Richard Smith considers the benefits and harmful effects of the various types of fuel, and makes a case for compressed natural gas.

A U-turn on diesel

In recent months, a good deal of media interest has focused on plans announced by Boris Johnson to charge drivers of diesel vehicles an extra £10 to drive in a new London ultra low emission zone (ULEZ) following EU pressure on Britain to cut air pollution levels. It was claimed that this was a “betrayal” of diesel car drivers who had previously been encouraged by EU and UK government policy to switch to diesel vehicles because they are more fuel-efficient and produce less carbon dioxide.

This was one policy that has proved all too successful. In 1994, 7.4% of the nearly 23 million cars on the road had diesel engines with the remaining 92.6% fuelled by petrol. In 2013, not only had the total number of cars risen to just over 29 million but 34.5% of these were now fuelled by diesel and 64.8% by petrol; the other 0.3% was accounted for by hybrids (some with diesel engines), gas or full electric cars, none of which existed in 1994.

So while the number of petrol-powered cars had fallen slightly over the period, the number of diesel-powered ones had risen almost 10 times.

This change may have produced a tiny benefit in terms of fuel consumption and carbon dioxide emissions, but the adverse effect on air quality in towns and cities from the increase in particulates and nitrogen dioxide has led the European Environment Agency to claim that London is the worst of Europe’s cities for air pollution. Although sound arguments to refute this claim have been put forward by the Mayor of London, there is, nevertheless, cause for concern over air quality; hence the suggestion of the additional £10 daily charge. This charge would apply to all pre-Euro 6 diesels and also to petrol cars registered before 2006.

The reason for the inclusion of these petrol cars is not readily apparent since all such vehicles produced since the beginning of the 1990s have catalytic converters and emit almost no nitrogen oxides, and never did produce particulates — leading some to suggest that their exhaust gas is cleaner than the air the engines takes in.

The encouragement of “dieselisation” was always misconceived and, apart from the emissions problem, has led to many car drivers buying vehicles ill-suited to their needs and, as a consequence, facing increased capital and maintenance costs. The blame for this has been laid at the door of EU legislators concentrating on climate change to the exclusion of air pollution.

How vehicle emissions legislation started

Vehicle emissions legislation has a long history, dating back to 1961 in California, with limits for hydrocarbon (HC) and carbon monoxide (CO) first being set by both the UN ECE and EEC in 1970. The limits were directed at these two main pollutants as they were the first ones known to have an adverse effect on health individually, and on the atmosphere in combination.

Oxides of nitrogen (nitrogen oxide and nitrogen dioxide, collectively referred to as NOx) are also harmful individually, and in combination with the two other regulated pollutants (HC and CO) contribute to the formation of atmospheric ozone, which is also damaging. Limits for NOx were first set in 1977. All of this early legislation was directed at petrol-fuelled vehicles with spark-ignition (SI) engines and there were no European standards set for CI engines until 1991.

Euro 1

The 1991 European standards published as Directive 91/441/EEC — the so-called Euro 1 standards for light duty engines — dramatically reduced the allowable limits for CO, HC and NOx and introduced for the first time limits for CI engines, including a limit on the production of particulates. Particulates (in the form of soot) result from even normal combustion in CI engines only and can lodge in the lungs, causing cancers. The first standards for heavy-duty engines were published the following year and became known as Euro 1.

Euro 1 was significant not only because it introduced standards for diesel engines, but because the limits introduced for SI engines CO, HC and NOx, together with the short time scale set for compliance, meant that exhaust gas after-treatment using the three-way catalytic converter was really the only practicable solution for manufacturers, though it was not specifically required as some believe. For correct operation the three-way catalyst has to operate with an air:fuel ratio of 14.7:1 (the “chemically correct” or stoichiometric ratio) and engines fitted with catalysts have closed-loop control of fuelling to maintain this ratio.

This requirement effectively killed the development of lean-burn engines that could have provided much greater fuel economy, as well as the required emission levels without the need for exhaust gas after-treatment, but was not ready for production. It has to be said, though, that in the intervening period, manufacturers have managed to achieve dramatic reductions in fuel consumption while retaining the catalyst for emissions control.


