Energy Conversion and Management 98 (2015) 89–97

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Experimental investigation of the effects of direct water injection parameters on engine performance in a six-stroke engine b _ Emre Arabaci a, Yakup Içingür , Hamit Solmaz b, Ahmet Uyumaz c,⇑, Emre Yilmaz b a

Mehmet Akif Ersoy University, Emin Gülmez High Vocational School of Technical Sciences, Department of Automotive Technology, Burdur, Turkey Gazi University, Faculty of Technology, Department of Automotive Engineering, Ankara, Turkey c Mehmet Akif Ersoy University, High Vocational School of Technical Sciences, Department of Automotive Technology, Burdur, Turkey b

a r t i c l e

i n f o

Article history: Received 17 December 2014 Accepted 12 March 2015

Keywords: Six-stroke engine Exhaust heat recovery Water injection Engine performance

a b s t r a c t In this study, the effects of water injection quantity and injection timing were investigated on engine performance and exhaust emissions in a six-stroke engine. For this purpose, a single cylinder, four-stroke gasoline engine was converted to six-stroke engine modifying a new cam mechanism and adapting the water injection system. The experiments were conducted at stoichometric air/fuel ratio (k = 1) between 2250 and 3500 rpm engine speed at full load with liquid petroleum gas. Water injection was performed at three different stages as before top dead center, top dead center and after top dead center at constant injection duration and four different injection pressure 25, 50, 75 and 100 bar. The test results showed that exhaust gas temperature and specific fuel consumption decreased by about 7% and 9% respectively. In contrast, fuel consumption and power output increased 2% and 10% respectively with water injection. Thermal efficiency increased by about 8.72% with water injection. CO and HC emissions decreased 21.97% and 18.23% until 3000 rpm respectively. NO emissions decreased with water injection as the temperature decreased at the end of cycle. As a result, it was seen that engine performance improved when suitable injection timing and injected water quantity were selected due to effect of exhaust heat recovery with water injection. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Many investigations have been recently performed for high engine performance and lower fuel consumption in the internal combustion engines. It can be clearly said that small-volume and high efficiency engines can be easily produced with developing the technology especially after the computer control systems are developed [1]. Six-stroke engines are considered as an alternative engine instead of two and four-stroke engines in the reciprocating engines. Many patent studies have been performed on the six-stroke engines and a lot of six-stroke engines were produced [1–4]. Six-stroke cycle is based on the addition of steam stroke to the four-stroke cycle. Exhaust heat energy can be used in six-stroke engines [5]. Different engine cycles were applied to the internal combustion engines such as Miller and Atkinson unlike six-stroke engines. Miller and Atkinson cycles are the alternative cycle ⇑ Corresponding author. E-mail addresses: [email protected] (E. Arabaci), [email protected] _ (Y. Içingür), [email protected] (H. Solmaz), [email protected] (A. Uyumaz), [email protected] (E. Yilmaz). http://dx.doi.org/10.1016/j.enconman.2015.03.045 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.

approaches in the internal combustion engines. However, power output losses occured with Miller and Atkinson cycles while thermal efficiency increased. For this reason, these engines have been used as hybrid. Similarly, power output decreases in six-stroke engines. On the contrary, thermal efficiency increases. It was also mentioned that specific fuel consumption decreased with the increase of thermal efficiency in six-stroke engines [5,6]. The first investigation was performed by Griffin [7] on six-stroke engines. In griffin’s study two extra stroke was added to four stroke engines. Extra two stroke were performed as steam cycle. The operating principle of the most six stroke engines rely on the principle. Griffin [7] performed an experimental study in a single effect, sliding valve and six-stroke engine. In Griffin’s study, extra air was delivered into the cylinder at the end of exhaust stroke. Second expansion stroke was obtained unlike four-stroke engine. A cycle was completed at 1080° crank angle degrees (CA). Schimanek [8] studied on the six-stroke engines. He proposed to increase the power output via increasing the volumetric efficiency. He also tried to add extra two strokes to four-stroke engines in his study. Liedtke [9] proposed to use the waste heat formed on cylinder walls and piston surface along the combustion by injection of steam. He also aimed to obtain the second expansion stroke. He emphasised the

