1 Introduction

World’s primary energy supply is still greatly dominated by the gradually depleting fossil fuels. Amid the raising energy demands and environmental concerns around the globe, research has been driven towards developing much sustainable and renewable energy systems. Biomass is not only one of such energy resources but also can provide various high value-added products and bio chemicals such as composite materials, paints, adhesives, etc. [1, 2]. According to reports [3], bio energy contributes a whopping 67% of the share amongst all available renewable energy sources and meanwhile the liquid bio fuels are being considered as the promising renewable solution for the versatile transport sector, wherein the CI engines plays a vital role. Biofuels (during 2000–2018), grown at 13% annual rate with sustainable development and witnessed (more than) 9 times the increased production from 17.3 billion litres to 160 billion litres. Being the world’s third-largest crude oil importer and fourth primary energy consumer, the Govt. of India by 2022 targeted produce 10 GW of bio energy by encouraging the utilization of alternative fuels, implementing strategic Bio-Fuel policies, expanding domestic mass production, launching various bioenergy schemes and programmes, etc. [4]. Frequently increasing costs of diesel, stringent emission regulations and the dwindling of the conventional energy-resources (Coal, crude oil, etc.) are some of the main reasons to necessarily switch over to alternative fuels. Blending diesel with a renewable biofuel could considerably reduce its consumption and so are the CO2 emissions but also improve the fuel quality as well as engine operation. Cashew nut shells are the abundantly generated agro-wastes from cashew nut shell processing industries every year and were proven to be amongst the potential bio-based renewable materials. Cashew nuts production (globally) has been reported to have increased by 32% in the past 10 years wherein the India alone covered 25% of total production [5]. An almost linear trend in the cashew production worldwide during the recent years leads to mounting cashew nut shell agro-waste to be dealt with.

2 Paper structure

2.1 Introduction

The section throws light on the depletion of the conventional fuel reserves and further need to opt alternative energy sources. Cashew nut shells as biomass; statistical reports regarding production of liquid biofuels, cashew nuts production and the role of bioenergy in meeting the existing energy demands were outlined.

2.2 Literature review

This section not only deals with the previous research studies covering the areas of fast pyrolysis, utilising cashew nut shell oil in different blend ratios as alternative fuel to evaluating the engine performance and emission characteristics such as BTE, BSFC, CO, NOx, UBHC etc., incorporating aromatic chemicals like toluene in fuel mixtures and also briefs the key findings. Research gap, novelty and objectives of the study are also addressed.

2.3 Materials and methodology

Here, a detailed insight to the adopted research plan and methodology for executing the work is presented. The section covers treating the materials and instruments used, describes various processes like pyro oil extraction, sample fuels preparation and characterization, detailed experimental procedure, etc.

2.4 Results and discussions

This section plots the obtained experimental data of all the tested fuel samples and discusses the associated logics, reasons behind their variation. Results are explained and also compared with previous works.

2.5 Conclusion

This part presents the key finding and conclusions drawn off the investigations.

3 Literature review

James Mgaya et al. [5] reviewed the latest trends and developments on Cashew nut shells, its extracts and isolates, as clean energy sources and also discussed various attempts made on the agro-waste utilization in energy sector.

Vinod [6] briefly reviewed various developments and status of biomass pyrolysis technologies over the past thirty years and realized that, bio-oils extracted through fast pyrolysis are found more expedient in transportation, storage and handling aspects. Hossain and Davies [7], after reviewing about twenty-four confirmed experiments on the pyro liquids (both crude and upgraded) in diesel engines realised that only quarter of them were deemed successful to stabilise engine performance. Summarised that long ignition delay, engine components erosion and corrosion, coking of cylinder and piston liners, sudden rise of cylinder pressure, lower BTE, extended combustion duration, higher SFC and CO emissions, etc. could be the associated reasons.

