EFFECT OF AMBIENT TEMPERATURE AND DENSITY ON SHOCK WAVE GENERATION IN A DIESEL ENGINE

Shock-wave generation by high-pressure diesel sprays has been investigated over a range of ambient temperature and density in a high-temperature, high-pressure optical vessel. In the past, shock wave generation was considered unlikely for diesel sprays because of the high-temperature environment and low fuel injection pressure. However, recent trends toward very high injection pressures, and earlier (lower temperature) injection in modern diesel engines, can potentially produce shock waves. Through high-speed schlieren imaging, injection-resolved shock waves have been captured at injection pressures of 150 MPa. Marked changes in shock wave generation are shown to occur with variation in ambient conditions that are experienced in an engine, including the following: (1) Shock waves diminish in strength with increasing temperature and eventually disappear due to the increasing speed of sound. (2) At low ambient temperature, high-density ambient gas quickly decelerates the tip of the spray, causing shock waves to detach from the spray. (3) At high ambient temperature, the high density decelerates the spray tip during injection-rate ramp-up before the spray reaches the speed of sound, such that no shock waves are produced.