1–1 Two-stroke engines
Two-stroke engines are an idea specific to simple light weight engines which have been mostly used in portable engines and motorcycles, where these advantages are given more importance [1]. Due to their simple design, light weight, the ability of producing high power better low temperature cold start capability, and relatively low cost, these engines are on demand and are used as a well-known power source in two-wheel tractors, tree cutting saws, lawn mowers, small engines of power generation, motor boats, motorcycles, etc. [2]. There are over eight million motorcycles in Iran that two-stroke-engine ones constitute their main part. Since 2-stroke engines do not have a closed crankcase like 4-stroke engines, as they use the crankcase as part of the induction tract, the oil must be mixed with the gasoline, for distribution throughout the engine for lubrication (Fig. 1) [3].
The mixing of fuels and lubricants is one of the main reasons for the increase of emissions in these engines. It has changed vehicles with 2-stroke gasoline engines into one of the main sources of air pollution, especially in Asia and has increasingly raised the global concern about the high levels of toxic hydrocarbon emissions of these engines [4]. The production of two-stroke gasoline motorcycles was banned in Iran in 2004 in order to reduce the air pollution. During the same year, 37,000 units of two-stroke motorcycles were dismantled by means of paying subsidies, but the process was very slow in later years. In general, due to the large number of these motors and the relative popularity, this task is very difficult and economically costly [5]. There are two quick and simple solutions to this problem namely the use of lubricants with less polluting effects and the regular maintenance of these engines, which will ultimately lead to the improvement of air quality.
The lubrication of two-stroke gasoline engines is always carried out by mixing the lubricant with gasoline in the proper ratio. Conventionally, the lubricant has been made up of petroleum based oils, additives of performance enhancers, and one solvent (petroleum products with a relatively low flash point) to improve the mixing of gasoline and lubricant [6]. The solvents used here have poor lubricating properties; therefore, they cause more piston surface wear and more friction which ultimately leads to the inability of the piston to supply the appropriate compression ratio and produce more smoke. In such lubricants, when the mixture of fuel and lubricant enters the crank chamber, the solvent evaporates due to overcoming the high temperatures and depending on engine temperature, disrupts the mixing ratio of fuel and lubricant. In general, the use of solvent, poor lubrication of mineral-based oils and incomplete combustion of fuel are the main problems of two-stroke petroleum-based lubricants [3].
Vegetable oils are well-known for good lubricating properties and contribute to the completion of the combustion process due to the existence of oxygen atom, in their structures. They also cause the reduction of engine fatigue, sediments of spark plugs and combustion chamber, the prevention of ring sticking and piston stop, keeping the engine clean at low operating temperatures and the prevention of wear of the bearings in low ratios of oil to fuel. Two major shortcomings of vegetable oils are the high pour point and low oxidation stability, which are revisable through chemical modification [7]. The structure of vegetable oils is composed of triacylglycerol molecules, which include three fatty acid chains that are connected to the molecules of hydroxyl groups via ester bonds. Physical and chemical properties of vegetable oils are largely influenced by the composition and distribution of fatty acids and the glycerol structure of triacylglycerol molecules [8]. Triacylglycerol molecules have more interactions with metal surfaces than hydrocarbons and create a very strong lubricating film on metal surface. Therefore, they are more effective in reducing friction and wear [9]. Castor oil is an inedible vegetable that is found in abundance around the world and has unique features and very suitable lubrication properties because of having a certain type of fatty acid in its structure (Ricinoleic acid).
Castor oil has a high viscosity and low friction coefficient and due to having hydroxyl functional group in its structure, in comparison with other vegetable oils; it has the solubility in alcohol, which makes it needless to use solvents in the lubrication of two-stroke engines [10]. The castor oil has been used previously as a lubricant and as the first motor oil produced in Castor lubricant production company based on castor oil [11]. Sivasankaran (1988) produced mixtures of two-stroke engine oils based on the vegetable oil of jojoba and examined the chemical and physical characteristics, fatigue, wear, and sediments of the engine. The results showed that the performance of jojoba oil in two-stroke engines with petrol fuel is equal to that of the commercial engine oils [12]. Zhou and Ye (1998) tested two types of new two-stroke engine oils on gasoline burning scooters, where oxygen additives and catalysts had been used. They examined engine exhaust particulates through gas chromatography. They found that using these two types of engine oils reduces exhaust particulates of scooter engines to 33–36 percent [13]. Singh (2011) produced a 2T engine oil from castor plant oil through epoxidation and found that this engine oil reduces more than 50 percent of smoke compared to petroleum-based two-stroke engine oils [3]. However, the use of biolubricants in internal combustion engines is not limited only to engine oil and covers a full range of lubricants used in cars [14]. Various applications of biolubricants in a truck is shown in Figure (2). Since the use of biolubricant are becoming wide spread and their application is environment friendly, therefore in the present investigation, the effect of castor oil biolubricant on a two stroke SI engine exhaust emission is experimentally examined.
