Fuel properties , performance and emission characterization of waste cooking oil ( WCO ) in a variable compression ratio ( VCR ) diesel engine §

The current study investigates the fuel properties, performance and emission characteristics in a variable compression ratio (VCR) diesel of the biodiesel produced from the waste cooking oil (WCO). The WCO samples were collected from the university and converted to biodiesel fuel with a two-step transesterification reaction. The fuel property tests showed that the properties of the WCO biodiesel were within the biodiesel standards. Diesel, WCO biodiesel and diesel-WCO biodiesel blend (B20) was used as fuel in a VCR engine. The performance and emission characteristics of the engine were measured at two different compression ratios (14:1 and 16:1) under partial load conditions. The experimental results showed that WCO biodiesel slightly decreased the brake thermal efficiency and thus increased specific fuel consumption. Biodiesel usage improved CO emissions up to 21,75% compared to diesel fuel. But, biodiesel usage increased CO2 and NOx emission due to higher combustion temperature and extra oxygen content of the biodiesel.


INTRODUCTION
Fossil fuels which are the main energy source for transportation of people and goods are depleting as it is known widely and the price of the fuel is increasing due to demand and supply facts [1].Also, enviromental effects of the fossil fuels threaten the human heallt and nature of all world.The exhaust emissions of the internal combustion engines are formed by the combustion fuels and these gases include toxic pollutants such as carbon monoxide (CO), carbon dioxide (CO 2 ), oxides of nitrogen (NO x ), unburned hydrocarbons (UHC), sulphur dioxide (SO 2 ), and etc.These toxic gases are dangereosly harmful for human healt and the nature [2].
The known facts force researchers to look for alternative energy sources for internal combustion engines.Biodiesel, which can be derived from vegetable oils and animal fats has a potential of being a substitute since biodiesel fuels are less toxic and renewable [3].Many raw materials have been studied by many researchers [1,[4][5][6][7][8][9][10][11].Also, biodiesels are not significantly worse than fossil fuels in the means of engine performance.Biodiesels are mostly derived with transesterification reaction or thermal cracking methods.The main raw material of biodiesel are mostly non-edible vegetable oils, animal fats and waste oils.The necessity of large agricultural areas and the high effort for production of vegetable oils are one of the most important drawbacks of biodiesel usage.But, tonnes of oils are used all over the world for different purposes.Especially, the food industry produce a high amount of waste frying oil.The waste of food industries and even the waste oils used in the houses are hazardous for enviroment unless the wastes are managed properly.The trend of renewable processes have been started to spread recently.Even the local city corporations are aware of the huge amount of oil wastes and have some efforts on this particular subject.Recycle of waste materials is a popular research subject.Reproduction processes from the waste material are eco-friendly and economically useful since the raw materials are already used up and the unit reproduction cost can be pulled down of the original production cost in some cases.
In literature, there are many investigations on the production, performance, emission and combustion characteristics of biodiesels [12][13][14][15][16][17][18][19].Hwang et al., (2016) published an article that investigates effects of biodiesel usage produced from waste cooking oil on a compression ignition engine.The authors reported that waste cooking oil (WCO) biodiesel usage decreased the CO, hydrocarbon (HC) and smoke emissions, and also WCO biodiesel caused a slight decrement of in-cylinder pressure [20].Man et al., (2016) reported a study that studies the effects of WCO-diesel blends on emission characteristics of a diesel engines.The study revealed that, WCO-diesel blend usage caused to increase of NOx emissions.But, CO, HC and particulate matter emisssion were decreased by WCO-diesel blends usage [21].Piker et al., (2016) studied the on the biodiesel production from the waste oil bu using egg shells as catalyst.The authors reported that fatty acid methyl ester yield of 97 wt.% was obtained after 11 h at ambient temperature and pressure with egg shells [22].In this study, fuel properties of waste cooking oil biodiesel and the effects of the biodiesel produced from waste frying oil that collected from the university on the performance and emission characteristics of a variable compression ratio (VCR) diesel engine were investigated.

Experimental Fuels
The experimental study was conducted in Petroleum Research and Automotive Engineering Laboratories of the Department of Automotive Engineering at Çukurova University.Waste cooking oil (WCO) samples were used as raw material for biodiesel production.WCO biodiesel was produced with two-step transesterification reaction.First, the collected oil samples were filtered prior to reaction in order to clean the contaminants.The free fatty acid (FFA) value of the cleaned oil was measured with the standard titration method.The FFA of the WCO was measured as 1,93 wt.%.Transesterification reaction was performed twice the FFA of WCO is high.The first reaction was performed at 65 o C for 60 minutes by stirring.Methanol 20 wt.% and 0,5 wt.% sodium hydroxide was used as reactant and catalyst, respectively.The methoxide was obtained before transesterification reaction by mixing methanol and sodium hydroxide.The second transesterification reaction was performed with methanol 10 wt.% and 0,25 wt.% sodium hydroxide under same conditions.After the reaction period, the mixture were batched in a separating funnel for 8 hours and at the end of the batching period the glycerine was separated from the mixture.After the separating the glycerine from the mixture, crude WCO biodiesel samples were obtained.Then, the crude biodiesel was washed with warm water and dried at 105 o C for one hour.Finally, in order to purify and to refine biodiesel the crude biodiesel was filtered.The biodiesel production flow diagram was shown in Figure 1.In the study, low sulphur diesel fuel (conventional diesel), WCO biodiesel (waste cooking oil methyl ester), diesel-biodiesel blend B20 (80% diesel + 20% WCO) were used as fuel in a variable compression ratio (VCR) diesel engine in order to investigate the effects of waste cooking oil biodiesel on the performance and emission characteristics of a VCR diesel engine at different compression ratios.
The psychical properties of the test fuels were measured before testing them in the test engine.Instruments used for analysing the products were; Zeltex ZX 440 NIR petroleum analyser with an accuracy of ±0.5 for determining cetane number; Tanaka AFP-102 for cold filter plugging point; Tanaka AKV-202 Auto Kinematic Viscosity test for determining the viscosity; Kyoto electronics DA-130 for density measurement, Tanaka flash point control unit FC-7 for flash point determination and IKA Werke C2000 bomb calorimeter for determination of heating value.The fuel quality measurements were performed according to EN 14214 and EN 590.

