The Influence of Exhaust Gas Recirculation on Performance and Emission Characteristics of a Diesel Engine Using Waste Plastic Pyrolysis Oil Blends and Conventional Diesel

Abstract

Through an experimental study, this work focused on finding the influence of exhaust gas recirculation (EGR) on waste plastic pyrolysis oils (WPPOs) with diesel as a base comparison fuel. The results show the amount of carbon monoxide emissions seemed to decrease at low engine loads up to intermediate loads of (50%), thereafter continued to increase significantly but marginally. Among fuels tested, blend WPPOB100 reported the highest BSFC, at 0% EGR flow rate. The value was 0.4751g/kW.hr. compared with 0.7235 g/kW.hr. at 30% EGR flow rate. Increased blend ratio had a direct decrease in brake power linearly. At 30% engine load, CD, WPPOB10, WPPOB20, WPPOB30 and WPPOB40 recorded values of 2.125 kW, 2.15 kW, 2.05 kW, 1.98 kW, 1.86 kW and 1.75 kW, respectively. Exhaust gas temperature (EGT) at 30% EGR flow rate, blend WPPOB10 had the highest reduction in temperature compared with the any other WPPO blends at 320°C. Increased blend ratio and EGR percentage flow rate increased smoke emissions within the test fuels blends. At 15% EGR flow rate, the following data were recorded: 7.53%, 7.1%, 6.72%, 6.25%, 6.0% and 5.4% for CD, WWPO10, WPPO20, WPPO30, WPPO40 and WPPO100, respectively.

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 NO_X, 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 NO_X 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 day-today 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 high-density (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].

Figure 1.

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

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].

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 NO_X 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 NO_X formation as it reacts with high-temperature combustion mixture, thereby increasing the availability of NO_X [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 NO_X as in the findings of [18, 19, 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 NO_X 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 NO_X 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 NO_X, 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 NO_X, 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 NO_X 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 NO_X, 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 NO_X emissions [31]. On the other hand, in a study by [32], the researchers observed increased BTE and NO_X emissions. This was identical to the findings on emissions of NOx, with reduced fuel consumption, CO and UHC [31].

2. The experimental set-up and equipment

2.1 Introduction

As discussed in the introduction and literature review, one of the gray areas is the fewer literature works for plastic pyrolysis oil biodiesel on EGR. The few numbers of experimental works on the waste plastic pyrolysis oil biodiesel, the interaction and influences motivate this work. Considering this trend and the level of experimental work thus far achieved, it becomes important to mount an experiment to increase and deepen understanding. Therefore, using an experimental approach aims to achieve study aims and objectives as set forth to study the effects of exhaust gas recirculation (EGR) on the performance parameters of a diesel engine, using WPPO biodiesel. Figure 2 shows engine experimental set-up schematic diagram.

Figure 2.

Experimental engine schematic diagram set-up.

2.2 Experimental apparatus and equipment

Figure 3 shows the EGR schematic loop and definitions; 1. is the direction of EGR gases, 2. is the $x$ subscript representing the exhaust molar gas quantity, 3. is the direction of inlet gases fresh charge, 4. is the $z$ subscript representing remainder of the intake charge, 5. is $n_f\text{,}$ which is the fuel molar quantity, 6. is the EGR valve, 7. is the R_x molar gas ratio, 8. is the subscript $y$ representing the inlet intake molar gas quantity, 9. is the direction of the exhaust gases exit, 10. is the engine unit.

Figure 3.

The EGR modification nomenclature schematic diagram.

2.3 Waste plastic preparation and the conversion process

1. The waste plastic materials are acquired from a municipal solid waste management site and taken to the sorting section of the pyrolysis plant. The dust and other fine wastes collected are filtered by the cyclone filter system and disposed through a vent with a particle size monitoring system.

2. After the sorting and removal of unwanted materials and dust. The waste plastics are taken through a conveyor machine for pressure cleaning and conveyed to the shredding machine, which reduces them to the required pyrolysis reactor size of 25.4–50 mm.

3. Loading into the pyrolysis reactor uses an automatic feeding machine for the waste material, and the reactor door is air tight locked to begin the next phase of pyrolysis process. As a caution, a manual loading system is provided in case of system power failure.

4. Using a power control panel by a machine operator, the system is started and operated. The preceding processes subsequently run automatically as the flow chart in Figure 4 indicates. The first stage heats up the dry waste plastic materials, as the reactor’s temperature increases to the required values of 400–500°C.

