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Fuels for Thought



 

 

While many operators base their fuel supply primarily on purchase price, consideration must be given not only to short term cost when the bowsers` nozzle is hung up, but the long term cost and implications when inferior quality fuel is used.

In the early seventies a major world-wide oil crisis created an overnight fuel shortage. As a result fuel prices rose sharply and have remained high since. This situation brought about a decrease in the quality of fuels, and the challenge for fuel refiners today, and the future, is to maintain a high fuel quality as the demand continues to increase.

To gauge the effect this fuel shortage had on the transport industry we journey back prior to the supply crisis when the cost of fuel amounted to roughly 30 percent of the engine`s operating expense. Today this figure fluctuates between 60 and 90 percent. This factor alone is forcing many operators to look at cheaper alternatives to reduce overheads, which in some cases includes inferior fuels. However, unit injectors and fuel pumps in modern engines won’t tolerate low quality fuels.

Here in Australia the Australian standard 3570-1988 for Automotive Diesel Fuel (AS3570) provides the parameters at which diesel must be supplied. The AS3570 was put together by representatives from oil companies, engine manufacturers and end users to provide a standard by which the performance, wear and emission factors of diesel fuel, its quality, may be judged.

Before attempting to judge the quality of fuel it is first necessary to understand the basic properties of fuel.

The operation of a diesel engine is a simple process. We know the engine produces power when fuel is atomized and mixed with air in the combustion chamber. This occurs as the piston rises in the cylinder, compressing the air and increasing the cylinder pressure. This, in turn, causes a rapid temperature rise. When the fuel is injected, the fuel air mixture ignites, releasing the energy and forcing the piston down and hence causes the crankshaft to turn. In a perfect situation the fuel would burn completely, without leaving any residue or producing any smoke.

Now look at tables one and two. The first test in the `Density` column measures the heat content of the fuel. The greater the density of the fuel, the greater the heat content per unit volume. The AS3570 indicates diesel fuel should have a density between 0.82 to 0.87kg/L at 15 degrees Celsius. If a fuel`s reading is below 0.82 it is known as a lighter fuel. Conversely, if the reading is greater than 0.87 it is a heavier fuel.

A lighter fuel will not produce the rated power because it has less heat per unit volume. This means it will produce less energy once inside the combustion chamber. On the other hand, a heavier fuel will produce more power as the heat per unit volume is greater.

There are a number of draw backs with both light and heavy fuels. Because light fuel produces less energy, more fuel is needed to produce the engine`s rated power and in turn will increase fuel consumption and reduce engine power.

Lighter fuels also have low viscosity. This prevents the fuel system from receiving proper lubrication. Consequently, the life of fuel system components is decreased. On the other hand, heavier fuels may produce more combustion chamber deposit formations. This can cause excessive wear of the cylinder liner and rings.

The next series of tests are for `Distillation` (refer to tables).

The correct distillation characteristics of a fuel are necessary for efficient combustion. Correct distillation is achieved by carefully balancing the light and heavy fractions during the distillation process.

The light fractions provide easier starting and a more complete burn, under various engine conditions. The lighter fuels are more volatile, and it is the volatility of the fuel that plays a major part in engine emissions.

The heavier fractions provide better volumetric fuel economy due to higher energy content. However the end point fractions may only partially burn, which can cause carbon deposits to form in the combustion chamber, and may contribute to excessive black smoke.

The blend of these two fractions is measured by the distillation test. A low 90 percent end point temperature is ideal. This tends to ensure low carbon residuals and minimum crankcase dilution, with good vaporization during severe cold weather and prolonged engine idling.

The ‘Cetane’ number is a measure of a fuel`s ignition characteristics; this can affect the fuel`s ability to start the engine. A reading above 50 means the fuel is more readily ignited, while a reading of 40 or below indicates a fuel is more difficult to ignite. An easily ignited fuel is ideal for engines started in colder climates. Under cold conditions, a high igniting fuel will reduce the amount of white smoke emitted and eliminate diesel knock on start-up.

Conversely, engines started with a low Cetane number fuel may suffer from engine knock and blow white clouds of smoke during engine warm-up in severe cold weather.