Shiva Kumar et al. [8] revealed that running CI engine with pure CPO resulted in inferior performance and recorded reduced HC, CO, smoke emissions along with raised NOx levels. Authors also highlighted that, up to 40% replacement of cashew nut shell bio-oil is practicable without compromising on engine performance. Impact of adding Hexanol [9], pentanol [10] to cashew nut biodiesel on engine parameters is correspondingly investigated and in both cases marginal change in BTE, significant drop in BSFC were noticed along with the increased NOx levels and decreased CO, HC, smoke emissions. Kasiraman et al. [11] tested blends of Cashew nut shell oil with 10%, 20%, 30% (by volume) camphor oil as fuels on a diesel engine and observed that ‘30% camphor oil blend with 70% cashew nut shell oil’ showed the performance close to diesel fuel with 3.6% higher BTE, 5.8% higher NOx and 7.4% higher smoke emissions and concluded that neat cashew nut shell oil performance can get significantly better with camphor oil blends and 30% camphor oil blend could be substituted for diesel. Bupesh Raja and Jaya Prabhakar [12] investigated the performance and emission characteristics of Diesel engine using ‘20%, 40% and 60% (by volume) proportions of Cashew nut shell oil blended with Diesel’ as fuels and reported that 20% blend showed a closer but however lower BTE to Diesel and achieved reduced CO, HC, CO2 emissions with a bit higher NOx levels over Diesel fuel. Pushparaj et al. [13] tested the effect of 10%, 20% and 30% by volume Cashew nut shell biodiesel blended with Diesel on the performance and emission evaluation in diesel engine and found that blend20 showed improved power output over diesel and improved performance parameters like up to 45% higher SFC along with the 37% reduced CO emissions and slightly increased Nox, etc. Venkatesan [14] tested the 10%, 20%, 30%, 40% and 50% by volume proportions ‘Cashew nut shell pyro oil blended with 20% Jathropa biodiesel-diesel mixture’ as fuels to diesel engine and found that ‘10% cashew nut shell pyro oil blended with 20% Jathropa-Diesel’ closely resembled diesel properties with slight increments in BTE, BSFC and also decreased the CO, HC and Smoke emissions with marginal rise in NOx levels. Venkatesan et al. [14] investigated the performance and emission characteristics of cashew nut shell pyrolysed oil—waste cooking oil with diesel fuel in a four stoke DI diesel engine. Blends of pyro oil with diesel blends are CPO5WC5D90, CPO10WC5D85 and CPO15WC5D80 resulted in comparable performance with slight reduction in BTE at all power outputs. Several metal based additives have been found to improve combustion quality when added to the biodiesels. Nano alumina particles added Cashe nut shell biodiesel was tesed in CI engine by Radhakrishnan et al. [15] and results showed 10%, 8.8%, 12.4% and 8.4% reductions in CO, HC, NOx and smoke emissions.

The studies covered in the literature generally examined the CI engine charcteristics by adding biodiesel/bio-oil to diesel fuel. However, this work assesses the impact of adding toluene into CPO-Diesel mixture on the engine performance and emissions. Toluene is a volatile and a flammable chemical that comes with similar chemical properties like propanol, hexane, and heptane.

Salih Ozer [16], found that when toluene was added to diesel fuel or to biodiesel in CI engine emissions like BSFC, HC, NOx were increased and CO, smoke emission levels were dropped. Xiuxiu Sun et al. [17] learnt from a theoretical study that increasing the toluene content in the surrogate fuel of diesel engine, increased the soot and NOx emissions. Karthikeyan et al. [18] improved the CI engine characteristics by adding a mono philic antioxidant called butylated hydroxyl toluene to B20 (20% rice bran biodiesel + 80% diesel) and in comparision to B20, the results have shown 0.5% increased BTE, 1.2% decreased BSFC, 9.6% decreased NOx, 9.6% and 16.6% and 11.2% increments in HC and CO emissions respectively. Hellier et al. [19] conducted experiments on a diesel engine supplied with binary mixture of toluene & n-heptane and found that with increasing toluene in the mixture, ignition delay timing was also increased which influenced the exhaust emissions by emanating higher NOx levels. Yu et al. [20] reported that the particulate emissions could be reduced by adding chemicals like xylene and toluene into the diesel fuel in CI engine. Joy et al. [21] improved the CI engine attributes by adding di mithyl ether (an ignition enhancer) in cashewnut shell biodiesel with 1.6% increased BTE and 4% improved BSFC as well as reduced emissions level.over pure biodiesel. Midhat et al. [22] concluded that exhaust emissions were observed to be largely dependent on the number of methyl branches present in the fuel used. Also found that fuel blends consisting more than 30% alkyl benzene proportions may lead to increased emissions like CO and UBHC, PM, etc.

