Fuel Property Testing: Ignition Quality

Hannu Jääskeläinen

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• Introduction
• Cetane Number
• Cetane Index
• Ignition Delay and Derived Cetane Number
• Heavy Fuels

Introduction

The ignition quality of diesel fuel is linked to the ignition delay time, the time between the start of injection and the start of combustion. Fuels of high ignition quality are characterized by short ignition delay and vice versa, fuels of poor ignition quality produce long ignition delay. The ignition delay was discussed in more detail in the paper on diesel combustion.

While ignition quality is a property of the fuel itself and is determined by the molecular composition of the fuel, the ignition delay time used to characterize it is not. Ignition delay time is also strongly affected by conditions such as the temperature and pressure of the environment into which the fuel is injected. Therefore, tests developed to measure ignition quality of a particular fuel have to be carried out under carefully controlled test conditions to ensure that only fuel effects are being measured.

A number of different tests have evolved for the quantification of ignition quality, which can be grouped into three types of methods:

• Cetane number is the most important and universally accepted ignition quality test. The cetane number test uses a standard single cylinder variable compression ratio diesel engine.
• Cetane index is a calculated value, which is derived from relatively easily measured fuel properties. Thus, cetane index can provide a measure of fuel ignition quality without the need to run the costly cetane number test.
• Ignition delay is measured using the more recent constant volume combustion chamber method. Based on the ignition delay, the derived cetane number is determined.

Cetane Number

Cetane numbers are measured using a method developed in the 1930s by the Cooperative Fuel Research (CFR) Committee, and later standardized as ASTM D613. The test involves running the fuel in a single cylinder, continuously variable compression ratio CFR Cetane Engine. Two primary reference fuels (hydrocarbons) define the cetane number scale:

• n-hexadecane (also called cetane, n-C₁₆H₃₄), which has very good ignition quality, was assigned the cetane number of 100;
• 1-methylnaphthalene, which has a poor ignition quality, was assigned a cetane number of zero.

In 1962, the low cetane number reference fuel was replaced with 2,2,4,4,6,8,8-heptamethylnonane (also called isocetane or HMN), which had better oxidation stability and was easier to use in the CFR engine. When measured against the two original standards, 2,2,4,4,6,8,8-heptamethylnonane has a cetane number of 15. When a fuel has the same ignition delay period as a mixture of the two primary reference fuels, its cetane number is derived from the volume percent of cetane and heptamethylnonane, as follows:

Cetane number = % cetane + 0.15 (% heptamethylnonane)(1)

In routine operations, the two primary reference fuels are replaced by two secondary reference fuels: T-fuel and U-fuel. These fuels are calibrated against the primary reference fuels and made available to testing labs by Chevron Phillips, the sole source supplier. The fuel supplier provides blend ratio instructions to achieve cetane numbers bounded by the values for the U and T fuels. Table 1 provides an example of T and U reference fuels, as well as their specifications.

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