An Experimental Study of Operating Range, Combustion and Emission Characteristics in an RCCI Engine Fueled with Iso-Propanol/n-Heptane

Abstract

Recently, studies have been carried out using environmentally sustainable technologies with more efficient energy conversion to fulfill emission requirements. One of these technologies, reactivity controlled compression ignition (RCCI), is a low-temperature combustion mode and has a dual fuel strategy. The controllability of combustion, high thermal efficiency and low nitrogen oxide (NO_x) and soot emissions are some of the most prominent advantages of this combustion mode. In this study, the effects of the premixed ratio (PR) and intake air temperature (IAT) on the operating range, combustion characteristics and emissions were investigated experimentally. In the experiments, iso-propanol and n-heptane were used as fuels. The experiments were carried out for two different case studies. In the first case, the experiments were performed at a 50 °C intake air temperature and three different premix ratios (PR25, PR50, PR75). The minimum brake-specific fuel consumption (BSFC) was 268 g/kWh and the widest operating range was obtained with PR25. In addition, the lowest emission values in NO_x, hydrocarbon (HC) and carbon monoxide (CO) emission formation were recorded with the use of PR25 fuel. In the other case, experiments were conducted at three different intake air temperatures (30 °C, 50 °C, 70 °C) with PR50. The minimum BSFC was measured as 268 g/kWh and the widest operating range was observed at a 70 °C intake air temperature. At the same time, the lowest NO_x emission values were obtained at a 30 °C intake air temperature. The maximum HC emission was determined as 586 ppm at a 30 °C intake air temperature. In addition, the minimum CO emission was measured as 0.142% by volume at a 70 °C intake air temperature.

1. Introduction

The world is facing serious problems such as decreasing petroleum resources and increasing air pollution due to the excessive use of motor vehicles [1,2]. Internal combustion engines (ICEs) are one of the greatest contributors to the occurrence of these issues, although they are used in many areas due to their high power [3]. In order to make the most effective use of these technologically remarkable machines, harmful emissions should be reduced and more efficient energy conversion should be provided [4,5]. The effect of high compression ratios in ICEs enables higher thermal efficiency to be achieved and thus emissions can be reduced [6]. Knock formation, which has a significant impact on engine performance, prevents spark ignition (SI) engines from operating at high compression ratios. On the other hand, compression ignition (CI) engines can operate at higher compression ratios, but the high nitrogen oxide (NO_x) and soot emissions are a disadvantage [7]. In particular, the simultaneous reduction of these emissions is not possible without expensive after-combustion techniques (selective catalytic reduction (SCR), three-way catalytic converters, diesel particulate filters (DPFs), exhaust gas recirculation (EGR), etc.) [8,9]. Moreover, the use of these systems carries high costs. In consideration of the problems encountered in SI and CI engines, researchers have focused on new combustion strategies such as homogenous charged compression ignition (HCCI) [10,11,12,13,14], premixed charged compression ignition (PCCI) [15,16,17,18,19] and reactivity-controlled compression ignition (RCCI) [20,21,22,23,24]. Compared to conventional combustion in internal combustion engines, combustion based on the low-temperature combustion (LTC) principle results in lower emissions (especially NO_x and soot) and higher thermal efficiency due to the reduced temperature rise by operating in lean mixtures [25,26,27,28,29]. However, while the low combustion temperature creates an advantage, on the other hand, it causes an increase in hydrocarbon (HC) and carbon monoxide (CO) emissions. The greatest problem to be overcome in LTC is the inability to control the combustion initiation and combustion phase. In addition, especially in the HCCI combustion strategy, the operating range of the engine remains at quite limited levels due to the knock increase at high level loads and the misfire problem at low loads [10,11].

