Next generation Engines

ABSTRACT

              This paper deals with the recent evolution in SI engines, that is, GDI technology along with turbocharging and emission control.

                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.

GASOLINE DIRECT INJECTION

I] INTRODUCTION

        Continued drawbacks from the conventional carburetor have tended to develop new techniques in SI engines. The consistent draw backs are the higher fuel consumption, greater emissions & lower output, GDI is the recent technology which is becoming a dominant solution over these limitations.

       Direct injection has started to get a grip on the petrol engine market and today we have really entered the age of gasoline direct injection. The demand for more efficient engines offering reduced fuel consumption but maintaining high output has been behind the evolution of latest GDI engines. GDI engines are characterized by injection of fuel at high pressure directly into the combustion chamber by specially developed injectors. During the induction stroke the air flows into the cylinder. The beginning of the end of intake manifold injection technology is marked by the introduction of GDI engines. The 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.

        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.

                   

Figure 1.  A gasoline direct injection (GDI) system

GDI can also increase cylinder emptying during the exhaust stroke.

П] MAJOR OBJECTIVES OF GDI ENGINE

1.      Ultra low fuel consumption.

2.      Superior power to conventional MPFI engine.

1.      The difference between new GDI and current MPFI

For fuel supply, conventional engines use a fuel injection system, which replaced the carburetion system. MPFI or Multi-Point Fuel Injection, where the fuel is injected to each intake port, is currently the one of the most widely used systems. However, even in MPFI engines there are limits to fuel supply response and the combustion control because the fuel mixes with air before entering the cylinder. Now day’s companies are developing an engine where gasoline is directly injected into the cylinder as in a diesel engine, and moreover, where injection timings are precisely controlled to match load conditions. The GDI engine achieved the following outstanding characteristics.

                                                           

                Fig shows comparison of GDI with other fuel injection systems.

                  Fig. Transition of fuel supply system

The GDI technology have assisted the engine to acquire certain outstanding features such as

1]   Extremely precise control of fuel supply to achieve fuel efficiency that approaches to that of diesel engines by enabling combustion of ultra lean mixture.

2]    Very efficient intakes and relatively higher compression ratio.

                       

                                  

2.      MAJOR SPECIFICATIONS

PARAMETER	GDI	CONVENTIONAL MPFI
Compression ratio	12	10.5
Combustion chamber	Curved -top piston	Flat -top piston
Intake port	Upright straight	Standard
Fuel system	In cylinder direct injection	Port injection
Fuel pressure(MPa)	50	3.3

             Fuel injection allows the fuel to burn completely in the cylinder, so that, unburnt charge would be negligible which lacks any knocking or precombustion in the engine. Higher compressions can be possible which will increase power output, thermal efficiency without knocking.

     3. Technical features

• Upright straight intake ports for optimal airflow control in the cylinder
• Curved-top pistons for better combustion
• High pressure fuel pump to feed pressurized fuel into the injectors
• High-pressure swirl injectors for optimum air-fuel mixture

III] Major characteristics of the GDI engine

1. Lower fuel consumption and higher output

A] OPERATING MODES IN GDI ENGINES

       1]   Stratified operation mode

           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. 

 2] Homogenous operation mode

            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.

3] Homogenous lean operation mode

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.

B] The GDI engines foundation technologies

     There are four technical features that make up the foundation technology. The Upright Straight Intake Port supplies optimal airflow into the cylinder. The Curved-top Piston controls combustion by helping shape the air-fuel mixture. The High Pressure Fuel Pump supplies the high pressure needed for direct in-cylinder injection. And the High Pressure Swirl Injector controls the vaporization and dispersion of the fuel spray.

1] In cylinder air flow       

     The GDI engine has upright straight intake ports rather than horizontal intake ports used in conventional engines. The upright straight intake ports efficiently direct the airflow down at the curved-top piston, which redirects the airflow into a strong reverse tumble for optimal fuel injection.

                                                                  

2] Fuel Spray

Newly developed high-pressure swirl injectors provide the ideal spray pattern to match each engine operational modes. And at the same time by applying highly swirling motion to the entire fuel spray, they enable sufficient fuel atomization that is mandatory for the GDI even with a relatively low fuel pressure of 50kg/cm2.

3] Piston shape

     The curved-top piston controls the shape of the air-fuel mixture as well as the airflow inside the combustion chamber, and has an important role in maintaining a compact air fuel mixture. The mixture, which is injected late in the compression stroke, is carried toward the spark plug before it can disperse.

