1 Introduction

Global energy demand is increasing rapidly due to the growing population and booming economy. About 992 million people in the world still do not have access to electricity. In India around 18% of people dwelling in rural areas are still not electrified [1]. Rural electrification is a challenging task because of the remoteness of the villages from the grid, the lower population in rural areas, and the cost of transmission and maintenance cost required for power gird extension [2]. Rural electrification depends mainly on conventional fuels, which contribute most to global warming and pollution. Due to rapid the depletion of fossil fuels, energy security is a suspicious threat to future generations. Therefore, renewable energy technologies can be used as an alternative to grid extension and are best suited for decentralised power generation, reducing atmospheric emissions [3,4,5].

Biomass is one among the renewable energy sources that can help to fill the gap between energy production and energy demand [6, 7]. Biomass gasification is a thermochemical process that converts solid feedstock into usable gases [8]. The main advantage of gaseous fuels is their higher combustion efficiency (70–80%) and higher heat release rate. In addition, it is easy to control and adjust the rate of energy output and can be used for various power sources [9]. The formation of tar, which is difficult to purify and reduces the output of H2, is the main challenge in the gasification process [10, 11]. The most common locally available biomass materials such as wood and non woody agricultural residues such as coconut shells, rubber seed kernels and coir pith can be gasified in a gasifier to producer gas that can be utilised for heat and power applications [12]. From the findings from the existing literature, appropriate examinations on the parametric analysis of the various types of biomass resources utilised in the gasification process of the gasifiers may significantly increase the system's performance in terms of calorific value and cold gas efficiency depending on the type of feedstock, equivalence ratio, flow rate, and gasifying agents [13]. Experimental studies carried out using a 15MWe downdraft gasifier reported that the coir pith shows poor performance, whereas coconut shell shows maximum efficiency because of its higher combustible species concentration [14]. Due to the high carbon concentration in sugarcane bagasse, gasification of coconut shells proved superior to sugarcane bagasse. When coconut shells were employed as a producer gas, the tar concentration decreased from 4018 to 160 mg/Nm3 [15].

Due to fast depletion, high impacts of environmental pollution, and growing prices of conventional fuels, the use of alternative fuels in engines is an attractive research area [16, 17]. When compared to petrol, dual fuel (producer gas-fossil fuel) operation resulted in considerable reductions in HC, CO, and NOx emissions. The findings of this study will be valuable for researchers working on the development of gasifiers and utilisation of producer gas as a supplemental fuel for SI and CI engine operations. The utilisation of biomass-derived producer gas fuelled I.C. engine can assist developed countries reduce their fossil fuel consumption [18]. Biodiesel, a liquid biofuel made from a variety of biomass sources can be used in both single and dual fuel modes. However, gaseous biofuels such as producer gas and bio gas must be run on dual fuel mode since gaseous fuel will not ignite under the prevailing conditions of temperature and pressure inside the engine cylinder [19]. Hence a considerable amount of diesel has to be supplied as an ignition source for making the engine run on dual fuel mode. Retrofitting the existing SI engines for producer gas operation is not attractive because they suffer from a high-power derating of about 40–70%. Modification of existing diesel engine for producer gas operation is simple and also power derating is limited to 20–30% [20]. Therefore, dual fuel operation using producer gas in CI engines is an effective approach for diesel conservation.

Experiments on a gasifier engine system using coirpith and woodchips as feedstock were conducted to analyse the engine’s performance characteristics at various producer gas flow rates and load conditions. At 70% load condition, the highest brake thermal efficiency (BTE) of 20% and 21% was obtained in dual fuel mode as coirpith and woodchips respectively. When compared to producer gas (PG) generated from coirpith, wood-generated producer gas had a higher calorific value and had a higher BTE. The amount of PG permitted into the engine cylinder increases as the load increases. Hence proper combustion does not take place due to the insufficient oxygen in the air-gas mixture which is the reason for decrease in BTE at higher loads [21]. Electrical power output and engine-generator efficiency for a multistage hybrid gasifier-engine system were on dual fuel operation. The results showed a maximum feasible electrical load of 11.5 kWe with a system efficiency of 15%, which was much lower than the system efficiency of 22.5% achieved during diesel mode operation. The lower calorific value of PG and the lower air fuel ratio for dual fuel operation were the reasons for the drop in efficiency [22]. Under different loading conditions, using producer gas as a supplemental fuel decreases BTE for all mixtures of diesel producer gas. When compared to diesel alone mode, it yielded adequate performance when 30% producer gas was used. The BTE diminishes as the percentage of producer gas increases. In dual-fuel mode, NOx pollution for all types of fuel blends was observed to decrease than for sole diesel fuel engines [23]. Another study was conducted on a dual fuel engine using methyl ester rice bran oil and producer gas at different loads. It was discovered that methyl ester rice bran oil blended with producer gas gives higher brake thermal efficiency than diesel and producer gas mixture. The percentage of diesel substitution was determined by the load, the quality (CV) and quantity (flow rate) of producer gas. At 63% engine load, a maximum diesel savings of 78% was observed [24]. Pre-treating or preheating the biofuels can significantly affect the performance of the system. Experiments using preheated blends of refined rice bran oil and PG revealed a reduction in BTE and SEC when compared to diesel only mode. A blend of refined rice bran oil preheated to 60 °C and mixed with diesel in the ratio of 1:1 operated at 84% engine load showed the highest BTE and lowest SEC compared to other blends. The SEC in dual fuel mode is comparatively higher than diesel only mode because when producer gas enters the cylinder at a higher temperature than ambient condition at the starting of compression stroke, it substantially increases the temperature inside the cylinder which results in higher energy input [25]. Compression ratio is one of the crucial process parameters of a dual fuel engine because the CR of engine varies with respect to engine design. Therefore, studies with various compression ratio engines must be carried out in order to assess its performance. The CR in VCR engines is usually made by adjusting the cylinder head, thus changing the swept volume. Sawdust and cotton stalks derived PG was employed for dual fuel operation. The engine’s performance and emission characteristics were studied at CR 12–18, and the results revealed that as CR increased, the percentage of diesel fuel saving increased. This is because at higher CR, the temperature and pressure inside the engine cylinder increase, thus improving the combustion efficiency resulting in higher diesel replacement. However, at lower loads, due to the demand for a rich mixture, diesel replacement is lower [26].

