Gas Oil and Driving Performance
Gas Oil and Driving Performance
Several operating characteristics influence engine performance, and their relative importance depends on engine type and duty cycle (for example, truck, passenger car, stationary generator, marine vessel, etc.). These characteristics are:
- Starting ease
- Low noise
- Low wear (high lubricity)
- Long filter life (stability and fuel cleanliness)
- Sufficient power
- Good fuel economy
- Low temperature operability
- Low emissions
Engine design has the greatest impact on most of these characteristics. However, because the focus of this publication is fuel, this chapter discusses how these characteristics are affected by fuel properties.
Leaks and heat loss reduce the pressure and temperature of the fuel/air mixture at the end of the compression stroke. Thus, a cold diesel engine is more difficult to start and the mixture more difficult to ignite when compared to a hot diesel engine. Engines are equipped with start-assist systems that increase the air temperature to aid ignition. These controls in the diesel engine can also decrease starting engine noise, white smoke, and cranking time.
Gas Oil that readily burns, or has good ignition quality, improves cold start performance. The cetane number of the fuel defines its ignition quality. It is believed that fuels meeting the ASTM D 975 Standard Specification for Gas Oil Oils minimum cetane number requirement of 40 provide adequate performance in modern diesel engines.
The minimum cetane number in Europe is 51. Some researchers claim that a number of modern engines can benefit from a higher cetane number when starting in very cold climates.
Smoothness of operation, misfire, smoke emissions, noise, and ease of starting are all dependent on the ignition quality of the fuel. At temperatures below freezing, starting aids may be necessary regardless of the cetane number.
Power is determined by the engine design. Diesel engines are rated at the brake horsepower developed at the smoke limit. For a given engine, varying fuel properties within the ASTM D 975 specification range does not alter power significantly.
However, fuel viscosity outside of the ASTM D 975 specification range causes poor atomization, leading to poor combustion, which leads to loss of power and fuel economy.
In one study, for example, seven fuels with varying distillation profiles and aromatics contents were tested in three engines. In each engine, power at peak torque and at rated speed (at full load) for the seven fuels was relatively constant.
The noise produced by a diesel engine is a combination of combustion and mechanical noise. Fuel properties can affect combustion noise directly.
In a diesel engine, fuel ignites spontaneously shortly after injection begins. During this delay, the fuel is vaporizing and mixing with the air in the combustion chamber. Combustion causes a rapid heat release and a rapid rise of combustion chamber pressure. The rapid rise in pressure is responsible for the knock that is very audible in some diesel engines.
By increasing the cetane number of the fuel, the knock intensity is decreased by the shortened ignition delay. Fuels with high cetane numbers ignite before most of the fuel is injected into the combustion chamber. The rates of heat release and pressure rise are then controlled primarily by the rate of injection and fuel-air mixing, and smoother engine operation results. A recent development is the common rail electronic fuel injection system. The use of a common rail allows engine manufacturers to reduce exhaust emissions and, especially, to lower engine noise.
Here again, engine design is more important than fuel properties. However, for a given engine used for a particular duty, fuel economy is related to the heating value of the fuel. In North America fuel economy is customarily expressed as output per unit volume, e.g., miles per gallon. The fuel economy standard in other parts of the world is expressed as volume used per unit distance – liters per 100 kilometers. Therefore, the relevant units for heating value are heat per volume (British thermal unit [Btu] per gallon or kilojoules per liter/cubic meter). Heating value per volume is directly proportional to density when other fuel properties are unchanged. Each degree increase in American Petroleum Industry (API) gravity (0.0054 specific gravity decrease) equates to approximately two percent decrease in fuel energy content.
ASTM International specifications limit how much the heating value of a particular fuel can be increased. Increasing density involves changing the fuel’s chemistry – by increasing aromatics content – or changing its distillation profile by raising the initial boiling point, end point, or both. Increasing aromatics is limited by the cetane number requirement (aromatics have lower cetane numbers), and changing the distillation profile is limited by the 90 percent distillation temperature requirement. The API gravity at 60°F (15.6°C) for No. 2 Gas Oil is between 30 and 42. The specific gravity, at 60/60°F, and the density, at 15.6°C, are between 0.88 and 0.82.
Combustion catalysts may be the most vigorously promoted Gas Oil aftermarket additive. However, the Southwest Research Institute, under the auspices of the U.S. Transportation Research Board, ran back-to-back tests of fuels with and without a variety of combustion catalysts. These tests showed that a catalyst usually made “almost no change in either fuel economy or exhaust soot levels.” While some combustion catalysts can reduce emissions, it is not surprising that they do not have a measurable impact on fuel economy. To be effective in improving fuel economy, a catalyst must cause the engine to burn fuel more completely. However, there is not much room for improvement. With unadditized fuel, diesel engine combustion efficiency is typically greater than 98 percent. Many ongoing design improvements to reduce emissions may have some potential for improving fuel economy. However, several modern emissions control strategies clearly reduce fuel economy, sometimes up to several percent.
