Heavy vehicles are dirty. They are a major source of street-level pollution. Overall, most traffic-related pollution comes from cars, but heavy vehicles are particular culprits in some of the denser parts of towns and cities, where there are more of them and the intensity of their use is greater.
However, not all heavy vehicles are the same. With buses there is an important consideration which applies less to lorries. Buses carry large numbers of people efficiently, compared with cars. When looked at from the perspective of moving people around, rather than vehicles, buses are squeaky clean compared to single-occupancy cars. But it might be difficult to remember that whilst being assailed by the distinctive aroma of diesel fumes on Oxford Street or in Piccadilly Circus.
In 1995, London Transport Buses published an assessment of the contribution to traffic pollution from buses. This analysis shows buses to be much cleaner per passenger kilometre than cars for most pollutants, and taxis really stand out as energy inefficient, as seen from the carbon dioxide (CO2) score (Table 1).
However, whilst motorists see buses as dirty, as wrong as that perception might be, they have one more reason not to use them. Conversely, a bus that is perceived to be clean (as well as being clean) is more likely to attract passengers from cars and thus contribute to a general environmental improvement. Of course, the cleaner buses actually are, then the greater their environmental advantages over cars. Furthermore, low-emission buses will make bus stations and other centres of their activity (such as Oxford Street) more pleasant places.
There are some pollutants where buses are a relative problem, such as sulphur dioxide (SO2) and particulate matter (PM). To its credit, L.T.B. is trying to make improvements here, as exemplified by its recent decision to subsidise the use of low sulphur diesel (less than 0.005% sulphur by weight, resulting in lower emissions at point of use and also allowing the use of two-way catalytic converters). The European Commission is also pressing for cleaner engines, bringing in progressively tighter emission standards (Table 3). However, the 'Euro II' standards which have just come into force are easily met by today's new engines and the 'Euro III' standards have yet to be confirmed.
There is a further problem with the European standards, in that they are based upon an engine test cycle (the 'R49 13-mode cycle') which merely measures emissions at different, constant engine operating rates. Acceleration, deceleration, load and cold starts are ignored. Clearly, buses are likely to be dirtier in real life than the standards would suggest because of their repeated stop-start operation and highly variable load factors. The U.S.A. uses a 'transient' test cycle which does take into account changes in speed and load, and something similar may be introduced in association with the Euro III standards in 2000 (Cahm, 1995).
A major drawback with usual emission tests is that they can be fairly unspecific about some important pollutants. For instance, there are many different hydrocarbons which occur in exhaust emissions, some relatively innocuous, others highly dangerous, including carcinogenic and teratogenic substances. Fuels like diesel and (to a slightly lesser extent) petrol are mixtures of many, complex hydrocarbons. Some other fuels are much simpler in composition and thus emit fewer of the really nasty complex hydrocarbons. For instance, natural gas is predominantly methane and liquefied petroleum gas (L.P.G.) is predominantly propane, both quite simple chemical compounds.
Indeed, alternatives to diesel and petrol are being examined around the world, with varying degrees of success. There is an increasing number of trials in the U.K. with various fuels and an important assessment of the performance of alternative fuels by the Energy Technology Support Unit was published earlier this year (Gover et al., 1996). Whilst many alternative fuels give marked improvements at the point of use (i.e. in terms of emissions from the exhaust pipe), the situation can be very different when looked at from the perspective of the full life-cycle of the fuels, i.e. taking into account extraction, production and transport of the fuels as well as final use.
The E.T.S.U. report examines the life-cycle performance of diesel, petrol, L.P.G., compressed natural gas (C.N.G.), biomethanol, bioethanol, 'biodiesel' (Rape Methyl Ester, R.M.E.) and battery electricity. The analysis was carried out for cars, light goods vehicles (L.G.V.s), heavy goods vehicles (H.G.V.s) and buses. The bus data includes two categories, 'old' and 'new', the latter being post-1990 buses, but not meeting the Euro I standards, with 36 or more seats (except for battery electric, where only midibuses are considered). However, the comparisons are performed by scaling up or down the emissions with petrol or diesel for the alternative fuel in question, and the bus base data is not based on real measurements from buses, but is extrapolated from those for H.G.V.s. There is thus a degree of uncertainty about the data, but the analysis does still present what appears to be the best life-cycle analysis so far carried out.