Researchers tested many pyro oil blends opus extracted from coconut shells [23] waste tyres and waste plastics [24, 25], hazelnut shells [26], waste cooking oil [27], date tree waste mixtures and date seeds [28], etc. as substitute fuels on CI engine to improve engine performance and reduce emission characteristics. Nonetheless no remarkable experimental investigations have been reported so far on CI engine using cashew nut shell pyro oil-toluene-diesel blends.

From the comprehensive literature surveyed, it is clear that Cashew nut shell liquid has the potential which upon proper treatment could possibly turn into a better alternative to diesel. Cashew nut shell biomass is selected, owing to cashew nut shell liquid’s 100% miscibility and its significant potential for blending towards diesel [8]. Most of the investigations on toluene-mixed-fuels used in CI engines are found to be typically theoretical [17, 29, 22, 30,31,32,33]. There hasn’t been any promulgated research in the accessible literature covering the feasibility of toluene addition to Pyro oil blends. This has paved the way to conduct this research.

This work was taken up to analyse the improvement of CPO’s performance in CI engine when blended with Diesel and Toluene. Specific objectives of this dissertation include.

  • Pre-treatment of cashew nut shell biomass and production of CPO through fast pyrolysis.

  • Preparation & characterization of the tested blends.

  • Assessing the performance and emissions characteristics of the CI engine using CPOT5, CPOT10, CPOT15 blends as fuels and finding the optimal one.

4 Materials and methodology

4.1 Overview of the materials and process description

4.1.1 Feedstock and pre-treatment

Cashew nut shells were collected from a cashew nut processing industry located in the sub urban Guntur, India. Shells were washed, sun dried (for 36 h to remove the dust and excess moisture), milled in a laboratory ball mill. and the samples were stored in a refrigerator at 4-5 \(^\circ \)C for further experimentation. Figure 1 shows the photographic views of the collected samples of Cashew Nuts, Cashew Nut Shells and the ball mill used.

Fig. 1
figure 1

Collected samples of a: cashew nuts, b, c cashew nut shells, d lab scale ball mil

Full size image

4.1.2 CPO production

Among the several available ways of transforming biomass into fuel, fast pyrolysis technique is considered more economical and an effective thermal conversion technique, wherein the biomass is quickly heated to 400–550 °C in absence of oxygen, converted directly to aun high yield bio-oil of around 74–75% (by wt) accompanied with lower yields of 13 wt% flue gas and 12 wt% bio-char as by-products [34]. Owing to pyro oil’s hydrophilic and high polarity nature, adding co-solvents could enhance its solubility.

The CPO is produced through a lab scale fast pyrolysis apparatus, photographic view of the same is shown in Fig. 2.

Fig. 2
figure 2

Lab scale fast pyrolysis setup

Full size image

A stainless steel made thermal reactor (350 mm length, 200 mm inlet diameter, 10 mm wall thickness) having an inlet for Nitrogen (N2) gas supply and an outlet for exiting the volatile gasses, was placed in an electric furnace of capacity 7.5 k-Wh. K-type thermocouples were attached at various axial and radial positions of reactor to record the temperatures at regular time intervals. The outlet of the reactor, through a high temperature-withstanding stainless steel pipe (of 130 mm length), was connected to a counter flow type stainless steel made condenser (of 500 mm length, 30 mm inlet diameter and 50 mm outlet diameter) which was linked to gas-oil separator (of 350 mm length and 100 mm diameter).

