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Fuel for thought

11th April 1991, Page 26
11th April 1991
Page 26
Page 27
Page 26, 11th April 1991 — Fuel for thought
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Which of the following most accurately describes the problem?

• The Gulf War served as an uncomfortable reminder of our dependence on oil and the countries that produce it. We also know that it is a limited resource and will not last indefinitely. Pressure to look for alternatives was originally driven by that knowledge, but in recent years environmental concerns have stepped up research into other energy sources.

The recent agreement by EC ministers firms up existing proposals for diesel emissions, but the stringent phase two particulate limit will force engine makers to rethink some aspects of engine design; naturally aspirated diesels will virtually disappear as a result. Turbocharged and charge-cooled engines produce lower levels of Nitrogen Oxides (NOx) by cooling the intake air before it enters the engine. NOx has two unpleasant effects: it causes the photochemical smog which persists over cities like Los Angeles; and it contributes to acid rain.

Particulate matter (PM), or soot particles, is a greater problem with diesel exhaust. This contains a mix of solids and liquids including sulphates, nitrates, carbon particles and ash. All alternative fuels under serious consideration produce lower PM levels than diesel.

The possible alternatives include methanol, ethanol, natural gas and hydrogen. The favourites for commercial vehicles appear to be methanol, ethanol and natural gas.

BENEFITS

Reformulated diesel offers several benefits over conventional diesel fuel, combining low levels of aromatic hydrocarbons and sulphur with a high cetane rating; the higher the cetane rating the better the fuel. The result is improved combustion, and lower NOx and PM emissions. Added attractions include straight substitution for existing fuel with no need for engine modifications.

Methanol, like ethanol, is an alcohol. Exhaust emissions are reduced in several

areas compared with a diesel engine. NOx and Hydrocarbon (HC) emissions are considerably lower, as are Carbon Monoxide (CO) and Sulphur Dioxide (SO2). Hydrocarbon emissions contribute to the Greenhouse Effect, while CO is another cause of smog and can have acute health effects; SO2, like NOx, is a cause of acid rain. Diesel engines can be adapted to run on methanol either by adding a cetane improver or by mixing it with diesel fuel.

Alternatively, glow plugs can be used, as tried by Detroit Diesel, or a conventional spark ignition system added. Methanol engines produce about 3-5% more torque back-up than a diesel, but they need about three times as much fuel to produce the same energy as diesel. Consequently, more fuel needs to be carried on the vehicle, meaning larger fuel tanks and payload penalties. Catalytic convertors are also needed.

DRAWBACKS

There are more serious drawbacks, however. Firstly, methanol is highly corrosive, which gives rise to storage and transport problems; tanks, fuel lines and engine parts must be specially coated against corrosion. It is also highly toxic and can be fatal if swallowed even in relatively small quantities. Skin contact is also dangerous; servicing and maintenance operations require great caution. Finally, methanol is soluble in water and could pose a serious health hazard if it contaminates water supplies.

Despite these problems several manufacturers have carried out experiments with methanol power. California, which has the most stringent emissions regulations in the world, will ban the use of petrol and diesel engines from the year 2007, but methanol engines will be permitted and this has spurred further research. Engine designer Ricardo has been working with methanol, with California in mind.

Volvo also sees a future for alcohol-based fuels such as ethanol or methanol. This is because they provide the best compromise for the environment and can be produced from non-fossil fuels, reducing oil dependence and the depletion of the earth's natural resources.

Scania has also been researching alcohol power, with most of its work concentrating on ethanol engines for buses. Leyland Daf has carried out wide ranging research which includes ethanol and methanol engines, but at present it is concentrating its efforts on natural gas.

Natural gas is also an alcohol-based fossil fuel and is often associated with oil or coal deposits where it has been produced by decay and fermentation over thousands of years; its principal constituent is methane.

For transport use it needs to be compressed to form CNG or liquified to form LNG. Compared with diesel it can also produce lower levels of exhaust emissions. Claims of how much vary according to their sources but CO, HC and NOx are all reduced. Further reductions in NOx emissions can be made using a catalytic convertor. Particulate emissions are drastically reduced and, as with methanol, natural gas is sulphur-free.

Diesel engines need to be converted to spark the ignition with natural gas and as it is pressurised a regulating system is also required. Natural gas is not as hazardous to handle as methanol, but it still presents storage and fuelling problems for commercial vehicles. CNG needs to be stored at pressures of up to 240bar, so the vehicle fuel tanks are bulky and heavy, with obvious payload penalties. Refuelling can also be a particularly lengthy process taking between five and eight hours per vehicle.

There are similar problems with LNG; bulky tanks are still required. To liquify natural gas it must be cooled under pressure to -160'C. It should then be stored in insulated tanks to maintain the temperature. Again these are bulky and heavy but the greater density of the liquid means that less fuel needs to be carried to cover the same distance as CNG. Generally, gas tanks need to be six times bigger than equivalent diesel tanks and between 31 and seven times the amount of diesel is needed to give a similar range.

CNG appears to be the current favourite among truck manufacturers. MAN recently exhibited a gas-powered 19.232 at the Brussels Show, powered by a 11.9-litre engine producing 171kW (230hp) at 2,200rpm converted to spark ignition. Carrying 640 litres of gas gives a claimed range of 300km. Iveco Ford and Cummins also see natural gas as the best option, following research with a gas-powered bus.

ALTERNATIVES

In the short-term diesel will continue to provide power for commercial vehicles. It is currently the most effective fuel and environmentally it produces a lower overall level of emissions than petrol. Alternatives could be in use by the year 2000, according to Anders Johannesson of Volvo: "In our opinion, alcohol-fuelled trucks can become a feasible alternative in the second half of the 1990s".

However, there are many problems still to be resolved, concerning technical development, fuel availability and pricing. There is no point in having squeaky clean vehicle emissions if the production processes cause more enviromental damage than the engine exhaust gases.

The Brazilian Proalcool scheme, begun in the 1970s, supports this point. It was designed to reduce Brazil's dependence on imported oil and large-scale production of sugar-cane was started. Ethanol was then produced from the sugar cane as a substitute for petrol. In the process large areas of rain forest were destroyed to provide plantations for the sugar-cane. Destroying the environment to provide an "environmentally friendly" fuel cannot be the right way ahead.

J by John Kendall

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