Diesel engine performance and emissions with fuels derived from waste tyres

Abstract

The disposal of waste rubber and scrap tyres is a significant issue globally; disposal into stockpiles and landfill poses a serious threat to the environment, in addition to creating ecological problems. Fuel production from tyre waste could form part of the solution to this global issue. Therefore, this paper studies the potential of fuels derived from waste tyres as alternatives to diesel. Production methods and the influence of reactor operating parameters (such as reactor temperature and catalyst type) on oil yield are outlined. These have a major effect on the performance and emission characteristics of diesel engines when using tyre derived fuels. In general, tyre derived fuels increase the brake specific fuel consumption and decrease the brake thermal efficiency. The majority of studies indicate that NOx emissions increase with waste tyre derived fuels; however, a few studies have reported the opposite trend. A similar increasing trend has been observed for CO and CO2 emissions. Although most studies reported an increase in HC emission owing to lower cetane number and higher density, some studies have reported reduced HC emissions. It has been found that the higher aromatic content in such fuels can lead to increased particulate matter emissions.

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Diesel engine performance and
emissions with fuels derived from
waste tyres
Puneet Verma1,2, Ali Zare
1, Mohammad Jafari
1,2, Timothy A. Bodisco
3, Thomas Rainey1,
Zoran D. Ristovski1,2 & Richard J. Brown1
The disposal of waste rubber and scrap tyres is a signicant issue globally; disposal into stockpiles and
landll poses a serious threat to the environment, in addition to creating ecological problems. Fuel
production from tyre waste could form part of the solution to this global issue. Therefore, this paper
studies the potential of fuels derived from waste tyres as alternatives to diesel. Production methods
and the inuence of reactor operating parameters (such as reactor temperature and catalyst type) on
oil yield are outlined. These have a major eect on the performance and emission characteristics of
diesel engines when using tyre derived fuels. In general, tyre derived fuels increase the brake specic
fuel consumption and decrease the brake thermal eciency. The majority of studies indicate that NOx
emissions increase with waste tyre derived fuels; however, a few studies have reported the opposite
trend. A similar increasing trend has been observed for CO and CO2 emissions. Although most studies
reported an increase in HC emission owing to lower cetane number and higher density, some studies
have reported reduced HC emissions. It has been found that the higher aromatic content in such fuels
can lead to increased particulate matter emissions.
Rapidly depleting petroleum resources and rising public concern related to climate change has prompted much
research into alternative energy resources1. e conversion of waste to energy presents an opportunity to replace
conventional fuels at the same time as reducing the waste burden on the planet2. Solid waste disposal is a serious
global issue that is leading to economic and environmental complications3. In the manufacturing and automotive
industries, there has been an increase in the quantity of waste rubber and tyres4,5, an issue that needs a sustainable
solution. It has been reported that approximately 1.5 billion waste tyres are generated every year, globally6.
Disposing of end-of-life tyres is not easy and inevitably gives rise to some collateral pollution7. For this rea-
son, the treatment of scrap tyres is being addressed by the relevant governing bodies and private industry8. 48.5
million tyre Equivalent Passenger Units (EPUs) contributed to Australia’s waste in 2009–2010 compared to 41.8
million in 2007–089. An EPU is a standardised measure of the quantity of tyres, given that the weight for a new
standard passenger vehicle tyre is 9.5 kg, whereas a used tyre is standardised as 8.0 kg.
e contribution of each Australian state or territory to the total end-of-life tyres (ELTs) in Australia and nor-
malised EPUs by the number of vehicles in 2012 is shown in Fig.1. New South Wales, Queensland and Victoria
dominate the generation of ELTs, which is related to the relatively high number of vehicles in these states10. On
the other hand, Western Australia and Northern Territory lead in trends for normalised EPU by the number of
vehicles in the state, which is most likely due to their low population density and large size. ELTs are converted in
tyre recycling plants into tyre crumbs, reinforcing bre and steel. Styrene butadiene, polybutadiene, nitrile and
chloroprene rubbers together with natural rubber are the main constituents of tyre crumbs5,7–9. eir chemical
structure with aromatic and aliphatic constituents plays an important role in determining the composition of
the oils derived from tyre crumbs. e reinforcing bre is usually recovered as a heterogeneous u made up of
polymeric bres that contain rubber11.
Tyre pyrolysis oil (TPO), derived from scrap tyres, has been shown to have potential as an alternative to
diesel6,12–19 with the added benefit of recycling the waste rubber and decreasing our reliance on natural
resources14,20,21. Pyrolysis involves an oxygen-free thermal decomposition process. e rubber decomposes to
1Biofuel Engine Research Facility, Queensland University of Technology (QUT), Brisbane, QLD-4000, Australia.
2International Laboratory of Air Quality and Health, Queensland University of Technology (QUT), Brisbane, QLD-
4000, Australia. 3School of Engineering, Deakin University, VIC, 3216, Australia. Correspondence and requests for
materials should be addressed to A.Z. (email: ali.zare@qut.edu.au)
Received: 3 October 2017
Accepted: 27 December 2017
Published: xx xx xxxx
OPEN
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form gases and the condensed vapours (i.e. liquids) can be used as fuels22. To this end, the recovery of solid and
liquid material is achieved11,23.