About 250 g of the pre-treated samples was fed into reactor for each run. Nitrogen gas was supplied into the reactor to make the environment inert and the samples were heated to an optimum condition of temperatures range 450–650 °C. Vapours emanating out of the reactor entered the condenser and got condensed by the action of circulating water. After that, the condensate was let dribbled into the gas-oil separator and the non-condensable gases were risen to the neck of a separate pipe, exited to the gas burner through an exhaust passage. CPO was collected off the gas-oil separator. Heating was ceased when no substantive changes in exhaust gases and in temperatures were observed. Then the reactor was allowed to cool down for nearly 6 h.

4.1.3 Toluene

Toluene was purchased from a local commercial vendor in Vijayawada, India and was a colourless, clear, water-insoluble, aromatic hydro carbon solvent, smelling typical to paint thinners. Due to being a pure hydrocarbon (C7H8) containing only carbon and hydrogen atoms, toluene’s complete combustion gives rise to CO2 and H2O ensuring the vehicle’s emission control system such as sensors etc. unaffected. Owing to its hazardous impact on living beings, toluene is generally employed in smaller quantities to the fuel samples. For this purpose, 5% (by vol) toluene was added to the tested blends. Toluene’s characteristics like low sensitivity, effective anti-knock/anti-pinging, more energy density, heating values etc. might boost the engine operation.

4.1.4 Blends preparation and characterization

The three CPO-TU-Diesel blends viz. CPOT5, CPOT10 and CPOT15 were each prepared (on volume basis) by taking the corresponding proportions separately in a beaker and the mixture was vigorously stirred up at a maintained speed of 850 rpm in a magnetic stirrer continually for about 45 min till a consistent mixture was observed. The treated mixture was, then, heated as well as sonicated in a probe sonicator for nearly 20 min to ensure the long lasting (over months) stability of the blend. Figure 3 shows the photographic views of the samples of neat CPO, Toluene, neat Diesel and the samples tested fuels and also the equipment used to prepare blends.

Fig. 3
figure 3

Photographic views of the samples used, tested blends and instruments used for blending

Full size image

The characterization of the properties of diesel, CPO, Toluene and the fuel blends was conducted at K L University, Andhra Pradesh, India and the testing methods were conformed to ASTM standards. Table 1 illustrates the average of the three consecutive observations for each property.

Table 1 Characterised physicochemical properties of the neat and blended fuels
Full size table

4.2 Experiment setup and procedure

Schematic diagram of the experimental test rig and the engine specifications are shown in Fig. 4 and Table 2 respectively. Experiments were conducted on a naturally aspirated, single cylinder, four stroke, water cooled, KIRLOSKAR make Diesel engine developing 4.4 kW output power at 1500 rpm. Test rig consisted of a CI engine, Dynamometer, Exhaust gas analyser, smoke meter, DAS (digital type) linked to a Computer, various sensors and measuring systems. An eddy current dynamometer with load sensor was coupled to CI engine shaft to provide the brake load. A non-contact type rpm sensor was attached close to flywheel to measure the speed. Rate of fuel consumption was measured in minutes/cc with the help of a solenoid regulated automatic Burette. A pipe-type calorimeter of 20–200 L/h water flow rate was employed to measure EGT and the water is made to circulate in the calorimeter by using a self-priming pump. Cooling water flow rate of 40–400 L/h to the engine and dynamometer was controlled by providing separate rota meter. The air box next to an in-lined air filter was included of differential pressure transmitters to measure air flow rates. Meanwhile, a fuel measurement system right after to the in-lined fuel tank measures fuel consumption rates. A surge tank in the upstream of air intake manifold was employed to ensure a steady air flow by damping out the pulsations. A piezo-electric pressure transducer and a TDC (Top Dead Centre) pulse pick up were fitted to engine cylinder to read the Cylinder pressure and crank angle respectively. A High-speed DAS with 16-bit, NI USB-6210 collected and stored the required data to analyse in offline. K-type thermocouples were used to read the temperatures across different points of the engine.

