OBJECTIVES • Define the terms used to describe diesel fuel. • Describe how the cetane number of a diesel fuel is determined. • Outline the minimum requirements of a highway diesel fuel. • State what constitutes low- and ultra-low sulfur (ULS) diesel fuel. • Identify the consequences of running DPF-equipped engines with anything but ULS. • Calculate how much ignition accelerator is required to restore original CN value.
OBJECTIVES (Cont.) • Determine the calorific or heating value of a fuel. • Understand some of the problems associated with storing fuel. • Identify degraded diesel fuel. • Explain the effects of contaminated or degraded fuel on a typical fuel subsystem. • Explain how cloud point and pour point specs affect cold weather engine performance. • Outline the constituents of a typical aftermarket diesel fuel conditioner.
INTRODUCTION • In a couple of earlier chapters, we suggested that diesel engines will happily burn just about any fuel introduced into the engine cylinders and this is true. • For sure, an engine oil can fuel an engine to a runaway destruction in just seconds.
FUEL TERMINOLOGY • Before studying some basic fuel chemistry and what happens in the cylinder combustion process, it helps to understand some key terms. • This chapter uses the building blocks of the combustion chemistry introduced in the previous chapter, and it uses some of those terms that apply to the characteristics of diesel fuel.
FUEL TERMINOLOGY (Cont.) • AFR: • air-to-fuel ratio. • Ash: • Diesel fuels normally contain a certain quantity of suspended solids or soluble metallic compounds such as sodium and vanadium. • ASTM: • American Society for Testing and Materials. • ASTM #1D Fuel: • Fuel recommended for use in high-speed, on-highway diesel engines required to operate under variable load and variable speeds. • ASTM #2D Fuel: • Fuel recommended for use in high-speed, on-highway diesel engines required to operate under constant loads and speeds.
FUEL TERMINOLOGY (Cont.) • Boil point: • The temperature at which a liquid vaporizes. • Calorific value: • Heat energy. • Catalyst: • A substance that enables a chemical reaction without itself undergoing any change. • Cetane number (CN): • A measure of the ignition quality of a diesel fuel. • Cloud point: • The temperature at which the normal paraffins in a fuel become less soluble and begin to precipitate as wax crystals. • Compressed natural gas • Crude oil: • Raw petroleum. • (CNG): • See Natural gas.
FUEL TERMINOLOGY (Cont.) • Diesel fuel: • Term used to describe distillate petroleum compounds and fractions formulated for use in on-highway compression ignition (CI) engines. • Distillate: • Term sometimes used to describe diesel fuel formulated for on-highway use. • Fire point: • The temperature at which a liquid HC fuel evaporates sufficient flammable vapor to burn continuously in air. • Flame front: • The forward boundary of the reacting zone in cylinder combustion.
FUEL TERMINOLOGY (Cont.) • Flame propagation: • The way in which a fuel combusts inside the engine cylinder as determined by the manner the flame front spreads. • Flash point: • The temperature at which a liquid HC fuel evaporates sufficient flammable vapor to ignite momentarily when a flame is brought near its surface. • Fractions: • A portion of a mixture separated by distillation or a cracking procedure such as hydrocracking or catalytic cracking. • Fuel-air ratio: • Commonly used to describe the ratio of fuel to air in diesel combustion. • Gasoline: • The group of liquid petroleum fuels blended for use in spark ignited (SI) engines.
FUEL TERMINOLOGY (Cont.) • Kerosene: • Made up of heavier fractions than gasoline, kerosene is widely used in heating oil and jet fuel. • Liquefied natural gas (LPG): • See Natural gas. • Microorganism growth: • Airborne bacteria and fungi commonly enter vehicle and storage tanks through their venting systems. • Natural gas: • The gaseous product of petroleum either suspended above liquid crude oil or dissolved in it. • Octane rating: • A measure of the antiknock quality of a fuel, usually a gasoline. • Oxidation stability: • The products of oxidizing stored diesel fuel can result in deposits, filter plugging, and lacquering of fuel injection equipment.
