Overview
Diesel fuel is a middle-distillate petroleum product (boiling range ~180-360°C, C₁₀-C₂₅ hydrocarbons) used in compression-ignition (diesel) engines and as heating oil. It is denser, more viscous, and contains more energy per liter than gasoline, and ignites by compression heat rather than spark. Diesel engines are dominant in trucks, ships, rail, construction equipment, and stationary power generation.
Grades & Classifications
| Grade | Description | Use |
|---|---|---|
| #2 Diesel (No. 2-D) | Standard highway diesel | Automotive, trucks (most common) |
| #1 Diesel (No. 1-D) | Lighter, lower viscosity, lower cloud point | Arctic/winter blend, blending with #2 |
| Heating oil (#2 fuel oil) | Similar to #2 diesel, slightly broader spec | Building heating, boilers |
| B5/B20/B100 | Biodiesel blends (5/20/100%) | Renewable diesel (with elastomer compatibility caveats) |
| Marine diesel (MDO) | Marine distillate | Ships (not residual bunker fuel) |
Physical Properties (#2 Diesel, 15°C)
| Property | Value | Notes |
|---|---|---|
| Density at 15°C | 820-845 kg/m³ (EN590) | Lower than water |
| Density at 20°C | ~840 kg/m³ | |
| Dynamic Viscosity (40°C) | 2.0-4.5 × 10⁻³ Pa·s (2-4.5 cSt) | Must flow at low temperatures |
| Cetane Number (CN) | 40-55 (min 40 US; min 51 EU) | Higher = easier ignition |
| Flash Point | >52°C (typical 60-80°C) | Combustible (not flammable like gasoline) |
| Cloud Point | -10 to -34°C | Wax crystals start forming |
| Pour Point | -20 to -40°C | Will no longer pour |
| Lower Heating Value | 43 MJ/kg ≈ 36 MJ/L | ~12% more energy/liter than gasoline |
| Sulfur Content (ULSD) | <15 ppm (US/EU ultra-low sulfur) | Was up to 5000 ppm before ULSD |
| Lubricity | Important for fuel injection pumps | ULSD requires additives |
Cold Weather Behavior
Diesel contains paraffin wax that crystallizes at low temperatures — the dominant winter operability issue:
| Temperature | Problem |
|---|---|
| Cloud Point (CP) | Wax crystals begin to form; fuel clouds; can clog filters |
| Cold Filter Plugging Point (CFPP) | Fuel will not pass through standard filters |
| Pour Point (PP) | Fuel ceases to flow entirely |
Winter solutions: blend #1 diesel (lower wax), add cold-flow improvers, use fuel heaters, store tanks indoors.
Engineering Applications
Engine Performance
- Fuel injection pressure: modern common rail at 2000-3000 bar (very high — requires fuel lubricity)
- Viscosity critical for fuel injection spray pattern; too viscous = poor atomization, too thin = injector leakage
- Cetane number controls ignition delay; higher CN = quieter, cleaner combustion, better cold start
- Fuel consumption calculations use density to convert volume to mass
Pumping & Pipeline
- Density 840 kg/m³ means centrifugal pump power is 16% less than for water at same head/flow
- Higher viscosity (2-4 cSt) requires viscosity corrections for pump performance (minor at this level)
- Diesel oil pipelines use same infrastructure as gasoline/jet fuel in batching
Storage
- Above-ground and underground tanks similar to gasoline but lower fire risk
- Flash point >52°C means diesel is classified as combustible (not flammable) — less stringent area classification
- Water accumulation at tank bottom (diesel can hold dissolved water; free water causes microbial growth)
- Algae/bacteria at water-fuel interface ("diesel bug") requires biocides and tank maintenance
Emissions & Environment
- ULSD required since 2006-2010 globally for on-road use
- Diesel particulate filters (DPF) capture soot (post-2007 engines)
- SCR (Selective Catalytic Reduction) uses DEF (AdBlue, urea solution) to reduce NOx
- Biodiesel (B20 and below) generally compatible with existing infrastructure; B100 requires special materials
Summary
Diesel fuel (density ~840 kg/m³, #2 grade) is the primary fuel for heavy transportation and power generation due to high energy density and engine efficiency. Its higher viscosity compared to gasoline is essential for fuel injection lubrication and spray patterns, while its flash point above 52°C makes it significantly safer to handle than gasoline. Cold-flow properties (cloud point, CFPP) are critical engineering parameters in winter operation. Ultra-low sulfur diesel (ULSD, <15 ppm) is now the global standard.