Fig. 4
figure 4

Schematic view of the experimental test rig

Full size image
Table 2 Engine specifications
Full size table

Initially, the engine was started and allowed to reach a steady state level. Both gas analyser and smoke meter were stabilized for a while before taking the observations. Fuel injection timing was set at 270 before TDC with 180 bar injection pressure. First, diesel was supplied to the engine as the CI engines are generally opted to be operated on diesel fuel and the baseline data is obtained. Then the fuel line was flushed out and the tank was filled with next fuel blend to be tested and the experimental data was collected. The process continued for all three blends tested viz. CPOT5, CPOT10, and CPOT15. At a given brake load, BSFC was calculated using the EXCEL computations in the computer. BTE was computed at the given load, by giving the inputs such as fuel consumption, Calorific value, density to the EXCEL sheet. Exhaust gas temperatures were measured by the pipe-type calorimeter. Emissions like HC, NOx, CO were measured by Exhaust gas analyser and the smoke emissions are measured by the smoke meter. Combustion parameters like cylinder pressure, HRR, ignition delay, etc. are averaged to multiple cycles and computed by amplifying the output signal of the transducer. To ensure the recorded results to be accurate, the average values were considered after conducting each set of experiments thrice.

5 Results and discussion

5.1 Performance parameters

5.1.1 Brake thermal efficiency

The useful amount of energy obtained by combusting fuel is generally specified by Brake Thermal Efficiency.

Variation of BTE against BP for all the tested fuels is shown in Fig. 5 and can be seen that BTE increased with increasing BP. CPOT5 attained higher BTE at all BP conditions over rest of the fuels. At rated output, 31% maximum BTE was attained for CPOT5 whereas neat diesel, CPOT10 and CPOT15 reached 30.5%, 29.5% and 28.5% BTE respectively.

Fig. 5
figure 5

Variation of BTE with BP

Full size image

Owing to having far lower flash and ignition points compared to diesel and CPO, toluene addition to the mixture seems to have triggered the primary phase combustion of the spray before the expected point and thus lowered the duration of combustion. Higher Calorific Value and Shorter Combustion durations could be the possible reasons behind the marginal improvement in BTE for CPOT5. Secondary atomization of fuel which resulted in an optimized mixture formation for CPOT5 might also be the associated reason. At all the BP conditions, BTE for CPOT10 and CPOT15 was lower than Diesel due to the poor spray formations resulted from the higher viscosities, lower CV and higher densities.

5.1.2 Brake specific fuel consumption

BSFC conveys the engine performance with reference to fuel economy. It measures the amount of fuel consumed in unit time to deliver unit Brake Power Output. Figure depicts the variation of BSFC with BP for all the tested fuels. From the Fig. 6 it can be seen that BSFC decreased with increasing BP outputs for all the tested blends. CPOT5 consumed lower fuel when compared to the rest at all the BP outputs. The rise in BSFC for CPOT10 and CPOT15 over Diesel could be associated to their comparative lower Calorific values, higher viscosity and densities. Toluene presence in the fuel mixtures makes the combustion time shorter thereby leading to an earlier pressure rise before the expected point on the combustion curve. This condition inhibits the intended contrived operation inside the cylinder and thus more fuel has to be fed to deliver the same power output, which results into an increased BSFC. However, the low thermal value of the chemicals added (like toluene) to the fuel can also increase BSFC and there are studies which advocates the same [35, 36] which mentions lower cetane and lower thermal value of the added chemical as the plausible reasons.

Fig. 6
figure 6

Variation of BSFC with BP

Full size image

At the rated BP condition, CPOT5 showed 4.7% lower BSFC over neat diesel. Having greater calorific value and higher energy density could be the reasons. Studies [22, 37] shows the BSFC reduced due to the improved combustion when toluene like chemical with low flash point and ignition temperatures are added.

