Abstract
Lasers can be used as an alternative ignition source to spark ignition in engines where low cycle to cycle variations in the ignition point is in great demand. Previous studies have proved that laser ignition in SI engines can be used to decrease the cycle to cycle variations in the combustion normally caused by quenching in the spark plug electrode gap. Laser ignition is also known to reduce the ignition delay and the combustion duration. Another advantage is the possibility to place the ignition source in a more suitable location in the combustion chamber, for example to suppress knock. Simultaneous ignition sources is a further measure to affect these parameters and implementation is just a matter of optical arrangement. Laser ignition can also be used to support HCCI combustion during operating conditions that are not favourable such as misfire or late combustion phasing. In this study laser ignition has been tested in an HCCI engine running with natural gas as fuel. A Scania D12 diesel engine was converted to single cylinder HCCI operation by the use of port fuel injection and inlet air preheating. Optical access into the combustion chamber was enabled by a bowditch piston extension with a quartz piston window and a mirror. Chemiluminescence imaging was used to monitor the effect of the laser plasma and further the onset of combustion. A single shot intensified camera system was used to retrieve pictures of the chemiluminescence at one specific crank angle each cycle. A high speed framing camera was also used to monitor the combustion event from the point of ignition to complete combustion with a resolution of 0.25 CAD. The laser used for these experiments was an Nd:YAG laser with a laser pulse energy of about 30 mJ and a pulse duration of 5 nsec. The effect of the laser plasma was investigated for different running conditions, as laser energy, ignition timing, ignition location, combustion phasing, load, dilution with EGR and trapped residuals. The lean limit for an effect of the laser plasma was found to be at lambda 2.5. With the help of the laser ignition the combustion phasing could be advanced up to 8 CAD from unstable running conditions. Different ignition locations were investigated, one in the centre of the combustion chamber and one in the side of the combustion chamber close to the cylinder wall. The latter one turned out to be the most effective location due to the fact that the HCCI combustion starts at the walls and propagates towards the centre of the combustion chamber in this particular engine. The engine was also equipped with a pneumatic valve system that enables continuously variable valve timings and valve lift. Thus it was possible to test the effect of laser ignition when running the engine with negative valve overlap and thus high residual rates as an alternative to EGR.
Keywords:Homogeneous Charge Compression Ignition, Laser Ignition, Natural Gas, Chemiluminescence Imaging, Initial Slow Heat Release