# Experimental Activities on a Hydrogen-Fueled Spark-Ignition Engine for Light-Duty Applications
## 2 Materials and Methods  
### Engine Architecture and Test Cell Characteristics  
The experiments were conducted using a single-cylinder SI engine with a displacement of 454.2 cm\({}^{3}\). These experimental facilities were previously employed in research conducted by [PERSON] et al. [27,28]. For reference, the key specifications of the engine are provided in Table 1. The engine has an 86 mm stroke, a compression ratio (CR) of 10.7:1, and a 4-valve pent-roof cylinder head. All the experimental tests utilized a conventional spark plug ignition system. Two different injection systems were used for the fuel supply: one being a PFI system capable of a maximum injection pressure of 5 bar, and the other a medium-pressure DI injection system with a maximum pressure of 30 bar. The DI system was specifically designed to enable a brief injection duration, ensuring the precise delivery of the necessary fuel quantity as the injection moved closer to the top dead center (TDC). The control of the fuel injected was managed by regulating the injection duration as a pivotal parameter. The hydrogen was stored and supplied in pressurized tanks at 200 bar. The engine was installed in a fully instrumented test cell, designed as illustrated in Figure 1. The initial test bench layout, originally intended for gasoline as fuel, was adapted for the use of pure hydrogen. Previous research included investigations into CNG and HCNG dual fuel systems [28]. For safety measures, an intake manifold flame arrester was installed upstream of the EGR mixer, and a dedicated hydrogen supply system was implemented to provide fuel to the injectors. The experimental facility offers comprehensive control over each relevant parameter during engine operation. High-diluted air conditions were achieved using an external compressor, while exhaust back-pressure control was maintained through a knife-gate valve situated in the exhaust line. Automated valves were employed to manage the hydrogen injection pressure when transitioning between the PFI and DI modes. \begin{table}
\begin{tabular}{c c} \hline Number of cylinders & 1 \\ Number of strokes & 4 \\ Displaced volume & 454.2 cm\({}^{3}\) \\ Stroke & 86.0 mm \\ Injection systems & PFI/DI \\ Ignition system & Spark plug \\ Cylinder diameter & 82.0 mm \\ Compression ratio & 10.7 \\ Connecting rod length & 144.0 mm \\ Valves per cylinder & 2 intake, 2 exhaust \\ Engine management system & AVL PREMS GDI \\ Combustion system & 4-valve pent-roof CDI \\ Intake valve opening (IVO) * & \(-\)380 CAD \\ Intake valve closing (IVC) * & \(-\)135 CAD \\ Exhaust valve opening (EVO) * & \(-\)600 CAD \\ Exhaust valve closing (EVC) * & \(-\)338 CAD \\ \hline \end{tabular} \({}^{*}\) with respect to the firing TDC (0 CAD).  
\end{table} 
Table 1: Main engine specifications. The test bench was fully equipped with state-of-the-art measurement devices. Incylinder pressure was measured using a piezoelectric sensor, while the intake and exhaust pressures were monitored using two distinct piezoresistive sensors. A Bronkhorst F-113 AC-1M0-AAD-55-V flowmeter was employed to record the hydrogen flow rate. Three distinct methods were employed to measure and estimate the air-to-fuel ratio: an exhaust gas analyzer, an \(\lambda\) sensor positioned at the exhaust tailpipe, and consideration of the various flows within the intake manifold (comprising air and fuel). All signals were captured at a sampling frequency of 0.2 CAD. For the analysis of NOx emissions and other relevant exhaust gases, HORIBA MEXA-7600 EGR equipment was utilized. Table 2 provides an overview of the instrumentation along with its corresponding accuracy. Hydrogen exhibits highly specific combustion properties suitable for internal combustion engines, and the most significant properties are summarized in Table 3. Notably, it possesses a wide ignition range in air, spanning from 4 to 73% by volume, allowing for stable combustion even with extremely lean mixtures.