Hydrotreated vegetable oil (HVO) diesel fuel is a promising biofuel candidate that can complement or substitute traditional diesel fuel in engines. It has been already reported that by changing the fuel from conventional EN590 diesel to HVO decreases exhaust emissions. However, as the fuels have certain chemical and physical differences, it is clear that the full advantage of HVO cannot be realized unless the engine is optimized for the new fuel.

—Happonen et al.

The studied HVO fuel is fully paraffinic—it contains no aromatics, sulfur, or oxygen. The HVO fuel meets the EN590 standard in all respects except in density.

The engine used in the study was a single-cylinder research engine based on a commercial 6-cylinder off-road engine. The commercial engine fulfilled EU 97/68/EC Stage III A and EPA 40 CFR 89 Tier 3 emission standards. EGR was simulated using neat nitrogen in the charge air; the EGR percentage was defined as the percentage of added nitrogen in the total inlet air mass flow.

The team studied the results of different combinations of advanced intake valve closing (IVC); exhaust gas recirculation (EGR) percentage; injection pressure (P_inj); and start-of-injection timing (SOI) on 50%, 75%, and 100% loads. When IVC was advanced, it was accompanied with an increase in charge air pressure in order to keep the intake air mass flow constant.

Three different engine conditions were chosen for each load: low-NO_x conditions (LN); low-smoke conditions (LS); and conditions where both NO_x and smoke are relatively low (LNLS). The low-NO_x and low-smoke conditions represent the cases where one emission can be reduced to a minimum using engine parameter adjustments, while the other emission can be reduced using suitable after-treatment techniques—i.e., selective catalytic reduction (SCR) or particulate filter (e.g., DPF). The third condition, where both NO_x and smoke are rather low, shows how large emission reductions can be expected with HVO fuel by engine parameter adjustments only.

IMEP of the 50%, 75%, and 100% loads were 10.8, 16.2, and 21.4 bar, respectively. The engine speed was 1500 rpm at all the measured conditions. The researchers first established baseline emissions under standard emission conditions at all three loads.

Among the findings of the study:

• LN conditions were achieved with the use of EGR, advancing IVC by 70 degrees and delaying the start of injection from 0 to 2 degrees depending on the load. In addition, injection pressure was increased from 30 to 70%. With these adjustments, an approximately 60% decrease in NO_x was achieved with the studied loads relative to the standard conditions with the HVO fuel.

  With the exception of increased injection pressure, all the adjustments resulted in a net increase in particulate emissions.

• To achieve LS conditions, they increased the injection pressure which enhanced combustion conditions in the cylinder. Also, on the 50% load, SOI was advanced allowing better combustion. Both of these changes thereby enhanced combustion conditions decreasing particulate emission and increasing NO_x.

  To keep the NO_x emission below the level of emission at reference conditions, a small EGR percentage and advanced IVC were used. The results show that NO_x emission remained at or slightly below the reference value, but PM emission was decreased 45−68% at LS conditions depending on the load.

• LNLS conditions were achieved by reducing NO_x with advanced IVC and a small percentage of EGR and by reducing particulate emission with a 30−70% increase in injection pressure depending on load. NO_x emission was decreased 30−50% depending on load and PM 25−33%.

The results show that considerable reductions of exhaust emissions are possible by adjusting the engine settings to better suit the HVO fuel. Nevertheless, it should be noted that the emission reductions obtainable with a given engine are strongly dependent on applied engine technology and aftertreatment systems. At least with the current strictening limits for particles, it is impossible that light-duty vehicles or heavy-duty engines could achieve these limits without the use of diesel particulate filters (DPF). However, by adjusting the engine to reduce NO_x emissions, the use of separate NO_x aftertreatment (i.e., SCR) might be avoided. In addition, it becomes evident from the results that the amount of particulate emission reductions are strongly dependent on the chosen particulate property that is being focused on.

In order to obtain a more comprehensive view of the health effects of emission particles, particle emission properties such as chemical composition, PM, particle number concentration, particle surface area, and particle diameter should be known. FSN, on the other hand, could be the only value related to particulates that is measured in an engine laboratory.

—Happonen et al.

The team thus also compared different indicators of particulate emissions, including filter smoke number (FSN), total particle number, total particle surface area, and geometric mean diameter (GMD) of the emitted particle size distribution. As a result of this comparison, they found a linear correlation between FSN and total particulate surface area at low FSN region.

Resources

• Matti Happonen, Juha Heikkilä, Timo Murtonen, Kalle Lehto, Teemu Sarjovaara, Martti Larmi, Jorma Keskinen, and Annele Virtanen (2012) Reductions in Particulate and NO_x Emissions by Diesel Engine Parameter Adjustments with HVO Fuel. Environmental Science & Technology doi: 10.1021/es300447t