Investigation of particle formation mechanisms in GDI engines during transient heating operation English

  • Category Technical paper
  • Edition SIA
  • Date 12/04/2013
  • Author G. L. Pilla, L. de Francqueville - IFP Energies Nouvelles
  • Language English
  • Type PDF file (3.69 Mo)
    (Downloadable immediately on receipt of online payment)
  • Number of pages 9
  • Code R-2013-06-08
  • Fee from 8.00 € to 10.00 €

New Euro 6 emission standards will become effective in September 2014. For the first time, both particle matter (PM) mass and particle number (PN) will be limited for spark-ignition engines. Particle emissions can reach more than 50% of the total emitted particles during the first minute of the NEDC cycle. Consequently reducing particle emissions in cold-start operation is a crucial step towards development of new GDI engines. This paper aims at presenting a methodology dedicated to investigate particle formation processes occurring in a GDI engine during transient heating operation. The engine was operated in both allmetal and optical configuration. In-cylinder wall temperature was measured by laser-induced phosphorescence (LIP). The wall temperature in optical configuration was ensured to be representative of all-metal configuration by using sapphire for optical accesses. Wall temperature reaches approximately 100°C after 60s of operation.
Several optical diagnostics were used: Mie scattering to visualize fuel sprays and liquid droplets on the piston and on the liner surface, direct visualization of self-illuminated combustion to characterize flame propagation and incandescent soot. Both diagnostics were performed with rapid cameras (4 kHz). Total internal reflection laserinduced fluorescence (TIR-LIF) was successfully used for the first time ever to evidence fuel liquid film on the liner surface. Conventional LIF was used to localize fuel film on the piston surface. Several parametric variations were performed to investigate the main mechanisms involved in particle formation: targeting and spray-plume angle of the multi-hole injector, injection pressure, injection strategy, injector fouling and in-cylinder aerodynamics.
It could be shown that large spray-plume angle significantly improves atomization and limits wall  impacts. The spray-cone angle must be such that sprays are contained in the piston bowl in order to avoid fuel accumulation phenomena in the top-land region. Multi-injection strategy, which consists in splitting the injection into several ones is efficient to limit liquid penetration and fuel impact on the walls. A very short injection is performed close to ignition to stabilize combustion in catalyst warm-up operation. This late injection hardly evaporates because of the lack of time between injection and ignition. Increasing injection pressure was shown to be very efficient to improve its atomization. In case of multi-injection, enhancing in-cylinder aerodynamics significantly limits the impact of the first injection on the piston, whereas it has a very little effect on a single injection strategy. Depending if a single or a multi injection strategy is used, it was found that air/fuel mixing processes is differently driven. Single injection in mainly controlled by wall impact, therefore reducing injection pressure can be helpful to limit the fuel quantity impacted on the piston. On the other hand, multi-injection in mainly controlled by atomization: the higher the injection pressure, the better the atomization and the reduction of PN emissions.


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