Elsevier

Applied Thermal Engineering

Volume 150, 5 March 2019, Pages 1090-1103
Applied Thermal Engineering

Research Paper
The effects of compression ratio and EGR on the performance and emission characteristics of diesel-biogas dual fuel engine

https://doi.org/10.1016/j.applthermaleng.2019.01.080Get rights and content

Highlights

  • Performance and emission features of diesel-biogas dual fuel engine are studied.

  • Effects of compression ratio, EGR and EGR temperature are investigated.

  • Exergy analyses is applied and various sources of irreversibilities are quantified.

  • Higher compression ratio improves the diesel substitution and efficiency.

  • Rate of EGR and EGR temperature also affects the engine characteristics.

Abstract

In this study, an experimental investigation on diesel-biogas dual fuel (DF) engine is presented based on energy and exergy analyses. The effects of change in compression ratio (CR), exhaust gas recirculation (EGR) and EGR temperature on the performance and emission characteristics of DF engine have been studied. In the first stage, engine was studied with increasing CRs of 16.5, 17.5, 18.5 and 19.5 in stepwise manner. It was found that the higher CRs were not only advantageous to the engine performance from first and second-law point of view but also to the exhaust emissions. In the second stage, DF engine was studied at the highest CR (19.5) and the effects of EGR were analysed. The engine was studied with EGR percentages of 5%, 10% and 15%, which caused slight improvements in engine efficiency at low load and simultaneous decrease in oxides of nitrogen (NOx) emissions. However, high EGR percentages at high loads showed slight decrease in engine efficiency. In the third stage, hot EGR was employed and the results obtained were compared with the cold EGR case. The results showed that the highest efficiencies both at low and high loads were obtained with hot EGR cases and at the same time exhaust emissions could also be kept in check.

Introduction

The energy demand has escalated sharply in the last few decades mainly driven by the strong economic growth and industrialization [1]. Most of the world’s energy demands are fulfilled by the fossil based energy sources, burning of which has considerably strained the ecological stability and have brought about serious environmental changes [1], [2]. The petroleum and other liquid fuels provide highest energy share of the world’s total energy demand with transportation sector being the largest (∼54%) consumer [1]. This indicates that transportation sector is one of the leading contributors in the present environmental instability and will lead in the future due to its fast-growing demand. The finite nature of fossil fuels and ecological damages caused by their consumption question the sustainability of a growth model solely based on them. Therefore, world’s attention has been shifted towards low-carbon fuels in the future fuel mix. The utilization of alternative and renewable fuels in the existing and future prime movers will play a key role in defining a sustainable development model. In this context, biogas is one of the important renewable fuels, especially in the developing countries [3], [4], [5].

Among the number of pathways of energy exploitation from biogas, its utilization in internal combustion engines (ICEs) is of prime interest [6]. The conventional diesel engines can be modified to work with biogas in dual fuel (DF) mode in which gaseous fuel (biogas) acts as the main fuel, i.e. provides major energy share, and liquid fuel acts as the pilot fuel, i.e. used in small quantities [7], [8]. The purpose of the pilot fuel is to ignite the gaseous fuel-air mixture and initiate the combustion so that it can further be sustained by combustion of the main fuel [8]. The DF operation of a conventional diesel engine has many advantages, such as good efficiency, low emission and fuel flexibility depending upon the types of fuel used [9]. In addition to that, with little modifications, it is possible to target the widely accepted diesel engine technology, and therefore maximize the scope of renewable fuel usage. However, there persist many technical challenges in the development of a DF technology. In the case of diesel-biogas DF technology, the major difficulties lie with relatively low conversion efficiency, high hydrocarbon (HC) and carbon monoxide (CO) emissions and limited percentage of gaseous fuel utilization [9], [10]. The biogas, which is typically a mixture of methane (CH4), carbon dioxide (CO2) and small percentages of other gases has very low calorific value. In addition to that the presence of inert gases like CO2 reduces the in-cylinder temperature by absorbing some amount of heat causing poor combustion [10], [11], [12]. Some of these problems, however at the different levels, have been reported with various fuel combinations in a DF engine.

