49.5
2
4.5
-1.5
-5.5
49.5
2
4.5
-1.5
-5.5
The Cetane Number Calculator estimates the Calculated Cetane Index (CCI) of diesel fuel using its physical properties: density and mid-boiling point temperature (T50). The cetane number is the diesel equivalent of the octane number for gasoline but measures ignition quality rather than knock resistance. A higher cetane number indicates shorter ignition delay, meaning the fuel ignites more readily after injection into the hot compressed air of a diesel engine. This property directly affects cold-start performance, combustion noise, exhaust emissions, and engine efficiency. The calculated cetane index, defined by ASTM D4737, provides an economical alternative to the costly engine-based cetane number test (ASTM D613), making it invaluable for fuel quality monitoring in refineries, distribution terminals, and regulatory compliance laboratories worldwide.
The Calculated Cetane Index (CCI) uses the ASTM D4737 four-variable equation simplified to two primary physical properties:
$$CCI = 454.74 - 1641.416 \cdot D + 774.74 \cdot D^2 - 0.554 \cdot T_{50} + 97.803 \cdot (\log_{10} T_{50})^2$$
where D is the fuel density in g/mL at 15°C and T50 is the 50% distillation recovery temperature in °C (the mid-boiling point from ASTM D86 distillation).
The equation captures the fundamental relationship between fuel molecular structure and ignition quality. Density correlates with the aromatic content of diesel fuel; aromatics have poor ignition quality and increase density. The mid-boiling point reflects the average molecular weight and volatility of the fuel; heavier, longer-chain paraffins tend to have higher cetane numbers and higher boiling points.
The actual cetane number (CN) is measured by running the fuel in a standardized Cooperative Fuel Research (CFR) engine and comparing its ignition delay to blends of cetane (hexadecane, CN = 100) and alpha-methylnaphthalene (CN = 0). The CCI approximation is accurate to within 2 cetane units for fuels without cetane-improving additives.
Most countries require a minimum cetane number of 40-51 for on-road diesel. European EN 590 specifies a minimum of 51, while the US ASTM D975 requires a minimum of 40. Premium diesel fuels typically have cetane numbers of 50-55 or higher.
A CCI of 55 or above indicates premium diesel fuel with excellent ignition quality, quiet combustion, easy cold starting, and low emissions. Values of 45-55 represent standard diesel fuel meeting most regulatory requirements. A CCI of 40-45 is the minimum acceptable range, and fuel may exhibit increased combustion noise and difficulty starting in cold weather. Below 40, the fuel falls below most regulatory standards and will likely cause rough idling, increased white smoke on startup, higher NOx emissions, and potential engine damage from excessive combustion pressure rise rates.
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A diesel fuel with density 0.835 g/mL and T50 of 280°C yields a CCI of approximately 52, meeting the European EN 590 minimum of 51.
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Higher density (0.870 g/mL) indicating more aromatics produces a lower CCI of about 44, barely meeting the US ASTM D975 minimum of 40.
The cetane number (CN) is measured directly in a standardized test engine (ASTM D613) by comparing ignition delay to reference fuels. The calculated cetane index (CCI) is estimated from physical properties (density and distillation temperatures) using ASTM D4737. CCI is accurate to within 2 units for fuels without cetane-improving additives but cannot detect additive effects.
Cetane number directly affects ignition delay, which determines combustion quality. Higher cetane means shorter ignition delay, resulting in smoother combustion, lower peak pressures, reduced noise (diesel knock), better cold starting, lower HC and CO emissions, and improved fuel economy. Low cetane causes rough combustion, excessive noise, and higher emissions.
Requirements vary by region: the US ASTM D975 specifies a minimum of 40, European EN 590 requires at least 51, and many Asian countries require 48-51. These minimums ensure acceptable combustion quality, emissions compliance, and engine protection.
Higher density generally indicates higher aromatic content, which lowers cetane number. Aromatics have complex ring structures that resist autoignition. Paraffinic (straight-chain) hydrocarbons have lower density and higher cetane numbers because their molecular structure promotes easier thermal decomposition and ignition.
Cetane improvers, primarily 2-ethylhexyl nitrate (2-EHN) and di-tert-butyl peroxide (DTBP), decompose at lower temperatures to produce free radicals that initiate combustion earlier. Adding 0.1-0.5% 2-EHN can increase cetane number by 3-8 units. The CCI equation cannot detect these additives, so actual CN may exceed calculated CCI for treated fuels.
T50 reflects the average molecular weight of the fuel. Higher T50 generally means heavier molecules with longer carbon chains, which tend to have higher cetane numbers. However, if the high boiling point is due to heavy aromatics rather than long paraffins, the cetane improvement is diminished, which is why both density and T50 are needed for accurate estimation.
Biodiesel (fatty acid methyl esters, FAME) typically has cetane numbers of 47-65, depending on the feedstock. Soybean biodiesel is typically 47-52, rapeseed 54-58, and coconut/palm can reach 60-65. The long straight-chain structure of fatty acid esters naturally promotes good ignition quality.
The equation can theoretically produce very low values for extremely dense fuels with low boiling points, but such fluids would not be practical diesel fuels. For normal diesel fuel ranges (density 0.80-0.88 g/mL, T50 200-350°C), the equation produces realistic values of 30-70.
Cetane and octane measure opposite combustion properties: cetane measures ease of autoignition (desirable in diesel), while octane measures resistance to autoignition (desirable in gasoline). Fuels with high cetane tend to have low octane and vice versa. N-hexadecane has CN 100 but poor octane, while isooctane has octane 100 but poor cetane.
ASTM D4737 states that the CCI is accurate to within 2 cetane units of the measured cetane number for straight-run diesel fuels without cetane-improving additives. Accuracy decreases for fuels containing more than 5% biodiesel, synthetic fuels (Fischer-Tropsch), or significant amounts of cetane improvers.
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