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Concerns over the effects of combustion chamber deposits (CCD) on engine performance coupled with increasing environmental pressures on vehicle exhaust emissions, have forced the original equipment manufacturers (OEMs) to call for control of fuel compositions to preclude or minimise deposit formation in the combustion chamber. However, this is currently not felt to be feasible, given the scarcity of data on the effect of CCD on emissions, and the lack of an industry standard measurement technique. This paper describes a two-year project which demonstrates the positive impact that a fully synthetic CCD control additive system has on engine performance and emissions. The same programme demonstrates that tailpipe HC, CO, and NOx all directionally increased after the deposit accumulation period, but only NOx decreased with the removal of CCD. Engine-out NOx emissions increase with heavy CCD weight.

Diesel fuel detergent additives are increasingly linked with high quality automotive diesel fuels. Both in Europe and in the USA, field problems associated with fuel injector coking or fouling have been experienced. In Europe indirect injection (IDI) light duty engines used in passenger cars were affected, while in the USA, a direct injection (DI) engine in heavy duty truck applications experienced field problems. In both cases, a fuel additive detergent performance test has evolved using an engine linked with the original field problem, although engine design modifications employed by the manufacturers have ensured improved operation in service. Increasing awareness of the potential for injector nozzle coking to cause deterioration in engine performance is coupled with a need to meet ever more stringent exhaust emissions legislation. These two requirements indicate that the use of detergency additives will continue to be associated with high quality diesel fuels.

In the quest for improved fuel efficiency and reduced CO2 emissions, motor manufacturers are increasingly turning to the High Speed Direct Injection (HSDI) diesel engine for passenger car use. To achieve acceptable levels of noise and emissions at low loads two stage injection is being utilised. Such injection systems are prone to nozzle coking due to the small fuel metering holes, low opening pressures and low fuel flow rates under part load operation. This coking leads to a rapid deterioration of emissions performance. This paper describes work done to investigate conditions leading to this phenomena and the possible mechanisms involved.