Although there are some literature studies available on the use of waste tyre derived (WTD) fuels, such as
TPO, as an alternative fuel to diesel7,14,24, these focused on production aspects only. A thorough search of the
literature could not nd any papers reviewing the engine performance and emission characteristics of TPO in a
comprehensive manner. us, this paper aims to review the potential of waste TPO to be used as a substitute for
diesel and its engine performance, combustion and emission characteristics. is study has been divided into
sections as follows: the introduction is given in Section 1, Section 2 follows with a historical overview of the publi-
cations related to WTD fuels from waste tyres such as TPO, carbon black and light fraction pyrolysis oil and their
application in diesel engines. Section 3 focuses on waste tyres as a source of biofuel, followed by Section 4, which
discusses the production techniques. Section 5 contains a detailed analysis of the eect of WTD fuels on engine
performance parameters, such as brake specic fuel consumption (BSFC) and brake thermal eciency (BTE) and
exhaust emissions such as nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons
(HC), particle mass (PM) and particle number (PN).
Historical overview
Despite the obvious attraction of producing fuel from waste tyres, there has been a limited amount of literature
focused on the use of WTD fuels. TableA, given in the appendix, shows a list of articles published in this eld,
focussing on the use of WTD fuels in diesel engines. e list of papers given in TableA has been retrieved from
Scopus with search terms such as waste tyre pyrolysis oil AND (diesel engine or performance or exhaust or diesel
particulate matter); tyre derived fuel AND (diesel engine or performance or exhaust or diesel particulate matter);
synthetic fuel AND (diesel engine or performance or exhaust or diesel particulate matter). Some of the papers
from the search terms focused on production aspects of WTD fuel, which have been reviewed separately in
Section 4. Following the literature listed in TableA, the following has been observed:
• e diesel engines used in the literature have been either direct injection or had a common rail fuel injection
system.
• Of the various WTD fuels derived from scrap tyres, the majority of researchers experimented with TPO.
However, some research has also focused on fuels from by-products and derivatives of pyrolysis oils such as
carbon black, light fraction pyrolysis oil and low sulphur tyre fuel.
• Interest in the use of WTD fuels in diesel engines surged in the late 2000s, as shown in Fig.2 (retrieved from
TableA), and has gained signicant attention again recently.
• As observed in Fig.3 (retrieved from TableA), the majority of the studies adopted 5–20% blends of WTD fuel
with diesel. However, across the literature the whole spectrum of blend percentages (including pure WTD
fuel, 100%) have been represented.
Figure 1. End-of-life EPUs for states or territories in Australia9.
Figure 2. History of published investigations in the use of WTD fuels in a diesel engine (TableA).
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• e majority of the studies analysed diesel engine performance under steady-state conditions. Only three
studies investigated transient conditions.
• ere is very limited literature reporting the eect of WTD fuels on PM and PN emissions.
Waste tyre – a source of alternative fuel
Waste tyres pose an environmental issue as they do not readily biodegrade and recovering their constituent com-
ponents is quite dicult20, an issue compounded by the vast quantity of waste tyres discarded annually6. Tyres are
primarily made of rubber (45–65 wt. %), carbon black (21.5–35 wt. %), and steel (16.5–25 wt. %), but also consist
of zinc, sulphur and additives25. In addition, the composition varies according the application of the type e.g. a car
tyre typically has 14% natural rubber and 27% synthetic rubber whereas truck tyres normally have more natural
rubber (27%) and lesser synthetic rubber (14%)26. e rest of the components such as llers, chemical additives
(e.g. sulphur), plasticisers and metals are the same for car and truck tyres26. e rubber component is present as
hydrocarbons compounded with brous materials7.
Most of the research related to waste tyres has focused on the production of fuel (TPO) which has been used
for blending with diesel. In addition, researchers have also attempted to blend carbon black with diesel27. Carbon
black is the solid waste collected in the pyrolysis reactor upon completion of the pyrolysis of waste tyres28. It has
been found that ELTs from passenger cars have more sulphur and aromatic content compared to ELTs from heavy
duty trucks29. Table1 gives the breakdown of tyre recycling and recovery processes in Australia.
As seen in Table1, the major portion of recycled rubber (49.3%) is granulated or powdered and disposed of in
landlls. Crumb rubber can be used to replace the traditional polymer modied binder in spray seal pavements
for its plasticity and waterproong properties. Recycled rubber granulate can be used in a range of moulded
products, ooring and matting, which can be used in sporting grounds and playgrounds. In 2013–2014, only 8.7%
of the waste tyres were used to produce fuel29. us, there is excellent potential for producing biofuel from ELTs
via the pyrolysis process. As a value-add this will help reduce the nancial burden on government bodies to treat
waste rubber and prevent landlling29.
Production methodologies and fuel characterisation
Dierent studies have been conducted to convert waste automobile tyres into liquid fuels30–32. Figure4 shows the
typical process steps in the pyrolysis of waste tyre rubber.
Initially during vehicle tyre treatment, tyres are cut into smaller sized pieces followed by the removal of fabrics,
beads and steel wires. is leads to chip formation, which includes chipping thick portions of rubber from the
periphery of the tyre. During chip treatment, small rubber chips are washed, dried and fed into a pyrolysis reactor.
In the nal step, products from pyrolysis in vapour form are fed into a water-condenser and ultimately stored in
the form of a liquid fuel. Upon distillation, TPO is collected which is then analysed for its fuel character.
