Abstract: Adjustment of injection strategy has been considered as an effective and reliable way for the emission control ofdirectly injected natural gas engines.However, the system investigation on the effects of pilot diesel quantity and injectionseparation is scarce.In order to achieve better emission characteristics by optimizing the injection strategy of directlyinjected natural gas engine, experimental investigation was conducted on a 6 cylinder pilot ignited natural gas engine tostudy the effects of diesel injection quantity (DIQ) and separation between diesel and natural gas injection (SDN).The testengine was modified from a diesel engine with bore diameter of 126 mm, stroke of 155 mm and compression ratio of 17.The design of cylinder head was redesigned to adapt to the installation of the dual fuel injector, which had two concentricneedles and two electronically controlled solenoid valves.An integrated pressure regulating module was added to control the injection pressure of diesel and natural gas.The supply system of natural gas, including compressor and buffer tank,was also added to provide compressed pipeline natural gas.Besides, the control module of natural gas injection wasintegrated into the original electronic controlled unit to realize the accurate control of both diesel and natural gas injection.In the experiment process, the intake temperature was fixed at 22° CA and the intake air pressure was maintained at 101kPa while the diesel rail pressure was adjusted to 18MPa.To prevent natural gas leaking into diesel, the rail pressure ofnatural gas was slightly lower than that of diesel (17.5 MPa).The tested operating condition was at engine speed of 1 275 r/min with brake specific effective pressure of 0.54Mpa, which was a typical operating point of European steady state cycle.To evaluate the effects of diesel injection quantity(DIQ) and separation between diesel and natural gas injection(SDN),the injection timing of natural gas was kept constant at 8° BTDC while diesel injection quantity was varied from 4.0 to 11.5mg under four different injection separations(0.5 ms, 0.8 ms, 1.1 ms and 1.4 ms).The natural gas pulse width was adjustedin accordance with the diesel injection quantity to maintain the fixed engine brake specific power.The emissions weremeasured by a Horiba MEXA 7 200 exhaust gas analyzer.CO emissions were tested by nondispersive infrared technology.HC emissions were tested by flame ionization detector.NO_x emissions were tested by chemiluminescent detector (CLD).The emissions of each operating point were collected after 5 minutes of steady operation and all the emissions wererecorded three times to obtain the averaged values for further analysis.The experimental results showed that HC emissionsdecreased with the increase of diesel injection quantity at the same injection separation and increased with increasinginjection separation at the diesel injection quantity of 4.0 mg; in the diesel injection quantity range of 6.0~11.5 mg.HCemissions changed slightly when the injection separation varied from 0.5 ms to 1.1ms, however, when injection separationextended to 1.4 ms, the rising trend of HC emissions became more obvious.CO emissions exhibited a first decrease thenincrease trend with the increase of diesel injection quantity, and the minimum value occurred at the diesel injectionquantity of 6.0 mg or 8.5 mg; a decrease with the increase of injection separation at all diesel injection quantities can alsobe observed.NO_x emissions firstly declined and then rise with the increasing diesel injection quantity; additionally, at theinjection separation of 0.5 ms, NO_x emissions were relatively lower while reached the peak value at the injection separationof 1.4 ms.It can be concluded that the increase of diesel injection quantity has beneficial effects on HC emissions andnegative effects on NO_x emissions while exerts little influence on CO emissions; the extension of injection separationresults in higher HC and NO_x emissions as well as reduced CO emissions.