Two- and three-wheeled vehicles featuring small-bore engines represent a significant portion of the automotive market in several countries across the globe. Due to stricter emission norms, these engines are now equipped with port-fuel injection (PFI) for fuel delivery instead of carburetted system used earlier. Direct-injection (DI) technology, which offers several advantages over PFI, however, is not yet implemented on small-bore production engines. In GDI engines, fuel is directly injected into the engine cylinder, and therefore, increases the volumetric efficiency due to charge cooling effect. This results in lower heat loss to the cylinder walls and also reduces knocking tendency because of which, a greater compression ratio can be employed compared to those of PFI engines. The higher volumetric efficiency and compression ratio result in an increased power output as well as increases the fuel economy due to an improved thermal efficiency. Furthermore, since the fuel is directly injected, there is no formation of fuel films on the manifold walls. This causes an improved transient response of GDI engines. Also, less fuel has to be injected during cold start conditions which greatly reduces the unburnt hydrocarbon emissions. However, since the fuel is directly injected into GDI engines, there are challenges involved such as limited time for fuel-air mixture preparation and impingement of fuel on in-cylinder surfaces. These challenges are further complicated in small-bore engines.
Therefore, comprehending the intricate in-cylinder processes becomes crucial for the development of small-bore GDI engines. Consequently, the necessity arises to develop a small-bore GDI optical engine for studying in-cylinder processes. It is to be noted that optical engines feature a transparent cylinder and piston top on an elongated piston to provide optical access to the combustion area of the engine. In this context, these transparent components must have the capability to endure complex loads and boundary conditions. Furthermore, the presence of the elongated piston assembly introduces additional mass, resulting in an increase of unbalanced forces. It becomes imperative to counteract these increased unbalanced forces by implementing an appropriate balancing mechanism.
Therefore, in the present study, first, a computer-aided design (CAD) model of a small-bore GDI optical engine (displacement volume of 200 cm3) is developed using SolidWorks. It is followed by a coupled temperature-displacement finite element analysis (FEA) performed in ABAQUS/CAE for the investigation of temperature, combined stress and displacements in the transparent components. Further, three different balancing mechanisms are designed in SolidWorks. Subsequently, dynamic analysis of these mechanisms is conducted using ADAMS/View to investigate the unbalanced forces.
The results showed that analysing temperature and combined stress distributions within the transparent cylinder and piston top facilitated the determination of the optimum thickness for these components. Following the dynamic analysis, crankshaft counterweights are chosen, and a comparison of three balancing mechanisms and their associated unbalanced forces is conducted, revealing that the investigated small-bore GDI optical engine's balancing mechanism eliminates the need for a secondary balancer shaft. Following these analyses and using the designed CAD model, the optical engine is developed. Subsequently, this engine is evaluated through motoring to facilitate particle image velocimetry (PIV) experiments