Diesel Fuel Subsystems & Diesel Fuel Characteristics – Study Notes

Diesel Fuel Subsystems & Diesel Fuel Characteristics – Study Notes

Fuel Subsystems – Definition, purpose, and structure

  • Definition: the group of components responsible for fuel storage and its transfer to the injection pumping apparatus.
    • Source: Fuel Subsystems – Chapter 19, page 340.
  • Objectives of the fuel subsystems:
    • Store fuel in tanks until required.
    • Remove moisture from fuel.
    • Filter fuel to remove abrasives.
    • Deliver fuel to injection components at correct temperature.
  • Typical layout:
    • Transfer system divided into primary (suction) circuit and secondary (charge) circuit.
    • Circuits separated by the transfer pump.
    • Some transfer pumps are located in the fuel tank; some systems (e.g., Cummins HPI-TP) are entirely under suction.
    • The secondary filter traps smaller-sized particles than the primary filter.
  • Significance:
    • Ensures reliable fuel supply, protects injectors, maintains fuel temperature, and reduces contaminants.
  • References to visuals:
    • Fig 19-1 (p. 341)
    • CAT C7 & C9 engines illustrated.

Fuel Tanks

  • Tank materials: aluminum, steel, plastic.
  • Functions:
    • Store and transport fuel.
    • Act as a heat exchanger to cool excess fuel.
    • Excess fuel used for injector cooling, injector lubrication, ECM cooling.
    • Aerodynamic considerations (on-highway).
  • Maintenance and components:
    • Inspection of caps and vents (atmosphere, breather filters).
    • Pickup tubes.
    • Fuel level sending units.
  • Visual references:
    • Fig 19-3, pg 343: Dual tanks.

Filters

  • Types and roles:
    • Primary filter: nominal filtration.
    • Secondary filter: finer filtration under charge pressure.
  • Micron ratings (examples):
    • Nominal rating: $40\,\mu\text{m}$.
    • Absolute rating: $10\,\mu\text{m}$.
  • Water management:
    • Water separators (WDS) and Water-in-Fuel (WIF) sensors exist to detect water in fuel.
    • WIF sensors are referenced (p. 351).
  • Heating and priming:
    • Fuel heaters for cold climates.
    • Hand primers used to purge air from the system.
  • Maintenance notes:
    • Secondary filters should be primed after installation (not pre-filled).
    • Common source of air leaks is the double-gasket of a filter under suction.
    • Inlet restriction values are specified by the manufacturer and should be checked; typically tested on the suction side of the system.
  • Figures:
    • Fig 19-7 (p. 347), Fig 19-11 (p. 350).

Suction Lines and Transfer Pumps

  • Suction lines: introduce air into the line if not properly sealed.
  • Low-pressure lines are part of the suction/transfer path.
  • Transfer Pumps (pg 352):
    • Mechanical vs. electric transfer pumps.
    • Two primary types: reciprocating plunger and gear mechanisms.
    • Both types are positive displacement pumps.
  • Hand primer:
    • Used to purge air from the system.
  • Summary notes:
    • Some systems have transfer pumps entirely within the tank; others, like Cummins HPI-TP, are entirely under suction.

Sensors in the Fuel System

  • Water-in-fuel (WIF) sensors.
  • Fuel Pressure Sensors (FPS).
  • Fuel Temperature Sensors (FTS).
  • Sending unit (fuel level in tank).

Summary: Key Points about Fuel Subsystems

  • Definition recap: fuel subsystem is a group of components for storage and transfer to injection pumping apparatus.
  • Transfer system structure: primary (suction) and secondary (charge), separated by the transfer pump.
  • Tank placement: some transfer pumps reside in the tank; others operate under suction as in Cummins HPI-TP.
  • Filtration: secondary filter traps smaller particles; primary is coarser.
  • Tanks provide cooling (heat exchanger) and can serve injector cooling and ECM cooling via excess fuel.
  • Water management: water separator removes free-state and emulsified water; WIF sensors indicate water buildup.
  • Inlet restriction: check manufacturer values; test on suction side.
  • Air leakage: common source is double-gasket under suction on filters.
  • Cold climate considerations: fuel heaters are used.
  • Priming: hand priming purges air; secondary filters should be primed post-installation (not pre-filled).
  • References: Chapter 19, pages 340–350; Fig 19-1, Fig 19-3, Fig 19-7, Fig 19-11.

