Performance and emission characteristics of CNG

Environmental Science and Pollution Research https://doi.org/10.1007/s11356-018-3681-8

RESEARCH ARTICLE

Performance and emission characteristics of CNG-fueled compression ignition engine with Ricinus communis methyl ester as pilot fuel Sunil Kumar Mahla 1

&

Amit Dhir 2

Received: 20 February 2018 / Accepted: 5 November 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract Surge in petroleum prices, its drying sources and degradation in air quality focused interest on renewable energy sources as substitute for existing fuels for internal combustion engines. This study highlights the combustion, performance, and emission characteristics of diesel engines fueled with compressed natural gas (CNG) as primary fuel and castor (Ricinus communis) oil methyl ester (COME) as pilot fuel. COME was produced from non-edible grade Ricinus communis oil. The biodiesel fuel properties and characterization was done as per ASTM D6751 specifications. The CNG was inducted through inlet manifold fumigation at a consistent flow rate of 15 l/min under dual-fuel mode. It is evident from the test results that B20-CNG yields brake thermal efficiency of 23.6% when compared to 25 and 27% for D-CNG and diesel fuel, respectively. The peak cylinder gas pressure was lower in dual-fuel mode when compared to conventional diesel. The emission results show increase in NOx emission by 24.5 and 28.4% for D-CNG and B20-CNG, respectively when compared to baseline diesel fuel at full engine load. There was increase in HC emission by 6.7 and 11% whereas CO emissions decreased by 31.6 and 37.4% for B20-CNG and DCNG, respectively at similar operating conditions. Reduction in smoke opacity by 49.4 and 59.6% was achieved respectively for D-CNG and B20-CNG under dual-fuel mode. On the whole, COME exhibits a better pilot fuel choice for dual-fuel combustion mode in comparison to conventional fossil petroleum diesel in terms of combustion, performance, and emissions characteristics. Keywords Dual fuel . Smoke . Biodiesel . Emissions . CNG

Nomenclature CNG Compressed natural gas COME Castor oil methyl ester LCV Lower calorific value DFM Dual-fuel mode ASTM American Society for Testing and Methods LPM Liter per minute NOx Oxides of nitrogen NaOH Sodium hydroxide SI Spark ignition engine

Responsible editor: Philippe Garrigues * Sunil Kumar Mahla [email protected] 1

Department of Mechanical Engineering, I.K. Gujral Punjab Technical University Campus, Hoshiarpur, India

2

School of Energy and Environment, Thapar Institute of Engineering and Technology, Patiala, India

CI TDC SOI BTE BSEC CO CO2 HC rpm

Compression ignition engine Top dead center Start of injection Brake thermal efficiency Brake-specific energy consumption Carbon monoxide Carbon dioxide Hydrocarbons Revolutions per minute

Introduction Scarcity of fossil fuel sources coupled with environmental degradation has led to an intensive research towards alternative renewable clean fuel technologies around the globe. It is always desirable to improve thermal efficiency and reduce exhaust emissions for conserving energy and addressing climate change issues. Diesel has been the major source of fuel for the transportation sector because of

