Car care & driving
Fuel Quality Indicators: What Octane and Cetane Numbers Mean
Autopapyrus
Fuel quality indicators fall into two groups: those required for every type of fuel (water content, mechanical impurities, sulfur) and those required only for specific fuels.
For automotive diesel fuel and gasoline, the second group includes the cetane and octane numbers, tetraethyl lead content, distillation range (fractional composition), pour point and cloud point, and vapor pressure.
The main quality indicators for gasoline are knock resistance, chemical stability, and distillation range.
1) Knock behavior describes how evenly and smoothly gasoline burns. Normal combustion speed of the air-fuel mixture is about 20-40 m/s (65-130 ft/s), while knocking combustion can exceed 2,000 m/s (6,500 ft/s). Metallic knocking or pinging sounds coming from the engine cylinders are the main sign of knocking combustion.
2) Knock resistance is measured by the octane number. It is determined on a special test engine by comparing a given gasoline with reference fuels (heptane is 0, isooctane is 100). The octane number directly depends on the engine's compression ratio. The higher the compression ratio, the higher the octane number of the gasoline the engine requires.
3) Gasoline's ability to evaporate is described by its distillation range, which records the initial and final boiling points.
4) Gasoline's resistance to forming carbon deposits and gums is described by its chemical stability. Gasoline should not contain sulfur compounds, sulfur, or water. Sulfur leads to corrosion, and water makes the engine harder to start.
The main quality indicators for diesel fuel are the cetane number, cloud point, and the content of mechanical impurities, sulfur, and water.
1) The cetane number determines how the fuel burns and how quickly pressure rises in the cylinders. Standard fuel has a cetane number of 40-45. Engine wear increases with fuel below 40, and the highest-quality fuel rates 45-50. Above 60, combustion completeness drops sharply, and exhaust smoke and fuel consumption increase.
2) The pour point affects how fuel is delivered to the engine cylinders. Summer diesel is rated at -5° (23°F), winter diesel at -35° (-31°F), and arctic diesel at -55° (-67°F).
3) The cloud point is the temperature at which microcrystals of ice and paraffin-type hydrocarbons begin to form. They collect on the filter and build up a film that fuel cannot pass through.
4) Excess sulfur and mechanical impurities can clog injectors, filters, and the passages of the fuel system.
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