In previous studies, various methodologies have been implemented to enhance the performance and emission characteristics of a DF engine. Yang et al. [13] investigated the effects of pilot injection timing on the particle emissions and combustion noise of a diesel-natural gas dual-fuel engine. It was found that by advancing the injection timing both performance and emission characteristics can be improved. However, combustion noise was found as the limiting factor, which also deteriorated with the advanced injection timing. It was also found that particle emissions were very sensitive to injection timing and can be reduced with advancement. The effects of pilot fuel quantity was studied by Abagnale et al. [14], and it was shown that 50% of methane ratio was optimum for pollutant and CO2 control. Banapurmath et al. [15] analysed the effect of compression ratio (CR) on a biodiesel-CNG DF engine. The experiments were performed by varying the CR from 15 to 17.5 keeping all the other operating parameters (such as injection timing, injection pressure, flow rates etc.) constant. It was reported that increasing the CR resulted in higher cylinder pressure and heat release rates, which rendered improvements in the brake thermal efficiency. By increasing the CR from 15 to 17.5, 1.4% improvement in the brake thermal efficiency was observed. Along with that HC, smoke and CO emissions were considerably decreased, whereas oxides of nitrogen (NOx) emissions were slightly increased. Sayin and Gumus [16] experimentally investigated the effects of CR and injection parameters on a biodiesel-diesel blended direct-injection diesel engine. It was found that by increasing the CR from 17 to 18 and 19, brake thermal efficiency was increased by 0.55% and 1.33% respectively. It was reasoned that with higher CRs, the fuel was injected at hotter region leading to improvement in the combustion process. The improvement in thermal efficiency and lower exhaust gas temperature with increased CR have also been reported by Muralidharan and Vasudevan [17] with waste cooking oil as biodiesel and diesel blend in a variable compression ratio diesel engine. This investigation also revealed that at increased CR, biodiesel-diesel blend showed higher combustion pressure, higher rate of pressure rise and lower heat release rate as compared to that with the diesel. The thermodynamic analysis on the effect of varying the CR was studied by Debnath et al. [18] with palm oil methyl ester as the biodiesel fuel. They found that higher CRs showed increase in shaft power, but also increased the energy loss to cooling water and exhaust gases. However, on exergetic terms the variations of these losses were found very small. Therefore, at the higher CRs, exergy destruction was decreased showing better thermodynamic performance of the engine.

Bora and Saha [19] studied a biogas run diesel engine in DF mode by varying the injection timing (26°, 29° and 32° before top dead centre (BTDC)) and CR (18, 17.5, 17 and 16). The results indicated that the highest brake thermal efficiency of 25.44% was obtained at the CR of 18 and the injection timing of 29° BTDC. However, at these operating conditions both NOx and CO2 emission were also highest. Masood et al. [20] investigated on the performance and emission characteristics of a diesel-hydrogen DF engine with different CRs. They reported that the percentage change in brake thermal efficiency when the CR was changed from 16.35 to 24.5 was about 27% at 90% hydrogen substitution and full load. At the same time reductions in HC, CO and particulate matter emissions were 17%, 21% and 16% respectively, however, NOx emissions were increased by 38%. Effect of CR was also investigated by Gnanamoorthi and Devaradjane [21] on a direct-injection diesel engine with various ethanol-diesel blends. They found that the highest compression ratio (19.5) generated the highest efficiency. The increase in CR also improved a range of engine performance parameters along with reduced HC and CO emissions, however, NOx emissions were still reported to be high. Selim [22] examined the effect of CR on combustion noise, knock and ignition limits of a DF engine with LPG, methane and compressed natural gas (CNG) as the main gaseous fuels and diesel as the pilot fuel. It was observed that increasing the CR generally increases the combustion noise, especially with the gaseous fuels of low self-ignition temperature. The descending order of combustion noise from DF engine was LPG, methane and CNG.

Therefore, even though the literature suggests better engine performance with higher CRs, the high NOx emissions remains a major constraint. In one of the studies, Laguitton et al. [23] even suggested to reduce the CR along with the injection timings to achieve very low NOx emissions in a premixed charge compression ignition (PCCI) diesel engine; however, the fuel consumption was increased, especially at the high loads. One of the methodologies to curb NOx emissions from high CR DF engines could be the use of EGR. Based on a two-zone phenomenological model, Papagiannakis [24] investigated the effect of inlet air preheating and EGR on a natural gas DF engine. It was found that increasing the inlet air temperature improved the engine efficiency, however, the in-cylinder pressure was also significantly high posing the risk of mechanical damage. With the use of EGR not only the in-cylinder pressure was controlled but also the NOx and CO emissions were reduced. Abdelaal and Hegab [25] reported that the use of EGR on a natural gas-diesel DF engine reduced the peak cylinder pressure and hence prolonged the engine life. Hu et al. [26] studied the effect of EGR and hydrogen addition in a natural gas operated spark ignition engine. It was found that increase in EGR first increased and then decreased the thermal efficiency of engine. At high EGR, addition of hydrogen in natural gas was found as a favourable approach to achieve better efficiency along with low emissions. It was explained that at high EGR condition, coefficient of variation of indicated mean effective pressure and cycle by cycle variations were significantly increased, which can be controlled by addition of hydrogen and therefore a stable low-temperature combustion can be realized [27], [28]. A quasi-two-zone combustion model was proposed by Hosseinzadeh et al. [29] to study the second-law analysis of diesel-natural gas DF engine under part load conditions. It was proposed to study the different cases of chemical, radical and thermal EGR on the performance of DF engine at low load. They found that without EGR, 28% of total input chemical exergy was wasted as unburned fuel in the exhaust. Furthermore, radical, thermal and their combined cases of EGR were found to have positive effects on availability terms, whereas, chemical case showed the opposite effect. Similar results were reported by Jafarmadar and Nemati [30] using a simulation model to study the effects of EGR fractions on hydrogen-diesel DF operations. As EGR fractions were increased from 0% to 30%, the exhaust exergy was increased from 14.9% to 56.7% and the exergy efficiency was decreased from 42.4% to 14.1%.