5. The heavy dense gas oil collects into the oil tank while the light oils go into the condenser where it is cooled and drops into the oil tank. However, the small quantities of liquefied gases, which failed to be converted into oil, are collected by the recycling system and burnt as fuel gas. The remaining gas is cleaned by the after-treatment system with the removal of sulphurets and black carbon, while the smoke and flue are emitted in the atmosphere.

6. After production of a batch, the system requires a cooling period of 4–5 hours through natural cooling. However, for faster cooling, nitrogen and carbon dioxide gases are utilized as cooling agents to shorten this process. This enables the removal of the carbon black compound without contamination or pollution to the environment.

7. Removal of steel and other metals is the final operation from the pyrolysis reactor plant, as this requires the opening of the reactor door in preparation for the next batch of the pyrolysis process.

Figure 4.

Waste plastic pyrolysis processing plant flow chart. Flow chart nomenclature 1. Pyrolysis reactor, 2. Carbon black discharge, 3. Carbon black deep processing, 4. Exhaust smoke discharge, 5. Gas separator, 6. Smoke scrubber to take out color and odor, 7. Condenser, 8. Chimney, 9. Oil tank, 10. Synchronized gas purification, 11. Synchronized gas-recycling system, 12. Extra gas burning, 13. Heating furnace during operation, 14. Loading of material.

2.4 Waste plastic pyrolysis oil (WPPO) physical properties

Table 1 shows physical properties of the waste plastic pyrolysis oil obtained through the pyrolysis process using waste plastics from municipal solid waste (MSW) management sites. Compared with the properties of conventional diesel fuel oil, these properties are in optimized conditions. The oil is yellowish in color as shown by the picture shot in Figure 5. The liquid distillate, which is free of visible particulate sediments, has a flash point of 20°C and a gross calorific value (GCV) of 40.15 KJ/kg. This calorific value of the waste pyrolytic oil compares with the range of petroleum fuels including conventional diesel; thus, making it capable of giving the same comparative working performance results in diesel internal combustion engines.

Parameters	Position value
Ignition Type	4 (Stroke) DICI
Number Of Cylinders	1
Model	TV 1
Cooling Medium	Water
Manufacturer	Kirloskar
Revolutions Per Minute	1500
Brake Power	3.5 Kw
Cylinder Bore	87.5 mm
Piston Stroke	110 mm
Compression Ratio	18.5:1
Connecting-Rod Length	234
Engine Capacity	661 cc
Dynamometer Make	234
Injection Timing	23.4˚ bTDC
Maximum Torque	28 N-M@1500
Injection Pressure	250 Bar

Table 1.

The engine specifications and position value.

Figure 5.

The liquid distillate samples from the waste plastic pyrolysis oil.

The distillation report analysis shows that the waste plastic pyrolysis oil (WPPO) has an initial boiling point (IBP) of 119°C–353.5°C. This indicates a presence of other fuel oil components such as kerosene, gasoline and, to some extent, diesel oil in the tested samples. Therefore, it is possible for this oil to be a biofuel feedstock in future, if upgraded into a lighter compound as diesel fuel or any liquid fuel in the near future (see Table 2).

Property	Unit	CD	WPPO	STANDARD
Appearance	—	Clear/brown	Clear/amber	Visual
Density @20	kg/M^3	838.8	788.9	ASTM D1298
Kinematic viscosity @40 °C	cSt	2.32	2.17	ASTM D445
Flash point	^0C	56.0	20.0	ASTM D93
Cetane index	—	46	65	ASTM D4737
Hydrogen	%	12.38	11.77	ASTM D7171
Cu corrosion	3 hrs@100 °C	—	1B	ASTM D130
Carbon	%	74.99	79.60	ASTM D 7662
Oxygen	%	12.45	7.83	ASTM D5622
Sulfur content	%	<0.0124	0.15	ASTM D4294
IBP temperature	^0C	160	119	ASTM D86
FBP temperature	^0C	353.5	353.5	ASTM D86
Recovery	%		98	—
Residue and loss	%		2.0	—
Gross calorific value	kJ/kg	44.84	40.15	ASTM D4868

Table 2.

Waste plastic pyrolysis oil test fuel properties, units of measurement, test standard methods, compared to conventional diesel.

2.5 Experimental procedure

1. The experimental engine is a Kirloskar variable compression engine, four-stroke single cylinder; water-cooled, developing 3.75 kW of power at 1500 rpm. The schematic diagram is provided in Figure 1.

2. The experimental engine’s technical specifications are indicated in Table 3. Engine load was provided by a dynamometer during the experimentation. Using a standard orifice mechanism, an air box was fitted to the engine intake manifold system, thus enabling engine intake airflow measurements.

