Effect of Gasoline Fuel Additives on Combustion and Engine Performance

Figure 3.9: End view of combustion vessel. Visible items: injector nozzle, electrodes & 
mixing fan 
3.2.1.2
 
Ignition System 
Figure 3.9 displays the electrodes in the combustion vessel. The electrodes 
were placed 55mm vertically below the injector nozzle, close to the centre of the 
vessel. For spray combustion, the electrodes were rotated slightly to shift the electrode 
gap towards the fringes of the spray where a more favourable air to fuel ratio, 
compared to the centre of the spray plume, was experienced. This is due to electrode 
wetting occurring at the central position that terminates the spark without ignition 
occurring. Electrode gap was fixed at 1mm for all experiments. 
Ignition system included a set of purpose built electrodes and a commercially 
available ignition coil. The electrodes were adjustable in the horizontal and vertical 
directions through different sized spacers and current carrying anode and cathode were 
separated from the pressure chamber through PEEK isolating rods. Although the 
anode in the circuitry was grounded to the pressure chamber and would not need 
isolation, for manufacturing purposes both the anode and cathode were manufactured 
following the same design. An assembly of the system can be seen in Figure 3.10.  

---

3.2 Combustion Experiments 
85 
Figure 3.10: Electrode assembly drawing 
The spark was generated through a commercially available coil-on-plug (COP) 
type inductive ignition coil. The system benefitted from easy set-up, low cost and 
readily adjustable spark duration through changes in dwell timing (coil charge time). 
This was especially important when spray combustion was concerned. Although Dale 
et al. [221] state that shorter spark durations are more efficient as less energy is lost to 
the electrodes, longer discharge periods increase ignition probability by exposing 
larger fraction of the air-fuel mixture to the related deviations in combustion 
characteristics, spark energy measurements for all combustions were performed.
Available spark discharge energy can be found by using the following 
relationship: 
𝐸
𝑆𝑝𝑎𝑟𝑘
= ∫[(𝑉
𝑃𝑟𝑜𝑏𝑒
− 𝐼
𝑃𝑟𝑜𝑏𝑒
× 𝑅
𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒
) × 𝐼
𝑃𝑟𝑜𝑏𝑒
] 𝑑𝑡
3.3 
Energy is calculated over the duration of the spark. Spark duration is defined 
as the time during which the measured current is above 0 A. Spark voltage and current 
data was obtained using a Tektronix P6015A high tension probe and Pearson Model 

---

3.2 Combustion Experiments 
86 
110 current monitor and sent to a Tektronix DPO3014 digital oscilloscope for data 
display and storage.
A typical voltage and current traces obtained with the aforementioned 
hardware with a signal trigger signal as a reference are presented in Figure 3.11. At t
1
the coil charging commences with voltages around 300-400 V applied to the electrode 
gap. A magnetic field, which is created between the primary and secondary windings 
of the ignition coil during this stage, collapses once power to the primary coil is 
removed, as represented by the end of spark signal. This leads to the pre-breakdown 
phase [221]. In this stage the potential difference between the anode and cathode 
caused by the collapsing magnetic field causes the free electrons in the gap to 
accelerate towards the anode. The higher the voltage input the higher the number of 
molecules that are ionised. Once the electrode gap impedance has been reduced to the 
point where current starts flowing, breakdown phase (t
2
) of the spark discharge 
phenomena is reached. In this stage the current and voltage increase rapidly and further 
ionise molecules between the electrodes. The energies in the arc channel are reported 
to be high enough to fully ionise the gas molecules and raise the temperature to 60 000 
K [221].