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importance of the usage of waste exhaust heat in the cylinder. Dyer [10] discussed the operation of six-stroke engines in a more detail. He mentioned that engine efficiency increased due to usage of the waste heat. It was also implied that exhaust system was improved and cooling system was simplified with six-stroke engine. Thus, water is compressed by a water pump. Moreover, it was noticed that cylinder pressure increased with the water injection into the hot exhaust gases. At this point, water was vaporized. Singh [11] developed the water injection system and controlled with computer in the six stroke engine. He performed a detailed study on six-stroke engine. In his work, sensors were mounted on the spark ignition engine and signals delivered by the sensors were processed in the computer. So, the water was injected using sensors and computer. The water was compressed by a high pressure water pump. In addition, the water was injected into the cylinder using high pressure direct injector. Injected water was warmed up by transferred heat from the engine to the coolant. Crower [12] claimed that waste heat could be used via injection of the water into the cylinder after combustion. He also said that NOx (nitrogen oxides) emissions decreased due to cooler operation of the engine. He showed that the water was vaporized with the injection of water and the second expansion stroke was obtained. Furthermore, the requirement of the cooling system is expected to be reduced. The required cooling capacity will be also reduced in the engine. Kelem [13] explained the method and studied on six-stroke engine including extra two strokes. Exhaust valve do not close and fresh air is delivered to the cylinder after intake, compression, expansion and exhaust stroke unlike four-stroke engines. So, fifth stroke occurs. The exhaust gases are expelled from the cylinder as piston moves toward to top dead center (TDC) at second exhaust stroke. Szybist [14] conducted a theoretical study on high efficiency six stroke engine. He proposed to increase the thermal efficiency of six-stroke engine. Whole exhaust gases did not expel from the cylinder. Some exhaust gases were trapped in the cylinder. The water was vaporized injecting the water into the burnt hot gases with usage of the waste heat energy of water. It was seen a slight increase on cylinder pressure but cylinder temperature when the water was suddenly vaporized. Water droplets were suddenly vaporized via injection the water on warmer surface. This is called Liedenfrost effect [15–20]. So, a second expansion stroke was obtained. There are many theoretical studies or patents on six stroke engines in the literature [15–36]. Mears [37] investigated the usage of the waste exhaust heat in water distillation. He showed that water injection could be used in the internal combustion engines. Domingues et al. [38] studied the usage of R123 and R245 in Rankine cycle. It was found that thermal efficiency increased by about 1.4–3.52%, 10.16–15.95% for mechanical efficiency. Fu et al. [39] also studied on the usage of exhaust heat recovery with steam turbocharge. Fu et al. [40,41] also studied on exhaust heat recovery with steam ve steam-assisted turbocharging. It is expected that engine brake torque increased by % 25. Yu et al. [42], performed an investigation on utilization of waste exhaust heat using thermoelectric generator. Wang et al. [43], studied on Rankine cycle based exhaust energy recovery system for heavy duty diesel ve light duty gasoline engine. Gasimi et al. [17], Kiran [44] and Manglik [45], have performed theoretical studies regarding the conversion of four stroke engines to six stroke engines. Karmalkar et al. [46] analyzed the usage of six stroke engines in hybrid vehicles. It was not seen experimental study on six stroke engines in the literature. All researches about the six stroke engines are theoretical. The main objective of this study is to investigate the effects of exhaust heat recovery with direct water injection on engine performance and exhaust emissions in a six stroke engine fueled with LPG. In this study, a single cylinder, four-stroke, spark ignition engine was converted to the six-stroke engine and the conversion

was performed using the knowledges in Dyer’s and Szybist’s study. The cam shaft of the test engine was modified and remanufactured. High pressure direct injector was mounted in the cylinder head in order to inject the water to the cylinder. A simple electronic control unit was developed and used in order to vary the water injection quantity and injection timing. The experiments were conducted at stoichometric condition and full load using LPG as test fuel. The effects of water injection quantity and injection timing were investigated on engine performance and exhaust emissions in six stroke engine.

