By Dave Scott
FleetWatch Technical Correspondent

South African diesel fuel creeps into conversations due to cost factors, quality perceptions, pollution issues, impact on engine wear, illegal dilution with illuminating paraffin and supposed lack of parity with countries producing new hi-tech power trains for modern trucks. However, many trucking people are clueless on diesel fuel terminology. Conversations on SA diesel are littered with hearsay, myth and general impressions. Yet diesel fuel can represent an astounding 45% of total costs - both fixed and variable - in terms of operating a truck fleet. 

In meetings with Patrick Swan, a well-known failure analysis expert in our road transport industry, I discovered my own diesel ignorance. It was time to straighten the record. When fuel is such a vital component of profit or loss, trucking people must have a better 'handle' on diesel fuel terminology and the way we use this information. In this feature, we will discuss issues such as:

• What is a cetane number and why is this number an important reference point?
• Is sulphur only a negative factor? Does sulphur in diesel have any benefit?
• How can one detect that diesel is premixed with illuminating paraffin?
• How does SA diesel fuel match international standards?
• How can truckers ensure the quality of diesel which is delivered and stored?

To kick off, let's look at the basic requirements for a diesel fuel. They are, that it:

• Ignites spontaneously when vaporised, compressed and heated.
• Burns cleanly, quietly and economically in the engine.
• Is suitable for the engine's injection equipment.
•  Is suitable for convenient handling at all stages from the refinery to the fuel tank.
• Does not degrade under normal circumstances.
• Does not harm any surface with which it normally comes into contact.
• Does not contain contaminants and separated wax .

A general perception exists in the market that South African diesel fuel is of a poor quality. Reasons most often cited are high sulphur content, low cetane number or that fuel pumps and injectors last longer in Europe or America. However, many local critics are talking off shaky platforms of weak maintenance, poor housekeeping and fuel storage, bad driving techniques and watching the price of oil instead of the cost of lubrication. 

The bulk of diesel fuel is refined from crude oil, the balance being made by Sasol from processing local coal reserves. After years of isolation and crude oil sanctions, local refineries are relatively sophisticated and flexible, being able to process various crude types purchased on spot markets at that time. A comparison of various fuel standards from around the world reveals the information as contained in Table 1.

An engine's heart is the combustion chamber. Surrounding a combustion chamber, a couple of relatively simple mechanical devices convert a piston's reciprocating motion into rotary motion, while valves control gas flow through the chamber. Engine efficiency means clean combustion, more power, low fuel consumption and less exhaust emissions, all completely dependent on combustion efficacy. It also means less soot and fuel dilution of crankcase oil, translating into longer and safer drain periods with reduced engine wear.

The standard of fuel injected into a diesel engine combustion chamber is critical - unmentionable things happen to engine combustion chambers and component life if these standards change.

Flash point
The flash point is the temperature at which enough vapour evaporates from the liquid fuel to ignite when a flame is applied. 

The flash point of diesel is important for the safe handling of product. Too low a flash point makes the fuel a fire hazard. Diesel with a very low flash might suggest a contamination with a more volatile and explosive fuel such as petrol. For this reason, there are mandatory minimum values for flash point.

The flash point of a fuel has no effect on engine performance; it does not influence the auto-ignition temperature or other combustion characteristics.

The flash point is measured by heating a sample slowly in a closed cup and exposing the vapours to a small flame at regular intervals.

SABS 342 specifies a minimum of 55ºC. A typical flash point of local diesel is 65-70ºC, making South African diesel safer, less volatile and more suited to our high ambient temperatures. 

Volatility
The volatility of a fuel is a measure of how readily it evaporates. The lower the boiling point, the greater the volatility. Diesel fuel is a mixture of hydrocarbon compounds with a range of boiling points. The volatility or distillation of diesel strongly influences it's performance especially in medium and high speed engines. The volatility influences other properties such as viscosity, flash point, auto-ignition temperature, cetane number and density.

The volatility of a diesel fuel has a major effect on engine performance because it determines how much fuel burns. For example:

When volatility is too low, the fuel spray is poorly atomised and the fuel burns poorly. This reduces power output and increases fuel consumption. As a rule, volatility should be as high as possible without adversely affecting other characteristics of the fuel. In general, high volatile fuels are best suited for engines that experience frequently changing loads and speeds. They give better fuel economy and produce less exhaust smoke, odour, deposits, dilution of crankcase oil and engine wear.

