Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.4.4.2
 
Unburned Hydrocarbons 
Unburned hydrocarbon (HC) emissions are an outcome of several processes. 
HC can result from leakage of air fuel mixture during the compression stroke through 
exhaust valves, small crevices within the combustion chamber, un-atomised fuel, 
layers of lubricant oil on combustion chamber walls or other cold surfaces that can 
quench flames [198]. Shen et al. [199] found the fuel hydrocarbon composition to 
have a profound effect on HC emissions through the aforementioned methods. They 
reported a decrease in HC emissions of up to 45 % with decreasing aromatic levels 
and increasing olefin levels in gasoline fuels.
Cold start HC emission levels were investigated by Henein and Tagomori 
[200]. HC emissions were contributed to low temperature combustion instability at 
start up. Additionally, low efficiency of the three-way-catalyst at low temperatures 
was mentioned. In order to reduce heat up time, in high performance vehicles this has 
resulted in catalytic converters being fitted on the exhaust manifolds or very near them 
[201, 202]. 
2.4.4.3
 
Nitrogen Oxides 
Nitrogen oxides (NO
x
) that result from combustion are nitric oxide (NO) and 
nitrogen dioxide (NO
2
). It is widely accepted that the oxides form as a result of 
oxidation of atmospheric nitrogen although in small quantities, it is possible for NO
x
emissions to originate from fuel bound nitrogen compounds [48]. Increased NOx 
emissions are contributed to increased combustion temperatures that enable oxidation 
of atmospheric nitrogen into nitrogen oxide and nitrogen dioxide. NO
2
emissions are 
only notable in compression and not in SI engines. NO
x 
output is dependent upon 
temperatures within the combustion chamber. Main heat induced reactions that 
contribute towards NO formation are [203]:
𝑁
2
+ 𝑂 ↔ 𝑁𝑂 + 𝑁
2.19 

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2.4 Combustion Analysis 
69 
𝑁 + 𝑂
2
↔ 𝑁𝑂 + 𝑂