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1. Introduction
The basic goals of the automotive industry; a high power, low specific fuel consumption,
low emissions, low noise and better drive comfort. With increasing the vehicle number, the
role of the vehicles in air pollution has been increasing significantly day by day. The
environment protection agencies have drawn down the emission limits annually.
Furthermore, continuously increasing price of the fuel necessitates improving the engine
efficiency. Since the engines with carburetor do not hold the air fuel ratio close to the
stoichiometric at different working conditions, catalytic converter cannot be used in these
engines. Therefore these engines have high emission values and low efficiency. Electronic
controlled Port Fuel Injection (PFI) systems instead of fuel system with carburetor have been
used since 1980’s. In fuel injection systems, induced air can be metered precisely and the
fuel is injected in the manifold to air amount. By using the lambda sensor in exhaust system,
air/fuel ratio is held of stable value. Fuel systems without electronic controlled it is
impossible to comply with the increasingly emissions legislation.
If port fuel injection system is compared with carburetor system, it is seen that has some
advantages. These are;
1. Lower exhaust emissions.
2. Increased volumetric efficiency and therefore increased output power and torque.
The carburetor venturi prevents air and, in turn, volumetric efficiency decrease.
3. Low specific fuel consumption. In the engine with carburetor, fuel cannot be
delivered the same amount and the same air/fuel ratio per cycle, for each cylinder.
4. The more rapid engine response to changes in throttle position. This increases the
drive comfort.
5. For less rotation components in fuel injection system, the noise decreases
(Heywood, 2000; Ferguson, 1986).
Though the port fuel injection system has some advantages, it cannot be meet continuously
increased the demands about performance, emission legislation and fuel economy, at the
present day (Stone, 1999). The electronic controlled gasoline direct injection systems were
started to be used instead of port fuel injection system since 1990’s.
2 Fuel Injection
The Gasoline Direct Injection (GDI) engines give a number of features, which could not be
realized with port injected engines: avoiding fuel wall film in the manifold, improved
accuracy of air/fuel ratio during dynamics, reducing throttling losses of the gas exchange by
stratified and homogeneous lean operation, higher thermal efficiency by stratified operation
and increased compression ratio, decreasing the fuel consumption and CO2 emissions, lower
heat losses, fast heating of the catalyst by injection during the gas expansion phase,
increased performance and volumetric efficiency due to cooling of air charge, better coldstart
performance and better the drive comfort (Zhao et al., 1999; Karamangil, 2004; Smith et
al., 2006).
2. The Performance and Exhaust Emissions of The Gasoline Direct Injection
(GDI) Engine
2.1 Performance of the GDI Engine
The parameters that have the greatest influence on engine efficiency are compression ratio
and air/fuel ratio. The effect of raising compression ratio is to increase the power output
and to reduce the fuel consumption. The maximum efficiency (or minimum specific fuel
consumption) occurs with a mixture that is weaker than stoichiometric (Çelik, 2007).
Because the port fuel injection engines work at stoichiometric air/fuel ratio, it is impossible
to see more improvement in the fuel economy. In these engines, the compression ratio is
about 9/1-10/1. To prevent the knock, the compression ratio cannot be increased more. For
the same engine volume, the increasing volumetric efficiency also raises the engine power
output.
GDI engine operate with lean mixture and unthrottled at part loads, this operation provide
significantly improvements in fuel economy. At full load, as the GDI engine operates with
homogeneous charge and stoichiometric or slightly rich mixture, this engine gives a better
power output (Spicher et al., 2000). In GDI engine, fuel is injected into cylinder before spark
plug ignites at low and medium loads. At this condition, Air/Fuel (A/F) ratio in cylinder
vary, that is, mixture in front of spark plug is rich, in other places is lean. In all cylinder A/F
ratio is lean and A/F ratio can access until 40/1. In homogeneous operation, fuel starts
injecting into cylinder at intake stroke at full loads (Alger et al., 2000; Çnar, 2001). The fuel,
which is injected in the intake stoke, evaporates in the cylinder. The evaporation of the fuel
cools the intake charge. The cooling effect permits higher compression ratios and increasing
of the volumetric efficiency and thus higher torque is obtained (Muñoz et al., 2005). In the
GDI engines, compression ratio can gain until 12/1 (Kume, 1996). The knock does not occur
because only air is compressed at low and medium loads. At full load, since fuel is injected
into cylinder, the charge air cool and this, in turn, decreases knock tendency.
