Improving the combustion process by determining the optimum percentage of liquefied petroleum gas (LPG) via response surface methodology (RSM) in a spark ignition (SI) engine running on gasoline-LPG blends

Highlights

• RSM was utilized to optimize the engine performance and emissions.

• Percentage of LPG and engine load have been chosen as input parameters.

• The proposed RSM model can reveal engine responses with high accuracy.

• Optimized results are 35% LPG ratio and 2400 W engine load.

Abstract

In the current research, it is aimed to determine the optimum ratio of liquefied petroleum gas (LPG) to be used efficiently in terms of performance and emissions in a spark-ignition (SI) engine running on gasoline-LPG blends with response surface methodology (RSM). To create the RSM model, LPG and engine load were selected as input variables, while performance and emission responses affected by input variables were selected as brake specific fuel consumption (BSFC), brake thermal efficiency (BTE), carbon monoxide (CO), carbon dioxide (CO₂), and hydrocarbon (HC). Analysis of variance (ANOVA) supported RSM analysis was performed according to the selected factors and responses, it was found that LPG had a significant effect on all responses. Moreover, it was concluded that BSFC and BTE are the most affected responses to LPG ratio change. Also, according to the optimization results, the optimum factor levels were determined as 35% and 2400 W for LPG and engine load, respectively. According to the verification study, the maximum error between the experimental results and the optimization results was found as 3.75%. As a result, it is concluded that the SI engine fueled with LPG can be successfully modeled with low error rates by using RSM.

Introduction

In addition to the air pollution caused by the development of industry and urbanization, air pollution caused by internal combustion engine vehicles have also negative effects on human health and other living creatures [[1], [2], [3]]. Fossil fuels, which have been used since the invention of internal combustion engines, have adversely affected the environment [4,5]. At first, this effect occurred at relatively low levels, as the environment reduced this effect within its natural mechanism. Today, due to the rapidly increasing human population, a significant part of people live in regions (especially in big cities) where air quality standards deteriorate [6]. In addition to the increasing population, the inevitable increase in the number of vehicles also affects the environment negatively and exceeds the natural self-protection capacity of the environment [[7], [8], [9]]. Until recent years, the fuels utilized in internal combustion engines were gasoline and diesel. Increasing fuel prices due to decreasing fossil fuel reserves as well as air pollution have led researchers and users to seek alternative fuels to diesel and gasoline in terms of both emission and cost [[10], [11], [12]]. LPG is an alternative fuel within the meaning of the European Union Directive (2014/94/UE), as it is an alternative for energy sources derived from crude oil [13]. LPG has attracted a great deal of attention in recent years, especially because it is cheap and generally produces less emissions compared to fossil fuels [14]. Lead tetra ethyl, which is used to increase the octane number of gasoline, is not found in LPG fuel. This ensures that the pollutant emissions in the exhaust are reduced by the use of LPG. In addition, due to the absence of sulfur in LPG, sulfur oxide emissions and soot and particulate emissions, which are often seen in diesel engines, do not occur. The relationship between the price of LPG and Gasoline in 2020 was very well compared in the article [13,15] additionally, they showed another relationship regarding the reduction of pollution for the environment after the use of LPG [13] and the reduction of fuel consumption [13,15]. LPG is a colorless, odorless, heavier than air, and flammable gas obtained during the distillation of crude oil in refineries or by separating the natural gas on oil deposits and liquefied under pressure [16,17]. Although the first use of LPG in internal combustion engines dates back to the 1910s, it became widespread in the 1950s [18]. Even though this fuel primarily consists of propane and butane, it may also contain different hydrocarbons such as propane, iso-butane, and n-butane in different percentages [19,20]. LPG is an attractive alternative fuel for SI engines because of its higher octane number than gasoline and lowers exhaust emission [21,22]. The high-octane number of LPG enables it to work without knocking with a higher compression ratio in SI engines. The effect of LPG use on the performance and emission responses of an SI engine has been extensively investigated in previous studies. Duc and Duy [23] conducted an experimental study to assess the combustion and emission performance of four-cylinder, four-stroke SI engine under idle engine conditions and using LPG/gasoline. The results obtained from the study revealed that there was a 4.5% decrease in-cylinder pressure with LPG compared to gasoline, and emissions such as CO, HC, and nitrogen oxide (NO_x) were reduced with LPG compared to gasoline. Simsek and Uslu [24] experimentally determined the effects of using LPG at different throttle positions in a single-cylinder, four-stroke, SI engine on engine performance and emissions. In the evaluation made in terms of engine performance, they determined an increase of 34.19% in BSFC and a decrease of 8.95% in BTE compared to gasoline usage by using LPG at full throttle opening. On the other hand, they found a reduction of 63% in HC emissions, 62.03% in CO emissions, and 56.42% in CO₂ emissions with the utilization of LPG. The authors indicated that the BSFC rose by 45.51% and the BTE reduced by 20.22% in the half gas opening. Besides, they found a reduction of 47.65% in CO emissions, 62.38% in HC emissions, and 69.54% in CO₂ emissions. Usman et al. [25] carried out their experiments in the engine speed range varying from 1600 rpm to 3400 rpm and 60% gas opening to determine the effects of LPG use on SI engine. They stated that emissions related to LPG such as CO, CO₂, and HC were reduced by 21%, 9%, and 21.8%, respectively. Bin Mohd Zain et al. [26] compared the use of LPG in the SI engine with the use of gasoline in terms of engine performance. The authors suggested that LPG produces lower engine performance than gasoline and to optimize fuel consumption, the best setting for injector fuel mapping for LPG fueled engine. Sabariah et al. [27] simulated the effect of LPG usage on performance, combustion process and emissions in a four-cylinder SI engine and compared to gasoline usage. The authors stated that lower HC and NO_x emissions occur with the use of LPG compared to gasoline, and the simulation application can be used successfully to calculate engine performance at different operating points. Wargula et al. [13] have compared the use of LPG and gasoline in a SI engine used as a woodchipper in terms of fuel consumption, emissions, and price. The authors stated that with the use of LPG, CO and NO_x emissions increased by 22% and 27%, respectively, while fuel consumption decreased by 28% and CO₂ and HC emissions by 37% and 83%, respectively.

