Effect of Gasoline Fuel Additives on Combustion and Engine Performance

4.2.1
 
Spray Combustion 
Initial combustion investigations were carried out using the DI fuel delivery 
system. Fuel was pressurised to the rated 110 bar and air in bomb raised to 5 bar. This 
configuration allowed for numerous combustion events to be carried out under closed 
vessel conditions without the diminishing oxygen levels having an effect subsequent 
combustions. Also, as described by Ghasemi et al. [233], increased back pressure 
results in an increased ambient density and leads to a wider spray cone angle. As a 
result of a wider cone angle, more of the quiescent air is entrained by the spray jet and 
lower localised equivalence ratio is achieved. In the current study, this improvement 
was observed through a reduced number of misfires. 
Typical pressure and heat release rate traces under ambient temperature 
conditions can be seen in Figure 4.17 and high speed imaging for a similar event in 
Figure 4.18. The pressure displayed was above the vessel starting pressure of 5 bar. A 
peak pressure of just below 0.04 bar was observed. This amounts to less than 1 % of 
what is experienced in a gasoline engine. Furthermore, over 20 combustion events, a 
standard deviation in peak pressure of more than 15 % and over 30 % in peak heat 
release rate was experienced. This is likely to be caused by the spray and spark 
variability. Peak heat release rate was experienced at around 35 ms after start of 
ignition signal and total combustion duration was approximately 90 ms. This is 
significantly longer compared to engine conditions where peak heat release can be 
reached in 2-3 ms and total duration remains below 10 ms [234, 235]. This was likely 
caused by the dense fuel spray not enabling close to stoichiometric air-fuel ratios in 
the central areas of the spray, thus, affecting combustion characteristics. Furthermore, 
the low energy state of the combustion environment would have had an effect as well 
as seen in previous sections, fuel spray delivery from the injector nozzle to the 
combustion region could have hindered the processes. 

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4.2 Combustion Investigations 
107