Chapter 17 Diesel Fuel Characteristics

Copyright © 2009 Delmar, Cengage Learning

Medium/Heavy Duty Truck Engines, Fuel & Computerized

Management Systems, 3E

Chapter 17

Diesel Fuel Characteristics


Introduction

Diesel engines will burn just about any fuel introduced into the engine cylinders

To maximize engine performance & service life, only fuels meeting the highest standards are used

Diesel fuel is a petroleum based product What is described as “diesel fuel” today will likely

change a decade from now Although in North America, diesel power is primarily

confined to the commercial sector, 50% of the automobiles in Europe use diesel power

Presenter

Presentation Notes

There is some increase in the use of biodiesel but this is mainly confined to low percentage cuts into petroleum based diesel fuel.


Fuel Terminology

AFR … air-to-fuel ratio, actual ratio of air to fuel in a combustion reaction

Ash … suspended solids or soluble metallic compounds in diesel fuels

ASTM … American Society for Testing & Materials Organization, classifies fuels to a standard

ASTM #1D Fuel … recommended for engines required to operate under variable loads & speeds

ASTM #2D Fuel … recommended for engines required to operate under constant loads & speeds


Fuel Terminology

Boil Point … temperature at which a liquid vaporizes Calorific Value …potential heat energy measured in

British Thermal Units (BTUs) 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 … temperature at which normal paraffins

in fuel become less soluble & begin to precipitate as wax crystals


Fuel Terminology

Crude Oil … raw petroleum Diesel Fuel … term to describe distillate petroleum

compounds formulated for use in on-highway compression ignition (CI) engines

Distillate … alternate name for diesel fuel Fire Point … temperature at which a liquid HC fuel

evaporates sufficient flammable vapor to burn continuously in air

Flame Front … forward boundary of the reacting zone in-cylinder combustion


Fuel Terminology

Flame Propagation … the way in which a fuel combusts inside the engine cylinder as determined by the manner the flame front spreads

Flash Point … temperature at which a liquid HC fuel evaporates sufficient flammable vapor to momentarily ignite when a flame is brought to the surface

Fractions … portion of a mixture separated by distillation or a cracking procedure


Fuel Terminology

Fuel-Air Ratio … used to describe ratio of fuel to air in diesel combustion

Gasoline … group of liquid petroleum fuels for use in spark ignited (SI) engines

Kerosene … widely used as heating oil & jet fuel

Microorganism Growth … airborne bacteria & fungi entering vehicle & storage tanks through venting systems


Fuel Terminology

Natural Gas (also liquefied natural gas: LPG & compressed natural gas: CNG) … gaseous product of petroleum either suspended above liquid crude oil or dissolved in it

Octane Rating … measure of the antiknock quality of a fuel (usually gasoline)

Oxidation Stability … products of oxidizing stored diesel fuel can result in deposits, filter plugging & lacquering of fuel injection equipment


Fuel Terminology

Photochemical Smog … reaction of HCs & NOxs with sunlight in the lower atmosphere

Pour Point … lowest temperature in which fuel can be pumped

Specific Gravity … the weight of volume of a liquid compared to the weight of the same volume of water

Stoichiometric Ratio … the exact ratio of reactants required for a chemical reaction to take place


Fuel Terminology

Sulfur Content … maximum sulfur amount in fuel as recommended by American Petroleum Institute (API) & Environmental Protection Agency (EPA)

Viscosity … measure of a liquid’s resistance to shear, a value that generally decreases as temperature increases

Volatility … the tendency of a liquid to vaporize


Petroleum

HC fossil fuels are found in the upper strata of the earth’s crust

70% of energy consumed in North America & 40% worldwide is derived from petroleum products

An unrenewable energy source with limited reserves


Crude Oil

Three types:

Asphalt-based

Paraffin-based

Mixed-base

Crude petroleum must be refined to separate the fractions used to formulate fuels identified as diesel fuel, gasoline, kerosene & heavy furnace oil


Refining Petroleum

Fractions separated Fractions graded & extracted by cut points Gas taken off the top is natural gas In descending order of boil: Gasoline Naptha Kerosene Light gas oil Heavy gas oil

Liquid remaining at bottom known as residuum


Thermal & Catalytic Cracking

Cracking is a process by which heavier fuel oils may be chemically modified by dividing heavy molecules into smaller light molecules

Thermal cracking subjects crude to high temperatures & pressures in a cylindrical tower

Catalytic cracking is a more efficient method of accomplishing same objective

Presenter

Presentation Notes

Crudes from various regions naturally yield various percentages of products from refining. Today, this natural extraction may not represent market demand. Cracking allows the crude to be refined more in accordance with the products required for the market.


Thermal & Catalytic Cracking

Hydrocracking is a catalytic cracking process undertaken in the presence of hydrogen, used to produce higher volatility, lighter fractions required in fuels



Cetane Number (CN) … measure of the ignition quality

Increasing CN reduces fuel density & therefore fuel mileage

As CN increases, ignition temperature decreases

Ignition Accelerators … fuel additives that increase the CN value

Cyclohexanol nitrate will raise CN by 7 points

Presenter

Presentation Notes

Ignition accelerators do not improve fuel; they merely lower ignition temperature & thereby reduce ignition lag. They should only be used after analysis of fuel by fuel supplier & then only a prescribed. Excessive quantities of ignition accelerator in fuel have the effect of advancing ignition timing which can cause engine damage.



Heating Value (Calorific Value)

Heat energy released during combustion

Value used to reckon the thermal efficiency of engine’s ability to produce power



API Gravity is a measure of how the weight of a petroleum liquid compares with water

Measured with a hydrometer

Understanding API gravity helps troubleshoot persistent low-power & marginal emissions test failures



Ultra-Low Sulfur Fuels

Sulfur dioxide produced from combustion of diesel fuel is recognized as environmentally hazardous

Sulfur content of diesel fuels progressively legislated to lower levels

Off-Highway Fuels

Bunker fuels cut with glycol & other antigel compounds similar to jet fuel


Fuel Storage, Deterioration & Performance

Fuel chemistry is adjusted seasonally by the refiner/supplier due to temperature variables

Storage tanks may contain water, drain the water periodically, helping to prevent water buildup

Water in fuel presents two main problems: If it collects in crossover pipe, becomes vulnerable to

freeze-up It promotes microorganism growth. Metabolic waste

is acidic & has a corrosive effect on fuel system components. More common in systems that run only a couple of times a year

Presenter

Presentation Notes

It’s a good practice to keep fuel tanks full. This keeps moisture-laden air out of the tanks and reduces water-in-fuel problems.


Fuel Storage, Deterioration & Performance

Climate Influence

When fuel is stored & exposed to high temperatures, it’s more volatile fractions are boiled off, lowering the CN

Fuel heaters sometimes used to help compensate for deficiencies in cloud point, pour point & viscosity ratings in colder climates


Fuel Conditioners

Most OEMs disapprove of their use

Vague mixture of cetane improvers, cleansing additives & pour point depressants in an alcohol base

Generally create more problems than they solve, however there are occasions when they have to be used

Use only when no other option available

Always follow manufacturer’s recommendations


Fuel Conditioners

Fuel conditioners are not normally recommended by OEMs

If required to use them, the technician should understand their functionality


Costs of Operating a Truck

Fuel represents major expense in operating a truck

The diesel fuel of the future?

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Chapter 17 Diesel Fuel Characteristics