A comparison has been extracted from the report for the 'new bus' category, and is shown in Tables 4 and 5. Table 4 shows the emissions from vehicle operation, which is of direct concern in the street, particularly in relation to CO, HC, NOx and PM. Here, not surprisingly, electric power performs best, with good showings from biodiesel, biomethanol and natural gas. However, biodiesel shows up poorly in respect of NOx and PM and the catalyst equipped gas bus surprisingly poorly in relation to CO, with the lean-burn version showing relatively poorly in relation to NOx; there is clearly a trade-off here. Bioethanol gives a poor performance, however it should be noted that the buses powered by the alcohols and by biodiesel are not fitted with catalytic converters, which would reduce emissions significantly. L.P.G. does quite well except for CO (the HC emissions are relatively simply compounds).
It will be noted that natural gas does poorly in relation to HC. However, this emission is predominantly methane, fairly innocuous at street level. It is a powerful 'greenhouse gas', however, but it represents a drop in the atmosphere set aside the difference in CO2 emissions compared with diesel.
Table 5 shows the life-cycle emissions from the various fuels. This represents the regional and global impact, where CO is less important (except near processing plants or transport corridors), oxidising to add a very small amount to CO2 levels. Here, battery power does less well, largely due to the U.K.'s relatively inefficient electricity generation (note also that the battery data are for midibuses). The SO2 levels are of particular concern, but depend very much on the power station fuel. Good performances are had from the biofuels, when the comments about catalysts above are considered. This is particularly true for energy and CO2 emissions, although not as good as is perhaps commonly believed; the renewability of these fuels depends crucially on the amount of energy going into production and transport. L.P.G. does relatively poorly as the extra hydrocarbons it contributes to the greenhouse effect are not compensated for by reductions in CO2 emissions. Diesel does surprisingly well for HC, but it has to be remembered that these emissions include more complex and toxic compounds than from most of the alternative fuels.
There are also some emissions in some of the alternatives which are not found in diesel, particularly acetaldehyde, which may be carcinogenic, from ethanol. These, however, appear to be treated adequately by a catalytic converter.
The E.T.S.U. report does not cover all alternative fuel options, although it does address those most commonly tested so far. Other options are included in the discussion below of the different fuel technologies.
Natural gas is supplied to most Londoners' homes for use in cooking and/or heating. It is predominantly methane and is derived from gas fields, such as in the North Sea, where it has formed in a process linked to that of petroleum oil. When compressed for use as a vehicle fuel (usually to 200 bar), it is known as Compressed Natural Gas. Only steel gas tanks are currently legal, adding considerable weight to the vehicle. For instance, the C.N.G. buses operating in Gothenburg (Sweden) have 18 steel tanks in pods on the roof, weighing 1.5 tonnes, which reduces capacity by between 20 and 22 passengers. The A.S.T.I. C.N.G. minibuses in Camden have two such tanks each, mounted under the floor. Other trials with C.N.G. buses in the U.K. are taking place in Bristol, Ipswich, Newbury and Southampton. One of the features of methane operation is very quiet running.
Filling usually takes place overnight, as fast filling equipment is very expensive. Methane is comparatively safe as it is lighter than air and would float upwards and dissipate in the unlikely event of a leakage. In the case of a fire, safety valves allow venting such that there is no real risk of explosion.
Natural gas is a fossil fuel, but with a lower carbon content than diesel. It is possible to use biomethane from sewage treatment plants and landfill sites after purification, however the supply is then inevitably local and of limited capacity. It is, however, ideal for a discrete fleet of, for instance, urban buses or refuse lorries.
Liquefied petroleum gas is derived from crude oil and has the safety disadvantage, compared to methane, of being heavier than air. It does, however, have the advantage over fuel oils of being of relatively simply composition, usually propane. Storage requires less space than C.N.G., but more than diesel. U.K. trials with L.P.G. are in Bath, Stratford-upon-Avon (tourist), London (tourist) and Warwickshire.
