HCNG) Fueled Supercharged Engine

Laser Ignition of Hydrogen Enriched Compressed Natural Gas (HCNG) Fueled Supercharged Engine

Avinash Kumar Agarwal*, Rajesh Kumar Prasad Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India *Corresponding Author’s Email: [email protected]

Abstract: In this study, the effect of enriching CNG with H2 (called hythane) on engine combustion, performance and emissions were investigated using a prototype single cylinder supercharged engine at two different operating conditions (naturally aspirated & supercharged) with different ignition sources [spark ignition (SI) and laser ignition (LI)]. The blending ratios were varied using a customized dynamic mixing system. The effect on flame kernel growth and start of combustion (SoC) using Nd:YAG laser (@1064 nm; 40 mJ/ pulse) for different HCNG blends were studied in constant volume combustion chamber (CVCC) at initial chamber filling pressures of 5 and 10 bar and 1000C temperature respectively. CVCC results showed that increased initial chamber filling pressure from 5 bar to 10 bar, the 10% mfb or SoC became faster, however combustion duration (CD) increased. Increasing H2 fraction reduced 10% mfb and CD for both pressures. Engine investigations showed that 30HCNG showed relatively earlier SoC, and lower coefficient of variation of indicated mean effective pressure (COVIMEP) at all load conditions. Laser ignition increased the brake thermal efficiency (BTE) at lean operating condition (approx. 42% for 30HCNG), and extended lean limit to 1.7 and 1.8 for 30HCNG and 40HCNG respectively. BSNOx decreased as λ increased but increased with increasing H2 fraction in the test fuel. Boosting resulted in slightly higher HC emissions due to lower in-cylinder temperatures.

Introduction Natural gas (NG) is one of most promising alternative fuels used in transportation sector due to its high H/C ratio, research octane number (RON), good anti-knocking properties [1]. The difficulties associated with CNG operated engines are such as slower flame speed and lower power output could be improved by adding small fraction of H2 with NG [2]. Minimum ignition energy (MIE) required for lean mixture at λ = 1.6 was 40 mJ/ pulse with focused beam numerical aperture (NA) was 0.04 [3]. It was found that LI led to a shorter ignition delay (ID), and higher laminar burning velocity compared to SI based on 10% mfb for lean NG-air mixtures [4]. The LI of NG-air mixtures required 26 mJ energy and gave at least 30 % higher pressure than SI systems and the lean limit was extended up to λ = 1.48 [5]. Herdin et al. [6] performed LI experiments in engine fueled by NG and observed that increasing in-cylinder pressure resulted in lower energy needed for plasma generation and shorter ID. Richardson et al. [7] performed LI (Nd:YAG laser; 1064 nm wavelength with pulse duration of 5 ns) experiments on a single cylinder engine fueled by H2-NG blends with H2 fraction ranging from 0-40% (v/v) and reported that for LI, the lean misfire limit was extended compared to SI.

Experimental Setup

Fig. 1. Schematic of experimental setup for CVCC Figure 1 shows a CVCC, which has four optical windows in the same plane. Two in longitudinal direction, which can be used for laser (Nd: YAG laser: Litron, Nano-L-200-30) beam propagation; and the other two in lateral direction, which can be used to illuminate the desired area using white light source (Thorlabs, OSL1) 1

and capture images using high speed camera (Photron, Fastcam SA1.1; @ 54000 fps). A single cylinder water cooled diesel engine (Kirloskar, DM-10; 948 CC; CR=11) was modified to operate on HCNG. Figure 2 shows the schematic of the engine setup with DC dynamometer. A custom build high flow rate solenoid gas injector was used to supply HCNG into the intake port. An injector triggering circuit was designed using a microcontroller (Arduino; Uno) to actuate the injector, which generated a 5V TTL pulse that activates the injector driver module.

Fig. 2. Schematic of engine experimental setup The injector driver module then converts this 5V pulse to 8A-2A peak and hold current output, which controls opening and closing of the solenoid injector. The amount of air inducted into the combustion chamber was controlled by a throttle valve and could be measured with the help of an orifice-plate fitted in the laminar flow element (Merium; 50MC2-2F). For supercharging, the test bench was equipped with an air compressor (Kaeser, SM12) which delivered compressed air at 6.0 bar pressure. Engine experimental were performed at constant speed of 1500 rpm with varying load (from no load to 30 Nm load) and varying λ. For laser ignition, plasma was created inside the engine cylinder using 2.5 mm diameter aperture fitted on the laser head and converging lens of 50 mm focal length in the laser spark plug.

Results and Discussion For CVCC investigations two different λ’s (1.1 and 1.3) were chosen to study effect of increasing H2 fraction and initial chamber filling pressures on 10% mfb and CD for HCNG blends. It was observed that immediately after plasma breakdown, plasma expanded asymmetrically and moved towards the lens faster than along the laser beam propagation direction.

Fig. 3. (a) Flame kernel growth of HCNG-air mixtures, (b) 10% mfb and CD for different HCNG-air mixtures Figure 3(a) clearly indicates that CD increased as λ increased due to slower flame kernel growth. It was also observed that increasing H2 fraction in HCNG, flame kernel growth increased at typical lambda and initial chamber filling pressure but retarded in all directions, if initial chamber filling pressure increased from 5 bar to 10 bar. Figure 3(b) shows the effect of initial chamber filling pressure, H2 %, and λ of unburnt fuel-air mixture on 10% mfb and CD. Reduction in flame initiation period could mitigate the problem of cycle-to-cycle variations in the engine [8]. As initial pressure increased from 5 bar to 10 bar, the flame initiation became faster however total CD increased. Increasing the H2 fraction in HCNG reduced 10% mfb as well as CD for both 5 bar 2

and 10 bar pressure initial chamber filling pressures but increased with increasing λ. In naturally aspirated engine operating condition (Figure 4), H2 addition shifts the peak pressure position towards TDC with increasing load. Rate of pressure rise (RoPR) increased with H2 addition but boosting lowered RoPR. 30HCNG showed early SoC at any load, followed by 40HCNG, and 20HCNG. Increasing H2 fraction increased CD for both SI and LI mode. LI increased the CD, which indicated more complete combustion. COVIMEP were below 6% for all HCNG blends in both SI and LI modes. 30HCNG showed relatively lower COVIMEP for both SI and LI modes. Lean burn limit of HCNG blends extended with increased H2 fraction and this extension was higher in case of LI mode. Compared to SI mode, LI increased BTE at lean operating conditions and lowered the exhaust gas temperature. BTE were approx. 42% for 30HCNG during lean operation. LI mode increased lean limit 1.7 and 1.8 for 30HCNG and 40HCNG respectively. Increasing H2 fraction in HCNG blends resulted in lower BSCO2 emissions. BSNOx decreased with increasing λ but increased with increasing H2 fraction. BSNOx decreased as λ increased and also with LI mode of operation.

Fig. 4. Comparison of BTE,BSFC and BSNOx using LI and SI for naturally aspirated and supercharged conditions Presence of H2 in the test blend enhanced combustion for higher boost pressure. BTE initially increased till optimum boosting and then decreased with increasing boost. Increased BTE showed more power output from the engine but decreased further due to leaner operation. Decreased exhaust gas temperature was due to higher H/C ratio. Boosting resulted in slightly more HC emissions due to lower in-cylinder temperatures. Using LI, 60% lower BSNOx was observed at 0.1 bar boost pressure.

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