Experimental investigations of Soyabean and Rapeseed SVO and biodiesels on engine noise, vibrations, and engine characteristics

Fuel 238 (2019) 86–97

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Full Length Article

Experimental investigations of Soyabean and Rapeseed SVO and biodiesels on engine noise, vibrations, and engine characteristics Chetankumar Patela,b, Nachiketa Tiwarib, Avinash Kumar Agarwala,b, a b

T

⁎

Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Soyabean biodiesel Rapeseed biodiesel Combustion Noise and vibrations Emission characteristics

Genset engines are popular for power backup as well as for agricultural applications in rural areas. However these engines are noisy and emit toxic pollutants. It is therefore important to develop noise reduction methodologies for genset engines since noise regulations are becoming increasingly stringent globally. Experimental investigations were carried out on a single cylinder engine to assess combustion, noise and vibration characteristics using Soyabean and Rapeseed biofuels (straight vegetable oil (SVO) and Biodiesel). In addition, performance and emission characteristics of these fuels were also investigated. Through this study, it has been found that there is a strong correlation between the heat release rate (HRR) and combustion noise as well as external noise. Higher was the HRR, higher were the magnitudes of combustion noise and external noise. Similar correlation also existed for vibrations.

1. Introduction Indian economy has been growing rapidly over the last few decades. To meet rapidly growing energy demand, India imports crude oil in very large quantity to fulfil requirement of petroleum products in various sectors of the economy. It is important for a developing country like India to look for alternatives such as biofuels for attaining selfsustainability in a price-sensitive and highly volatile crude oil market. Fossil fuels such as diesel, gasoline, compressed natural gas (CNG) are utilized in internal combustion (IC) engines to meet the energy requirements. Compression ignition (CI) engines are largely used for catering to small-scale and large-scale stationary applications such as power generation as well as for mobile applications such as in transport sector, but they are largely dependent on fossil fuels such as mineral diesel. Vegetable oils are easily available renewable alternative liquid fuels, which can be utilized in CI engines. However their high viscosity makes it difficult to use them directly in the engines. Vegetable oils can be utilized in IC engines in five different ways: (i) direct use/blending, (ii) micro-emulsion, (iii) pyrolysis, (iv) transesterification, and (v) hydrotreated vegetable oil [1,2]. Biodiesel is produced by transesterification process using primary alcohols such as methanol and ethanol and basic catalysts such as NaOH or KOH/acidic catalysts/enzymes. Biodiesel produced from vegetable oils by using methanol/ethanol are called fatty acid methyl/ethyl esters (FME/FEE) with glycerol as a byproduct. Biofuels are emerging as attractive eco-friendly renewable ⁎

alternative fuels to displace conventional mineral diesel. Many researchers have investigated performance, combustion and emission characteristics of biodiesel produced from different feedstocks. Noise and vibrations investigations of IC engines have also attracted attention of researchers in last decade because of increasing awareness about their adverse health effects. Several researchers have conducted combustion investigations of biofuels vis-à-vis baseline mineral diesel. It is reported that the maximum heat release rate (HRRmax) is lower for biodiesels/blends compared to baseline mineral diesel [3–8]. Shahabuddin [3] exhibited that fuel properties such as relatively lower calorific value, shorter ignition delay and higher viscosity of biodiesels were responsible for slightly lower HRR of the biodiesel-fueled CI engine. Can [4] reported reduction in HRRmax for 5 and 10% blends of waste cooking oil (WCO) biodiesel vis-à-vis baseline mineral diesel at full load and part loads. This was due to lower fuel quantity available in the combustion chamber at the time of ignition because of shorter ignition delay period and relatively lower calorific value of biodiesel. Agarwal and Dhar [8] reported relatively lower HRRmax in premixed combustion phase of Karanja oil and its blends compared to baseline mineral diesel. Several researchers investigated noise and vibrations characteristics of biodiesel fuelled engines. Higher combustion noise from biodiesel and its blends compared to baseline mineral diesel was the general trend reported by several researchers [9–13]. Differences in combustion phasing and injection rates were responsible for relatively higher

Corresponding author at: Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India. E-mail address: [email protected] (A.K. Agarwal).

https://doi.org/10.1016/j.fuel.2018.10.068 Received 22 May 2018; Received in revised form 9 September 2018; Accepted 10 October 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.

