Experimental Studies of Low-Load Limit in a Stoichiometric Micro-Pilot Diesel Natural Gas Engine
Abstract
:1. Introduction
2. Materials and Methods
2.1. Micro-Pilot Diesel Natural Gas Engine
2.2. Instrumentation and Measurement Uncertainty
2.3. Fuels
2.4. Experimental Procedure
3. Results and Discussion
3.1. CA50 Study
- The higher compression ratio of the micro-pilot engine vs. the SI engine;
- Different combustion chamber geometries;
- Natural gas premix flame speed versus gasoline premix flame speed.
3.2. Diesel Injection Pressure Study
3.3. EGR Study
3.4. Diesel Pilot Injection Quantity Study
3.5. Lowest Stable Load Achievable in Stoichiometric Operation
4. Summary and Conclusions
- The compression ratio impacts the minimum intake manifold pressure that maintains combustion stable. By reducing the compression ratio from 17.3:1 to 15.0:1, the intake manifold pressure had to be increased by 5 kPa, which raised the lowest load achievable by 1 bar NMEP due to the extra premix fuel required to maintain a globally stoichiometric premix.
- Combustion phasing retard controlled by the diesel micro-pilot start of injection is a way to achieve lower loads. This is a common strategy utilized in spark-ignited engines. A modified version of the correlation for SI engines is proposed in this work to fit the diesel micro-pilot natural gas combustion system. The micro-pilot engine combustion phasing retard has a slightly stronger impact on the minimum possible load than an SI engine. This shows the importance of the optimum combustion phasing in diesel micro-pilot NG engines.
- The reduction of pilot injection pressure improves combustion stability by the observed shortening of ignition delay. The lower injection pressure helps keep the pilot fuel-rich zones near the injector tip, facilitating the flame to initiate and spread inside the combustion chamber. The combustion phasing retard caused by the change in injection pressure agrees with the micro-pilot correlation, indicating that the change in injection pressure does not affect the negative impact of combustion phasing retard on NMEP.
- The effect of diesel injection pressure was observed to be more dominant in low in-cylinder temperatures (i.e., T < 900 K). This is an important finding to help the development of low-temperature combustion strategies. As the in-cylinder charge temperature is near the diesel auto-ignition limit, the lower injection pressure can improve flame development and combustion stability. This effect becomes negligible as the in-cylinder temperature increases.
- Adding exhaust gas to the intake can reduce the total fuel ingested while maintaining a stoichiometric mixture, leading to longer diesel pilot ignition delays. The exhaust gas recirculation limit at low load was observed to be 12% as increasing delay in ignition results in unstable combustion beyond this limit, independent of injection pressure and timing.
- Increasing the diesel pilot quantity can improve the combustion stability at a given operating condition by providing a robust and stable ignition source. This comes at the cost of reducing the total diesel substitution ratio.
- Lean operations enabled lower loads by reducing the total fuel quantity in the intake premix for a given intake pressure. Lean operation increased NOx and unburned hydrocarbon emissions by 40% and 160%, respectively, while lean operation precludes using a TWC to control NOx emissions.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Nomenclature
AFR | Air to Fuel Ratio | LHV | Lower Heating Value |
BMEP | Brake Mean Effective Pressure | IMEP | Indicated Mean Effective Pressure |
BSNOx | Brake Specific NOx | MAP | Manifold Air Pressure |
BSTHC | Brake Specific THC | MBT | Maximum Brake Torque |
CA50 | Crank Angle position at which 50% of the heat from combustion has been released | NG | Natural Gas |
CO | Carbon Monoxide | NMEP | Net Mean Effective Pressure |
COV | Coefficient of Variance | NOx | Oxides of Nitrogen |
CR | Compression Ratio | P | Pressure |
DSR | Diesel Substitution Ratio | SI | Spark-Ignition |
EGR | Exhaust Gas Recirculation | SOC | Start of Combustion |
EQR | Equivalence Ratio | SOI | Start of Injection |
FS | Full Scale | T | Temperature |
GHG | Green House Gases | THC | Total unburned Hydrocarbons |
HP | High Pressure | TWC | Three-Way-Catalyst |
