Car
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Description
For this article, a car is defined as any conventional motor-driven road vehicle designed primarily for the private transport of people and goods. Generally, its unladen weight will be less than about 2 tonnes, although there may be exceptions. Commercial vehicles based on the same chassis of a car are also included.
Detailed description
Without doubt, light road vehicles are a major source of pollution today. These include the pick-ups and 4x4 SUVs, so beloved by many in various countries to take their kids to school or for going to the mall.
Even in countries where annual testing is mandatory, some of them can be seen on the roads spewing clouds of smoke from their exhaust pipes. This is certainly increasing local pollution levels and could be stopped overnight by strong action.
However, this is only a small part of the problem. There are seven types of polluting emissions coming from cars, even as they are driven away from the showroom, with an eighth type on cars which burn petrol (gasoline) containing lead in those countries where it is still permitted:
- carbon mono- and di-oxides
- volatile organic compounds (VOCs)
- nitrogen oxides (NOxs)
- heavy hydrocarbons
- sulfur compounds
- particulate matter from exhausts
- particulate matter from friction between tyres and road surfaces
- lead compounds.
As a general rule, the larger the car, the higher is the amount of pollution produced.
Petrol (gasoline) or diesel fuel?
Assuming that a fossil-fuelled internal combustion engine is necessary under a given set of circumstances, the choice of fuel is paramount. It is true that diesel-engined cars consume less fuel per distance run than their petrol-driven counterparts, of equivalent power, even though they may have slightly larger and heavier engines for a given power output. However, this is no advantage in terms of pollution. The amount of carbon in diesel fuel is higher than in the more volatile petrol, so that the carbon dioxide produced per unit energy produced is substantially equal (typically 150 - 450 g CO2/km, depending on engine size and other factors). All the other of the first six pollutants in the above list are higher with diesels than with petrol-engined vehicles fitted with a catalytic converter, even if the diesel burns desulfurised fuel and is fitted with a particle filter, which is effective only for the larger particles.
That having been said, some of the latest diesel engines are much less polluting than earlier ones, provided they are correctly maintained. However, the advantage of this is reduced as the wear on the engine components increases, typically after having run 75,000 - 100,000 km.
The big advantage of diesel cars used to be the very considerably lower cost of diesel fuel. However, this advantage has diminished and is expected to do so even more in the future. In many EU countries, the cost of diesel fuel is either the same as petrol or even higher, in attempts to reduce pollution.
Some very small diesel cars are very economical, with practical consumption figures on a par with larger mainstream hybrid cars.
With current technology, for mainstream mid-sized passenger cars, the least polluting option would seem to be petrol-powered hybrid cars. For urban commuting of one or two persons, a diesel-powered small car may be advantageous.
2-WD or 4-WD?
Anecdotal evidence suggests that 95% of privately-owned, non-agricultural, 4-wheel drive cars are never used, in their lifetime, under conditions which requires the all-round drive. Even in winter, with snow, 4-WD may occasionally be helpful, but special snow tyres on a 2-WD vehicle are more effective than ordinary ones on a 4-WD. Furthermore, steering and braking control on a 4-WD are worse than on a 2-WD under snow or icy conditions. So, why have this feature which adds heavily to the cost of a car and adds to the weight and therefore the fuel consumption? It is interesting to see that some Japanese light-SUV makers ("half-roaders") have broken with tradition and are now offering 2-wheel drive versions of their vehicles.
This is not to say that such vehicles are unnecessary for professional off-road use.
Catalytic converter?
What is a converter? Such a system meters some air into the exhaust gases before they go into the converter. This is a container containing thousand of ceramic beads or ceramic nets, each coated with a microscopically thin layer of platinum. As the hot exhaust gases hit the platinum, the latter reacts chemically with them, without being consumed, and this reaction is what is called exothermic, which means that the temperature rises. In fact, it reaches a bright red heat and this allows any residual combustible material in the gases to be burnt. The result is considerably reduced emissions of carbon monoxide, VOCs, NOxs, heavy hydrocarbons, sulfur compounds, etc., all of them pollutants. It should be noted that converters do not reduce carbon dioxide emissions, another pollutant.
