\JEW FLASHPOINT FOR DIESEL
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Oil industry moves to lower cetane ratings for diesel are likely to cause operational problems for users. John Dickson-Simpson examines the effects of reducing the fuel's ignitability
DEEP suspicion of the petroleum industry persists in transport circles. "They've had us over an barrel for years, clocking up huge ofits while we've been struggling to ake ends meet," is a view often pressed by transport managers.. So when the oil companies say they ant to lower the quality of diesel fuel, uliers are inclined to treat such a itement as code language for
eserving profits on oil products. The aspect of diesel fuel quality that is risidered central is the ignitability, or tane, figure. This immediately creates .nfusion because there are two
ethods of cetane measurement: Cetane dex (calculated) and Cetane Number 70M a test-engine's performance). Whatever the precise definition of tam:, the proposed drop in the British andard minimum from 50 to 47 is .odest. However, the automotive and ansport industry camps regard the tempt to lower cetane as much a .atter of principle as of technical mcern. Even if the practical effects of a nail drop in cetane are small, they arc
nevertheless being proposed at a sensitive time in terms of environmental restrictions.
Tests that have been done are hardly comprehensive, hut there are certainly detectable (if variable) effects from lowering mane: engine noise can increase by a decibel; smoke density increases by a tiny ;itnount; white smoke lingers 30 per cent longer after initial start-up; extensions in cold-start time of between 25 and 50 per cent have been recorded; and firing pressures go up wheal the injection timing is advanced to compensate for the longer ignition delay.
A decibel on noise may not seem much, bur some diesel-engined vehicles already struggle to meet the more stringent noise limits steadily being introduced. In that context, therefore, a decibel can become critical.
Exhaust -smoke limits are not such a problem under existing legislation. Again, though, there are worries for the future when legislation moves towards defining precise analytical limits for particulates, hydrocarbons and nitrogen oxides in diesel exhaust. A deterioration in fuel quality will not he helpful.
White smoke is of more immediate concern, especially to British operators. This is not because of any limits defined in regulations; instead there is a general sensitivity to nuisance at depots of bus and truck operators which can now attract public objections to licence applications. No operator is anxious to do anything that might invite complaints from residential neighbours.
Regarding prolonged cold-start times, these can be reduced through adjustments in engine tuning, but there are inevitable consequences in the relationships between firing pressures, power and fuel consumption. These are all becoming more finely balanced as engine development progresses.
Last on the list of the effects of lower cetane values on diesel engine is increased tiring pressures. However, with the latest high-power turbocharged diesels much is made of the greater power obtained without raising firing pressures and therefore without
hazarding engine life or reliability. More fuel and air is being digested, but the injection timing is retarded so that the internal explosion occurs at a more advantageous crank angle. This has been done by injecting the fuel faster and by speeding and improving the fuel-air mixing process. Because a lower cetane fuel does not burn readily, the injection will have to be advanced again.
Further engine development work might be done to ensure that the final burning still occurs at a favourable crank angle, and that more vigorous air-swirl keeps the fuel in suspension during the longer delay period. In the meantime, however, operators are running existing engines. Using lower cetane fuel will reduce their power output and lengthen cold-start times if existing timing is retained. On the other hand, if timing is advanced to restore power and coldstarting response, fuel consumption and carbon formation will suffer. Fuel will be injected before full swirl vigour (especially from squish) has developed, resulting in wetting of the chamber walls. A net loss in efficiency is likely even though lower cetane fuels tend to be heavier, which means more fuel for a given volume.
RUNNING on fuels with varying cetane values will present practical difficulties to operators during the transition to lower cetane fuel. Timing advanced to suit lowcetane diesel will cause premature burning with high-cctane fuel. The firing pressure will soar, bringing risk of damage to the engine. It becomes increasingly important, therefore, for fleet engineers to know the characteristics of the fuel they buy, to monitor the quality and adopt an engine-tune policy that is not likely to impair durability.
Indeed, this problem of cetane-spread already occurs. When the Institute of Road Transport Engineers did a fuelanalysis survey last year the Cetane Number (of more direct practical significance than Cetane Index as far as engines arc concerned) varied between 50.5 and 56. Most attained 52 Cetane Number.
This reality about fuels is also why transport engineers are making more fuss about the proposed reduction in cetane than might at first seem reasonable. The practical situation, they point out, is that they are faced not merely with a reduction from 50 to 47 but with a reduction from as much as 56 to 47 which makes the oil companies' proposals more far reaching than they seem.
The petroleum industry counters by saying it is looking to the future. It predicts that by the end of this century the proportion of crude oil required for transport will rise from 34 per cent to 52 per cent. There is no way that this extra take of lighter fractions can come from
the straightforwardly refined product and preserve today's ignition quality. Furthermore, there is more take of North Sea oil, which inherently has lower cetane.
The frustrating refusal of the oilmen to entertain any compromise on the cetane issue does nothing to dispell the fuel users' deep suspicion of their motives. After all, the refiners do still have the option of incorporating cetaneenhancing additives, so why don't they do so? Such a suggestion is being resisted because of extra cost. The argument is that fuel users would resent paying more.
Since this price issue was raised, however, fuel prices have tumbled. In any case, the additive content is tiny less than 0.5 per cent is typical. The argument is also weakened by the IRTE's discovery that sonic diesel fuels already contain cetane improvers.
The most constructive compromise to he put forward so far has come from the IRTE, which suggested to the British Standards committee that cetaneimprover be added just to winter grade fuel. The IRTE argument is: "Look, you already lace winter-grade fuel with anti-waxing additive. What's so difficult about introducing a cetane-improving additive at the same time? The need for high cetane is most acute in cold weather and cold-starting performance is the most affected aspect of low-cetane fuel. That would permit offsetting cost-savings by producing fuels with cetane even as low as 45 in summer grade without upsetting operational performance intolerably. Yet it would still provide an official fuel with at least 50 cetane for meeting environmental legislation such as noise."
The oil industry representatives have so far refused to accept the IRTE
compromise. "I here are murmurings about confusion in the seasonal changeover periods, but these do not convince the transport engineers, who point out that climatic temperatures clt not drop suddenly enough to make mixed cetane fuel a problem in changeover periods.
In the face of such implacable opposition is there another way out? There might be. Change the rules.
This would not be cheating, it wouI be simply realism. There is now too much discrepancy between Cetatic Ind. and Cetane Number. Although it is Cetane Number that is of more real-lit relevance to an engine, it is a fairly expensive and cumbersome business tc keep doing engine tests. So Cetane Index was invented to estimate cetane by calculation based on the density am mid boiling point of the fuel (the temperature to achieve 50 per cent vaporisation). A review of the calculation method can be justified to get a closer approximation to realised Cetane Number. Then a 47 Cetane Index would be acceptable.
Alternatively (and preferably), stick 50 Cetane Number, leaving it to the refiners to decide how they achieve it economically as they wish.
If the oil companies choose to do nothing, plain market forces will have decide the outcome. Some independen body will have to monitor fuel quality and periodically publish the results so that users can decide buying policies. The IRTE has done useful pioneer wot here and will be publishing a 1986 survey in the next issue of its journal, although there is no reason why some other trustworthy body should not do similar job on a committed regular bas and funded by those with a vested interest in quality control.