Environ Sci Pollut Res

higher efficiency and higher power output of the diesel engine. Diesel engines are notorious for high emissions of NOx, particulate matters, and different types of polycyclic aromatic hydrocarbons (Jindal et al. 2015; Singh et al. 2017, Mohsin et al. 2014). Vehicular emissions are mainly responsible for deteriorating air quality in urban areas, which has prompted enactment of stringent emission laws at global level. Dual-fuel operation of conventional CI (compression ignition) engines by using a gaseous fuel as the primary fuel and a small amount of liquid fuel as the pilot fuel is an attractive option especially in developing nations (Paul et al. 2015; Barik and Murugen 2014; Kumar and Kumar 2016; Ashok et al. 2015). It makes a promising way to strike a good balance among sustainable development, energy conservation, and environmental preservation. CNG has a high octane number and is suitable for engines with high compression ratios, with possible improvement in thermal efficiency and power output (Karabektas et al. 2014; Mahla et al. 2018a, b, c; Kumar and Kumar 2016). It mixes rapidly with air to form a homogenous air-fuel mixture for efficient combustion inside engine cylinder and substantial reduction in harmful emissions (Abdelaal and Hegab 2012; Mansour et al. 2001; Qi et al. 2011; Papagiannakis and Hountalas 2003). A dual-fuel engine has characteristic features of both SI (spark ignition) and CI engines. Both liquid and gas fuels are combusted simultaneously under dual-fuel mode. The gaseous fuel mainly acts as primary fuel, with large energy contribution whereas pilot liquid fuel acts as an ignition source with small energy contributes (Mahla et al. 2018b; Mohsin et al. 2014; Semin 2008). The advantage of higher compression ratio of dual-fuel diesel engine permits the use of various gaseous and liquid alternative fuels. However, at certain engine loads, dual-fuel engine shows poor performance and higher emissions of HC and CO (Ramesh and Reddy 2004; Mansour et al. 2001). At higher engine loads, the performance and emissions get improved due to complete combustion of fuels. The literature indicates significant reduction in particulate and soot emissions with natural gas as secondary fuel (Gharehghani et al. 2015; Tarabet et al. 2014; Lounici et al. 2014; Sahoo et al. 2009). Engine modifications (leading to suitable compression ratio, pilot fuel quantity, advancing of injection timing, and increase in pilot fuel injection pressure) and exhaust after treatment such as EGR show some positive impact on the engine performance and emission characteristics (Paul et al. 2015; Ryu 2013; Mustafi et al. 2013). However, a number of researchers have reported a higher NOx emission due to the peak gas flame combustion temperature at higher engine operating loads under dual-fuel mode (Senthilraja et al. 2016; Namasivayam et al. 2010; Nwafor 2000; Papagiannakis 2013). CNG is a low cetane

fuel and cannot be directly used in conventional CI engine. A high cetane pilot fuel is required to initiate ignition in CNG-fueled dual-fuel diesel engine (Gunea et al. 1998; Karabektas et al. 2014; Liu et al. 2013). The nature of pilot fuel under dual-fuel mode has strong influence on the combustion process. The combustion phenomenon under dual-fuel operation is rather complex when compared to CI engine counterparts (Bora and Saha 2016; Hosmath et al. 2016; Mahla et al. 2018c). Biodiesel (mono-alkyl ester) is considered as a good renewable fuel candidate for diesel engines. It is produced through variety of feedstock’s including edible and nonedible oils, animal wastes, sewage sludge, etc. Biodiesel is an environmental friendly fuel and has superior fuel properties when compared to fossil petro-diesel such as renewable nature, better lubricity, non-toxic, free from aromatics and sulfur (Atabani et al. 2012; Pali et al. 2015). Many investigators have reported that biodiesel usage in diesel engine gives lower CO, HC, PM, and sulfur emissions but higher level of NO x emissions (Sahoo et al. 2009; Singh et al. 2017; Hosmath et al. 2016; Mohsin et al. 2014). Among all oils, castor oil has two interesting features; on the one hand, it does not compete with edible oils; on the other, its cultivation does not need high input cost (Keera et al. 2018). India is one of the largest exporters of castor oil (Mahla et al. 2018a). Castor oil extract from castor beans contains 40–55% of oil content, a very high potential when compared to other non-edible oil seeds (Keera et al. 2018; Saez-Bastante et al. 2015). As castor bean is not suitable for human consumption, the utilization of castor seed as energy crop does not compete with food scarcity. The only disadvantage associated with castor oil is high viscosity for its use as biodiesel fuel. To overcome this challenge, its blend with diesel can be used in CI engine which meets the standard specifications. Few scattered documents are available on the utilization of CNG and biodiesel in conventional CI engine under dual-fuel mode. The usage of CNG and biodiesel under dual-fuel mode is eco-friendly due to its clean burning characteristics (Namasivayam et al. 2010; Hosmath et al. 2016; Kumar and Kumar 2016; Paul et al. 2015; Mohsin et al. 2014). Literature reported reduction in CO, HC, and NOx emission levels using diethyl ether (DEE) as an additive along with diesel/biodiesel-CNG dual-fuel operation (Karabektas et al. 2014). Literature lacks on the evaluation of engine combustion, performance, and emissions characteristics using COME with CNG under dual-fuel mode. An attempt has been made in the present study to evaluate the combustion, performance, and emissions characteristics of the unmodified conventional CI engine with CNG-COME combinations at different engine operating loads in comparison to baseline fossil petro-diesel.