Among the LTC strategies, RCCI [30] is particularly important given its advantages, such as better control of combustion and a wider operating range compared to others. In the RCCI combustion strategy, two different fuels or fuel mixtures are used: low-reactivity fuel (LRF) and high-reactivity fuel (HRF). LRF (gasoline, ethanol, butanol, propanol, etc.) is sprayed into the intake manifold and mixed with air; then, this air–fuel mixture is taken into the cylinder. HRF (diesel, n-heptane, diethyl ether, etc.) is sprayed directly into the cylinder before the piston reaches the top dead center (TDC) [31,32]. This allows the timing between the start of ignition and the start of injection to be increased. In this way, the more homogeneous formation of the mixture is ensured. It becomes possible to achieve the controllability of combustion, and objectives such as saving fuel and reducing emissions are realized when intervening in the injection timing [5]. In order to achieve better thermal efficiency values, RCCI and other LTC strategies can be developed by optimizing, through improvement studies, parameters such as the injection timing, exhaust gas temperature, amount of fuel injected into the intake port or into the cylinder and intake air temperature (IAT) [33,34,35,36]. It has been stated that more efficient RCCI combustion can be achieved with the recently revealed reverse reactivity stratifying (R-RCCI) mode. Unlike the RCCI combustion principle, in R-RCCI, a small amount of high-cetane fuel (HRF) is injected into the intake port before it would normally be injected to ignite the high-octane fuel (LRF) during the compression process. In this proposed combustion mode, much less air–fuel mixture penetrates the cavities in the combustion chamber than in RCCI, as a small amount of HRF fuel is included in the preformed mixture. In this way, combustion can be started easily with a small amount of HRF, and more efficient combustion can be achieved [37].

Recently, researchers have been focused on obtaining wider operating ranges, more efficient combustion processes and lower emission values by using alternative fuels and new fuel mixtures instead of conventional fuels. In a study, RCCI combustion was investigated using diesel and gasoline fuels to reduce the emission levels over a wide operating range in a heavy-duty engine. It has been observed that emission values that meet the EURO VI criteria can be reached by examining the influences of parameters such as the EGR ratio and advanced SOI. It was observed that low NO_x emissions, 45% HC emissions and a 76% CO emission improvement were achieved with a reduction in the EGR ratio. By advancing the injection timing from 8 °CA to 32 °CA bTDC, a reduction in HC and CO emissions was observed and a 3.5% saving in fuel consumption was recorded [38]. In another study performed in RCCI combustion mode, in which gasoline and methanol fuels were used as the LRF and diesel fuel was used as the HRF, the effects of these fuels on the combustion characteristics were investigated. Regarding the use of methanol–diesel fuels, it has been stated that CA50 should be retarded in order to overcome the negative effects caused by high combustion speeds. It has been determined that the combustion efficiency increases with a shorter combustion duration and higher efficiency is achieved compared to the use of other fuel pairs [39]. Benajes et al. [40] conducted research to achieve operating range expansion and low emission targets by using 20% ethanol and 80% gasoline fuels by volume as the LRF and a diesel–biodiesel fuel mixture as the HRF in a diesel engine with a compression ratio of 17.5, operated in RCCI combustion mode. The maximum operating loads were limited to 35% at all engine speeds due to the maximum pressure and maximum pressure rise rate. In addition, it has been observed that when the exhaust gas temperature rises above 200 °C in this combustion process, oxidation occurs effectively and a reduction in HC and CO emissions is achieved. In research conducted on an engine operated using the LTC concept, compressed natural gas (CNG), methanol, ethanol and gasoline fuels as the LRF and neem biodiesel and diesel fuels as the HRF were used. The usability of these fuels was examined by conducting a series of tests to obtain the lowest NO_x and soot emissions. The selection of methanol–gasoline–diesel fuels and the dilution of the air taken into the cylinder with EGR provided an improvement in thermal efficiency. It has been stated that the increase in pressure rise rates is due to the use of a high-octane-number fuel with excessive dilution using a high-rate EGR. This situation also causes negative effects such as knocking and an increase in cyclical differences [41]. Uyumaz et al. [42] investigated the importance of lambda for combustion characteristics by using iso-octane/n-heptane fuels in the RCCI combustion strategy. It was determined that the start of combustion was retarded with the increase in the fuel injected into the cylinder. This fact was attributed to a decrease in cylinder temperature due to greater fuel evaporation. Moreover, the increase in lambda provided an increase in thermal efficiency and fuel savings. In the study conducted by Li et al. [43], iso-propanol–butanol–ethanol (IBE) and diesel fuel mixtures were used and the effects of these fuels on the combustion, performance and emissions were determined in a diesel engine. It has been stated that higher thermal efficiency is obtained with the use of 15% IBE and 85% diesel in terms of volume, compared to the use of only diesel fuel. Moreover, HC and CO emissions tended to increase and low NO emission values were observed with the increase in the amount of diluted gas in all experiments. In another study, combustion and emission characteristics were experimentally investigated by using iso-propanol–biodiesel–diesel fuel mixtures in an engine operating in RCCI mode. It was determined that an increase in the rate of combustion, a longer ignition delay and lower levels of NO emissions occurred with the increase in the amount of iso-propanol [44].