2. Material and method A single cylinder, four stroke, spark ignition Honda GX 270 model test engine was used in the experiments. The technical specifications of the test engine are given in Table 1. It is hoped that the thermal efficiency of six-stroke engines is higher than four stroke engine due to extra expansion stroke with water injection. More energy can be converted to the useful work via the extra expansion stroke [13–15]. Cam shaft should be redesigned and the water injection system should be mounted in the engine in order to convert four-stroke engine to the six-stroke engine. The structural differences are seen between four- and six-stroke engines in Fig. 1. Cam-crank gear ratio was changed in the test engine. For this purpose, new cam profiles were determined and designed at original valve timing of the test engine. Partial exhaust process was added to the valve timing diagram of the test engine as exhaust valve opened twice in a cycle. The view of the modified six-stroke engine is seen in Fig. 2.

2.1. Water injection system Injector drive circuit was used in order to operate the water injector. Moreover, high pressure system was also used in order to compress the water. A electronic control unit was utilized in order to control the injector. For this purpose, a rotary encoder was mounted in the cam shaft of the test engine in order to determine the camshaft angle. High pressure (160 bar) water pump was used in order to compress the water. 70 W power is needed to operate high pressure water pump [47]. Pressure limiting valve was used for keeping constant the water pump pressure. The water injection system operation is seen in Fig. 3. Before the electronic control unit was designed a simple water injection system was developed in order to determine the injector characteristics. Injector delay time and water injection quantity were determined. According to obtained values, electronic control unit was programmed in order to provide desirable values. The technical properties of the injector are given in Table 2. Bosch HDEV 5.2 model high performance injector was used in the experiments. Water injection advance can be controlled via the control unit. The water injection quantity depends on the water pressure and

Table 1 The technical specifications of the test engine. Model

Honda GX 270

Valve system Number of cylinder Diameter/stroke (mm) Swept volume (cm3) Compression ratio Ignition timing (°CA, before TDC) Cooling system

OHV 1 77/58 270 8.2:1 20 Air cooled

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Fig. 1. The structural differences between four- and six-stroke engines.

_ inj;r was determined using injection duration. Injector flow rate m Eq. (1) as below.

_ inj;r ¼ 0:14663nP 0:3138 ½tt  td 105 m ai

ð1Þ

In Eq. (1), tt and td define the theoretical injection duration and injector delay time respectively. Pai and n define the effective injection pressure and engine speed respectively. 2.2. Experimental procedure

Fig. 2. The view of the modified six stroke engine.

The test engine was coupled with Cussons P8160 model DC dynamometer which was rated 10 kW at 4000 rpm engine speed. Load cell was used in order to measure brake torque. Engine speed was measured by pick-up sensor mounted on the dynamometer shaft. The schematic view of the experimental setup is seen in Fig. 4. Engine wall temperature was measured with K-type thermocouple (TC) placed below the cylinder head. Engine oil

Water Injection System

Fig. 3. The water injection system operation.

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Table 2 The technical properties of the injector. Model

Bosch HDEV 5.2

Maximum pressure (bar) Ohmic resistance (X, at 23 °C) Boost voltage (V) Boost current (amper) Boost duration (ls)

200 1.5 65 10.5 410

temperature was also measured with K-type thermocouple. Engine oil temperature was kept constant at 85 °C. Exhaust gas temperature was measured K-type thermocouple mounted in the exhaust line. The measurement points are seen in Fig. 5. Fuel consumption was measured using precision balance. Sun MGA 1500 exhaust gas analyzer was used in order to measure lambda and exhaust emissions. The technical specifications of exhaust gas analyzer are given in Table 3. The experiments were conducted at full load condition between 2250 and 3500 rpm engine speed. Ignition timing and valve timing were held constant. Injection duration should be varied in order to control the water injection quantity at constant pressure. The closing timing of the injector changes with changing the injection duration at the constant engine speed and the same injection pressure. If the closing timing of the injector was assumed to be a constant reference point, the start of injection was affected by the injection duration. So, the effects of injection advance on engine performance could not be clearly determined as the injection duration was affected by the start and end of the injection at constant injection pressure. For this reason, water injection quantity and injection pressure were controlled by keeping constant injection duration. Injector delay time was determined as 1.3 ms in the experiments. It was also seen that there was a slight effect of injection pressure on injector delay time [1]. It is preffered that water injection starts between 720 and 723 °CA and ends at 730 °CA in the literature [5,13–15]. The optimum injection duration was determined as 1.2 ms in the experiments. If the injection duration decreased below the 1.2 ms, injection pressure had to be increased for the desirable water injection quantity. In contrast, the time dependent on the crank angle increased. Fig. 6 shows the variation of the water injection quantity dependent on the water injection and different injection durations. As water injection pressure increased, water injection increased per cycle. Similarly, water injection increased with the increase of injection duration. In the

