A broadly ranging single injection event was used in a Waukesha diesel CFR engine in order to explore various conventional and alternative combustion modes at light load (2 bar GMEP) using n-heptane fuel. Start of injection (SOI) was varied from the start of the intake valve open (IVO) event all the way past TDC at the end of the compression stroke. Emissions, including detailed particulate, were collected at all of the operating points. Additionally, further experiments were performed with port fuel injection in order to create a homogeneous charge compression ignition (HCCI) combustion mode as well as partially premixed combustion (PPC) using both port and direct fuel injection.
HCCI and PPC combustion modes were achieved with the characteristic rise in carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions with however, a corresponding decrease in NOx emissions as compared to conventional direct (into cylinder) injection combustion modes. For conventional diesel operation with progressive advancement of SOI it was seen that start of combustion (SOC) advanced and then retarded slightly before stabilizing. This was associated with a general lengthening of ignition delay (IGD) with progressive SOI advancement. Even with very early intake valve open (IVO) injection events, the emissions behavior did not approach HCCI or PPC, suggesting that the charge mixture homogeneity of companion port injection could not be achieved in this engine using direct injection alone.
High speed optical natural light filming of the combustion events through a large quartz window showed conventional diesel combustion with strong diffusion flames, reducing in intensity with PPC operation, and then no visible combustion with HCCI.