Overview
Gasoline (petrol) is a refined petroleum product consisting of a mixture of hydrocarbons (predominantly C₄-C₁₂, boiling range ~30-200°C) optimized for spark-ignition internal combustion engines. It is one of the most widely handled flammable liquids in the world. Density, volatility (Reid vapor pressure), and octane rating are the primary engineering properties.
Physical Properties
| Property | Value (typical, 20°C) |
|---|---|
| Density | 720-780 kg/m³ (average 750 kg/m³) |
| Density (summer blend) | ~760 kg/m³ (heavier components) |
| Density (winter blend) | ~740 kg/m³ (more volatile for cold starts) |
| Dynamic Viscosity | ~0.6 × 10⁻³ Pa·s (0.6 cP — less viscous than water) |
| Kinematic Viscosity | ~0.8 cSt |
| Specific Heat | 2200 J/kg·K |
| Thermal Conductivity | 0.13 W/m·K |
| Boiling Range | 30-200°C |
| Flash Point | -43°C (-45°F) — extremely flammable |
| Autoignition Temperature | ~280°C |
| Reid Vapor Pressure (RVP) | 7-15 psi (seasonal) |
| Octane Rating (AKI/RON) | 87-93 AKI (91-98 RON) |
| Lower Heating Value | 44 MJ/kg |
Volatility Classes (Reid Vapor Pressure)
RVP is a key specification controlling gasoline volatility, regulated seasonally and geographically:
| Class | RVP (psi) | Season | Purpose |
|---|---|---|---|
| Summer blend | 7.0-9.0 | Hot months | Minimize evaporative emissions, prevent vapor lock |
| Transition | 9.0-11.5 | Spring/fall | Moderate conditions |
| Winter blend | 11.5-15.0 | Cold months | Good cold start, easy vaporization |
Octane Rating
Octane rating measures resistance to engine knock (auto-ignition under compression):
- AKI (Anti-Knock Index): U.S. pump rating, (RON+MON)/2 — typically 87 regular, 89 mid, 93 premium
- RON (Research Octane Number): Europe/Asia rating — typically 91, 95, 98
- Higher octane allows higher compression ratios (more power/efficiency) but does not contain more energy per liter
Energy Content
Gasoline energy is ~44 MJ/kg, which is ~34 MJ/liter due to density of 750 kg/m³. Diesel has similar energy per kilogram but ~12% more per liter due to higher density.
Engineering Applications
Storage & Handling
- Underground/above-ground storage tanks (UST/AST) at gasoline stations — typically double-wall fiberglass/steel
- Vapor recovery systems mandatory in most jurisdictions (Stage I/II) to capture VOC emissions
- Hydrostatic pressure in tanks: ~7.5 kPa per meter depth (vs 10 kPa/m for water)
- Tank material compatibility: steel, HDPE, fiberglass (NOT natural rubber, certain plastics)
Fuel Dispensing
- Submersible turbine pumps or suction pumps dispense at ~0.2-0.5 MPa
- Metered delivery to 0.5% accuracy (legal metrology requirements)
- Vapor recovery nozzles for emissions control
Pipeline & Transport
- Batched in multi-product pipelines (between diesel and jet fuel, separated by interface)
- Lower density means pump power is 25% less than water for equivalent flow
- Centrifugal pump performance unaffected by gasoline viscosity (much less viscous than crude oil)
- Pipeline leak detection: gasoline floats and spreads rapidly
Environmental Considerations
- LUST (Leaking Underground Storage Tanks) historically major groundwater contamination source
- MTBE phased out as oxygenate; replaced by ethanol (E10, E85)
- Ethanol-blended gasoline behaves differently — phase separation with water, different material compatibility
Summary
Gasoline (density ~750 kg/m³) is the lightest common refined fuel, highly volatile and flammable. Its low density reduces pump power requirements compared to water or diesel, but its extreme flammability (flash point -43°C) governs all storage, dispensing, and transport design. Seasonal RVP and density variations (winter/summer blends) affect both engine performance and storage tank design.