These are alcohol fuels derived from crops, principally short-rotation coppice wood for methanol and grain for ethanol, although potatoes, sugar beet, cellulose and even refuse can be used. The largest fleet of ethanol buses operates in Stockholm (180). Storage space is somewhat more than for diesel. For every joule of energy produced as methanol, 0.51 joules must be used in the production and transport process. For grain-based ethanol, the input is 0.98 joules per joule produced, which makes ethanol an energy store, like batteries, rather than a pure fuel. A combustion enhancer has to be added to ethanol to work in a 'diesel' engine, usually at 5%, the resultant fuel being coded 'E95'. The safety precautions for alcohols are similar to those for petrol.
Rape Methyl Ester is produced from rape seed and can be used in a diesel engine without modification, either on its own, or in a mixture with ordinary diesel, especially as R.M.E. tends to solidify at low temperatures. 0.88 joules of energy must be put into the production of every joule of energy in R.M.E. fuel, which also puts R.M.E. largely into the category of an energy store. U.K. bus trials with R.M.E. have taken place in Gravesend, Maidstone and Reading.
Hydrogen is another energy store, rather than a pure fuel, unless hydrogen can be obtained as a by-product of an industrial process (which will have its own environmental impacts). Hydrogen is produced by splitting water (H2O) molecules by electrolysis. This is an energy-inefficient process, requiring electricity. It is not clear what the energy loss is likely to be in operation, but it is likely to be more than with batteries.
Hydrogen can be used in vehicles in two ways. It can either be combusted as a fuel (as in the buses on trial in Munich, Germany, and at the E.U.'s joint research centre in Belgium) or it can be used in a 'fuel cell', wherein hydrogen and oxygen ions are mixed, generating electricity to power a motor in the same way as a battery. Safety is likely to be similar to that with petrol power.
Electricity has the major advantage of being fume-free at the point of use and generally giving extremely quiet operation. It can be used directly or stored in a battery (or in the form of an energy carrier fuel, such as hydrogen). Most electric public transport vehicles use external power supplies - trams, trains and trolleybuses. Trams have been reported on before in these pages. Trolleybuses (including the 'Guided Light Transit' dual-mode trolleybus, which can run as a diesel-electric hybrid) are common in a number of countries on the continent, particularly Switzerland and in the former Eastern Bloc, as well as in the U.S.A. Their disadvantage is that they require significant investment in infrastructure, needing a relatively high level of patronage to justify it. Battery vehicles can provide similar benefits for smaller scale operations and situations where route flexibility is needed. One of the advantages of electric vehicles is that no power is used when they are stationary or coasting, and the motor can be used to provide braking whilst acting as a generator to return power to the external supply or the battery (regenerative braking).
Until recently, the main application for battery vehicles in the U.K. was milk floats and the power source was generally considered inappropriate for vehicles with higher performance requirements. However, advances have been made in recent years, not the least of which is the 'Wavedriver' device which manages power flow to the motor and ensures the batteries are charged efficiently, making batteries more practicable and economic for wider use. Now, battery buses can make use of 'opportunity charging' - topping up the battery charge when convenient, rather than waiting until the batteries are exhausted and then doing a full charge; the 'Wavedriver' allows full battery potential at all times. The battery buses in the A.S.T.I. project and the Thamesway (Badgerline) Dennis Dart, due to run in service between Southend and Hadleigh and Basildon in 1997, use the 'Wavedriver', whereas the Optare midibuses used in Oxford's Electric Bus Project did not, and indeed, there were problems with insufficient discharge until the buses were redeployed such that each of the four rested every fourth day and the others travel further (Beeton et al., 1995).
A further advantage with batteries is that they can be charged with night-time cheap-rate electricity supplies. However, there are question-marks as regards their eventual disposal, especially of the advanced nickel/iron and nickel/cadmium batteries being developed, with greater energy density than the standard lead/acid variety. Nickel and cadmium are also expensive, cadmium likely to become more so as it becomes scarcer.
An ingenious variation on the above themes is to use a hybrid vehicle, with power coming from electricity and from a fluid fuel. Dual-mode trolleybuses ('duobuses'), running alternatively on externally-supplied electricity and on diesel are running in a number of cities, including Essen (Germany), Nancy (France), Bergen (Norway) and Copenhagen. There are also versions with batteries instead of the diesel engine. The Guided Light Transit vehicle (produced by manufacturer BN) runs on external electricity when in guided mode and on electricity from a diesel generator when running as a non-guided bus. Similarly, diesel-electric trams are in the pipeline.