Diesel Density vs Temperature
Diesel density varies significantly with temperature — critical for custody transfer and flow measurement:
| Temperature (°C) | Density (kg/m³) | Correction Factor |
|---|---|---|
| 0 | 858 | 1.021 |
| 5 | 854 | 1.016 |
| 10 | 849 | 1.011 |
| 15 | 845 | 1.006 (reference) |
| 20 | 840 | 1.000 |
| 25 | 836 | 0.995 |
| 30 | 832 | 0.990 |
| 40 | 823 | 0.980 |
| 50 | 815 | 0.970 |
The correction coefficient is approximately 0.7 kg/m³ per °C (density decreases as temperature rises). For API gravity conversion: API = (141.5 / SG) - 131.5, where SG = density/1000 at 15°C.
Diesel vs Gasoline Density Comparison
| Property | Diesel | Gasoline | Difference |
|---|---|---|---|
| Density (15°C) | 820-845 kg/m³ | 715-770 kg/m³ | Diesel ~10% denser |
| Energy per liter | 36 MJ/L | 32 MJ/L | Diesel ~12% more |
| Energy per kg | 43 MJ/kg | 44 MJ/kg | Similar |
| Viscosity (40°C) | 2-4.5 cSt | 0.5-0.8 cSt | Diesel 4-6× higher |
| Flash point | >52°C | -43°C | Diesel much safer |
| Cetane/Octane | CN 40-55 | RON 91-98 | Opposite scales |
Diesel Weight Calculation
To calculate diesel weight from volume:
Example: 1000 liters of diesel at 20°C:
- W = 1.0 m³ × 840 kg/m³ = 840 kg
- Compare: 1000 liters of water = 1000 kg (diesel is ~16% lighter)
- Compare: 1000 liters of gasoline = ~740 kg (diesel is ~14% heavier)
Related Engineering Tools
- Fluid Density Calculator — Calculate density of any fluid
- Pump Power Calculator — Diesel pumping power requirements
- Motor Power Calculator — Engine power output estimation
- Flow Rate Calculator — Pipeline flow calculations
- Heat Transfer Calculator — Diesel heating/cooling systems
Diesel Storage Engineering
Tank Sizing
- ULSD storage tanks typically sized for 7-30 days of consumption
- Fill factor 0.85-0.90 (allow ullage for thermal expansion)
- Underground tanks: double-wall with leak detection
- Above-ground tanks: dyke/bund capacity ≥ 110% of largest tank
Material Compatibility
- Carbon steel: standard for diesel storage (no corrosion issues with ULSD)
- Stainless steel 304/316: for biodiesel blends (B20+ may affect elastomers)
- FRP (fiberglass): underground storage, compatible with all diesel grades
- Seals/gaskets: Buna-N (NBR) for petroleum diesel; Viton for biodiesel blends
Frequently Asked Questions
What is the density of diesel fuel in kg/m³? Standard #2 diesel density is 820-845 kg/m³ at 15°C (EN590 specification). At 20°C, typical diesel density is approximately 840 kg/m³. Diesel is about 10% denser than gasoline.
How much does 1 liter of diesel weigh? One liter of diesel at 20°C weighs approximately 0.84 kg (840 grams). This is lighter than water (1.0 kg/L) but heavier than gasoline (~0.74 kg/L).
What is the specific gravity of diesel? Specific gravity (SG) of diesel at 15°C is typically 0.82-0.845 (relative to water at 4°C). API gravity is approximately 36-38 degrees.
How does temperature affect diesel density? Diesel density decreases by approximately 0.7 kg/m³ per 1°C increase in temperature. At 40°C, diesel density drops to ~823 kg/m³. This correction is critical for fuel custody transfer.
What is the difference between diesel density and API gravity? API gravity = (141.5 / SG) - 131.5. For diesel with SG 0.84: API = (141.5/0.84) - 131.5 = 37.0°API. Lower API gravity means denser fluid.