3. Using a digital fuel gauge, the fuel flow rate to the experimental engine was measured and with aid of a stopwatch mounted to fuel outlet valve, time taken for the fuel consumed was measured.

4. Temperature thermocouples of k-2 type provide measurements for the exhaust the exhaust gas temperatures. This measurement for EGR temperature is done before it mixes with the fresh intake air charge and the combustion constituents.

5. In order to monitor cylinder combustion pressure in the engine cylinder head, a cylinder pressure transducer is mounted to collect data values through a system charge amplifier connected to a computer data acquisition machine.

6. The crank angle (crankshaft position) is monitored and measured through a mounted encoder near the crankshaft pulley area.

7. Experimental emissions were monitored through a five-channel gas exhaust analyzer, while for measurements for the smoke intensity, an AVL 437C smoke meter was utilized.

8. Since it is a variable compression engine which develops maximum power at 1500 rpm. The experiments were conducted based on nominal engine speed, at part load and full load. However, other data were obtained from different engine load as specified in the set-up. In this experiment, part engine load is 50% load, engine full load is engine running at 100% load, all with a fixed compression ratio of 18.5:1.

9. The modified experimental engine EGR system and data collection are shown in Figure 3. The exhaust gases were tapped from the exhaust pipe and joined to the intake manifold air intake system via the air flow meter box through a manually controlled gate valve, which makes it possible for the mixing of EGR gases and the fresh air intake.

10. The EGR percentage flow rate was divided into the following modes: 0%, 5%, 10%, 15%, 20%, 25% and 30% at intervals of 5%. The waste plastic pyrolysis oil fuel blends were prepared in the following percentages order and mixed with diesel fuel in 10%, 20%, 30%, 40% and 100%. For example, WPPO10 blend is 90% conventional diesel fuel and 10% waste plastic pyrolysis oil (WPPO) fuel in that order. Therefore, throughout this experiment, blends will be referred to as in this format WPPOB10 with digits denoting the percentage blend ratio of plastic oil by volume to conventional diesel supplied.

11. To avoid contamination, each test was conducted after a thorough evacuation procedure on the previous preceding experiment. The fuel lines and the fuel injection system mechanism were completely evacuated before a new set of experiment was initiated. This made it possible to collect good data with inputs from the test mode only, as there is no fear of contamination, poor results or error.

Molecular formula	Percentage composition
C10	66.32
C10-C15	4.38
C15-C20	12.66
C20-C25	8.22
C25-C30	8.42

Table 3.

The waste pyrolysis plastic oil chemical composition.

2.6 Percentage uncertainties and error analysis

In order to identify the accuracy and precision of the measuring tools and instruments used in this experiment work, this process error analysis was performed. Experimental errors occur due to conditions outside the experiment itself such as poor calibration of the instruments, observational errors, manufacturing errors, errors associated with experimental set-up and planning, besides environmental conditions existing during the experiment [33]. Table 4 is a list of instruments, percentages of uncertainties of CO, CO₂, UHC, NO_X, exhaust gas temperature (EGT) and smoke opacity and percentage error analysis. These percentages of error analysis are derived from the root sum square method formula and expressed in equation form [1] as in Eq. (1).

Instrument	Accuracy	Measuring Range	Percentage inaccuracies
AVL 437C (smoke meter) Smoke intensity	±1%	0–100%	±1
AVL pressure transducer GH14D	±0.01 bar	0–250 bar	±0.01
AVL 365C Angle encoder	±1^0	—	±0.2
AVL Digas 444 (Five Gas Analyzer)
CO	±0.03% to ±5%	0–10% by vol	±0.3
CO2	±0.5% to ±5% by vol	0–20% vol	±0.2
O2	±5% by vol	0–22% by vol	±0.3
HC	±0.1% to ±5%	0-20,000 ppm by vol	±0.2
NOx	±10%	0–5000 ppm by vol	±0.2
K-2 Thermocouple	±1 °C	0-1250°C	±0.2
Digital Stop Watch	±0.2 s		±0.2
Digital Fuel Gauge	±1 mm		±2
Burette	±0.2 cc	1-30 cc	±1.5

Table 4.

