**1. Introduction**

Modern-day transport systems are important and critical, especially the transportation of goods, transport services and people. Internal combustion engines with diesel fuel as the primary source of energy form the bulk of commercial and personal transport. This is owing to their numerous advantages compared with other forms or types of propulsion in internal combustion engines.

Diesel engines are inherently lean burn engines, generating low carbon dioxide emissions compared with petrol-propelled internal combustion engines. Diesel engines have other merits such as high thermal efficiencies, durability and construction robustness [1]. This endears them to users, thus expanding application use as more countries move into urbanization and industrialization. However, there has been a formidable challenge to phase them out, based on environmental and human health issues due to the high levels of NOX, smoke and PM emissions.

Therefore, there has been continued increase in stringent emission regulations enacted by global industrial powers, United States of America and the European Union environmental protection agencies the G-7 and G-20. The diesel engine has been accused as a pollutant, hence the search for alternative fuels in the interest of reducing energy consumption, environmental degradation and air pollution from NOX gases, which diesel engines emit, thus decelerating atmospheric carbon concentration globally. The road transport sector is an environmental concern due to its rapid expansion. This expansion has eroded all the technological developments and improvements achieved in the war against air pollution from diesel engines. Climatic change, erratic energy prices, uncertainty of future fossil fuel supplies, unending internal conflicts in major oil-producing countries create a compelling case for alternative fuels [2].

The alternative sources of fuel energy supply increase food insecurity as it makes the use of plant-based feedstocks for biodiesel fuel. This makes this option a far less viable option leading to high food prices and inflation [3]. Therefore, waste plastic from municipal solid waste management sites is increasingly becoming a popular alternative source of fuel and energy due to the widespread use of plastics in daytoday activities. Despite the greater factor of environmental effects of plastic waste and disposal costs, plastics are still applied widely in daily economic and social activities. Plastic waste has created havoc to the environment due to challenges of proper disposal and non-biodegradable nature of plastics [4].

There are two types of plastics widely used today, namely PVC (poly-vinyl chloride) and HPDE (high-density polyethylene) also known as polyethylene highdensity (PEHD) [5].

Globally plastic waste accounts for 8–12% of waste with a projected annual increase of 9–13% by 2025 [6, 7]. Back here at home in South Africa, 24,115,402 metric tonnes of general waste was produced, 6% of which is 1,446,924 metric tonnes of plastic waste with a national average waste production annual increment projection of 2–3%, since 2008 [8] as in **Figure 1**. This makes a sustainability case in managing waste into energy, using technology to degrade waste plastic mass into energy. Using techniques such as pyrolysis results in hydrocarbons similar in quality and characteristics to petroleum fuels due to its high yield achieved by pyrolysis [9, 10].

Originally, pyrolysis is a word coined from two Greek words pyro-'fire' and lysis-'decomposition' [11]. Pyrolysis is a chemical decomposition process of making fuel from plastic waste by heating [12]. Pyrolysis has been recommended as one of the solutions to ending the menace of plastic waste in the world. During pyrolysis, assorted waste plastic is introduced into a reactor and subjected to high temperatures of 400–600°C or sometimes 900°C at atmospheric pressure in the absence of oxygen for 3–4 hours to produce oil and other plastic waste by-products [13]. As a method of transforming waste plastic into biodiesel pyrolysis has been recommended by researchers and commercial entities. This is because of its cost-effectiveness and its high energy conversion rate besides the high yield compared with any other method of plastic waste extraction [14].

