‘Raison d’etre’ of the petroleum refining industry is the manufacture of fuels for cars, trucks, aircrafts & ships. I have conducted courses for non-engineering refinery staff on this subject. Since it is quite complex, I seek indulgence of the honourable members to clarify some points that has been raised in this thread.
Before we start, we must understand the difference between workings of petrol & a diesel engine because only then we can understand the differences in the fuel requirements and effect of various fuels on the environment.
Normal petrol cars are fitted with a 4 stroke engine where the air fuel mixture is ignited just before the piston reaches Top dead centre (TDC) thru a spark plug. (TDC is the highest level the piston will reach in cylinder in upward stroke). We therefore need a fuel which evaporates quickly as it needs to be sucked in the car by the carburettor (before injection came on the scene). Also since the compression generates heat, fuel should burn easily but not too early or too late. Additionally it should safe to store it in the car fuel tank even in very hot weather.
It was found that Iso-octane (CH3(CH2)6CH3) burnt the best and it was given arbitrary octane rating of 100. Normal heptane H3C(CH2)5CH3 burnt the worst and was given octane rating of zero.
Octane rating of a gasoline is the percentage of Iso-octane in a mixture of Iso-octane & n-heptane which has the same burning characteristics as the gasoline in question. Car with lower compression ratio say 7:1 will run okay on about 82/83 RON but cars with 9:1 or more need close to 90 RON gasoline (petrol in Pakistan) for efficient operation.
Compared to regular petrol, high-octane petrol requires higher temperature and pressure to ignite, so it can withstand higher compression without igniting. Therefore, it's less likely to result in premature ignition, and therefore prevents pinging and/or knocking. In other words, it's a form of protection for high compression engines.
On the other hand higher octane provides no additional benefit for the cars designed to run on regular gas. For the record in the UK regular gasoline is 95 RON.
Generally hydrocarbons with branch chain & aromatics have higher Octane rating and straight or normal hydrocarbons have lower Octane.
Modern diesel engines on the other hand have compression ratio in excess of 20:1. Diesel engine has no spark plugs. Instead the high compression compresses the air in the cylinders so much that the heat from compression ignites the fuel. Quite often diesel engines have a ‘Glow’ plug which is just an electric heater that sticks into the cylinder to warm things up when it is cold.
Diesel engine has the diesel fuel injected straight into cylinder at the top of the compression stroke. This meant that fuel explodes and expands very quickly, making a rattly sound. Here it is important the fuel does not ignite too early. Hence diesel fuel consists of long straight chain hydrocarbons. Cetane (C16H34)ignites very easily under compression; for this reason, it is assigned a Cetane number of 100, and serves as a reference for other fuel mixtures. Normal automotive diesel in the UK has Cetane number of 50.
A diesel won't rev much above 5000rpm (petrol engines will do 7000 or even 8000rpm), but its torque is produced at low revs. Brilliant for towing, not so well for flat out 0-60mph times. A diesel engine is heavy. It can make a car seem more stable, but can spoil the cornering/handling. It makes the steering heavy too and I wouldn't recommend buying a diesel car without power steering
Modern diesel cars have become refined to the point where you may even struggle to tell the difference in some upmarket models. Often, the visible give away is the rev counter, which will have a much lower rev limit than one for a petrol car. Diesel cars are more efficient, and use less fuel for a given mileage, potentially causing less damage to the environment especially if fitted with a diesel particulate filter.
Turbo charging a diesel is easier than turbo charging a petrol engine. One problem for a petrol engine is that if the compression ratio is too high, and the pressure in the cylinder gets too high during the inlet stroke, then the fuel/air mixture can start to burn too soon, while the piston is still on the way up. A turbo increases the pressure in the cylinder making this problem worse. With a diesel engine, there is no fuel in the cylinder during the compression stroke, so a turbo can be used to pack as much air in there as desired without causing problems.
Petrol models tend to be faster, smoother and quieter than their diesel counterparts. Emissions from petrol cars have been dramatically reduced by the introduction of catalytic converters, which oxidise pollutants such as CO to less harmful gases such as CO2. When compared to petrol cars without catalysts, catalyst cars have much lower CO, HC and NOx emissions, at the expense of CO2emissions, which increase due to the oxidation of carbon monoxide to CO2. As a consequence of this, a catalyst car will also use slightly more fuel and become less efficient. However, despite these improvements, petrol cars with catalysts still produce more CO and HC than diesel cars, although exhaust emissions of NOx and particulates are much lower than diesel cars. In fact particulate emissions from petrol cars are so low that they are not routinely measured.
Diesel fuel contains more energy per litre than petrol and coupled with the fact that diesel engines are more efficient than petrol engines, diesel cars are more efficient to run. Diesel fuel contains no lead and emissions of the regulated pollutants (carbon monoxide, hydrocarbons and nitrogen oxides) are lower than those from petrol cars without a catalyst. However, when compared to petrol cars with a catalyst, diesels have higher emissions of NOx and much higher emissions of particulate matter.
Petrol cars always had very little sulphur because feed for the reformer (which converts low octane to higher octane) is hydro desulphurised. Diesel used to have high sulphur (0.5% max) in imported diesel in Pakistan. But currently trend is toward low sulphur and European norm is now ULSD (15 ppm max). So both fuels are about equal in sulphur dioxide emissions. Since no one uses lead any more, there is no lead pollution from petrol engine exhaust.
Emissions from cars are greatest when an engine is cold. On a cold day a petrol car may take up to 10km to warm up and operate at maximum efficiency; a diesel car may only take 5km. Consequently, diesel cars produce less unburned fuel during a cold start, which will result in lower emissions of carbon monoxide and hydrocarbons. Diesel cars could make a significant impact on air quality in urban areas where most cold starts occur, especially when it is considered that a catalyst on a petrol car would take several minutes to reach its operating temperature. Overall, diesel cars emit less hydrocarbons, carbon monoxide and lead pollution than petrol cars, but produce more noxious gases and significantly more particulates.
Despite much debate over which car, petrol or diesel, is cleaner, weighing up the advantages and disadvantages is not easy. For example, diesel cars have been promoted, as they produce less CO and HC on average when compared to petrol cars, and they have greater fuel economy producing less CO2 per km. However recent health concerns about particulate matter have given diesels a less environmentally-friendly image, as have the higher emissions of nitrogen oxides compared with petrol cars. As a comparison, petrol cars produce virtually no particulate matter, take longer to warm up, produce more carbon dioxide per mile on average, and emissions of the regulated pollutants are higher.
My personal vehicle is turbo charged petrol whereas my wife drives a diesel car. Thus I have no favourites.