                                                                  

2. Realization of lower fuel consumption

 (1) Basic Concept

            In conventional gasoline engines, dispersion of an air-fuel mixture with the ideal density around the spark plug was very difficult. However, this is possible in the GDI engine. Furthermore, extremely low fuel consumption is achieved because ideal stratification enables fuel injected late in the compression stroke to maintain an ultra-lean air-fuel mixture.
           An engine for analysis purpose has proved that the air-fuel mixture with the optimum density gathers around the spark plug in a stratified charge. This is also borne out by analyzing the behavior of the fuel spray immediately before ignition and the air.

                                                                        

(2) Combustion of Ultra-lean Mixture

                    In conventional MPI engines, there were limits to the mixtures leanness due to large changes in combustion characteristics. However, the stratified mixture of the GDI enabled greatly decreasing the air-fuel ratio without leading to poorer combustion. For example, during idling when combustion is most inactive and unstable, the GDI engine maintains a stable and fast combustion even with an extremely lean mixture of 40 to 1 air-fuel ratio.

 (3) Vehicle Fuel Consumption

   Fuel Consumption during Idling

The GDI engine maintains stable combustion even at low idle speeds. Moreover, it offers greater flexibility in setting the idle speed. Compared to conventional engines, its fuel consumption during idling is 40% less.

                                                               

                        

Fuel Consumption during Cruising Drive

         At 40km/h, for example, the GDI engine uses 35% less fuel than a comparably sized conventional engine.

                                                                 

Fuel Consumption in City Driving

              The GDI engine used 35% less fuel than comparably sized conventional gasoline engines. Moreover, these results indicate that the GDI engine uses less fuel than even diesel engines.

                                                            

 Emission Control

          Unregulated emissions such as benzene, 1-3butadiene, formaldehyde, and acetaldehyde are the vehicular hydrocarbon emission components coming out from the GDI engines, which will be targeted near future. Hcs are removed by a catalyst at normal operating conditions, but the conversion efficiency is low at the cold start conditions.

          Previous efforts to burn a lean air-fuel mixture have resulted in difficulty to control NOx emission. However, in the case of GDI engine, 97% NOx reduction is achieved by utilizing high-rate EGR (Exhaust Gas Ratio) such as 30% that is allowed by the stable combustion unique to the GDI as well as a use of a newly developed lean-NOx catalyst.

                                                                      

3. Realization of Superior Output

(1)   Basic concept

            To achieve power superior to conventional MPI engines, the GDI engine has a high

Compression ratio and a highly efficient air intake system, which result in improved volumetric efficiency.

 Improved Volumetric Efficiency

             The upright straight intake ports enable smoother air intake. And the vaporization of fuel, which occurs in the cylinder at a late stage of the compression stroke, cools the air for better volumetric efficiency.

                                                         
                                                                  

Increased Compression Ratio

                The cooling of air inside the cylinder by the vaporization of fuel has another benefit, to minimize engine knocking. This allows a high compression ratio of 12, and thus improved combustion efficiency.

                                                                 

(2)   Achievement

Engine performance

       Compared to conventional MPI engines of a comparable size, the GDI engine provides approximately 10% greater outputs and torque at all speeds. 

                                                                 

                                                        

                                                              

Vehicle Acceleration

                   In high-output mode, the GDI engine provides outstanding acceleration.
The following chart compares the performance of the GDI engine with a conventional MPI engine. 

                                                                     

                                                                

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.

Figure 16.  This is a picture of a turbocharger with twin-scroll turbine housing.  Notice the dual inlets that allow the separation of exhaust from interacting cylinders

Gasoline for GDI engine

                     The GDI engine is persisting fundamental drawback with sulfur content in the gasoline, which increases NOx emissions during stratified operation mode. The sulfur content in the gasoline should be restricted to 5ppm compared to 338ppm present actually in the gasoline.

SUMMERY & CONCLUSION

SUMMARY

               GDI though developed long before in 1930s, its configuration and the new electronic control are among the top of the new inventions. GDI on the way to satisfy today’s fuel saving requirements and increasing environmental demands. Flexibility to adopt changing vehicle requirements is the key benefit of the GDI which separate it from other conventional engines.

               Emissions coming out from the burning of fuel at low temperature during stratified operation mode are the major concerns ahead. Turbocharging and new emission control techniques can be used for their subsequent regulation and control.

               All the major car manufactures are now shifting towards GDI and MPFI soon is replaced by it. GDI engines will spread quickly in the countries having strict standards about pollution control and the fuel quality being used.  

CONCLUSION

                   From this paper it can be concluded that GDI helps improving fuel savings, thermal efficiency pioneered by its different operating modes. Restriction of sulfur to 5ppm in gasoline is the key requirement for emission control. Like the all the fuel injection systems that have come before it, the new direct injection engines will still require replacement parts and will likely suffer from similar injector woes that plague today's engines. In fact, direct injection injectors may prove to be even more troublesome than today's indirect injectors because they're exposed directly to the heat of combustion.