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The optimum modulation of fuel injection has been investigated by applying fuel injection research tools to several direct injection diesel engines. These studies provided insight into the fuel injection and combustion processes which has assisted the evaluation of the currently available practical devices. Two stage lift devices have been developed to reduce combustion noise at idle, light load and low speed engine operating conditions. However, once these devices enter their second stage of lift, the combustion noise increases rapidly. Two pilot injection devices have been tested in two high speed direct injection (HSDI) engines on test beds and both have reduced combustion noise over a wide speed range. When run in a vehicle, they have been particularly effective in reducing “acceleration noise” which can be very obtrusive in cars and light commercial vehicles fitted with high speed direct injection engines.