Single-Zone Model Combustion chamber is treated as a single, ideal gas (air). Bulk temperature and pressure profiles are predicted using the ideal gas law. Used to predict a pressure trace, engine power. Bulk, average temperature is ineffective in predicting emissions. Air
Single-Zone Modifications Engine friction can be modeled linearly. Empirical equations can be found for many applications (motorcycle shown) The specific heat ratio can be modeled as a function of temperature. A curve-fitted equation was used in this model.
Heat Transfer Prediction Methods Woschni’s method splits process into respective combustion periods. Woschni assumed gas velocities were proportional to the mean piston speed. Annand included radiative terms. Annand assumed “pipe flow characteristics”.
Modeling NO Formation Formation Mechanisms (1)Zeldovich mechanism High Temperature Combustion (above 1800 K) (2) The prompt mechanism Low Combustion Temperatures, When Fuel Contains Nitrogen (3) The N 2 O-intermediate mechanism Lean, Low Temperature Combustion (4) The NNH mechanism Least Understood, Important in Hydrogen Combustion Only the general Zeldovich mechanism was used in this model:
Modeling NO Formation Continued Water-Gas Shift Reaction Rate of NO formation is expressed as:
HC Emissions Due to Crevices Flames require a minimum distance in order to propagate through an opening, otherwise the flame is extinguished. In cold-engine operation, crevices and cylinder walls also work as a heat sink creating fuel pooling. This model included piston ring pack crevices and a correction factor due to spark-plug offsets.
HC Oil Layer Absorption and Desorption HC molecules absorb into the oil film on the intake and compression cycles. HC continue to absorb past saturation due to rising vapor pressures (reference Henry’s Law). The post-combustion imbalance in concentrations (between the oil layer and combustion chamber gases) causes HC desorption. Henry’s Law
MATLAB Model The main MATLAB simulation was stored in a script. Functions were generated to predict emissions and volumetric efficiencies. Sub-functions were created to plot BSFC and emissions maps. Main Script NO Emissions Function HC Emissions Function Volumetric Efficiency Correction Function
Single-Zone Outputs The indicated cylinder pressure is predicted using the single-zone model. EVO cuts the plot off. IVC causes pressure to begin building. The bulk cylinder temperature is produced using the single-zone model. This is the average of the burned and unburned zones.
Two-Zone Outputs The two-zone model splits the bulk temperature into unburned and burned-zones. The burned-zone temperature is elevated in comparison to the bulk gas temperature. HC and NO emissions are displayed in the command window. Frozen NO composition was assumed at 90% of the peak burned-zone temperature.
Comparing Model Outputs Woschni’s method under-predicted the heat transfer coefficient Woschni’s method over-predicted torque and NO emissions. Since NO formation is strongly temperature dependent, the 20% relative error shown was due to a difference in formation temperatures of approximately 120[K] VariableWoschniAnnandRelative Error NO429.57[PPM]341.19[PPM]20.57 [%] HC2.47 [%]2.27 [%]8.10[%] Power24.40 [kW]22.76 [kW]6.72[%] BMEP1117.20[kPa]1042.10 [kPa]6.72[%] BSFC334.30 [g/kW-h]353.51 [g/kW-h]5.75[%] Mechanical Efficiency88.81 [%] 0[%] Combustion Efficiency85.09 [%] 0[%]
Acquired Data The BSFC map showed a trough in the plot at the upper right corner (~8500 RPM, 70% throttle). During Testing, the engine seized, so a complete map wasn’t possible. Over testing range, minimum BSFC values were ~280[g/kW-h]
Comparing Data to Model Power, Torque, BSFC NO Emissions
In-Class Activities Spark-Timing Optimization BSFC Mapping Emissions Mapping Assume Burn Duration Create Call Function Assume Timing, Efficiencies at Operating Points Simulate Operating Points With Call Function Assume Timing, Efficiencies at Operating Points Simulate Operating Points With Call Function
Spark-Timing and BSFC Plots An MBT spark-timing plot was constructed. This required an assumed air-fuel ratio and burn duration. The BSFC map required functions that simulated many loads, RPM, and assumed air-fuel ratios. A burn-duration model would improve the accuracy of the plot shown. The model also predicts throttle positions that may not be possible to operate at (i.e. 4000 RPM 100% throttle)
Emissions Maps The same functions that were used to create BSFC maps were used to create emissions maps. NO emissions ranged relative to combustion temperatures and air- fuel ratios. HC emissions were high at low loads. HC emissions were related to combustion temperatures and the “quality” of combustion.
Conclusions The model was valid at operating points from 50%- 70% throttle. No data points above 70% throttle and at high engine speeds were gathered due to engine failure. A volumetric efficiency map at low throttle positions would be necessary. It’s expected that relative error in BSFC calculations could be minimized by documenting fuel temperatures.
Future Work MATLAB Model Empirical Models for a Specific Engine Chemical Model Baseline Maps EES Fluid Properties Class Projects FMEP Model Volumetric Efficiency Alternative Fuels Complete Emissions Burn Duration Model HC Emissions