Air travel is by no means a modern invention; Leonardo Da Vinci designed a flying machine in the 15th century. Various methods of powering flight have been tried over the course of the centuries. Hot air balloons dominated the early days of air travel; an aircraft was powered by a steam engine in 1852; and today we mostly use jet planes, or propeller aircraft with petrol engines.
High Octane Fuel for Propeller Aircraft
Propeller aircraft engines are similar to car engines but more powerful. Not only do aeroplanes travel faster, they also have to overcome gravitational forces during take-off. Generally speaking, the more powerful the engine, the higher octane petrol it requires. In a high compression engine, petrol with a low octane rating may ignite prematurely due to pressure. Consequently some of the fuel ignites in the cylinder before the spark is produced, the rest is ignited by the spark, and a double explosion occurs. Performance is affected and the engine can be damaged.
Petrol is obtained through the fractional distillation of crude oil, and in its basic form is a random mixture of different hydrocarbon compounds. Certain hydrocarbons auto-ignite under pressure more easily than others. At the refinery, hydrocarbons can be modified and petrol can be blended to produce different grades of fuel. High octane blends are achieved either by using additives such as methanol and ethanol that increase the octane rating, or by chemically processing the hydrocarbons to change their molecular structure and reduce their tendency to auto-ignite.
Modern avgas contains a complex blend of additives, including lead, and is very expensive.
Most commercial airlines use jet planes powered by gas turbine engines. Jets cruise at around 500mph, much faster than propeller planes. Jet engines are also used in boats, for high-speed land travel, and in space rockets. The power of a jet engine comes from the rear ‘thrust’ of the hot exhaust gases that are forcibly ejected after combustion.
Briefly, cold air is sucked into the engine from the front; the air is then compressed, mixed with kerosene and ignited in the combustion chamber. Because of the high pressure and temperature, combustion is vigorous; hot gases are forced out through a tapering nozzle, and this force drives the plane forward. Various fuels including hydrogen could theoretically be used for combustion, but in practice all jet fuel is kerosene.
Will Jet Planes Go Green?
In response to the need to seek more environmentally-acceptable energy sources, trials have been carried out using biofuel blended with standard kerosene. These have proved remarkably successful; indeed, it seems that biofuel can actually enhance performance. There are plans to continue exploring the use of biofuels with a view to using a 50:50 blend; but this then raises issues over the wisdom of producing biofuel crops at the expense of food crops.
A possible way round this could be to use algae as a biofuel source. Algae can be cultivated on land that could not be used for food crops; however, its suitability is as yet unproven, though the results of early research have been encouraging.
Using hydrogen as jet fuel is another possibility. Hydrogen produced by nuclear power stations would be a carbon-free alternative; the main drawback is that liquid hydrogen is very bulky, so designing onboard storage for a sufficient volume of hydrogen would pose a new challenge.
During the first half of the 20th century, airships seemed a very promising form of air travel. Airships filled with helium or hydrogen are lighter than air, and can travel long distances using very little fuel. But after the Hindenburg disaster in 1937 they lost credibility. The horrific explosion that killed 36 passengers on the Hindenburg was thought to have been caused by a spark of static electricity that ignited leaking hydrogen, and thereafter people lost confidence in airships, deeming them unsafe. Now, with the benefit of new materials and better technology, there are signs of a possible airship revival.
High altitude airships are already being used for various purposes, including telecommunications, environmental monitoring and surveillance; they can rise into the stratosphere in an hour and a half, and use very little energy in doing so. Passenger airships are becoming increasingly popular for tourism, and there have been plans to use airships for intercity travel around Europe.
Environmental Impact of Air Travel
At present, air travel produces less pollution globally than either road transport or domestic energy consumption. However, this may change; the automotive industry is looking to use cleaner energy, and air traffic is predicted to increase significantly. The air industry has begun looking for environmentally-friendly, sustainable alternatives to fossil fuels, but the use of kerosene is likely to continue for the foreseeable future.