Figure 3. History of published investigations for use of WTD fuels in a diesel engine based on the blending
ratio (TableA).
Form Destination Typical applications Size Proportion in
market (%)
Whole tyres/shredded tyres Tyre derived fuel Energy recovery (e.g. pyrolysis) >200 mm 8.7
Whole tyres Stockpiling for
future recycling Pyrolysis, crumbing Not applicable 31
Whole tyres/granules/
shredded tyres Civil engineering Civil construction 10–60 mm 11
Granulated, crumbed or
powdered Rubber crumb,
granules, landll Road construction, explosives,
adhesives, disposal and ooring 1–10 mm 49.3
Table 1. Summary of domestic tyre recycling and recovery markets29.
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Pyrolysis involves the thermal breakdown of biomass at high temperature. e initial temperature for pyrol-
ysis is approximately 400 °C and reactions can continue to occur up to 1000 °C26. Much research has focused
on the conversion of waste tyres into fuel using a broad range of pyrolysis processes4,6,11,22,33–36. Pyrolysis can be
classied in terms of several approaches: according to residence time (e.g. fast pyrolysis, intermediate and slow
pyrolysis)37, reactor type (e.g. xed, moving, uidised bed and vacuum reactor pyrolysis) or novel technologies
(e.g. microwave and ultrasonic)12,38. Fast pyrolysis oers some promising advantages in the conversion of bio-
mass. Generally, fast pyrolysis (residence time < 2 s) is employed to maximise the liquid product yield, while slow
pyrolysis is employed to maximise the solid product yield. Pyrolysis has shown potential for transforming used
tyres into three components of useful products: carbonaceous solids, liquid hydrocarbons and non-condensable
gases32. Some major pyrolysis variants such as vacuum pyrolysis39, and microwave pyrolysis3. Ultrasonic devul-
canisation38,40 and supercritical uid depolymerisation41,42 are shown in Fig.5. It has been mentioned in the liter-
ature that compared to vacuum pyrolysis, rubber tyres are found to be poor microwave absorbers3,8,43. In order to
make the process more ecient, several materials such as particulate carbon, glycerol, biomass char, ionic liquids
and graphite have been used which aid in improving the microwave absorbing capacity of rubber tyres8.
Eect of reactor operating parameters. e following description of the eect of reactor operating
parameters, such as temperature and catalyst (type and concentration), is provided to give a brief overview.
Note, this is provided as supplementary information to lay the foundation for exploring the performance (BSFC
and BTE) and emission characteristics (NOx, CO, CO2, HC and PM emissions) of a diesel engine fuelled with
WTD fuels. For interested readers, a detailed review of reactor parameters for pyrolysis process can be found
elsewhere26.
Eect of temperature. In general, researchers have investigated the eect of reactor temperature ranging between
200–700 °C. A reaction temperature over 500 °C has been found to have a less noteworthy eect on fuel yield and
composition4. Products obtained were found to be a complex mixture of C5-C20 organic compounds with a high
fraction of aromatic compounds and high caloric value (42 MJ/kg)4. is was further veried by Murillo et al.23
who varied the reaction temperature from 400 to 600 °C and observed that 500 °C was the optimum temperature.
A similar variation in temperature from 400–700 °C at 50 °C increments was investigated by İlkılıç and Aydın17
and it was observed that as the temperature reached 400 °C, the rst signs of the liquid product were observed.
Liquid yield increased with increase in temperature and achieved its peak value at 500 °C and then dropped
marginally to become stagnant. Higher temperatures had a negligible inuence on liquid yield. On other hand,
gaseous yield kept on increasing at an approximately linearly rate with an increase in reactor temperature. Similar
results were observed in another study by the same authors35.
ere have been other studies in the literature which focused on the eect of reactor temperature during pyrol-
ysis within the range of 200–600 °C30,44. It has been observed that the char yield dropped signicantly with a rise
Figure 4. Process chart for dierent steps in waste tyre pyrolysis16,28,32,44,75.
Figure 5. Dierent technologies for pyrolysis of waste tyres12,22,39,76.
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in pyrolysis temperature. e liquid fuel yield increased when the temperature increased to 350 °C from 280 °C.
Further increases in temperature resulted in a drop of the liquid fuel yield. On the other hand, gas yield kept on
increasing with increases in temperature from 280–400 °C30. In addition to the eect of the reactor temperature,
dierent zeolite catalysts, ultra-stable Y-type (USY) and Zeolite Socony Mobil–5 (ZSM-5) have also been stud-
ied44. It was observed that for the USY catalyst, 400 °C was the optimal temperature, beyond this temperature the
oil yield decreased and the gas yield increased. Similar behaviour was found for the ZSM-5 catalyst. Conesa et al.45
observed that higher liquid oil yields were obtained at a medium temperature range (300–500 °C) and that for the
higher temperature range (>500 °C) the oil yield was found to drop and a higher gas yield acquired.
