Experimental Investigation of Performance and Emissions for a Hybrid Electric Vehicle Operating on Gasoline, CNG, and Dual Fuel over the WLTC

Hybrid electric vehicles (HEVs) frequently cycle their internal combustion engines (ICE), potentially cooling the three-way catalyst (TWC). This challenges the use of compressed natural gas (CNG), as methane (CH4) requires high temperatures for TWC oxidation. This study experimentally investigates the performance, engine-out emissions (CO, NOx, CH4, NMHC, CO2), and catalyst temperatures of a Toyota RAV4 hybrid vehicle on gasoline (G), CNG, and dual fuel (MIX) during the WLTC. Engine-out emissions were measured upstream of the TWC. Results showed similar engine work output (~17.8 kWh/100 km), while CNG significantly reduced fuel mass consumption (−18.7%) and CO2 emissions (−27.5%) compared to gasoline, driven by both its higher LHV and higher average BTE. CO (−32.3%) and NOx (−34.0%) emissions were lower with CNG, linked to leaner operation and significantly retarded ignition timing for NOx control. However, CH4 emissions drastically increased with CNG. This study reveals a synergy between the same retarded ignition timing strategy used to successfully control engine-out NOx (−34.0%) and created a positive secondary effect, raising pre-TWC temperatures by 4.5%. Higher thermal condition is essential for the aftertreatment of chemically stable methane, highlighting a direct link between the engine’s NOx control logic and the potential to mitigate methane slip.