Researchers at Jilin University in China report in a papaer in the journal Fuel that coolant temperature in a gasoline direct-injection (GDI) engine under idle has a critical effect on particulate number (PN) emissions and that optimizing other operating parameters will further reduce PN emissions.

PN concentration with coolant temperatures coupled with excess air ratios, the ignition timing is 20 CAD BTDC, injection pressure is 3 MPa and injection timing is 90 CAD BTDC. Sun et al. Click to enlarge.

Compared to port-fuel injection (PFI) engines, gasoline direct injection (GDI) engines show a significant (5–15%) improvement in fuel economy, especially because of higher volumetric efficiency and higher knock resistance. Furthermore, the fuel injection control of GDI is more accurate than PFI, resulting in improved potential for higher thermal efficiency and fuel economy. GDI engines provide higher specific power output relative to PFI. However, GDI engines tend to emit higher particulate number (PN) than PFI engines and even the diesel engines equipped with diesel particulate filter (DPF).

… Because of the high residual gas fraction, low combustion temperature and weak charge flow in the cylinder, SI engines always consume more fuel and expel more toxic emissions at idle stage. Moreover, traffic congestion is very common in modern cities which make vehicles idle for rather long time. For example, vehicles idle time accounts for about 30% of total running time in Hong Kong, and about 38% of that in Shanghai during peak hours. Thus, improving engine idle performance becomes a valuable issue to enhance engine overall performance.

Due to the increased residual gas fraction and inhomogeneity of the air-fuel mixture, idle emissions also increase, especially for particulate emissions. Previous studies indicated that the majority of particle emissions were found in the first three minutes of the driving cycle at cold start, and this part is mainly idle stage.

… This paper focused on PN emission under idle stage which only has limited investigations. Besides, the influences of each parameter under different coolant temperatures are novel and useful in future studies.Sun et al.

The Jilin team conducted two sets of experiments to analyze the effects of different parameters associated with PN emissions under different coolant temperatures. The first set analyzed ignition timing and excess air ratio characteristics; the second one mainly investigated injection strategies. Coolant temperature was adjusted from 40°C to 80 °C for both sets to represent the cold-start process.

PN concentration with coolant temperatures coupled with injection pressures, the excess air ratio is 1, ignition timings is 20 CAD BTDC and injection timing is 80 CAD BTDC. Sun et al. Click to enlarge.

The test engine was a second-generation 1.8-liter GDI engine (118 kW, 250 N·m) which belongs to charge-guided type; mixture condition was close to homogeneous charge. The original engine head has a centrally mounted spark plug with the injector situated between the intake valves.

Among the major findings of the study:

• Total PN emissions declined with increasing coolant temperature in each group of experiments. The decreasing amplitude is limited in low coolant temperature range. When coolant temperature exceeded 70 °C, PN significantly decreased.

• 5–22 nm PN emissions increase and 22–50 nm PN emissions decrease with higher λ. A richer mixture has higher 50–150 nm PN than leaner ones. To enhance three-way catalyst (TWC) efficiency, an equivalent mixture is suitable without resulting in an obvious PN increase.

• Advancing ignition timing will decrease total PN under idle stage and the decrease is mainly from 5 to 22 nm. Particles belong to 22–50 nm and 50–150 nm are insensitive to ignition timings. Properly advanced ignition timing will make combustion phase shifting forward and significantly decrease PN.

• Total PN was minimized at 5 MPa injection pressure. Too low an injection pressure will deteriorate fuel evaporation, while too high an injection pressure will aggravate the particle nucleation process. An adequate injection pressure should be adopted which can fulfill fuel evaporation and restrict nucleation.

• Total PN minimized at 80 CAD BTDC injection timing and the value is only 14% for the minimum value in 140 CAD BTDC. Optimal injection timing for PN is significant under idle stage which should avoid fuel impingement both on piston head and the cylinder line.

In summary, coolant temperature is critical for particulate emissions under idle stage, so shortening the warm-up stage is important to reduce the PN emissions. Specifically, fixing excess air ratio at 1, properly advancing ignition timing, adopting 5 MPa injection pressure and injecting fuel at 80 CAD BTDC are optimal parameters for reducing particulate number emissions under idle stage in this experimental situation.

—Sun et al.

This work is supported by the National Natural Science Foundation of China. Resources

• Yao Sun, Wei Dong, Xiumin Yu (2018) “Effects of coolant temperature coupled with controlling strategies on particulate number emissions in GDI engine under idle stage,” Fuel, Volume 225, Pages 1-9 doi: 10.1016/j.fuel.2018.03.075