Considering the advantages obtained with the use of these low-carbon alcohol fuels in internal combustion engines, there is a gap in the literature, with few studies on the use of iso-propanol fuel as an LRF in engines operating based on the RCCI principle. In the RCCI combustion strategy, which scientists are currently trying to develop and improve, it is important to examine the effects of the use of iso-propanol as a low-reactivity fuel on the performance, combustion and emission characteristics. In this research, the changes in the operating range, combustion and emission characteristics with the use of an iso-propanol–n-heptane fuel pair in a single-cylinder, four-stroke, water-cooled diesel engine (compression ratio = 17.5:1) operated in RCCI combustion mode were investigated experimentally at different intake air temperatures and premixed ratios. In addition, a comprehensive investigation of the effects of the iso-propanol–n-heptane fuel pair in RCCI combustion mode by considering both the intake air temperature and premixed ratio is a novelty in the literature.

2. Materials and Methods

The experiments were performed in a single-cylinder, water-cooled Antor Lombardini diesel engine (LD510) operated in RCCI mode. The test engine was connected to a Cussons DC dynamometer (P8160) with a power of 10kW (at 4000 rpm); thus, it was able to run. The technical characteristics of the test engine used in the experiments, conducted in the Automotive Engineering Department Engine Laboratory at Gazi University, are listed in

Table 1. Technical characteristics of the test engine.

Engine	Compression Ignition (4-Stroke, Water-Cooled)
Combustion strategy	RCCI
Stroke × bore	90 mm × 85 mm
Number of cylinders	1
Compression ratio	17.5:1
Cylinder volume	510 cm3
Length of connecting rod	118 mm
Power	12 HP
Max. engine speed	3000 rpm
Max. torque	32.8 Nm @1800 rpm
Oil consumption	8 g/h
Oil reservoir capacity	1.75 lt

.

This study included the establishment of the process steps of the test system, the establishment of the real-time control system and software development, the establishment of the combustion analysis system, calibration operations, RCCI experiments for iso-propanol–n-heptane, data recording with the data acquisition card, the acquisition of outputs with the developed MATLAB code and the visualization and interpretation of the results. Figure 1 displays the flow chart of the research.

The RCCI combustion concept includes port fuel injection (PFI) and direct injection (DI) systems. Before the CI engine used in the experiments was converted to RCCI mode, a mechanical injection system was replaced with a fuel injection system of common rail. A PFI injector was fitted on the designed and manufactured intake manifold and it was assembled with the engine. Iso-propanol as the LRF was sprayed into the intake manifold by PFI, and n-heptane as the HRF was injected into the cylinder by DI. The specifications of the fuels are presented in

Table 2. The chemical specifications of test fuels [3,45].

Fuel	Octane Number	Density (kg/m3)	Boiling Point (°C)	Molar Mass (g/mol)	Lower Heating Value (kJ/kg)
Iso-propanol C3H8O	107	809	82	60.10	30447
n-heptane C7H16	-	679.5	98	100.16	44560

. Two separate fuel supply systems were constructed for both the DI and PFI systems. For the PFI system, a fuel pump (low pressure), regulator (pressure), fuel filter and port injector (low pressure—3 bar) were used. A fuel pump (low pressure), fuel filter, fuel pump (high pressure—up to 1500 bar), AC motor for fuel pump, rail line, direct injector (high pressure—750 bar) and fuel cooler were also used for the DI system. Parameters such as the fuel quantity and injection timing, which also affect RCCI combustion and other combustion modes, were controlled using the PFI and DI systems. The detailed experimental setup is presented in Figure 2.

One of the most effective parameters affecting RCCI combustion is the premixed ratio (PR), defined as the ratio of the energy of the LRF injected into the intake manifold to the total energy provided by all fuels. The injection characteristics and mass flow rate of both fuel injection units could be controlled by using the designed and manufactured fuel injection module. The flow rates of the test fuels were checked and calibrated by varying the solenoid injectors’ pulse widths. The calibration process was applied with an injection pressure of 750 bar in the DI injector and 3 bar in the PFI injector. An optical encoder (1000 pulses) was used to obtain pressure data with sufficient resolution. The use of a National Instruments (Model: NI-USB-6259) data acquisition card was preferred to obtain in-cylinder pressure signals. These signals were measured using a pressure transducer (Kistler 6056A) with accuracy of -20 pC/bar. The signals from the pressure sensor were amplified using a combustion analyzer (Cussons P4110).

