Low-temperature diesel combustion with a high rate of exhaust gas circulation is one approach to reducing in-cylinder formation of pollutants; another is chemical kinetics-controlled combustion modes, such as homogeneous charge compression ignition (HCCI) is another. While the first has been adopted in diesel engines successfully, the team points out, the latter is still at a prototype stage due primarily to:

1. Lack of a method for controlling combustion phasing.

2. Limited operating range: The spontaneous ignition of the entire charge almost simultaneously results in a high rate of pressure rise, which limits high-load engine operations. The low-load engine operation is also limited by the occurrence of overly lean or diluted mixture conditions, resulting in misfiring.

3. Cold-start issue. If liquid fuel is used, wall wetting during the cold-start condition becomes an issue, which requires additional intake air heating.

Many researchers have attempted dual-fuel combustion to resolve these issues. In dual-fueling diesel engines, one fuel is injected into the intake manifold to form a premixed charge, as in many kinetics-controlled combustion regimes. However, the start of combustion is not initiated by spontaneous autoignition of the premixed charge but by near top-dead-center injection of diesel as in conventional diesel engines. Upon the initiation of the ignition by diesel jet, the combustion of the premixed charge could occur either as turbulent flame propagation or by sequential autoignition. Ideally, the variation of proportion of two fuels and diesel injection timing can control the combustion phasing to avoid a high rate of pressure rise during the premixed combustion. Moreover, the engine can run only on diesel injection to resolve the wall-wetting issue during the cold-start period and the misfiring problem at low-load conditions.

Many have explored the potential of dual-fuel diesel combustion using various fuels. … Recently, there has been increased interest in ethanol dual-fueling. The idea is that ethanol can be produced from renewable feedstocks, and thus, it is a good alternate to fossil fuels. Ethanol has high octane rating, higher than conventional gasoline, meaning that it has low cetane number and extended ignition delay. This is desirable in a dual-fueling application to avoid knocking or unwanted early ignition that might occur with a lower octane premixed fuel. In addition, ethanol is an oxygenated fuel, which may help reduce soot emissions. The present study focuses on ethanol injection into the intake manifold and its impact on in-cylinder combustion and engine-out emissions.

—Padala et al.

The main focus of their study, published in the ACS journal Energy & Fuels, was the effect of the ethanol port-fuel-injector (PFI) position on dual-fuel combustion and engine-out emissions.

Illustration of the intake ports, manifold, and PFIs at two different PFI positions. Credit: ACS, Padala. Click to enlarge.

They examined two PFI positions while varying the ethanol energy fraction: one close to the hot intake valves, so that the sprays impinged upon the hot valve surface (Position A), and the other further upstream of the intake valves, allowing increased residence time for interactions between ethanol droplets and intake airflow (Position B).

Emissions. Among the findings about emissions were:

• Between PFI positions A and B, the overall trend with an increasing ethanol fraction is very similar. As the ethanol fraction increases, the CO emission decreases and then increases; the HC emission continues to increase; the NO_x emission increases until it decreases because of misfiring; and the smoke emission decreases until the incomplete combustion products (“white smoke”) from misfiring cycles increase the opacity level.

• The CO emissions show the highest value at misfiring conditions when the maximum ethanol fraction was applied (i.e., 50% for PFI position A and 35% for PFI position B). The HC emission shows the same increasing trend and highest values when the engine misfires. The exception was that the HC emission continues to increase with an increasing ethanol fraction with no decreasing trend. This was likely due to HC emissions originated from the crevice volume, they suggested.

• NO_x emission tends to increase while smoke emission decreases with an increasing ethanol fraction, until the misfiring reverses the trend. They suggested the increased NO_x emission was likely due to the increased combustion temperature, as evidenced by the increased peak aHRR. The decreased smoke emission was also expected because of the increased premixed combustion and the fact that ethanol is oxygenated fuel.

• CO and HC emissions are higher for PFI position B compared to PFI position A. While the trend is the same, the difference in values suggests an additional source of CO and HC increase other than the reaction quenching near the wall and ethanol charge trapped inside the crevice volume.

Combustion efficiency calculated using measured HC and CO emissions for the two PFI positions. Credit: ACS, Padala et al. Click to enlarge.

From the measured HC and CO emissions, the team estimated the combustion efficiency, which they found follows a reversed trend of CO emissions with a drastic increase of HC emissions at high ethanol fractions, attributing to a marked decrease in combustion efficiency.

Combustion efficiency was estimated higher than 97%—except in misfiring conditions—suggesting that the variations of combustion efficiency would not make a significant impact on dual-fuel combustion.

Major general findings from the study are:

1. Mie-scattered ethanol spray images suggest significant impacts of ambient airflow on ethanol droplets breakup. If an ethanol PFI is located further upstream of the intake valves, the interaction between the ethanol droplet and airflow is extended, resulting in smaller ethanol droplets.

2. Up to an ethanol energy fraction of 30%, global phenomena such as in-cylinder pressure and aHRR do not show variations for two different ethanol PFI positions tested in the present study; however, misfiring occurs at a higher ethanol energy fraction for the PFI located closer to the valves. This implies that an effect other than the droplet residence time in the intake system plays a significant role.

3. Positioning the injector closer to the intake valves can take great advantage of hot surface boiling of ethanol droplets, resulting in increased vaporization and reduced wall wetting. Measured engine-out emissions of HC and CO are consistent with this explanation, showing lower values when the injector is placed closer to the intake valves.

Resources

• Srinivas Padala, Sanghoon Kook, and Evatt R. Hawkes (2013) “Effect of Ethanol Port-Fuel-Injector Position on Dual-Fuel Combustion in an Automotive-Size Diesel Engine,” Energy & Fuels doi: 10.1021/ef401479s