Shows measuring instruments, range of measurement, percentages accuracies and inaccuracies, as calculated from Eq. (2).

$$R=\sqrt{{\displaystyle \sum }_{i=1}^n{X}_i^2}$$E1

Where R is the total uncertainty percentage, $X_i$ is the individual uncertainty of all the calculated operating parameters, $n$ is the total number of the parameters in the experiment and $i$ is the i^th term of the computed parameters. The total percentage of the uncertainty is thus calculated based on Eq. (2) as follows:

$$R=\sqrt{X_1^2+X_2^2+X_3^2}+\dots X_i^2$$E2

3. Results and discussion

3.1 Brake-specific fuel consumption (BSFC)

Figure 6 is the variation of brake-specific fuel consumption with full load under the effects of EGR percentage flow rate. Lower ratio blends of WPPOB10 and WPPOB20 show minimal reduction in the brake-specific fuel consumption (BSFC) at 0% EGR flow rate in Figure 6 compared with the values of conventional diesel and WPPOB100, which are showing significantly high values of brake-specific fuel consumption (BSFC) at that mode. At 0% EGR, flow rate conventional diesel has a brake-specific fuel consumption value of 0.4 g/kW.hr., compared with WPPOB100 with a value of 0.4751 g/kW.hr. In other words, blend WPPOB100 has a higher brake-specific fuel consumption than diesel. The values for the other blends of WPPO are placed at 0.3225 g/kW.hr., 0.3615 g/kW.hr., 0.3645 g/kW.hr. and 0.3715 g/kW.hr., for WPPOB10, WPPOB20, WPPOB30 and WPPOB40 at this point, respectively.

Figure 6.

Brake-specific fuel consumption (BSFC) versus EGR percentage flow rate full load engine conditions.

Similar trends are reported with application of EGR percentage flow rate such as at 20–25% EGR flow rate. At this point, the BSFC showed increasing tendencies, which is identical to the findings of [34]. This phenomenon is due to the effects of dilution of the fresh air intake as it mixes with exhaust gases. This mixture comes from the recirculated EGR system gases leading to incomplete combustion of the inducted mixture, hence a drop in power and engine torque. This scenario increases engine fuel consumption to maintain constant engine speed to meet increased load demand, which reflected increased BSFC. These findings are identical to the findings of [35].

The WPPO biodiesel blends showed better fuel economy with EGR percentage flow rate application. This is true especially for lower blend ratios of WPPOB10 and WPPOB20 compared with conventional diesel test fuels. However, increased EGR percentage flow rate increased the BSFC across all the test fuels used. For example, at 0% EGR, conventional diesel was 0.4 g/kW.hr. compared with 0.495 g/kW.hr. with application of 30% EGR flow rate. On the other hand, blend WPPOB10 was 0.3225 g/kW.hr. compared with 0.5780 g/kW.hr. with application of 30% EGR flow rate.

Figure 6 shows that the highest BSFC among the blends of diesel and conventional diesel test fuel is from blend WPPOB100. This blend at 0% EGR flow rate had a value of 0.4751 g/kW.hr. compared with 0.7235 g/kW.hr. at 30% EGR flow rate. During experimentation with 10% EGR flow rate, the values for the BSFC across all the test fuel seemed to pick a lineal increment trend as in Figure 6. This was indicated by the flattening of the graph curves with close-packed value trends.

3.2 Brake thermal efficiency

The aim of brake thermal efficiency is to help us to understand the ability of the combustion system to utilize the fuel provided. Furthermore, it is a means of comparing and assessing how efficiently fuel conversion was carried by turning it into mechanical output [27, 28]. Figure 7 is brake thermal efficiency (BTE) % variations, under different blends of WPPO and conventional diesel fuel, with EGR % flow rate.

Figure 7.

Brake thermal efficiency (BTE) versus engine load percentage.

Figure 8 is variation of the brake thermal efficiency with EGR % flow rate using different blends of WPPO and CD. In this figure, a decrease in the brake thermal efficiency with all high blend ratio fuel such as WPPOB40 and WPPOB100 is indicated compared with conventional diesel fuel. However, blend WPPOB100 has the lowest decrease of brake thermal efficiency at 7.05%, with 10% EGR flow rate. The value drops further to 2.35% with application of 30% EGR flow rate.

Figure 8.

Brake thermal efficiency (BTE) % versus EGR % flow rate.

In other words, there is a reduction in the brake thermal efficiency due to the application of EGR percentage flow rate as shown in Figure 8. For example, at 0% EGR flow rate, brake thermal efficiency for conventional diesel is 12.15% compared with WPPOB10 and WPPOB20 at 13.25% and 13.05%. The WPPOB100 blend has the lowest value for thermal efficiency for all EGR rate flow modes than any other test fuel as shown in Figure 8.

