Experimental Investigation of Fuel Consumption and Exhaust Emissions of a 16V Pent-Roof Engine Fueled by Gasoline and CNG

A systematic experimental investigation was undertaken to compare the fuel consumption and exhaust emissions of a production SI engine fueled by either gasoline or compressed natural gas (CNG). The investigation was carried out on a two-liter four-cylinder engine featuring a fast-burn pent-roof chamber, one centrally located spark plug, four valves per cylinder and variable intake-system geometry. The engine was originally designed at Fiat to operate with unleaded gasoline and was then converted at Politecnico di Torino to run on CNG. A Magneti Marelli IAW electronic module for injection-duration and spark-advance setting was used to obtain a carefully controlled multipoint sequential injection for both fuels. Besides the design features and the air-fuel control system, which are of particular interest for natural gas operation due to the lack of data on their combined application to CNG vehicles, the relatively high compression ratio of ≈ 10.5, that is almost suitable for natural gas fueling, makes the engine attractive for bi-fuel operation, i.e. the capability of running on either CNG or gasoline, where operation with natural gas is usually compromised.

The engine was operated with gasoline and natural gas under steady state conditions at four different speeds and loads. MBT spark timing and stoichiometric air-fuel ratio were used first. Then for each point in the speed and load matrix, tests were carried out both as “spark sweeps” centered on MBT spark advance and as “RAFR sweeps” around stoichiometric air-fuel ratio. At each test with both fuels, the engine performance analysis was carried out in terms of specific fuel consumption and exhaust emissions. An upgraded multipurpose exhaust-gas analyzer, including a device to separate methane from non-methane HC pollutants, was used for emission quantity measurements. The effect of engine variables on fuel conversion efficiency and exhaust emissions was thoroughly investigated.