Environ Sci Pollut Res Table 1

Fatty acid profile of Ricinus communis (castor) oil

S. no.

Fatty acid name

Structure

wt% age

1

Ricinoleic acid

18:1

83.97

2

Palmitic

16:0

0.46

3 4

Stearic Oleic

18:0 18:1

0.52 2.28

5 6

Linoleic Linolenic

18:2 18:3

0.61 0.33

7

Dihydroxylstearic

18:0

0.24

Materials and methods

increase with an addition of 20% of biodiesel in the diesel. The physicochemical properties of the biodiesel and their blends were determined in accordance with international ASTM specifications as mentioned in Tables 2 and 3.

Test fuel properties and characterization The instrument used for measuring fuel properties as per ASTM specifications is depicted in Table 4. Method for measuring some important fuel properties is given below:

Kinematic viscosity

Oil and gas The castor oil was purchased from the local oil retailer. The fatty acid profile of Ricinus communis (castor oil) is depicted in Table 1. The chemicals used for biodiesel production were procured from Merck Ltd. (Methanol, NaOH, etc.). The CNG was procured from Indraprashta Gas Limited. The CNG used in this work is a combination of methane and several other hydrocarbons. The composition of CNG is given in Tables 2 and 3. The detailed composition of CNG used in testing was obtained from the Gas Authority of India Limited (GAIL). The following analysis was used for all calculations involving natural gas. (Density at 15 °C = 0.79 kg/m3, LCV = 50,000 kJ/kg).

Preparation of castor (Ricinus communis) oil methyl ester The transesterification reaction was carried out in a 2-l threeneck glass reactor equipped with temperature indicator, reflux condenser, and a variac for temperature controlled oil bath. The process optimization was performed with 0.5–1.5% w/w of catalyst amount; 3:1–12:1 methanol to oil molar ratio; 30 °C–65 °C reaction temperature for 15–90 min. The optimized conditions for biodiesel production are 1% wt. of catalyst (NaOH), 6:1 methanol to oil molar ratio, 30 °C room temperature, and 45 min as duration for reaction temperature. Under these conditions, a biodiesel yield of 96.4% was obtained. After transesterification reaction, the mixture was then allowed to settle in a separating funnel for overnight so as to separate the glycerol. After settling of glycerol, the remainder upper layer of methyl ester was washed with distilled water 3– 4 times to remove catalyst and excess methanol. It was then heated at 120 °C to remove the traces of moisture. Finally, the COME so obtained was cooled and stored. However, the calorific value of the biodiesel (COME) was found to be 39,969 kJ/kg, which is below the calorific value of the diesel (43,560 kJ/kg). The flash point, fire point, cloud point, and pour point of diesel-biodiesel fuel blends were found to

The viscosity was measured as per ASTM D445 specification by water bath capillary tube viscometer. The instrument was electronically controlled by thermostat switch to maintain desired temperature of 40 °C during measurement.

Flash point and fire point The flash and fire point was measured by Able’s flash point apparatus (Make: Widsons Ltd.) as per procedure laid by ASTM D93 specifications.

Cloud point and pour point Both the fuel properties was measured by cloud and pour point apparatus (Make: Widsons Ltd.) as per ASTM D2500 procedures.

Cetane number The values of cetane number (CN) in Tables 2 and 3 were calculated from the fatty acid profile, iodine value (IV), and saponification number (SN) as per protocol (Ruhul et al. 2016; Krisnangkura 1986). Table 2 CNG composition

Component

% vol.

Methane

96.113

Ethane Propane i-Butane n-Butane i-Pentane Nitrogen CO2 Hexanes

2.571 0.359 0.05 0.09 0.01 0.598 0.149 0.06


Properties of test fuels

Properties/fuels

Diesel

B20

B100

CNG

ASTM D6751

Relative Density at 15 °C (kg/m3) Viscosity at 40 °C, cst Lower heating value (MJ/Kg)

0.8445 2.6803 43.56

0.8526

0.899

0.79

0.880–0.900

3.651 42.54

7.437 40.45

– 50

1.9–6.0 > 33

Flash point (°C) Fire point (°C)

65 71

78 83

170 176

– –

100–170

Cloud point (°C)