Diesel Fuel Ratings & Classifications

  • Regulatory context:
    • Fuel standards set by EPA and governed by ASTM.
  • On-highway categories:
    • ASTM #1D.
    • ASTM #2D.
  • Subcategories:
    • ASTM #1D is commonly used in subarctic environments; has a lower calorific value than #2D and may contain no more than 0.0015% sulfur (15 ppm) per EPA 2007 spec.
    • ASTM #2D LS (Low Sulfur) / ULSD (Ultra Low Sulfur Diesel): higher calorific value than #1D; however, LS/ULSD has less lubricity and historically caused problems for older injection pumps/injectors; lubricity enhancers are often required for compatibility with older engines; many external emissions devices require ULSD for proper operation (e.g., DPF).
  • Equations/quantitative notes:
    • Sulfur content limit: $S \le 0.0015\% = 15\ \text{ppm}$ (EPA 2007 spec).
  • Practical implications:
    • Use of ULSD is mandated for post-2007 engines with DPF; using other fuels can damage DPF.

Cetane Number (CN)

  • CN overview:
    • Average CN in NA for 1D & 2D fuels is around $CN\approx 47$; legal minimum CN for 1D/2D is $CN_{min}=40$.
  • CN defines ignition quality and ignition delay characteristics:
    • A higher CN corresponds to shorter ignition delay (ignition temperature is effectively lower).
    • Qualitative relation: Ignition delay∝1CN\text{Ignition delay} \propto \frac{1}{CN}
  • CN and operating conditions:
    • Cetane requirement depends on operating conditions and engine design; CN requirement increases with decreasing compression temperature.
  • Consequences of CN levels:
    • Higher CN is desirable for easy cold-starts, reduced roughness, and less diesel knock.
    • Low CN leads to long ignition delay, hard starting, white smoke, misfiring, roughness, and diesel knock.

Heating Value (Calorific Value) of Diesel Fuels

  • Specific gravity and calorific value relationship:
    • For CN $\ge 40$ and 15°C, the specific gravity (density) typically ranges from $SG = 0.870$ to $0.780$.
    • Higher CN generally corresponds to lower specific gravity (denser fuels tend to have different compositions affecting CN and density).
  • Typical energy content (BTU per US gallon):
    • $\text{ASTM }1D\text{ diesel} \approx 137{,}000\ \text{BTU/gal}$.
    • $\text{ASTM }2D\text{ diesel} \approx 142{,}000\ \text{BTU/gal}$.
    • Comparison benchmarks:
    • Average pump gasoline $\approx 125{,}000\ \text{BTU/gal}$.
    • Butane $\approx 130{,}000\ \text{BTU/gal}$.
    • Propane $\approx 93{,}000\ \text{BTU/gal}$.
  • Significance: heating value influences thermal efficiency and power output of the engine; related to density of the fuel.
  • CN and heating value are interconnected via fuel composition and energy density.

API Gravity and Fuel Density

  • API gravity definition:
    • API gravity measures how the weight of a petroleum liquid compares with water.
    • Higher API gravity means lighter-than-water liquid; API gravity must fall within typical diesel windows: about $25$–$48$ degrees for many diesel fuels; fuels heavier than water have API gravity $<10$.
    • Range for petroleum products usually spans roughly $10$–$70$ API gravity.
  • Practical uses:
    • API gravity helps diagnose lower-power problems and monitor fuel quality changes.
  • Notation:
    • API = American Petroleum Institute.

Cloud Point, Pour Point, and Flash Point

  • Cloud Point:
    • Temperature at which wax crystals become large enough to make fuel appear hazy.
    • Typically about $5^{\circ}\text{F}$ ($3^{\circ}\text{C}$) above the pour point.
    • Indicates the lowest temperature at which fuel can be pumped through the system before wax precipitation.
  • Pour Point:
    • Lowest temperature at which the liquid can be pumped; generally just above the gel point.
  • Flash Point:
    • Temperature at which sufficient vapor can be ignited momentarily with a flame.

Viscosity and Fluidity

  • Viscosity definition:
    • Measure of a liquid's resistance to shear; decreases as temperature increases.
  • Measured in Saybolt Universal (SSU):
    • $1D$ fuels typically about $34.4\,SSU$.
    • $2D$ fuels typically about $40\,SSU$.
  • Significance:
    • Viscosity influences fuel flow through filters and injectors, especially at startup and low temperatures.