3.3 Brake power (BP)

Figure 9 shows brake power variations with different blends of WPPO and conventional diesel fuel at full engine load. The results obtained show that there is a lineal increase in the brake power for all the test fuels applied with increase in engine load. Conventional diesel fuel has the highest increase in brake power values compared with the blends of WPPO. At 20% engine load, conventional diesel is at 1.45 kW while WPPOB10 has a value of 1.350 kW representing a difference of 6.8% in BTE when the two fuels are compared.

Figure 9.

Engine brake power versus varying engine load percentage.

Figure 9 also shows very close unitary increments with increase in engine load conditions. It also shows a decrease in brake power as the blend ratio increased for all the fuels tested. In other words, the increase in blend ratio showed a direct decrease in brake power linearly. For example, at 30% engine load CD, WPPOB10, WPPOB20, WPPOB30 and WPPOB40 reported values of 2.125 kW, 2.15 kW, 2.05 kW,1.98 kW, 1.86 kW and 1.75 kW, respectively, thus, showing a decrease in the value of the engine brake power throughout the experimentation and analysis period. Blend WPPOB100 showed the lowest values for the engine brake power compared with the blends of WPPOB10, WPPOB20, WPPOB30 and WPPOB40. These findings are identical with the findings of a research on WPPO blends [20].

The application of EGR percentage flow rate does not show significant changes in brake power. However, there is a negligible drop in the engine brake power application of EGR flow rate except for the blend WPPOB10. The blend has almost identical values to conventional diesel as the curve of the two fuels indicates in Figure 9; therefore, leading to a conclusion that the blends of WPPO have identical brake power values with conventional diesel.

3.4 Exhaust gas temperature (EGT)

Temperature is one of the key factors in determining the formation of engine exhaust emissions, besides providing or helping in the analysis and study of combustion processes in relation to fuel [36]. The result in Figure 10 is showing a variation in exhaust gas temperature (EGT) with different fuel blends of WPPO and conventional diesel with the application of EGR percentage flow rate. The result indicates that EGT decreases with different blends of WPPO compared with conventional diesel test fuel.

Figure 10.

Exhaust gas temperature (EGT ⁰ C) versus EGR percentage flow rate.

The difference between conventional diesel and WPPO blends is the temperature increases in all the test conditions reported. However, it should be mentioned that as the blend ratio increased with EGR % flow rate application, the exhaust gas temperatures reduced significantly especially for WPPOB30 and WPPOB40 at 0% EGR flow rate, the highest temperature value obtained for conventional diesel is 456°C compared with WPPOB100 blend at 490°C. needless to mention at 30% EGR flow rate this blend has most reduction in temperature compared with the other WPPO blends with a temperature value of 320°C.

Applying increasing rates of EGR % flow rate modes as in Figure 10 reduces exhaust gas temperature. For example, at EGR percentage flow rate of 5%, the highest value for conventional diesel test fuel obtained was 440°C. The minimum value was 340°C obtained at 30% EGR flow rate. This trend is repeatedly shown for other WPPO blends with the application of EGR percentage flow rate such as blend WPPOB10 with a high value of 467°C and the lowest being 362°C at 5% and 30%, respectively. On the other hand, WPPOB40 shows its highest value as 472°C and the lowest as 330°C when applying 5% and 30% EGR flow rate, respectively.

The reduction in exhaust gas temperature among the different blends of WPPO was due to low calorific value of the blends and the low exhaust loss. This result is identical to the findings of [37, 38]. According to results shown in Table 2, WPPO has a calorific value of 40.15 kJ/kg compared with the calorific value of conventional diesel at 44.84 kJ/kg. The third cause is the effects of dilution, chemical and thermal factors brought through exhaust gas recirculation rate flow [39, 40].

3.5 Hydrocarbon emissions

Figure 11 is a variation of hydrocarbon emissions in parts per million under full engine load with the application EGR percentage flow rates, using different blends of WPPO and conventional diesel (CD). All the blends of WPPO tested indicated significantly higher hydrocarbon emissions, especially with higher engine load conditions as shown in Figure 11. However, conventional diesel still produced more and higher values of hydrocarbon emissions compared with all the blends of WPPO across all the engine loading conditions and operating modes.

Figure 11.

Unburnt hydrocarbons emissions versus EGR percentage flow rate.

For example, when the EGR percentage flow rate is 0%, in other words, no application effect, Figure 11 shows there is less hydrocarbon emissions for all the test fuels. The following values were reported 22 ppm, 23 ppm, 21 ppm, 20 ppm, 19 ppm and 17 ppm for WPPOB10, WPPOB20, WPPOB30, WPPOB40 and WPPOB100 respectively compared with 20% EGR percentage flow rates with 77 ppm, 68 ppm, 52 ppm, 46 ppm, 44 ppm and 40 ppm, respectively.