While an engine may appear to operate normally if low Cetane fuel is used for prolonged periods, harmful fuel derived deposits will accumulate in the combustion chamber. This can cause engine damage such as piston erosion.

The next test is for the ‘Sulphur’ content. Fuels with high sulphur content produce increased quantities of sulphur gases upon combustion. These gases, combined with moisture from low jacket water temperature and intermittent operation, produce acids that result in increased engine corrosive wear.

There are a number of precautions that can be taken to prevent sulphur related damage to an engine. Firstly, keep the operating temperature of the cooling system above 80 degrees Centigrade. This helps limit the condensation of sulphur compound vapours on cylinder liner walls.

Secondly, maintain the crankcase breather system to prevent condensation in the crankcase. Condensation will cause a rapid depletion of the Total Base Number (TBN) in the oil. Use a regular used oil analysis program and follow the standard oil change intervals, unless oil analysis indicates otherwise.

The last test on the table is `Viscosity`. While only two samples have this reading, it is worth noting the difference between them to understand the effect diesel viscosity can have on an engine.

Viscosity is the measure of a liquid`s resistance to flow. A high viscosity reading indicates the liquid is thick and will not flow easily. A low reading indicates the liquid is thin and is more free-flowing. Fuels with either too high or too low viscosity can cause damage to the fuel system components.

High viscosity fuel causes the injection pressure to rise. This increases the wear on the cam followers and gear train in the fuel pump assembly. This type of fuel atomizes less efficiently, and the engine will be harder to start.

Low viscosity fuel, on the other hand, may not have the ability to adequately lubricate the plungers, barrels and injectors. This will cause premature failure of the fuel system components. The use of a low viscosity fuel should be evaluated carefully. It should be remembered that fluctuations in temperature will vary the viscosity of a given fuel to some degree.

Table 2 shows the random results of fuel taken from major oil companies in capital cities. A quick glance at these results indicates they all meet the AS3570 standard.

The results fluctuate somewhat with the quality and type of crude oil available at the time diesel is drawn off the barrel.

Table 1 lists four different fuel samples. Take a look at sample A. The density reading of 0.864 is on the heavier side of the parameters set by Australian standards. This reading suggests this fuel will enable the engine to produce the recommended power with good fuel consumption. The 90 percent end point is 334.5°C, which is 22.5°C under the AS3570 standard maximum. This means the fuel should burn efficiently, with minimum emission and smoke. The Cetane number of 47 is only two figures away from the AS3570 minimum. This fuel may, in cold conditions, make the engine hard to start even though it falls within the standard.

From these figures we can assume that this fuel will produce the engine`s recommended horsepower, returning good fuel economy, and producing normal wear on fuel system components.

Now look at sample B. Straight away the density reading of 0.818 reveals that this fuel is out of AS3570 specifications. It is a light fuel, which means it will not have the energy per volume to produce the recommended horsepower of the engine.

Analysis of this fuel reveals that, although an AS3570 figure isn`t given for Initial Boil Point (IBP), this sample is very low at 131.5°C. This tends to suggest that it may be bordering on JET A-1 kerosene, and the flash point may be too low for a diesel fuel.

While the flash point doesn`t directly relate to engine performance, it does have important ramifications in the safety aspects of handling and storage.

The 90 percent end point figure is a little higher than the AS3570, which indicates it has a heavy end. The Cetane figure suggests this fuel will start an engine relatively easily in cold conditions.

Because there is such a big margin between the IBP and Final Boil Point (FBP), it can be assumed this fuel is a blend of heavy and light fuels. The heavy end of this fuel may only partially burn in the combustion chamber, causing carbon build up inside the engine and emissions of black smoke from the exhaust.

Sample B was taken from a truck where the operator was experiencing a low power complaint. Meantime sample A came from a truck in the same area that was operating to the manufacturer`s and owner`s expectations.

The relevant figure in sample C is the viscosity reading. The viscosity, at 16.928 Centistokes (cSt), is extremely high, meaning this fuel will not flow easily. This will affect the fuel delivery and operation of advance units and hydraulic governor mechanisms including excessive pressure on the fuel injection system.