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consisted of a resistive load bank, a voltmeter, and an ammeter. Intake air-flow rate measurement was done using an orifice plate mounted on a surge tank and a U-tube manometer connected across the orifice plate. Fuel consumption rate was measured using a volumetric fuel flow rate measurement unit. A five gas emission analyser (AVL, 4000) was used for the measurement of exhaust gas emission species, while the smoke opacity was measured by smoke opacimeter (AVL, 437). Table 3 shows the measurement principle, range and resolution of exhaust species measured by the emission analyser. In-cylinder pressure signals were acquired by a piezoelectric pressure transducer (Kistler, 6613CQ09-01) which were amplified by a charge amplifier (Kistler; 5015). A precision shaft encoder (Encoders India; ENC 58/6-720ABZ) was connected to the engine crankshaft, which provided accurate information of the piston position with 0.5° crank angle resolution. A high speed combustion data acquisition system (Hi-Techniques, MeDAQ) was used to acquire in-cylinder pressure data and shaft encoder signals simultaneously for further analysis. In-cylinder pressure-crank angle (P-θ) data was further analysed to calculate the heat release rate (HRR).

combustion noise from biodiesels/blends [10]. Lee et al. [13] reported strong correlation between HRRmax and engine vibrations. Patel et al. [14] also reported strong correlation between HRRmax and combustion noise/vibrations in the direction of piston movement of the engine. Giakoumis et al. [15] suggested that higher combustion noise was a result of higher HRRmax in the premixed combustion phase. Patel et al. [16] reported higher combustion noise in case of 20% (v/v) Karanja biodiesel blend (KB20) due to relatively shorter ignition delay and higher HRRmax, while it was relatively lower for Karanja biodiesel (KB100). Giaokoumis et al. [17] reported slightly higher noise with 30% biodiesel blend compared to baseline mineral diesel. However some researchers also reported reduction in noise and vibrations [18–23] with biodiesels/blends. These was attributed to enhanced lubricity, vibration damping properties and relatively lower HRRmax of biodiesel [19]. Patel et al. [20] reported 1–3 dB(A) reduction in combustion noise and external noise for 20% (v/v) SVO blended with mineral diesel compared to baseline mineral diesel. Uldumar et al. [23] investigated noise and vibrations characteristics of an unmodified four-stroke, four-cylinder, CI engine using 20% and 40% (v/v) blends of sunflower, canola and corn biodiesels with mineral diesel. They reported reduction in vibrations of the engine block for biodiesel blends. They also reported slight reduction in sound pressure level (dB(A)) for biodiesel blends. Taghizadeh-Alisaraei et al. [24] investigated two four-stroke diesel engines, a single cylinder engine and a six- cylinder engine using six test-fuels namely B20, B40, B60, B80, biodiesel (B100) and mineral diesel (D100). They reported relatively higher vibrations from B20 and B40, while lower vibrations from B100 and B80, compared to baseline mineral diesel. Taghizadeh-Alisarai and Rezaei-Asl [25] reported 4.79% increase in engine vibrations with 6% (v/v) bioethanol blended with mineral diesel. They concluded that increase in vibrations was due to variations in cylinder pressure as well as the inertia of engine components. Many researchers conducted noise and vibration investigations on automotive engines, which use relatively higher fuel injection pressures (FIP) and operate at variable engine speeds. Present study was conducted on a genset engine, which has relatively lower FIP due to deployment of a mechanical fuel pump and injector, and they operate at a constant engine speed. It is equally important to carry out such a study in Indian context, due to presence of more than 40 million such engines in India in decentralized power generation, agriculture and water pumping sectors of the economy. Literature review suggests that noise and vibrations characteristics and their correlation with combustion characteristics especially for Soybean and Rapeseed biofuels is not available in open literature. To establish correlation between these characteristics and to fill the research void, comparative experimental investigations were carried out using Soyabean biodiesel (SB100), 20% (v/v) Soyabean biodiesel blend (SB20), 20% (v/v) Soyabean oil blend (S20), Rapeseed Biodiesel (RB100), 20% (v/v) Rapeseed biodiesel blend (RB20), 20% (v/v) Rapeseed oil blend (R20) and baseline mineral diesel, as test fuels. In addition, performance and emissions characteristics of these test fuels were also investigated and an attempt has been made to correlate these characteristics with engine’s noise and vibration characteristics.