−1

−5

4

–

− 2 to 12

Pour point (°C) Cetane number

−6 49

−1 51

2 55

– –

− 15 to 10 40 to 55

Experimental setup The test rig comprises a single cylinder, 4-stroke, air cooled direct injection diesel engine, commonly used in agricultural pump sets and farm machinery in India. The engine was tested at 20%, 40%, 60%, 80%, and 100% brake load conditions. The detailed specifications of the test engine are given in Table 4. The schematic diagram of the test setup is shown in Fig. 1 (Mahla et al. 2018a, b, c). All the experiments were conducted at constant rated speed of 1500 rpm under diesel and dual-fuel operation mode. The CNG flow rate was set at 15 l/min and this flow rate was maintained by control valve at all engine operating loads. The pilot fuel supply was manually regulated to maintain desired engine power output keeping CNG flow rate constant, at all operating engine loads. The diesel engine fuel injection system was mechanically controlled to regulate the pilot fuel supply. The start of injection (SOI) of fuel under mono-diesel and dual-fuel combustion mode was set at 26° BTDC as recommended by the manufacturer. The engine power output was measured by applying field voltage across electrical generator through a 5-kW electrical load bank. The air flow, fuel flow, temperatures and engine power output were monitored at all engine load conditions. For measuring peak cylinder gas pressure, a piezoelectric pressure transducer, Kistler make, Model 701A was used. The pressure transducer was fitted in cylinder head for measurement of gas pressure. For monitoring TDC (top dead center) position, Table 4

a magnetic pick, Electro make, Model 3010 Ama was used and fitted to the crank shaft of the engine. The 400 pressure cycle data was averaged and recorded for each operating load and stored in host computer for further n analysis. The CNG was inducted in the engine cylinder through inlet manifold fumigation. The CNG entered through a flame trap which was fitted with a pressure gauge, rotameter, regulating valve and safety release valve. A small surge tank was also provided between flame trap and inlet manifold to damp out the fluctuations of the flow and to measure flow rate properly. The CNG volumetric flow rate was consistently maintained at 15 LPM through inlet manifold of the engine while diesel fuel supply was regulated manually to maintain constant engine speed of 1500 rpm. However, beyond 15 LPM flow rate of CNG induction, severe knocking was encountered at higher engine operating load conditions, so the maximum flow rate was restricted to 15 LPM and was considered as the basis for comparison.

Performance analysis The performance parameters (such as brake power, brake thermal efficiency, and brake specific energy consumption) used for evaluation of engine operation were calculated as per standard protocols (Sahoo et al. 2009; Mahla et al. 2018a, b, c) and are given below:

Fuel measuring apparatus

Apparatus

Fuel property

ASTM standards

Unit

Range

Accuracy

Abel’s apparatus (Make: Widsons Ltd.) Viscometer (U-tube water bath) (Make: Widsons Ltd.) Bomb calorimeter (Make: Widsons Ltd.) Cloud and pour point (Make: Widsons Ltd.)

Flash and fire point Kinematic viscosity Calorific value Cloud point

D93 D445 D240 D2500

°C cst MJ/kg °C

− 5 to 250 1.6 to 35 0–45,000 − 15 to 18

± 1% ± 0.35% ± 1% ± 1%

Environ Sci Pollut Res t The heat release rate dQ dθ can be calculated by the following formula:

dQt γ dV 1 dP dQht ¼ P þ V þ dθ dθ ðγ−1Þ dθ ðγ−1Þ dθ where γ

Cp/Cv (for diesel, its value is 1.3 to 1.35) Heat transfer Instantaneous cylinder pressure Instantaneous cylinder volume

dQht dθ

P V Fig. 1 Experimental setup layout

B:P: ¼

Estimation of heat release across cylinder walls

V I KW 0:88 1000

where, B.P. is brake power, V is voltage and I is current in Amperes, ηgen = 0.88 (generator efficiency)

B:T :E: ¼

B:P: 3600 100 ðmCNG LCVCNG þ mdf LCVdf Þ

where, B.T.E. is brake thermal efficiency, mbio and mdf is mass of CNG (kg/h) and pilot diesel fuel, respectively. LCVbio and LCVdf are lower calorific value of CNG (kJ/ kg) and diesel fuel (kJ/kg) respectively.

∑ mtotal fuel LCV total fuel B:S:E:C: ¼ B:P:

where, B.S.E.C. is brake-specific energy consumption; mtotal fuel and LCVtotal fuel are mass and lower calorific value of total amount of fuel respectively.

Combustion analysis In-cylinder gas pressure was recorded using a piezoelectric transducer. For measuring TDC position an inductive magnetic pickup was installed near the disc mounted on the cam shaft of the engine. Using incylinder pressure data and TDC position it is possible to calculate combustion parameters such as heat release rate. The heat rate analysis can be calculated by applying first law of thermodynamics expression (Heywood 1988).