A side issue at the time was the removal of lead from petrol. Tetra-ethyl lead was included in petrol to prevent wear on valve seats and allow high compression ratios for increased performance but, when burnt, it produced metallic lead in the exhaust. Lead was known to be a cause of limited mental development in children and there was a campaign to remove it from petrol for this reason.

The data on the effect of lead on mental development were mostly collected at a time when lead pipes were commonly used to carry drinking water but this data was then related to high measured concentrations of lead in roadside dirt at places such as the Gravelly Hill interchange, without much consideration of how such lead could be ingested by children compared with the direct ingestion from lead-contaminated drinking water.

The catalytic converter cannot operate with leaded fuel so Euro 1 effectively achieved two separate targets for the environmental lobby.

Global warming?

Not regulated at all at this time was carbon dioxide (CO2), which is not harmful to humans in normal concentrations and is actually essential for green plant growth. CO2 is an inescapable product of combustion; in fact, the better the combustion (ie the better the fuel economy), the higher the amount of CO2 produced and so the introduction of the catalytic converter actually increased slightly the amount of CO2 in the exhaust.

CO2 has always been known as a so-called “greenhouse gas”: one that allows solar radiation in the visible spectrum through to earth from the sun, but prevents infrared (heat) radiation from escaping from the earth to outer space. This had previously always been regarded as a benefit because, without the greenhouse effect keeping heat in, the earth would have been too cold to support human life. Since the beginning of time the Earth’s temperature has moved in cycles of hotter and colder, and in the 1970s the worry was that we were suffering a period of global cooling, with the doomsters forecasting another ice age. CO2 was, therefore, not of concern.

From 1988 onwards the global cooling scare was superseded by the great global warming scare, which implicated the production of CO2 by humans and predicted disaster from global temperature rise. This led the politicians into concentrating on CO2 reduction and took their attention away from the other emissions. This resulted in Vehicle Excise Duty being based on CO2 production, the publication of official fuel consumption figures and reduction in fuel duty for diesel. These policies led to the marked upturn in the number of diesel-powered cars on the roads.

We might add at this point that, while global temperatures did indeed increase from the late 1970s to the late 90s, that was followed by a period of stability and there has since been another cooling trend noted.

Meanwhile, in the real world…

With market share at stake, manufacturers understandably produced vehicles that performed well on the approval tests and thus fell into the lowest possible VED band; low official fuel consumption figures also helped sales. Unfortunately, the official tests bear little or no resemblance to real life and buyers have rarely been able to achieve the figures claimed.

The same is true for emissions, particularly from diesel engines, which were again optimised for the test and not the road. This coupled with the huge increase in diesel vehicles on the roads and their need for (expensive and therefore often skipped) maintenance to prevent malfunction probably meant that the hopes of legislators and buyers were not to be realised.

In their preoccupation with global warming the legislators and environmentalists had forgotten (or ignored) the particulates that are produced by CI engines, but not by SI ones, and the increase in use of diesel engines together with the unrealistic emission test figures brought air quality back on to the agenda.

The response was to reduce the particulate emission levels at approval time (even though the test was still unrealistic) requiring diesel particulate filters (DPFs) to be fitted to new vehicles. This actually increases fuel consumption slightly and was not a suitable technology for many of the people who were being encouraged to buy diesel cars (and indeed vans for local deliveries), costing them a great deal of money for cleaning or replacing clogged DPFs.

Having “solved” the particulate problem the next concern was NOx emissions. Ever since the introduction of the three-way catalyst these had been effectively zero for petrol engines, but lack of reality in type approval testing and sheer numbers meant that, in cities, there were still unacceptable concentrations caused by the increasing number of diesel engines. The solution this time was more bolt-on after-treatment in the form of a reduction catalyst to convert the NOx to nitrogen.

And then there were bio-fuels

Yet another problem is the finite nature of the source of the fuel, whether diesel or petrol.

Crude oil was created millions of years ago and is being used at a rate that means reserves must at some time eventually be exhausted. When exactly that might be, as usual, depends on who you listen to and the particular point they wish to make. Whichever estimate you care to choose, however, the fact remains that not only is the resource finite, but it largely occurs in parts of the world that are unstable and prone to potential disruption of supply either by volume or price. The idea of producing a fuel locally, not only free of such disruptions, but also renewable over a very short period, is therefore attractive.

