Laser Ignition in Gasoline Direct Injection Engines, 03-9354Printer Friendly Version
Inclusive Dates: 10/01/02 - 07/01/04
Background - The objective of this research is to improve current Gasoline Direct Injection (GDI) engine technology for use in automotive applications. GDI engines use stratified-charge, unthrottled operation to achieve improved part-load fuel economy in comparison to conventional Port Fuel Injection (PFI) engines. One disadvantage of current GDI technology is that the fuel spray dynamics required for proper stratified charge operation usually involves liquid fuel impingement on the piston. The piston is used to redirect the fuel spray such that a near-stoichiometric fuel/air mixture is produced near the wall-mounted spark plug, and a fuel-lean mixture is produced in the remaining volume of the combustion chamber. However, a portion of the liquid fuel remains on the piston and does not participate in the combustion process, resulting in high levels of unburned hydrocarbon emissions and less-than-optimal fuel economy. The premise of this project was that a laser ignition system could replace the wall-mounted spark plug and ignite the mixture at a location away from the chamber walls, negating the need for intentional fuel and piston interaction. This approach was expected to result in improved fuel economy and emissions.
Approach - An existing single-cylinder research engine was modified to accept the GDI system and laser ignition system. A pulsed Nd:YAG laser was used in conjunction with a series of mirrors and a lens to transmit and focus the laser beam to the desired ignition location within the combustion chamber. A fused silica window installed in the cylinder head provided the optical path into the combustion chamber. A 60-degree hollow cone-style fuel injector was positioned to avoid piston impingement. The variables that were studied included the laser focal point location, laser timing, fuel injection timing, and ignition spot energy density. High-speed cylinder pressure data was used to evaluate the performance of the engine.
Accomplishments - A complex interdependence between the fuel injection timing, the laser focal point location, and the laser ignition timing was observed. On the whole, the engine operated with highest fuel conversion efficiency when the mixture was ignited along the spray centerline. This result confirmed the benefit of laser ignition in controlling the ignition location. Additionally, it was noted that the laser was able to ignite mixtures that would normally be considered over-mixed (or insufficiently stratified) in a spark-ignited GDI engine.
Experiments with homogeneous air/fuel mixtures were performed to study the effect of ignition spot energy density and the lean ignitability limit. The lean operating limit was extended down to an equivalence ratio of 0.5 using the laser, in comparison to the limit of 0.6 using the conventional spark ignition system. Results also suggest that there is an optimum ignition spot energy density, beyond which engine performance no longer improves.