Battery power has been limited to midibuses, but hybrid technologies can allow larger buses to make use of electricity even if the cost of trolley infrastructure is not justified. The largest trial with such buses is in Stockholm, where nine purpose-built buses are each equipped with an electric motor and a car-sized petrol engine. The latter runs at a constant rate, thus avoiding the surge of energy use and pollutant production on acceleration and, especially, starting, and powers a generator, which feeds the motor, modulated by batteries. In particularly sensitive environments, such as confined city centre streets, the engine can be turned off and the bus is powered solely by the batteries. The plan is to eventually use ethanol in the engines instead of petrol. Similar, if smaller scale, projects are in progress in this country in Portsmouth (diesel/electric), Exeter (petrol/electric) and in Kew Gardens, where a trial battery bus receives part of its power from a photovoltaic array on the roof. There is, however, a weight penalty from having two power systems on board.
The E.T.S.U. report (Gover et al., 1996) considers the life-cycle economics of various fuels. The analysis is dependent upon many variables, but the following conclusions are drawn, based upon a comparison of variously fuelled L.G.V.s:
An important factor is that current trials are small-scale. The costs of vehicles and fuelling facilities are uncompetitively high, unless large-scale orders can be made. Semi-prototype vehicles are obviously more expensive than those in series production and fuelling stations are not widespread, as they are for petrol and diesel. Even a bus company will have to invest in a new fuelling system in addition to its diesel supply. The opening up of the market is a major aim of the Europe-wide Z.E.U.S. project, co-ordinated in the U.K. by the London Borough of Sutton and part-funded from the E.U.'s T.H.E.R.M.I.E. programme for energy efficiency in transport. According to Z.E.U.S.'s U.K. Project Manager, Sutton's Helmut Lusser, its main objective is the stimulation of the market for low-emissions vehicles in general, without pushing any one technology in particular.
Fuel duty is a major handicap, as diesel is cheaper than the alternative fluid fuels. The imposition of tax on the alternatives thus hinders their introduction, even though it is diesel, the dirtier fuel, which ought to be made less attractive in order to encourage the use of the alternatives.
Aside from the cost of providing new fuelling stations, some of the alternative fuels are necessarily limited geographically in their production. The prime example of this is biomethane from landfill sites or sewage treatment works, as mentioned above. A locally-orientated fleet of buses, delivery vehicles or waste collection vehicles could very usefully be powered by biogas, however. The fossil fuels can be limited too; natural gas from the North Sea has already past its supply peak. How long the 'dash for gas' will be a viable proposition is uncertain. The future difficulty of supply of cadmium for nickel/cadmium batteries has already been mentioned.
There are wider supply questions too, as far as renewable fuels are concerned. Renewable energy sources are in principle inexhaustible (the energy input needed for their production is discussed above), but this is only true as long as they are used sustainably. If the supply of crop-based fuels is over-exploited, it can be depleted. There is also only a limited supply of renewable energy at any given time. There is a limit to the number of fields of rape for biodiesel, or wind farm sites for electricity which can be accommodated for technological, environmental or political reasons. Much of the work on alternative fuels is being done on cars, and the costs of these may come down considerably. If the valuable renewable energy supplies are used to increase the amount of private motoring, then they are being wasted, purely and simply.
This can be seen most clearly in the case of battery electric cars, where the fuel costs are significantly lower than for petrol. If battery costs are reduced significantly (reasonably likely), then the cost of motoring will fall. This is likely to increase the amount of car travel undertaken, wiping out or even reversing the benefits from the cleaner power source. Widespread (inevitably cheap) use of electric cars will have to be taxed more heavily than petrol and diesel currently are, in order to at least retain fiscal neutrality and maintain environmental benefits. Yet, the reverse appears to be happening, with various benefits, such as free city-centre parking, being made available to drivers of battery cars!
In the long term, it would not be unreasonable for the cleanest fuels to be reserved to some extent for the most important vehicles, such as buses and local delivery and waste collection vehicles.
