Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.3
 
Spray Characterisation 
The previous section described the different fuel introduction and atomisation 
strategies available to IC engines. Although carburettors allow for excellent air-fuel 
mixing characteristics, lack of control over fuel quantity and the subsequent reductions 
in fuel efficiency mean that the use in modern engines has all but stopped and the 
following sections will concentrate on fuel spray development through pressure 
atomisers. Furthermore, variables that affect atomisation quality and methods in 
determining droplet sizes are presented.
2.3.1
 
Fuel Atomisation 
Dependent upon the injection pressure, fuel and ambient conditions, sprays go 
through the process of liquid flow ejection, primary and secondary spray plume and 
droplet break-up. The liquid jet leaving a nozzle will start to deform on the liquid 
interface. As the spray progresses, the disturbances on the jet surface grow and result 
in the jet breaking into fragments and ligaments. This mechanism is referred to as the 
primary break-up of the spray. The size of these fragments is influenced by liquid 
density, ρ
l
, dynamic viscosity, 
µ
l
, flow velocity, u and a characteristic dimension D. 
This can be described by a dimensionless Reynolds Number, Re: 

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2.3 Spray Characterisation 
47 
𝑅𝑒 =
𝜌
𝑙
𝑢𝐷
µ
𝑙
2.3
 
After primary separation of fragments and ligaments from the liquid jet, the 
elements can further disintegrate into smaller droplets. This process is the secondary 
break-up regime and can continue to take place until the surface tension forces are 
strong enough to withstand the disruptive hydrodynamic inertial forces [115]. The 
ratio between the inertial and surface tension forces can be expressed as the 
dimensionless Webber number, We: 
𝑊𝑒
𝑔
=
𝜌
𝑔
𝑢
2
D
𝜎
𝑙
2.4
 
Weber number can also be expressed in terms of the liquid properties at the 
spray boundary where the subscript ‘g’ would be replaced by ‘l’ [116]. A critical 
Weber number above which secondary droplet break-up occurs has been shown to be 
around 12 [117]. However, the ranges of Weber numbers within which different 
secondary droplet break-up mechanisms can occur vary widely, as reviewed by Jaalal 
and Mehravaran [118]. The review refers to varied droplet generation techniques 
employed by different researchers as the principal cause. Acroumanis et al. [119] 
suggest a classification of secondary droplet break-up mechanisms as shown in Table 
2.2.
Weber Number 
Break-Up Mechanism 
We < 12 
Vibrational 
12 ≤ We < 18 
Bag 
18 ≤ We < 45 
Bag-and-Stamen 
45 ≤ We < 100 
Chaotic 
100 ≤ We < 350 
Sheet Stripping 
350 ≤ We < 1000 
Wave Crest Stripping 
We < 1000 
Catastrophic