In their study, the U Mich researchers examined engine combustion performance, PM, NO_x, CO, total hydrocarbon (THC) emissions, particle size distribution, soot morphology, and nanostructure for conventional combustion and various PCCI conditions.

Advanced combustion modes for internal combustion engines have drawn much attention from researchers due to their com- bination of advantages relative to traditional spark-ignition (SI) and compression-ignition (CI) engines. Homogenous charge compression ignition (HCCI) engines can achieve high thermal efficiency while significantly reducing NO_x and particulate matter (PM) emissions. Because it is very difficult to control ignition timing and combustion rate, HCCI combustion mode is limited to low loads. Several other combustion modes are derived from the HCCI combustion concept: low temperature combustion (LTC), partially premixed charge compression ignition (PCCI), and reactivity-controlled compression ignition (RCCI) combustion modes.

… PCCI combustion provides an effective way to reduce NO_x and PM emissions at low-to-medium load. Its basic operating mechanism is to apply combinations of very early or late injection timing, high injection pressure, low compression ratio, and high exhaust gas recirculation (EGR) levels to achieve a separation between the end of injection and the start of combustion, so that a premixed charge can be achieved before ignition occurs. The PCCI combustion mode can be considered a diesel combustion with a reduced fraction of mixing controlled combustion. Soot and NO_x emissions from PCCI can be higher than from HCCI but much less than emissions from the traditional CI combustion mode.

… Previous work has shown that high n-alkane content, high cetane number LTFT fuel can reduce PM, NO_x, CO, and THC emissions compared to those of ULSD. The goal of this study is to investigate if RD can also provide the same benefit because RD can become a practical and widespread fuel at much lower cost than FT fuels and has an inherently low carbon footprint. This work compares RD to ULSD and LTFT fuel using engine experiments under both PCCI and conventional operation conditions.

—Sun et al.

In the experiments, the researchers used a 425 cm³ single-cylinder engine based on a GM/Isuzu 1.7 L, 4-cylinder, common-rail, direct injection diesel engine. The compression ratio was reduced from 19:1 to 15:1 to achieve PCCI combustion. The original piston was changed to one with a larger combustion bowl. The cylinder has 4 valves and a centrally placed fuel injector. An EGR cooler and EGR valve cool and meter the exhaust into the engine intake air.

ULSD, the baseline fuel for the study, has much lower cetane number and higher aromatic content compared to the other two fuels; LTFT and RD have no or very little amount of aromatics. LTFT has very high n-alkane content, up to 72%—shown to produce very low PM and NO_x emissions in previous studies. RD has lower n-alkane content than LTFT, but a similar amount of saturates. The renewable diesel used in the study has a similar cetane number, density, and H/C ratio as those of LTFT fuel. However, the viscosity of RD is much higher.

Sun et al. Click to enlarge.

The engine operated at the steady state operating point of 1500 rpm, and 3.5 bar indicated mean effective pressure (IMEP)—a low load operating point commonly used by passenger car diesel engines. Engine speed and load were kept constant for all fuels during both conventional and PCCI combustion modes to enable straightforward comparisons.

Both conventional and PCCI combustion modes used a single injection per cycle. Conventional combustion used an injection pressure of 300 bar; this was increased to 700 bar for PCCI modes to advance the end of injection (EOI) and increase mixing before start of combustion (SOC). Higher injection pressure also gives more momentum to the fuel droplets, which helps fuel−air mixing before ignition occurs.

For conventional combustion, the EGR rate was set to 25%, and injection timing was adjusted to set CA50 (the crank angle of 50% burn) at 10 degrees after top dead center (ATDC). For PCCI combustion, the EGR rate was set to 40%, and injection timing was swept from the knock limit to the misfire limit in increments of 2 degrees for each of the three fuels.

Among the findings of the study:

• LTFT fuel has the best performance due to its high n-alkane content and low viscosity. LTFT fuel also has significantly lower PM emissions due to its low soluble organic fraction (SOF) and has reduced CO, THC, and NO_x emissions compared to those of ULSD.

• RD also reduces CO, THC, and NO_x emissions compared to those of ULSD. However, because of RD’s lower n-alkane content and higher viscosity and distillation curve compared to those of LTFT, it produces slightly higher CO and THC emissions and has significantly higher SOF within PM compared to those of the LTFT fuel.

  Nonetheless, RD with an optimized formulation can perform equally well as the LTFT fuel while providing a low carbon footprint and much lower cost.

  —Sun et al.

• High cetane number and low aromatic content fuels can effectively reduce PM and NO_x emissions simultaneously. Both LTFT and RD appear to benefit from having a low critical equivalence ratio achieved through different species (n-alkanes versus mildly branched iso-alkanes). In addition, low viscosity and low distillation temperature were shown to be important factors for the reduction of emissions.

• PCCI combustion as achieved in this study—i.e., with increased EGR and injection pressure—provided an effective way to reduce NO_x and soot emissions, albeit with increased CO and THC emissions. PCCI combustion resulted in higher CO, similar THC, lower NO_x, higher PM, and lower soot emissions.

• The soot nanostructure was not significantly influenced by the combustion mode or the type of fuel in contrast to some prior work, which suggested less ordered soot with PCCI combustion and more ordered soot with LTFT fuel. Low and high resolution TEM images revealed that soot produced from late injection PCCI combustion has larger primary soot particles and from early injection PCCI combustion has smaller primary soot particles.

Resources

• Chenxi Sun, Dongil Kang, Stanislav V. Bohac, and Andre L. Boehman (2016) “Impact of Fuel and Injection Timing on Partially Premixed Charge Compression Ignition Combustion” Energy & Fuels doi: 10.1021/acs.energyfuels.6b00257