From the review of existing literature, it is found that several experimental studies on a biomass gasifier-based power generation system have been reported. Only a limited theoretical studies have been identified. It is critical to theoretically model and analyse the system in order to identify the major governing factors so that the system may be validated and optimised for operation with improved efficiency. Theoretical modelling of downdraft fixed-bed reactors, using thermodynamic equilibrium model have been utilised to forecast the final composition of the production gas. Other studies employed models that combined chemical and thermodynamic equilibrium to forecast the final composition of the production gas and reaction temperature for certain reactor designs [27]. The dual fuel process in engines can be modelled in a number of ways. Zero-dimensional, multi-zone, and 3D CFD techniques are some of the modelling methodologies that have been published in the literatures to simulate dual fuel combustion [28, 29]. In practical engines the thermodynamic processes are never in equilibrium. Considering this keynote, non-equilibrium thermodynamics modelling called as Finite Time thermodynamics have been used for solving practical problems. The FTT model is simple for doing numerical computations and as they are more practical oriented, the results obtained are much realistic and reliable compared to other modelling approaches [30]. Many thermodynamic analyses, comparisons, and optimizations have been employed by design engineers to aid in the development of novel design concepts. In a study employing Finite Time thermodynamics, the relationships between net power and efficiency for Diesel, Otto, and Dual cycles were determined theoretically by taking heat-transfer losses into account [30]. Previous literature provided a study to determine the characteristics of power and efficiency for otto, dual cycles considering the heat transfers and friction losses. In one of the preceding studies, losses arising from heat resistance and friction were determined and the performance of an irreversible diesel cycle was predicted [31].

Due to the menacing environmental impacts of conventional fuels, the world is mitigating towards renewable biofuels. Experimental evaluation on dual fuel engines using PG generated from various feedstock are available in numerous numbers in the literature. Theoretical investigation for dual fuel engine technology needs major attention as they help to identify the most influential and critical parameters in experimentation. However, only a few theoretical studies have been reported in the literature. In this regard, this study uses the finite time thermodynamics (FTT) model, developed for predicting the performance parameters of a variable compression ratio (VCR) CI engine fuelled with diesel-producer gas (PG) combination. The results of the theoretical model are compared with the experimental results in terms of Brake thermal efficiency at different compression ratio and load conditions for engine operated with PG generated all the feedstock. Using the proposed model, the impacts of engine load and compression ratio on engine performance were also studied. There is no study available that looks at the impact of engine load and CR on engine performance using PG generated from different biomass feedstock using experimentation and the finite time thermodynamics (FTT) model. This study will be useful for researchers and engine design engineers working in the field of gaseous biofuels to identify the most influential design factors for efficient operation.

2 Materials and Methods

A generalised flow chart for this research work has been presented in Fig. 1. This research involves three phases, namely identification of study area and the abundant biomass resources available within the study area, utilisation of the selected biomass for production of producer gas using biomass gasifiers for experimental evaluation of dual fuel engine, theoretical evaluation of the dual fuel engine using Finite time thermodynamics model, and Comparison of experimental and theoretical results.

Fig. 1
figure 1

General flow chart of this Research work

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2.1 Selection of Study Area and Available Biomass Resources

A research was conducted in a few remote villages in the southern region of India to efficiently utilise the existing biomass energy resources in remote places. The study area is located along the south-west coast of Kanyakumari district, Tamil Nadu, India. The geographical coordinates extend from 77° 9′ 49.20′′ E to 77° 34′ 15.00′′ E longitude and 8° 6′ 32.60′′ N to 8° 14′ 15.30′′ N latitude [32, 33]. The survey emphasised on agricultural operations in the rural areas and crop plantations. Furthermore, the financial feasibility of technology and the impact of technology on rural areas were investigated using previous work provided by several researchers. Field surveys focusing on rural households and personal interviews were conducted in the villages to obtain data on the provision of renewable feed stocks, current energy usage, and so on. The database is being used in this research project to collect relevant information from the field of research [34, 35]. In the present study, production of agricultural biomass from all the major crops within the study area is identified. The major BM energy resources noted in this study are, rice husk, coconut shell, and rubber shell. Gross residue potential (GRP) is the total amount of residue produced and is determined by using residue-to product ratio (RPR). The energy potential of each of the biomass feedstock is calculated by using the following formula [36]

$${\text{GRP}} = {\text{AAC}} \times {\text{AY}} \times {\text{RPR}}$$
(1)

AAC—Available area under cultivation (Ha), AY—Average yield (kg/ha), RPR—Residue to product ratio.