Some moving parts of Gas Oil pumps and injectors are protected from wear by the fuel. To avoid excessive wear, the fuel must have some minimum level of lubricity. Lubricity is the ability to reduce friction between solid surfaces in relative motion. The lubrication mechanism is a combination of hydrodynamic lubrication and boundary lubrication. In hydrodynamic lubrication, a layer of liquid prevents contact between the opposing surfaces. For Gas Oil pumps and injectors, the liquid is the fuel itself and viscosity is the key fuel property. Fuels with higher viscosities will provide better hydrodynamic lubrication. Gas Oils with viscosities within the ASTM D 975 specification range provide adequate hydrodynamic lubrication.
Boundary lubrication becomes important when high load and/or low speed have squeezed out much of the liquid that provides hydrodynamic lubrication, leaving small areas of the opposing surfaces in contact. Boundary lubricants are compounds that form a protective anti-wear layer by adhering to the solid surfaces.
The less-processed Gas Oils of the past were good boundary lubricants. This was not caused by the hydrocarbons that constitute the bulk of the fuel, but was attributed to trace amounts of oxygen- and nitrogen-containing compounds and certain classes of aromatic compounds. Evidence for the role of trace quantities is the fact that the lubricity of a fuel can be restored with the addition of as little as 10 parts per million (ppm) of an additive.
Lubricity enhancing compounds are naturally present in Gas Oil derived from petroleum crude by distillation. They can be altered or changed by hydro-treating, the process used to reduce sulfur and aromatic contents. However, lowering sulfur or aromatics, per se, does not necessarily lower fuel lubricity.
The use of fuels with poor lubricity can increase fuel pump and injector wear and, at the extreme, cause catastrophic failure. Such failures occurred in Sweden in 1991 when two classes of “city” diesel (with very low sulfur and aromatics contents) were mandated.
Heavy hydro-treating was needed to make these fuels. The problem was solved by treating the fuel with a lubricity additive. As regions regulate lower sulfur levels, mostly accomplished with more severe hydro-treating, the general trend is lower levels of lubricity in unfinished, unadditized fuels. The additized finished fuel in the market, however, should have adequate lubricity because of the fuel specifications in place.
Various laboratory test methods exist to determine fuel lubricity. One method widely used is the high frequency reciprocating rig (HFRR). Many regions of the world have fuel specifications based on this test method.
Inadequate lubricity is not the only cause of wear in diesel engine fuel systems. Gas Oil can cause abrasive wear of the fuel system and the piston rings if it is contaminated with abrasive inorganic particles. Fuel injectors and fuel injection pumps are particularly susceptible to wear because the high liquid pressures they generate require extremely close tolerances between parts moving relative to one another.
ASTM D 975 limits the ash content of most Gas Oils to a maximum of 100 ppm. (Inorganic particles and oil-soluble, metallo-organic compounds both contribute to the ash content; but, only inorganic particles will cause wear.) The U.S. government has a tighter specification of 10 mg/L (approximately 12 ppm) for all particulate matter. However, neither specification addresses particle size. While most fuel filters recommended by engine manufacturers have a nominal pore size of 10 microns, studies by the Southwest Research Institute reveal that the critical particle size for initiating significant abrasive wear in rotary injection fuel pumps and in high-pressure fuel injection systems is from six to seven microns.
However, as engine designs to reduce emissions result in higher fuel rail and injector pressures, the tighter clearances will have less tolerance for solids and impurities in the fuel.
Consequently, some engine manufacturers are now specifying filters with pore size as low as two microns.
Organic acids in Gas Oil can also cause corrosive wear of the fuel system. While this may be a significant wear mechanism for high sulfur diesel, it is less significant for low sulfur diesel because hydro-treating to reduce sulfur also destroys organic acids. With the introduction of bioGas Oil, there is some indication that organic acids could potentially increase.
Low temperature operability is an issue with middle distillate fuels because they contain straight and branched chain hydrocarbons (paraffin waxes) that become solid at ambient winter temperatures in colder geographic areas. Wax formation can also be exacerbated by blends of biodiesel with conventional Gas Oil. Wax may plug the fuel filter or completely gel the fuel, making it impossible for the fuel system to deliver fuel to the engine.
Engine design changes to address this problem include locating the fuel pump and filter where they will receive the most heat from the engine. The practice of pumping more fuel to the injectors than the engine requires is also beneficial because the warmed excess fuel is circulated back to the tank. While the primary purpose of this recycle is to cool the injectors, it also heats the fuel in the fuel tank.
