Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.2.3
 
Gasoline Direct Injection 
Gasoline direct injection (GDI) refers to systems where gasoline is injected 
directly into the combustion chamber. First GDI equipped spark-ignition (SI) engines 
were developed in 1950s but complex injector control electronics meant commercial 
use in light duty vehicles was not achieved on large scale until the end of 20
th
century. 
An extensive review of spark ignited direct injection gasoline engines was carried out 
by Zhao et.al. [106]. They bring attention to better atomisation of fuel with GDI 
compared to PFI due to higher injection pressures. Injection pressures of 250 bar and 
above can be reached [107]. Typical droplet size mean values are around 16 µm for 
GDI as opposed to 120 µm for PFI technology enabling faster evaporation of fuel 
[108, 109, 110]. This leads to reduced wetting of cylinder walls and significantly 
improved low temperature unburned hydrocarbon emissions. Unburned hydrocarbon 
emissions reductions of up to 70 % have been reported when experimentally used in a 
2-stroke engine [111]. 

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2.2 Fuel Metering Systems 
45 
In case the fuel is injected during the induction stroke, injection strategy is 
called homogeneous, whilst if in compression stroke, it is stratified. Stratified injection 
strategy allows for overall lean air-fuel ratios whilst stoichiometric conditions are 
achieved around the sparking point, enabling for air-fuel ratios as high as 25:1 
compared to stoichiometric 14.7:1. Reduced NO
x
emissions are thought to be possible 
with direct injection through charge cooling whereby relatively cool fuel is injected 
into the combustion chamber, thus reducing temperature within the cylinder. 
However, often the improved lean-burn capability and the subsequently increased 
availability of ambient nitrogen in GDI engines results in higher quantities of nitrogen 
oxides. 
Two types of injectors are used with GDI technology – pressure-swirl and 
multi-hole injectors [105]. Pressure-swirl injectors use a single hole at the tip of the 
injector with an internal nozzle geometry that gives the fuel a high angular velocity at 
exit from the nozzle, thus, creating a swirl effect. Swirl effect in pressure-swirl 
injectors is used to enable better air entrainment during mixing. As the fuel leaves the 
nozzle, it mostly produces a hollow conical sheet that can easily break down to create 
small droplets. As will be explained further in later sections, fuel mixing is directly 
related to atomisation of fuel spray which in turn is a function of the injector nozzle 
geometry. In multi-hole type injectors, several smaller diameter holes are used in the 
nozzle geometry. Air is entrained by dispersing several smaller jets into a wider 
volume and swirl effect is not needed. Additionally, smaller diameter nozzle holes will 
produce smaller size distributions during atomisation which increases fuel evaporation 
rates. 
In research and especially when spray characterisation is concerned, single-
hole injectors are often preferred over multi-hole injectors. Difficulties with multi-
hole injector analysis arise from the need to separate the spray plumes which can 
otherwise interact with one another resulting in non-symmetric radial velocity and 
drop size distributions at given distances from the nozzle exit. A non-symmetric radial 
drop size profile was measured by Fdida et.al. [110], who carried out experiments on 
drop sizing with a 6-hole GDI injector using laser diffraction and phase Doppler 
anemometry techniques. Serras-Pereira et al. [112] avoided interactions between spray 
plumes by using a custom-made ‘bowl’ to collect all but one spray plume and Patel 
[113] replaced a 6-hole diesel injector nozzle with a custom made 2-hole one where 
injection directions were radially 180° apart. Baert et al. [114] compared spray 

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2.3 Spray Characterisation 
46 
characteristics between one multi hole injector with a ‘bowl’ that blocked all but one 
spray plume and another with the holes welded closed. They found welding the holes 
to increase the momentum force of the spray by almost 10 % and as such 
recommended the ‘bowl’ type spray plume blocking technique. 
Due to high injection pressures, reliability of fuel pumps and injectors can 
become problematic and subsequently great expectations are put on the properties of 
fuel for removing deposits and lubricating the moving components of the fuel system. 
Additional disadvantage of the GDI system is its high dependence of accurate spark 
timing in stratified mode whereby early spark could miss the spray completely or late 
one be influenced by spark plug wetting due to high liquid fuel concentration. 
Furthermore, due to possibility of liquid fuel combustion, high level of soot formation 
is possible. Advantages include accurate fuel metering control and the possibility of 
very lean fuel mixtures promises greatly increased fuel economy and reduced 
emissions.