The important observations of the energy feed stock and their estimated Gross residue potential from the study are given in Table 1. The proximate and the ultimate analysis of the biomass feedstock were determined using the standard procedure at the Food Safety and Quality Testing Laboratory (Indian Institute of Food Processing Technology, Tanjavur, Tamil Nadu) and are given in Table 2.

Table 1 Observations of the energy feed stock and their estimated Gross residue potential from the study
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Table 2 Ultimate and proximate analysis of biomass feedstock used in the biomass gasifier
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2.2 Experimental Setup and Procedure

An experimental setup, as illustrated in Fig. 2a, has been designed and developed to carry out the investigation. It consists of a gasifier unit, diesel fuel tank, CI engine, and measurements apparatus. For the production of producer gas from the selected biomass feedstock (rice husk, coconut shell and rubber shell), a downdraft gasifier was used. The specifications of the downdraft biomass gasifier are presented in Table 3. The gasifier unit consists of downdraft gasifier where biomass feedstocks (rice husk, coconut shell and rubber shell) are fed into the top of the reactor maintained under controlled conditions. Air is supplied in limited amount by means of tyures provided on the sides of the gasifier for the gasification process to take place inside the chamber. The gas generated is passed through a scrubber and filter unit in-order to remove the impurities. The ultraclean gas obtained was stored in the of 10 m3 gas collection bag because the composition of producer gas (Table 4) may vary persistently due to pressure drop and lead to incomplete combustion and misfiring when it is directly fixed to the engine. Hence storage bags were used to maintain same gas composition throughout the operation. From the storage bags the producer gas was mixed with air in the intake pipe in T joint arrangement (90°) and sent into the engine cylinder.

Fig. 2
figure 2

a Schematic representation of the experimental setup. b Pictorial view of the experimentation setup

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Table 3 Specification of the downdraft biomass gasifier
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Table 4 Composition of producer gas
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The power generation unit consisted of a single cylinder, four stroke, multi-fuel, variable compression ratio engine of rated capacity 3.75 kW coupled to an eddy current dynamometer making the generator setup. The detailed specifications of the engine are presented in Table 5. A load cell was connected to the generator setup to provide the electrical resistance. The engine was made to run at a rated speed of 1500 rpm and diesel was injected by a diesel injector at 23° before top dead centre (BTDC) at an injection pressure of 230 bar. Being a variable compression ratio (VCR) diesel engine, compression ratio of the engine is variable and can vary from 12 to 22. The engine was run at different engine loads, compression ratio, and blend ratio of producer gas, and the experimental measurements were recorded. The pictorial view of the experimental setup used in the experimentation is shown in Fig. 2b.

Table 5 Specification of the CI engine
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The performance of the variable compression ratio (VCR)diesel engine is tested in two modes. (1) Diesel only mode and (2) Dual fuel (diesel- producer gas) mode. The performance of the engine is measured in terms of brake thermal efficiency, brake specific fuel consumption and brake specific energy consumption for diesel only mode and dual fuel (Diesel/Producer gas) mode.

As a prior preparation for experimentation, the identified biomass feedstock (Rice husk, coconut shell or rubber shell) is fed into the gasifier by a hooper provided on the top. Air is forced into the gasifier by means of a blower and the combustion of biomass is initiated by pouring a little quantity of diesel. Thus, the combustion process in gasifier takes place and the gas generated leaves the bottom of the gasifier. This gas generated is then passed through a coarse filter and scrubber unit to remove the impurities and dust particles. The ultra clean gas obtained is then cooled and passed through the filter unit consisting of a passive filter, fine filter and fabric filter. The filtered producer gas obtained is introduced into the engine cylinder by mixing with the intake air. Initially the engine is run on diesel only mode and then switched to dual fuel (diesel-producer gas) mode by adjusting the producer gas inlet valve. The experimental readings were carried out for various compression ratio and load by varying the knob in the eddy current dynamometer. Experiments were conducted in diesel only mode and dual fuel mode with varying braking power (0.5–3.5 kW) and compression ratio (12–18). At each brake power and compression ratio, three trials were carried out in order to avoid error.

2.2.1 Measurements of Performance Parameters

To evaluate the performance parameters, an eddy current dynamometer, a piezoelectric transducer and digital PT-100 type temperature sensor was calibrated and used in the computerised CI engine set up along with a digital data acquisition system. Brake power, brake thermal efficiency for only diesel mode, brake thermal efficiency of dual fuel (diesel-producer gas) mode, brake specific fuel consumption and brake specific energy consumption were measured during experimentation of the dual fuel CI engine setup.

The performance of the engine is evaluated by the relation between power developed, specific energy consumption, and specific fuel consumption at each operating condition within the effective speed and load range. An engine’s fuel consumption is obtained by calculating the required time to consume a given volume of fuel. The mass of fuel was determined by multiplying the volumetric fuel consumption by the density of the fuel. For precise volumetric assessment of air measurement, an air box with orifice metre and manometer was utilised, and the mass flow rate was obtained. Temperature was measured using a digital PT-100 type temperature sensor.