5.1.3 Exhaust gas temperature

Energy lost to the exhaust i.e. the effective energy utilization is typically reflected by the Exhaust gas temperatures. Figure 7 represents the variation of EGT with BP for all tested fuels.

Fig. 7
figure 7

Variation of EGT with BP

Full size image

It can be understood from Fig. 7, that with increase in BP outputs EGT also increased for all the fuels.. This could be explained by the cause that more amount of the fuel has to be injected to produce increased BP outputs, which in turn leads to increasing EGTs. Extended EGTs over neat diesel fuel for the higher blends (CPOT10 and CPOT15) were might be due to the occurrence of partly more combustion in the stage of diffusion and their late burning nature (owing to excess amount of oxygen). For CPOT5, EGT was found closer but however lower to Diesel. At the maximum BP condition, highest EGT observed for CPOT15, CPOT10, Diesel and CPOT5 were 337 \(^\circ \)C, 326 \(^\circ \)C, 314 \(^\circ \)C and 305 \(^\circ \)C respectively. Though few literature studies [38] assessing the fuels similar to toluene showed reduced EGTs for all the tested fuels, in this work effect of toluene in reducing EGTs for CPOT10 and CPOT15 blends seems to have been outmatched by the impact of excess oxygen combustion.

5.2 Emission parameters

5.2.1 CO emission

CO emissions off the CI engine are generally lower than SI engine as the CI engines are operated in Oxygen enriched atmosphere. Main cause of CO emissions would depend on air–fuel mixtures, inferiorly formed combustion mixture, shorter periods of combustion, lack of sufficient Oxygen, etc. [39].

Figure 8 depicts the variation of percentage of CO emission with increasing BP output for all the tested fuels and the graph conveys that CO emissions increased with increasing Brake load, but with a steep rise in 3.3–4.4 kW range. There are some research works in the literature whose results found in agreement with the case [40,41,42,43,44]. From the Fig. 8 it can be realized that CO emissions for Diesel are lower than CPOT10 and CPOT15 as the Diesel is a free-from-Oxygen fuel. CPOT5 was observed to be the fuel with lowest CO emissions amongst the tested as the packed energy density of the CPOT5 is superior to the rest. At the maximum BP condition17%, 15%, 10% and 8% CO emissions were recorded for CPOT15, CPOT10, Diesel and CPOT5 respectively. Fuel richness, poor energy density and inferior Calorific values of the blends CPOT10, CPOT15 led to the partial oxidation of carbon atoms and resulted in formation of CO. CPOT5 was demonstrated to be the optimally proportioned fuel blend. It is observed that there are some previous studies conveying both decreased [45] and increased [46] CO emissions when the low ignition chemicals are mixed with fuels.

Fig. 8
figure 8

Variation of CO emissions with BP

Full size image

5.2.2 UBHC emissions

The Comparative difference in the un burnt hydro carbon emissions discharged off the CI engine supplied with neat Diesel, CPOT5, CPOT10, CPOT15 were presented in Fig. 9 which instantiate that the UBHC emissions increased with increasing BP.

Fig. 9
figure 9

Variation of UBHC with BP

Full size image

At the maximum Brake power conditions, the recorded UBHC were 99 ppm, 97 ppm, 91 ppm and 87 ppm for CPOT15, CPOT10, Diesel and CPOT5 respectively. Higher UBHC emissions for CPOT15 and CPOT10 over Diesel might be due to the latent heat of vaporization of the moisture as well as toluene content present, which could results in quenching of flame at the chamber walls. Relatively higher densities, viscosities and bulky oxidation reactions taking place at low temperature conditions might also be the possible reasons for higher emission of UBHC over Diesel. These results found similar to some of the previous studies of literature [47, 48]. CPOT5 was found to be with lowest UBHC emissions among the tested and the associated reasons could be the higher Heat release rates, optimal mixture formation which assists in proper and qualitative combustion.