Some studies have reported the eect of varying other reactor parameters, such as heating rate and ow rate,
along with the eect of temperature. Banar et al.46 experimented with two dierent heating rates viz. 5 °C/min and
35 °C/min between 350 and 600 °C reactor temperature and observed that an increase in the heating rate resulted
in a decrease in the oil yield. e maximum oil yield dropped to 31.1% at a heating rate of 35 °C/min compared to
that of 38.8% at 5 °C/min. It was noted that for both heating rates the oil yield increased up to 400 °C and then the
commonly reported downward trend observed. Frigo et al.47 produced diesel like fuel from the pyrolysis of scrap
tyres and varied the temperature from 300 to 500 °C. e authors mentioned that the yield of dierent products
was reliant on the ow rate of crushed tyres into a reactor. ey achieved a maximum oil yield of ~45% by opti-
mising the ow rate of the crushed tyres.
Temperature eects were investigated in the microwave assisted pyrolysis of scrap tyres with and without
activated carbon as a catalyst48. For pyrolysis without a catalyst, an increase in temperature resulted in a decrease
of char yield initially which then reversed—the opposite trend was observed for the gas yield. e optimum tem-
perature for microwave pyrolysis without a catalyst was found to be between 550–600 °C, the oil yield achieved
was between 19.03 to 28.63% (in contrast, with use of a catalyst the maximum oil yield (54.39%) was achieved at
500 °C).
From the above studies, it can be concluded that: the temperature aects the product yields from the pyrolysis
reaction signicantly; an increase in pyrolysis temperature causes a decrease in yield of carbon black, whereas the
yield of gaseous products always increases with temperature; and, the liquid product yield initially increases with
an increase in temperature then drops with further increases in temperature.
Eect of catalyst. Catalyst type and concentration play an important role in pyrolytic treatment of waste tyres.
Shah et al.30 varied the catalyst (calcium carbide) to tyre rubber ratio from 0.1 to 0.5 and observed that the max-
imum oil conversion occurred at a ratio of 0.3. Furthermore, it was observed that using a 0.3 calcium carbide to
crushed tyre ratio increased the liquid fuel conversion to 38.4%, from 22.8% when no catalyst was used. Boxiong
et al.44 also reported that the use of catalysts USY and ZSM-5 increased the conversion percentage for oil yield in
the pyrolysis reaction. e oil yield increased to 70.8% for USY and 56.8% for ZSM-5 compared to that of 35% at
non-catalytic conditions.
Dung et al.49 experimented with two dierent catalysts at 500 °C during slow pyrolysis: Mobil Composition
of Matter No. 41 (MCM-41) prepared with silatrane; and Ru/MCM-41. Compared to the uncatalysed reaction,
the oil yield increased with MCM-41 and further increased with Ru/MCM-41. On the other hand, the gas yield
decreased and char yield did not change signicantly. e presence of acidic sites promotes the conversion of
heavy compounds to lighter ones owing to cracking activity, thereby increasing the gas yield. From the above
studies, it is seen that the use of dierent catalysts such as USY, ASM-5, MCM-41 and activated carbon has been
benecial for increasing product yield in waste tyre pyrolysis.
Fuel characterisation. Fuel properties play a key role in engine performance and emission characteristics.
Table2 shows the comparison of fuel properties of dierent WTD fuels. Apart from TPO, studies have investi-
gated the carbon black and also TPO which has been distilled. e density of diesel is lower than that of other
fuels. e higher oxygen content found in TPO decreases the caloric value, which in turn reduces engine power.
In addition, the higher kinematic viscosity of TPO is related to the degree of unsaturation, and it also increases
with the oxygen content of the fuel. is can adversely aect the atomisation of the fuel spray and evaporation
characteristics of the fuel during combustion. Sulphur and aromatic content is substantially higher in TPO com-
pared to diesel.
Engine performance and emissions
ere are a number of studies in the literature which have focused on engine operation with waste TPO blended
with diesel. Engine operation analysis of TPO is divided into two sections, performance characteristics and emis-
sion characteristics.
Performance characteristics. Some authors have attributed reduced engine performance (BTE) on TPO
fuel to its lower caloric value. Wang et al.6 observed that BSFC increased with TPO percentage in blended fuels.
is was attributed to the lower caloric value of TPO fuel (42.21 MJ/kg) compared to diesel (43.8 MJ/kg). Lower
caloric value directly eects BSFC as more fuel is consumed to operate the engine for the same power output. Ilkiliç
and Aydin17 also concluded that the lower caloric value of TPO contributed to decreased engine power and higher
BSFC for diesel-TPO blends. For higher blends of TPO, engine power and torque decreased gradually. Engine
power was lowest for pure TPO compared to blends of diesel with 75% and 50% TPO, the reason cited to which
was lower caloric value of TPO. BSFC for TPO fuel for 5 to 100% blend increased from 322.91 to 367.17 g/kWh,
which was again related to the lower caloric value of TPO.
Figures6 and 7 show the variation in BTE and BSFC for dierent blending ratios of WTD fuels, as retrieved
from TableA. It has been observed that BTE decreases with an increase in blend percentage, whereas BSFC
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increases for higher blends of WTD fuels. Martinez et al.50 compared the characteristics of TPO and diesel fuel
and observed that even a 5% addition of TPO caused an increase in BSFC at low engine loads. A 4% increase in
BSFC of the TPO blended fuel was accounted for based on the lower energy content of TPO5 i.e. 42.23 MJ/kg in
comparison to 42.31 MJ/kg of neat diesel fuel. At high engine loads, BSFC and BTE were found to be comparable
for both types of fuels. High density of TPO fuel is helpful in getting comparable performance characteristics at
high engine loads. Fuel composition (cold lter plugging point, viscous fraction and residue aer distillation)
plays a key role in deciding the BTE and BSFC at low loads because of poor atomisation of fuel and encounters
diculties in mixing with air at low temperatures in the combustion chamber. On the other hand, at higher
engine loads the dierence in BTE and BSFC for diesel and a blend of 5% of TPO is reduced because the ratio
of mechanical to pumping losses-to-fuel energy is reduced for higher loads. Hariharan et al.18 also reported that
the lower BTE of the TPO blended fuel is due to the lower heating value compared to diesel. ey also reported
that the lower heat release rate (HRR) during the premixed combustion phase is the reason for the lower thermal
eciency of TPO fuel operation up to part load.