Since the vehicles are used usually in urban traffic, studies on improving the urban driving
fuel economy have increased. Engines have run usually at part loads (low and medium
loads) in urban driving. Volumetric efficiency is lower at part loads, so engine effective
compression ratio decreases (e.g. from 8/1 to 3/1-4/1), engine efficiency decreases and fuel
consumption increases. The urban driving fuel economy of the vehicles is very high (Çelik,
1999). Distinction between the highway fuel economies of vehicles is very little. As majority
Gasoline direct injection 3
of the life time of the vehicles pass in the urban driving, the owners of the vehicles prefer the
vehicles of which the urban driving fuel economy is low.
At full load, as the GDI engine operate with throttle, only a small reduction of fuel
consumption can be obtained to the PFI engine. There is the more fuel economy potential at
part load. At compression stroke, since air is given the cylinders without throttle for
stratified charge mode, pumping losses of the GDI engine is minimum at part loads, Fig.1
(Baumgarten, 2006). The improvements in thermal efficiency have been obtained as a result
of reduced pumping losses, higher compression ratios and further extension of the lean
operating limit under stratified combustion conditions at low engine loads. In the DI
gasoline engines, fuel consumption can be decreased by up to 20%, and a 10% power output
improvement can be achieved over traditional PFI engines (Fan et al., 1999).
Fig. 1. Reduction of throttle losses in the stratified-charge combustion (Baumgarten, 2006).
The CO2 emissions, which are one of the gases, bring about the global warming. To decrease
CO2 emitted from vehicles, it is required to decrease fuel consumption. Downsizing
(reduction of the engine size) is seen as a major way of improving fuel consumption and
reducing greenhouse emissions of spark ignited engines. In the same weight and size,
significant decreases in CO2 emissions, more power and higher break mean effective
pressure can be obtained. GDI engines are very suitable for turbocharger applications. The
use of GDI engine with turbocharger provides also high engine knock resistance especially
at high load and low engine speed where PFI turbocharged engines are still limited
(Lecointe & Monnier, 2003; Stoffels, 2005). Turbocharged GDI engines have showed great
potential to meet the contradictory targets of lower fuel consumption as well as high torque
and power output (Kleeberg, 2006).
4 Fuel Injection
In GDI engine, by using twin charging system drive comfort, engine torque and power can
be increased for the same engine size. For example, Volkswagen (VW) has used the dual
charging system in TSI (twin charged stratified injection) engine. The system includes a
roots-type supercharger as well as a turbocharger. The supercharger is basically an air
compressor. A mechanical device driven off the engine's crankshaft, it employs rotating
vanes which spin in opposite directions to compress air in the engine's intake system. The
high and constant torque is obtained at wide range speed by activate supercharger at low
speeds and turbo charger at high speeds (Anon, 2006).
In Table 1, it is given specifications of the two different engines belonging to the 2009 model
VW Passat vehicle, for example. TSI engine urban driving fuel economy is 18% lower than
that of PFI engine. CO2 emission is 12% lower than that of PFI engine. Although TSI engine
swept volume is lower than PFI engine, power and torque is higher by 20% and 35%,
respectively (Table 1). As engine torque is maximum at interval 1500-4000 1/min, shifting is
not necessary at the acceleration and thus drive comfort increase (Anon, 2009).
2.2 Exhaust Emissions of the GDI Engine
CO emission is very low in GDI engine. CO varies depending on air /fuel ratio. CO is high
at rich mixtures. Since GDI engines operate with lean mixture at part loads and
stoichiometric mixture at full load, CO is not a problem for these engines. In GDI engine,
due to the wetting of the piston and the cylinder walls with liquid fuel, HC emission can
increase. Hydrocarbon (HC) emissions are a function of engine temperature and, therefore it
can rise during cold start. The cold starts characteristics vary depending on the fuel
distribution