FUEL TERMINOLOGY (Cont.) • Photochemical smog: • Results from the photochemical reaction of HCs and oxides of nitrogen (NOx) with sunlight in the lower atmosphere. • Pour point: • As fuel temperature drops below the cloud point, paraffin wax crystals increase in size, and the pour point generally denotes the lowest temperature at which the fuel can be pumped. • Specific gravity: • The specific gravity of a liquid is the weight of a volume of the liquid compared to the weight of the same volume of water. • Stoichiometric ratio: • The stoichiometric ratio is an expression of the exact ratio of the reactants required for a chemical reaction to take place.
FUEL TERMINOLOGY (Cont.) • Viscosity: • A measure of a liquid’s resistance to Shear. • Volatility: • The tendency of a liquid to vaporize.
PETROLEUM • The word petroleum is derived from the Latin words “petra,” meaning rock, and “oleum,” meaning oil. • Somewhere around 70% of the energy consumed in North America and 40% worldwide is derived from petroleum products. • The origin of the carbon and hydrogen that are the elemental components of petroleum is in the organic materials that made up the primordial (existing at the beginning) Earth. • The theory is that these elemental components passed through an organic phase, usually single-celled plants and algae, and mostly located in aquatic environments. • Note: there are alternate theories that the Earth produces oil by other (non-biological) means which seem to have some validity.
Crude Oil • Crude oils are loosely classified by their content, which indicates exactly what can be extracted from them in refining processes. • There are three types of crude oils: • Asphalt-based crudes • Paraffin-based crudes • Mixed-base crudes
REFINING PETROLEUM • The refining of crude petroleum begins with two processes: vaporization and condensation. • The gas taken off the top of the distilling column (or tower) is natural gas and, in descending order of boil temperature: gasoline, naphtha, kerosene, light gas oil, and heavy gas oil. • Refining crude yields oils, greases, gasoline, diesel fuel, and other compounds. • Motor oils & fluids – 5% • Gasoline – 43% • Kerosene/Fuel Oil – 24% • Other Products – 28%
THERMAL AND CATALYTIC CRACKING • Most highway fuels are a “brew” of fractions carefully blended by the refiners. • Cracking describes processes by which heavier fuel oils may be chemically modified by dividing their heavy molecules into smaller, light molecules.
THERMAL AND CATALYTIC CRACKING • Hydrocracking is a catalytic cracking process undertaken in the presence of hydrogen. • Before fuels are blended, their composite fractions must be purified. This generally refers to the removal of salt, sulfur, and water impurities.
DIESEL FUEL CHARACTERISTICS • Because we have become accustomed to readily available, good quality, uncontaminated highway fuels in North America, we are often slow to attribute a fuel system or engine problem to the actual fuel being used. • Fuel filtration is especially important before testing an engine on a dynamometer or running fuel economy profiles.
CETANE NUMBER • Cetane number (CN) is a measure of the ignition quality of a diesel fuel. • As the CN of a diesel fuel increases, its ignition temperature decreases.
IGNITION ACCELERATORS • Ignition accelerators (a.k.a. cetane improvers) are fuel additives that increase the CN value of a fuel. • Engine performance factors that are influenced by the ignition quality of the fuel are cold starting, engine warm-up, combustion roughness, acceleration, and exhaust smoke density.
TECH TIP • Ignition accelerators do not improve fuel; they merely lower the ignition temperature and thereby reduce ignition lag. • Ignition accelerators should only be used after analysis of fuel by the fuel supplier and then only exactly as prescribed. • Excessive quantities of ignition accelerator in fuel have the effect of advancing ignition timing, which can cause engine damage.
HEATING VALUE (CALORIFIC VALUE) • An important property of a diesel fuel is the heat energy it releases during combustion. • Heating values may be expressed as: • J/kg joules per kilogram • J/L joules per liter
HEATING VALUE(CALORIFIC VALUE) (Cont.) • cal/g calories per gram • cal/L calories per liter • Btu/lb Btu per pound • Btu/gal. Btu per gallon
API GRAVITY • American Petroleum Institute gravity, usually known as API gravity, is a measure of how the weight of a petroleum liquid compares with the weight of water.
ULTRA-LOW SULFUR FUELS • Sulfur is present in most crude petroleum, and it is more prominent in the heavier residual fractions from the refining process. • Today, ultra-low sulfur (ULS) fuel is classified as diesel fuel containing 0.0015% sulfur or less.
