Most people don’t really understand the science behind petrol and diesel fuels. Jake Venter takes a close look at both, and gives some advice on the use of additives
Petrol and diesel fuels are mixtures of particular combinations of hydrogen and carbon called alkanes. Both these fuels are obtained mostly from crude oil, also called petroleum — from the Greek petra (rock) and the Latin oleum (oil).
Fuel is a mixture of many compounds. The initial distillation of the average barrel of crude contains refinery gas (1,0 to 2,0 per cent), virgin petrol (15% to 30%), kerosene (10% to 15%), diesel oil (15% to 20%) and a residue (40% to 50%).
A petrol engine is designed to burn fuel in a controlled manner. This means the flame has to ignite only after the spark has occurred, and spread across the combustion chamber without the excess heat causing other flames to start. Uncontrolled combustion is called detonation and petrol has to be specially formulated to avoid it. This is done by adding octane improvers.
The octane rating has nothing to do with the energy content of the fuel. It refers only to the fuel’s tendency to burn without detonation. An engine designed for 92-octane petrol will not benefit from a higher octane fuel. In addition, an engine designed for 92-octane will suffer from using lower 90-octane fuel at large throttle openings because that is when detonation is likely to occur. Also, modern knock sensors will usually retard the timing rather than allow an engine to detonate, because the latter can damage an engine quite quickly if it is severe.
This differs considerably from petrol. It is less volatile and oilier, but its heating value is practically the same. Petrol weighs 0,745 kg/l but diesel is 11% denser. This is one of the minor reasons why diesel engines are more fuel-efficient.
One major reason is that most of the time diesel engines run very lean. Even when the driver demands maximum output the engine runs with about 20% excess air. Other reasons are the lack of a restricting throttle butterfly, and the high compression ratio of more than 15:1.
Technically, diesel fuel is expected to behave exactly the opposite to petrol. It must first vapourise inside the combustion chamber and then burst into flames without the benefit of a spark. This process relies entirely on the heat arising from the high compression ratio, as well as the heat from the previous combustion event.
Cold starting is taken care of by glow plugs, or fuel heaters in the case of large marine or stationary diesel engines.
This means there is no single heat source. There is a multitude of flames, each arising from fuel molecules finding sufficient oxygen to combust. This freedom to burn when conditions are right rather than when a spark occurs means that diesel engines can run on a variety of fuel oils.
Diesel fuel’s viscosity varies significantly with temperature. At something like -15C it ceases to flow and this means that in winter fuel suppliers have to add an additive to keep it in a liquid state at low temperatures.
Diesel’s equivalent of petrol’s octane number is the cetane number (CN). It is a measure of the time delay between the fuel entering the combustion chamber and the first measurable pressure rise due to combustion. This time delay is the reason that diesel engines do not like to rev much over 4 000 r/min. At such a speed the flames only arise when the pistons are already so far down the bore that the engine cannot gain enough energy to accelerate.
Higher CN values usually imply a shorter delay period. Modern high speed automotive diesels prefer a CN number of over 50, and additives are used by the suppliers to improve CN values when they are too low. SA diesel has a CN number of over 51.
Using aftermarket fuel additives
Very few aftermarket fuel additives are worth the extra trouble and expense. The major oil companies spend so much time and money on improving their product that it’s very difficult for a small outfit to come up with a worthwhile improvement.
When low-sulphur diesel became available in SA, some of the press releases mentioned that the process that removes sulphur from the fuel has the effect of reducing the fuel’s lubricity. The releases also mentioned that fuel suppliers were adding a lubricity enhancer to the fuel.
Some people argued that “more is better” and started to add a small quantity of two-stroke oil to each tankful of diesel in the hope that it would enhance the lubricity even further.
Adrian Velaers, a South African automotive engineer, has conducted tests on an engine dynamometer — an injector test rig — and on a high-frequency lubricity evaluator to see if the addition of two-stroke oil is beneficial. His report, submitted at the 11th International Tribology Conference earlier this year, concluded that such an addition cannot be recommended. The reasons are:
- Unlike petrol, diesel does not evaporate before entering the combustion chamber, hence the oil droplets cannot form a deposit on the injectors and other close-tolerance metal parts. It therefore cannot enhance lubricity.
- Most two-stroke oils are rich in zinc, which is a harmful contaminant and the cause of fuel injector deposits. Such deposits will lead to a drop in power. The zinc also causes an ash build-up in diesel particulate filters. – Jake Venter