Volatility and Contaminants

  • Volatility:
    • Tendency of a liquid to evaporate; critical for summer operation.
    • Higher volatility leads to greater fuel loss through boil-off, which can affect cetane and combustion characteristics.
  • Contaminants:
    • Diesel fuels may contain suspended solids or soluble metallic compounds (e.g., sodium, vanadium).
    • Ash content affects injectors, fuel injection pumps, piston rings, exhaust valves, and turbochargers.
  • Additional notes:
    • Volatility and contaminants are influenced by climate and storage conditions.

Fuel Fractions, Distillation, and Classification

  • Fractions:
    • Fuels are mixtures of crude petroleum fractions separated by distillation or cracking (e.g., hydrocracking, catalytic cracking).
    • Classified by volatility and applicable boiling range.
  • Practical implications:
    • Fractions are carefully balanced to optimize combustion, lubricity, lubricity enhancers, and emissions compatibility.

Climate, Storage, and Degradation Effects

  • Climate effect on CN and fuel stability:
    • Average CN of NA 1D & 2D fuels is $CN\approx 47$; legal minimum is $CN_{min}=40$.
    • Stored fuel at high temperatures can lose volatile fractions, lowering CN.
  • Micro-organisms in stored fuel:
    • Micro-organisms can grow in stored fuel, especially in gensesets and emergency power systems.
    • Metabolic waste from micro-organisms can be acidic and corrosive, plugging filters.
  • Cloud point and pour point considerations are important for cold-weather operation.

Fuel Additives and Contaminant Control (Table 18-7 reference)

  • Fuel additives and their functions:
    • Biocides: inhibit growth of bacteria and fungi to prevent filter clogging.
    • Demulsifiers and Dehazers: improve separation of water and prevent haze.
    • Rust & Corrosion Inhibitors: prevent rust and corrosion in fuel subsystems, pipelines, and storage facilities.
    • Fuel Stability: stabilizers to prevent gum formation and oxidation.
    • Metal Detectors: inhibit gum formation; (note: practical implementation varies by system).
    • Oxidation Inhibitors: minimize oxidation, gum, and precipitate formation.
    • Dispersants: prevent agglomeration and disperse residue; may peptize injector deposits and increase filter life.
    • Engine Performance Detergents: prevent injector deposits and increase injector life.
    • Cetane Improvers: increase Cetane number.
    • Smoke Suppressants: minimize exhaust smoke.
    • Pour Point Depressants: reduce pour point and improve low-temperature flow.
    • Cloud Point Depressants: reduce cloud point and improve low-temperature filterability.
    • De-Icers: reduce freezing point of small amounts of water to prevent fuel line plugging.
    • Antifoam: minimizes formation of fuel foam.
  • Reference: Fuel Conditioners & Additives, table 18-7 (p. 337).

Summary: Diesel Fuel Characteristics (Consolidated)

  • On ignition quality:
    • Ignition quality is rated by Cetane Number (CN).
    • CN correlates with ignition temperature: higher CN generally lowers ignition delay.
    • Minimum CN for on-highway 1D & 2D fuels is legislated at $CN_{min}=40$ in NA.
  • Density and energy:
    • There is a correlation between fuel density and heating value: higher CN often corresponds to lower density; higher heating value contributes to better energy content per volume.
  • Fuel deterioration and storage:
    • Fuel deteriorates chemically over time; micro-organic contamination can occur with prolonged storage.
  • Sulfur content:
    • EPA mandated standard for sulfur content is $0.0015\%$.
  • Emissions and post-2007 engines:
    • Use of non-ULSD fuels in engines with DPF can damage the DPF within a short period (often <1 hour).
  • Practical takeaways for operation and maintenance:
    • Monitor CN, heating value, API gravity, cloud/Pour points, viscosity, and volatility to ensure reliable operation.
    • Maintain and service fuel filters, water separators, and WIF sensors to avoid fuel-related failures.
    • Use appropriate additives for lubricity, cetane enhancement, and cold-weather performance as per manufacturer recommendations.

Reference

  • Source: Bennett, S. (2013). Medium/Heavy duty truck engines, fuel & computerized management systems (4th ed). Clifton Park, NY: Delmar Cengage Learning.
  • Diesel Fuel Characteristics – Chapter 18, page 325.