Effect of Air-Fuel Ratio on Engine-Out Exhaust Hydrocarbon Species from a Direct Injected Gasoline Engine

Stanislav  Bohac, University of Michigan

Exhaust HC speciation from port fuel injected (PFI) engines has been studied extensively, but investigations of exhaust HC species from direct injected gasoline (GDI) engines are limited in number. HCs from GDI engines may differ from PFI engines because the fuel/air mixture in a GDI engine tends to be less homogeneous due to wall wetting and reduced time for fuel evaporation.

In this study, engine-out hydrocarbons are speciated from a stock MY2010 GM Ecotec LNF 2.0L turbocharged GDI engine that uses side mounted wall guided injectors and is operated on E10 93 octane pump gasoline. A GC-FID is used to speciate un-oxidized C1-C10 HC and an FTIR is used to speciate oxygenated HC. The engine is operated rich (lambda=0.90), stoichiometric (lambda=1.00), and lean (lambda=1.10), all at 1600 rpm, 8 bar BMEP.

For all lambdas, exhaust paraffins (37-47%) and olefins (35-40%) dominate aromatics (6-10%) and oxygenates (8-19%) in terms of carbon mass.

As lambda is swept from rich (lambda=0.90) to lean (lambda=1.10), the concentration of paraffins (638-316 ppmC1), olefins (471-316 ppmC1), and aromatics (127-54 ppmC1) decrease, and the concentration of oxygenates (110-164 ppmC1) increases. The fraction of paraffins (paraffins/total HC) decreases and the fraction of oxygenates increases. C1-C4 HCs (679-447 ppmC1), C5-C8 HCs (580-330 ppmC1), and C9-C10 HCs (846-408 ppmC1) all decrease, and the total partial pressure of exhaust HCs decreases from 0.50 to 0.21 mbar.

During rich operation, the top three HCs (in terms of ppmC1) are methane, ethene, and acetylene. During stoichiometric and lean operation, the three top HCs are olefins: propene, ethene, and 2-methylpropene. The highest aldehyde concentration observed is 56 ppmC1 formaldehyde at λ=1.10 and the highest aromatic concentration observed is 62 ppmC1 m&p-xylene at λ=0.90.

As lambda is swept rich to lean, average molecular weight of the HCs increases from 43.2 to 50.0 g/mol, and the average molecular composition changes from C2.91H6.41O0.11 to C3.13H6.60O0.360. During rich operation, average H:C and O:C ratios of exhaust HCs (2.20 and 0.0392) closely match those of the test fuel (2.22 and 0.0377), in agreement prior speciation studies investigating PFI engines. During lean operation, the H:C ratio of exhaust HCs becomes lower than the fuel due to reduced paraffin fraction, and the O:C ratio of exhaust HC is higher than the fuel due to an increased oxygenate fraction.

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