Sample D is another light fuel. If this fuel was used in a truck engine, it would probably experience a low power complaint. The 90 percent end point is 42°C, below the As3570 maximum which also indicates that this fuel is light. Lubrication of the fuel system components may be a problem with this fuel.

From the information gathered in these samples, enough data was collected to accurately assess each of the fuels, except for sample C, where the distillation data was not available.

Putting the fuels to the test, a Caterpillar 3406C rated at 460 hp was chosen as the test engine. It was felt that engines of this generation would give a result relevant to both older mechanical styles and the electronic engines used today.

Two fuel samples were run through the engine. The first (sample D) was chosen as it had the lowest density reading. The second sample (sample A) came from a vehicle that operated to both customer and manufacturer`s expectations.

A brief look at the chart plotted from the dynamometer results indicates a big difference between the two samples. At around 1200 rpm there`s a difference of 22 hp. The difference increases to 35 hp at 1800 rpm. Therefore, the biggest drop in horsepower occurs in the engine`s working rev range from 1500 rpm to 1900 rpm.

What does this mean in real terms, and how much does it cost?

As far as performance goes, initially the vehicle may be down half a gear here and a gear there, while at the same time using a little more fuel. However, as time goes on, continued use of this type of fuel may cause accelerated wear to the injector pump and injectors. As the injectors continue to wear, the injector may dribble fuel on the piston causing an over fuel situation. Or alternatively, the spray pattern may be affected. This can cause fuel to be sprayed onto the cylinder wall, washing away the lubrication oil. Fuel may also be sprayed onto the outer edge of the piston, which is not designed to take the full load of the air/fuel mixture as it ignites. Spillage in this area may result in cracks forming on the piston top.

Meantime, in most instances, the operator is unaware of the extent of the internal damage until such time as the engine is pulled down. Needless to say, repairing an engine when it gets to this condition is very expensive.

The chart indicates that at normal cruising revs of 1700 rpm, with sample A the engine produced 458 hp. In the same rev range, sample D produce 423 hp a 35 hp drop, or a 7.64 percent power reduction.

To analyse the costs, use the example of a truck that runs between Melbourne and Sydney five nights a week, a distance of 868 km. Add on 132 kays for running around, doing pick-ups and deliveries, and that`s 1000 kays per day. It averages 2km/L and takes fuel from sample A which costs $0.76 per litre.

The total bowser costs at the end of every day will be $380 for 500 litres of fuel. The weekly fuel cost will be $1900 for 2500 litres. Taking into consideration that this truck will have a few nights off during the year for various reasons and assuming it only runs 48 weeks in the year, then the total fuel bill will be $91,200 for 120,000 litres of fuel.

A second truck does the same run in a vehicle of the same spec. Only it uses sample D, because it’s cheaper. Given that the chart showed a 7.64 percent loss of power when using sample D, we can assume fuel consumption will be slightly greater. This is due to the lesser density in sample D when compared with sample A, producing a lesser amount of energy per unit volume.

False Economy. A reduction of 7.64 percent from 2km/L equates to 1.85km/L. While fuel economy is worse, the bowser price of sample D is cheaper at $0.66 per litre. So it uses roughly 540 litres a day on the same run and the bowser cost will be $356.40 per day. The weekly fuel bill will be $1782 for 2700 litres. At the end of 48 weeks it will have used 9,600 litres more fuel with a total of 129,600 litres, however the total fuel bill is $8,040 less at $83,160.

While these figures can only be used as a theoretical example, given the wide parameters of operating conditions which all play a part in engine fuel consumption, they do give an indication of the amount of savings at the fuel bowser.

It is up to the individual operator to ascertain if the bowser savings outweigh the accelerated engine wear and additional fuel filters that may be required when using fuel that doesn`t meet the Australian standard AS3570.

Another point worth considering, especially with new engines still under warranty, is that use of fuel and oil that do not meet the manufacturer`s recommendations may void warranty if it is found that poor quality fuel or oil contributed to premature engine failure.



April 17, 2014 | Posted in: Tech Tips

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