The application of EGR percentage flow rate reduces the amount of hydrocarbon emissions emitted across board all test fuel blends. However, conventional diesel fuel produced more hydrocarbon emissions compared with all WPPO blends tested. For example, Figure 10 shows that at EGR flow rates of 5%, 10%, 15%, 20%, 25% and 30%, conventional diesel had 43 ppm, 57 ppm, 70 ppm, 82 ppm and 85 ppm, respectively. On the other hand, the values for WPPOB10 were 23 ppm, 35 ppm, 40 ppm, 48 ppm, 50 ppm and 52 ppm, respectively. Therefore, the application of EGR percentage flow rate increased hydrocarbon emissions values as presented in Figure 10.

3.6 NO_X emissions

The formation of NO_X emission is dependent on cylinder temperature, the concentration of oxygen and the residence time spent in the combustion chamber by the fuel-air mixture during phase of pre-mixing [41]. All tested blends indicated a drop in NO_X emissions with the application of EGR percentage flow rate, at all engine load conditions. This was due to the rise in the total heat capacity of the working gases as EGR % flow rate increased, which was identical with the studies and findings of [42, 43, 44]. Figure 12 shows NO_X emissions value for the conventional diesel was 920 ppm at full load without EGR percentage flow rate, compared with WPPOB100 at 1270 ppm. However, with application of EGR percentage flow rate of 30%, the values reduced to 401 ppm for CD and 432 ppm for WPPO100, respectively.

Figure 12.

EGR percentage flow rate variations with NO_X emissions.

However, during study, engine part load values in Figure 13 for NO_X emissions reported lower values compared with the full load engine conditions for the same test fuels. The NO_X emission for conventional diesel at 50% (engine part load) was 635 ppm compared with full load at 1100 ppm. On the other hand, the value for WPPOB100 at 50% (engine part load) was 850 ppm compared with 1250 ppm at full engine load. This result concurs that at 50% (engine part load), the values of NO_X emissions emitted by tested blends of WPPO except WPPO100 were lower compared with full engine load conditions.

Figure 13.

Variations of NO_X emissions (ppm) versus varying engine load percentage without application of EGR flow rate.

3.7 Carbon monoxide emission

Figure 14 shows variations of carbon monoxide emission percentage with varying load under the effects of EGR percentage flow rate, with different fuel blends of WPPO and conventional diesel fuel. As a gas, carbon monoxide is toxic and requires control to acceptable levels. Carbon monoxide is a product of poor combustion of hydrocarbon fuels due to dependency on the air-fuel ratio relative to the stoichiometric proportions [42].

Figure 14.

Carbon monoxide emissions percentage versus varying engine load.

In the experiment conducted, the amount of carbon monoxide emissions decreased with engine loads up to part load (50%). For example, at 0% engine load, the value of conventional diesel is 0.051% compared with 50% engine load when the value dropped to 0.03% by volume. However, the CO emissions continued to increase significantly but marginally as in Figure 14 as the load increased from this point. Increasing the engine load from 50% recorded continuous but marginal increases of carbon emissions by volume across all the test fuels irrespective of the EGR percentage flow rate. For example, at 80% engine load, the value for WPPOB100 is 0.02% up from 0.0165% by volume. The other WPPO biodiesel blends also show a similar trend and concurrency. WPPOB20 and WPPO30 test fuels at 50% engine load condition have values of 2.25% and 2.15% as compared with 3.36% and 2.95% respectively, at 80% engine load.

Figure 15 is the variation of carbon monoxide with EGR percentage flow rate application under conventional diesel and different blends of WPPO. The WPPO blends produced continuous increase in smoke emissions almost doubling values with the application of EGR percentage flow rate. For example, at 10% EGR flow rate, the carbon monoxide emission values were 9.79%, 10.46%, 10.91%, 11.25% and 12.75% for WPPO10, WPPO20, WPPO30, WPPO40 and WPPO100, respectively. The application of same EGR percentage flow rate reports the lowest carbon emissions with a value of 7.65% for conventional diesel test fuel.

Figure 15.

Carbon monoxide VS EGR percentage flow rate application.

The blend ratio and EGR percentage flow rate have a correlation on the amount of CO emissions produced. In other words, increased blend ratio increased carbon monoxide emissions within the blends as the EGR percentage flow rate increased. For example, at 20% EGR flow rate, CO emission values recorded were 18.25%, 21.35%, 22.65%, 24.55%, 26.95% and 28.85%, respectively. These values are for CD, WPPO10, WPPO20, WPPO30, WPPO40 and WPPO100. However, blend WPPO30 reports values of 4.85%, 7.28%, 10.91%, 16.37%, 24.55%, 35.75% and 52.69% as the EGR flow rate increased to 30%, respectively. This is caused by dilution, thermal and chemical effects of EGR % flow rate application as some of the oxygen in the inlet charge is replaced with recirculated exhaust gas that causes incomplete combustion.

