Of all the types of power unit that were tried out during the early years of the automobile, it was eventually the petrol-fuelled four-stroke internal combustion engine that proved most popular – so much so, that for the greater part of the 20th century many motorists never dreamed of buying any other type of car.
Petrol engines are not intrinsically as efficient as diesel engines. Although petrol and diesel engines in cars look very similar and, as far as the end user is concerned, have almost identical operating procedures, in fact they work differently. In petrol engines, ignition is by a spark, whereas in diesel engines it is heat produced by compression that ignites the fuel.
Because petrol is more flammable, petrol engines can run at higher speeds but at lower compression. Also, because combustion is less complete in a petrol engine than in a diesel, more fuel is used and more CO2 is produced. This is especially true when running from cold. Petrol engines take significantly longer than diesels to reach their optimum operating temperature, and during this time emissions are particularly high.
The emissions produced by car engines were not generally perceived as a major problem until scientists made the connection between fossil fuels, CO2 emissions and climate change; but since then, attitudes have changed radically. The government has structured the taxation system to give motorists strong incentives for choosing less polluting vehicles with better fuel economy, and consequently the cars being produced today reflect this change in priorities. Fifty years ago, many family petrol saloons did no more than 20mpg; now, most will do at least 30 mpg, and there are petrol cars on the market that can achieve 50mpg or more.
Recent Developments in Petrol Engine Design
Petrol engine technology is continually advancing, as car manufacturers compete with each other to meet the needs of the marketplace and the wishes of their customers. When petrol was cheap, the focus was on achieving higher speeds and faster acceleration. Increases in petrol prices led to a generation of cars offering better fuel economy. Next came a strong drive to reduce pollution from petrol engines.
Catalytic converters (CATs) were an early step forward in cutting pollution (though not CO2 or fuel consumption), and their use has become almost universal, despite initial resistance by motorists who were concerned that CATs lowered performance. A major advance in both fuel-efficiency and cleaner motoring was the introduction of direct injection to replace carburettors. The carburettor mixed petrol and air outside the engine; injectors are positioned inside the combustion chamber, resulting in less pollution, and are used in almost all modern petrol engines.
Now, faced with concerns over CO2 and fossil fuels, manufacturers are providing options that combine petrol derived from crude oil with another energy source . We have seen already the launch of flexfuel cars that use a petrol/ethanol mix, and petrol/electric hybrids that use an electric motor to share the work of the petrol engine.
Future Developments of Petrol Engines
Understandably there is great reluctance to abandon an energy form that has proved so successful for so long; so we are likely to see a succession of attempts to make petrol cars acceptable again. One concept that may become available within the next five years is ‘homogenous charge’ engines which, whilst running on petrol, will operate more like a diesel engine, without spark plugs and with high cylinder pressures for combustion; the aim here is to achieve cleaner exhaust emissions and better fuel economy than is possible from either a normal petrol engine or a normal diesel.
Recognising that new models account for only a proportion of cars on the road, pioneering methods of making older cars more eco-friendly are also being explored; for instance, one concept that has yet to be fully developed is a tiny hydrogen generator that can be retro-fitted to petrol cars. Hot exhaust emissions convert liquid organic hydride to hydrogen gas, which then goes into the air intake to induce lean burn; this system, it is claimed, can improve fuel efficiency by almost one-third, and cut CO2 gases, again by almost a third.
Other Types of Energy for Automotive Use
The perceived need to not only reduce car emissions in the short term, but to phase out the use of fossil fuels altogether in the long term, is proving a powerful incentive for innovation in the automotive world. Alongside work to make petrol engines more environmentally-friendly, more radical research is exploring other types of energy for automotive use, such as fuel cell energy. It is beginning to seem that, after dominating the marketplace for around a century, the petrol car will one day be supplanted as the most popular type of automotion.