Natural gas stands out as a promising alternative fuel, and utilizing late intake valve close (LIVC) can further enhance its potential by improving internal combustion engine performance. The present study investigated the effect of LIVC on the performance of a Nissan Qashqai J10 four-cylinder internal combustion ignition engine (ICE) operating on gasoline (G) and natural gas (NG), with a focus on both energy and ecological aspects at stoichiometric points. Experimental tests were performed under the usual engine operating conditions, with engine speeds of 2000 and 3000 rpm and brake mean effective pressures (BMEPs) of 0.31, 0.55, and 0.79 MPa, while the intake valve closing moment was delayed at 24°, 31°, 38°, 45°, 52°, and 59° after bottom dead center (aBDC). The software AVL BOOST™ (version R2021.2) and its utility BURN were used to calculate the rate of heat release (ROHR), mass fraction burned (MFB), in-cylinder temperature, and the rate of temperature rise. The substitution of natural gas for gasoline substantially decreases CO₂ and NO_x emissions while enhancing the engine’s energy efficiency. Implementing a LIVC strategy can further boost brake thermal efficiency and reduce CO₂, though it negatively impacts CO, HC, and NO_x emissions. Optimal performance necessitates balancing efficiency improvements and CO₂ reduction against the control of other pollutants, potentially through combining LIVC with alternative engine control methodologies.

This study explores the impact of intake air cooling intensity, defined by heat exchanger effectiveness (HEE) and exhaust gas recirculation (EGR), on the energy and environmental performance of a turbocharged compression ignition (CI) engine. Experimental investigations were conducted on a 1.9-litre CI engine operating at 2000 rpm under three brake mean effective pressure (BMEP) conditions (0.2, 0.4, and 0.6 MPa), which correspond to part-load engine operation. HEE was varied at 0%, 50%, and 100%, in both EGR-on and EGR-off modes. Additional numerical simulations were carried out using AVL BOOST software to analyze combustion dynamics, including engine operating cycle modeling to validate the accuracy of the combustion analysis. The results demonstrate that increasing HEE significantly improves cylinder filling and excess air ratio, leading to enhanced combustion efficiency and lower in-cylinder temperatures. This, in turn, reduces specific NO_x emissions by approximately 40% with EGR and approximately 60% without EGR; however, under EGR-on conditions, the reduced combustion intensity leads to increased smoke and unburned hydrocarbon emissions—particularly at high cooling intensities. This effect is primarily associated with the engine control unit’s (ECU) limitations on intake air mass flow to maintain the target EGR ratio. Integrated control of HEE and EGR systems improves engine performance and reduces emissions across varying conditions, while highlighting trade-offs that inform the refinement of air management strategies.

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, NO_x, 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 NO_x (−34.0%) emissions were lower with CNG, linked to leaner operation and significantly retarded ignition timing for NO_x 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 NO_x (−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 NO_x control logic and the potential to mitigate methane slip.