present study, partial exhaust valve opening and closing timing are 66 °CA degree before bottom dead center and 44 °CA before top dead center respectively. Valve lift of partial exhaust process is 3.3 mm and 5.5 mm for the second exhaust process. So, some exhaust gases are trapped in the combustion chamber. It affects the emissions directly. Emissions are affected by water injection during the second exhaust stroke. Combustion products after combustion are mixed with the combustion products which produced after water injection at the end of six stroke. So, measured emissions values are the mixture of combustion products which obtained after combustion and water injection. If the injector delay time was assumed to be 1.3 ms, injection duration became 2.5 ms. Injection pressures were changed as 25, 50, 75 and 100 bar in the experiments. Injection duration was kept constant. Table 4 shows the injection timing values. Injection timing was changed according to engine speed as seen in Table 4. The test engine could not be operated well with gasoline owing to carburettor problems. Because air/fuel ratio was not properly changed for a desirable value. The structure of the carburettor prevented to operate the engine stably when the gasoline was used as a test fuel in six stroke engine mode. So, unstable operation was observed due to deterioration of air/fuel ratio. Hence, LPG (including 70% butane and 30% propane) was used to be a test fuel in case of gasoline in the experiments. In addition, lambda was held constant stoichometric air/fuel ratio (k = 1). The properties of the test fuel are given in Table 5.

3. Results and discussion Six-stroke engines provide energy economy in case of higher power output compared to four stroke engines. It is aimed that the thermal efficiency increases with the usage of waste exhaust heat. So, portion of waste exhaust heat can be converted to useful work [1,13–18]. The experiments were conducted at four different injection pressure (25, 50, 75 and 100 bar) and three different injection advance (before TDC, TDC and after TDC) in order to investigate the variation of brake torque. Before the experiments, six stroke engine was first operated without water injection and brake torque and fuel consumption were measured. The priority target of this study is to improve the performance of six stroke engine for a stable operating conditions. Thus, cylinder pressure was not measured and cycle analysis was not investigated. The water injection quantity was determined changing injection

Experimental Setup Water Injection System

Fig. 4. The experimental setup operation.

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Exhaust Gas Temperature Thermocouple

Engine Wall Temperature Thermocouple Engine Oil Temperature Thermocouple Fig. 5. Measurement points for engine oil, engine wall and exhaust gas temperatures.

Table 3 The technical specifications of the exhaust gas analyzer.

CO (%) HC (ppm) NO (ppm) CO2 (%) O2 (%) Lambda

Operating range

Accuracy

0–15 0–9999 0–5000 0–20 0–25 0.6–4

0.001 1 ppm 1 ppm 0.1 0.01% 0.001

Water Injection (mg/cyc)

pressure at constant injection duration. The effects of the water injection quantity and injection advance on brake torque were investigated. Fig. 7 illustrates the variation of brake torque dependent on the water injection. Charge mixture composition and temperature are affected by the waste exhaust gas fraction which remained in the previous cycle. Because, injected water quantity and injection timing were changed at the end of first exhaust stroke. So, exhaust gas concentration is also altered. Ceviz and Yüksel [49] determined that coefficient of variation of indicated mean effective pressure decreased when LPG was used as the test fuel compared to gasoline in spark ignition engine. They also mentioned that higher laminar flame speed of LPG and good mixing properties decreased cyclic variations. The experiments were repeated under stable operating conditions and Fig. 7 was obtained. It is possible to say that brake torque generally decreases with the increase of water injection quantity. The most effective water injection was seen between 2750 and 3250 rpm engine speed. Furthermore, brake torque increased with the water injection. The test results showed that the water injection quantity was limited in order to increase engine performance at lowest and highest engine speeds. Unstable operation occured when the engine run at 2250 and 3500 rpm engine speeds with water injection. The reason of the decrease the brake torque after a certain water injection is to

