic converters to motorcycles already in use. While aimed specifically at London, the contents of this paper are relevant to any urban conurbation where air quality is an issue.


Powered two-wheelers (PTW) comprise power assisted cycles, mopeds (up to 50 cc/45km/h), light motorcycles (up to 11kW/125cc), motorcycles, scooters and roofed urban access vehicles. PTWs are increasingly considered as alternative transport to cars. They offer the door to door convenience of personal powered transportation while tackling congestion, reducing demand for land, particularly parking and address social exclusion. The vulnerability of users and questions concerning their environmental credentials are often held against them.

PTWs and Emissions

Some low-powered PTWs are electrical and emission-free at the point of use but are responsible for emissions from the coal and gas fired power stations which generate the electricity that they use. They are inherently inefficiency from energy losses in transmitting electricity and its storage. This may be overcome by the use of fuel cells but it is questionable as to whether this technology will readily lend itself to PTWs.

All other PTWs use internal combustion engines. There are effectively none in the parc using diesel power although some prototypes and specials do exist. Like all petrol engines PTWs' combustion products include carbon dioxide (CO2) which is not a pollutant but a greenhouse gas and a range of noxious pollutants.

Most of the smaller PTWs and mopeds in particular use the two-stroke cycle and hence in addition to the usual pollutants, they produce particulates from partially burnt lubricating oil. These tend to be low as a result of improved quality low ash oils and decreasing oil/fuel ratios. Pollutant emissions similar to those of four-stroke engines (see below) have been reduced through direct injection and other mechanical innovations.

The rest of the PTW parc uses engines which employ the four-stroke which are increasingly replacing the smaller two-strokes. They are not dissimilar to those used by cars so the pollutant emissions are typically, unburnt hydrocarbons (HC) carbon monoxide (CO) and oxides of nitrogen (NOx). There is currently a question of the volume of NOx which is generated from PTWs which has arisen while an appropriate test cycle was being devised.

The volume of CO2 and pollutant emissions depends not only on the characteristics and capacity of individual engines but the manner in which they are used. Most motorised traffic is subject to intermittent progress in much of London with long periods when their engines are idling while stationary coupled with acceleration and deceleration which is an inefficient cycle of use. PTWs in making better progress through congested traffic are subject to more efficient cycles conducive to lower emissions. This benefit has not been quantified but a study is in progress to do so. Such factors should be considered when emissions limits are being considered.

Legislative Measures to Reduce PTW Emissions

All PTWs regardless of whether they use a two- or four-stroke cycle are subject to emissions limits imposed under European Community legislation.

Under EC Type-Approval for Motorcycles and Three-Wheelers, pollutant emissions limits (Euro 1) were set for the first time and apply to new models introduced since mid 1999. All new vehicles are or shortly will be subject to the same limits. A study in Italy has suggested that this has achieved a 70% reduction of pollutant emissions from unregulated vehicles. In the European Union, it is customary to ratchet down the limits initially imposed and those applying to exhaust emissions are no exception. An emissions directive is currently under consideration using the Conciliation Process which has just been agreed by the European Parliament. Euro 2 limits are to be introduced in 2003 with Euro 3 following in 2006/7. They will be assessed by either a modified car test cycle or a world motorcycle test cycle. There had been problems in developing the latter resulting in the modified car test cycle being considered. The Italian study previously referred to suggests that Euro 2 will achieve a 95% reduction of pollutants over those produced by unregulated vehicles. Euro 3 is intended to achieve emissions levels close to currently required for new cars.

Practical Measures to Reduce Pollutant Emissions

Two-stroke emissions have been reduced by innovative engineering measures such as direct injection. Future designs may involve the separation of the lubrication system from the fuel system eliminating particulate emissions.

Most four-stroke machines have achieved Euro 1 limits by more efficient fuel metering by through fuel injection and/or air injection to achieve more complete combustion of the exhaust products. The latter can be used with machines employing carburettors.

Catalytic converters can further reduce emissions. A catalytic converter is a relatively simple device. It is made of precious metals and has a large surface area such as a honeycomb construction. It must be located in the exhaust system as near to the engine as possible to allow it to achieve a high working temperature. While not taking part in the reaction, it causes exhaust products to react to completion. An undesirable side-effect is that the cat reaches high temperatures requiring shielding which may create difficulties for PTWs where space is limited. It can cause minor restrictions in the exhaust system with a small but significant effect on efficiency unless this is taken into account in the design of the engine. A cat require time to reach its operating temperatures and is not, therefore, ideal for a vehicle undertaking a short journey or subject to long periods with the engine idling at low ambient temperatures where the cat could fall below its operating temperature. Through its extensive use of precious metals a cat is expensive and requires unleaded petrol since lead compounds will contaminate it.

A simple or oxidising catalytic is as described above and by completing the reactions of the fuel/air mix convert HC and CO into CO2 and water vapour. Similar reductions can be achieved by lean burn technology in which a higher proportion of air is used in the combustion process. It does not reduce levels of NOx which is caused by high temperatures in the engine causing the oxygen and nitrogen from the air to react with one another.
To reduce levels of NOx as well as HC and CO, a closed loop or three-way catalytic converter is necessary. While the catalytic converter is no different from a simple cat, it requires an engine management system using electronically controlled fuel injection and a lambda sensor in the exhaust system immediately upstream of the cat. As the engine operates, the sensor controls the fuel injection system by setting the air/fuel ratio for a stoichiometric mix or one in which its active components will react to completion. This is achieved by increasing proportion of fuel in the air/fuel mix. The result is that the oxidising reaction promoted by the simple cat is supplemented by a reducing reaction of the NOx freeing the oxygen to react with CO and HC so that the final combustion products are CO2, water vapour and nitrogen. Closed loop catalytic converters increase fuel consumption and hence CO2 emissions so that in addressing problems associated with pollutants, they are replaced by greater fuel consumption greenhouse gas emissions. Cat-equipped motorcycles tend to suffer from significantly greater fuel consumption than similar models not so equipped and require a complex engine management system to function. These factors could militate against the use of utilitarian PTWs nullifying the usual advantages of a low purchase price and frugal fuel consumption. A closed loop catalytic converter seems to be an unduly complex and inherently inefficient means of addressing pollutant emission. The BMF's view is that a greater latitude in the setting of emissions limits would give manufactures an opportunity to develop systems which would allow engines to operate more efficiently.

Euro 2 limits should be readily achievable by more efficient fuel management systems, air injection and, in some instances, simple catalytic converters.

Some manufacturers consider that Euro 3 limits can be met by air injection and simple catalytic converters although, in some cases, closed loop catalytic converters may be necessary.

Retrofitting of Catalytic Converters

The most cost-effective means of reducing pollution from PTWs is to ensure that they are well maintained and operate at peak efficiency. Some could be retuned for efficiency and better mid-range response rather than maximum performance.

With appropriate exhaust systems subject to suitable locations and heat shielding, it should be possible to retrofit simple catalytic converters to most PTWs. They would not be expected to cause a significant reduction in fuel efficiency or increase CO2 emissions while having the potential to reduce pollutant emissions. Financial incentives such as grants towards cost of installation could apply.

Closed loop catalytic converters have few advantages over simple cats and many drawbacks. They cannot be used for PTWs using carburettors and those with fuel injection systems would require significant modification to be compatible. They are, therefore, not considered to be suitable for retrofitting.

Trevor A Magner Bsc

Last reviewed/updated 24/09/04