2.1. Engine and Simulation Model
In this study, the simulation model was built based on the natural gas engine test bench and the model was calibrated based on the experimental data. The engine specifications are shown in
Table 1. The schematic diagram of the partial oxidation reforming system for a natural gas engine is shown in
Figure 1.
Figure 1 is mainly used to illustrate the structure of the fuel reforming system, and an air filter was added into the intake system, which is important for measuring the real air flow rate and maintaining the stability of the air intake system [
21].
As shown in
Figure 2, a one-dimensional engine simulation model was constructed for a natural gas engine with the software GT-Power (GT-SUITE v2016). In order to provide a good prediction of in-cylinder combustion, the model involves the engine’s cylinder geometry, ignition timing, fuel characteristics, etc. A turbulent flame combustion model was used to predict in-cylinder burn rate, emissions, and flame–wall interactions and calculated based on detailed cylinder geometry. The in-cylinder heat transfer was calculated with the classical Woschni correlation. And, the flow model was used to calculate the swirl, tumble, and turbulence parameters according to the piston and cylinder geometry.
In this model, an Explicit Euler integration scheme that is second-order accurate is used in the solver. The solver was considered to converge when the error in the parameters of temperature, pressure, and flow within the engine model was less than 2% between two consecutive steady state cycles. The time step of this simulation model is a single cycle, and the engine parameters at the end of the previous cycle will be used in the initial state of the next cycle.
Since the fuel used in the model is a mixture of natural gas and its reforming products, it is necessary to provide relevant parameters for the fuel in the engine simulation model. The basic combustion parameters of the fuel mixture were calculated using the software Chemkin-pro (Chemkin 4.5). The “Foundational Fuel Chemistry Model (FFCM-1)” was chosen to calculate the combustion parameters of the fuel mixture. For POFR, the H
2, CO, CH
4, and a small number of CO
2 components of the reforming natural gas are injected directly into the inlet pipe as part of the fuel in the injector of the engine model together with the natural gas. The N
2 component in the reforming gas is influenced by adjusting the N/O ratio in the inlet air model. The FFCM-1 reaction mechanism was used to simulate the combustion of CH
4 and the combustion of H
2, CO, CH
2O, and other components in good agreement with the actual experimental results [
22]. The fuel mixture’s maximum laminar flame propagation velocity and the corresponding reaction equivalent ratio at this velocity can be obtained. In addition, as shown in Equations (1) and (2), parameters such as flame decay velocity, temperature coefficient, and pressure coefficient of the mixed fuel were calculated using empirical methods [
23,
24].
where
denotes the laminar flame burning velocity of the mixed fuel;
denotes the mass fraction of dilution gas in the air–fuel mixture;
denotes the laminar flame propagation velocity of the mixed fuel in the reference state;
denotes the maximum laminar flame burning speed of the fuel mixture;
denotes the laminar flame decay velocity of the fuel mixture;
denotes the actual equivalent ratio for the natural gas engine at different operating conditions; and
denotes the equivalent ratio for the conditions corresponding to the maximum laminar flame burning speed of the fuel mixture.
To obtain an explicit expression about laminar burning velocities dependent on pressure and temperature, the measured laminar burning velocities have been fit to a simple power law relation at the datum temperature
(300 K) and the datum pressure
(0.1 Mpa).
and
are the in-cylinder temperature and pressure, respectively. Moreover, both the temperature coefficient
and the pressure coefficient
are functions of the chemical equivalence ratio and are calculated using Equations (3) and (4):
Moreover, Equation (5) demonstrates that POFR is achieved by reforming natural gas with air, resulting in fast self-heating that can be sustained without a continuous heat supply. An Rh-based catalyst is used to carry out POFR at an inlet gas temperature of 300 °C, an inlet pressure of 1 bar, and a reaction velocity of 25,000 h
-1. Studies indicate that hydrogen production rates can reach up to 60% when the fuel-reforming ratio ranges between 3% and 12% [
18].
Regarding the blended fuel of the natural gas engine, the mixture components can be calculated after blending different proportions of fuel-reforming gas based on the POFR products. In this study, the fuel blends for the natural gas engine were mixed with 0%, 3%, 6%, 9%, and 12% of fuel reforming gas, and the key information is listed in
Table 2. Furthermore, as shown in Equation (6), a parameter, fuel-reforming ratio (
), is introduced to help clearly illustrate the ratio of fuel which is involved in POFR.
where
denotes the fuel mass flow rate participating in the reforming reaction, and its unit is kg/h;
denotes the fuel consumption under the same load conditions for the natural gas engine, and its unit is kg/h.