Global warming is involved here, too. Carbon-neutrality is a big element in the drive to reduce CO2 production; the idea being that the CO2 produced when the fuel is burnt is matched by the CO2 absorbed at some other stage. As green plants absorb CO2 from the atmosphere while growing, if they are used to produce fuels then that will offset the CO2 produced by their combustion. Many plants can be used to produce substitutes for petrol, and diesel fuel and sugar cane has been used to produce fuel in Brazil since the 1980s.

The EU Renewable Energy Directive requires 10% of road transport energy to be from renewable sources by 2020 and UK petrol supplies at the moment may contain up to 5% ethanol (E5) from plant sources with up to 10% (E10) permitted; higher ratios — up to 100% — are possible. Diesel fuel can also be made from plant sources and, in the UK currently, 7% bio content is used (B7) although again up to 100% is possible and B20/B30 is available.

While both petrol and diesel-containing bio element contribute to carbon neutrality, power output from petrol engines is impaired above E5 levels so more fuel (including the non-renewable element) needs to be used. The bio element in petrol also causes problems in vehicles not designed for it and, while new cars will run on E10, older ones may suffer from corrosion of plastics and metals in the fuel system and fuel blockages due to deposits loosened by the increased solvent effect. Bio diesel in concentrations less than 100% is claimed to have no effect on fuel consumption in diesel engines, but problems have occurred with clogged fuel filters and waxing at low temperatures.

It should be noted that bio fuels do not produce less CO2 than conventional mineral ones. The chemistry is inescapable: all combustible fuels contain carbon that oxidises to CO2 when burnt. It is just a question of the carbon balance; that is the CO2 produced in combustion is balanced out by the CO2 absorbed while the plant source of the fuel was growing to give a net figure of (hopefully) zero.

We should also bear in mind that growing crops for fuel takes up land that cannot then be used for food crops, so we might question whether that is such a good idea when large numbers of people around the world struggle to get enough to eat as it is.

The zero emissions misnomer

Much attention has been paid to zero emissions vehicles (ZEVs) dating back to the 1990 requirement for a certain proportion of vehicles sold in California to be ZEVs. The intervention of reality meant that the original target of 10% ZEVs by 2003 has been much revised and fudged around with a complex system that actually includes gasoline vehicles and a system of carbon credits.

The ZEV definition means that only vehicles powered by a battery or hydrogen fuel cell are categorised as pure zero emission — and even the latter are not really that since they emit water vapour. However, when looked at as a wider system, these are far from being really zero emission and may actually be worse than some of the other power sources because the emissions are simply shifted from the vehicle tailpipe to the electricity generation plant.

All that said, there will be a significant local advantage in cities and other areas where geography and climate conspire to create extremely poor local air quality.

Hybrid technology

Hybrid power units have been in use for some time in cars, truck and buses (and now F1 cars) and many operators have some in their fleet. Reading Buses, for example, has 31 hybrid double deckers in its fleet of 160 and reckons that they use about 30% less diesel and that emissions are reduced by about a corresponding amount. However, they cost about half as much again as the ordinary diesel version, still use energy from a non-renewable source, and still produce about 70% of the emissions of a full diesel, so require a particulate trap and selective catalytic reduction to comply with latest Euro standards.

Hybrids, too, are packed with complicated technology, which is something else to go wrong, and use heavy and expensive batteries with a planned life of about seven years and a replacement cost of £30,000. Apart from the cost to the operator, in global terms there is also the considerable environmental impact of the mining, processing and movement of the (often toxic) materials used in making the battery — and a further environmental impact from their disposal after a quite short working life. All of this means that, overall, the hybrid vehicle is not really as green as it is often claimed to be.

Batteries not included

Notionally, ZEVs have for a long time been suggested as the way ahead, but for an equally long time, battery technology has failed to provide an adequate duty ratio for most applications: a range of a few tens of miles with a recharge time of many hours can hardly compete with the range of hundreds of miles and refill time of a few minutes provided by petrol or diesel vehicles.

However, recent developments have changed this situation and full electric vehicles are now a much more viable proposition. There are several examples of the use of full electric buses where recharging is carried out at stops in little more than the time that it takes passengers to alight and board.

