Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.20 
𝑁 + 𝑂𝐻 ↔ 𝑁𝑂 + 𝐻
2.21 
Further possibilities have been proposed where recombination process of N
2
and O occurs [198, 204]: 
𝑁
2
+ 𝑂 + 𝑀 ↔ 𝑁
2
𝑂 + 𝑀
2.22 
𝑁
2
𝑂 + 𝑂 ↔ 𝑁𝑂 + 𝑁𝑂
2.23 
And 
𝑁
2
+ 𝐻 + 𝑀 ↔ 𝑁𝑁𝐻 + 𝑀
2.24 
𝑁𝑁𝐻 + 𝑂 ↔ 𝑁𝑂 + 𝑁𝐻
2.25 
Several investigations have been carried out under different conditions and 
with different fuels to see effects on NO
x
emissions. Chen et al. [205] studied the effect 
of alcohol-gasoline fuel blends and measured NO
x
reduction of up to 30 %. However, 
this was accompanied with a 10 % reduction in torque and even greater output losses 
and increased emissions were recorded under high load conditions. Wang et al. [206] 
investigated the effect of gasoline mixed with hydrogen or hydrogen-oxygen mixture 
on engine performance and emissions. They found a decrease in CO and HC emissions 
but up to 70 % increase in NO emissions with hydrogen-oxygen blends. This was 
contributed to increase combustion chamber temperature and increased air availability 
within the combustion chamber.
The primary technique used in modern automotive engines to reduce the NO
x 
emissions is exhaust gas recirculation (EGR). Part of the clean air introduced to the 
engine is replaced by exhaust gases from the previous cycle. This increases the water 
vapour content and reduces available oxygen levels in the cylinder. Although small 
gains can be achieved through depletion of available oxygen, the main advantages 
result from addition of water vapour. The water vapour increases the heat capacity of 
the gas mixture within the cylinder and as a result, the peak temperatures reached are 
reduced, thus, reducing NO
x
emissions [207].

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2.4 Combustion Analysis 
70 
Three-way catalysts used in modern vehicles have proven to be inefficient in 
reducing NO
x
emissions in lean burn engines [208]. Problems arise from the increased 
amount of atmospheric nitrogen in the air fuel mixture resulting in increased nitric 
oxide levels and the efficiency of catalyst at high exhaust oxygen concentrations. 
However, research is on-going in finding more efficient ways in which to tackle the 
issue both experimentally as well as by modelling methods [209, 210] and has in part 
been addressed by the creation of the lean NO
x
trap (LNT) [208, 211] and the Selective 
Catalytic Reduction (SCR) technology [212, 213] 
LNT technology allows for NO to oxidise on the alkali metal and alkaline earth 
promoters present in the catalysts to form NO
2
. The oxidation is followed by 
formation of stable nitrites with the alkali metals or alkaline earth materials. The trap 
has, however, an absorption limit and will need to be ‘purged’ after a while which 
currently is achieved by reducing the air-fuel ratio for short periods of time. The SCR 
technology injects a reducing agent (typically urea) into the exhaust of a vehicle with 
reported reductions of up to 50 % in NO
x
been reported [214]. However, these 
technologies are primarily still in early stages of development and can come at a 
significant cost.