Figure 3 shows the calculation results of natural gas fuel laminar flame speed, where F0 is the fuel-reforming ratio of 0 (pure natural gas) and F12 is the fuel-reforming ratio of 12%. It can be seen that as the fuel-reforming ratio increases from 0% to 12%, the maximum laminar flame burning speed increases with the increasing H
2 content in the fuel, with the maximum laminar flame velocity corresponding to equivalent ratios ranging from 1.05 to 1.10 for different types of fuel blends. The maximum laminar flame velocities for F0 and F12 are each 0.37 m/s and 0.41 m/s, while both have the maximum laminar flame velocity at an equivalence ratio of around 1.08.
Table 3 gives the low calorific values of the mixed fuel in the natural gas engine simulation model.
2.2. Natural Gas Engine Simulation Model Validation
The natural gas engine has been tested on the engine test bench under low- and medium-load conditions with different fuel-reforming ratios. Therefore, these test data can be used to validate the POFR natural gas engine simulation model.
During the bench test, the emission of gaseous pollutants was measured with an AVL 493D GAS PEMS device, and the fuel flow rate and air flow rate were measured separately with TOCEIL CMF050 and FMT700-P. The engine combustion parameters (in-cylinder pressure, heat release rate, etc.) were measured and calculated with AVL Indicom.
Table 4 shows the accuracy of all the measuring equipment.
The reformed fuel and air flow rates were controlled with flow meters and pressure-regulating valves, respectively, and the reformed gases were mixed with the intake air before the inlet of the supercharger. And, after the engine had run steadily for 3 minutes, the relevant equipment recorded the engine performance and emission parameters under this operating condition. The flow rates of fuel and air involved in the reforming reaction were calculated on the basis of the fuel-reforming ratio.
According to the basic combustion parameters calculated with Chemkin-pro, the combustion model and fuel model have been set up and calibrated using experimental data from the natural gas engine test bench of
Figure 1. The intake and exhaust boundary conditions of the simulation model were consistent with the atmospheric environment in the laboratory when the engine was under partial-load conditions, and the intake and exhaust pressures were set according to the actual measurements on the test bench. Under full-load conditions, the engine simulation model’s pressure and temperature after the outlet of the supercharger were set to be consistent with the bench test values (under pure natural gas fuel conditions), and the exhaust boundary conditions were consistent with the laboratory atmosphere.
The cylinder pressure was compared and validated at partial-load conditions at 1500 rpm with a 12% reforming natural gas. As shown in
Figure 4, the experimental and the simulated data of cylinder pressure show consistency in terms of maximum peak pressure and combustion phase, indicating that the model could better reflect the actual combustion situation in the engine.
Moreover,
Figure 5 compares the engine’s maximum power, intake air flow rate, and brake-specific fuel consumption (BSFC) under different operating conditions. The relevant in-cylinder combustion equivalent ratios and ignition advance angles at full-load conditions are shown in
Table 5. The maximum power result shows a strong correlation between the experimentally obtained data and the model-calculated values, with negligible deviation observed between 1300 rpm and 1500 rpm. As the engine speed increases, the discrepancy between the measured and simulated values was found to be 3.4% at 1700rpm. Moreover, the disparity in BSFC tends to decrease with increasing speeds, with the highest difference of 4.1% observed at 1300 rpm. For the partial-load conditions, the measured and simulated data showed comparable values for BSFC and intake air mass flow rate, with the exception of the 10% load at 1500rpm. In this case, the BSFC error was noted as 9.2%. The error between the experimental and simulated results for most of the compared conditions does not exceed 5%. Hence, the calculation results by the simulation model are relatively accurate and predictive.
In general, the numerical study mainly includes two sub-studies: one is to investigate the combustion characteristics of combustion and performance, and the other is for the emission characteristics. Furthermore, the two sub-studies were conducted under both partial and full loads. The engine speed of partial loads in the simulation was set to be 1500 rpm. Regarding the full load, the engine speed was set at 1200–1700 rpm with the same inlet manifold pressure.