Fuel tank draws the gasoline fuel into the fuel pump to be pressurized before passing into the combustion chamber. When it is pressurized, the fuel changes state from liquid to gas. The formation of this vapor is easily mixed with the air molecules in the chamber and ready to be ignited by the spark plug.
The result of this phase change will bring all the active particles to be cooled in the chamber while in PFI, the air is cooled in the intake manifold beforehand. Before injection timing occurs, it has a higher potential of producing increased power if only the density of incoming air rises after it is cooled such as in PFI. Hence, accidental detonation is more likely to occur in GDI since the temperature of combustion chamber is increasingly high due to the pressurized fuel. There are few advantages and drawbacks of using each type of fuel injection. However, the latest technology being investigated is a dual-injection concept where it utilizes both types of fuel injection techniques and compensate each flaw, which commonly can be found in CI engines.
One of the reasons are for its environmental impact. The in-cylinder temperature is responsible for the production of NOx. Hence by lowering the temperature, it could reduce the NOx and soot emissions 1-3. It is usually done by adding water from a PI system while diesel will be supplied in a separate DI system, as to improve the hydrogen energy share in a diesel DI engine 4. The application of dual-injection concept is relatively new in SI engines compare to CI engines. An alcohol-gasoline engine was being researched by Wang which he used gasoline in PI and alcohol in DI.
He found that the engine has a higher efficiency performance than any other injection strategies due to the alcohol in DI better utilized the high enthalpy of vaporization of alcohol fuels 5-6. This showed that the engine has a high and quick combustion rate than a gasoline PI. In this development, late EDI timing is desired due to its effectiveness on knock mitigation. This also deteriorated the combustion and emission performance of the engine.
Some of the few common processes used by researchers in industry to investigate the issue are as follows. The Rosin-Rammler Diameter Distribution Method was used to model the primary breakup process (blob injection concept) 7. This consequent droplets breakup process is induced by the relative velocity of gas and liquid phases and usually modeled by the WAVE model that only suitable for high Weber number (We>100) flows 8-9. The droplet distortion and drag will be sent for a Dynamic Drag model where it will assumed that droplet drag coefficient is related to the magnitude of the drop distortion 10. Finally, the Convection/ Diffusion Controlled Model will be used for the evaporation of the fuel which takes into account of its gradient diffusion (governed by fuel saturation vapor pressure) and convection (governed by flow velocity) on the droplet evaporation 11.
One of the reasons direct injection timing could affect the performance of the vehicle is wall-wetting. The fuel is released at the nozzle exit and depending on the interactions of gas and liquid phases by the source of fuel to be exerted to a certain amount of pressure that releases as the type of form desired to be by researching further its partial differential equations of the gas phase. As a liquid is released from its nozzle, the liquid droplets may stick, rebound, spread or splash when it collides with the wall depending on the wall surface temperature and impact energy 12. A partially premixed combustion in the combustion chamber of DI SI engines due to the mixing and evaporating process is still occurring by the time of ignition, hence making the mixture to not be congruent. In EDI of the formation of mixture, the high intake flow rate increases the heat and mass transfer between the fuel droplets and the surrounding gas, thus accelerates the fuel evaporation and enhances the mixing.
The in-cylinder flows become much slower in the compression stroke. When injection timing is slow in the compression stroke, the volume of combustion chamber becomes smaller and the in-cylinder flow rate reduces. At early timing, the liquid droplets has more time to absorb the heat surrounding from the cylinder walls and evaporate before combustion takes place. The cooling effect of fuel injection is compromised and the engine anti-knock ability is reduced. The late injection timing that made the increased wall film is also the cause of formation of hydrocarbon and carbon monoxide emissions.
This brings extreme cooling effect and over-rich mixture in the chamber which leads to incomplete combustion. The heat transfer dramatically drops before fuel injection because of the high air temperature, hence the cooling effect leads to a lower in-cylinder temperature at the time of ignition as well as lower knocking tendency 13. In GPI, gasoline is injected into the intake port and most of the fuel impinges to the metal surfaces of the port and valves 14. The port and valves of increased temperature has a hot surface that helps to evaporate the fuel to the fuel film whereas in EDI, the fuel absorbs heat from the charges and then evaporates.
Hence, this makes the cooling effect of GPI less effective to EDI. Moreover the octane number and latent heat of vaporization of gasoline are lower than ethanol. In DI, early injection timing is also said to improve the engine power output but weaker in mitigating the knock than late injection timing. Early injection timing can be further improved with a highly homogeneous mixture that improves the burning, an increased in the efficiency of volumetric of the pistons and the enhancement of spark timing. The volumetric efficiency can be improved by lowering the volume and temperature of the fresh charge that is partly due to the evaporation limit of the fuel. When the charge temperature is reduced, this will result a high volumetric efficiency in early injection timing.
Usually the evaporation of ethanol needs more heat transfer from the new charge and the cylinder wall. The ethanol may easily impinge on the cylinder surface as the piston passes the top dead center that caused the heat removal from piston surface and cylinder wall rather than from fresh charge hence lowers the volumetric efficiency. In late injection timing, the volumetric efficiency has no impact of its timing. Since the injection occurs after the inlet valve is closed, the volumetric efficiency stays constant 13. The duration of initiating the combustion does not affect or relate to the engine’s efficiency. It Is likely to be related to the mixture stability in combusting and the homogeneous quality including the in-cylinder temperature before igniting 15.
A long duration taken meaning mixture is hard to be burned. The longer the duration of combustion, the more heat is lost through the cylinder wall. Late injection timing has the longest combustion duration but it is effective against knocking. The thermal efficiency is almost independent of the early injection timing. If it were to compare, the early injection timing of thermal efficiency is better than the late injection timing and GPI conditions 13. In addition if ethanol was used as a fuel, the high energy content of the stoichiometric mixture per unit mass of air and the increased volumetric efficiency may be useful to thermal efficiency as well.
References
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http://dx.doi.org/10.4271/2012-01-1285.