ICE | Internal Combustion Engine | VGT | Variable Geometry Turbocharger |
IMAT | Intake Manifold Air Temperature |
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No. of Cylinders | 6 |
Bore and Stroke | 107 × 124 mm |
Connecting Rod Length | 192 mm |
Displacement Volume | 6.7 L |
Compression Ratio | 15.0:1 |
Aspiration | Turbocharged (VGT) + Charge Air Cooler + HP EGR + Throttle Valve |
Diesel Micro-Pilot Injection System | HPCR injector 8 holes (168 microns diameter) |
Diesel Injection Pressure | 600 to 2000 bar |
Diesel Micro-Pilot Minimum Fuel Quantity | 3.3 mg/injection |
Measurement | Accuracy | Full Scale | Uncertainty (Absolute) | Unit |
---|---|---|---|---|
In-Cylinder Pressure Transducer | ±0.30% FS | 0–250 | ±0.75 | bar |
Engine Crank Angle Degree (Encoder) | ±1.00 | 360:1 | ±1.00 | Deg |
Fuel Flow Rate (Diesel and NG) | ±0.05 FS | 0–30 | ±0.015 | g/s |
Thermocouples | ±2.20 | 0–800 | ±2.20 | °C |
Intake/Exhaust Manifold Pressure Transducer | ±0.08% FS | 0–6.89 | ±0.0055 | bar |
Oil and Fuel Inlet Pressure | ±0.50% FS | 0–7/0–2 | ±0.035/0.010 | bar |
Torque | ±0.05% FS | 0–5000 | ±2.50 | Nm |
NOx Analyzer | ±1.00% FS | 0–5000 | ±50.0 | ppm |
THC Analyzer | ±1.00% FS | 0–10,000 | ±100.0 | ppm |
Measurement | Range | Uncertainty (Absolute) | Unit |
---|---|---|---|
NMEP | 4.70–8.00 | ±0.15 | Bar |
Combustion Stability (COVNMEP) | 0.40–9.0 | ±0.05 | % |
Ignition Delay | 1.70–5.0 | ± 0.08 | ms |
Diesel Substitution Ratio | 86.0–93.2 | ±0.50 | % |
CA50 | 12.0–32.0 | ±0.60 | °aTDC |
Exhaust Gas Recirculation | 0.00–12.0 | ±0.20 | % |
Equivalence Ratio | 0.80–1.0 | ±0.01 | - |
BSNOx | 0.20–133 | ±34.1 | g/kWh |
BSTHC | 2.10–37.0 | ±5.00 | g/kWh |
Diesel (ULSD) | Density (kg/m3) at 15.6 °C, 1 atm | Lower Heating Value (MJ/kg) | Stoichiometric AFR | H/C | Cetane Number |
851.6 | 42.8 | 14.60 | 1.85 | 51.7 | |
CNG | Density (kg/m3) at 20 °C, 1 atm | Lower Heating Value (MJ/kg) | Stoichiometric AFR | H/C | Methane Number * |
0.727 | 47.5 | 16.30 | 3.80 | 83.0 |
Range of Operating Conditions Investigated | |||||||||
---|---|---|---|---|---|---|---|---|---|
Speed | EQR | MAP | Intake Temp. | CR | Diesel Pilot SOI | Diesel Inj. Pressure | EGR | Diesel Pilot Quantity | |
RPM | - | kPa | °C | - | °bTDC | bar | % | mg/injection | |
CA50 Study | 1200 | 1.0 | 80 (CR17.3); 85 (CR15.0) | 35 | 15.0; 17.3 | 2.0 to 15 | 1000 | 0 | 3.3 |
Diesel Inj. Pressure Study | 15.0; 17.3 | 2.5 (CR17.3); 7.0 (CR15.0) | 600 to 1200 | 0 | 3.3 | ||||
EGR Study | 15.0 | 7.0 to 15 | 600 | 0 to 12 | 3.3 | ||||
Pilot quantity Study | 15.0 | 15 | 600 | 12 | 3.3 to 6.6 |
Reduced Compression Ratio (15.0) | Original Compression Ratio (17.3) | |
---|---|---|
Load (NMEP) [bar] | 6.20 | 5.20 |
Indicated Fuel Conversion Efficiency [%] | 35.7 | 36.3 |
Diesel Substitution Ratio [%] | 93.2 | 91.0 |
Equivalence Ratio [-] | 1.00 | 1.00 |
EGR [%] | 12.0 | 12.0 |
Pilot Injection Pressure [bar] | 600 | 600 |
Pilot Injection Quantity [mg/inj] | 3.30 | 3.30 |
CA50 [°aTDC] | 26.0 | 24.0 |
COV NMEP [%] | 3.50 | 3.30 |
MAP [kPa] | 85.0 | 80.0 |
Intake Manifold Temperature [°C] | 35.0 | 35.0 |
EGT [°C] | 587 | 515 |
Brake Specific NOx [g/kWh] | 0.50 | 0.25 |
Brake Specific CO2 [g/kWh] | 590 | 538 |
Brake Specific THC [g/kWh] | 4.20 | 8.90 |
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Bonfochi Vinhaes, V.; McTaggart-Cowan, G.; Munshi, S.; Shahbakhti, M.; Naber, J.D. Experimental Studies of Low-Load Limit in a Stoichiometric Micro-Pilot Diesel Natural Gas Engine. Energies 2022, 15, 728. https://doi.org/10.3390/en15030728
Bonfochi Vinhaes V, McTaggart-Cowan G, Munshi S, Shahbakhti M, Naber JD. Experimental Studies of Low-Load Limit in a Stoichiometric Micro-Pilot Diesel Natural Gas Engine. Energies. 2022; 15(3):728. https://doi.org/10.3390/en15030728
Chicago/Turabian StyleBonfochi Vinhaes, Vinicius, Gordon McTaggart-Cowan, Sandeep Munshi, Mahdi Shahbakhti, and Jeffrey D. Naber. 2022. "Experimental Studies of Low-Load Limit in a Stoichiometric Micro-Pilot Diesel Natural Gas Engine" Energies 15, no. 3: 728. https://doi.org/10.3390/en15030728
APA StyleBonfochi Vinhaes, V., McTaggart-Cowan, G., Munshi, S., Shahbakhti, M., & Naber, J. D. (2022). Experimental Studies of Low-Load Limit in a Stoichiometric Micro-Pilot Diesel Natural Gas Engine. Energies, 15(3), 728. https://doi.org/10.3390/en15030728