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Almost all published work involving injector nozzle cleanliness has been performed by means of keep-clean tests. Another area of interest in the performance of detergents is that of clean-up, which is the ability of a detergent to remove excessive deposits built up in previous service. A study is now reported in which the degree of clean-up is established for a modern commercial detergent used in a low-sulphur diesel fuel. Measurements are presented from a bench engine and also from road vehicles operated under controlled urban driving conditions. Substantial clean-up is demonstrated using detergent treat-rates representative of modern formulation technology, regaining the state of cleanliness that provides optimum performance of the injection equipment. The fouling levels after clean-up are comparable with the results of keep-clean tests, run under identical conditions. Accumulated driving distance and fuel usage to provide this clean-up effect are also discussed.

The benefits of diesel fuel additives have been demonstrated in a broad range of performance and operational areas, from the refinery, through storage and distribution, to fuel dispensing and vehicle operation. The customer is certainly aware of their effects on fuel performance in many of these respects, such as cold-weather operation, ease of starting, foaming, odour, etc. An area of particular interest in customer perception, however, is fuel economy. Excluding the use of after-market fuel-treatment devices, it is claimed that additives of different types can improve fuel economy, for example by improving combustion, by maintaining injection equipment in optimum condition, or by reducing engine frictional losses.

A method is proposed to quantify the activity of a detergent at a selected engine operating condition by measuring the amount of additive required to reduce nozzle fouling to zero in a well-defined engine test procedure. To determine this critical detergent concentration, a series of engine tests is performed using increasing levels of additive, until a zero level of fouling is achieved. These results are then used to construct a detergent concentration response map, defining unambiguously the additive activity and allowing quantitative comparisons of different detergents or additive packages. Concentration response plots are presented from both bench engine test series and vehicle road trials which demonstrate the wide variability in the amount of different detergents required to achieve comparable deposit control. From these maps the performance of the detergents may be compared in terms of keep-clean and clean-up activity for the range of engines and operating conditions.

The role and required function of a diesel fuel detergent are discussed and related to the process of nozzle deposit formation. A bench engine test method is described which uses an air-flow technique to measure the fouling produced in IDI nozzles and hence quantify the effectiveness of detergents in the engine test. Data are presented demonstrating the discrimination in detergent performance which may be obtained by applying this method at different operating conditions. Results of ECE 15.04 emissions tests are presented from vehicles having nozzle fouling levels ranging between 1-70%. These emission levels are plotted versus nozzle fouling. From these data, it is concluded that reducing fouling produces a systematic reduction in unburnt hydrocarbon emissions. An optimum fouling level of 15-40% is identified to minimise particulate emissions from this engine.