In addition to its advantages, it also has a disadvantage such as a decrease in power output due to the decrease in volumetric efficiency in SI engines in the use of LPG. LPG, which is in the form of gas in carburetion or manifold injection, usually causes a decrease in volumetric efficiency and power, as it replaces some of the inlet air [28,29]. Therefore, it is extremely important to determine the optimum LPG ratio to obtain optimum performance and emission levels in the use of LPG in SI engines. RSM is one of the optimization techniques used in internal combustion engines. RSM is a mathematical method aimed at simultaneously determining the optimum levels of responses affected by many factors and the corresponding optimum factor levels [30,31]. RSM is a computer application that can provide high accuracy results with less experimentation compared to classical experimental methods. While the determination of optimum operating parameters (injection advance, ignition advance, engine load, injection pressure, compression ratio, mixing ratio, etc.) in internal combustion engines can be made with thousands of experiments by experimental methods, it can be determined in the range of 15-50 experiment numbers depending on the number of parameters with RSM. On the other hand, although RSM has the potential to evaluate the interaction effects of independent input parameters, it also has the disadvantage that it cannot be used to explain why an interaction(s) occur. In addition, another disadvantage is that the RSM is poor at predicting potential outcomes for a system operating outside of the operating range under consideration. The ability and accuracy of RSM techniques in optimizing the performance and emission responses of a SI engine have been extensively studied in previous studies. Abdalla et al. [32] aimed to optimize the effects of fusel oil-gasoline fuel mixtures on engine performance and emissions using RSM. The results of the study revealed that the optimum operating parameters were engine load corresponding to 60% throttle opening, 20% fusel oil ratio, and 4500 rpm engine speed. Responses based on optimum conditions were determined as 67.6 kW brake power, 235.17 g/kWh BSFC, 0.118% CO, and 1931.4 ppm NO_x. In another study in the literature, Simsek and Uslu [33] experimentally examined the effects of using fusel oil in different ratios in a SI engine operated under different compression ratios and engine load conditions and then optimized with RSM. It was stated that the optimum operating conditions, which were stated to be achieved with a high desirability value, were 30% mixing ratio, 8.39 compression ratio, and 3777 W engine load. The authors stated that the results obtained from RSM were successful and it could be used under different operating conditions in the SI engine fueled fusel oil as an alternative fuel. In another optimization study, Yusri et al. [34] used RSM to determine the optimum ratio of secondary butyl alcohol that they tested as an alternative fuel in the SI engine. According to the results obtained from RSM, fuel with 15% secondary butyl alcohol content is the optimum ratio in terms of performance and emission.

There are many studies in the literature on the experimental investigation of the use of LPG in a SI engine. Two main aspects distinguish this study from other studies in the literature. As the first novelty, experimental studies were carried out in this study using different proportions of LPG. When the literature studies are examined, in the experimental studies conducted with the usage of LPG in SI engines, a single LPG ratio was generally preferred. The amount of research on the utilization of LPG at different rates is limited. On the other hand, there is no study in the literature about optimizing the LPG percentage with an optimization method. In this study, the aim of determining the optimum LPG percentage with the RSM to achieve the best performance and emission values is another novelty.