Brake thermal efficiency (BTE) is the ratio of the available thermal energy in the fuel to the power delivered by the engine. In this experimentation, brake thermal efficiency (BTE, %) for diesel only mode and the dual fuel (diesel-producer gas) mode was determined as the following equations [41],

Brake thermal efficiency for diesel only mode (\({\mathrm{BTE}}_{\mathrm{d}}\),%),

$${\text{BTE}}_{{\text{d}}} = \frac{{{\text{BP}} \times 3600 \times 100}}{{m_{{\text{d}}} \times H_{{\text{d}}} }}$$
(2)

where \({m}_{\mathrm{d}}\) is the mass flow rate of diesel in kg/h; \({H}_{\mathrm{d}}\) is the calorific value of diesel in kJ/kg; BP is the brake power, which is investigated by using eddy current dynamometer in kW and is expressed as [42],

$${\text{BP}} = T\left( {\frac{2\pi N}{{60}}} \right) \times 10^{ - 3} = f\left( {{\text{VI}}} \right)$$
(3)

where N is the engine speed in rev/min, T is the Torque in Nm, V is the voltage and I is the current in amperes.

Brake thermal efficiency for dual fuel (diesel/producer gas) mode (\({\mathrm{BTE}}_{\mathrm{dual}}\), %) [41],

$${\text{BTE}}_{{{\text{dual}}}} = \frac{{{\text{BP}} \times 3600 \times 100}}{{\left\{ {\left( {m_{{\text{d}}} \times H_{{\text{d}}} } \right) + \left( {m_{{{\text{pg}}}} \times H_{{{\text{pg}}}} } \right)} \right\}}}$$
(4)

where \({m}_{\mathrm{pg}}\) is the mass flow rate of producer gas in kg/h; \({H}_{\mathrm{pg}}\) is the calorific value of producer gas in kJ/kg.

Brake specific fuel consumption (BSFC) is defined as the rate of fuel flow per unit power generation. It is a measure of the engine’s efficiency in utilising the fuel supplied to produce work. The fuel consumption of an engine is calculated by using a glass pipette to determine the time taken to consume a particular volume of fuel. By multiplying volumetric fuel consumption by density, the mass of fuel was estimated. For efficient volumetric evaluation of air consumption, an air box with orifice metre and manometer was utilised, and the mass flow rate was obtained. This is one of the most crucial criteria when comparing the different fuel sources. Brake specific fuel consumption (BSFC, kg/kWh) is expressed as follows [41]:

$${\text{BSFC}}_{{\text{d}}} = \frac{{m_{{\text{d}}} }}{{{\text{BP}}}}$$
(5)
$${\text{BSEC}}_{{{\text{dual}}}} = \frac{{m_{{\text{d}}} + m_{{{\text{PG}}}} }}{{{\text{BP}}}}$$
(6)

where \({m}_{\mathrm{d}},{m}_{\mathrm{PG}}\) are the fuel consumption rates of diesel and producer gas and \(\mathrm{BP}\) is the brake power in kW.

Brake specific energy consumption (BSEC) is another measure of engine performance which is more reliable than BSFC. The BSEC gives the effective utilisation of fuel in terms of energy. The BSEC for diesel only and dual fuel (diesel-producer gas) modes are calculated by the following equation [41],

$${\text{BSEC}}_{{\text{d}}} = \frac{{m_{{\text{d}}} \times H_{{\text{d}}} }}{{{\text{BP}}}}$$
(7)
$${\text{BSEC}}_{{{\text{dual}}}} = \frac{{\left( {m_{{\text{d}}} \times H_{{\text{d}}} } \right) + \left( {m_{{{\text{pg}}}} \times H_{{{\text{pg}}}} } \right)}}{{{\text{BP}}}}$$
(8)

where \({\mathrm{BSEC}}_{\mathrm{d}}\) is the brake specific energy consumption for diesel only mode in kJ/kWh; \({\mathrm{BSEC}}_{\mathrm{dual}}\) is the brake specific energy consumption for dual fuel (diesel/producer gas) mode in kJ/kWh; \({m}_{\mathrm{d}}\) is the mass flow rate of diesel in kg/h; \({H}_{\mathrm{d}}\) is the calorific value of diesel in kJ/kg; \({m}_{\mathrm{pg}}\) is the mass flow rate of producer gas in kg/h; \({H}_{\mathrm{pg}}\) is the calorific value of producer gas in kJ/kg.

The use of producer gas with diesel reduced the consumption of diesel fuel. The percentage of diesel savings for experimentation is calculated using the given formula [43]

$$\% {\text{DS}} = \frac{{m_{{\text{diesel }}} - m_{{\text{dual }}} }}{{m_{{\text{diesel }}} }}$$
(9)

where \({m}_{\mathrm{diesel} },{m}_{\mathrm{dual}}\) are the diesel consumption rates in diesel only mode and dual fuel mode, respectively.

2.2.2 Uncertainty Analysis

The error or uncertainty analysis in an experimental work quantifies the difference between measured and true value of a thermo physical quantity of a material. The uncertainty in the estimate of the true value provides a rational way of evaluating the significance of the scatter on repeated trials [44].

$$\Delta q = \sqrt {\left( {\frac{\partial q}{{\partial x_{1} }} \cdot \Delta x_{1} } \right)^{2} + \left( {\frac{\partial q}{{\partial x_{2} }} \cdot \Delta x_{2} } \right)^{2} + \left( {\frac{\partial q}{{\partial x_{3} }} \cdot \Delta x_{3} } \right)^{2} + \cdots }$$
(10)
$$q = \frac{\Delta q}{{\left| q \right|}} \times 100$$
(11)

The Gaussian distribution was used to estimate the overall uncertainty of the experimentation by quantifying the observed quantities such as Temperature, load, engine speed, Time, manometer and flow meter from their respective uncertainties as given in Table 6. For the calculated parameters such as brake thermal efficiency, brake specific fuel consumption and brake specific energy consumption the overall uncertainty of ± 1.2% was observed which were assessed using Eqs. (10) and (11).