Under dual-fuel operation, the heat release rate calculated from the gross is the total one due to the combustion of liquid fuel and the gaseous one. The correlation for the instantaneous heat transfer considers truly the conditions present in a direct injection diesel engine (Hohenberg 1979). This correlation is based on extensive experiments done on a direct injection diesel engine. The instantaneous heat transfer across the walls for the engine was estimated using the following equation. 130pc 0:8 ⋅ vp þ 1:4 hc ¼ V 0:06 ⋅T g 0:4

0:8

where hc νp V Tg

Heat transfer coefficient Piston velocity Instantaneous cylinder volume Cylinder charge temperature

Emission analysis The emission analysis was done as per the previous literature (Mahla et al. 2018a, b, c). The exhaust gas emissions in terms of CO, HC, and NOx were measured by a five gas analyzer (AVL Digas 444 N). The smoke opacity of the engine exhaust was measured with a diesel smoke meter (AVL 437C). The gas analyzer probe was inserted in the exhaust pipe to monitor the exhaust emissions. The analyzers were allowed to reach a thermal stability before use. In order to ensure continuous flow of exhaust gas before measurement, the exhaust surge tank was fitted to the engine exhaust manifold pipe. The gas emissions were measured in strict compliance with ASTM-D6522 standards.


Engine specifications

Make and model

Kirloskar Oil India Ltd. (DAF 8)

Bore × stroke Rated power

95 × 110 mm 5.9 kW (8 BHP)

Rated speed Number of cylinder

1500 rpm Single

Compression ratio

17.5:1

Type of cooling

Air cooled

Lubrication type Displacement volume

Forced feed 780 cc

Nozzle opening pressure No. of holes (diesel injector)

200 bar 4

Static injection timing

26° bTDC

Injection type Injector type

DI (direct injection) Single nozzle hole (Pintle)

Inlet valve opening (degree) Inlet valve closed (degree)

4.5° bTDC 35.5° aBDC

Exhaust valve opening (degree) Exhaust valve closed (degree) Alternator specifications Manufacturer

35.5° bBDC 4.5° aTDC

estimate of the measurement accuracy. Special emphasis was given to the measurement error for pollutant emissions. To ensure that the accuracy of the measured values, the gas analyzer probe was heated up for some time to avoid condensation of exhaust gases. The smoke meter was also allowed to be adjusted for its zero point before taking each measurement. The uncertainty analysis of the various parameters is depicted in Tables 5 and 6.

Results and discussions This section presents the combustion, performance, and emission characteristics of CI engine fueled with CNG and COME blend under dual-fuel combustion mode. The results of various test fuels have been compared with baseline conventional diesel fuel at different engine operating loads. All the tests were performed at a constant speed of 1500 rpm at different engine operating loads under single and dual-fuel combustion mode.

Kirloskar Pvt. Ltd.

Dynamometer Rated speed Rated output Voltage rating

AC Alternator, 50 Hz, single phase 1500 rev/min 5 kVA 230 V

Combustion characteristics

Current rating Power factor

21.7 A 1.0

Figure 2 illustrates the variation of peak cylinder gas pressure with crank angle at full load engine condition. The peak cylinder gas pressure is found to be lower under dual-fuel mode for both pilot fuels as compared to diesel mode. These finding are in agreement with previous reported literature (Papagiannakis and Hountalas 2003; Mustafi et al. 2013; Bora and Saha 2016; Mahla et al. 2018b). This is mainly due to lower combustion rate of gaseous fuel-air mixture during premixed controlled combustion stage and later ignition under dual-fuel mode when compared to mono diesel operation (Mustafi et al. 2013). At full load, the peak cylinder gas

Data measurement and uncertainty analysis Any experimental measurement, irrespective of the type of instrument used, possesses a certain amount of uncertainty or error. All the measurements were replicated thrice to get a reasonable value. This enabled to determine the repeatability of the measured data and have an Table 6

Effect on cylinder gas pressure

Uncertainty analysis

Serial no. Instrument name

Range

Accuracy

Measurement technique

Percentage uncertainty

1 2 3

60 Nm 0–100 kg 0–50 cc

± 0.25 Nm ± 0.1 kg ± 0.1 cc

Opposing eddy current Strain gauge type load cell Volumetric type

± 0.25 ± 0.2 ± 0.1

± 100 kg/h 0–10,000 rpm

± 0.01 kg/h Turbine flow principle ± 10 rpm Magnetic pick up type

Dynamometer Load indicator Fuel measurement

4 Air flow meter 5 Speed 6 BTE (dual-fuel mode) 7 BSEC (dual-fuel mode) Exhaust gas analyzer 6 Carbon monoxide 7 Hydrocarbons 8 Oxides of nitrogen