3.8 Carbon dioxide emissions

Carbon dioxide is the principal composition of the exhaust gas recirculation gases. However, carbon dioxide gas and the exhaust temperatures are indicators of combustion quality in the combustion chamber [6]. Carbon dioxide gas has a higher heat capacity making it a thermal heat sink during the combustion process. This helps in reducing peak cylinder temperatures, hence the reduction in the NO_X emissions.

The value of CO₂ is considerably high without EGR percentage flow rate coupled with lower engine loads for all the fuel blends tested. For example, at 20% engine load blend WPPOB100 has 4.65% compared with all the other test fuels and is the highest carbon emissions value. The other blends reported are CD, WPPO10, WPPO20, WPPO30 and WPPO40 with 3%, 2.50%, 1.5% and 1.85% respectively, as shown in Figure 16.

Figure 16.

Variation of carbon dioxide percentage emissions versus engine load percentage, with different types of fuel blends of WPPO and conventional diesel.

Additionally, Figure 16 shows that the amount of carbon dioxide increased with increased engine load. For example, as the engine load increases to 40%, the value of WPPOB40 is 2.75% compared with WPPOB30 at 3.25% while at 70% engine load, the values are 4.5% and 5.25%, respectively. The observation is that as the engine load is increased with increased blend ratios, lower-ratio blends are observed to emit more carbon dioxide emissions as compared with those blends with high ratios except blend WPPOB100 that releases more carbon emissions than any test fuel as aforementioned earlier. At full engine load, the value of carbon dioxide emissions is at the highest values as in Figure 16 across all the test fuels. The values are 12.75%, 10.85%, 9.65%, 8.75%, 8.35% and 8% for WPPOB100, CD, WPPOB10, WPPOB20, WPPOB30 and WPPOB40, respectively.

The application of EGR percentage flow rate increases the carbon dioxide emissions exponentially by almost doubling the values as can be seen in Figure 17. For example, at 10% EGR flow rate, the value of conventional diesel is 5.35% compared with WPPOB100 at 7.25%, WPPOB10 at 4.75%, WPPOB20 at 4.25%, WPPOB30 at 3.95% and WPPOB40 at 3.65%, respectively. This result reinforces the idea that there exists a correlation between blending and the EGR percentage flow rate with carbon dioxide emissions. Hence, the conclusion that the higher the blend ratio, the higher the emissions values and vice versa with application of EGR percentage flow rate. For example, at 30% EGR flow rate, all test fuels show high carbon dioxide emission, such as conventional diesel at 10.95%, WPPOB10 at 9.95%, WPPOB20 at 9.65%, WPPOB30 at 8.85% and WPPOB100 at 14.35%, respectively.

Figure 17.

Variations of carbon dioxide percentage versus EGR percentage flow rate, with different types of WPPO fuel blends and conventional diesel.

3.9 Opacity emissions

Smoke opacity emissions are defined as the solid hydrocarbon soot particles found in the exhaust system exit gases and linked to the formation of smoke emissions [45]. All tested blends of WPPO showed increased and aggravated levels of smoke emissions. However, they were of lower values compared to conventional diesel values.

Figure 18 smoke emissions under the influence of EGR % flow rate using different WPPO blend and conventional diesel.

Figure 18.

Variation of smoke emissions or opacity % versus EGR % flow rate, with different WPPO blends and conventional diesel.

The incessant increase in smoke emissions in this figure is explained by high kinematic viscosity and the low volatility values of WPPO blends compared with conventional diesel test fuel. Furthermore, the poor injection and spray characteristics of WPPO blends of fuels compared with the spray and injection characteristics of conventional diesel fuel cause this phenomenon. WPPO blends are also associated with the high aromatic compounds compared with conventional diesel test fuel, hence the poor spray and injection quality.

The application of EGR percentage flow rate shows significant increases in the values of smoke emissions across all the test fuels. Blend WPPOB10 shows smoke emission of 7.2% lower compared with conventional diesel at 0% EGR flow rate. On the other hand, conventional diesel reports 11.5% higher emissions than WPPOB100 blend fuel when EGR flow rate is at 30%. This result is identical to the study findings of the following researchers [46].