There is one proviso with converters; if leaded petrol is used, even in minute quantities, the platinum becomes "poisoned". One tankful of leaded petrol will stop a converter from working, for ever. It is quite an expensive component to replace, but this cost would have to be borne by the careless motorist, because the annual exhaust test will reveal the problem. It is for this reason that the filler tubes of cars with converters have a diaphragm which will not allow the filling nozzle of a leaded petrol pump to enter.
Diesel cars cannot be fitted with catalytic converters and this is one of the reasons why they are more polluting. The only filtration is capturing and burning particles greater than about 1 µm diameter; smaller particles, which are the most dangerous as they remain suspended in the air longest, pass through unaltered, as do many of the toxic gaseous pollutants.
Pollution
Let's have a look at the pollutants in the above list.
Carbon oxides
Carbon dioxide is the major cause of global climate change. Signatories of the Kyoto Protocol and other agreements have a legal obligation to minimise emissions of carbon dioxide. Reducing these may involve:
- improved public transport
- reducing the use of cars (e.g., walking the kids to school)
- using cars with lower fuel consumption
- using cars which do not need fossil fuel (e.g., electric, biological fuels)
- heavy carbon taxes
Carbon monoxide, which is a highly toxic gas causing semi-chronic cumulative damage to the oxygen-carrying capacity of the blood, oxidises in the air to carbon dioxide within a day or two, depending on the abundance of hydroxyl radicals. Carbon monoxide is burnt in catalytic converters and little is emitted from vehicles thus fitted.
Volatile organic compounds (VOCs)
VOCs are organic gases or vapours. Typically, in this context, they are vapours or decomposition products derived from unburnt or partially burnt fuel. Their effect is essentially local and they are a component, along with NOxs, of the smog, haze and ozone that affects most towns, especially in summer. Anyone having seen the pollution in Cairo, Delhi, Bangkok, Mexico City or a host of other cities will know the problem. This is largely due to motor traffic. The mechanism is a chemical reaction between the VOCs and the NOxs, triggered by sunlight. The resultant reaction is also the precursor of a second photochemical one, which produces tropospheric ozone, a highly toxic and irritant gas when it is at ground level. It requires only trace amounts to cause a reduction of immune system responses and this is the probable cause of a number of diseases, including asthma in children.
There are some natural producers of VOCs, such as terpenes and terpenoids emitted by a few aromatic plant species. The quantities in cities from these sources are negligible. On the other hand, in forested areas, natural VOCs may approach similar levels to those produced by vehicles.
Nitrogen oxides (NOxs)
There are several different kinds of nitrogen oxides. The only normal major natural source is the reaction between the nitrogen and oxygen due to the heating effect of lightning strikes. Most of this is immediately washed out by rain. All combustion at temperatures exceeding about 500°C will produce NOxs and that in the cylinders of a car engine is no exception. This is the other component, with VOCs, required to produce smog, haze and ozone, especially in cities (see the preceding section). Diesel engines, even with particle filters, produce more than petrol engines with a catalytic converter.
Heavy hydrocarbons
This is a phenomenon normally associated principally with diesel engines, although it does occur, to a small extent, in petrol engines, especially if badly maintained. Unburnt fuel, in contact with the cooled cylinder walls, and oil which passes the piston rings and valve guides, often undergoes a number of reactions because of the heat and pressure. These often produce polymers of the alkane groups and other heavy hydrocarbons, as well as VOCs. Many of these heavy molecules are dangerous to lung tissue and some may even be carcinogenic (causing cancers). Even if you cannot see exhaust gases, these heavy hydrocarbons are inevitably produced. If the exhaust is actually visible, then the problem is severe. More HCs are produced during starting and acceleration than in running a hot engine.