This, of course, requires the charging points to be built into stops all along any given route and eliminates the possibility of a late-running service making up time by an immediate turnaround if it must recharge first. However, at least this is possible; a similar system for goods vehicles is much harder to imagine.

The same problems regarding the cost, life and environmental impact of the batteries also apply to full electric vehicles as to hybrids but, in this case, since the charging is provided from an external source, the overall environmental credentials are even more questionable. Because electric vehicles only really shift the point of production of the emissions from the vehicle itself to the power station, there may be no overall reduction — and it might be worse — depending on the source used to generate the electricity.

It is unthinkable that renewables will ever be able to provide sufficient generation capacity for a developed country and the grid will be supplied from a variety of sources, so the battery ZEV might be being recharged with energy generated by a grossly polluting, coal-fired power station somewhere.

That said, there would be advantages to their use in city centres, although when the UK is predicted to be in electrical energy supply deficit in a few years’ time after the decommissioning of old power stations and the lack of any reliable replacements, one wonders whether this might not be at the expense of the lights going out everywhere else while they recharge.

Compressed natural gas

One option that appears to provide wins all round is the use of compressed natural gas (CNG) as a fuel. The engine is a normal piston engine using spark ignition and there are no huge batteries or complex control systems. The gas used is the same as that used in ordinary domestic heating and cooking — indeed is drawn from the same gas main.

The charging station at the operator’s depot takes this gas, compresses it, stores it in cylinders on site and then refills the vehicles from these cylinders in about the same time as it takes to fill a diesel tank (a low pressure, slow-fill option is also available, though refilling would then be overnight).

Reading Buses has 34 such vehicles with a conversion cost of only just over 25% of the price of the basic (single deck) vehicle. Tailpipe emissions are effectively confined to water, and Euro VI standards are met without the need for extensive exhaust gas after-treatment. The use of such a clean-burning fuel in a gaseous form also reduces maintenance, but the really significant point concerns the production of CO2, for which CNG betters the Euro VI target by about 24% according to a study by the German Energy Agency (DENA) in 2010. In the same study, 100% Bio-CNG beat the target by 97% with a greenhouse gas emission figure of 5gmCO2 equivalent/km — the same as electricity generated from wind power.

The Reading Buses CNG vehicles operate under a contract with the Gas Bus Alliance (part of the Gas Alliance Group), which provides a full service from feasibility to gas supply. Although users draw gas from the standard gas grid, an equivalent amount of Bio-CNG is injected into the grid from a bio-methane production plant. This plant uses anaerobic digesters to convert organic waste from homes, farms, etc into methane and thus recycles waste that would otherwise have to be disposed of by other means.

Of course, the same comment applies as with electricity: just as users on green electricity tariffs might actually be using electricity generated from burning coal, so the gas being used for the fill is not necessarily the same gas generated by the digesters. One does at least know that someone somewhere is using it, so there is, overall, a carbon balance.

While CO2 is identified as a greenhouse gas, large quantities of it are used industrially, much of which (including all that used by the drinks industry) is imported. CO2 is a waste product of the Bio-CNG production and, if captured rather than vented, can be used to replace expensive imports. Furthermore, the use of waste material that would otherwise have to be disposed of by burning or in landfill, emitting CO2 and methane (a greenhouse gas) in the process can convert a merely carbon-neutral process into a carbon-negative one, ie it effectively removes greenhouse gases from the environment. Also, the production of Bio-CNG within the UK not only saves money, but provides greater energy security through reduced reliance on the import of gas or oil from volatile regions of the world.


While all policies with respect to vehicle emissions have always been devised with the best of intentions and all the objectives sought are desirable in their own right, the implementation has frequently left much to be desired with legislators swaying to and fro in the breezes created by vociferous (often single-issue) pressure groups.

A lack of technical knowledge (not really to be expected) by buyers has led them to acquire vehicles perhaps not best suited to their needs and not achieving the benefits the buyers had been led to expect. If they feel that they have been led to make wrong decisions by politicians, they may very well be right. The legislators should be expected to take a holistic view based on unbiased technical advice and not succumb to partisan lobbying.

For the foreseeable future it is likely that the optimum solution will be a mix of the power unit types discussed above, not excluding ordinary diesel engines, and Bio-CNG has great potential as a national asset and deserves attention.

Last reviewed 8 September 2014