5.2.3 NOx emissions

Figure 10 portrays the variation of NOx emissions for all the tested fuels with increasing BP and conveys that NOx emissions are increased with increasing BP outputs for all the fuels tested. Ignition delay, in-cylinder temperatures and air/fuel mixture proportions are some of the factors that influence the NOx formations.

Fig. 10
figure 10

Variation of NOx with BP

Full size image

It can be seen from the graph that at peak load conditions, NOx emissions registered for CPOT15, CPOT10, Diesel and CPOT5 are 498 ppm, 478 ppm, 458 ppm and 426 ppm respectively CPOT10 and CPOT15 blends resulted in higher NOx when compared to neat diesel due to excess oxygen amount causing higher heats released and thereby making the cylinder environment favourable to form NOx. Sudden rise in cylinder pressure and temperatures due to the addition of toluene could also be the reasons to increase NOx levels. CPOT5 have shown 8% decreased NOx emissions over neat diesel at the rated BP. Lower latent heat of vaporization of toluene could be an attributed reason. There are literature studies indicating both reduced [49] and increased [47] NOx emissions upon adding chemicals like toluene.

5.2.4 Smoke emission levels

The suspended soot particles in the exhaust gases off the engine generated in the diffusion stage of the combustions and higher levels of Smoke emissions reflects the combustion inefficiency. Figure 11 shows the Smoke emission levels variation with varying BP conditions and it can be understood from the graphs that with increasing BP, Smoke levels also increased almost linearly as the increased amounts of fuel was drawn into the cylinder to produce BP.

Fig. 11
figure 11

Variation of smoke levels with BP

Full size image

From Fig. 11 it can be learnt that smoke emission levels for CPOT15, CPOT10 and CPOT5 are lower than neat Diesel at all the engine BP outputs. Water droplets in the form of moisture present in the Pyro Oil could results into the secondary atomization due to the turbulent explosions off the heating and leads to the emission of comparatively lesser Smoke over neat diesel.. In case of neat diesel, the sucked-in fuel droplets were large enough (due to poor atomization) to cause inferior spray formation and also Diesel being an oxygen–free-fuel combined with inadequate combustion time paves the way to the emission of solid carbon particles in the exhaust stream of gases. Adding toluene makes the fuel mixture poorer and mitigates the rich mixture regions and thus expected to reduce the smoke levels. Some literature findings [50,51,52] are found sound similarity with the current results. AT the rated brake power, 12.5%, 6.9% and 4.1% decreased smoke levels over neat diesel are recorded for CPOT15, CPOT10 and CPOT5 blends.

5.3 Combustion parameters

5.3.1 Ignition delay

The ignition delay (ID) is defined as the duration between the start of fuel injection for the process of pre-ignition and the start of dectectable combustion. Figure 12 shows the variation of ignition delay period with engine brake power output for all the tested fuels. Compared to CPOT5, remaining tested fuels showed higher ID at all power outputs. It was noted as 10° CA ID for CPOT5 and 10.3 \(^\circ \)CA, 10.8 \(^\circ \)CA, 11.3 \(^\circ \)CA ignition delays were found with diesel, CPOT10 and CPOT15 repectively, at full load condition. The increase in ignition delay with the blends of pyro oils could be due to the lower temperatures of the combustion chamber, and aslo might be owing to relatively higher water content present in CPOT10 and CPOT15. The high latent heat of vaporisation of water resulted in lowering the temperature of the charge during compression and increased the ignition delay. This was more pronounced at low engine power outputs for all the fuels..