Table 6 Accuracies and uncertainties of the measurand measured in experimentation
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2.3 Theoretical Modelling

The dual fuel combustion technology uses two different fuels, a primary fuel and a secondary fuel. In this study, Producer gas the secondary fuel, is mixed with intake air, and this premixed mixture is introduced inside the engine cylinder. During the compression stroke, the temperature and pressure of the premixed mixture inside the engine cylinder increase. At the end of the compression a minor quantity of diesel (primary fuel) is injected in order to start the combustion. In the next process, auto ignition of the premixed mixture occurs as the attained temperature inside the cylinder is much higher than the self-ignition temperature of the premixed mixture. The dual fuel combustion of the premixed flame is a complex mechanism compared to the diesel combustion and almost occurs in an uncontrolled manner due to the turbulence of the gaseous fuel and occurs for a longer period contributing to most of the energy input. The final process is the expansion process wherein the combusted mass is exhausted out of the engine cylinder. The processes taking place in dual fuel diesel engine are in relevance with dual cycle which is shown Fig. 3 has been considered for modelling. A novel Finite Time Thermodynamics (FTT) model for diesel engines running on diesel-producer gas combination has been made in this analysis [45] and numerically computed using MATLAB software.

Fig. 3
figure 3

Thermodynamic cycle (P–V and T–S) of dual fuel engine

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In dual fuel mode the main parameter is diesel replacement which primarily depends upon the compression ratio, heating value of fuel, load percentage, and the capacity of the engine. From the conduct of experiments and reviewing the past literatures [46], an equation has been developed using multivariable regression analysis to calculate the percentage of diesel saving.

$$\% DS = ((X_{1} \times CR) - (X_{2} \times CV) - (X_{3} \times L) - X_{4} ) \times E$$
(12)

where \(X_{1}\), \(X_{2}\), \(X_{3}\), and \(X_{4}\) are the regression coefficients obtained by multivariable regression and their values of \(X_{1}\) ranges from 1.6 to 1.8, \(X_{2}\) from 0.85 to 1.15, \(X_{3}\) from 0.01 to 0.029, and \(X_{4}\) from 10.22 to 25.26 respectively.

The effective efficiency (\(\eta_{{{\text{ef}}}}\)) and effective power (\(P_{{{\text{ef}}}}\)) of the dual fuel cycle is given by

$$\eta_{{{\text{ef}}}} = \frac{{P_{{{\text{ef}}}} }}{{Q_{{\text{f}}} }}$$
(13)

where \({Q}_{\mathrm{f}}\) is the total heat potential of the fuel in kJ/kg.

$$P_{{{\text{ef}}}} = Q_{{{\text{in}}}} - Q_{{{\text{out}}}}$$
(14)

where \(Q_{{{\text{in}}}}\) is the total heat added during constant volume heat addition (\({Q}_{in1})\) process (process 2–3) and constant pressure heat addition (\({Q}_{\mathrm{in}2})\) process (process 3–4) and is given by

$$Q_{{{\text{in}}}} = Q_{{{\text{in}}_{{1}} }} + Q_{{{\text{in}}_{{2}} }} = m_{t} \left[ {\int\limits_{{T_{2} }}^{{T_{3} }} {C_{{\text{v}}} {\text{d}}T + \int\limits_{{T_{3} }}^{{T_{4} }} {C_{{\text{p}}} {\text{d}}T} } } \right]$$
(15)

The total heat rejection from the cycle \(Q_{{{\text{out}}}}\) is given by

$$Q_{{{\text{out}}}} = m_{t} \left[ {\int\limits_{{T_{1} }}^{{T_{5} }} {C_{{\text{v}}} {\text{d}}T} } \right]$$
(16)

where \({m}_{\mathrm{t}}\) is the total mass flow rate of fuel and air entering into the engine cylinder in kg/s and is given by

$$m_{{\text{t}}} = m_{{\text{a}}} + m_{{\text{f}}}$$
(17)

where \(m_{{\text{a}}}\) is the mass flow rate of air and \(m_{{\text{f}}}\) is the mass flow rate of fuel and is given by

$$m_{{\text{f}}} = m_{{\text{d}}} + m_{{{\text{pg}}}}$$
(18)

where \(m_{{\text{d}}}\) and \(m_{{{\text{pg}}}}\) are the mass flow rates of diesel and producer gas, respectively.