dQ (θ) 1 ⎛ dP (θ) dV (θ) ⎞ V (θ) = + γP (θ) dθ γ − 1⎝ dθ dθ ⎠

(1)

whereQ = heatrelease, P= in-cylinder pressure, V = cylinder volume and γ = polytropic coefficient of combustion gas. Cumulative heat release (CHR) was calculated by integrating the HRR curve. The maximum and saturated values of CHR was set at 100% mass burn fraction (MBF). This parameter was then used to identify the crank angle position corresponding to MBF of 10% and 90%. The difference in crank angle positions for these values of MBF were taken as ‘combustion duration’. Combustion noise data was acquired and processed by another data acquisition system (NI Drivven; µDCAT) using the in-cylinder pressure traces. Crank angle based pressure signals were converted into time based cylinder pressure signals. FFT of these time-based pressure signals was done to convert them into frequency domain. Sum of all harmonics between the 1/3rd octave frequencies were done to convert this frequency domain data into the 1/3rd octave spectrum or band. These data sets were further filtered through human ear (A-weighting) and structure attenuation filters. These results were presented in the combustion noise (dB(A)) format. External engine noise was measured by a micro-phone (B&K; 4192) placed at 1 m distance from the engine, as per standard practice. B&K 4192 microphone is an externally polarized pressure field microphone having a sensitivity of 12.5 mV/Pa. These signals were acquired using a software (NI; LabView signal express), a high speed data acquisition chassis (NI; cDAQ 9178) and another card module (NI; 9232) at a sampling rate of 102.4 kS/s. Engine noise was measured as sound pressure level (SPL) using Eq. (2).

P Sound pressure level (SPL) = 20 log10 ⎛ rms ⎞ dB ⎝ P0 ⎠ ⎜

⎟

(2)

where,

2. Experimental setup

Prms = rms value of sound pressure, measured by the microphone; P0 = Reference sound pressure = 2 × 10−05 Pa

Fig. 1(a) shows the experimental setup for measurement of engine performance, combustion, emissions, noise and vibrations characteristics. Fig. 1(b) shows the accelerometer positions on the engine and Fig. 1(c) shows the directions of the vibration measurements. Table 1 shows important properties of seven test fuels and Table 2 shows the specifications of the test engine. Experimental investigations were carried out in a single cylinder direct injection compression ignition (DICI) engine, which is typically used for gensets and agricultural farm machine applications world-over. This engine was coupled to an AC generator and a control panel, which

Since human ear has varying sensitivities for different frequencies, A-weighting filter was utilized for representation of noise data. This data was represented in rms value. Vibrations were measured in vertical, longitudinal and lateral directions using miniature tear drop CCLD accelerometers (B&K; 4517). A miniature tear drop CCLD accelerometer (B&K; 4517) was used to measure the lateral vibrations. It has a sensitivity of 10 mV/g. One accelerometer was used for measurement of vibrations in a single direction only. Vibrations data was also presented in rms value. 87

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Fig. 1. (a) Schematic of the experimental setup, and (b) Accelerometer positions on the engine (c) Vibration directions.

3. Results and discussion

Combustion in diesel engine is largely affected by factors such as quality of fuel-air mixture, cetane number, presence of long or short chain unsaturated fatty acids etc. These factors largely affect premixed combustion and mixing controlled combustion phases. SB100 has higher cetane number, presence of long-chain unsaturated fatty acids and inferior air-fuel mixing characteristics, which lead to reduction in HRRmax during premixed combustion phase. However, this was favourable for mixing controlled combustion phase. SB100 exhibited higher HRR in mixing controlled combustion phase compared to baseline mineral diesel due to higher cetane number, and higher load improved combustion in this phase. HRRmax was the highest for baseline mineral diesel, followed by S20, SB20 and SB100 in the premixed combustion phase. Ignition delay for SB20 was lower than other test fuels. Biofuel blends have superior evaporation rate compared to SB100

3.1. Combustion investigations Fig. 2(a) shows the variations in cylinder pressure for select test fuels with varying engine load. It was observed that in-cylinder pressure increased with increasing engine load. Diesel showed higher in-cylinder pressure from no load and 60% load, while at 100% load, SB20 showed slightly higher in-cylinder pressure compared to other test fuels. Fig. 2(b) shows the HRR for select test fuels. Typical HRR curve of a diesel engine has three important stages namely ignition delay, premixed combustion, and mixing controlled combustion. Large part of combustion heat release takes place in premixed combustion phase, followed by mixing controlled combustion phase for all test fuels.