Some studies relate the lower BTE and higher BSFC of TPO blended fuel to density and viscosity. Tudu et al.15
reported that the higher density of TPO fuel (910 kg/m3) compared to diesel (830 kg/m3) results in poor atomisa-
tion and spray characteristics, causing incomplete combustion of fuel. Wamankar and Murugan51 also concluded
that the higher density and viscosity of carbon black led to poor atomisation, thus, had higher BSFC compared
to diesel. Murugan et al.12 used 10% to 50% of TPO blended in diesel and observed that TPO blended fuels had
lower BTE compared to diesel, which was due to higher viscosity and lower heating value of TPO fuel. However,
BTE for TPO blends improved with an increase in TPO percentage, though remained lower than that of diesel.
Improvement in BTE, BSFC and brake specic energy consumption (BSEC) for higher blends of TPO can be
attributed to better lubricity of the blend due to the additional use of TPO. Tudu et al.19 also said that the presence
of aromatic content and higher boiling point of light fraction pyrolysis oil are the key reasons behind higher use-
ful work for TPO blended fuel compared to neat diesel.
In another study, Murugan36,52 studied the eect of distillation on TPO and used this fuel in a diesel engine. It
was observed that 30% blends of TPO and distilled TPO had marginally lower BTE compared to neat diesel. e
authors related the reduced BTE for blends with distilled TPO to a decrease in viscosity. In their study, the fuel
spray did not propagate as deeply into the combustion chamber and therefore some fuel remained unburnt. is
incomplete combustion resulted in a reduction in eciency. Further, a 30% blend of distilled TPO with diesel had
lower BTE than a 30% blend of TPO with diesel, which might have been due to the presence of volatile fractions
resulting in further incomplete combustion.
Wamankar and Murugan53 studied the inuence of dierent injection timings and nozzle opening pressures
on a direct injection diesel engine operated with a slurry of carbon black, water and diesel. Maximum BTE was
observed at an advanced injection timing of 26° before top dead centre (BTDC) and 220 bar nozzle opening
Fuel property Diesel TPO Distilled TPO Carbon black
Density @ 15 °C (kg/m3) 0.8–0.83 0.92–0.935 0.871 0.86
Flash point (°C) 50 43 36 —
Kinematic viscosity at 40 °C (cSt) 2 3.2 1.7 —
Caloric value (MJ/kg) 42.7–43 38–42.8 45.6 32.6–32.86
Oxygen (% m/m) — 0.10–3.96 — 1
Sulphur (% m/m) <0.001 0.72–0.96 0.03 0.02
Carbon (% m/m) 87 83.45–85.60 — 86.4
Nitrogen (% m/m) — 0.40–1.05 — 0.3
Hydrogen (% m/m) 13 9.59–11.73 — 2.86
Ash content (%) 0.01 0.31 — 10.24
Aromatic content (% m/m) 26 39.3–63 — —
Table 2. Fuel properties for dierent WTD fuels28,32,52,77.
Figure 6. Variation in BTE for dierent blending ratios of biodiesel compared to neat diesel15,16,32,51,52,60.
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pressure. Advancement in injection timing increased the in-cylinder pressure. e maximum peak pressure also
occurred at an earlier crank angle because combustion started relatively earlier. On the other hand, an increase in
nozzle pressure resulted in a shorter ignition delay. e authors stated that advanced injection timings resulted in
a higher heat release rate, thus, reecting the higher BTE.
Emission characteristics. Dierent researchers have studied emission characteristics i.e. NOx, CO, HC,
CO2, PM and PN emissions of diesel engine operated on TPO, diesel and biodiesel and results are summarised
below.
Nitrogen oxide emissions. e fundamental mechanisms behind NOx formation in diesel engines are thermal,
prompt, and fuel-bound nitrogen. One of the sources of nitrogen for NOx formation during combustion of diesel
and alternative fuels is atmospheric (molecular) nitrogen. Most of the literature studies showed varying trends
in terms of the inuence of alternative fuels on NOx emission from diesel engines54. ermal NOx refers to the
NOx formed through the high temperature oxidation of nitrogen (N2) in the combustion chamber. NO emis-
sion formation in diesel engine is driven by the following key parameters i.e. in-cylinder temperature, air/fuel
ratio, oxygen concentration and residence time for the reaction to take place51. In general, dierent authors have
reported an increase in NOx emission with the use of TPO compared to diesel, as observed from the trend given
in Fig.8. Barring a few studies, with an increase in blend percentage, NOx emissions were found to increase with
TPO compared to diesel.
Broadly, NOx variation can be discussed on the basis of two parameters: injection related and temperature
related.
