Study of natural gas injection timings on a downsized and boosted, multi-cylinder direct injection (DI) engine

This study presents experiments conducted on a 4-cylinder, downsized and boosted, direct injection (DI) engine fuelled with compressed natural gas (CNG). The impact of injection timing on engine performance was investigated by varying the start of injection (SOI) from an advanced timing during the induction stroke to a retarded timing close to intake valve closure. Injection timings that are too advanced or too retarded result in lower brake thermal efficiency (BTE), whereas intermediate SOI timings increase BTE. An analysis of mixing time, intake air flow, and combustion process are performed to understand the impact of injection timing on the DI CNG engine performance. Introduction Improvement in internal combustion engine (ICE) efficiency is imperative given the cost uncertainty and diminishing supplies of petroleum-based fuels, as well as the demands of increasingly strict emissions regulations. Compressed natural gas (CNG) has emerged as a promising alternative to conventional refinery stream fuels due to its favorable properties including low energy specific CO2 emissions, high antiknock resistance and an extended lean flammability limit. Current natural gas fuelled automotive engines using port fuel injection (PFI) technology have limitations on peak torque and power compared to gasoline fuelled equivalents as a result of intake air displacement by the natural gas [1, 2]. This has provided much of the motivation to study natural gas DI engines, in an effort to overcome the limitations of NG PFI. Ferrera et al. [3] found 1-3% BTE improvement over CNG PFI operation due to the implementation of direct injection. Several authors [4, 5, 6] studied different aspects of CNG DI engine behavior under various operating parameter such as rail pressure, fuel injection, spark timing etc. Husted et al. [7] showed that up to two-thirds of the lost torque in NG PFI operation compared to GDI operation can be recovered with DI CNG. Sevik et al. [8] studied the effect of injection timings on the performance of a single cylinder engine equipped with NG DI, NG PFI and E10 PFI system and showed similar efficiencies at part load operation for both E10 and CNG. Although these studies have provided a preliminary characterisation of DI CNG engine performance, there is still uncertainty as to how DI CNG will operate in a modern, DISI production engine and how best to optimise the injection strategy for such an engine. Therefore, this study aims to investigate the performance of an advanced DISI production engine using injectors optimised for CNG delivery at various SOI timings. Experimental Set-up and Procedure A Ford 2.0L EcoBoost (4-cylinder, downsized and boosted, DI) engine was used for this experimental study. The engine configuration is given in table 1. The specifications of mains natural gas used for this study are given in table 2. An outward opening, Table 1: Engine Specification Bore 87.5 mm Stroke 83.1 mm Displacement 2 L Compression Ratio 9.3 Injection Pressure 16 bar Injector Orientation Side Mounted Ignition Source Spark Plug Table 2: Natural gas specification Composition by Volume [9] Methane (CH4) 92.25 % Ethane (C2H6) 4.42 % Propane (C3H8) 0.69 % Nitrogen (N2) 0.84% Carbon-Di-Oxide (CO2) 1.8% Property LHV 47.3 MJ/kg AFR Stoich 16.0 prototype valve type injector from Continental was used to supply CNG at a constant 16 bar pressure to the cylinder. A Bosch hot-film, air-mass meter and a Coriolis flow-meter were used to measure air and the CNG flow rate, respectively. A Bosch LSU 4.9 lambda sensor was installed on exhaust pipe to measure and monitor the air-fuel ratio on-line. The engine was run with an open, developmental MoTec M142 ECU. Four Kistler piezoelectric pressure sensors and a crank angle encoder with 0.1 CAD resolution were installed to record the in-cylinder pressure history. The ensemble average of 300 successive pressure traces was used to conduct heat release analysis in GT-Power based on the following non-predictive combustion methodology. The experiment was conducted at an engine speed of 1500 Reverse Run Model • Single Zone combustion • Assume Heat Transfer • Checking Prms In-Cylinder Pressure Test Parameters Apparent Burn Rate Profile Forward Run Model • Two Zone combustion • Wall Temperature Solver • Checking Prms Actual Heat Transfer Rate Reverse Run Model • Final Burn Rate Forward Run Model • Combustion Result RPM and 6 bar BMEP. Several CNG SOI timings ranging from 120 BTDCF to 320 BTDCF were tested at λ = 1. Generally, the most retarded SOI timing is limited by the choked fuel flow condition and available fuel-air mixing time between the end of injection and spark timing, whereas the most advanced SOI is limited by excessive CNG flow back into the intake manifold. Maximum brake torque (MBT) timing was maintained in all test conditions by adjusting spark timing to target 50% MFB location ∼ 8-10 CAD ATDCF . In present experimental study, SOI timings are categorized into three zones as shown in Figure 1 and defined as advanced SOI, i.e. fuel injection proceeding towards intake valve opening (IVO), retarded SOI, i.e. injection towards intake valve closing (IVC) and intermediate SOI as injection between these two.