There are trade-offs to be made in respect of biofuels. In the Swedish town of Linköping, six buses were converted to run on biomethane from the local sewage works. This methane is used to provide electric power and hot water within the sewage works, which switched to using electricity from the local power station, which uses some fossil fuels and some wood, for the six hours every night when the buses were being fuelled. However, there was still an energy benefit here, even though it sounds like a swap. The internal combustion engine is relatively inefficient, wasting most of the energy in the fuel as heat and noise. The power station, however, is a Combined Heat and Power (C.H.P.) station, producing steam for district heating as well as electricity, giving an overall efficiency of 70-80% (similar to the sewage works' own turbine). A saving in non-renewable energy and CO2 emissions was thus achieved.
Of perhaps greater potential concern is the trade-off between the production of fuel and that of food on agricultural land. Elvingson (1995) reports on work in the Netherlands on (inter alia) the environmental impacts of growing crops for fuels compared with food production. Whilst the overall situation is fairly similar, fuel production can lead to the need to import some food or fodder crops, with the associated environmental costs of their transport. Furthermore, fuel cropping may fit in with 'set-aside' policies for reducing farm production, but it does not allow the environmental impact of farming to be reduced by the alternative method of the reduction of food surpluses, i.e. deintensification of agriculture, which inevitably requires more land than chemical-dependent, intensive farming.
Even in terms of biomass energy, the use of food crops for fuel is questioned by the International Energy Agency (1995). It finds that 'energy forestry' (short-rotation coppicing for electricity) is preferable environmental grounds. Of course, methanol is produced from wood (and ethanol can be), but the inputs are greater in growing and conversion than with combustion for electricity.
At the end of the day, it seems that a number of alternative fuels have very real benefits for transport, particularly in heavy vehicle fleets, such as of buses. However, the benefits have to be considered carefully, especially in relation to the trade-offs and supply limitations discussed above. Some fuels are ideally suited to localised operations, such as biogas. Others represent a valuable use of a by-product, such as Stockholm's ethanol operation. It does seem that, if battery problems (not least of which are environmental) can be overcome, electricity offers one of the best solutions, including fume-free operation and flexibility of generation; Britain's electricity generation is generally inefficient, but various agencies are working on this, just as others are working on improving the environmental efficiency of transport.
One of the most interesting alternative fuels projects is to be found at Camden Community Transport (C.C.T.). The Accessible Sustainable Transport Integration (A.S.T.I.) project is part-funded by the E.U. Life programme and brings together C.C.T., L.B. Camden, Camden & Islington Health Authority, the Motor Industry Research Association (M.I.R.A.) and private sector partners, including British Gas, Powergen and London Electricity. Three battery electric and three C.N.G. minibuses run on a variety of C.C.T. duties, including the semi-routed 'PlusBus' service. One is dedicated to a low-cost, door-to-door service for taking Camden residents to the Camden Town Sainsbury superstore, which sponsors the service as a result of a condition of its planning permission. The bus carries the Sainsbury corporate livery.
The buses are Iveco 49-12 TurboDaily van conversions, seating 13 passengers, or six-seated and three wheelchair-users. Passenger access is by means of a driver-operated, semi-electric, fold-down step at the side door, or a driver-operated wheelchair lift at the rear. The vehicle was chosen as C.C.T. has a number of diesel Iveco buses amongst its 46-strong fleet, and so a familiar design was appropriate. M.I.R.A. organised the conversion to battery and gas operation, with engines built by Iveco specifically for gas in the latter case. The three-way-catalyst-equipped gas buses each have two 60l, 200 bar, steel tanks under the floor. The battery version has approximately one tonne of lead/acid gel batteries in a similar position. These buses also feature the 'Wavedriver' power management unit, as mentioned above.
Fuelling takes 5-8 hours for the gas buses, usually overnight, although advantage is taken of opportunity fuelling as well. The gas is compressed from the normal domestic-pressure supply. A full pair of tanks gives the bus a range of 90 to 100 miles and a maximum speed comparable to diesel. Drivability is similar to diesel power, but far quieter and cleaner. The electric buses are fully charged in 2.5 hours and the Wavedriver ensures that this is done efficiently. This unit also allows opportunity charging whilst maintaining full battery potential at all times. Charging takes place at 420V A.C., up to 20kW. The range with a full charge is at least 50 miles, depending upon driving style, and the maximum speed is 40 m.p.h., more than adequate for the urban driving environment. The Wavedriver unit also manages the supply of power from the batteries to the motor and allows the pattern of acceleration and deceleration to be preset.
