Introduction
Converted internal combustion engines refer to engines whose systems are modified from their original configurations to meet the requirements of another engine type, such as converting a diesel engine to an engine that operates on natural gas as fuel, which can reduce the formation of nitrogen oxides (NOx), carbon monoxide (CO), and soot emissions. Now, many worldwide companies involved in the design and manufacturing of internal combustion engines have converted diesel engines to natural gas-fueled engines; Fait Powertrain is an example. This company has successfully presented some commercial types of converted diesel-to-natural gas-fueled engines. Given Iran's natural gas resources and the lower cost of producing and handling natural gas compared to gasoline/diesel, as well as the lower pollution from natural gas engines, the conversion of gasoline-fuelled engines to natural gas-fuelled engines has been successfully implemented in Iran. However, converting diesel-fuelled engines to natural gas-fuelled engines received less attention. The literature survey also indicated that the simultaneous investigation of compression ratio, ignition timing, and turbocharger map variables on the performance and emission parameters of a diesel engine converted to a natural gas engine has received limited attention. The present study aimed to investigate the effects of compression ratio, ignition timing, and turbocharger map variables on the performance and emission parameters of a diesel engine converted to natural gas using one-dimensional computer simulation. The findings of this study, achieved through advanced modeling of a converted diesel-to-natural gas-fuelled engine using the efficient GT-Power software, can lead to the proper selection of the aforementioned variable values to obtain nearly equal performance parameters, along with reduced emissions, compared to the base diesel engine.
Material and Methods
A four-stroke, four-cylinder, water-cooled, direct-injection, turbocharged diesel engine from Fiat Powertrain was studied. The engine was installed in IPCo's engine test room. The dynamometer and other instruments in the test room measured the engine's performance parameters. The diesel engine was simulated using GT-Power software. The simulated model consisted of a fuel tank, transfer fuel pump, injector pump, fuel common rail, fuel filtration, pressure regulator, injectors, turbocharger, exhaust gas recirculation system, input and output manifolds, input and output valves, cylinder, crankcase, and so on. The developed diesel engine model was validated using experimental data. The necessary modifications were applied to the simulated diesel engine model to enable operation on natural gas. The effects of independent variables, including engine speed at 11 levels, compression ratio at 3 levels, ignition timing at 3 levels, and turbocharger operating map at 5 levels, were investigated on the performance parameters and emissions of the converted-to-natural-gas engine model. Based on the considered levels of the independent variables, modeling of the dependent variables was performed at 495 operating points. The dependent variables were included brake torque, brake mean effective pressure (BMEP), carbon monoxide (CO), carbon dioxide (CO₂), nitrogen oxides (NOₓ), and unburned hydrocarbons (HC).
Results and Discussion
The validation of diesel engine simulation results against experimentally measured data from IPCo and manufacturer catalogs demonstrated that the developed model has acceptable accuracy and can be used as a reliable tool for analyzing engine performance. The results showed that the converted engine BMEP and brake torque values are highly dependent on the ignition timing. For all independent variable levels, the maximum and minimum values of the converted engine BMEP and torque were observed at 13° before top dead center (BTDC) and 21° BTDC, respectively. These results indicated that at 13° BTDC, combustion occurred at an optimal point in the engine cycle, facilitating timely combustion and generating maximum effective mechanical work. An examination of the engine BMEP and output torque values revealed that the maximum BMEP (13.77 bars) and brake torque (328 N.m) can be achieved with proper selection of the turbocharger parameters. However, at turbocharger operating maps 4 and 5, with a compression ratio of 14 and an ignition time of 13° BTDC, the engine model would produce a BMEP of about 13.40 bars and a brake torque of about 320 N.m at 2500-3250 rpm, which could be preferred for heavy-duty vehicles. The investigation of engine combustion Nox and CO emissions indicated that the greatest and lowest values occurred at ignition timings of 21° BTDC and 13° BTDC, respectively. A comparison of NOx and CO emissions between the base diesel engine and the converted natural gas engine demonstrated that the NOx and CO levels for the converted engine were at least 60% and 50% lower than those of the base diesel engine, respectively. An investigation of CO2 emissions in the converted engine across all studied cases indicated that the highest values occurred at 21° BTDC and the lowest at 13° BTDC. The maximum CO2 pollutant level of the converted engine decreased by at least 40% compared to the base diesel engine.
Conclusions
This study demonstrated that an appropriate combination of compression ratio, ignition timing, and turbocharger map parameters for the converted engine can lead to an optimal design that simultaneously achieves comparable performance metrics to the base diesel engine while significantly reducing exhaust emissions. Considering that the converted engine would be used in place of a diesel engine in commercial vehicles, it is recommended that further research be conducted to design turbocharger operating maps that enable the converted engine to produce maximum power and torque within almost the same engine speed ranges as the base diesel engine.