ULS Introduction Schedule • It is a federal requirement that from December 2010, ULS standard fuel is universal for all on-highway diesel-powered vehicles. The following is also true: • ULS became mandatory in all off-highway fuel retail outlets from December 2010. • Marine and locomotive industries began transitioning to ULS fuel in 2009. The transition schedule is due for completion in 2014.
ULS Introduction Schedule • Initiatives to reduce sulfur content in bunker grade oils used in large marine and generating stations are currently being worked on. • By 2014, ULS will be the only fuel that refiners will be allowed to dispense to retail outlets.
CAUTION • ULS must be used in all diesel engines equipped with a diesel particulate filter (DPF). • Running the pre-2007 spec LS fuel in a post-2007 engine can destroy a DPF in as little as 1 hour of operation. • Running ULS in legacy engines (pre-2007) neither increases nor decreases engine performance.
FUEL STORAGE, FUEL DETERIORATION, AND PERFORMANCE • The fuel chemistry of both gasolines and diesel fuels is adjusted seasonally by the refiner/supplier largely because what is required of a fuel varies with temperature. • Whenever fuel is drawn out of a tank, it is replaced by air.
FUEL STORAGE, FUEL DETERIORATION, AND PERFORMANCE (Cont.) • Replacement air always contains some percentage of moisture and, in areas of high humidity such as the Great Lakes region, this can be extreme. • When temperatures cool, vaporized moisture in the tank condenses to water, which because of its greater weight settles at the bottom of the fuel tank.
WATER IN FUEL • Water presents two main problems while it is actually in a fuel tank. • However, it presents a host of other problems should it actually be drawn out and pumped through a fuel system.
MICROORGANISM GROWTH • Another problem associated with water in tanks is microorganism growth. • Airborne source bacteria reside in the tank sump water and feed off the HCs in the fuel, causing a certain amount of degradation of the fuel.
TECH TIP • It is good practice to keep fuel tanks full. • This keeps moisture-laden air out of the tanks and reduces water-in-fuel problems. • Fill truck fuel tanks before parking overnight.
INFLUENCE OF CLIMATE • Hot weather presents other problems. • The year-round average CN value of 1D and 2D ASTM fuels sold in North America is around 47. • The legal minimum for both fuels is 40.
Cloud Point and Pour Point • Cloud point and pour point are important diesel fuel characteristics in frigid temperature conditions. • The cloud point value indicates the temperature at which filters can become plugged. • Pour point is the lowest temperature at which a fuel can be pumped through the fuel system.
FUEL CONDITIONERS • There are no Society of Automotive Engineers (SAE)/ ASTM standards that apply to diesel fuel conditioners. • While most engine OEMs disapprove of their use, they are commonly sold in their dealerships, a measure of the profitability of the product rather than its worth.
FUEL CONDITIONERS (Cont.) • Diesel fuel conditioner should always be used according to the engine manufacturer’s recommendations rather than those of the additive manufacturer.
COSTS OF OPERATING A TRUCK • The major expense in truck-operating costs is fuel. • The increase in fuel prices since 2002 has shifted business away from brokers and small operations into the major fleets. • Here’s a pie chart that represents the operating costs of a truck in 2005 with fuel costs representing 75%.
FUTURE FUEL SOURCE? • We can summarize this chapter with a reminder that a diesel engine can run on non-petroleum based fuels, including one that flows as sap in trees.
Fuel Efficiency Versus Fuel Economy • Using the term fuel economy in commercial trucking is so misleading, it should be avoided. • Fuel economy relates to miles per gallon and says nothing about what is being done with the truck to produce the data.
SUMMARY • Crude petroleum is the basis of diesel fuel and many other HC fuels. • Petroleum is (theoretically) a nonrenewable fossil fuel with limited reserves. • Crude petroleum must be refined to separate the fractions used to formulate fuels identified as diesel fuel, gasoline, kerosene, and heavy furnace oil. • Several different cracking processes may be used to obtain the lighter fractions constituent in diesel fuels and gasolines. • Diesel fuels and gasolines are a brew of many different petroleum fractions.
SUMMARY (Cont.) • Gasolines are usually more chemically complex brews than are diesel fuels. • The ignition quality of a diesel fuel is rated by its CN. • A CN number can be correlated to an actual ignition temperature. • Minimum CN values for on-highway 1D and 2D diesel fuels are legislated in North America at 40. • There is a correlation between fuel density and its heating value.