Grade 4 Diesel, also known as No. 4 Fuel Oil (FO-4), is a heavy distillate fuel positioned between lighter diesel fuels (Grade 2) and residual fuel oils (Grades 5 and 6). It is engineered for industrial and commercial combustion systems requiring high heat output, stable combustion, and long continuous operating cycles.
This fuel exhibits significantly higher viscosity than automotive diesel and typically requires preheating systems for effective pumping, atomization, and efficient combustion.
Physical & Chemical Properties
Density & Viscosity:
High density due to long-chain hydrocarbons
Kinematic viscosity substantially higher than Grade 2 diesel
Preheating required (typically 40–70°C)
Poor cold-flow behavior without heating
Energy Content:
Gross calorific value: ~42 – 44 MJ/kg
Delivers sustained heat output for continuous industrial operations
Volatility & Flash Point:
Low volatility
High flash point (>55–65°C) improves storage and handling safety
Minimal evaporation losses
Sulfur Content:
Traditionally higher than Grade 2 diesel
Varies based on refinery treatment and regional regulations
May require emission control systems in regulated zones
Refining & Production
Extracted from mid-to-heavy distillate fractions of crude oil
Boiling range: ~250°C – 350°C
Often blended from heavy gas oil and lighter residual streams to adjust:
Viscosity
Pour point
Combustion stability
May undergo hydrotreating or thermal cracking to reduce sulfur and improve flow characteristics. Final specifications depend on application and regulatory requirements.
Combustion Characteristics
Slow, steady burn rate ideal for large burners
Requires higher injection pressures and preheated fuel systems
Produces higher flame temperature and longer flame length
Improper combustion may lead to soot formation, slagging, and fouling of heat-exchange surfaces.
Storage & Handling
Stored in heated or insulated tanks
Heated pipelines and positive displacement pumps required
Filtration needed to remove sediments
Periodic tank cleaning recommended due to asphaltene content
Industrial Applications
Industrial boilers and furnaces
Power generation plants (base load or backup)
Cement, steel, and glass manufacturing
Large stationary diesel engines
Marine auxiliary engines (non-road use)
Why Grade 4 Diesel is Preferred
High energy density
Cost-effective for bulk industrial consumption
Reliable under continuous high-load operation
Environmental & Regulatory Considerations
Higher particulate and sulfur emissions than light diesel
Requires emission controls such as scrubbers or ESPs
Often restricted to non-road or permitted stationary installations
Comparison with Grade 2 Diesel
| Property | Grade 2 Diesel | Grade 4 Diesel |
|---|---|---|
| Viscosity | Low | High |
| Energy Density | Moderate | High |
| Sulfur Content | Low (ULSD) | Moderate to High |
| Preheating Required | No | Yes |
| Typical Use | Vehicles | Industrial Systems |
Advantages & Limitations
Advantages:
High thermal efficiency
Stable combustion for continuous operations
Lower cost per unit of heat
Limitations:
Requires heating infrastructure
Higher emissions without treatment
Not suitable for automotive engines