In most cycle models, the working fluid is assumed to behave as an ideal gas with constant specific heats. The specific heats at constant volume \(C_{{\text{v}}}\) and pressure \(C_{{\text{p}}}\) can be expressed as a function of Temperature as given as follows [47]:

$$\begin{aligned} C_{{\text{p}}} = & 2.506.10^{{ - 11}} T^{2} + 1.454.10^{{ - 7}} T^{{1.5}} \\ & - 4.246.10^{{ - 7}} T + 3.162.10^{{ - 5}} T^{{0.5}} \\ & + 1.3301 - 1.512.10^{4} T^{{ - 1.5}} \\ & + 3.063.10^{5} T^{{ - 2}} - 2.21.10^{7} T^{{ - 3}} \\ \end{aligned}$$
(19)
$$\begin{aligned} C_{{\text{v}}} = & 2.506.10^{{ - 11}} T^{2} + 1.454.10^{{ - 7}} T^{{1.5}} \\ & - 4.246.10^{{ - 7}} T + 3.162.10^{{ - 5}} T^{{0.5}} \\ & + 1.0433 - 1.512.10^{4} T^{{ - 1.5}} \\ & + 3.063.10^{5} T^{{ - 2}} - 2.21.10^{7} T^{{ - 3}} \\ \end{aligned}$$
(20)

Total heat potential of the fuel (\(Q_{{\text{f}}}\)) is the amount of heat released during the combustion of the fuel and is expressed as

$$Q_{{\text{f}}} = m_{{\text{d}}} \times H_{{\text{d}}} + m_{{{\text{pg}}}} \times H_{{{\text{pg}}}}$$
(21)

where \(H_{{\text{d}}}\) and \(H_{{{\text{pg}}}}\) are the calorific values of diesel KJ/kg and producer gas KJ/Nm3, respectively.

The diesel saving of a dual fuel engine is calculated by variation in air fuel ratio of the fuel mixture. The equivalence ratio is given by

$$\Phi = \frac{{{\raise0.7ex\hbox{${m_{{\text{f}}} }$} \!\mathord{\left/ {\vphantom {{m_{{\text{f}}} } {m_{{\text{a}}} }}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${m_{{\text{a}}} }$}}}}{{F_{{{\text{st}}}} }}$$
(22)

where \(F_{{{\text{st}}}}\) is the stoichiometric air fuel ratio and is calculated using Eq. (23) [48]. By balancing the equation, the stoichiometric fuel air ratios for producer gas derived from rice husk, coconut shell and rubber shell was found as 1.26, 1.57 and 1.69, respectively.

$$\begin{aligned} & C_{{\text{m}}} H_{{\text{n}}} + {\text{ }}\left( {m{\text{ }} + \frac{n}{4}} \right){\text{ }}\left( {{\text{O}}_{2} + 3.76\,N_{2} } \right) \\ & \quad \to m{\text{CO}}_{2} + \frac{n}{2}{\text{H}}_{2} {\text{O}} + 3.76\left( {m{\text{ }} + \frac{n}{4}} \right)N_{2} \\ \end{aligned}$$
(23)

The engine is assumed to run on constant speed mode at 1500 rpm, and the governing system was used to control the required power output. On only diesel mode of operation, generally, the amount of air drawn by the engine was in excess of stoichiometric requirement. When producer gas was introduced into the system, the amount of air drawn by the engine eventually decreased. Hence air–fuel ratio was reduced. The methodology for modelling study for a dual cycle powered diesel producer gas engine using Finite Time Thermodynamics is as shown in Fig. 4.

Fig. 4
figure 4

Flow chart showing Finite Time Thermodynamic Modelling for a dual fuel engine

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3 Results and Discussion

The performance of a 5 HP (3.75 kW) diesel engine was assessed using both the diesel and dual fuel mode. A 150 kWth capacity downdraft gasifier was employed to generate producer gas for executing the engine performance trials in dual fuel mode. Three agro-residues, namely as rice husk, coconut shell, and rubber shell were utilised to evaluate the performance of the biomass gasifier operated dual fuel engine. Experiments were carried out with different brake power (0.5–3.75 kW) and compression ratios (12–18) in both diesel and dual fuel modes on the combustion of three chosen agro-residues and the results are presented below in detail.

3.1 Effect on Brake Thermal Efficiency (BTE) with the Engine Load and CR

The variation of brake thermal efficiency of the engine operated with diesel only mode and dual fuel (diesel-producer gas) modes with respect to engine loads and various CR for the selected feedstocks are as shown in Figs. 5 and 6. It shows that the brake thermal efficiency increases with increase in engine load. A similar trend has been observed in the previous studies also [49]. It is found that the maximum brake thermal efficiency of diesel only mode was 24.28% at the engine load of 3.44 kW. Dual-fuel engines are always less efficient than the stand alone diesel fuelled engines. Because of the lower combustion rate caused by the low calorific value of producer gas, poor efficiency is observed at lower loads of dual fuel operation. Diesel also causes poor ignition and combustion of the lean air-gas mixture at these low loads. As a result, a small effect of producer gas on part-load efficiency has been detected. However, beyond 40% load, the brake thermal efficiency of dual fuel operation is improved. This is due to the greater amount of premixing of the fuel producer gas combination and the quicker combustion rate of the producer gas. The thermal efficiency of a diesel-fuelled engine is higher because more diesel implies more energy released, resulting in higher thermal efficiency. The efficiency of the engine improves as the calorific value of the producer gas rises. It is also observed that the BTE of rice husk was 19.13% at CR 16 and 3.44 kW load. Similarly, at CR14 and 3.44 kW, the BTE of 19.44% was recorded for coconut shell. Rubber shell had the highest BTE of 19.80% at CR16 and 2.56 kW. Based on the trends, Rubber shell feed stock has a highest BTE than other feed stocks because the calorific value of producer gas is greater.