1) Injection related parameters:
a) e use of fuels with higher bulk modulus in hydraulically operated injectors (not in common rail
systems) leads to an earlier pressure rise and advances the start of injection. Advanced injection-tim-
ing results in an increase in ignition delay, residence time, and duration of premixed phase, thus,
resulting in higher in-cylinder temperature and increased NOx formation. Additionally, higher oxy-
gen content allows fuel to premix completely during ignition delay resulting in more heat release dur-
ing the premixed-burn phase of combustion at ignition and thus accountable for an increase in NOx
emissions55. Wamankar and Murugan53 stated that advancing the injection timing results in higher
NO emissions for all fuels owing to rapid combustion and high in-cylinder temperature. Kegl56 stated
that injection timing is more important than degree of in-cylinder temperature and HRR. Advanced
injection results in the maximum in-cylinder temperature and HRR being reached earlier in the cycle,
which greatly aects NOx emissions.
b) Dierences in the viscosity of fuels also plays a key role in advanced fuel injection systems operating
with a mechanical pump. Fuels with low viscosity have increased fuel leakage, leading to a decrease in
the rate of pressure rise and thus delays in the start of injection54. In general TPO has higher viscos-
ity than diesel16,57, therefore less fuel leakage and higher pressure rise, which ultimately results in
advanced injection timing.
2) Temperature related parameters:
a) Higher nitrogen content in pure TPO fuel (0.79 wt.%) compared to diesel (0.00 wt.%)50,58 is a further
reason behind the noted increase in nitrogen oxides.
b) Additionally, higher adiabatic ame temperatures increase NOx formation. More aromatic content,
lower H/C ratio and fuel bound oxygen result in higher adiabatic ame temperature for engine opera-
tion with TPO fuel50,58, which results in higher NOx emissions59.
Contrary to the general trend in the published literature, some studies reported a decrease in NOx emissions
with the use of TPO fuel.
Figure 7. Variation in BSFC for dierent blending ratios of biodiesel compared to neat diesel17,60,65.
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a) Murugan et al.52 related NOx variation to the stoichiometry of fuel and in-cylinder temperature. It has
been reported that when the in-cylinder temperature goes beyond 1800 K, NOx formation starts60. If there
is lean stoichiometry during the combustion process, NOx formation is lower, which is generally at lower
load conditions. On other hand, due to the diusive mixing of fuel and air occurring along the spray enve-
lope, combustion takes place near stoichiometric, forming higher NOx. It was reported that lower blends
of distilled TPO (10% to 20%) with diesel have low NOx emissions, compared to higher blends such as
90% blend of distilled TPO resulting in an increase in NOx, which was linked to higher HRR in the case of
higher blends.
b) Aydın and İlkılıç60 related lower NOx emission for low sulphur TPO with in-cylinder temperature. It was
concluded that NOx emissions reduced by 29.3 and 40.2% for 50% and 75% blends of low sulphur TPO
compared to diesel. e primary reason cited for lower NOx emission for low sulphur TPO blended fuel
was lower in-cylinder temperature.
c) Other important factors that lead to a reduction in NOx are considered to be the lower cetane number of
low sulphur tyre fuel, which results in delayed combustion and higher heat of vaporisation that results in
decreased combustion temperature60. Tudu et al.19 stated that lower peak pressure and in-cylinder temper-
ature are the key reasons for lower NO emissions for fuel blended with light fraction pyrolysis oil (LFPO)
compared to neat diesel.
d) Further, it has been reported that operating the engine with exhaust gas recirculation (EGR), aids in
inhibiting NOx formation. Martinez et al.50,58 used TPO blended fuel in a diesel engine and expressed that
higher EGR values lowers the oxygen concentration and ame temperature, thereby helping in reducing
thermal NOx emissions.
CO and CO2 emissions. CO emissions are the result of incomplete combustion typically due to a lack of oxygen
or available time in the cycle for the completion of combustion. In general, an increase in engine load or speed
results in a decrease in CO emissions; however, an excessive increase in engine speed results in higher CO emis-
sions. At very high engine speeds, a lower combustion ame temperature limits CO oxidation into CO2; therefore,
the CO emission values at very high engine speeds tend to also be higher53,60,61. Literature relating to the use of
TPO in diesel engines indicate an increase in CO emissions with TPO blended fuel17,19,52,60, which is clear from
the trend given in Fig.9. With an increase in blending ratio, CO emissions gradually increased compared to die-
sel, although attempts have been made to decrease the CO emissions with use of TPO by blending it with other
fuels such as biodiesel15,57.
Ilkiliç and Aydin17,60 stated that higher density of TPO fuel results in more injection of fuel on a mass basis
leading to lower air-to-fuel ratio, which has been considered crucial for higher CO emissions due to decreased
availability of air. is was further supported by Wamankar et al.51 and Tudu et al.19 higher fuel density leads to
lower air-to-fuel ratio. Moreover, the higher viscosity of TPO results in poor atomisation characteristics of fuel
resulting in incomplete combustion, further compounding the issue with CO emissions. Contrary to general
statements in the literature, Murugan et al.52 stressed a leaner fuel air mixture in the cylinder and said that the
ame does not propagate through some of the mixtures nearer to the wall and crevice volume. is causes incom-
plete combustion leading to higher CO emission for distilled TPO blended fuel when compared to diesel.