0–9.99% vol. ± 1% 0–10,000 ppm ± 20 ppm 0–10,000 ppm ± 10 ppm

– ± 0.1 ± 1.5 ± 1.7

Non-dispersive infra-red sensor – Chemi-luminescence principle, electro chemical sensor ± 0.20 Flame-ionization detector-FID ± 0.20


Fig. 2 Variation of cylinder gas pressure with crank angle at full engine load

combustion stage is strongly influenced by period of ignition delay, mixture formation, and rate of combustion in the initial phases of pre-ignition and combustion process (Barik and Murugen 2014). At full engine load, the HRR value was found to be 120 and 114 J/°CA for CNG-D and CNG-B20 respectively as compared to 77 J/°CA for normal diesel operation. The high heat release rate is also responsible for higher NOx formation in premixed combustion stage. The castor biodiesel blend of B20 as pilot fuel has shown a slightly higher heat release rate than mono diesel fuel which is attributed to higher oxygen content in fuel which promotes better combustion and is responsible for higher heat release rate.

Performance characteristics pressure was found to be 63.4 and 66.5 bar for CNG-D and CNG-B20 respectively as compared to 69.7 bar for mono diesel operation. Under dual-fuel mode, B20 as pilot fuel produces slightly higher peak cylinder gas pressure as compared to diesel as pilot fuel.

Effect on gross heat release rate Figure 3 represents variation of gross heat release rate with crank angle at full load. It is clear from Fig. 3 that high heat release rate occurs in dual-fuel operation as compared to diesel-fuel mode. The high heat release rate in dual-fuel operations is probably due to the richer natural gas-air mixture which burns more rapidly. During the diffusive combustion phase, the heat release rate under dual-fuel mode was observed to be higher revealing later combustion of gaseous fuel-air mixture, which is in agreement with previous findings (Papgiannakis et al. 2003; Mustafi et al. 2013). The heat release rate in the premixed

Fig. 3 Variation of gross heat release rate with crank angle at full engine load

Effect on brake-specific energy consumption Brake-specific energy consumption (BSEC) is a reliable performance parameter during comparison of fuels with different calorific values. It is a measure of combustion quality of the fuels. Figure 4 shows the variation of BSEC as a function of engine load under dual-fuel operation for both pilot fuels. At lower engine loads, BSEC for dualfuel operation for pilot fuels, diesel, and B20 was considerably higher as compared to the one under neat diesel operation. This is because of poor utilization of the gaseous fuel mainly due to the lower combustion temperature and lower air-fuel ratio at light load (Namasivayam et al. 2010). At 20% engine load, the BSEC values were found to be 43 and 44.2 MJ/kWh for D-CNG and B20-CNG, respectively when compared to 32.1 MJ/kWh for diesel fuel mode. On the other hand, at higher loads, the improvement in fuel utilization, higher heating value, and

Fig. 4 Variation of BSEC with engine load


when compared to B20-CNG combustion mode throughout engine load spectrum. This may be attributed to shorter ignition delay and higher calorific value of D-CNG when compared to B20-CNG. At 100% engine load, the BTE values were found to be 27, 25, and 23.8% for diesel, D-CNG and B20-CNG, respectively.

Emissions characteristics In this section, the main engine exhausts tailpipe pollutants (HC, CO, NOx, and smoke) were compared for dual-fuel and conventional diesel mode at different engine-operating loads. Fig. 5 Variation of BTE with engine load

