Cetane Number (CN): Rating Diesel Ignition Quality
Cetane number quantifies the propensity of a diesel fuel to auto-ignite promptly after injection into high-pressure, high-temperature air. Higher numbers correspond to shorter ignition delay, smoother pressure rise, and lower combustion noise, making cetane a critical quality metric for compression-ignition engines powering transportation, generators, and industrial equipment.
Use this guide alongside the octane rating article to compare how the two major fuel quality indices influence hardware design, and reference the generator fuel planning calculator when budgeting for premium blends.
Definition and Measurement Basis
Cetane number is dimensionless and anchored to two primary reference fuels: n-hexadecane (cetane), assigned a value of 100 for its extremely short ignition delay, and heptamethylnonane, assigned a value of 15 for its poor ignition quality. A diesel sample’s cetane number equals the percentage by volume of cetane in a cetane/heptamethylnonane blend exhibiting the same ignition delay in a standard test engine.
Modern laboratories often report derived cetane number (DCN) obtained from constant-volume combustion chambers. DCN correlates with the primary standard but relies on digital pressure transducers and algorithmic curve fitting, enabling faster, automated testing that suits refinery quality-control loops and pipeline custody-transfer checks.
Historical Evolution
The cetane scale emerged in the 1930s as Rudolf Diesel’s engine concept matured from stationary machinery to highway vehicles. Engineers needed a reproducible index to ensure fuels ignited reliably in engines with ever-higher compression ratios. ASTM D613 codified the Cooperative Fuel Research (CFR) engine test procedure, which remains the legal reference in many jurisdictions.
As global supply chains diversified, refiners blended paraffinic, naphthenic, and aromatic streams to meet cold-flow and emissions mandates. Cetane improver additives, such as 2-ethylhexyl nitrate, became common during the mid-20th century, allowing refiners to boost cetane without sacrificing yield. Renewable diesel from hydrotreated vegetable oils now delivers cetane numbers above 70, illustrating the metric’s ongoing relevance to sustainability strategies.
Concepts and Influencing Factors
Chemical Composition
Straight-chain paraffins ignite readily, boosting cetane, whereas aromatics and branched molecules resist auto-ignition. Refinery units such as hydrocrackers and hydrotreaters adjust molecular distribution to achieve target cetane while managing sulphur, density, and cold-flow constraints documented in API gravity specifications.
Test Conditions
Ambient temperature, intake air moisture, and the compression ratio of the CFR engine influence ignition delay. Standard methods tightly control these variables, but field performance can deviate—especially in high-altitude or cold environments—so engine manufacturers often specify minimum cetane values above regulatory baselines.
Relationship to Emissions
Higher cetane typically reduces hydrocarbon and carbon monoxide emissions because combustion starts earlier and completes more fully. However, excessively high cetane may raise nitrogen oxides as advanced combustion timing elevates peak temperatures. Calibrated engine control units balance cetane with exhaust-gas recirculation, injection timing, and aftertreatment hardware to meet regulatory limits.
Applications Across Sectors
On-road and off-road vehicles. Heavy-duty fleets track cetane to maintain reliable cold starts and minimise injector deposits. Fleet managers leverage fuel analytics dashboards alongside the transport emissions calculator to quantify efficiency gains from higher-quality fuel.
Standby power systems. Hospitals, data centres, and telecom hubs specify premium cetane levels to guarantee rapid load acceptance during outages. Planning tools such as the microgrid autonomy calculator clarify inventory requirements when switching between seasonal blends.
Marine propulsion. Medium-speed marine diesels rely on consistent ignition delay to control cylinder pressure rise. International Maritime Organization sulfur caps have shifted blending practices, making cetane analytics a core part of bunker-fuel procurement.
Why Cetane Number Matters
Cetane number underpins engine durability, noise, and efficiency. Poor ignition quality increases combustion delay, causing rough operation and higher particulate emissions that challenge diesel particulate filters. By contrast, fuels with adequate cetane support advanced strategies such as homogeneous charge compression ignition and dual-fuel pilot injection.
Documenting cetane alongside density, heating value, and sulphur ensures transparent procurement. Integrating cetane metrics into enterprise resource planning or maintenance logs helps operators correlate field performance with fuel batches, closing the loop between laboratory testing and real-world outcomes.