All six buses are fitted with data loggers, which monitor a variety of factors, including speed, acceleration, time, temperature, power and use of the tail-lifts. They are also linked to a satellite tracking system, 'Omnitrak', which monitors vehicle location to within 10m and allows efficient dispatch and small detours to be made.
C.C.T.'s Fleet Manager, Karl Claydon, estimates a production version of the battery vehicles to cost £45-50,000, compared with about £35,000 for the standard diesel version. He believes this difference can be saved over the lifetime of the vehicle, especially if lower-cost, night-time electricity is used. The batteries are expected to last between three and five years, by which time, Claydon points out, battery technology is likely to have moved on. He estimates a production gas bus to cost £40-£45,000.
The C.C.T. drivers have reacted very favourably to the A.S.T.I. buses, preferring them to the diesels, for both environmental reasons and the driving experience.
Carbon monoxide is a respiratory inhibitor, causing reduced oxygen supply to the tissues as it is taken up by the blood more readily than oxygen.
Carbon dioxide is the most important, longest-lived contributor to global warming (the 'Greenhouse Effect'), which cannot be removed by exhaust cleaning equipment; its emission is an inevitable consequence of burning carbon-based fuels. In the case of fossil fuels, such as diesel, C.N.G. or L.P.G., this carbon has been held out of the natural atmospheric carbon cycle for millions of years; its release by combustion alters the balance of gases in the atmosphere. However, CO2 emissions from the combustion of renewable fuels can be assumed to be equivalent to that taken up by living things in the very recent past and thus to make no nett contribution to global warming. The energy needed to produce the fuel is another matter. Global warming is likely to lead to major shifts in climatic zones, flooding of low-lying lands, changes in agricultural potentials and the spreading of tropical diseases.
Hydrocarbon emissions are inevitable when hydrocarbon fuels are burned. The only non-hydrocarbon, fluid fuel considered in this article is hydrogen. Emissions result both as by-products of combustion and from incomplete combustion of the fuel itself. There are a vast number of HC compounds, some of which are relatively innocuous; others are highly dangerous. Simple fuels, like natural gas, produce relatively simple HC emissions, like methane (CH4), which are, most importantly, 'greenhouse' gases. At the other end of the scale are a number of polycyclic aromatic hydrocarbons (PAH), based around one or more benzine rings, many of which are, or are believed to be, carcinogenic (cancer-inducing), mutagenic (mutation-inducing) and/or teratogenic (inducing 'birth defects'). In the middle of the scale, many HC react with NOx in the presence of sunlight, to form O3.
Nitrogen oxides consist of nitrogen oxide (NO) and nitrogen dioxide (NO2), between which (together with N2O), there exists a chemical equilibrium in the atmosphere. NOx contributes to environmental acidification through the formation of nitrous and nitric acids, as well as to the over-nutrification of watercourses. NOx is also responsible for respiratory problems, with NO2 being more toxic than NO. In the presence of sunlight, NOx reacts with various HC to form O3.
Nitrous oxide exists in chemical equilibrium with NO and NO2 in the atmosphere and contributes to acidification and over-nutrification. N2O is also an important 'greenhouse' gas.
Ozone is a respiratory inhibitor, the principle agent in forest dieback and has a damaging impact on crops. Low-level (Tropospheric) O3 is produced by the reaction of various HC and NOx in the presence of the ultra-violet light in sunlight, most spectacularly in the photochemical smogs encountered in a number of coastal cities around the world, where temperature inversions block the escape of pollutants. However, naturally occurring, high-level (Stratospheric) O3 is vital to the existence of higher life on this planet, reducing the amount of ultra-violet radiation reaching the lower atmosphere.