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Fig. 1. (continued)

Fig. 3 shows the variations in HRRmax for different test fuels compared to baseline mineral diesel. It is seen that all three test fuels (S20, SB20, SB100) exhibit relatively lower HRRmax at all loads compared to baseline mineral diesel. SB100 generated the lowest HRRmax. RB100 generated 17.5% to 31% lower HRRmax compared to baseline mineral diesel. S20, SB20, R20 and RB20 also generated lower HRRmax by 8.5% to 31%. Lower HRRmax for Soyabean and Rapeseed biodiesels were mainly attributed to their relatively inferior spray atomization characteristics and lower calorific value compared to baseline mineral diesel. Biodiesels have lower calorific value, higher viscosity and higher inherent fuel oxygen content compared to baseline mineral diesel. Evaporation rates of biodiesels are also lower than baseline mineral diesel. This results in delayed combustion, which lowers the HRRmax for Soybean and Rapeseed biodiesels. However biodiesel blends show improvement in combustion, resulting in slightly higher HRRmax compared to 100% biodiesel. Evaporation rate of sprays of biodiesel blends improved due to their lower SMD compared to 100% biodiesel. Inherent oxygen content helps in obtaining superior combustion [26–30]. Similar results were also reported by Gumus et al. [31]. They reported that compared to biodiesels, mineral diesel fuelled engines exhibited higher HRR. Authors attributed this to higher volatility, superior fuel-air mixing, smaller spray droplet size distribution and higher spray droplet velocities in case of mineral diesel compared to biodiesels/blends. HRR decreased with increasing biodiesel content in the test blends. In another study, Nagaraja [31] also reported similar results. He also reported that increased biodiesel content in biodiesel blends led to reduction in HRR. Similarly, Can [4], Ozturk [7], and Agarwal and Dhar [8] reported reduction in HRR with increasing biodiesel content in the test blends. Premixed combustion phase is always followed by mixing controlled combustion phase in a CI engine. In mixing controlled combsution phase, slightly higher HRR was observed for biodiesels, their blends and SVO blends compared to baseline mineral diesel (Fig. 2(b)). This trend was due to relatively slower evaporation, mixing and combustion of biofuels in the premixed combustion phase. Fig. 4 shows cumulative heat release (CHR)-crank angle variations for select test fuels. It is observed that CHR was lower for SB100 compared to baseline mineral diesel, while S20 and SB20 exhibited lower CHR between −10 and 10° CA and higher CHR beyond 10° CA compared to baseline mineral diesel. This was due to relatively slower burning of biodiesels/blends in initial stages of combustion due to their relatively higher viscosity and lower calorific value compared to baseline mineral diesel. Fig. 5(a) shows the start-of-combustion (SoC), the end-of-combustion (EoC) and overall combustion duration (CD) in crank angle degrees

Table 1 Important properties of test fuels. Test fuel

Calorific value (MJ/kg)

Density (g/cm3) (ASTM-D 4052)

Kinematic viscosity @ 40 °C (cSt) (ASTM-D445)

Cetane number (CN)

Mineral Diesel S20 SB20 SB100 R20 RB20 RB100

43.07

0.82

2.71

52

42.7 42.7 41.3 42.6 42.6 41.6

0.836 0.823 0.873 0.834 0.829 0.872

4.17 3.14 4.55 4.20 2.87 4.493

– – 65 – – 54

Table 2 Specifications of the test engine. Engine parameters

Specifications

Manufacturer Engine type

Kirloskar Oil Engines Ltd., India Vertical, four-stroke, single-cylinder, constant speed, direct injection CI engine 7.4 kW @ 1500 rpm 102 mm/116 mm 0.948 l 17.5 26° bTDC 200 bar Water Cooling 685/532/850 mm 6.21 bar

Rated power Bore/stroke Displacement volume Compression ratio Start of fuel injection timing Nozzle opening pressure Cooling type Length/width/height BMEP at 1500 rpm

Table 3 Measurement principle, range and resolution of different species in the emission analyser. Emissions

Resolution

Range

Measurement principle

NOX CO HC CO2

1 PPM 0.01% (v/v) 1 to 10 PPM 0.1% (v/v)

0–5000 PPM 0–10% (v/v) 0–20000 PPM 0–20% (v/v)

Electrochemical Sensor NDIR NDIR NDIR

along with lower presence of long-chain unsaturated fatty acids due to blending with mineral diesel. This leads to superior spray atomization and vaporisation of SB20, and improved combustion characteristics resulting in higher HRRmax compared to SB100. 89

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Fig. 2. (a) Variations in cylinder pressure w.r.t. crank angle; and (b) Variations in HRR w.r.t. crank angle for various biofuels.

Fig. 4. Variations in cumulative heat release with crank angle for various biofuels. Fig. 3. Variations in HRRmax for various biofuels.