Fig. 12
figure 12

Variation of ignition delay with BP

Full size image

5.3.2 Maximum rate of pressure rise

The variation of maximum rate of pressure rise (MRPR) at different power outputs for Diesel and tested pyro oil blends with BP are reported in Fig. 13. All the tested fuels resulted in lower peak pressure as compared to CPOT5 at all operating conditions. In CI engines, peak pressure inside the cylinder is affected mainly by the amount of fuel charge involved in the first stage of combustion (called the uncontrolled combustion). The high viscosity and density of the blends of pyro oil resulted in poor mixture preparation with air, and hence the rate of pressure rise was reduced. Longer ingnition delays causes the more fuel accumulation before combustion begins and results in higher rates of pressure rise. Thus, CPOT5 and diesel showed increased MRPR over the rest.

Fig. 13
figure 13

Variation max. rate of pressure rise with BP

Full size image

5.3.3 Combustion duration

The combustion duration in CI engine is known as the time period between the ignition point (which accounts for 10% of the fuel burnt) and the end of combustion (which accounts for 95% of the burned fuel). The total combustion duration increased with the CPOT10 and CPT15 as compared to diesel and CPOT5 at all power outputs as seen in Fig. 14. Late combustion phases for CPOT10 and CPOT15 were seems delayed due to their higher density and viscosity value. CD was noted as 42 \(^\circ \)CA, 41 \(^\circ \)CA and 39 \(^\circ \)CA respectively with CPOT15, CPOT10 and CPOT5 at peak power output.

Fig. 14
figure 14

Variation combustion duration with BP

Full size image

5.3.4 Heat release rate

Figure 15 illustrate the variation of HRR for diesel, CPOT5, CPOT10, and CPOT15 at the rated power outputs of engine. HRR is calculated from the energy balancing equations using pressure and crank angle diagram data. It is observed that CPOT5 resulted in the maximum HRR and the amount of the fuel consumed during the initial phase of pre-mixed combustion was lower as well as the rate of diffusion combustion was slightly higher for diesel, CPOT10 and CPOT15 as compared to CPOT5. As the fuel gets evaporated during the ignition delay period, led to negative HRR at the beginning i.e. before starting of combustion as seen in Fig. 15. It can be explained that the CPOT15 owing to high moisture content, its cetane number gets decreased and this resulted in prolonged ignition delay and thus with CPOT10 and diesel as compared to CPOT5. Owing to the high viscosity and density of the blends, later stage combustion occurred. As expected, CPOT10, CPOT15 showed lower peaks when compared to CPOT5.

Fig. 15
figure 15

Variation of heat release rate with crank angle

Full size image

6 Conclusions

CI engine performance and emissions characteristics have been explored when 5(vol.) % of toluene was added to various CPO-Diesel mixtures.

Toluene was found to be well miscible in CPO-Diesel blends and no phase separation occurred over a week. Impact of Toluene addition to fuel in ameliorating engine characteristics has been found moderate. Performance results of CPOT5 revealed 1.64% higher BTE, 5% lower BSFC and 2.9% lower EGTs than neat diesel at rated power output. Meanwhile 4.4%, 20%, 7% and 4.17% decreased emissions of UBHC, CO, NOx and smoke were recorded. Compared to diesel, decreased smoke emissions were observed at all BP outputs for tested fuels. 13%, 8% reductions in ID were observed for CPOT5 over CPOT10 and CPIOT15 respectively. Reduced CD, increased HRR and MRPR were showed for CPOT5 fuel. Engine characteristics with CPOT10, CPOT15 fuels were found inferior to diesel.

Though CPOT5 was found optimal among the tested, its viability to be an effective alternative fuel seems feeble as the improvements observed were marginal.

7 Future scope

The research could further be extended to analyse various blend ratios of toluene in different bio-oils and their impact on engine parameters. So far, most of the research works focussed on improving engine attributes such as BTE, BSFC, exhaust emissions by employing various alternative fuels. However, the studies revealing the long-term impacts of these substitutes on engine i.e. extent of oil contamination, injector nozzle, piston geometry, combustion chamber, etc. are found scanty and recommended for future work. Recovering the harmful aromatic organic compounds such as toluene from the exhaust emissions (if any) is also a challenge.