An assessment of the ultra-lean combustion direct-injection LPG (liquefied petroleum gas) engine for passenger-car applications under the FTP-75 mode

Fuel economy and emissions characteristics of the lean-burn combustion strategy were assessed in a direct-injection liquefied petroleum gas engine under urban driving conditions. This study investigated: (1) engine experiments under steady-state conditions that frequently occur during city driving and (2) zero-dimensional drive cycle simulation under the FTP-75 mode. From the engine experiments, two sets of operation maps were constructed for the lean and conventional combustion modes. The lean combustion set consisted of the lean operation between 20 and 80 Nm and the stoichiometric operation for the rest of the operating envelope. The selected lean operation range was found to be the largest possible to operate under the lean condition. The entire conventional combustion set was taken at the stoichiometric condition for comparison with the lean combustion set. Then, the steady-state performance maps of the fuel consumption, emissions, and exhaust temperature of the two combustion sets were used in a vehicle model of the cycle simulation. The drive cycle simulation results showed that the fuel economy of the lean combustion set was 10.65 km/l, which is lower than that of the conventional combustion set by 6.3%. Carbon dioxide emission was 146.25 g/km, down by 8 g/km from the conventional combustion set. The time-resolved emissions of the particle mass and particle number of the lean combustion set were 0.005 g/km and 3.2 × 1011 #/km, respectively. The particle emissions of the lean combustion set were higher than those of the conventional combustion set, but considered as acceptable levels based on the upcoming emission standards.