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Fig. 5. (a) Start-of-combustion (SoC), end-of-combustion (EoC) and combustion for duration; and (b) Variations in combustion duration for various biofuels.

noise [32]. Combustion noise is caused by rapid cyclic pressure rise in the engine combustion chamber. Mechanical noise is primarily attributed to rotating and reciprocating engine components. It originates from inertial forces, which cause piston slap (impact of piston on the cylinder wall), most notably when piston moves from top dead centre (TDC) to the bottom dead centre (BDC) during expansion stroke, and from gears, tappets, valve trains, timing drives, fuel injection equipment and bearings [33]. Combustion characteristics such as cylinder pressure rise and heat release play significant role in noise emanating from the engine [22,32,33]. Fig. 6(a) shows combustion noise characteristics for select test fuels. Combustion noise initially increased with increasing engine load, and peaked at approximately 40% rated load before starting to decrease again. Since sound pressure level can be directly correlated with changes in cylinder pressure (a parameter strongly coupled to the HRR), it makes sense to examine the correlation between combustion noise and HRR. From Fig. 2(b), it was noted that very much like combustion noise level, HRRmax also initially increased with increasing engine load and peaked at ∼60% load in premixed combustion phase. Beyond this load, it reduced. Another important observation for SB100 was that it had relatively lower HRRmax and it was also reflected by the lowest combustion noise for SB100 amongst all test fuels. Fig. 7(a) shows variations in combustion noise for all test fuels compared to baseline mineral diesel. Comparing this data with that in Fig. 4, it is evident that there is a strong correlation between HRRmax and combustion noise. Higher the HRRmax, higher is the combustion noise. These observations are valid for all test fuels. It was also observed that most test fuels produced lower combustion noise compared to baseline mineral diesel at all engine loads. Combustion noise for most test conditions were ∼0.25 to 7 dB(A) lower than that for mineral diesel. Finally, it can also be noted that increasing biodiesel content in the test fuels reduced the combustion noise. This observation can also be explained in reference to the observations made for HRRmax. Combustion noise also depends on the cetane number of the test fuel. Biodiesel (B100) has higher cetane number than baseline mineral diesel,

for different test fuels at varying engine loads. These plots were derived from cumulative heat release (CHR) curves shown in Fig. 4. It is observed from Fig. 5(a) that at low loads, all test fuels start burning at roughly the same time, since the overall spread of SoC at these loads does not exceed 3 CAD. However the EoC differs significantly amongst these test fuels. Specifically EoC for baseline diesel was the earliest at all engine loads, while for SB100, it was the farthest. As a result, the overall combustion duration for baseline mineral diesel was the smallest, while for SB100, it was the longest. CD for S20 was longer than the baseline mineral diesel. CD is mainly affected by the evaporation rate of biodiesels. All biodiesels have different evaporation rates due to their different molecular structures. Evaporation rates of biodiesels were generally observed to be slower than mineral diesel. Slower evaporation rate of biodiesels (RB100, SB100) were due to their larger droplet size distribution and relatively inferior spray atomization characteritics compared to baseline mineral diesel [27]. This resulted in longer CD at lower engine loads for biodiesels. However CD for mineral diesel and SB100 were almost similar at 100% load due to relatively higher incylinder temperatures for SB100. This resulted in improvement in combustion characteristics and shortening of CD for SB100 compared to baseline mineral diesel. With increasing engine load, fuel-air ratio increased. Consequently, more time was required for higher quantieis of test fuels injected to burn in the combustion chamber. Hence CD for all test fuels increased appreciably at higher engine loads. This increase in CD due to increasing engine load was the largest for mineral diesel and the smallest for SB100. Hence the difference in CD vis-à-vis baseline diesel (Fig. 5(b)) reduced with increasing engine load. Similar trends were also observed for other test fuels namely RB20 and RB100. 3.2. Noise and vibration investigations There are three main sources of noise in an engine. These are: (i) combustion noise, (ii) mechanical noise, and (iii) intake and exhaust 91

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Fig. 6. (a) Combustion noise, and (b) Engine noise for various biofuels.

in the range of 92.2–95.3 dB(A) and it was higher than the combustion noise at all loads. Majority of biofuels exhibited ∼1 to 3 dB(A) lower external noise compared to mineral diesel except RB20 at no load and full load and RB100 at full load. In general, external noise increased with increasing engine load. To understand this phenomenon of external noise, it must be understood that external noise from an engine has many sources, including engine combustion noise from the combustion chamber,

which leads to a reduction of a premixed combustion phase, that is directly related to reduction in rate of pressure rise (RoPR) and HRR. Hence this leads to reduction of combustion noise [Rabl et al.]. It is observed that HRR and RoPR are always higher in the premixed combustion phase compared to diffusion combustion phase, which results in higher combustion noise [34]. Fig. 6(b) shows the engine noise for select test fuels. It was observed that external noise for the engine fuelled by baseline mineral diesel was