1.2 ms 1.4 ms 1.0 ms

18 16 14 12 10 8 6 25

50 75 100 Water Injection Pressure (bar)

Fig. 6. The variation of the water injection quantity dependent on the water injection and different injection durations.

decrease of in-cylinder temperature. Thus, exhaust heat recovery could not be effectively occured. Fig. 8 shows the effect of water injection on engine performance. ‘‘Green dashed line’’ shows the results which water pump losses were considered in the calculations and it was called as ‘‘modified’’. The highest brake torque values were used when the water injection occured. The biggest effect of the water injection was seen between 2500 and 3250 rpm engine speed on brake torque. It can be clearly said that engine operated stable between 2500 and 3250 rpm engine speed. It can be also concluded from Fig. 8 that engine wall and exhaust temperatures decreased with water injection. In contrast, fuel consumption increased with the water injection due to increase of volumetric efficiency. But specific fuel consumption decreased. It is possible to say that brake torque increases because of the exhaust heat recovery with water injection. Brake torque and brake power increased with the water injection. It was also seen that specific fuel consumption decreased with the water injection. It can be explained that there is no significant difference between fuel consumptions with and without water injections. However, specific fuel consumption decreased with the water injection as exhaust heat energy was converted to useful work via water injection. It results also the decrease of exhaust gas temperature. It was found that the water injection caused to reduce in-cylinder temperature and engine wall temperature. It can be also mentioned that injected water into the burnt gases led to reduce the in-cylinder temperature in the last cycle. Because, the water was vaporized and cooled the combustion chamber. Fig. 9 illustrates the effects of water injection on modified brake power versus engine speed. As the water injection quantity increased, brake power increased until maximum power engine speed. It can be also said that maximum water injection should occur at maximum power engine speed. It was seen that there was a good accordance with brake power and water injection. Fig. 10 depicts the effects of water injection on thermal efficiency versus engine speed. ‘‘Green dashed line’’ shows the results which water pump losses were considered in the calculations and it was called as ‘‘modified’’ in Fig. 10. It could be clearly said that thermal efficiency increased with water injection compared to without water injection at each engine speed. Maximum thermal efficiency was obtained with water injection at 2750 rpm engine speed. Thermal efficiency increased by about 8.72% with water injection compared to without water injection. In addition, thermal efficiency increased by about 12.61% with water injection at maximum brake torque and brake power engine speeds. It can be concluded that engine performance improved when the water was injected. CO emissions are produced due to incomplete combustion. CO emissions decrease with the increase of air excessive coefficient at lean burn operation condition [49–53]. Moreover, insufficient

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Table 4 The variation of injection timing. Engine Speed (rpm)

Injection duration

Injection timing (°CA) Before TDC

2250 2500 2750 3000 3250 3500

TDC

After TDC

ms/cyc

°CA

Start

Stop

Start

Stop

Start

Stop

1.2 (1.3 ms/cyc injection delay)

16.2 18.0 19.8 21.6 23.4 25.2

33.0 33.9 34.8 36.6 37.5 38.4

16.8 15.9 15.0 15.0 14.1 13.2

18.0 18.9 19.8 21.6 22.5 23.4

1.8 0.9 0.0 0.0 0.9 1.8

3.0 3.9 4.8 6.6 7.5 8.4

13.2 14.1 15.0 15.0 15.9 16.8

Table 5 The properties of propane and butane [1,15,16,48].

