Preparing Mixes For Determining Octane And Cetane

PC-controlled automatic dosing devices from Metrohm can automate preparation of surrogate mixtures for determining octane and cetane numbers. Commercially available fuels are complex mixtures of hundreds of different hydrocarbons. For the calibration of the test engines or advanced experimental and computational research they are modelled by means of multi-component surrogate mixtures that adequately represent the desired physical and chemical characteristics.

By definition, every octane and cetane number corresponds to a specific mixing ratio of primary reference fuels (PRFs). Based on this information, the tiamo-controlled automatic dosing device prepares the surrogate mixtures. The setup drastically minimises time-consuming and error-prone manual-preparation steps and contact with hazardous solvents. Additionally, precise and accurate results are displayed on customisable reports that fully comply with all current GLP and GMP requirements. The octane rating of gasoline is a parameter that indicates how much a fuel can be compressed before it spontaneously ignites.

This spontaneous ignition is also known as engine knocking and adversely affects engine performance. The higher the octane rating of gasoline, the higher the resistance to knocking. By definition, the octane rating of isooctane, a highly branched alkane that burns smoothly, is set to 100. The rating of n-heptane, an unbranched alkane with a tendency for premature ignition, is set to zero. An octane rating of 85 means the fuel has the same knocking properties as a mixture of 85 per cent isooctane and 15 per cent n-heptane. There are different octane ratings such as the research octane number (RON) or the motor octane number (MON).

Both are determined using the fuel on a test engine (such as the CFR ASTM Test Engine) and comparing the obtained results with those for mixtures of isooctane and n-heptane. The different ratings result in different working conditions of the test engine. As the preparation of the solvent mixtures significantly affects the accuracy of the determination of the octane rating, strictly accurate solvent mixtures are of paramount importance. PC-controlled automatic dosing devices allow a straightforward, reproducible and accurate preparation of solvent mixtures. Additionally, the manual handling with hazardous solvents is minimised, which translates into increased operator safety.

The instruments required for this are: the 846 Dosing Interface, Dosino 800, dosing unit and 840 Touch Control Surrogate mixtures. The wide range of hydrocarbon species in commercial gasoline complicates the understanding of fundamental physical and chemical processes in the test engine. A substantial number of surrogate mixtures, mainly consisting of the primary reference fuels (PRFs) n-heptane, iso-octane and toluene, have been developed to mimic concrete fuel properties. The calibration of internal combustion engines used for measuring the motor octane numbers (MONs) of gasoline, for example, requires the accurate preparation of a high number of surrogate mixtures with known MON.

An automated dosing system that is fully controlled by tiamo replaces the tedious, time-consuming and error-prone manual preparation of surrogate mixtures. Whereas for gasoline the desirable property is the ability to resist ignition when injected into the engine, the desired property of diesel fuel is to auto-ignite. The relevant parameter is the cetane number. A compound that has a high octane number tends to have a low cetane number. Binary surrogate mixtures, consisting of the PRFs cetane and a-methylnaphthalene, are needed to adequately describe the chemical complexity of conventional diesel fuels.

Advantages include: upgradable custom-designed setup - surrogate mixtures of numerous primary reference fuels can be prepared; preparation of variable sample volumes between 0.1 and one litre; fully automated dosing minimises manual handling with hazardous solvents; accurate and precise dosing provides highly reproducible surrogate mixtures; detailed and customisable reports are in accordance with GLP and GMP; and the parallel dosing of the primary reference fuels saves time - the preparation of a 200ml surrogate mixture takes less than 5min.


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