ere have been attempts made to reduce the CO emissions even with use of WTD fuels. Koc and Abdullah57
observed that all the dierent biofuel blends (TPO or biodiesel) had CO emissions lower than neat diesel. At
lower engine speeds, a 10% blend of biodiesel with diesel had the highest CO emission, whereas, 10% each of
biodiesel and TPO in diesel (80% diesel) had the lowest CO emissions. Tudu et al.15 concluded that using 10%
dimethyl carbonate (DMC) in light fraction pyrolysis oil resulted in a 66% reduction in CO emissions. is could
be due to the additional oxygen content of DMC, resulting in a more homogeneous mixture of air-fuel resulting
in more complete combustion. e other reasons may be the turbulent motion by an internal jet helps make the
air-fuel mixture inside the combustion chamber more homogeneous, which leads to more complete combus-
tion15. With an advancement in fuel injection timing, 16–18% lower CO emissions are observed, which is mainly
due to higher cylinder temperatures and more rapid oxidation between C and O2 molecules. Retardation in fuel
injection timing causes CO emissions to increase53.
Figure 8. Variation in NOx for dierent blending ratios of biodiesel compared to diesel16,32,50,52,57,65,77.
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Similar to CO emissions, lower engine speeds under load result in higher CO2 emissions57. Completely oxi-
dised carbons in the fuel give rise to higher CO2 emissions. Ternary blends like 5% to 10% each of biodiesel and
TPO in diesel had lower CO2 emission than neat diesel57. Tudu et al.15 found an improvement in CO2 emission
for fuel blended with LFPO or DMC compared to neat diesel. Additionally, provision of turbulence with help of
internal jet piston arrangement helps in further reduction of CO2 emission15.
Hydrocarbon emissions. e unburnt hydrocarbon emissions are a direct result of incomplete combustion, due
to the incomplete mixing of the air and fuel. e concentration of unburned hydrocarbon decreases with engine
load. HC emissions for dierent emulsions were found to be higher than diesel due to the higher density of emul-
sion fuels, resulting in poor atomisation51.
In general, the literature reports an increase in HC emissions for WTD fuels, as observed in the trend (Fig.10).
Murugan et al.52 stated that an increase in HC emissions for distilled TPO blended fuel was due to the presence of
unsaturated hydrocarbons, which did not break down during the combustion process. e literature also reports
that the fuel spray does not propagate into the combustion chamber properly, resulting in gaseous hydrocarbons
on the inner walls of the cylinders and crevice volumes being le unburnt, causing incomplete combustion.
Aydın and İlkılıç17,60 related the lower cetane number of low sulphur TPO blended fuel, which resulted in
higher ignition delay. In addition, higher density and nal distillation temperature in low sulphur TPO blends
were the main reasons behind the higher HC emissions, compared to diesel. Murugan et al.32 said that poor vol-
atility, higher density and viscosity are the key reasons for higher HC emission with TPO blended fuel. Similar
observations were made by Wamankar and Murugan28,51,53,62–64 for the emulsion of carbon black, water and diesel.
In this study, higher HC emissions were related to poor atomisation of the fuel, as a consequence of: the higher
density and viscosity, inferior fuel spray quality and a lower compression ratio of the fuel. Tudu et al.15 also
reported higher HC emissions for a fuel blended with the light fraction of pyrolysis oil, but observed a reduction
with the use of an internal jet piston. is dierence was attributed to higher turbulence by the internal jet piston
motion. Martinez et al.58 related higher HC emission for TPO fuel due to higher sulphur content compared to
diesel fuel.
In contrast to the general nding, some authors reported a decrease in HC emissions for TPO blended fuels.
Frigo et al.65 reported that the relatively lower viscosity (2.9 cSt) of TPO compared to diesel (3.5 cSt) resulting in
better spray atomisation and that the aliphatic hydrocarbon and aromatic content aided TPO vaporisation and
ultimately the combustion velocity. All of these factors contributed towards lower HC emissions compared to
diesel. Öztop et al.66 observed lower HC emissions for TPO blended fuel compared to gasoline at higher engine
speeds; however, they observed an increase in HC emission at low engine speed. e reason for this could be poor
atomisation at low engine speeds due to the higher fuel viscosity. However, at higher engine loads, air movement
causes a more favourable air-to-fuel ratio, resulting in complete combustion, thereby reducing HC emission.
Diesel particulate matter emissions. PM is a solid and liquid mixture suspended in a gas. In combustion cham-
bers of compression ignition engines, fuel is injected and mixed with an oxidant; consequently, a great degree of
heterogeneity characterises the combustion process. is process, which is the main cause of PM emission, is
called diusion ame combustion67. PM is a complicated pollutant that is not chemically well-dened in terms of
its composition, formation and control. PM composition depends on factors such as, engine speed and load, fuel
type, lubricant type, engine maintenance, aer-treatment systems and the level of dilution aer being emitted68,69.
TPO has the potential of reducing PM emissions owing to its oxygen content, which ranges from 0.10–
3.96 wt%. (as shown in Table2). Most studies in the literature have indicated that alternative fuels such as biodies-
els can decrease PM emissions59,70,71 owing to their oxygen content70–72. Soot formation generally occurs in the
fuel-rich zone, under high temperature decomposition during combustion. It mainly takes place in the fuel spray
core region. Since these fuels are partially oxygenated, the fuel-rich zone is reduced, preventing higher soot for-
mation and aiding the soot oxidation process leading to lower PM emissions59. However, PM formation is aected
by dierent parameters which can either reinforce or cancel the eect of one another under dierent conditions.