Density (kg/L at 15 °C) RON MON Low heating value (kJ/kg) Freezing point (°C)

Propane

Butane

0.510 100 95.4 46,296 187.7

0.580 92 89 45,277 138

oxygen and low temperature at the end of combustion cause to release CO emissions. Fig. 11 shows the variation of CO emissions dependent on the engine speed. It can be said that CO emissions decrease with the water injection. CO emissions decreased 21.97% until 3000 rpm engine speed. Furthermore, the water

injection quantity increased between mentioned engine speeds. CO emissions started to increase after 3000 rpm engine speed. Because, sufficient oxygen could not be delivered into the cylinder due to flow losses at higher engine speeds. Akansu and Bayrak [50] showed that CO emissions were higher with CH4/H2 mixture fuel compared to LPG. It was also concluded that CO emissions decreased with the increase of air excessive coefficient from 0.8 to 1.2. Morganti et al. [51] have found that physically appropriate concentrations of CO and HC had slight effect on autoignition compared with NO when propane, n-butane and isobutane were examined. Fig. 12 illustrates the variation of HC emissions. HC emissions decrease by about 18.23% until 3000 rpm engine speed. After 3000 rpm, HC emissions tend to increase like CO emissions. Thermal efficiency also decreased after 3000 rpm engine speed.

4.5 Brake Torque (Nm)

Brake Torque (Nm)

3.5 3 2.5 2 1.5

2250 rpm

1 9 10 11 12 13 Water Injection (mg/cyc)

3 2500 rpm 8

14

4.5

9 10 11 12 13 Water Injection (mg/cyc)

14

4.5

Brake Torque (Nm)

Brake Torque (Nm)

3.5

2.5 8

4

3.5 2750 rpm 3

4

3.5 3000 rpm 3

8

9 10 11 12 13 Water Injection (mg/cyc)

14

8

4

9 10 11 12 13 Water Injection (mg/cyc)

14

3

Brake Torque (Nm)

Brake Torque (Nm)

4

3.5

3250 rpm 3

2.5 2 1.5 3500 rpm 1

8

9 10 11 12 13 Water Injection (mg/cyc)

Before TDC

14

TDC

8

9 10 11 12 13 Water Injection (mg/cyc)

After TDC

w/o injection

Fig. 7. The variation of brake torque dependent on the water injection.

14

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1.5

4 3.5 3 2.5

with water inj. w/o water inj.

Brake Power (kW)

Brake Torque (Nm)

4.5

with water inj. w/o water inj.

Engine Wall Temp. (K)

7 2250 2500 2750 3000 3250 3500 Engine Speed (rpm) 440

420

400 with water inj. w/o water inj. 380 2250 2500 2750 3000 3250 3500 Engine Speed (rpm)

Spec. Fuel Cons. (g/kWh)

9

0.9

w/o water inj. with water inj. with w. inj. (modified)

0.7

1000

Exhaust Gas Temp. (K)

Fuel Cons. (g/min)

11

1.1

0.5 2250 2500 2750 3000 3250 3500 Engine Speed (rpm)

2 2250 2500 2750 3000 3250 3500 Engine Speed (rpm) 13

1.3

with water inj. w/o water inj. with w. inj. (modified)

700

400 2250 2500 2750 3000 3250 3500 Engine Speed (rpm) 650

600 with water inj. w/o water inj. 550 2250 2500 2750 3000 3250 3500 Engine Speed (rpm)

12 11 1.2 10 1 9 8 0.8 Br. Power (Modified) 7 Water Injection 0.6 6 2250 2500 2750 3000 3250 3500 Engine Speed (rpm)

Thermal Efficiency (%)

Fig. 9. The effects of water injection on brake power.

In addition, water consumption decreases rapidly after 3000 rpm. So, unstable operation occured. At lower engine speeds, it is seen that heat losses increase due to heat transfer to the cylinder walls. Because, there is enough time to transfer the heat at lower engine speeds. It results the decrease of in-cylinder temperature. Thus, HC formation occurs. Moreover, HC emissions are higher, because flame goes out at cooler cylinder surface due to lower in-cylinder temperature at lower engine speeds. At higher engine speeds, flame is similarly quenched at cylinder surfaces due to heat losses and gas leakages. Bayraktar and Durgun [52] showed that in-cylinder pressure and temperature predicted for LPG was higher in a spark ignition engine. It is possible to say that HC emissions decrease at higher in-cylinder temperatures. In work [50],

18

4

15

3

12 9

w/o water inj. with water inj. with w. inj. (modified)

6 2250 2500 2750 3000 3250 3500 Engine Speed (rpm)

Fig. 10. The effects of water injection on thermal efficiency.