Sulphur dioxide causes respiratory problems, particularly when forming the basis for the formation of PM. SO2 is readily oxidised to sulphurous and sulphuric acids, which are far more aggressive than SO2, attacking the respiratory tract. Sulphurous and sulphuric acids are major causes of environmental acidification. Also, the presence of sulphur in fuels (above 0.05% by weight) prevents the use of catalytic converters as they oxidise it directly to sulphuric acid.
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| CO2 | CO | HC | NOx | SOx | PM | |
| Car | 165 | 12.9 | 1.5 | 1.4 | 0.08 | 0.05 |
| Bus | 77 | 1.4 | 0.4 | 1.2 | 0.10 | 0.11 |
| Taxi | 330 | 2.0 | 0.4 | 1.6 | 0.43 | 0.25 |
| Motorcycle | 115 | 8.9 | 1.1 | 1.0 | 0.06 | 0.04 |
| CO2 | CO | HC | NOx | SOx | PM | |
| Car | 237 | 18.5 | 2.2 | 2.0 | 0.11 | 0.07 |
| Large bus | 1035 | 17.5 | 4.7 | 14.6 | 1.22 | 1.30 |
| Midibus | 670 | 18.8 | 2.8 | 8.7 | 0.91 | 1.00 |
| Average bus in London | 944 | 17.5 | 4.7 | 14.6 | 1.22 | 1.30 |
| Taxi | 330 | 2.0 | 0.4 | 1.6 | 0.43 | 0.25 |
| Motorcycle | 119 | 9.2 | 1.1 | 1.0 | 0.06 | 0.04 |
| CO | HC | NOx | PM | Year | |
| Euro I | 4.5 | 1.1 | 8.0 | 0.36 | 1993 |
| Euro II | 4.0 | 1.1 | 7.0 | 0.15 | 1996 |
| Euro III | 2.0 | 0.6 | 5.5 | 0.15 | 2000 |
| Energy | CO2 | CO | HC | NOx | SO2 | PM | |
| Diesel | 13.2 | 885 | 4.3 | 0.4 | 14.1 | 0.3 | 1.1 |
| L.P.G. catalyst | 14.5 | 864 | 8.5 | 0.9 | 2.8 | --- | 0.3 |
| C.N.G. catalyst | 15.8 | 775 | 4.3 | 0.7 | 2.1 | --- | 0.2 |
| C.N.G. lean-burn | 14.0 | 690 | 0.9 | 0.4 | 7.8 | --- | 0.2 |
| Biomethanol | 13.2 | (717) | 3.5 | 0.3 | 6.1 | --- | 0.2 |
| Bioethanol E95 | 13.2 | (735) | 4.6 | 0.6 | 12.7 | --- | 0.2 |
| R.M.E. | 11.7 | (903) | 2.9 | 0.4 | 16.2 | --- | 0.7 |
| Battery (average generating mix) | 14.1 | --- | --- | --- | --- | --- | --- |
| Battery (night generating mix) | 13.8 | --- | --- | --- | --- | --- | --- |
| Energy | CO2 | CO | HC | NOx | SO2 | PM | |
| Diesel | 14.8 | 977 | 4.3 | 1.8 | 14.5 | 0.9 | 1.1 |
| L.P.G. catalyst | 16.3 | 975 | 8.6 | 2.1 | 3.4 | 0.6 | 0.3 |
| C.N.G. catalyst | 16.8 | 852 | 4.3 | 4.7 | 2.3 | 0.5 | 0.2 |
| C.N.G. lean-burn | 14.8 | 758 | 0.9 | 4.0 | 7.9 | 0.4 | 0.2 |
| Biomethanol | 6.8 | 357 | 3.8 | 2.9 | 7.3 | 0.3 | 0.3 |
| Bioethanol E95 | 13.0 | 593 | 5.1 | 4.3 | 14.9 | 2.0 | 0.4 |
| R.M.E. | 10.0 | 376 | 3.5 | 2.2 | 18.3 | 1.3 | 1.0 |
| Battery (average generating mix) | 14.1 | 801 | 0.2 | 2.4 | 2.2 | 6.7 | 0.2 |
| Battery (night generating mix) | 13.8 | 701 | 0.2 | 2.3 | 1.9 | 5.3 | 0.1 |
The author would like to thank Karl Claydon of Camden Community Transport for information and for demonstrating the A.S.T.I. minibuses.