Sulfur compounds
All fossil fuels contain sulfur compounds. These are mostly removed during refining, especially the specially desulfurised fuels, but there is always a small proportion remaining. These produce sulfur dioxide during combustion and this oxidises in the air to form sulfur trioxide. This combines with humidity to form sulfuric acid, a major component of "acid rain". This can cause various illnesses in plant and animal life, including a contribution to emphysema and asthma in humans. It also causes attack of the surfaces of marble and limestone (e.g., the problems with the Parthenon in Athens) and, when it does so, it releases carbon dioxide, increasing the atmospheric loading. It is also believed that sulfuric acid, resulting from the combustion of fossil fuels, was a major contributor to the deaths of some 4,000 persons during the smog in London in December 1952.
Particulate matter from exhausts
All vehicles emit particulate matter, such as tarry soots. These have been identified as probable carcinogens (similar to those produced by cigarette smoking). With cars fitted with catalytic converters, most of this particulate matter, with a well maintained engine, is burnt, except that the converter does not start to work until it becomes hot, usually after 5 or 6 km after starting from cold. This is also when the engine runs "rich" (i.e., with a higher fuel-to-air ratio), so that the combustion is incomplete and most soot is formed. A car with a converter is just as polluting as one without a converter in these first few kilometres. Diesel engines tend to produce more particulate matter than petrol engines and cars, of all types, with a high oil consumption especially so. The particles can be wind-borne over considerable distances, especially in dry conditions.
Particulate matter from friction between tyres and road surfaces
This is an inevitable part of road transport. We all know that a tyre wears down its tread at a rate of (very roughly) 1 mm/5,000 km, depending on the car, the tyre composition, the air pressure, the way it is driven and so on. This represents between 150 and 250 g of rubber compounds per tyre. For every 1,000,000 private vehicles running 15,000 km/year, on average, it means that about 2,500 tonnes of rubber compounds are lost each year. Most of this is transformed into dust and such dust cannot be healthy, even if it is inevitable.
At the same time, the road surfaces are worn, probably to a similar degree. Most of the surfaces are made from compacted hot melt petroleum bitumen aggregate (usually referred to, incorrectly, as asphalt). The bitumen is formed from the still bottoms after the refining of petroleum and consists of an unrefined mixture of many heavy hydrocarbons and carbon. Almost certainly, some of these hydrocarbons are suspected carcinogens. The quantity of bitumen converted to dust is unknown.
Lead compounds
Tetraethyl lead (TEL) has been added to petrol to prevent "knocking" in engines since about 1923. This allows the compression ratio of the engines to be increased, thereby obtaining a better efficiency without premature ignition and thus damage to the engine.
The quantity of TEL added to petrol was small, up to 0.8 ml per litre, but the total quantities of petrol sold are enormous, leading to an annual consumption of thousands of tonnes of the substance. Most of the lead passed through to the exhaust in a variety of compounds, some as gases, some as dust. Lead compounds are very toxic to humans and cause a variety of health problems. In particular, the brain and intellectual development of children is severely retarded when they are constantly exposed to lead compounds. It has been found that the lead content in the blood of children has been reduced by more than 75% in children in the USA since the sale of leaded petrol was banned.
For the anecdote, the man who discovered that TEL reduced "knocking", Thomas Midgeley, a mechanical engineer with no knowledge of chemistry, was also the same person who discovered CFCs, used in refrigerators, causing depletion of the ozone layer; this one man had on his shoulders the responsibility for two major and potentially dangerous environmental hazards!
Unfortunately, leaded petrol is still on sale in many countries, even though its bad effects have been known for over 30 years. This does nothing for anyone and I urge everyone, in the strongest terms, never to use anything but lead-free fuel. If their car is old and will not accept it, then the engine should be modified to allow it to be used (or taken off the road!). It was an anomaly that leaded petrol is still on sale, when it has been banned for many years in most developed nations and many developing ones.
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