Figure 9. Variation in CO emissions for dierent blending ratios of biodiesel compared to
diesel16,17,28,32,52,60,65,77.
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Recently, PN emissions, which is a count of individual particles, has gained more attention. is could be due
to the toxicity of particles, which increases by decreasing the particle size73. e EU Commission included PN in
the Euro 5b and Euro 6 emission standards for light-duty and heavy-duty vehicles74. However, there is still a lack
of regulation on the size of emitted particles. Small particles could be more harmful to health because they can
penetrate deeper into the lungs and cause inammation where deposited73.
Martinez et al.58 found that a 5% blend of TPO with diesel exhibited higher smoke and PN emissions com-
pared to diesel. is could be due to the higher aromatic and sulphur content. Lower biodiesel blends show
comparatively lower smoke emissions because of better combustion and fuel atomisation28. Martinez et al.50 con-
cluded that the higher aromatic content in TPO fuel is a key factor in increasing PM emissions because it aids
the formation of soot precursors, resulting in higher sooting tendency. In addition, authors could not nd any
research in the literature studying the morphology and nanostructural characteristics of PM emissions from
diesel engine fuelled with WTD fuels.
Conclusion
e use of alternative fuels in diesel engines has been attracting researchers over the last decade owing to the
gradual decrease in petroleum-based fuels and their subsequent environmental concerns. ere has been 48.5
million tyre EPUs of waste in Australia alone in 2009–2010 and the rate of waste accumulation has been increas-
ing. Using waste tyre pyrolysis oil could be a solution to this issue owing to the treatment of waste rubber and
scrap tyres by recycling. ere have been various studies on pyrolysis of scrap tyres, production of WTD fuels
and their use in diesel engines. is review paper studied the historical overview of the literature published on
the production of WTD fuels from waste tyres and their application in diesel engines. e application of WTD
fuels in diesel engines started mainly in the late 2000s and has gained the attention of researchers as alternative
fuels to diesel. Most of the studies have focused on the use of 5 to 20% of WTD fuel blended in diesel, whereas,
some studies have even tried complete replacement of diesel. is paper reviewed the production aspects of WTD
fuels from waste tyres such as methodologies, eect of operating parameters e.g. reactor temperature and catalyst.
Further, fuel properties, engine performance and emission characteristics for WTD fuels were reviewed with the
following ndings
1. Among dierent pyrolysis processes, microwave and vacuum pyrolysis stand out due to the high yield of
products and relatively lower processing time.
2. Reactor temperature plays a key role in the pyrolysis process. It was observed that an increase in temper-
ature increases the oil yield initially, but aer achieving the peak, oil yield drops gradually. On the other
hand, the gas yield increases and char yield decreases with an increase in reactor temperature. e opti-
mum temperature for the pyrolysis process has been found to be within 450 °C–500 °C.
3. e type of catalyst and its concentration also holds an important role in the pyrolysis process. It has been
concluded that the catalyst to rubber ratio within 0.3–1% to be optimum and USY, ASM-5, MCM-41 cata-
lysts have been found to be most eective.
4. TPO has a higher oxygen content (0.10–3.96%) compared to diesel, which could be eective in the reduc-
tion of PM emissions.
5. For performance parameters, in general BSFC and BTE are found to be higher for use in TPO blended fuel
compared to diesel possibly due to a lower caloric value (38–42.8 MJ/kg). In addition, higher density and
viscosity were also cited as the reasons for poor atomisation of the fuel leading to inferior BSFC and BTE.
6. For emission characteristics, mixed observations have been reported for NOx emissions. Some authors re-
ported an increase in NOx emissions with use of TPO blended fuel, attributed mainly to higher viscosity and
the higher nitrogen and oxygen content of TPO fuel. On other hand, some report a decrease in NOx emis-
sions, which was attributed to lower peak pressure and in-cylinder temperature and lower cetane number.
7. CO and CO2 emissions have been found to be higher with the use of TPO blended fuel due to the higher
density and viscosity of TPO, leading to a lower fuel to air ratio. A similar was the trend observed for HC
emissions.
Figure 10. Variation in HC emissions for dierent blending ratio of biodiesel compared to
diesel16,17,28,32,52,60,65,77.
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8. ere has not been signicant literature on DPM with TPO. Although, due to the higher oxygen content,
PM emissions should be lower than that for diesel but it has been reported that the higher aromatic content
in TPO resulted in higher PM and PN emissions none-the-less. ere is a signicant gap and scope of
research to analyse PM emission for TPO blended fuel in the future.
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Author Contributions
P.V. conceived the idea, did literature search, wrote the main manuscript text and prepared the tables and gures.
A.Z. assisted in data interpretation and presentation and critically revised the manuscript. M.J. assisted in
literature search and critically revised the manuscript. T.A.B. assisted in data analysis and critical revised the
manuscript. T.R. assisted in data analysis and critically revised Section 3 and 4. Z.D.R. and R.J.B. supervised and
critically revised the manuscript.
Additional Information
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-018-19330-0.
Competing Interests: e authors declare that they have no competing interests.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and
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Scientific RePoRts | (2018) 8:2457 | DOI:10.1038/s41598-018-19330-0
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