*The Influence of Exhaust Gas Recirculation on Performance and Emission Characteristics… DOI: http://dx.doi.org/10.5772/intechopen.105011*

**Figure 1.**

*Waste data analysis (from municipalities) for South Africa [8].*

Catalysts are employed to maintain and sustain high temperatures during pyrolysis reaction [15]. These catalysts include calcium oxide (CaO), silica dioxide (SiO2), aluminum tri-oxide (Al2O3) and zeolite (NaAlSi2O6-H2O) [16]. Pyrolysis breaks down large molecules of plastic waste into minute molecules producing hydrocarbons with smaller molecular mass. For example, the addition of ethane enables fractional distillation to be applied and obtain fuels, chemicals and by-products from the process. The pyrolysis process gives yields with a weight factor of 75% of liquid hydrocarbons in mixtures of petrol, diesel and kerosene, in the proportion of 5–6% as residue coke while the remaining balance as liquidified petroleum gas (LPG) [17].

The use of biodiesel thus calls for NOX reduction techniques such as exhaust gas recirculation (EGR) due to the oxygen content inherent in most biodiesel fuels. This is the single most factor responsible for NOX formation as it reacts with high-temperature combustion mixture, thereby increasing the availability of NOX [7]. Diesel fuels and biodiesel fuels both require fuel additives to improve engine lubricity, better ignition qualities and better mixing. Oxygenates in biodiesels provide reduction in PM emissions since the O2 content aids better combustion. It also lowers exhaust emissions with a clear-cut trade-off between PM and NOX as in the findings of [18–20]. Most of these researchers suggest modifications, for example, using thermal barrier coating [21]. Thermal coating improves efficiency, reduces NOx emissions and smoke density but minimally increases brake thermal efficiency with a decrease in fuel economy.

Saravanan, [22] Observed that with application of EGR percentage flow rate, a further reduction for both NOX and soot emission could be achieved with addition of n-pentanol. In an experiment conducted by [23], the authors reported a simultaneous reduction for both NOX and soot emissions using low-temperature combustion (LTC) strategy, with EGR % flow rates, late injection timing and n-pentanol blended diesel-biodiesel fuels. However, [24] reported a contrary finding with addition of n-pentanol to diesel-biodiesel resulting in increased BSFC and no decrease in BTE. This seems to confirm n-pentanol as a better fuel additive to waste plastic pyrolysis oil (WPPO) compared with n-butanol due to its high cetane number, better blend ratio stability and less hygroscopic nature [25].

In order to reduce combustion temperatures, ignition delay is suggested as it aids in the reduction of NOX, which is temperature-dependent. The use of cetane improvers is also an alternative technique in reducing NOx as the poor cetane index of WPPO fuel blends leads to poor ignition quality. Particularly when biodiesel fuels are used such as glycol ether, which reduces PM, UHC and CO emissions in common rail direct injection diesel engines. These cetane improvers decrease cylinder pressure, ignition delay, heat release rate and engine knock or noise [26]. The inclusion of n-pentanol in diesel-biodiesel blends has been reported to shorten combustion duration and increases the HRR, while significantly reducing the NOX, CO and UHC emissions [27].

In many experimental works, fuel additives have been utilized with diethyl ether as the most common. As an organic compound, diethyl ether has a high cetane number and capable of boosting ether cetane number [28]. When used as an additive, diethyl ether reduces ignition delay, cylinder peak pressure, heat release rate, CO, CO2 and NOX with a trade-off in which the BTE increased [29]. Other researchers found that diethyl ether reduces the ignition delay period, UHC, NOx, whereas BTE seemed to increased, but [15] using WPPO fuel observed ignition delay and higher heat release rate with diethyl ether blends.

Diesel engines run stably on most medium blended ratios of waste plastic oil, although they produce high NOX, UHC and CO emissions. However, to stabilize and improve performance for higher blend ratios, injection timing is a technique most recommended. This allows engine performance and stability without upgrading fuel, engine modification or fuel alteration through addition of additives [30]. Injection timing affects performance from WPPO and Jatropa blends of 20% tyre oil and 80%. This results into lower fuel consumption, CO, UHC and PM with increase in NOX emissions [31]. On the other hand, in a study by [32], the researchers observed increased BTE and NOX emissions. This was identical to the findings on emissions of NOx, with reduced fuel consumption, CO and UHC [31].
