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Growing interest in ways to assess coking
Page 118
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• Diesel fuel quality is no longer decreasing in most areas worldwide, and in several cases improvements have been reported, according to the Paramins annual survey.
The Paramins Division of Exxon Chemical is sponsoring a series of presentations on automotive trends in Europe and the first one was held at the Dorchester Hotel, London. The conference was told that premium diesel fuel had begun to be made available in most European countries in the past two years.
The premium grade consists of three groups.
The first are those claiming to improve low temperature operability. Then there are those addressing the issues of combustion efficiency, including reduced smoke, improved economy and reduced noise. Finally, there are handling characteristics, including improved lubricity, reduced foaming and better odour.
Of these, low temperature operability is undoubtedly the key issue.
Recent severe winters in Europe have highlighted the problem. They made some people question the validity of the cold filter plugging (CFPP) test, and the amount of protection offered by the specification levels in each country.
Initially, claims for premium diesel fuel low temperature properties were based on a lower CFPP figure. More recently, guarantees of operability have been given down to specific temperatures. But CFPP is still relevant. In 1982, a CEC committee examined the evidence and concluded that CFPP was still a good predictor of vehicle operability.
Paramins believes that the CFPP test will remain the basis for low-temperature operability prediction. There might be a complementary test to access performance in severe conditions.
A further cold weather problem with diesel fuel is its tendency to precipitate wax in storage or in vehicle tanks when the fuel cycles in temperature around its cloud point. The wax deposits sometimes cause fuel blockages. Wax antisettling additives have been developed which prevent this problem becoming troublesome.
Paramins believes that the sensible use of current additive technology, coupled with specification geared more closely to local climates, can protect diesel fuel in all but the most severe winters, without further reductions in fuel cloud points. The situation is less clear or the combustion and other handling characteristics of diese fuel. CEC committee PF-26, which has been examining diesel combustion tests, is aim looking at other performance aspects, such as light load hydrocarbons emissions, cold start performance, an so on.
Paramins believes that we are now nearer to needing performance testing for combustion quality. Performance features built into the CFR cetane test could be the answer, or perhaps a standard coking test. There is a growing interest in test procedures to assess injector coking properties of diesel fuel, although this characteristic does not directly correlate with cetane number.
The European Commission is progressing with CEN a diesel fuel specification. The effect of changing refinery processes on diesel fuel quality is perhaps not so severe as was at one time thought. Cetane number in Europe last year still averaged around 49.