Petrol and diesel have long been rival energies for road transport applications. Traditionally, petrol engines were preferred for cars because they were faster and quieter, whilst the greater pulling power of the diesel engine was considered better suited to commercial vehicles. More recently, refinements to diesel technology have made diesel cars more attractive. But concerns for the environment are causing us to re-evaluate both these types of energy, and future choices are likely to favour fuels that are able to establish the best eco credentials.
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History of Diesel Powered Cars
The compression ignition engine was invented towards the 19th century, although there is some debate as to whether credit should go to the German engineer Rudolph Diesel or the British engineer Herbert Akroyd-Stuart. However, the basic principle on which both men worked is that a piston can compress air to create a sufficiently high temperature, through pressure, to ignite heavy oil.
Compared with the petrol engine, diesel engines burn very efficiently. However, the performance of early diesel cars was not comparable to that of petrol cars because acceleration was slower and the engines were noisier. Diesels were therefore used in HGVs, where speed was less important than power and low fuel consumption was desirable. Subsequent design improvements resulted in more refined diesel engines that were a lot quieter and faster. Turbo-chargers, initially introduced on high-performance cars, are now added to most diesels to boost power and improve mid-range performance. More recently, the traditional direct fuel injection method is increasingly being replaced by the common rail system, which can further enhance performance and economy.
Diesel Engines and the Environment
All fossil fuels produce emissions when they burn. Whereas petrol combusts to give high levels of carbon monoxide, carbon dioxide and unburned hydrocarbons, diesel engines produce significantly less carbon dioxide per mile, and almost no carbon monoxide or hydrocarbons. However, there are significant particulate emissions –the tiny specks of soot in black smoke – from diesel, whereas petrol produces virtually none. Diesels also produce lower levels of nitric oxides than petrol cars with no catalytic converter, but more than those fitted with a CAT.
Opinions are therefore divided on which of the two is better for the environment. The perceived role of carbon dioxide in climate change has to be weighed against the effects of particulates and nitrogen; nitrogen oxides are known to cause acid rain and trigger respiratory problems, whilst suspicions are growing that particulate emissions may be carcinogenic as well as causing or aggravating respiratory problems. Diesel cars are now fitted with particulate filters, and must meet limits set by EU standards.
Cleaner Diesel Options
One partial solution is to modify the diesel itself so that it burns more cleanly. ‘City diesel’ available in the UK can, depending on the engine and the operating conditions, result in a quite dramatic reduction in particulates.
Another option is to replace petrol-diesel with biodiesel. At present, the Renewable Transport Fuel Obligation (RTFO) requires all diesel for road transport applications to be composed of at least 2.5 per cent biofuel; this is expected to be raised to 5 per cent in 2010. Biodiesel produces lower CO2 emissions and also has the advantage of being a renewable energy; the higher the proportion of biodiesel in the fuel, the more environmentally-friendly it is. Many vehicles already use 80 per cent blends of biodiesel, and some run on 100 per cent.
Driven by the need to compete in the marketplace, diesel vehicle manufacturers are constantly working to not only develop more eco-friendly engines, but also to improve fuel consumption – and therefore emission levels – by designing cars that are lighter and more aerodynamic. Various chemical additives are also being trialled, that can be used either in the fuel or in exhaust system to reduce noxious emissions, particularly of nitrogen.
A different approach is the use of hybrid technology, where a car is powered jointly by two separate fuel sources – electrical energy from a battery, and fossil-fuel energy from an engine. The first hybrids to go into production were petrol/electric, but diesel/electric hybrids have recently become available.
The Future of Diesel Vehicles
Modern diesel cars are already less polluting than their predecessors, and motorists are responding very positively to biodiesel. In the long term, widespread production of biodiesel from purpose-grown fuel crops may prove undesirable; but biodiesel can also be produced from waste oil and fats and from crop waste, and this is very environmentally-friendly.
There is also considerable public interest at this stage in using ‘home-made’ biodiesel from waste oils. At the same time, however, the motor industry is exploring many avenues to find a solution to the problems caused by fossil fuels, and it is conceivable that a different type of energy, possibly fuel cells, may be used in vehicles in the future.
