Cetane engine reference fuels

Unlike spark-induced combustion engines requiring fuel that resists autoignition, diesel engines require motor fuels, for vhich the reference compound is cetane, that are capable of auto-igniting easily. Additives improving the cetane number will promote the oxidation of paraffins. The only compound used is ethyl-2-hexyl nitrate. 

The cetane engine is a variable compression single cylinder engine very much like the octane engine. The engine is mn at 900 rpm and injection is timed to start at 13° before top dead center (BTDC). The compression ratio is adjusted so that the test fuel starts to ignite at exacdy top dead center (TDC), for an ignition delay of 13° or 2.4 ms. Reference fuels are chosen which bracket the sample and the cetane number of the sample is estimated by interpolation between the two reference fuels. 

Either primary or secondary reference fuels can be utilized in the cetane engine when determining the cetane number of distillate fuel. Primary reference fuels are n-cetane and heptamethylnonane. Secondary reference fuels are identified as T Fuel and U Fuel. The characteristics of each of these fuels are outlined in TABLE 4-7. 

The T Fuel and U Fuel references are commonly used for routine cetane engine determinations. Care should be taken to ensure that both T and U reference fuels are not exposed to long-term storage at temperatures below 32°F (0°C). Both fuels may gel and solidify at low temperatures. 

Cetane Number—The cetane number (C.N.) of a fuel is the percentage by volume of normal cetane in a mixture of cetane and alpha-methylnaphthalene which matches the unknown fuel in ignition quality when compared with a standard diesel engine under specified conditions. The C.N. scale ranges from 0 to 100 C.N. for fuels equivalent in ignition quality to alpha-methylnaphthalene and cetane, respectively. For routine-testing, secondary reference fuels having cetane values of about 25 and 74 are blended in any desired proportion. 

Cetane Number—The ignition quality of a diesel fuel measured using an engine test specified in ASTM Method D 613. Cetane number is determined using two pure hydrocarbon reference fuels cetane, which has a cetane rating of 100 and heptamethylnonane (also called isocetane), which has a cetane rating of IS [2.2]. 

The engine used was a one-cylinder direct-injection test engine at Saab-Scania, Sodertalje, Sweden. During the tests, maximum pressure, fuel consumption, exhaust temperature, CO, CO2, NO, NO2, O2, HC, and smoke were registered at varied injection timings, loads, and speeds. The performance of three microemulsion fuels was compared with a reference fuel of pure diesel oil with the cetane number of 43. The data of the microemulsion fuels are given in Table II. 

To determine the cetane number of any fuel, its ignition delay is compared in a standard test engine with a blend of reference fuels (ASTM D-613, IP 41). 

The capacity of diesel fuels for autoignition is characterized by the cetane number. Commercial diesel fuels have cetane numbers of between 48 and 55. This number is measured with the aid of a CFR diesel engine by comparison with mixtures of primary reference fuels cetane or n-hexadecane has a cetane number of 100 (by definition), and heptamethylnonane has a cetane number of 15. A binary mixture of these two species containing X % by volume of cetane has therefore a cetane number equal to X + 0.15 (100-X). 

Instantaneous maps of the mixture fraction, temperature, and main combustion products (H2O, CO2, CO) are shown in Figure 7.4 for the n-heptane. N-heptane is a fuel commonly used in engines. Its cetane number is approximately 56, which is typical for diesel fuel, because its properties of ignition and combustion are similar to those of diesel fuel [7]. The n-heptane has received substantial interest because it is a major component of the primary reference fuel (PRF) in internal combustion engine studies [6] and is considered a surrogate for liquid hydrocarbon fuels used in many propulsion and power generation systems [8]. 

---