CO (vol. %)

Brake Power (kW)

1.4

Water Injection (mg/cyc)

Fig. 8. The effects of water injection on engine performance.

with water inj. w/o water inj.

2 1 0 2250 2500 2750 3000 3250 3500 Engine Speed (rpm) Fig. 11. The variation of CO emissions.

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HC (vol. ppm)

800 600

with water inj. w/o water inj.

400 200 0 2250 2500 2750 3000 3250 3500 Engine Speed (rpm) Fig. 12. The variation of HC emissions.

NO (vol. ppm)

160 120 80 40

with water inj. w/o water inj.

0 2250 2500 2750 3000 3250 3500 Engine Speed (rpm) Fig. 13. The variation of NO emissions.

unburned hydrocarbon values of LPG were higher than of CH4/H2 fuel due to higher carbon level of LPG. Fig. 13 shows the variation of NO emissions. It can be concluded from Fig. 13 that NO emissions decrease with the water injection compared to without water injection. The reason of the decrease of NO emission is to reduce in-cylinder temperature at the end of combustion. NO formation increases very strongly with the increase of flame temperature [50,53]. NO emissions increase with maximum cylinder pressure and temperature [54]. As it is known that NO emissions are produced at higher combustion temperatures. But, water injection causes the decrease of in-cylinder temperature. So, NO formation is prevented. As water consumption increased, NO emissions decreased, because injected water prevented to increase in-cylinder temperature significantly. Bayraktar and Durgun [52] implied that LPG decreased the NO and CO mole fractions in a spark ignition engine. Li et al. [54] determined that real time NO emissions could be used to identify combustion. They have also seen that decrease of lambda caused to increase in NO emissions in a spark ignition engine fueled with LPG. In [50], NO emissions of CH4/H2 (70/30) are higher than LPG NO values. It can be said that low NO emissions were measured during the experiments. It was deduced that NO emissions are strongly affected by exhaust heat recovery in this study. Because, temperature at the end of six stroke decreases significantly. So, NO formation deteriorates due to lower cylinder temperature. 4. Conclusions The thermal efficiency can be increased with exhaust heat recovery in six stroke engines. In this study, a single cylinder, four-stroke, gasoline engine was converted to six-stroke engine using the knowledges mentioned by Dyer and Szybist. This study is important to be the first experimental study on exhaust heat recovery six stroke engine with water injection. The corrosive effect of water was not considered in this study. Water is injected into the high temperature-exhaust gases. Water is suddenly vaporized, after the injection and discharged from the exhaust line.

The aim of this study is to investigate the effects of water injection and injection advance on engine performance and exhaust emissions in a single cylinder six stroke engine. The experiments were first conducted without water injection in order to compare with water injection. The experiments were performed between 2250 and 3500 rpm engine speed in a modified six stroke test engine fueled with LPG at full load. The experiments were also performed at stoichometric air/fuel ratio. The test results showed as below:  Exhaust gas temperature and engine wall temperatures decreased 7% and 2% respectively with water injection. Moreover, volumetric effciency increased owing to cooling effect of water.  Brake power increased 10% with water injection. In contrast, specific fuel consumption decreased 9%.  The biggest effect of the water injection was observed between 2750 and 3250 rpm engine speed on engine performance. More water was injected between 2750 and 3250 rpm engine speed.  The test results showed that injection timing should be advanced with the increase of engine speed. There is no remarkable effect of the water injection quantity and injection advance on engine performance at lowest (2250 rpm) and highest (3500 rpm) engine speeds.  The thermal efficiency and engine performance improve with the proper water injection quantity and injection timing compared to without water injection in six stroke engine.  CO and HC emissions decreased 21.97% and 18.23% until 3000 rpm respectively. NO emissions decreased with water injection as the temperature decreased at the end of cycle.  It was deduced that lower NO emissions are the result of exhaust heat recovery in six stroke engines. NO emissions substantially decreased due to lower cylinder temperature. It is hoped that this study contributes the effects of water injection on engine performance and exhaust emissions in six stroke engines.

Acknowledgements This study was supported by Gazi University Scientific Research Foundation in frame of the project code of TEF-07/2012-05. As researchers, we thank Scientific Research Foundation of Gazi University.

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