Bosch BEA550 emission measuring equipment was used for the determination of exhaust gases. HC, NO_x, CO, O₂ and CO₂ emissions can be analyzed with this equipment. Additionally, this emission device can determine the lambda value by referring to Bretschneider’s equation.

Table 3. The technical information of the BEA550 emission analyzer.

	Measurement Range	Accuracy
Lambda	0.5 to 9.999	0.001
NO (ppm vol)	0 to 5000	1
HC (ppm vol)	0 to 9999	1
CO (% vol)	0 to 10	0.001
CO2 (% vol)	0–18	0.01
O2 (% vol)	0 to 22	0.01

includes the technical information related to the exhaust gas analyzer unit used in the tests.

The experiments were carried out by planning two different case studies.

Table 4. The case studies in all tests.

Parameters	Case-1	Case-2
Combustion mode	RCCI	RCCI
Fuel	PR25-PR50-PR75	PR50
Intake air temperature (°C)	50	30–50–70
Engine speed (rpm)	1000–2500	1000–2500
Lambda	1.40–4.00	1.40–4.00

includes all test conditions, which are described as follows.

In the first case study, all experiments were conducted at a constant IAT of 50 °C and the effects of the premixed ratio (PR25, PR50, PR75) on the operating range, characteristics of combustion and emission for the RCCI mode were investigated. The content of the fuels used was as follows: 25% iso-propanol + 75% n-heptane (PR25); 50% iso-propanol + 50% n-heptane (PR50); 75% iso-propanol + 25% n-heptane (PR75). Before starting the experiments, the engine was heated by running it in CI mode for a while. The engine speed was gradually increased, starting from 1000 rpm, to determine the operating range in RCCI mode for each PR fuel. For each PR fuel, the engine load was increased by increasing the total amount of fuel, keeping the premixed ratio constant and the engine speed approximately constant. In this manner, knock and misfire limits were determined for each PR fuel and the engine operating range was established.

In the second case study, all experiments were performed at one premixed ratio (PR50) and the effects of the IAT (30 °C, 50 °C, 70 °C) on the operating range, characteristics of combustion and emission for the RCCI mode were investigated. Similarly, the engine was heated up by running it in CI mode for a while before starting the experiments. The engine speed was gradually increased, starting from 1000 rpm to 2500 rpm, to determine the operating range in RCCI mode for each IAT with the PR50 fuel. For each IAT value, the engine load was increased by increasing the total amount of fuel, keeping the premixed ratio constant at 50% and the engine speed approximately constant. As in the previous case study, knock and misfire limits were determined for each IAT and the engine operating range was established.

The crank angle and in-cylinder pressure data were acquired with a crank angle sensitivity of 0.36°. The average of 50 cycles was taken with an algorithm developed in the MATLAB program, thus eliminating cyclic differences and providing an accurate data acquisition approach. The outputs, such as the in-cylinder pressure, IMEP, HRR, the coefficient of variance in IMEP (COV_imep), the initiation of combustion (CA10), the crank angle at which 50% of integrated heat release arose (CA50), the combustion duration (CA90-10), the maximum rate of pressure rise (MPRR), BSFC and the thermal efficiency were all calculated with this algorithm. In the experiments, the voltage signals received from the data acquisition card were converted into pressure data in bar units. The MATLAB code used included the calculation of the heat release rate, integrated heat release and combustion periods (CA10, CA50, CA90), with these pressure data depending on the HRR per crank angle. The HRR per crank angle was determined with the following equations, in accordance with the first law of thermodynamics [6].

$$\frac{dQ}{d\theta }=\frac{n_c}{n_c-1}P\frac{dV}{d\theta }+\frac{1}{n_c-1}V\frac{dP}{d\theta }+\frac{d{Q}_w}{d\theta }$$

(1)

$$\frac{d{Q}_w}{d\theta }=\frac{1}{6n}{h}_{g}A\left(T_g-T_w\right)$$

(2)