The WPPOB10 blend emits the highest levels of smoke emissions for the blended fuels compared with the other WPPO fuel blends. In experimental analysis, blend WPPOB100 is the second highest emitter of smoke emissions. However, as the blend ratio and the EGR percentage flow rate increased, the smoke emissions increased incessantly across all the test fuels compared with EGR percentage flow rate of 0%.

4. Conclusion

• Increase in the percentage of the blend ratio of WPPO shows a marked decrease in the engine brake power compared with conventional diesel. This is due to lower percentage of the energy content presented in Table 2 for WPPO test fuel compared with conventional diesel test fuel.

• WPPO biodiesel blends peak power produced did not match the peak power of conventional diesel. It ranged between 5% and 8% less compared with the peak power conventional diesel test fuel produced.

• Data presented in this work supports widespread use of WPPO as an alternative fuel for compression ignition engines as a feedstock. Through the experimental analysis conducted, this can be achieved with or without modifications to the engines, especially for blends WPPOB10 and WPPOB20.

• During part and intermediate engine load, it is observed that the engine operating under different blends of WPPO and conventional diesel fuel emits less NO_X emissions as compared with the full load mode condition as demonstrated in the result observed in Figure 12.

• As the percentage ratio of the blended WPPO increased, there was a significant increase in the brake-specific fuel consumption, as in Figure 5 for the blends of WPPO compared with the brake-specific fuel consumption (BSFC) values of conventional diesel test fuel.

• As the blend ratio is increased, there is a reduction in the percentage amount of CO emissions released by the test engine as shown in Figure 13. This is due to high oxygen content in WPPO blended fuels compared with conventional diesel, which aids combustion.

• The values of carbon monoxide emission obtained during experimentation for the two blends of WPPOB10 and WPPOB20 were close with minimal differences in terms of their volume percentage emissions produced by the test engine as in Figure 13.

• For all the test fuels, it was observed that at low engine load from 10 to 40% before it hits the 50% point, there is a decrease in emissions as shown in Figure 14. However, there is significant continuous and marginal increase in the percentage of carbon monoxide emissions by volume as the load is increased to 50% part load across all the test fuels irrespective of the EGR percentage flow rate.

• From Figure 16, it was observed during experimentation, there is an incessant increase in smoke emissions for all the blends of WPPO with or without EGR percentage flow with test fuel blend WPPOB10 producing the highest values of smoke emissions followed by WPPOB100 test fuel blend.

• Incessant increase in smoke emissions is observed due to WPPO blends of fuel having high kinematic of viscosity and low volatility compared with conventional diesel test fuel. Other possible causes could be the poor injection and spray characteristics associated with WPPO blends compared with conventional diesel fuel, with high spray qualities.

• NO_X emission using conventional diesel at engine part load (50%) is 635 ppm compared at full load at 1100 ppm. The value for WPPOB100 at engine part load (50%) is 850 ppm compared with 1250 ppm at full engine load. This result data indicates a concurrence that at part engine load (50%), the values of NO_X emissions by all the blends of WPPO are lower compared at full engine load condition except blend WPPO100.

Abbreviations

RX	Molar Gas Ratio
nf	Fuel Molar Quantity
Al2O3,	Aluminum Tri-Oxide
ASTM	American Society for Testing and Materials
BP	Brake Power
BSFC	Brake Specific Fuel Consumption
BTE	Brake Thermal Efficiency
CaO	Calcium Oxide
CD	Conventional Diesel
CO	Carbon Monoxide
CO2	Carbon Dioxide
Cu	Copper
DEA	Department of Environmental Affairs
EGR	Exhaust Gas Recirculation
EGT	Exhaust Gas Temperature
FBP	Final Boiling Point
G-20	Group of 20 Highly Industrialized Countries
G-7	Group of 7 Developed and Industrialized Countries
GC–MS	Gas Chromatography–Mass Spectrometry
GVC	Gross Calorific Value
HPDE	High-Density Polyethylene
HRR	Heat Release Rate
IBP	Indicated Brake Power
SiO2	Silica Dioxide
kW	Kilowatt
LPG	Liquid Petroleum Gas
LTC	Low Temperature Combustion
MSW	Municipal Solid Waste
NaAlSi2O6-H2O	Zeolite
NOX	Oxides Nitrogen
PEHD	Polyethylene High Density
PM	Particulate Matter
PVC	Poly Vinyl Chloride
UHC	Unburnt Hydrocarbons
WPPO	Waste Plastic Pyrolysis Oil
x	Exhaust Molar Gas Quantity
y	Inlet Intake Molar Gas Quantity
z	Remainder of the Intake Charge