South African diesel is distilled to international standards for a 90% recovery at 362oC. The remaining 10% evaporation is least volatile with the highest density and best calorific value, a most important distillate section for an operator looking for good fuel economy. South African diesel consumers are not short-changed in local distillation processes. 

Cetane Number
The cetane number of a fuel indicates how readily it is able to self ignite in the engine. As a measure of diesel fuel's ignition quality, it represents the time delay between injection and ignition. A number around 100 has a negligible time delay while 0 is excessively long. After injection begins, atomised fuel particles must be given time to evaporate and mix with the compressed air in the combustion chamber, eventually forming a flammable mixture of fuel vapour and air. Injection continues after ignition has occurred. The combustion chamber then contains swirling pockets of burning fuel together with other pockets of evaporating raw fuel.

Because combustion is inefficient during the flame propagation period, the ignition delay period which causes it should be as short as possible. This can be affected by aspects of engine design but the nature of the fuel is the most important factor in reducing ignition delay.

Local temperatures and air-fuel ratios vary greatly. Too high a cetane number means fuel will ignite too close to the injector, forming a fuel rich zone around the injector, while the remainder of the combustion chamber has a weak air-fuel ratio. Incomplete combustion and soot particle formations occur in fuel rich zones resulting in black smoke.

For typical on and off highway engines, a cetane number of 45-50 is considered ideal. Note that SABS 342 has a minimum cetane number of 45 compared to the USA 2D standard of 40. Actual SA cetane numbers are around 48 while in the USA, the average for 2D grade is 45.

Higher cetane numbers may be required for future high speed engines but this will depend on combustion chamber design and particularly, air swirl within the chamber. Under cold start conditions, higher cetane number fuels ignite more readily and at start up have lesser white smoke. According to failure analysis consultant Patrick Swan: "Balancing emissions between start up and a hot engine is a major priority for modern engine designers. A cetane number greater than 40 is considered adequate for modern diesel engines."

To the driver, the most noticeable effect of low cetane fuels is the familiar diesel knock (increased noise). Starting may also be difficult, especially at low temperatures. Other effects include rougher running and increased exhaust emissions. When using fuels with higher cetane numbers, the engine starts more easily with a shorter warm up period and a better fuel consumption is achieved. There is less white smoke (caused by unburned fuel). However if the cetane number is too high, the engine is more likely to produce black smoke. This is because the short ignition delay means that some raw fuel is sprayed into an established flame, producing soot.

Cetane Index
Cetane index is a value calculated from the physical properties of diesel fuel to predict it's cetane number. A cetane index may be two digits above the cetane number for diesel fuel derived from imported crude oil. The cetane number is actually determined by expensive tests conducted with a laboratory engine.

Illuminating paraffin (IP) has a low cetane number of about 40. When mixed with diesel in significant proportions, it would affect the cetane performance of diesel. 

Sulphur
As part of its chemical structure, crude oil contains a few percent sulphur. Fuels distilled from crude oil will contain some of this sulphur and its removal requires reaction of fuel with hydrogen under high pressure. During combustion, formation of sulphur oxides and water vapour combine resulting in sulphur acids (sulphuric and sulphurous acids). These acids are only a problem below their dew point - under 338ºC - above which they pass out of the engine as gasses.

During combustion, nitrogen from the air is also oxidised, forming nitrogen oxides which in turn become nitric and nitrous acids below their dew points (83ºC). Among a soup of other acids also formed in the engine, sulphur is thus only one source of a strong acid. While all these acids are extremely corrosive to engine components, good quality crankcase oils are fortified to neutralise them.

Sulphur has several benefits
It is a superb lubricity agent and acts as an anti-oxidant for the fuel. Fuels containing moderate amounts of sulphur protect a fuel pump and injectors from wear and deposit forming products of thermal degradation and oxidation. These problems are becoming more severe in modern high-pressure fuel systems that are also cooled by fuel. Sasol fuel contains essentially no sulphur and is fortified by a small amount of lubricity additive.

Sulphur in fuel is associated at a molecular level with complex structures that burn less easily. Low-sulphur fuels burn cleaner and reduce engine particulate emissions. Particulate emissions depend largely on engine design and are only partly derived from fuel. Combustion of crankcase oil entering the combustion chamber contributes from 20% to 80% of engine particulate emissions.

Low-sulphur diesel is thus produced primarily to reduce engine particulate emissions. Unless suitable additives are included in low-sulphur diesel, it increases the wear of fuel system components and adversely affects filter life due to clogging with deposit-forming sludge.

