Definition of Next Generation Engines
Gasoline direct injection (GDI) engine technology has received considerable attention over the last few years as a way to significantly improve fuel efficiency without making a major shift away from conventional internal combustion technology. In many respects, GDI technology represents a further step in the natural evolution of gasoline engine fueling systems. Each step of this evolution, from mechanically based carburation, to throttle body fuel injection, through multi-point and finally sequential multi-point fuel injection, has taken advantage of improvements in fuel injector and electronic control technology to achieve incremental gains in the control of internal combustion engines. Further advancements in these technologies, as well as continuing evolutionary advancements in combustion chamber and intake valve design and combustion chamber flow dynamics, have permitted the production of GDI engines for automotive applications.
GDI technology has potential applications in a wide segment of automotive industry. It is attractive to two stroke engine designer because of the inherent ability of in cylinder injection to eliminate the exhaust of uncombusted fuel during the period of overlap in intake and exhaust valve opening. The greatest fuel efficiency advantages of GDI can be realized in direct injection stratified charge lean combustion applications, significant fuel savings can be achieved even under stochiometric operation.
Use of gasoline direct injection (GDI) can reduce charge-air temperature while allowing for higher compression ratios. This has the effect of reducing the potential for detonation yet increasing gasoline engine efficiency. Instead of fuel and air mixing prior to entering the cylinder as with typical fuel injection, GDI uses a high-pressure injector nozzle to spray gasoline directly into the combustion chamber. An example of a GDI system is shown in Figure. One advantage of GDI is that as the fuel vaporizes, it absorbs energy from the charge. This "cooling effect" lowers the temperature of the air in the cylinder, thereby reducing its tendency to detonate.
GDI WITH TURBOCHARGING
In current turbocharged applications, the intake and exhaust valves are never open simultaneously. Unfortunately, lack of any valve overlap allows combustion gasses to remain in the cylinder after the exhaust stroke, which is a detriment to the next combustion process and can possibly increase NO X emissions. In GDI engines, though, the intake charge is air only-not an air-fuel mixture. This means that both intake and exhaust valves can be open at the end of the exhaust stroke and that fresh air can be used to flush out the cylinder.
Another recent innovation in turbocharger design that can further aid cylinder emptying during the exhaust stroke is the concept of twin-scroll turbine housing. Twin-scroll turbine housing serves to prevent pressure-wave interaction of the exhaust flows. Engines with an even number of cylinders, especially four-cylinder engines, frequently have a problem with exhaust pressure-waves from cylinders just beginning the exhaust stroke interacting with other cylinders that are nearing the end of the exhaust stroke. By using typical single-inlet turbine housing, approximately ten percent of the combustion gas remains in the cylinder after each exhaust stroke. Twin-scroll turbine housing, like that pictured in Figure, creates two separate inlets to the turbine section. Each inlet combines the exhaust flows from cylinders that are on different strokes in the cycle. Utilization of twin-scroll turbine housing significantly reduces the pressure-wave interaction between the cylinders, helping empty the cylinders of exhaust gasses more completely.
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