Fig. 7. (a) Variations in combustion noise, and (b) Variations in external noise for various biofuels. 92

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directions compared to Soyabean based biofuels. S20 and SB20 showed higher vibrations in longitudinal direction compared to baseline mineral diesel. How et al. [2] also reported reduction in vibrations due to relatively lower peak RoPR from biodiesel blends compared to baseline mineral diesel. Test fuel oxygen content and reduction in peak RoPR were responsible for lower engine vibrations in case of biodiesel blends [23]. Variations in HRR also reflected variations in RoPR. In this study, HRR variations of test fuels were taken into considerations for establishing correlation with vibrations. Fig. 9(a) shows the vibrations in vertical direction from biofuels compared to baseline mineral diesel. It was observed that for most SVOs and biodiesel blends, vibrations in vertical direction showed a reduction of ∼8 to 30% at all loads compared to baseline mineral diesel. This was consistent with earlier observations of combustion noise, HRR, HRRmax and external noise trends for these test fuels. Thus in general, vertical vibrations, combustion noise and HRR are positively correlated. If HRR in the premixed combustion phase decreases, so does the combustion noise, and so do the vertical vibrations. One test fuel, which didn’t follow this trend was RB20 for some unexplained reasons. Fig. 9(c) shows the comparison of vibrations in lateral direction for biofuels compared to baseline mineral diesel. Vibrations in lateral direction showed ∼1 to 35% reduction compared to baseline mineral diesel except for RB20 and RB100 at 40% load. Vibrations in lateral direction showed a strong correlation between HRRmax, combustion noise, and engine noise. Fig. 9(b) shows comparison of vibrations in the longitudinal direction compared to baseline mineral diesel. Vibrations in longitudinal direction increased for majority of biofuels compared to baseline mineral diesel. Table 4 shows the HRRmax and combustion noise of biofuels/blends compared to baseline mineral diesel and Table 5 shows the noise and vibrations in vertical direction for biofuels/blends compared to baseline mineral diesel. It is observed from the results that biofuels showed reduction in HRRmax compared to baseline mineral diesel. Similar reduction in combustion noise, engine noise and vibrations in vertical direction were also observed. These results helped us arrive at the conclusion that HRRmax has strong influence on the combustion noise, engine noise and vibrations in the vertical direction.

friction generated noise from various moving engine components, piston slap, and rotating/reciprocating motion generated noise from engine components such as flywheel, piston, connecting rod, crank shaft, radiator fan, pump, etc. All these sources contribute to overall engine noise. Given the complexity of various phenomena taking place inside/outside the engine leading to external noise, a direct one-to-one correlation between combustion parameters and HRR with the external noise may not be feasible. Fig. 7(b) shows the differences in the external noise for the engines fuelled by biofuels vis-à-vis baseline mineral diesel. Majority of biofuels showed reduction in engine noise except RB20 at no load and 20% load, and RB100 at 80% and 100% loads. Except these outliers, the difference in external noise for all test fuels at varying loads relative to baseline mineral diesel was within ± 2.0 dB (A). Such variation in external noise levels from an acoustic standpoint can be barely perceived. Despite numerous external noise sources and variability in the test environment, which remains beyond the experimental control, this study demonstrated good correlation amongst combustion noise, external noise, HRR and HRRmax. Seifi et al. [33] also reported close correlation between noise and HRR for water based emulsified fuels. Fig. 2(a) showed that higher engine loads were associated with higher in-cylinder pressures hence increasing engine load would lead to higher stress on the mechanical components to a greater extent, which in-turn would generate higher external noise. This was observed from Fig. 6(b) also. In addition, such a correlation between in-cylinder pressure and mechanical excitation would be particularly strong in the direction of piston motion and is also reflected in the vibration data corresponding to that direction. This is indeed observed from the plot of vibration data presented in Fig. 8. Fig. 8 shows the amplitude of vibrations in the three directions for the test fuels. Vibrations were observed to be the highest in vertical direction since this is the piston motion direction as well. Baseline mineral diesel exhibited higher vibrations in vertical and lateral

