Large natural gas engines that introduce premixed fuel and air into the engine cylinders allow a small fraction of fuel to evade combustion, which is undesirable. The premixed fuel and air combust via flame propagation. Ahead of the flame front, the unburned fuel and air are driven into crevices, where conditions are not favorable for oxidation. The unburned fuel is a form of waste and a source of potent greenhouse gas emissions. A concept to vent unburned fuel into the crankcase through built-in slots in the liner during the expansion stroke has been tested. This venting process occurs before the exhaust valve opens and the unburned fuel sent into the crankcase can be recycled to the intake side through a closed crankcase ventilation system. The increased communication between the cylinder and the crankcase changes the ring pack dynamics, which results in higher oil consumption. Oil consumption was measured using a sulfur tracer technique. Careful design is required to achieve the best tradeoff between reductions in unburned hydrocarbon emissions and oil control.
On the Cyclic Variability and Sources of Unburned Hydrocarbon Emissions in Low Temperature Diesel Combustion Systems