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The use of small-hole diesel injector tips and high injection pressures was investigated as a countermeasure to the increased particulate matter (PM) emissions formed when using exhaust gas recirculation (EGR) in diesel engines. This study examined the use of injector tip hole sizes down to about 0.09-mm (0.0035 in.), and injection pressures to 300 MPa (3000 bar, or 43,500 psi). The first phase of these studies was conducted in a high-temperature, high-pressure combustion bomb, with supporting calculations using a unit injector model, a jet-mixing model, and a diesel jet evaporation model. The second phase was conducted in a commercial diesel engine of 12.7-liter displacement designed to meet U.S. 1998 emissions levels. Engine tests were conducted with a baseline cam and a faster rise-rate cam, and three different hole tip sizes. The cams consisted of a baseline cam and a cam of similar design, but with a 12 percent faster rise rate.