1–2 Bio-based lubricants
Lubricants serve varieties of applications in industries, automobiles, agricultural machinery, aviation machinery, etc. by performing critical functions such as reducing friction, removal of wear particles, increasing efficiency, minimizing energy losses and uniformly distributing heat. The most common types of lubricants incurrent use are automotive transmission fluids, hydraulic fluids, metal working fluids, cold rolling oils, fire resistant hydraulic fluids, industrial gear oils, neat cutting oils and automotive gear lubricants [14]. In recent years, increasing environmental awareness and more restrictive regulations have accelerated the development of biodegradable lubricants all over the world [16–18]. Pollution caused by mineral based oil is severe and causes long term damage to both soil and waterways [19]. The uncertainty of the crude oil supply, pollutant emissions and its higher price give biodegradable biolubricants more advantages over mineral base oils. In general, vegetable oils have some excellent properties for their potential use as lubricants, such as high-viscosity index, high lubricity, low volatility, and, specially, both low toxicity and high biodegradability [16, 20]. However, vegetable oil in its natural form has limited application due to its poor oxidation stability [21] and inferior low temperature properties [22]. Problems are faced by vegetable oil due to existing double bond [23] and the presence of active sites in the β hydrogen of Triacylglycerol ester. The earlier researchers have taken two approaches to improve the properties of vegetable oil for use as base oil lubricant with improved flow property and oxidative stability [24]:
1. The fatty acids are removed from the glycerol and attached to another more stable polyols such as NPG (Neopentyl glycol), TMP (Trimethylol propane) and PE (Pentaerythritol) [9, 25, 26].
2. In another method, Epoxidation has been used to improve the oxidative stability in rapeseed oil based lubricant [27].
Also, genetic modification and selective hydrogenation were also considered in a previous study [28].
Some studies concerning the improvement of oxidative stability and low-temperature properties have
been reported during the last two decades [21, 22, 29]. The fatty acid polyol esters of vegetable oils are a potential biolubricant that may be derived from renewable sources. The modified vegetable oil esters are environmentally friendly and rapidly biodegradable base stock for lubricant production [30]. Polyol esters are produced by transesterification of vegetable oil fatty acids or fatty acid esters and polyols such as TMP, NPG and PE. The polyols are all branched polyols that have no labile hydrogen atoms in their structure that are equivalent to the β carbon of glycerol. Therefore, esters of these polyols have greater thermal stability than glycerol esters of the same fatty acids. Two transesterification reactions are needed to produce TMP-ester (Fig. 3).
Producing biolubricants is suitable for alternative energy applications because of their widespread sources. The types of biolubricant feedstock may differ from country to country and highly depend on geographical locations. Biolubricants are made from different crop oils. More than 350 oil-bearing crops are known, among which, only palm and palm kernel [31–34], soybean[35, 36], sunflower [37], coconut[38] ,rapeseed[39–41], cottonseed, and peanut oils are considered as potential alternative biolubricants[15]
In the present study, the castor oil was chosen as the raw material to produce biolubricant. The free fatty acids (FFA) content of castor oil was about 0.63% and the water content being 0.08%. Biolubricant was produced by transesterification, which was affected by many factors, such as castor oil methyl ester to TMP molar ratio, reaction temperature and catalyst concentration. Most of the studies on the transesterification changed one factor at a time (COST) [31, 34, 41–44]. However; reaction system influenced simultaneously by more than one factor can be poorly understood with the COST-approach. Therefore, the experiments were performed according to central composite design (CCD) and response surface methodology (RSM) to understand the relationship between the factors and conversion of oil to biolubricant, and to determine the optimum conditions for production of biolubricant. This method has been used in the optimization of biodiesel by the earlier researchers [45–51].