<div class="htmlview paragraph">The investigation of pumping efficiencies of classic FIE lead to the development of an hydraulic unit injector (HUI). In the HUI the pump plunger is not directly driven by a camshaft but by a distributor-type fuel-injection pump (VE-pump) which simultaneously operates the control.</div> <div class="htmlview paragraph">The HUI was designed with a computer model. The differential piston increases the pressure by the multiplier 4. The VE-pump produces only 30 MPa for an injection pressure of 100 MPa. The high pressure chamber is filled through a small check valve.</div> <div class="htmlview paragraph">Cavitation in the pipe is eliminated by a specially designed delivery valve and a preloaded spring for the differential piston.</div> <div class="htmlview paragraph">A reduced needle shaft (3 mm diameter) leads to a very fast opening and closing of the nozzle, thus allowing to set the opening pressure to 40 MPa.</div> <div class="htmlview paragraph">The HUI's were driven on the pump test bench up to the engine speed of 5000 rpm and reached a peak injection pressure of 120 MPa. On the engine test bench a bmep of 0.8 MPa at a smoke number 4 was reached with a 1.7 l naturally-aspirated engine. The minimum specific fuel consumption was 225 g/kWh. The ignition delays are 2-4°CA shorter than usual due to the small 0.17 mm nozzle holes. This results in a considerably lower NO_x concentration in the exhaust gas. On the other hand there is still a high smoke emission at part load which cannot be explained:Smoke number = 2 at bmep = 0.3 MPa and n = 3000 rpm.</div>