Fig. 5
figure 5

Variations of BTE with CR and BP in diesel only mode

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Fig. 6
figure 6

Variations of BTE with CR and BP in dual fuel (diesel-producer gas) mode as a rice husk b coconut shell c rubber shell as a feedstock

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3.2 Effect on Brake Specific Fuel Consumption (BSFC) with the Engine Load and CR

The BSFC value is a measure of efficiency of the fuel and it indicates how efficiently the engine converts supplied fuel into useful work. The variation of brake specific fuel consumption of the engine operated with diesel only mode and dual fuel (diesel-producer gas) modes with respect to engine loads and various CR for the selected feedstocks are as shown in Figs. 7 and 8. Equations (5) and (6) are used to compute brake specific fuel consumption in diesel and dual fuel modes. It is noticed that BSFC is lower at higher loads and increasing at lower loads. For all CR, the lowest BSFC (0.33 kg/kWh) is obtained at the maximum load (3.44 kW) and the highest BSFC (3.44 kW) is obtained at CR16 for diesel only mode. At CR12 and 3.44 kW, ricehusk had a minimum BSFC of 0.67 kg/kWh. Coconut shell yielded 0.66 kg/kWh at CR 12 and 3.44 kW, whereas rubber yielded 0.64 kg/kWh at CR 18 and 3.44 kW. The findings revealed that the calorific value of producer gas from various feedstocks influenced the BSFC of the engine. This is because with higher loads, more fuel is injected into the cylinder, resulting in better combustion and effective usage of the fuel consumed. According to the results, rubber shell based producer gas had a lower BSFC on dual fuel mode than rice husk and coconut shell, indicating that rubber shell derived PG is efficiently utilised for producing the required power. The BSFC decreases when the calorific value of the fuel decreases, since a decrease in the calorific value of a fuel leads in an increase in the engine's fuel consumption.

Fig. 7
figure 7

Variations of BSFC with CR and BP in diesel only mode

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Fig. 8
figure 8

Variations of BSFC with CR and BP in dual fuel (diesel-producer gas) mode as a rice husk b coconut shell c rubber shell as a feedstock

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3.3 Effect on Brake Specific Energy Consumption (BSEC) with the Engine Load and CR

The variation of brake specific energy consumption of the engine operated with diesel only mode and dual fuel (diesel-producer gas) modes with respect to the engine loads and various CR for the selected feedstocks are as shown in Figs. 9 and 10. For diesel only mode and dual fuel (diesel-producer gas) modes, the BSEC shows an opposite trend to BTE. The lower the BTE gives the lower the BSEC. Equations (7) and (8) were used to determine the BSEC. The maximum BSEC of 32.12 kJ/kWh was recorded at 0.86 kW for diesel only mode, and as load increases, the BSEC declines to 15.44 kJ/kWh at 3.44 kW and CR 16. The lowest BSEC was 15.05 kJ/kWh for rice husk and 15.26 kJ/kWh for coconut shell at CR14 and 3.44 kW, respectively. At CR 18 and 2.56 kW, a lower BSEC of 12.19 kJ/kWh was recorded for rubber shell. Because the calorific value of the air gas mixture is lower at greater loads and the fuel consumption of producer gas is higher, the BSEC is lower. At lower loads, incomplete combustion predominates, resulting in a rise in the BSEC of the engine.

Fig. 9
figure 9

Variations of BSEC with CR and BP in diesel only mode

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Fig. 10
figure 10

Variations of BSEC with CR and BP in dual fuel (diesel-producer gas) mode as a rice husk b coconut shell c rubber shell as a feedstock

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3.4 Variations of Biomass Consumption with the Engine Load and CR

Before a prototype can be developed for real-time applications, it is critical to calculate the biomass consumption rate for any biomass power producing system. As a result, the biomass consumption rate must be determined under various operating conditions. The efficiency of the gasifier system was chosen at 75% [50]. In this investigation, and the BM consumption rates for the rice husk, coconut shell and rubber shell feedstocks were calculated and represented as shown in Fig. 11. The figure showed that the BM consumption rate increased with load for all the feedstocks. For ricehusk biomass the BMC increases with increase in load and maximum biomass consumption of 4.56 kg/h is observed at 3.44 kW. This is because of the higher loads, more fuel is used to provide the required power, resulting in an increase in producer gas consumption. The BM consumption reduces as the CR ratio increases, and the lowest value of BMC is found at CR16 and 3.44 kW. The BMC grows upwards to 4.37 kW at CR 18. This rise in BMC usage to 4.37 kW shows that BMC is reliant on the engine's BTE. The BTE is slightly decreased at CR 18, resulting in an increase in BMC. The BSEC is greater for all feedstocks. Rubber shell and coconut shell both showed similar patterns. The BMC for rubber shell is 3.53 kg/h at CR18 and 3.44 kW, whereas the BMC for coconut shell is 3.80 kg/h. This is because as the CV of producer gas rises, the amount of fuel required to generate the required power drops.

Fig. 11
figure 11

Variations of BMC with CR and BP in dual fuel (diesel/producer gas) mode as a rice husk b coconut shell c rubber shell as a feedstock

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A generalised equation for the optimal percentage of diesel savings has been derived from the data reduction of experimental readings using multivariable regression analysis, as shown in Eq. (12). Using this optimal value of diesel savings, the optimal solution of BTE, CR, and BP can be examined, and this value is used as blend (per cent of PG) for theoretical analysis. Table 7 shows the optimum values of percentage of diesel savings, brake thermal efficiency, CR and BP.

