GASOLINE ENGINE OPERATING AT LEAN AIR-FUEL MIXTURE BY LASER IGNITION

Internal combustion engines for automobiles have undergone rapid changes in the past century. Driven by several factors including customer demand, the global concern on the impact of the use of engines on the environments and advancement in technology such as the development of electric cars, the performance of engines has been tremendously improved. The electric spark plug has been widely used in the engine ignition. However, concerning the difficulty in igniting diluted air-fuel mixtures and operating under high pressure in the engine combustion chamber when an electric spark plug system is used, various enhanced systems have been investigated for faster and robust combustion.

Among the proposed ignition systems, laser ignition has particularly attracted significant research attention owing to its potential advantages. For example, manipulation of the laser beam offers access to the internal parts of the cylinder to allow ignition at any point thus improving the engine efficiency. Unlike electric spark plug, laser beams allow the ignition of the diluted air-fuels mixtures at high-pressure operation.

For the past few years, the operation of different types of engines based on the laser ignition system has been investigated. To this effect, the National Institute for Laser, Plasma and Radiation Physics researchers: Dr. Nicolaie Pavel and Dr. Mihai Dinca in collaboration with Professor Radu Chiriac and Dr. Florin Draghici from the University of Politehnica of Bucharest and Dr. Adrian Birtas from Renault Technologie Roumanie assessed the performance of a conventional multipoint fuel injection passenger automobile engine operated by an own developed laser ignition system. Specifically, they investigated the performances of a laser ignition system in terms of power, stability and pollutant emissions. The work is currently published in the research journal, Optics Express.

The four-stroke, four-cylinder engine was operated at a constant speed and constant load condition of 2000 rpm – 2 bar at different ignition timings. This was used to simulate the city traffic conditions. Additionally, two air-fuel ratios for stoichiometric (λ~1) and lean (λ~1.25) mixture conditions were considered thus allowing measurements of parameters indicating engine performances, combustion stability, emission, and efficiency.

The research team observed general improvement in the engine brake power and combustion stability under the laser ignition system. For instance, in comparison with the electric spark plug, the laser ignition-based engine that was run at optimum ignition timing resulted in an increase in power up to approximately 29% on lean λ~1.25 mixture combustion. Also, changing of the engine ignition system from classical electric spark plug to laser ignition devices led to a decrease in the brake specific fuel consumption for both air-fuel ratios. It is worth pointing out that the tested mixtures exhibited a reduction in the carbon monoxide emission. On the other hand, however, the emission of nitrogen compounds rose for all the ignition timings under laser ignition. The results were attributed to more stable engine operation provided by faster ignition and more robust combustion initiation due to a higher efficiency of the ignition induced by laser in the breakdown stage of flame kernel formation, with a substantial reduction of the initial stage of combustion.

Regarding the benefits of using laser ignition system-based engines and the extensive research on the same, the world is slowly realizing single beam compact lasers resembling conventional spark plug devices. Some of these devices have been applied in passenger gasoline engine vehicles. Based on the presented results, further improvements on the laser ignition systems will lead to high engine performance and efficiency in terms of stability, emissions, and power. Therefore, the study performed by the Romanian researchers provides essential information that will pave way for further research and development of more advanced, reliable and cost-competitive laser ignition systems.