Port Injection of Secondary Fuel Can Simultaneously Lower NOx and PM in a Direct Injection Biodiesel Engine

8 October 2008

Effect of premixed fuels on the engine emissions at different equivalency ratios. Click to enlarge. Credit: ACS

Researchers at Shanghai Jiao Tong University found that port fuel injection of a secondary fuel with a lower boiling point in a biodiesel-fueled, direct injection engine can simultaneously reduce both NO_x and PM emissions. A paper on their work appeared online 4 October in the journal Energy & Fuels.

A number of studies have shown that the use of biodiesel in light- and heavy-duty diesel engines can lower PM, CO, SO_x, and HC emissions compared to standard diesel fuel combustion with comparable or even slightly better engine efficiency. However, NO_x levels can increase—a problem when confronting more stringent regulatory limits on NO_x emissions.

According to the flame model of the direct-injection (DI) combustion process, it is very difficult to suppress the NO_x and soot emissions to very low levels simultaneously because of the “trade-off” relationship between the NO_x and soot emissions. Therefore, it is necessary to develop a new combustion method to simultaneously improve the NOx and soot emissions for biodiesel engines.

In the present study, the partial HCCI or compound HCCI combustion mode was introduced into the biodiesel engines. By this method, the authors hoped to simultaneously reduce the NO_x and soot emissions of the biodiesel engine. To achieve this target, a moderate amount of premixing fuel with lower boiling point, such as n-heptane, DMM [dimethoxymethane], and ethanol, was injected into the intake port and used as a partial substitute of biodiesel. As a result, a leaner homogeneous fuel/air mixture could be formed because the premixing by port fuel injection allowed sufficient time for evaporation and air mixing.

—Lu et al. (2008)

The research engine was based on a single-cylinder, four-stroke, naturally-aspirated direct injection engine with 782 cc of engine displacement. Engine speed was fixed at 1,800 rpm in this study.

Premixed fuel was injected into the intake port by an electronic fuel injector approximately 0.35m upstream to the intake port, to allow the leaner homogeneous fuel/air mixture to form during the intake stroke and compression stroke. The original fuel injection system injected the biodiesel charge directly into the cylinder at near top dead center (TDC). An additional universal ECU, which was synchronized with an engine encoder and various sensors, controlled the amount of the secondary premixed fuel.

The study found that:

• The heat release curves of the n-heptane/biodiesel premixed combustion presents a three-stage combustion, namely, cool flame and hot flame reaction of n-heptane, as well as diffusion combustion of biodiesel, while the heat release curves show only one-stage heat release with the port injection of DMM and ethanol.

• A simultaneous reduction of NO_x and smoke opacity can be obtained with the premixed fuels, but ethanol premixing shows the most significant effects in the reduction of NO_x and smoke opacity compared to other kinds of premixed fuels.

• Under the same overall equivalence ratio and partial equivalence ratio of premixed fuel, premixing DMM exhibits larger indicated thermal efficiency, premixing n-heptane shows lower indicated thermal efficiency, and premixing ethanol displays comparable thermal efficiency when compared to the original neat biodiesel engine.

• For a fixed premixed ratio, premixing n-heptane shows the larger NO_x increase in the slope but premixing DMM shows a larger smoke opacity increase of the slope with the increase of the partial equivalence ratio of biodiesel. While the HC emissions seem to be mainly determined by the premixed fuel properties but not the equivalence ratio, ethanol premixing has larger HC levels than premixing n-heptane and DMM.

• For a fixed overall equivalence ratio, both HC and CO emissions remain stable but NO_x emissions decrease at first up to a critical premixed ratio. As the premixed ratio exceeds the critical value, NO_x and CO emissions start to increase gradually but HC emissions almost maintain the same level. Under the above conditions, smoke opacity always improves with the increase of the premixed ratio.

The remarkable difference in combustion characteristics is dominated by the physical and chemical properties of the premixed fuels. It is well-known that the n-heptane is a primary reference fuel with a cetane number of 56. The leaner n-heptane/air mixture can be easily operated in a HCCI engine. The addition of premixedn-heptane activates the low- and high-temperature chemical reactions and results in a slightly faster initiation of the heat release. Hence, it prevents the sharp increase of the heat release in the premixed burn region of conventional biodiesel combustion. On the other hand, ethanol is a renewable biofuel with a super low cetane number and a larger latent heat value of vaporization. As a result, the combustion event of the biodiesel engine with port injection of ethanol occurs very late. This promotes the sharp heat release in the premixed burn region of the overall combustion event. It is very interesting to find that the combustion characteristics with the premixed of DMM exhibit another phenomenon, which is different from the above two trends.

—Lu et al. (2008)

Resources

• Xingcai Lu, Libin Ji, Junjun Ma, and Zhen Huang (2008) Improved NO_x and Smoke Emission Characteristics of a Biodiesel-Fueled Engine with the Port Fuel Injection of Various Premixed Fuels. ASAP Energy Fuels, doi: 10.1021/ef800526e