Emissions

Vehicle emission levels have increasingly been one of the driving forces in engine and fuel technology research. All countries within the EU follow the same regulations for new cars. The regulated emissions include carbon monoxide, unburned hydrocarbon, nitric oxide, nitrogen dioxide and particulate emissions. Additionally from the start of EURO 6 standard, the total number of particulates (in addition to weight) will also be measured. As briefly mentioned in Section 2.1, fuel additives can be successfully utilised to reduce regulated emission levels.

2.4.4.1 Carbon Monoxide

Carbon monoxide (CO) primarily results from incomplete combustion that is caused by oxygen starvation during combustion that would allow for complete oxidation of the fuel. However, slow burning rates could also result in an increase in emissions. Incomplete combustion is mostly reached during cold start periods as well as transient events during full load situations where fuel rich mixtures are employed.

Better burning rates and lean mixtures enable lower CO levels. Ji and Wang [194]

experimented with hydrogen addition to fuels to improve the lean burn limits of gasoline. They found reductions of CO emissions by more than 75 % were possible with 4.5 % hydrogen in gasoline. Furthermore, reduction in cold start emissions was reported by Chen et al. [195] with gasoline-ethanol blends, where ethanol

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concentration was between 20-30 % in fuel. Similar results were reported by Al-Hasan [196] where 20 % ethanol blend gave lowest CO emission levels. Emissions are of great interest and even diesel-gasoline fuel blends have been studied for effects on cold start CO release rate [197].

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 (NOx) that result from combustion are nitric oxide (NO) and nitrogen dioxide (NO2). It is widely accepted that the oxides form as a result of oxidation of atmospheric nitrogen although in small quantities, it is possible for NOx

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. NO2 emissions are only notable in compression and not in SI engines. NOx output is dependent upon temperatures within the combustion chamber. Main heat induced reactions that contribute towards NO formation are [203]:

𝑁₂+ 𝑂 ↔ 𝑁𝑂 + 𝑁 2.19

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𝑁 + 𝑂₂ ↔ 𝑁𝑂 + 𝑂 2.20

𝑁 + 𝑂𝐻 ↔ 𝑁𝑂 + 𝐻 2.21

Further possibilities have been proposed where recombination process of N2

and O occurs [198, 204]:

𝑁₂+ 𝑂 + 𝑀 ↔ 𝑁₂𝑂 + 𝑀 2.22

𝑁₂𝑂 + 𝑂 ↔ 𝑁𝑂 + 𝑁𝑂 2.23

And

𝑁₂+ 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 NOx emissions. Chen et al. [205] studied the effect of alcohol-gasoline fuel blends and measured NOx 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 NOx

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 NOx emissions [207].

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Three-way catalysts used in modern vehicles have proven to be inefficient in reducing NOx 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 NOx 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 NO2. 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 NOx been reported [214]. However, these technologies are primarily still in early stages of development and can come at a significant cost.

2.4.4.4 Particulate Emissions

According to Myung et al. [215], particulate matter (PM) and particulate number (PN) formation in gasoline engines is related to DI engines. Namely, the emissions are related to non-uniform fuel air mixture and wetting of combustion chamber walls that can inherently occur under cold start and transient high fuel injection rate conditions. Cold conditions inhibit evaporation of fuel which hinders air-fuel mixing and results in fuel rich areas where insufficient oxygen levels cause pyrolysis of fuel to occur. Their experiments with liquefied petroleum gas addition showed significant promise of the fuel to reduce PN concentrations by up to 99 %.

Further improvements can be achieved by improved injector design. Using multi-hole DI injectors and spray-guided (as opposed to wall guided) injection systems where fuel is injected towards the ignition source can significantly reduce wetting of cylinder walls.

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