2.2.6. Diesel Mechanisms

Five diesel reduced mechanisms are available. The flowchart shown in Figure 2.6: Decision flowchart for selecting MFL diesel mechanism summarizes the applicability of the four mechanisms. More details of the four mechanisms are presented in the sub-sections below.

Figure 2.6: Decision flowchart for selecting MFL diesel mechanism

Decision flowchart for selecting MFL diesel mechanism

 

2.2.6.1. Single-component Diesel

The diesel_nheptane_MFL2017.cks chemistry represents diesel with n-heptane (mechanism name: nc7h16) as the surrogate. The focus of this chemistry set is on modeling combustion and emissions at low- to high-temperatures in diesel engines. The Model Fuel Library Validation Manual includes the mechanism validation plots. The mechanism can be used to model soot emissions using the pseudo-gas soot chemistry. This chemistry can be used to predict CO, HC (hydrocarbons), and NOx emissions from diesel engines.

The mechanism has been reduced for the following range of conditions:

  • 1. Pressure: 10–100 bar

  • 2. Temperature: 700–2000 K

  • 3. Equivalence ratio: 0.5–4

  • 4. EGR: 0–40%

This mechanism has been reduced from the full mechanism “Diesel_PAH_NOx” in the MFL database, which has been thoroughly validated against fundamental experimental data for the operating conditions of interest for diesel engines. The mechanism was reduced from this comprehensive full mechanism using the Reaction Workbench software, for the conditions listed above.

For the emissions, the following species predictions are expected to be accurately predicted:

  1. Soot-precursor species (pseudo-gas only):

    1. acetylene (c2h2)

    2. benzene (c6h6)

    3. phenyl (c6h5)

  2. CO (co)

  3. NOx (no and no2)

  4. Unburned hydrocarbons

Soot can be modeled as a pseudo-gas species; in the mechanism this species is named soot.

 

2.2.6.2. Multi-component Diesel

The 4-component diesel surrogate has a composition of 36 wt% n-hexadecane/9.7 wt% AMN/15.4 wt% HMN/38.9 wt% decalin. There are three chemistry sets for this surrogate:

  1. diesel_4-comp_MFL2017.cks

  2. diesel_4-comp_pseudo-gas-soot_MFL2017.cks

  3. diesel_4-comp_detailed-soot_MFL2017.cks

The difference between the three chemistry sets is the soot model they can be used with; they are any acetylene-based empirical soot model, the pseudo-gas soot model, and the Method of Moments soot model, respectively. The same reduced chemistry set may be used for a surrogate whose composition is different, as long as it consists of the same fuel components (or a subset) and includes a similar amount of the fastest-burning component (in this case n-hexadecane). The target application is modeling combustion at low- to high-temperatures in diesel engines. It can be used to track soot particle mass, number and size in Ansys Chemkin, and using the Method of Moments in Ansys Forte. This chemistry can be used to predict CO, HC (hydrocarbons), and NOx emissions from diesel engines.

For the composition used in the reduction, the surrogate has the following liquid-fuel properties:

Aromatics, vol%9.7
Cetane Number54.4
Molar H/C ratio1.87
Liq. Density, g/cm30.83
Distillation curve
T10, K487
T30, K497
T50, K514
T70, K537
T90, K554

The mechanism has been reduced for the following range of conditions:

  1. Pressure: 1–100 bar

  2. Temperature: 700–2000 K

  3. Equivalence ratio: 0.5–4

  4. EGR: 0–40%

This mechanism has been reduced from the full mechanism “Diesel_PAH_NOx” in the MFL database, which has been thoroughly validated against fundamental experimental data for the operating conditions of interest for diesel engines. The Model Fuel Library Validation Manual includes the mechanism validation plots. The mechanism was reduced from this comprehensive full mechanism using the Reaction Workbench software, for the conditions listed above.

For the emissions, the following species predictions are expected to be accurately predicted:

  1. Soot-precursor species:

    1. acetylene (c2h2)

    2. butadiyne (c4h2)

    3. propargyl (c3h3)

    4. benzene (c6h6)

    5. phenyl (c6h5)

    6. toluene (c6h5ch3)

    7. naphthalene (naph)

    8. acenaphthalene (a2r5)

    9. pyrene (a4)

    10. coronene (coronene)

  2. CO (co)

  3. NOx (no and no2)

  4. Unburned hydrocarbons

Soot can be modeled using the soot-surface mechanism and the Method of Moments soot model.

The species names in the chemistry file for the fuel species are:

  1. n-hexadecane is nc16h34.

  2. AMN is a2ch3.

  3. Heptamethylnonane is hmn.

  4. Decalin is decalin.