Diesel engines are widely used owing to their high thermal efficiency and low fuel consumption. Despite these benefits, particulates and NO_x emitted from diesel engines are of major concern for the environment. Though the global air fuel mixtures are lean in diesel engines, the local equivalence ratios can be greater than 2. These locally rich regions within the combustion chamber lead to favourable conditions for the formation of soot particles, which eventually results in higher exhaust tail pipe soot emissions.

...Stringent emissions regulations are proposed in Europe and US and around the globe to control and reduce the soot particulates...The number concentration of soot particulates emitted from diesel engine has also been a major concern. Euro 5 and Euro 6 aim to reduce PM emission to 0.005 g/km for a passenger cars but the future legislation has also introduced a limit on the particle number emissions.

The generation of soot in engines can be minimized by enhancing the mixing time to have a more premixed type of combustion, and also by operating the engine at conditions where the global in-cylinder temperatures are low enough to inhibit the in-cylinder soot formation. These conditions of improved mixing can be achieved through strategies such as high levels of EGR, high swirl and through advanced or retarded injection timing, which are normally referred to [as] partially premixed or low-temperature combustion.

The benefit of low soot emissions from these strategies are mainly limited to low and medium load operation. Limited information is available in the literature about the exhaust soot particle number size distribution under these operation conditions for diesel and RME fuels. So this work is focused toward exploring the exhaust soot particle number and size measurement under the strategies that favourlow temperature combustion.

—Labecki et al.

For the study, the researchers used a 4-cylinder, 2.0-liter high speed direct injection diesel with a compression ratio of 18.2:1. The injectors had six holes with a nozzle hole diameter of 0.154mm; the fuel was pressuried using a common rail system.

The main operating parameters investigated were injection timing, injection pressure and EGR under two different engine loads: 2.7 bar BMEP and 5.0 bar BMEP at 2000 rpm. Fuel injection pressure was varied from 800 to 1200 bar, while injection timing was varied from 9 to 3 deg bTDC, and for these conditions the level of EGR was also varied.

For each fuel injection timing, the effect of injection pressure and EGR was varied independnelty to study their effects on the soot particle number distribution. Under all operating conditions, they researchers measured particle number size distirbution thrice and reported the average from the scans.

Broadly, the other main findings from the study were:

1. At low engine load, more particles were observed in the nucleation mode. This condition provides low soot emissions due to lean mixture combustion. The nucleation mode particles were observed as a result of limited condensation and agglomeration. Increasing the engine load to 5 bar BMEP caused the combustion mixture to become richer and favorable for higher soot emissions. All small particles tended to coagulate and form aggregates of larger particles—hence the accumulation mode in the case of higher engine load, and wider particle size range.

2. Increasing the fuel injection pressure reduced the particle size and its concentration in the accumulation mode under low load operation. At higher loads, the nucleation mode increased as the fuel injection pressure increased, while the number conentration in the accumulation mode decreased with an increse in injection pressure.

3. The addition of EGR caused smaller particles from the nucleation mode to agglomerate to form larger size particles, which were observed in the accumulation mode. This effect was observed for EGR at both low and high loads. A combination of high load and EGR results in the detection of significanty larger particles. High levels of EGR cause the particle number size distribution to become uni-modal. The number concentration in the accumulation mode under high load operation increased with EGR.

4. The effect of injection timing did not have a clear and consistenet trend between diesel and RME fuels in the nucleation and accumulation mode under low load operation. Under high load operation, retarding the fuel injection timing from 9 deg bTDC to 3 deg bTDC caused the particle number concentration in the nucleation mode to decrease for diesel. The size distribution transformd from bi-modal to uni-modal distribution at the injection timing of 3 deg bTDC, but the accumulation mode remained the same for different injection timings. The particle number conentration for RME reduced both under nucleation and accumulation mode when the fuel injection timing was retarded at high load.

Resources

• Lukasz Labecki, Andreas Lindner, Wolfgang Winklmayr, Renate Uitz, Roger Cracknell, Lionel Ganippa (2013) Effects of injection parameters and EGR on exhaust soot particle number-size distribution for diesel and RME fuels in HSDI engines. Fuel, Volume 112 Pages 224-235 doi: 10.1016/j.fuel.2013.05.013.