In these equations, dQ represents heat release according to the crank angle differentiation dθ. P and V denote the in-cylinder pressure and volume of the cylinder, respectively. n_c is the rate of specific heat and dQ_w/dθ is the heat transfer from the cylinder wall. n, h_g, A, T_g and T_w denote the engine speed, coefficient of heat transfer (by modified Woschni heat transfer model) [46], surface area of heat transfer per crank angle, mean gas temperature in the cylinder and temperature of the cylinder wall, respectively. The thermal efficiency was calculated by the following equation [6]:

$${\eta }_T=\frac{W_{net}}{{\dot{m}}_{IP}\times Q_{LH{V}_{IP}}+{\dot{m}}_{nH}\times Q_{LH{V}_{nH}}}$$

(3)

where $W_{net}, {\dot{m}}_{IP}, {\dot{m}}_{nH}, Q_{LH{V}_{IP}}, Q_{LH{V}_{nH}}$ represent the net work, the amount of iso-propanol consumed, the amount of n-heptane consumed, the low heat value of iso-propanol and the low heat value of n-heptane, respectively. The brake-specific fuel consumption (BSFC) was determined using the following equations:

$$P=\frac{T\cdot n}{9549}$$

(4)

$$BSFC=\frac{{\dot{m}}_{IP}+{\dot{m}}_{nH}}{P\cdot 60000}$$

(5)

P, T, n and BSFC refer to the effective power, torque, engine speed and brake-specific fuel consumption, respectively, in these equations.

3. Results and Discussion

Many studies continue to be conducted on the promising RCCI combustion strategy. One of the main objectives is to identify fuel pairs with wider operating ranges and low emissions by controlling the combustion process. In this study, variations in the operating range, characteristics of combustion and emissions with the use of the iso-propanol–n-heptane fuel pair in RCCI mode were experimentally investigated at different temperatures and premixed ratios.

3.1. Premixed Ratio Effects

There are many parameters affecting RCCI combustion, such as the IAT, chemical properties of fuels and injection timing [5,35]. One of the most significant of these is the premixed ratio formed using the fuel pair. All tests were carried out at a constant IAT of 50 °C and the effects of the premixed ratio (PR25, PR50, PR75) on the operating range, combustion and emission characteristics in RCCI combustion mode were investigated. The fuels used were as follows: 25% iso-propanol + 75% n-heptane (PR25); 50% iso-propanol + 50% n-heptane (PR50); 75% iso-propanol + 25% n-heptane (PR75).

The effect of using fuels with three different premix ratios (PR25, PR50 and PR75) at 50 °C IAT on the operating range of the engine in the RCCI combustion regime is shown in Figure 3. The upper boundary line in the operating range represents the knock limit and the lower boundary line represents the misfire limit. The operating range of the LTC engines is expressed by the knock and misfire limits. The level of engine load is adjusted by the amount of fuel injected into the cylinder. When the operating map is examined, it is seen that the knock limit is reached as the mixture becomes richer and the misfire limit is reached as it becomes leaner. Knocking is caused by high heat release due to simultaneous combustion and suddenly occurring pressure increases in the cylinder [47]. When the IAT and premixed ratio were constant, the experiments were carried out at four different engine speeds (1000, 1500, 2000 and 2500 rpm). The load and speed range of the engine were determined by increasing the amount of fuel from the misfire limit to the knock limit at each engine speed. While the lambda value was high at low loads, this value decreased as the load increased for each fuel premixed ratio. Since the reactivity of the PR25 fuel was higher, its auto-ignition capability in the cylinder was better. In this way, the engine could be run in leaner regions in RCCI mode. In this regard, the operating range was extended by using the PR25 fuel at an IAT of 50 °C.