Tomita et al. [31] studied the effect of EGR on combustion and emission characteristics of diesel-natural gas DF engine. It was reported that when the EGR rate was increased, the flame area and the luminescence intensity of the pilot fuel flame were decreased. The high EGR rate caused the ignition delay to be lengthened and the start of heat release was delayed. Ghazikhani et al. [32] investigated the relationship between irreversibility and brake specific fuel consumption in an indirect injection diesel engine with various EGR ratios. It was found that at low load and low engine speed, induction of EGR led to considerable decrease in the total in-cylinder irreversibility, however, at high load the effect was opposite. At all the EGR ratios, irreversibility and brake specific fuel consumption showed similar trends. The effect of EGR on a DF (diesel-natural gas) HCCI mode was numerically studied by Jafarmadar et al. [33] based on exergy analysis. They found that with increase in EGR, high specific heat of EGR components and decreased oxygen concentration led to reduction in the heat release and therefore poor combustion. At the 30% EGR condition, exergy efficiency and burned fuel were decreased by 41.3% and 37.7% respectively causing 29% increase in irreversibility. Ismail and Mehta [34] found that as the EGR fraction was increased, the exergy destruction was also raised mainly due to lower product temperature. Though, this was also translated to reduction in product chemical exergy losses resulting in the improvement of exergy efficiency.

Based on the literature review, it has been found that the variation in CR and EGR addition are possible techniques to improve the engine performance and control the emissions respectively. In a few reported studies with DF engines, these techniques have been independently investigated with gaseous fuels, such as methane and hydrogen. However, biogas shows significantly different thermo-physical properties as compared to the fuels reported in the literature. Therefore, it is necessary to investigate the effects of these parameters on diesel-biogas DF engine in detail. It has been shown in this work that it is possible to improve the diesel-biogas DF engine performance without compromising with the emissions by optimising the CR and EGR. The results are presented with energy and exergy analyses, which has received little attention in the literature but gives vital information about the scope of improvements. The experimental study has been carried out to encompass the variations in engine parameters, such as CR, EGR fraction and EGR temperature. The emissions analyses for HC, CO, smoke and NOx emissions have also been performed for above engine parameters.

Section snippets

Test rig

The tests were conducted in a four-stroke, single-cylinder, direct-injection modified diesel engine. The engine comes with the original CR of 17.5, injection timing of 23 °BTDC and constant engine speed of 1500 rpm. It has a bowl-in-piston combustion chamber and MICO BOSCH, 3-hole diesel injector. The major specifications of the test engine are given in Table 1. The engine was coupled to an AC dynamometer, which was connected to an electrical loading panel to control and vary the engine loads.

Effect of compression ratio (CR)

Variation in the fuel flow rates (both diesel and biogas) with engine load is shown in Fig. 2 for different CRs. The fuel flow rates increase with increase in engine load to satisfy the high energy demand. It was also found that the biogas flow rate was always higher than the diesel flow rate at the same operating conditions because of the high pilot fuel substitution and lower calorific value of biogas. The effect of increase in CR was found positive on fuel demand as it lowered the fuel flow

Conclusions

In the present article a diesel-biogas DF engine was studied to investigate the effects of variation in CR and addition of EGR on performance and emission characteristics of the engine. The CR was varied from 16.5 to 19.5 in stepwise manner and engine operations were studied at maximum pilot fuel substitution settings. The engine operations were also studied at three EGR percentages of 5%, 10% and 15%. In addition to that the effect of EGR temperature was also investigated on engine performance

Acknowledgement

We would like to acknowledge the support of Council of Scientific & Industrial Research (CSIR), India. Research facility provided by Indian Institute of Technology Delhi (IITD), India is also gratefully acknowledged.

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