WHAT is your main consideration when deciding where to fill up? Convenient location, brand loyalty, advertising campaigns, or the special additive packages a particular brand of fuel offers? I bet the latter point is probably the last reason why you turn in at a certain forecourt, but as an informed motorist it should be high on your priority list. You wouldn’t, for example, use any old oil in your engine. You’d insist on the best, right? The additive packages included in pump fuel are therefore also worthy of your attention.
The “base fuel”
Local refineries produce fuel according to the South African National Standards (SANS); SANS1598 for petrol and SANS 342 for diesel. These are the minimum specifications fuel must comply to and this is called a base fuel. Base fuel is then sold in bulk to the oil companies and fuel distributors. It is up to these companies to modify the properties of the fuel through additives (fuel differentiation) before it is offered to the public.
Although running base fuel in a modern engine is feasible, it is far from ideal. And here’s why…
Additives used in petrol
Petrol or gasoline is a collective term for a range of fuels that are used in spark-ignition engines. The following additives are typically used.
Detergents
This is one of the most popular additives used in advertising campaigns because the general public easily understands the concept of “cleanliness”. Base petrol will result in deposits forming in the intake system and combustion chambers of engines.
The actual forming of deposits is due to a chemical interaction between the fuel, air, lubricating oil, blow by gases, exhaust-gas recirculation (EGR) and hot metallic surfaces. This is especially true in port-injection engines where injectors spray the fuel on the back of the intake valve during the intake stroke before it enters the combustion chamber.
The international-standard test for measuring inlet-valve deposits (IVD) was developed specifically for this and it entails running an old Mercedes-Benz M102E 2,3-litre, four-cylinder engine on a dyno according to a prescribed cycle for 60 hours. Before each test, the combustion chambers and intake system are cleaned and the new valves that are used for every test are weighed. After the test, the valves are weighed again to determine the accumulated mass of the deposits. A decent detergent additive will reduce the deposit level compared to the base fuel by up to 97 per cent and maintain engine efficiency.
Friction modifiers
Friction modifiers are also known as petrol fuel-economy additives. The economy benefit compared with a base fuel is achieved by reducing the internal friction of the engine. The moving parts that are in contact with the fuel are coated with a low-friction material (at microscopic level) that results in a measureable (albeit small) fuel consumption benefit. Friction additives are also beneficial for high-pressure fuel pumps and injectors because their service
lives are prolonged because of the reduction in friction.
Combustion improvers
The main aim of a combustion-improver additive is to lower the harmful emissions produced by the engine. As incomplete or partial combustion leads to a sharp increase in emissions, it is important to increase the chance of complete combustion by chemically modifying the base fuel.
Corrosion inhibitors
The SANS corrosion test for base fuel requires a standard of protection against copper corrosion, but does not account for silver and ferrous corrosion. It is up to oil companies to enhance the corrosion-protection quality of base fuel. This is beneficial to the customer, as components in contact with petrol will last longer.
Fuel stabilisers
Petrol quality degrades over time (have you ever smelled stale fuel?) and is measured in gum formation, acid number and oxidation stability. The SANS stability requirement does not cover fuel stability over prolonged fuel-storage periods and, again, oil companies have to expand the fuel’s shelf life with additives. This is especially needed in classic cars or vehicles that keep petrol in the fuel system for a long time.
Additives used in diesel
Diesel is a collective term for a range of fuels that are used in compression ignition engines.
The following additives are typically used.
Detergents
Even more critical than petrol, it is important to have good detergent qualities in diesel. The reason is that modern high-pressure injection systems (over 2 000 bar) deliver diesel to the combustion chamber via a number of tiny diameter holes (up to eight with a diameter of about 120 microns) in the injector tip (not visible to the naked eye). Any deposits in these holes would clog the flow of diesel, disrupt the spray pattern and lower the performance of the engine (see figure).
A clogged injector also negatively influences fuel economy and emissions.
Cetane-number enhancers
The cetane number of diesel denotes its ignition quality; a higher number denotes a greater tendency to ignite. SANS 342 specifies a minimum number of 45 for diesel in South Africa. Increasing the cetane number of diesel results in better cold starting, smoother combustion and lower fuel consumption compared with base diesel. The latter point was proved at the Sasol Fuels Application Centre by running the New European Drive Cycle (NEDC) on a test bed fitted with a modern 1,6 turbodiesel engine.
Anti-foaming
Base diesel tends to produce foam when dispensed through a pump nozzle into the tank of a vehicle. This slows down the refuelling process dramatically and may lead to diesel spillage if the fuel pump’s stop mechanism fails to identify the foam passing the nozzle. SANS 342 unfortunately does not include a foaming specification and only certain oil companies will add an anti-foaming additive to the benefit of the customer.
Corrosion inhibiter
Similar to the characteristic of a corrosion inhibiter in petrol, the diesel equivalent fulfils the same role of protecting components from corrosion damage when they come into contact with diesel.
Dyes
There are several reasons for adding a dye to a fuel, including brand identification or differentiation of fuel for tax purposes (for example, diesel used for agricultural purposes in Europe is dyed for easy detection). Of course, it is vital that the dye does not impact on the performance of the diesel.
The key take-outs
Oil companies spend vast amounts of money and engineering resources to develop a specific additive-fuel package and therefore not all fuels are the same. You should be wary of buying fuel with an unknown origin or brand. Worse still is to add your own additive, as this upsets the fine additive balance and might be very costly in the long run.
ADDITIVES IN LOCAL FUEL: THE CLAIMS
It is impossible to know the exact additive-package content of fuel brands without chemically analysing samples. Instead, we perused the marketing material of local fuel brands to see which additives are promoted.
Two-stroke oil in diesel?
A rumour currently doing the rounds claims that adding two-stroke oil to diesel improves the lubrication properties of the fuel and prolongs the life of diesel injectors and other fuel-system components. Sasol performed testing on diesel containing two-stroke oil at its Fuels Application Centre in Cape Town and showed that there is no measurable increase in lubricity when two-stroke oil is added to diesel. In fact, diesel base fuels that are sold by the refineries have to comply to a strict lubricity specification and in many cases they are already supplemented with special lubricity-improver additives. Furthermore, two-stroke oil contains certain metallic compounds that are known to cause injector fouling in modern common-rail diesel engines. Engine-test results showed significant injector fouling and an associated engine-power loss when two-stroke oil was added to diesel. This practice is therefore best avoided.
Sasol Fuels Application Centre
This state-of-the-art fuel research facility located close to Cape Town opened in June 2009. It consists of six engine test cells that are used to research the impact of Sasol’s range of synthetic and crude-oil-derived fuels on vehicle emissions and performance at sea level. Three of the advanced test cells are fitted with AC dynamometers that can replicate real-world drive cycles. This facility is also used to fine-tune the additive packages specifically for our local environment. Visit www.sasol.com for more info. Thanks to Sasol for allowing us access to its resources for this article and the accompanying photographs.
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