The effects of fuels with different premixed ratios on the in-cylinder pressure, heat release rate (HRR) and integrated heat release (IHR) at an IAT of 50 °C in RCCI combustion mode are shown in Figure 4 and Figure 5. In RCCI, the ignition timing is delayed with the increase in low-reactivity fuel, resulting in a reduction in in-cylinder pressure [48]. When the data obtained at a 2500 rpm engine speed and lambda value of 2.40 are analyzed, it can be stated that the maximum in-cylinder pressure increases with the use of a premixed fuel (PR25) with higher reactivity. When the premixed fuel content is considered, the premixed ratio is reduced as more high reactivity fuel is sprayed directly. This situation causes the formation of richer regions in the combustion chamber before the start of combustion. It can be stated that combustion control is achieved at low premixed ratios with the high-reactivity n-heptane fuel [49]. The less n-heptane fuel included in the mixture, the smaller the amount of iso-propanol fuel expected to combust because of the lower number of auto-ignition points. Lean regions where combustion does not occur may be formed with an increase in iso-propanol in the mixture, resulting in lower in-cylinder pressure values in RCCI mode [50]. This may also be caused by the low viscosity and density of iso-propanol [51]. It was found that as the iso-propanol content increased in the fuel mixture, the knock tendency was reduced depending on the high-octane value of iso-propanol. The maximum in-cylinder pressure was 65.87 bar (8.64 °CA aTDC) with the PR25 fuel. In a similar study, Wang et al. [48] stated that the in-cylinder pressure decreased as the amount of low-reactivity fuel in the mixture increased in LTC mode. It can also be stated that the start of combustion is retarded as the alcohol composition (i.e., iso-propanol) increases in the chemical structure of the fuel mixture. There was a 2 °CA delay in the start of combustion when using the PR75 fuel compared to the PR25 fuel. In addition, when the PR25, PR50 and PR75 fuels were used, the combustion durations were 49.32 °CA, 46.08 °CA and 43.20 °CA, respectively, due to the increase in low-reactivity fuel.

Figure 6 shows the BSFC maps obtained in RCCI mode for fuels with different premixed ratios at an IAT of 50 °C. These maps also display the operating ranges depending on the engine speed and engine torque. The axes and color scale are the same for each fuel, for a more accurate evaluation. Regarding the operating range, it can be concluded that the lowest BSFC distribution is obtained with the PR25 fuel, since it shows better combustion performance at an IAT of 50 °C, and the minimum BSFC is obtained as 268 g/kWh.

The formation of NO_x emissions with the use of fuels with different premixed ratios at an IAT of 50 °C in RCCI mode is illustrated in Figure 7. The free nitrogen molecules in the air combine with oxygen to form nitrous oxide at high temperatures. The parameters affecting the formation of nitrous oxide are the temperature of the combustion chamber and the lambda value of the mixture [52]. As the engine speed increased, NO_x emissions were reduced for all three premixed fuels due to the stability of the engine. After a certain engine speed, NO_x emission formation decreased due to the shorter combustion period. The minimum NO_x emissions were obtained as 16 ppm with the PR75 fuel. Although the lowest emission values were obtained with the PR75 fuel owing to its higher O₂ concentration in comparison to other fuels, the most appropriate NO_x emission distribution was observed with the use of the PR25 fuel when the operating ranges were considered.

Figure 8 displays the formation of HC emissions with the use of fuels with different premixed ratios at an IAT of 50 °C in RCCI mode. Hydrocarbon emissions form as a result of the incomplete combustion of the fuel molecules due to the low temperature and lack of oxygen in the regions close to the cylinder wall. In these regions, the temperature of the air–fuel mixture is noticeably lower than in the center of the cylinder [53]. The amount of oxygen in the mixture increases and the combustion quality improves when increasing the air–fuel ratio in the cylinder. However, if the mixture is too lean, HC emissions rise due to the low combustion quality [54]. Since the combustion duration is shorter at high engine speeds, some of the hydrocarbons cannot be fully combusted and the HC emissions increase. When HC emission formation was analyzed for different premixed ratios, the HC emission values increased as the engine speed gradually increased. The lowest level of HC emission formation was observed with the use of the PR25 fuel when the operating ranges were considered. The minimum HC emissions were recorded as 170 ppm with the PR25 fuel.

The formation of CO emissions with the use of fuels with different premixed ratios at an IAT of 50 °C in RCCI mode is shown in Figure 9. The hydrocarbon chains that compose the fuel are reacted and divided into smaller parts during combustion. Carbon atoms separated from hydrogen are reacted with O₂ molecules to form CO₂ molecules. However, if the oxygen content of the charge taken into the cylinder is low, the carbon atoms separated from hydrogen bond with a single oxygen atom to form a carbon monoxide (CO) molecule [36,55]. Despite the relatively higher CO emission values obtained with the PR75 fuel owing to its higher O₂ concentration in comparison to other fuels, the most appropriate CO emission distribution was observed with the use of the PR25 fuel when considering the wide operating range. The minimum CO emissions were obtained as 0.132% by volume with the PR25 fuel. In a relevant study, Uyumaz [3] stated that the minimum CO emissions were obtained with higher n-heptane content in the fuel mixture due to the higher combustion temperature and more rapid combustion due to knocking.

