1725 Cetane number and cetane improvers

Because cetane number is a measure of the ignitability of the fuel, Fig.17.13, a low value may render starting in cold weather difficult and increase the tendency towards the generation of white smoke, Fig. 17.14. It also increases the ignition delay (interval between injection and ignition). As a result, fuel must be injected into the combustion chamber earlier. Consequently, more fuel is injected before ignition occurs, so the ultimate rate of pressure rise is more rapid, and the engine therefore noisy. Also, because the fuel has less time to burn before the exhaust valve opens, the hydrocarbon emissions are increased.

On the other hand, if the cetane number is higher than that for which the injection system is timed, power will be lost because a high proportion of the pressure rise will occur when the piston is at or near TDC. Furthermore, the

Fig. 17.13 Combustion sequences photographed through a quartz window in the piston crown of an engine running on (top) a commercially available alternative fuel and (bottom) Shell Advanced Diesel. The two part circles are the inlet and exhaust valve heads and the bright spot to the left of the cylinder is an illuminated pointer over degree markings on the flywheel, which are too small to be visually identifiable here

Fig. 17.14 Two photographs showing white smoke in the exhaust gases of an engine running on (left) a commercially available fuel and (right) Shell Advanced Diesel fuel might ignite before it has mixed adequately with the air, so smoke and hydrocarbon emissions may be experienced. Consequently, fuels having high cetane numbers perform best when the injection is retarded. Conversely, with too much retard, there will not be enough time for complete combustion, so smoke and hydrocarbon emissions will again be the outcome. Since high cetane numbers are difficult to achieve, the regulations in most countries specify only the lowest limit that is acceptable.

Cetane improvers, mainly alkyl nitrates, are substances that decompose easily and form free radicals at the high temperatures in combustion chambers. Unfortunately, however, it is the fuels that have the lowest cetane numbers that tend to respond least to cetane improvers.

Combustion improvers differ from cetane improvers in that they are mainly organic compounds of metals such as barium, calcium, manganese or iron and are catalytic in action. Barium compounds could be toxic, so interest now centres mainly on the others. Although these compounds produce metalbased particulates, they also lower the auto-ignition temperature of the carbon-based deposits in both the cylinders and particle traps, causing them to be more easily ignited.

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