Effects Of Air-Fuel Ratio And Operating Conditions On Particle Emissions From A Small Diesel Engine

Automotive engineers typically increase the air-fuel ratio (AFR) of an engine to control the amount of smoke emitted, but it not quite known how this process affects particulate number (PN). In the work presented, AFR was independently varied to study its effects on PN. It was found that increasing the AFR reduced the concentrations of larger particles from 108 #/cm3 to 106 #/cm3 which is an effect observable as a reduction in smoke. However, the same increases in AFR only resulted in an energy specific PN change from 1015 #/kWh to 1014 #/kWh. The study was then extended to examine how different combinations of engine speed, torque and timing affected PN. It was found that variations in timing (17 oBTDC to 4 oBTDC), speed (2000RPM to 3000RPM), and load (0 ft-lb to 12 ft-lb) had negligible effects on the amount of normalized PN produced. A novel study was performed to investigate the contribution of lubricating oil on the PN, and it was found that changing the oil formulation changed the amount of nuclei (sub-20nm) mode particles produced, and running the engine with no oil substantially reduced the nuclei (sub-20nm) mode while leaving the large (over-20nm) particle mode unchanged

Nucleation-accumulation Mode Trade-off in Non-volatile Particle Emissions From a Small Non-road Small Diesel Engine

Small (\u3c 8 kW) non-road engines are a significant source of pollutants such as particle number (PN) emissions. Many small non-road engines do not have diesel particulate filters (DPFs). They are so designed that air–fuel ratio (AFR) can be adjusted to control visible diesel smoke and particulate matter (PM) resulting from larger accumulation mode particles. However, the effect of AFR variation on smaller nucleation mode nanoparticle emissions is not well understood. Several studies on larger engines have reported a trade-off between smaller and larger particles. In this study, AFR was independently varied over the entire engine map of a naturally aspirated (NA) non-road small diesel engine using forced induction (FI) of externally compressed air. AFR’s ranged from 57 to 239 compared to the design range of 23–92 for the engine, including unusually high AFR’s at full-load operation, not previously reported for conventional combustion. As expected, larger accumulation mode particles were lowered (up to 15 times) for FI operation. However, the smaller nucleation mode nanoparticles increased up to 15 times. Accumulation mode particles stopped decreasing above an AFR threshold while nucleation particles continuously increased. In-cylinder combustion analysis showed a slightly smaller ignition delay and higher burn rate for FI cases relative to NA operation. Much higher peak cylinder pressures were accompanied by much lower combustion and exhaust gas temperatures (EGT), due to higher in-cylinder mass during FI operation. Peak nucleation mode emissions were shown to be negatively correlated to EGT for all the data, collapsing on a single curve. This is consistent with some other studies reporting increased nucleation mode emissions (and higher accumulation mode particles) with decreased load, lower speed, lower EGR, advanced combustion phasing, and higher injection pressure, all of which reduce EGT. The nucleation-accumulation trade-off has been explained by the ‘adsorption hypothesis’ by some investigators. In the current work, an alternative/supplemental argument has been made for the possibility that lower cylinder temperatures during the late-burning phase (correlated to lower EGT) phase hampers oxidation of nucleation mode particles and increases nucleation mode emissions