Dual fuel engines have shown significant potential as high efficiency powerplants. In one example, SwRI® has run a high EGR, dual-fuel engine using gasoline as the main fuel and diesel as the ignition source, achieving high thermal efficiencies with near zero NOx and smoke emissions. However, assuming a tank size that could be reasonably packaged, the diesel fuel tank would need to be refilled often due to the relatively high fraction of diesel required. To reduce the refill interval, SwRI investigated various alternative fluids as potential ignition sources. The fluids included: Ultra Low Sulfur Diesel (ULSD), Biodiesel, NORPAR (a commercially available mixture of normal paraffins: n-pentadecane (normal C15H32), and n-hexadecane (normal C16H34)) and ashless lubrication oil. Lubrication oil was considered due to its high cetane number (CN) and high viscosity, hence high ignitability. These fluids were evaluated at a low speed, low load condition where smoke was problematic and shown to be a strong function of the ignition source.
Biodiesel and ULSD showed similar smoke levels with biodiesel producing a shorter ignition delay. With the same energy flux and constant gasoline flow rate, NORPAR had higher smoke than ULSD because of significantly shorter ignition delay and mixing time. Lubrication oil had even higher smoke emissions at a constant energy flux and gasoline flow rate, which was attributed to the high viscosity leading to mixing problems. However, because of both the high viscosity and high cetane number of the lubrication oil, the ratio of ignition fluid to gasoline could be reduced without impacting the combustion stability. When the fraction of ashless lubrication oil was reduced from 18% to 10%, the smoke was reduced to levels comparable to ULSD. Based on the test results, it is assumed that the ratio of ignition fluid could be reduced even further, which indicates that the use of ashless lube oil may be a realistic option as an ignition source in a dual-fuel engine.