Experimental Setup
The performance and emission measurements were performed with a single cylinder, multi fuel, and variable compression ratio diesel engine.In order to provide the accuracy of measurements, the experiments were done at partial load conditions (60% load).An eddy current dynamometer was used to measure performance characteristics of the engine.Brake thermal efficiency (BTHE), specific fuel consumption (SFC) and exhaust gas temperature (EGT) results of the engine for test fuels were studied at two different compression ratios (14:1 and 16:1).Also, the exhaust emissions of the engine were measured simultaneously.Carbon monoxide (CO), carbon dioxide (CO 2 ), and nitrogen oxides (NO x ) emissions were studied for all test fuels.Table 1 and 2 shows the technical specifications of the VCR test engine and exhaust gas analyser.

Fuel Properties
Some important physical fuel properties test fuels were measured.The fuel properties of the diesel, WCO and B20 fuel were given in table 3. Fuel property tests shows that density of WCO and B20 was higher than the diesel fuel, but the values were within the biodiesel standards EN 14214.The tests showed that WCO biodiesel has a higher kinematic viscosity than diesel fuel.High viscosity can damage fuel pump but, the viscosity of WCO biodiesel and B20 were within the standards and it can be used directly without any modification.Heating value of the WCO biodiesel and B20 was slightly lower than diesel fuel.Flash point of the WCO biodiesel was over 120 o C which is safer compared to diesel fuel.Also, blending WCO biodiesel with diesel fuel increased the flash point value of the diesel fuel which is an important criteria for storage of the fuel.Cold filter plugging point of the WCO was measured higher than diesel fuel.

Performance Characteristics
Brake thermal efficiency (BTHE) can be defined as the ratio of power output to heat input.BTHE values are also related with heating value of the fuels.Higher BTHE value indicates the better combustion of fuel which means higher cylinder pressure and higher power output for unit fuel used.Specific fuel consumption (SFC) is a measure of how efficiently fuel is used and this value is directly related with BTHE [23].Figure 2 and 3 shows the BTHE and SFC results of test fuels.It can be seen from the graphs increasing CR from 14:1 to 16:1 increased BTHE and decreased SFC values for all test fuels due to better combustion of fuels.Increasing CR from 14:1 16:1 increased BTHE values 2,29%, 6,75%, and 2,37% for diesel fuel, WCO biodiesel and B20 fuel, respectively.Higher compression ratio experiments resulted in 7,06%, 10,13%, and 9,92% lower SFC for diesel fuel, WCO biodiesel, and B20 fuel respectively.WCO biodiesel usage slightly decreased BTHE values compared to diesel fuel.BTHE results of WCO biodiesel were 8,62% and 4,64% lower compared to diesel fuel , at 14:1 and 16:1 compression ratios, respectively.Exhaust gas temperature (EGT) is an important criteria for internal combustion engines and it is related with combustion parameters such as compression ratio and fuel properties [24].EGT results are shown in Figure 4.It can be seen from the graph WCO usage increased EGT compared to diesel fuel due to higher oxygen content of biodiesel.Also, increasing compression ratio increased EGT significantly for all test fuels.

Emission Characteristics
Throughout the study; CO, CO 2 and NO x emissions of the test engine were measured.Figure 5, 6 and 7 shows the CO, CO 2 and NO x results of all test fuels, respectively.The study showed that increasing CR improved CO emissions for all test fuels due to better combustion of fuels [25].But, CO 2 and NO x emissions were increased when compression ratio was elevated.Higher compression ratios enhance the combustion and thus nitrogen oxides emissions significantly elevates due to higher in-cylinder temperature and higher flame velocity.Biodiesel usage usually improves CO emissions and increases CO 2 and NO x emissions due to higher combustion temperature and extra oxygen content in the chemical composition [26].

CONCLUSIONS
In this study waste cooking oil was converted to biodiesel by using transesterification reaction and the effects of diesel, waste cooking biodiesel and diesel-biodiesel blend usages were investigated at two different compression ratios in a variable compression ratio diesel engine.Throughout the study following conclusions were obtained; • Waste cooking oil has a high FFA value and thus two-step transesterification was performed.
• Fuel properties of waste cooking oil and its blend with diesel fuel were within the biodiesel standards.
• Increasing compression ratio improved BTHE and SFC values, and increased EGT for all test fuels.
• Increasing compression ratio improved CO emissions but caused to increase of CO 2 emissions and NO x emissions.
• Waste cooking oil biodiesel usage slightly decreased BTHE and SFC values and increased EGT.
• Waste cooking oil usage improved CO emissions and caused to increase of CO 2 emissions and NO x emissions.

Table 1
Technical specifications of the test engine

Table 2
Technical specifications of the emission device

Table 4
Fuel properties of test fuels