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The spray formation and mixing processes in a direct-injection gasoline engine are examined by using a sophisticated air flow calculation model and an original spray model. The spray model for a spiral injector can evaluate the droplet size and spatial distribution under a wide range of parameters such as the initial cone angle, back pressure and injection pressure. This model also includes the droplet breakup process due to wall impingement. The arbitrary constants used in the spray model are derived theoretically without using any experimental data. Fuel vapor distributions just before ignition and combustion processes are analyzed for both homogeneous and stratified charge conditions.

Essential mechanism underlying the instability of initial flame propagation, such as that of lean-burning, is analyzed by using the G-equation. We examine the “spatial” structures of the initial flame propagation process in detail, although a lot of efforts have devoted in order to investigate the “temporal” aspects such as turbulent intensity and flame speed. An important physical parameter, the ratio of the ignition width and the key-vortex diameter (L/D), is found to control the instability of initial flame propagation process. Optimization of the parameter L/D may stabilize initial flame propagation process for improving gasoline-engine performance.

Turbulent flows in a model engine having a square piston were analyzed in detail by using a numerical simulation method with higher-order accuracy to perform simulations on an orthogonal homogeneous grid without grid motions. Calculations were performed during several continuous engine cycles. A better understanding of the cycle-by-cycle differences, i.e., cyclic variations, in flow fields may lead to more effective ways of stabilizing combustion.

In recent years, a new type of engine (Fugine) based on the colliding of pulsed supermulti-jets was proposed by us, which indicates the potential for attaining very high thermal efficiencies and also less combustion noise. A prototype engine with eight nozzles for injecting octagonal pulsed supermulti-jets, which was developed with a low-cost gasoline injector and a double piston system, showed high thermal efficiency comparable to that of diesel engines and also less combustion noise comparable to that of traditional spark-ignition gasoline engines. Another type of prototype piston-less engine having fourteen bioctagonal nozzles was also developed and test results confirmed the occurrence of combustion, albeit it was unstable. In this work, time histories of pressure were measured in the combustion chamber of the piston-less prototype engine under a cold flow condition without combustion in order to examine the compression level obtained with the colliding supermulti-jets.

In our previous papers, a new concept of a compressive combustion engine (Fugine) was proposed based on the collision of pulsed supermulti-jets, which can enclose the burned gas around the chamber center leading to an air-insulation effect and also a lower exhaust gas temperature due to high single-point compression. In order to examine the compression level and air-insulation effect as basic data for application to automobiles, aircraft, and rockets, a prototype engine based on the concept, i.e., a piston-less prototype engine with collision of bi-octagonal pulsed multi-jets from fourteen nozzles, was developed. Some combustion results [Naitoh et al. SAE paper, 2016] were recently reported. However, there was only one measurement of wall temperature and pressure in the previous report. Thus, in this paper, more experimental data for pressures and temperatures on chamber walls and exhaust temperatures, are presented for the prototype engine.

Cyclic variations of stratified-charge and homogeneous-charge turbulent combustion in direct injection gasoline engine can be simulated over five continuous cycles based on the multi-level formation for the compressible Navier-Stokes equation and also a spray model. Computational result is compared with an experiment. Then, a factor generating the cyclic variations is revealed, which leads to an effective way to control instability of combustion at very lean burning conditions.

For various densities of gas jets including very light hydrogen and relatively heavy ones, the penetration length and diffusion process of a single high-speed gas fuel jet injected into air are computed by performing a large eddy simulation (LES) with fewer arbitrary constants applied for the unsteady three-dimensional compressible Navier-Stokes equation. In contrast, traditional ensemble models such as the Reynolds-averaged Navier-Stokes (RANS) equation have several arbitrary constants for fitting purposes. The cubic-interpolated pseudo-particle (CIP) method is employed for discretizing the nonlinear terms. Computations of single-component nitrogen and hydrogen jets were done under initial conditions of a fuel tank pressure of gas fuel = 10 MPa and back pressure of air = 3.5 MPa, i.e., the pressure level inside the combustion chamber after piston compression in the engine.

A synthesized spheroid particle (SSP) method is proposed on the basis of the first principles of fluid dynamics for calculating consistently the atomization behaviors of spray droplets and liquid sheets injected, the mixing of air and fuel, and the wall-impingement of droplet. The method includes a formulation for evaluating the collision of droplets accurately. The atomization process of the conical sheet of liquid fuel injected from a simplex nozzle is simulated by the SSP method for a wide range of back pressures and initial cone angles.

Fluid-dynamic principle for obtaining relatively stable combustion is found by performing cycle-resolved computations of turbulent flows in engines. Cycle-resolved computations are performed by using the implicit large eddy simulation (ILES) code, which we have proposed earlier. Calculations over continuous cycles show us the existence of “silent domain” in the engine cylinder, having weak cyclic-variations of flow. Time-dependent velocities averaged over six cycles, mean velocities, are also small in the silent domain. Moreover, we examine further on why cyclic variations of flow is weaker in the silent domain. This brings us a way for controlling cyclic variations for several engines.

Instability of the Initial flame propagation is examined after computing the flows during three continuous cycles of an engine. Cycle-resolved large eddy simulation (CLES) is employed for these computations. First, we calculated the compressible turbulent flows during three continuous cycles in a model engine having square piston. Then, the initial flame propagation processes are calculated by using G-equation at the flow condition of TDC. Grid system of 1,000,000 points is employed. Relation between cyclic-resolved turbulence and initial flame is qualitatively examined by the computational results.

We proposed a new compressive combustion principle for an inexpensive and relatively quiet engine reactor that has the potential to achieve incredible thermal efficiency. The high efficiency can be achieved with colliding supermulti-jets that create complete air insulation to encase burned gas around the chamber center. We developed a small prototype engine system for gasoline, which has a strongly-asymmetric double piston and the supermulti-jets colliding with pulse. In this report, we will show combustion experimental results at startup and at steady state operation. We obtained exhaust temperature over 100 degree Celsius and pressure data, which imply auto-ignition occurrence of gasoline.

A new engine concept based on pulsed supermulti-jets colliding at a small area around the chamber center was proposed in our previous research. It was expected to provide noiseless high compression ratio and nearly-complete air-insulation on chamber walls, leading to high thermal efficiency. In the previous reports, three-dimensional computations for the unsteady compressible Navier-Stokes equation were conducted, which were qualitative because of using regular grid method. This time, we develop a new numerical code in order to quantitatively simulate the compression level caused by the jets colliding with pulse. It is achieved by applying a staggered grid method to improve conservatibity of physical quantities at very high compression in combustion phenomena. Computations at a simple condition were fairly agreed with a theoretical value. Computational results obtained for a complex geometry of an engine by the new code had less error than one with previous codes.

This paper proposes a new compressive combustion principle for an inexpensive, lightweight, and relatively quiet engine reactor that has the potential to achieve incredible thermal efficiency over 60% even for small engines having strokes shorter than 100mm, whereas eco-friendly gasoline engines for today's automobiles use less than 35% of the supplied energy for work on average. This level of efficiency can be achieved with colliding supermulti-jets that create air insulation to encase burned gas around the chamber center, thereby avoiding contact with the chamber walls, including the piston. Emphasis is also placed on the fact that higher compression results in less combustion noise because of the encasing effect. We will first show that numerical computations done for two jets colliding in line quantitatively agree with shock-tube experiment and theoretical value based on compressible fluid mechanics.