better combustion characteristics of CNG lead to an improvement in brake-specific energy consumption. At 100% engine load, the BSEC values obtained were 15.2, 16.4, 17.58 MJ/kWh for diesel, D-CNG- and B20-CNGtested fuels, respectively. Effect on brake thermal efficiency Brake thermal efficiency is defined as ratio of brake power to the thermal energy input from the fuel. It evaluates how well an engine converts the heat obtained from a fuel into mechanical energy. Brake thermal efficiency increases with increase in engine operating loads for all tested fuels (Fig. 5), which may be attributed to improvement in combustion and higher cylinder temperature at higher engine operating loads (Weaver and Turner 1994; Barik and Murugen 2014). In comparison to other fuels, it was found that brake thermal efficiency (BTE) is higher in case of diesel when compared to dual-fuel operational mode throughout engine load spectrum. The similar trend was also observed by other researchers (Bora and Saha 2016; Mohsin et al. 2014; Nayak and Mishra 2016; Mahla et al. 2018b). It is observed that at lower engine load, the BTE is lower in dual-fuel operation with both pilot fuels in comparison to baseline diesel mode. At lower engine loads, the overall intake charge mixture is lean and the ignition source is also weak, as the pilot fuel quantity is small. It leads to poor ignition and combustion since natural gas share is high at light load. This results in slower combustion rates, thus leading to an incomplete combustion which causes a drop in thermal efficiency (Sahoo et al. 2009, Tarabet et al. 2014, Semin 2008). As the load increases, the mixture get richer which increase the combustion temperature of the fuel-air mixture inside engine cylinder, consequently leading to reduction in ignition delay period of CNG. Due to higher combustion temperature, an improved CNG utilization occurs inside the combustion chamber. D-CNG mode shows slightly higher BTE

Effect on oxides of nitrogen (NOx) Figure 6 portrays the variation of brake-specific NO x emission with engine load for all test fuels which depicts its higher concentration at low load for conventional diesel operation when compared to dual-fuel mode of either pilot fuel. In case of conventional diesel, the NOx decreased sharply with increase in engine load. The brakespecific NOx concentration for dual-fuel mode of either pilot fuel was lower than conventional diesel up to 60% engine loads. This is mainly due to the lower rate of premixed controlled combustion of the intake charge, which results in lower gas temperature inside combustion chamber (Mustafi et al. 2013; Paul et al. 2015; Lounici et al. 2014). The lean mixture, high-specific heat of premixed charge and slower flame propagation lowers the combustion temperature and suppresses the NOx concentration at low loads (Senthilraja et al. 2016). The similar trend was reported in literature (Mustafi et al. 2013; Barik and Murugen 2014; Bora and Saha 2016). At higher engine load, improvement in combustion rates due to high combustion temperature liberated higher NOx emissions.

Fig. 6 Variation of NOx with engine load


fuels (Senthilraja et al. 2016). At full engine load, the HC emissions were found to be increased by 6.7 and 11% when compared to conventional diesel. The B20 castor biodiesel blend used as pilot fuel under dual-fuel mode results in a drop in HC emissions as compared to diesel. The higher cetane number, shorter ignition delay and oxygen availability plays a vital role in improving combustion characteristics. Effect on carbon monoxide

Fig. 7 Variation of HC with engine load

At higher engine loads, dual-fuel operational mode of either pilot fuel with CNG enrichment produced higher NOx emissions as compared to conventional petro-diesel. At full engine load, the NOx emission increased by 24.5 and 28.6% for D-CNG and B20-CNG, respectively when compared to conventional diesel mode. Moreover, B20CNG dual-fuel combination produced higher brakespecific NOx emission than D-CNG throughout engine load spectrum which can be attributed to the higher availability of oxygen in biodiesel fuel. Effect on unburnt hydrocarbons The unburnt hydrocarbons are the fuel particles which remain entangled in the exhaust tailpipe and do not participate in fuel combustion. The variation of hydrocarbons (HC) emission as a function of engine load is given in Fig. 7 for all test fuels. It is clear from Fig. 7 that unburned hydrocarbon emissions are considerably higher for the dual fuel as compared to conventional diesel at entire engine operating loads. Due to the induction of gaseous fuel i.e., CNG, instead of fresh air, the flame propagation from the ignition zones of the pilot fuel is normally suppressed due to the low combustion flame temperature and air-fuel ratio (Qi et al. 2011; Nwafor 2000). Though there are some other reasons also which contribute to the higher emissions of HC in exhaust tailpipe emissions like (i) the fuel-air mixture filling crevice spaces during compression stroke remains unburned, (ii) valve overlapping period between inlet and exhaust to facilitate scavenging can also cause an increase in HC emissions from dual-fuel combustion as part of fuel left is unburnt (Abdelaal and Hegab 2012; Weaver and Turner 1994). As observed under dualfuel mode of either pilot fuels, difference in unburned hydrocarbon emission level is higher for low loads than high engine load. At higher engine loads, the higher combustion temperature and richer fuel-air mixture promotes better oxidation of

