Fuels and Combustion Questions and Answers

Answer: A

Correct answer: 100 and n-heptane is zero

The scale is defined by two reference fuels: iso-octane, which resists knock well, is assigned 100, and n-heptane, which knocks readily, is assigned zero. A fuel of octane number 90 knocks like a mixture of 90 per cent iso-octane and 10 per cent n-heptane in a standard test engine. Cetane belongs to the diesel scale and has no place in this definition.

Answer: B

Correct answer: ease of ignition under compression

A diesel has no spark, so the fuel must ignite of its own accord when injected into the hot compressed air, and cetane number measures how readily it does - the delay between injection and ignition. It is the opposite requirement to petrol, where the fuel must resist spontaneous ignition, which is why a high octane fuel is generally a poor diesel and the reverse.

Answer: C

Correct answer: poisons catalytic converters and is toxic

Tetraethyl lead is an extremely effective anti-knock additive, so it was not abandoned for lack of performance. It was abandoned because lead is a cumulative neurotoxin dispersed along every road, and because it destroys the noble metal catalyst in a catalytic converter, making the fuel incompatible with modern emission control. Octane is now raised by reforming, isomerisation and oxygenates instead.

Answer: D

Correct answer: propane and butane

LPG is the C3 and C4 fraction - propane and butane - which are gases at ambient conditions but liquefy under modest pressure, so a large amount of energy travels in a small cylinder. That is the whole basis of the fuel. Methane and ethane require far higher pressure or cryogenic temperature and are handled as CNG or LNG instead.

Answer: A

Because methane's critical temperature is far below ambient it cannot be liquefied by pressure alone and is stored as a compressed gas at about 200 bar. Its high hydrogen to carbon ratio makes it the cleanest of the fossil fuels per unit of heat.

Answer: B

Steam passed over incandescent coke gives the two gases in roughly equal volumes by an endothermic reaction, so the bed cools and the blast must be alternated with air. Both constituents burn, which is why its calorific value is respectable.

Answer: C

Correct answer: nitrogen from the air blown through the bed

Producer gas is made by blowing air through a hot fuel bed, and the nitrogen that accompanies the oxygen passes through unchanged, ending up as roughly half the product by volume. It carries no heat of combustion but must still be heated, so it dilutes the gas to about 1000 to 1200 kcal/m³. Water gas, made with steam instead of air, avoids the nitrogen and has a much higher value.

Answer: D

Hydrogen makes up about half the volume and methane a further third, with some carbon monoxide, so the calorific value is far above that of producer gas. Tar, ammonia and benzole are recovered as by-products of the same process.

Answer: A

Roughly sixty per cent methane with most of the balance carbon dioxide gives a calorific value about three fifths that of natural gas. Removing the carbon dioxide and the hydrogen sulphide upgrades it to a pipeline-quality gas.

Answer: B

Partial oxidation yields carbon monoxide and hydrogen rather than carbon dioxide and water, so the chemical energy is retained in a gas that can be transported or used as feedstock. Complete combustion would release all of it as heat at once.