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.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.Cavitation in the pipe is eliminated by a specially designed delivery valve and a preloaded spring for the differential piston.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.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 NOx 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.