Advanced Laser Ignition System (ALIS)
The primary constituent of natural gas is methane (>90%) which being a stable molecule is difficult to ignite. For methane, both chain branching reactions, as well as, chain propagation reactions are highly constrained by the available radical pool. As a result, ignition as well as flame propagation are difficult compared to other hydrocarbons.
With a view to comply with the ever stringent NOx emission standards, engine manufacturers have resorted to low-temperature combustion strategies such as lean-burn combustion and Exhaust Gas recirculation (EGR). To offset the loss in specific power of the engine in such strategies the intake charge is boosted by turbocharging. This along with the industry driven target to enhance typical engine BMEP ≈ 25 bar from the current values of BMEP ≈ 18 bar, results in extremely high in-cylinder charge densities across the spark plug electrodes that has an insulating effect. The Capacitance Discharge Ignition (CDI)Systems capable of generating voltages up to 40 kV across the spark gap and used prominently in the industry are not capable of overcoming such insulating effects and result in misfires. Additionally, at high spark gap voltages, the electrode erosion is accelerated to a point that spark plug life is highly compromised. Though the ARES desirable spark plug life is 8000 hrs, modern plugs have a life of 1000-3000 hrs.
With the above problems in mind, laser ignition appears to be a promising alternative. A series of tests were conducted by Argonne comparing the performance of conventional CDI ignition and laser ignition in a static chamber and a Rapid Compression Machine. Such tests have shown that sparking becomes easier at higher pressures in the case of laser ignition. Also, laser ignition is found to be capable of extending the lean-ignition limit all the way to the lean-combustion limit (flammability limit). Alternately, laser ignition allowed use of higher rates of charge dilution with exhaust gases or inert gases. Subsequent tests on a 11 liter displacement single-cylinder engine using laser ignition have shown lean-limit extension and accelerated combustion. Such a behavior with optimal ignition timing results in NOx reductions up to 70% or efficiency improvements up to 3% points. Additional performance improvements are possible through accelerated combustion resulting from placement of ignition kernel at the center of the combustion chamber and through multi-point ignition.
Encouraged by such results, efforts were pursued to develop a laser ignition system that has the functional blocks shown schematically below. The pulse train from a high-power laser is multiplexed and distributed to individual cylinders via fiber optic cables. At the distal end of the fiber optic cables, laser plugs – plugs with the same foot print as a conventional spark plug but containing a sapphire lens – refocus the laser energy to achieve sparking. Though most of the functional elements have been designed successfully, reliable transmission using fiber optic cables has remained a challenge. While efforts continue to develop better strategies for fiber-optic transmission, in the interim a system based on free-space laser transmission is being developed for use with a 6-cylinder engine. Preliminary tests performed on a Cummins 6-cylinder engine offer promise for lean-operation as compared to other advanced ignition systems.
Schematic representation of Advanced Laser Ignition System (ALIS).
A video showing prototype ALIS under test.
“Laser Based Ignition System for Natural Gas Reciprocating Engines, Laser Based Ignition System Having Capability to Detect Successful Ignition Event, And Distributor System for use with High-Powered Pulsed Lasers,” US Patent 7114858.
- Biruduganti, M., Gupta, S., Bihari, B., McConnell, S., Sekar, R., “Air Separation Membranes- An Alternative to EGR in Large Bore natural Gas Engines,” Journal of Engineering for Gas Turbines and Power GTP09-1189 (accepted for publication).
- Bihari, B., Gupta, S. B., Sekar, R. R., Gingrich, J. and Smith, J., “Development of Advanced Laser Ignition System for Stationary Natural Gas reciprocating engines,” ICEF2005-1325, ASME-ICE 2005 Fall Technical Conference, Ottawa, Canada, 2005.
- Biruduganti, M., Gupta, S., Bihari, B., Klett, G., and Sekar, R., “Performance Analysis of a Natural Gas Generator using Laser Ignition,” ICEF2004-983 Fall Technical Conference ASME 2004.
- Gupta, S. B., Sekar, R. R., Klett, G. M., and Ghaffarpour, M., “Ignition Characteristics of Methane-air Mixtures at Elevated Temperatures and Pressures,” SAE 2005-01-2189, SAE Transactions Journal of Fuels and Lubricants.
- Gupta, S., G. Klett, Biruduganti, M., Sekar, R., Saretto, S. R., Pal, S., and Santoro, R. J. “Laser Ignition for Natural gas Reciprocating Engines: A literature Review,” Paper No 204, CIMAC 2004.
Sreenath Gupta, firstname.lastname@example.org; Bipin Bihari, Bihari@anl.gov