Flow Characteristics of Compressed Natural Gas Delivery for Direct Injection Spark Ignition Engines 2015-01-0002
High pressure ratio (PR) compressed natural gas (CNG) jets are studied using optical diagnostics under quiescent but realistic spark-ignited (SI) engine conditions. CNG jets were issued impulsively from a bespoke direct injector. Observations into the delivery characteristics for a large pressure ratio (PR) range were carried out (8.3<PR<400). Non-dimensional scaling laws show that the penetration rate can be represented by a single relationship. The penetration rate is the primary determinant of fuel targeting. Additionally, the jet growth rate and spread angle are also presented. The evolution of the transient underexpanded jet features: the barrel length and Mach disc diameter are detailed and a strong dependency on the injector needle-lift is shown. These nearfield compressible features are recognised as flow structures which influence the mixing rate of a free jet and are also shown to control the size of the incompressible regions. The highly dynamic jet features are captured with high-fidelity schlieren high-speed video.
Particle image velocimetry (PIV) maps taken at multiple after start of injection (aSOI) instances detail the flow characteristics within the jet core and the ambient regions. Using the jet core centerline velocity, the Mach disc location (barrel length) is measured to within 7% of schlieren measurements. PIV measurements acquired at late injection (timing) conditions reveal detail on the influence of fuel delivery on the flow within the ignition zone. The mean flow velocity and kinetic energy are consequences of the jet's proximity to the ignition region with influences largely originating from the encroaching shear-layer and/or the rolling vortex.