Cetane Number

Cetane Number is a measure of the ignition delay characteristics of diesel fuels used in compression ignition engines.

​The test method determines the rating of diesel fuel oil in terms of an arbitrary scale of Cetane numbers using standard single cylinder, four stroke cycle, variable compression ratio, indirect injected, diesel engine (CFR F-5 Rating Unit). 

​The Cetane number scale covers the range from zero (0) to one hundred (100) but typical testing is in the range of 30 to 65 Cetane number. 


The Cetane number is calculated by interpolation of average readings of the variable compression plug hand wheel position proportioned to the Cetane numbers of the bracketing reference fuel blends. The calculated Cetane number is rounded to the nearest tenth and reported. 

Reference Fuels

Reference fuels used in this test method are designated “T Fuel” (high Cetane number) and “U Fuel” (low Cetane number) continue to be used in CFR testing laboratories today for routine ratings of diesel fuel. 

​Typically the “T Fuel” Cetane number value is approximately  73 – 75 while the “U Fuel” Cetane number is approximately    20 – 22. 

​Each matched – pair batch of these secondary reference fuels is engine calibrated by the ASTM Diesel National Exchange Group.

Blending tables of the Cetane numbers assigned to volumetric blends of “T Fuel” in “U Fuel” are published as each new pair are produced.


For quality reasons the Cetane Number must comply with a minimum Cetane specifications stipulated by its appropriate country’s legislation. 

Cetane  Engine Unit ASTM D 613
Compression Ratio8 – 36 
Standard Bore Diameter3.25″ 
Oil Pressure25 – 30 psi 
Crankcase Vacuum1″ – 6″ of vacuum H20
Oil Temperature135°F ± 15°F 
Intake Air Temperature 150°F ± 1°F
Coolant Temperature212°F ± 3°F 
Injector Water Temperature100°F ± 5°F
Injection Advance13° BTDC
Basic Injection Timing13° BTDC
Injection Quantity13.0ml in 60  +/-1 seconds
Injection Opening Pressure1500psi   +/-50 psi
Water flow1.5 us gallon water / minute
Valve Clearances0.008” ± 0.001”
Engine Speed900 RPM ±1%