3.2. Intake Air Temperature Effects

Another of the most important parameters affecting the RCCI combustion regime is the intake air temperature. All tests were performed with the PR50 fuel and the effects of the IAT (30 °C, 50 °C and 70 °C) on the operating range, characteristics of combustion and emissions in RCCI mode were investigated.

The effect of the IAT on the operating range of the engine in the RCCI combustion regime is shown in Figure 10. The upper boundary line in the operating range represents the knock limit and the lower boundary line represents the misfire limit. In LTC regimes, the operating range of the engine is expressed by the knock and misfire limits. The mean cycle temperature decreases with the reduction in the mass of fuel injected into the cylinder at low loads. Accordingly, the temperature of the cylinder wall and residual exhaust gases also decreases. These temperature drops make it difficult for the fuel to ignite spontaneously in the following cycles and lead to misfires. The level of engine load is adjusted by the amount of fuel injected into the cylinder. When the operating map is examined, it is clearly seen that the knock limit is reached as the mixture becomes richer and the misfire limit is reached as it becomes leaner. Knocking is caused by high heat release due to simultaneous combustion and suddenly occurring pressure increases in the cylinder [47]. When an experimental study was carried out for LTC engines at ambient temperature, the intake air temperature was already 40–50 °C. In addition, experiments were performed at an intake air temperature of 30 °C to examine the operating conditions in cold climatic conditions. Moreover, experiments were conducted at a 70 °C intake air temperature in order to extend the operating range in RCCI mode with the iso-propanol–n-heptane fuel pair. Intake air temperatures as high as 70 °C can be reached by heating the intake manifold with the temperature of the exhaust gases in real applications [56]. When the IAT and premixed ratio were constant, the experiments were conducted at four different engine speeds (1000, 1500, 2000 and 2500 rpm). The load and speed range of the engine were determined by increasing the amount of fuel from the misfire limit to the knock limit at each engine speed. While the lambda value was high at low loads, this value decreased as the load increased for each fuel premixed ratio. When the IAT was increased to 70 °C with the use of the PR50 fuel, the operating range expanded, especially towards the knock limit, and the widest operating range was obtained at an IAT of 70 °C.

Figure 11 and Figure 12 display the effects of the IAT on the in-cylinder pressure, HRR and IHR when using the PR50 fuel in RCCI mode. When the data obtained at a 2500 rpm engine speed and lambda value of 1.80 were analyzed, it was determined that the maximum cylinder pressure increased with the rising IAT and combustion started at earlier crank angles. It was observed that the combustion started at a 70 °C intake air temperature at the earliest. The maximum in-cylinder pressure value was 77.54 bar (8.64 °CA aTDC) with the PR50 fuel at this temperature. The higher IAT prevented the cylinder wall temperature from dropping too much and thus improved combustion. The cold cylinder wall causes a significant decrease in the kinetic reaction rate and adversely affects combustion in RCCI mode [57]. Uyumaz [3] also reported in a study that combustion was advanced and the in-cylinder pressure was measured at higher levels when increasing the intake air temperature from 313 K to 393 K in LTC mode using iso-propanol fuel. It can be stated that the combustion duration decreases as the IAT rises. As the IAT increases, the chemical reactions improve and the number of active molecules in the combustion process increases, resulting in shorter combustion periods. For IATs of 30 °C, 50 °C and 70 °C, the combustion periods were 57.24 °CA, 56.52 °CA and 54.36 °CA, respectively. The shortest combustion period was realized at 70 °C with the use of the PR50 fuel.

The BSFC maps obtained in RCCI combustion mode using the PR50 fuel for different IATs are illustrated in Figure 13. These maps also display the operating ranges depending on the engine speed and engine torque. The axes and color scale are the same for each fuel, for a more accurate evaluation. Considering the operating range, it can be said that the lowest BSFC distribution was obtained at an IAT of 70 °C because of the more stable combustion behavior observed at higher temperatures, and the minimum BSFC was recorded as 268 g/kWh.