3.3. Performance and emissions investigations From Fig. 10(a), it is observed that brake thermal efficiency (BTE) was marginally lower for most biofuels/blends compared to baseline mineral diesel. The reason is that higher viscosity of biodiesels lead to relatively inferior spray atomization. In addition, lower calorific value of biodiesels require more fuel quantity per cycle to be injected in order to generate the same power output as that of baseline mineral diesel. Blending SVO or biodiesels with mineral diesel therefore results in reduction in fuel viscosity and increase in the calorific value of the test blend. This also improves the spray characteristics, leading to superior combustion and comparable BTE, that is closer to baseline mineral diesel. Few SVO and biodiesel blends (SB20, R20 and RB20) even showed higher BTE than baseline mineral diesel. This improvement in BTE correlated well with corresponding reduction in fuel viscosity which improved the HRR. BTE decreased with increasing biodiesel content in the test blend as well. Increased BTE reduced the BSFC/BSEC. BSEC was slightly higher for biofuels/blends due to their lower calorific values and lower BTE’s compared to baseline mineral diesel. BTE and BSEC are inversely related parameters therefore the parameters responsible for a particular trend in BTE, are also responsible for the reverse trend in BSEC. Relatively inferior spray atomization due to higher fuel viscosity, higher density, higher SMD of the spray droplets and lower calorific value of test fuels were responsible for higher BSFC and lower BTE of biofuels/blends [8,27,35]. Fig. 11(a) shows comparison of brake specific CO emission from

Fig. 8. Cylinder head vibrations at different engine loads for various biofuels. 93

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Fig. 9. Variations in vibrations in (a) vertical, (b) longitudinal, and (c) lateral direction for various biofuels.

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Table 4 HRRmax and combustion noise of biofuels/blends compared to baseline mineral diesel. Property

HRRmax (%)

Combustion noise (dB(A))

Fuels/Load

0

60

100

0

60

100

S20 SB20 SB100 R20 RB20 RB100

−13.5 (↓) −13(↓) −35(↓) −21(↓) −25(↓) −17.5(↓)

−12(↓) −20(↓) −49(↓) −21(↓) −25.5(↓) −31(↓)

−9(↓) −14(↓) −46(↓) −20(↓) −24(↓) −26(↓)

−0.8(↓) −1(↓) −4.7(↓) −1.3(↓) −7.3(↓) −1.4(↓)

−1(↓) −1.3(↓) −2(↓) −2.5(↓) −0.3(↓) −2.1(↓)

−0.5(↓) −1.3(↓) −2.7(↓) −0.1(↓) −1.4(↓) −2(↓)

biofuels vis-à-vis baseline mineral diesel. Majority of test fuels emitted relatively higher CO than mineral diesel. Biofuels emitted higher CO primarily due to higher fuel viscosity, which resulted in higher SMD of spray droplets and relatively lower spray droplet velocities compared to baseline mineral diesel [27]. This led to slightly inferior fuel-air mixing and resultant combustion. Similar observations were also reported by Ozturk [7], wherein higher emission of CO was seen from B10. Valente et al. [36] also reported higher CO emission from B25, B50 and B75 due to (i) higher fuel quantity injected per cycle for biodiesel blends, and (ii) higher test fuel viscosity. Hellier [37] also reported a similar trend. Contradictory results were also reported by some researchers. Various factors affecting CO formation in an engine include fuel density, viscosity, flash point, SMD of spray droplets, latent heat of vaporisation of fuel, and droplet velocity distribution after fuel injection. These factors are responsible for air-fuel mixing in the engine. In case of biofuels, these factors affect fuel-air mixing adversely therefore higher CO emission is observed from biofuel operated engine in most studies. Incomplete combustion of fuel is the main reason for higher HC emissions. Fig. 11(b) shows brake specific HC emissions from biofuels vis-à-vis baseline mineral diesel. It was observed that HC emissions were lower from SB100 and RB100. This was primarily due to combined effect of presence of oxygen in biodiesel molecules, relatively larger SMD of spray droplets, shorter ignition delay but higher combustion duration of biodiesel. These factors lead to slightly lower HRRmax in the premixed combustion phase and lower HRR in mixing controlled combustion phase however longer CD, wherein significant portion of remaining HC in the combustion chamber burns, resulting in lower emissions of HCs. Mixed trends of HC emissions are reported in the open literature. Canacki et al. [38] reported reduction in HC emissions from biodiesel. Anand et al. [39] also reported a similar reduction in HC emission from biodiesels, which was attributed to two factors: (i) shortened premixed combustion phase, and (ii) reduction in stoichiometric air requirement owing to fuel-bound oxygen in biodiesels. However, contradictory results of higher HC emissions were reported by Banapuremath [40] for biodiesels and by Agarwal and Agarwal [41] for Jatropha oil/blends. Fig. 12 shows the variations in brake specific NOX emissions from biofuels compared to baseline mineral diesel. It was observed that majority of biofuels exhibited lower NOX emissions compared to baseline mineral diesel except SB20 and R20. Main NOX formation