The carbon monoxide (CO) emission of an internal combustion engine is a function of poor and incomplete oxidation of fuel due to lower cylinder temperature and limited availability of oxygen. Both of these factors control the rate of fuel decomposition and oxidation process (Bora and Saha 2016; Kumar and Kumar 2016). Figure 8 shows the variation of CO emission with engine load for diesel and dual-fuel mode. For all test fuel cases, it was found that CO emission were higher at low loads which then decreased at medium load and then increased at higher engine loads. This is primarily due to the incomplete combustion of CNG-air mixture owing to insufficient ignition source as well as lower combustion temperature and flame quenching (Mustafi et al. 2013; Pali et al. 2015; Liu et al. 2013; Gharehghani et al. 2015). As engine load was increased, the combustion temperature increment dominates and helps to promote more complete oxidation of fuel. At higher engine loads, rich fuel-air mixture and scarcity of oxygen produced higher CO formation with conventional petro-diesel as compared to low engine load condition (Kumar and Kumar 2016; Bora and Saha 2016). At full engine load, dual-fuel mode of either pilot fuel produced less CO when compared to conventional diesel. CO emission was decreased by 31.6 and 37.4% for D-CNG and B20-CNG, respectively in comparison to conventional diesel. The B20-CNG fuel combination remarkable lowers the CO emissions in comparison to D-CNG mode at all load condition.

Fig. 8 Variation of CO with engine load


Fig. 9 Variation of smoke opacity with engine load

Effect on smoke opacity Figure 9 illustrates the smoke opacity variation with engine load. Smoke opacity emission increases with increasing engine load for all tested fuels. It can be seen from the Fig. 9 smoke opacity emission is lower in dual-fuel mode as compared to baseline petro-diesel throughout engine operating loads. The measured smoke opacity emission level in exhaust tailpipe is high when fuel H/C ratio is less than 2 (Barik and Murugen 2014). The lower smoke opacity level under dual-fuel mode is due to low C/H ratio of CNG and combustion of premixed homogenous charge in dual-fuel mode (Nayak and Mishra 2016, Mohsin et al. 2014). The fuel composition also strongly influenced the smoke production inside the combustion cylinder. CNG mainly contains methane which being the lower member of paraffin family has lesser tendency to produce smoke emission level in engine exhaust (Papagiannakis and Hountalas 2003, Hosmath et al. 2016). Under dual-fuel mode, some fossil petro-diesel is replaced by the incoming CNG through inlet manifold and affects the smoke generation inside engine cylinder. The smoke emission is mainly associated with diesel combustion as CNG is a non-soot forming fuel. The lower smoke emissions under dual fuel may also due to absence of aromatic compounds in the CNG fuel. At full engine load, the smoke emission was reduced by 49.4 and 59.6% for D-CNG and B20-CNG as compared to conventional diesel. Therefore, dual-fuel combustion mode is an effective control strategy for reducing smoke opacity emissions from diesel engines.

Conclusions The current work explores the possibility of utilizing castor biodiesel as pilot fuel for CNG operated diesel engine under

dual-fuel mode. The results of this investigations revealed that peak cylinder pressure is lower under dual-fuel mode when compared to baseline diesel under similar engine operating load. The high heat release rates occur under dual-fuel mode when compared to diesel. B20-CNG dual-fuel mode yields thermal efficiency of 23.8% when compared to 25 and 28% for D-CNG and baseline diesel, respectively at full engine load. BSEC under dual-fuel mode is higher in comparison to petro-diesel at all engine loads. The investigation of emission characteristics reveals that under dual-fuel operational mode, NOx emissions increased by 24.5 and 28.6% for D-CNG and B20-CNG, respectively, and CO emission levels decreased by 31.6 and 37.4% for B20-CNG and D-CNG, respectively when compared to conventional diesel mode. There was appreciable reduction in smoke emission by 49.4 and 59.6% for D-CNG and B20-CNG, respectively in comparison to diesel fuel. On the whole, CNG-COME dual-fuel combinations exhibit excellent combustion, performance and emissions characteristics in view of conserving fossil petroleum diesel and degradation of ambient air pollution. The potential benefit of using this dual-fuel technology is reduction in emissions for generation of electric power and preservation of ambient air quality. Acknowledgements The authors are grateful to the staff members of the Department of Mechanical Engineering, I.K. Gujral Punjab Technical University Campus, Hoshiarpur and School of Energy and Environment, Thapar University Patiala for extending wholehearted support during the experimental work.

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