Sometimes operators may allow diesel equipment to idle in cold weather rather than turning the engine off when it is not in use. This practice is no longer allowed in certain regions. In some cases, the cost of the fuel may be less than the cost of winterizing the engine; vehicles designed for low-temperature operation are usually equipped with heated fuel tanks, insulated fuel lines, and heated fuel filters.
In a refinery, there are a number of approaches to improve a fuel’s low-temperature operability, such as:
- Manufacture it from less waxy crudes.
- Manufacture it to a lower distillation end point. (This excludes higher boiling waxy components with higher melting points.)
- Dilute it with a fuel with lower wax content (No. 1-D Gas Oil or kerosene).
- Treat it with a low-temperature operability additive.
After the fuel is in the distribution system, dilution with No. 1 diesel is the most practical way to improve low-temperature performance. Additives are used to improve low temperature filterability and lower the pour point. When they work, additives have several advantages over dilution: they are readily available in most areas of the world, treatment cost is less, and the treatment does not lower fuel density (thus heating value and fuel economy are not affected).
Low-temperature operability issues are also discussed on page 56. The tests to characterize a fuel’s low-temperature operability (cloud point [ASTM D 2500], pour point [ASTM D 97], cold filter plugging point [ASTM D 6371], and low-temperature flow test [ASTM D 4539]) are discussed on page 65.
FUEL STABILITY – FILTER LIFE
Unstable Gas Oils can form soluble gums or insoluble organic particulates. Both gums and particulates may contribute to injector deposits, and particulates can clog fuel filters.
The formation of gums and particulates may occur gradually during long-term storage or quickly during fuel system recirculation caused by fuel heating.
Storage stability of Gas Oil has been studied extensively because of governmental and military interest in fuel reserves. However, long-term (at ambient temperatures) storage stability is of little concern to the average user, because most Gas Oil is consumed within a few weeks of manufacture. Thermal (high-temperature) stability, on the other hand, is a necessary requirement for Gas Oil to function effectively as a heat transfer fluid. Thermal stability may become more important because diesel engine manufacturers expect future injector designs to employ higher pressures to achieve better combustion and lower emissions. The change will subject the fuel to higher temperatures and/or longer injector residence times.
Low sulfur Gas Oils tend to be more stable than their high sulfur predecessors because hydro-treating to remove sulfur also tends to destroy the precursors of insoluble organic particulates. However, hydro-treating also tends to destroy naturally occurring antioxidants. It may be necessary for the refiner to treat some low sulfur Gas Oils with a stabilizer to prevent the formation of peroxides that are the precursors of soluble gums.
The fuel system of a diesel engine is designed and calibrated so that it does not inject more fuel than the engine can consume completely through combustion. If an excess of fuel exists, the engine will be unable to consume it completely, and incomplete combustion will produce black smoke. The point at which smoke production begins is known as the smoke limit. Most countries set standards for exhaust smoke from high-speed, heavy-duty engines. In the U.S., the opacity of smoke may not exceed 20 percent during engine acceleration mode or 15 percent during engine lugging mode under specified test conditions.
Smoke that appears after engine warm-up is an indication of maintenance or adjustment problems. A restricted air filter may limit the amount of air, or a worn injector may introduce too much fuel. Other causes may be miscalibrated fuel pumps or maladjusted injection timing. Changes made to fuel pump calibration and injection timing to increase the power of an engine can lead to increased emissions.
Because smoke is an indication of mechanical problems, California and other states have programs to test the exhaust opacity of on-road heavy-duty trucks under maximum engine speed conditions (i.e., snap idle test). Owners of trucks that fail the test are required to demonstrate that they have made repairs to correct the problem. There are also smoke regulations for ships in port.
Variation of most fuel properties within the normal ranges will not lead to the high level of particulate matter (PM) represented by smoking. The exception is cetane number; fuel with a very high cetane number can cause smoking in some engines. The short ignition delay causes most of the fuel to be burned in the diffusion-controlled phase of combustion (see page 78), which can lead to higher PM emissions.
Fuel can indirectly lead to smoking by degrading injector performance over time, when:
- Gums in the fuel are deposited on the injectors, causing sticking, which interferes with fuel metering.
- Petroleum resid or inorganic salts in the fuel result in injector tip deposits that prevent the injector from creating the desired fuel spray pattern. (Some low-speed, large diesel engines are designed to burn fuel containing large amounts of petroleum resid. These are typically used in marine and power generation applications.)
- Abrasive contaminants or organic acids in the fuel, or inadequate fuel lubricity cause excessive abrasive or corrosive injector wear.