Fig. 10. Variations in BTE for various biofuels.

mechanism in CI engines is thermal NOX, primarily due to higher incylinder temperatures [42]. Thus reduction in NOX emissions correlates well with the reduction in HRRmax in the premixed combustion phase, as shown by lower HRRmax, which also showed lower in-cylinder temperatures. Szybist et al. [43] and Zhihao et al. [44] also reported similar correlations between HRRmax in the premixed combustion phase and NOx emissions. 4. Conclusions Experimental investigations were carried out to assess combustion, noise and vibrations, performance and emission characteristics of Soyabean and Rapeseed biofuels (SVOs and biodiesels/blends). Following conclusions emerged from this experimental study. Lower incylinder peak pressure and HRRmax were observed from biofuels compared to baseline mineral diesel. Combustion duration was relatively longer for biofuels compared to mineral diesel. Lower combustion noise (by ∼0.25 to 8 dB(A)) was also observed from biofuels compared to mineral diesel. Lower HRRmax correlated well with lower combustion noise from biofuels. Externally measured noise was relatively higher compared to combustion noise from all test fuels at all loads. Difference in external noise from biofuels compared to baseline mineral diesel was up to 2 dB(A). Despite these low differences, external noise correlated

Table 5 Noise and vibrations in vertical direction of biofuels/blends compared to baseline mineral diesel. Property

Noise (dB(A))

Vibration in vertical direction(g)

Fuels/Load

0

60

100

0

60

100

S20 SB20 SB100 R20 RB20 RB100

−1.2(↓) −2.3(↓) −1(↓) −0.8(↓) 0.7(↑) −0.4(↓)

−0.5(↓) −1.7(↓) −1.1(↓) −1.4(↓) −0.5(↓) 0(↔)

−0.8(↓) −1.1(↓) −0.5(↓) −1.3(↓) 0.8(↑) 1.6(↑)

−17(↓) −13(↓) −8(↓) −21.3(↓) 12(↑) −9.5(↓)

−21(↓) −18(↓) −17.5(↓) −27(↓) 13(↑) −3(↓)

−19(↓) −30(↓) −19(↓) 10(↑) 16(↑) 3(↑)

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Fig. 11. Variations in (a) CO, and (b) HC emissions for various biofuels.

well with the HRRmax. Noise and vibration signatures were found to be closely coupled in the context of CI engines. Similar correlations for vibrational amplitudes were observed. Vibrations in vertical direction were higher than other orthogonal directions, irrespective of the test fuel used. Lower vertical vibrations were observed from biofuels compared to baseline mineral diesel. This correlated well with lower HRRmax. Majority of biofuels exhibited reduction in lateral vibrations. This also correlated very well with HRRmax. Higher longitudinal vibrations were observed for most biofuels at higher engine loads compared to baseline mineral diesel. Biodiesels showed higher CO emission, primarily due to inferior spray atomization. Biodiesels however exhibited relatively lower HC emissions and lower NOx emissions compared to baseline mineral diesel, which correlated well with lower HRRmax. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2018.10.068. References [1] Agarwal AK. Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines. Prog Energy Combust Sci 2007;33(3):233–71. [2] How HG, Masjuki HH, Kalam MA, Teoh YH. An investigation of the engine performance, emissions and combustion characteristics of coconut biodiesel in a high pressure common-rail diesel engine. Energy 2014;69:749–59. [3] Shahabuddin M, Liaquat AM, Masjuki HH, Kalam MA, Mofijur M. Ignition delay, combustion and emission characteristics of diesel engine fueled with biodiesel. Renew Sustain Energy Rev 2013;21:623–32. [4] Can O. Combustion characteristics, performance & exhaust emissions of a diesel engine with a waste cooking oil biodiesel mixture. Energy Convers Manage 2014;87:676–86. [5] Chauhan B, Kumar N, Cho H, Lim H. A study on the performance and emission of a diesel engine fueled with Karanja biodiesel and its blends. Energy 2013;56:1–7. [6] Kegl B. Influence of biodiesel on engine combustion and emission characteristics. Appl Energy 2011;88:1803–12.

Fig. 12. Variations in NOX emissions from various biofuels.

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