South African specifications for sulphur in diesel fuel will move down from the current level of 0,55% by mass to 0,30% on 1 January 2002 - a welcomed improvement. (See FleetWatch article September 1998 - SA Diesel Fuel or Brimstone)

Viscosity
The viscosity of a fuel indicates it's resistance to flow. The higher the viscosity, the greater the resistance to flow. This characteristic is important because it affects the performance of the engine's fuel injection equipment. Correct fuel atomisation depends on fuel's viscosity, the injection pressure and injector hole size. To burn most cleanly, fuel must be injected such that the momentum of the atomised fuel disperses it evenly throughout a combustion chamber.

Higher viscosity fuels will atomise into larger droplets with a higher momentum and are more likely to collide with the relatively cooler liner wall, thus extinguishing the flame and increasing soot deposits (and emissions). In the long term, the fuel striking the cylinder walls washes away the film of lubricating oil and dilutes the oil in the crankcase leading to excessive engine wear.

A fuel with too low a viscosity produces a spray which is too soft and does not penetrate far enough into the combustion chamber, affecting combustion, reducing both power output and fuel economy. In addition, a fuel rich zone is produced around the injector, which leads to excessive soot formation. World-wide all fuels, including SA fuels, have similar viscosities.

Fuel Additives
It is generally accepted that fuel system components, particularly injectors, last far longer in service in Europe or the USA than in SA. The major reason for this extended life is that in these areas it is common for the fuel suppliers to add suitable additives to their fuel. Additives commonly used are:

a) Detergent, which keeps all fuel system components clean and can increase injector life considerably, thus also improving engine life as piston and general engine sludge and wear are reduced.
b) Oxidation inhibitors which reduce fuel degradation and propensity to form deposits.
c) Lubricity enhancers to improve injector life.
d) Cetane number improvers. Unless a fuel cetane number is below 45, this additive is wasted.
e) Combustion enhancers. Normal combustion is already in excess of 98% efficient, hence the addition of a combustion enhancer claiming 30% improvement can only improve combustion by 30% of about 1% of the remaining 2%. 

The only fuel additive currently available in SA diesel is a small amount of lubricity enhancer in Sasol's diesel to make up for its low viscosity and lack of sulphur. A widespread introduction of additized diesel fuel containing a detergent, oxidation inhibitors and lubricity enhancers would assist in giving the average SA trucker and off-road users significant maintenance savings. 

Conclusion
SA diesel fuel has a higher sulphur content than other First World countries, tending to increase particulate emissions potential. However, in all other aspects, it is equal to any other fuel world-wide - and a lot better than many. Failures are mainly due to injector wear or deposits, which can be overcome with the use of detergent additives commonly used in First World countries.

Our diesel fuel suppliers are slow off the mark on this issue due to increased cost structures. But what about the cost of owning and operating transport? Perhaps they should think of their customers? Improved maintenance procedures will also go a long way to reducing operating costs.

In the main it is not the fuel that is a problem - it is the way we manage and drive our fleets.


Common engine fuel delivery failures
Common injector failures include low injection pressure, dribbling and poor spray pattern.

a) Low injection pressure
Over time, during normal engine service, injection pressure drops until a point is reached where fuel is no longer atomised correctly. This results in incomplete combustion. Injection pressure is set by spring tension on the injector needle; any wear between the spring and needle will reduce injection pressure. Causes of abnormal wear are poor fuel lubricity, water or other contaminants in the fuel. To avoid this type of wear, Patrick Swan recommends water traps and good fuel filtration. A suitable lubricity agent, not common to SA fuels, can extend injector life. 

b) Dribbling
When the needle no longer seats correctly in a nozzle tip, fuel leaks past at moderate pressures. While water corrodes the needle or seat, a most common long-term cause of dribbling is hard varnish type deposits on the seat. These deposits result from fuel oxidation or thermal degradation where the fuel is unable to wash away. A suitable detergent added to fuel will lift and minimise these deposits.

c) Poor spray pattern
Blocked or partially blocked spray holes alter a spray pattern resulting in grossly fuel rich sections of the combustion chamber and incomplete combustion. Varnish type deposits block the holes. These would remain clean with use of a suitable detergent in the fuel.



Acknowledgements & References
Patrick Swan - Aswan Consulting
Raymond Abraham - Fuel Specialist, Shell SA
Shell Product Handbook.
Chevron/Oronite - Diesel Fuels - Technical review