Table 7 Optimum values of diesel savings, brake thermal efficiency, CR and BP
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According to the experimental results, the maximum BTE for biomass generated from rice husk is 19.13% at CR16 and 3.44 kW, while the % of DS value is only 7%. At CR14 and 0.86 kW, the maximum % of DS is 11.6%, but the equivalent efficiency is 9.98%. At CR18 and 2.56 kW, an optimal value of BTE 17.91% and DS 11.2% is obtained. At CR14 and 3.44 kW, a maximum BTE of 19.44% is attained for coconut shell biomass, while the corresponding DS value is only 13%. At CR18 and 1.72 kW, the maximum diesel savings is 27.14%, while the efficiency is just 16.95%. At CR 18 and 2.56 kW, there is a 17.54% improvement in diesel savings. At CR16 and 2.56 kW, a maximum BTE of 19.80% is achieved from rubber shell-based biomass, while the corresponding DS value is only 38%. At CR18 and 0.86 kW, the maximum DS value is 58.18%, however the efficiency is 12.5%. The optimal value is found at CR18, load 2.56 kW, and DS 48%, with a BTE of 21.68%. It was observed that the % of DS increased with calorific value of producer gas and CR allowed in to the engine cylinder. This is due to the fact that when the CR is high, more fuel (Diesel and PG) is admitted into the engine cylinder. Diesel and producer gas must supply the same amount of heat to create the required amount of electricity. Since producer gas obtained from rubber shells has a higher calorific value than producer gas derived from other feedstocks, it has a higher diesel replacement rate.

From the experimental results obtained it is found that the optimum values are found at 2.56 kW, which is obtained at the contributions for 48% for rubber shell based PG, 24% Coconut shell based PG, 11.2% for Rice husk based PG and this is taken as optimum value for further calculations. The values obtained are in similar range to those found in previous literature and are presented in Table 8. The BTE is computed and shown for theoretical and experimental for this optimal load (2.56 kW) and blend conditions (48% for rubber shell based PG, 24% Coconut shell based PG, 11.2% for Rice husk based PG) as shown in Fig. 12. It is evident from the plot that the theoretical model is closely in relevance to the experimental data. Figure 12 shows that as the compression ratio is increased, the BTE progressively rises and finally decreases. The fall in BTE at greater CR is due to a reduction in engine cylinder capacity, which leads to incomplete combustion and hence a reduction in efficiency. Also, due to incomplete combustion, the heat energy released by combustion is lower at greater loads, contributing to a drop in efficiency.

Table 8 Comparison of Experimental results with existing literature
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Fig. 12
figure 12

Experimental and theoretical comparison of brake thermal efficiency with compression ratio

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4 Conclusions

The experimental and theoretical performance parameters of a variable compression ratio (VCR) CI engine fulled with a diesel-producer gas (PG) combination obtained from rice husk, coconut shell, and rubber shell gasification were investigated in this work. Both the diesel and dual fuel modes were used to evaluate the performance of a 5 HP (3.75 kW) diesel engine. To provide producer gas for the engine performance measurements in dual fuel mode, a downdraft gasifier with a capacity of 150 kWth was used. Experimental results have been used to verify a novel theoretical model based on finite-time thermodynamics (FTT). Based on the performance of the VCR engine fuelled with a diesel-producer gas combination of three chosen agro-residues the following conclusions are drawn.

  • The maximum brake thermal efficiency of the diesel only mode was observed to be 24.28% at 3.44 kW engine load, which is greater than the dual fuel engine since more diesel implies more energy released, resulting in higher thermal efficiency.

  • The rubber shell-fueled dual fuel (DF) engine had the highest brake thermal efficiency of 19.80%, compared to 19.13% and 19.44% for rice husk and coconut shell-fueled DF engines, respectively. Since rubber shell has a higher calorific value than rice husk and coconut shell, which leads to the higher efficiency.

  • The study found that the BSFC of the engine was influenced by the calorific value of producer gas from various biomass feedstocks. The experimental results revealed that the rubber shell-derived producer gas had a lower BSFC (0.64 kg/kWh) in dual fuel mode than rice husk (0.67 kg/kWh) and coconut shell (0.66 kg/kWh), this implying that rubber shell-derived PG is effectively used to provide the required energy.

  • It is revealed that the rubber shell-derived producer gas had a lower BSEC of 12.19 kJ/kWh in dual fuel mode than rice husk and coconut shell as 15.05 kJ/kWh and 15.26 kJ/kWh respectively.

  • Rubber shell has a BMC of 3.53 kg/h at CR18 and 3.44 kW, while coconut shell has a BMC of 3.80 kg/h and the rice husk has a BMC of 4.56 kg/h. This is because the amount of fuel required for generating the required power decreases as the heating value of producer gas rises.

  • The optimum values are found at 2.56 kW of engine load, which is obtained at contributions of 48% for rubber shell based PG, 24% for Coconut shell based PG, and 11.2% for Rice husk based PG, according to the experimental results.

  • In this work, experimental results have been used to validate a new theoretical model based on finite-time thermodynamics (FTT). It is evident that the theoretical model is closely relevance to the experimental data.

  • It was concluded that the rubber shell derived producer gas fuelled dual fuel engine showed better BTE and diesel savings compared with the ricehusk and coconut shell based biomass fuel.

The findings from this research will be helpful for researchers and engine design engineers working in the field of biofuel powered dual fuel engines. The FTT model will help researchers to get reliable results for predicting the performance of the engine without doing experimentation on PG diesel-based dual fuel engines. As renewable biofuel technology is a promising area, this research has a lot of scope to be extended further by varying operating parameters like injection timing, using PG from a new source of biomass, pre-treating the biofuels, etc.