The formation of NO emissions at different IATs with the PR50 fuel in RCCI mode is illustrated in Figure 14. The parameters affecting the formation of nitrous oxide are the temperature of the combustion chamber and the lambda value of the mixture [52]. As the engine speed increased, NO_x emissions were reduced at all IATs due to the stability of the engine. High combustion temperatures are one of the most effective factors for NO_x formation. The fact that NO_x chemical formation mechanisms occur less at low combustion temperatures is one of the most important attributes of LTC regimes [3]. The minimum NO_x emissions were recorded as 46 ppm at IAT of 30 °C. In consideration of the operating ranges, it was observed that the lowest NO_x emission values were obtained at an IAT of 30 °C, while the highest NO_x emission distribution appeared at an IAT of 70 °C when using the PR50 fuel.

The formation of HC emissions at different IATs with the PR50 fuel in RCCI mode is shown in Figure 15. Hydrocarbon emissions form as a result of the incomplete combustion of the fuel molecules due to the low temperature (i.e., 30 °C) and lack of oxygen in the regions close to the cylinder wall [54]. The flame occurring during combustion is extinct because of the low cylinder wall temperature [47]. Therefore, flame propagation is not continuous in the cylinder, faulty auto-ignition occurs and poor combustion is observed. In general, HC emissions decrease with an increasing IAT due to the development of chemical reactions and the shorter duration of combustion at higher IATs. The highest level of HC emission formation was seen at an IAT of 30 °C when the operating ranges were considered. The maximum HC emissions were measured as 586 ppm.

The formation of CO emissions at different IATs with the PR50 fuel in RCCI mode is presented in Figure 16. Since CO molecules can be oxidized at higher combustion temperatures [36], the lowest CO emission distribution was observed at an IAT of 70 °C. Accordingly, it was determined that CO₂ formation improved and the level of CO emissions dropped. The lowest level of CO emission formation was obtained at an IAT of 70 °C when the operating ranges were considered. The minimum CO emissions were measured as 0.142% by volume with the PR50 fuel.

4. Conclusions

The RCCI combustion regime is a potentially promising combustion mode with respect to its combustion characteristics and emissions, and it continues to be developed. In the current study, the variations in the operating range, characteristics of combustion and emissions with the use of an iso-propanol–n-heptane fuel pair in RCCI combustion mode were experimentally investigated at different temperatures and premixed ratios. For this purpose, two different case series were planned and tests were conducted.

When the effects of the premix ratio were considered, the widest operating range was obtained with the PR25 fuel due to its high reactivity and better auto-ignition capability in the cylinder. Hence, the engine could be operated in RCCI mode both in the regions close to the knock limit and in the leaner regions. In addition, it was found that the knocking tendency declined with the increase in the iso-propanol content in the mixture because of the high octane number of iso-propanol. The results show that combustion is retarded as the amount of iso-propanol is increased. The lowest BSFC distribution was observed with the PR25 fuel due to its better combustion performance compared to other premixed fuels, and the minimum BSFC was noted as 268 g/kWh. In addition to these, the lowest emission values in NO_x, HC and CO emission formation were obtained with the use of the PR25 fuel.

The operating range was enlarged particularly up to the knock limit, and the most extensive operating range was obtained at an IAT of 70 °C using the PR50 fuel. It is observed that the combustion duration decreases as the IAT rises. As the IAT increased, the maximum cylinder pressure also increased and combustion was initiated at earlier crank angles. Considering the operating range, the lowest BSFC distribution was obtained at an IAT of 70 °C and the minimum BSFC was recorded as 268 g/kWh. The minimum NO_x emissions were measured as 46 ppm at an IAT of 30 °C. The production of the highest HC emissions was observed at an IAT of 30 °C when the operating ranges were considered. The maximum HC emissions were determined as 586 ppm. In addition, the minimum CO emissions were measured as 0.142% by volume at an IAT of 70 °C.

It can be concluded that the iso-propanol–n-heptane fuel pair is appropriate for the RCCI combustion regime as it provides a wide operating range, improvable combustion performance and acceptable emission values, as shown in this experimental study. Moreover, RCCI engines can be used as an internal combustion engine mode in electric–hybrid vehicles due to their high thermal efficiency, low BSFC and low emissions. In addition, it can be stated that the iso-propanol–n-heptane fuel pair is especially suitable for clean energy applications due to its emission capabilities. Optimization studies can be conducted by applying the response surface method and changing the reactivity with different premix ratios. In addition, the compression ratio can be changed to determine the most suitable and effective operating range.