Published on Apr 02, 2024
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.
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.
A] OPERATING MODES IN GDI ENGINES
The engine offers highest amount of fuel savings in the stratified lean operation mode with a large amount of excess air. As the fuel injected is small in quantity control over its injection timing is very important otherwise homogenization of the same would lead to no or very poor combustion. Therefore the fuel air mixture is concentrated by strategic injection no earlier than last third of the upwards movement of the piston so that the fuel will be concentrated exactly around the spark plug. The air fuel ratio at this mode is 30 to 40.
As there is no dependency of fuel injection with throttle opening the throttle remains wide open during the induction stroke, allowing the maximum air with proper circulation. The charge stratification allows engine to burn total cylinder mixtures with a much high concentration of air than conventional engines. The air fuel ratio can be as high as 55:1. During stratified charge operation, the injectors meter the fuel mass so precisely that unthrottled operation is possible which reduces pumping effect and lowers fuel consumption. Stratified mixture greatly decreases air fuel ratio without leading to poorer combustion. In addition, ignition and combustion occur centrally in the combustion chamber, surrounded by an insulating air cushion that reduces heat dissipation at the cylinder wall, thus improving the efficiency.
The characteristic-controlled cooling also somewhat increases the economy; during underloads, it lets the coolant temperature increase to 110 degrees Celsius, thus improving the efficiency of the engine. However, the especially economical stratified lean operation mode functions only in the case of underloads and low speeds (up to 3000 rpm). At higher speeds, the time is not sufficient to optimally prepare the fuel, which is injected very late during the stratified lean operation mode, and to control the emissions.
When the GDI engine is operating with higher loads or at higher speeds, fuel injection takes place during the intake stroke. This optimizes combustion by ensuring a homogeneous, cooler air-fuel mixture that minimized the possibility of engine knocking. If the driver requires increased engine performance, the engine controller automatically switches to the homogenous operation mode, with an evenly distributed fuel-air mixture in a stoichiometric relationship (lambda equals 1). Now, the fuel is injected into the air in the intake in time with the intake of air so that a homogenous, easily combustible fuel-air mixture forms within the entire combustion chamber.
This is not required at higher engine loads, where the switch valve opens so that the air can flow into the combustion chamber without any impediments. Another factor that reduces consumption in the homogenous operating mode is that the engine has a higher efficiency than conventional petrol engines with intake manifold injection due to higher compression.
The third operating mode of the engine at higher loads and speeds where stratified operation is no longer possible is the homogenous lean operating mode. In terms of performance characteristics, it can be said that this operating mode forms a belt between the stratified operation and the homogenous operating modes. In order to increase the turbulence and thus the inflammability of the lean mixture, injection and combustion run in a manner similar as in the homogenous operation mode, with the difference that more air is mixed in than is required for combustion. As a result, fuel consumption can be reduced.
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