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
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 NOX
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.
