Experimental Study on Effects of RCSL and RCTL Combustion Chamber for Combustion Process of Highly Intensified Diesel Engine

Abstract

In order to optimize the combustion chamber of a highly intensified single-cylinder diesel engine, including improvement of the air utilization rate in the combustion chamber, optimization of the fuel/air mixture process, reduction of the soot emission in the cylinder, and obtainment of the optimal combustion performance parameters, a re-entrant combustion chamber with step-typed lip (RCSL) and a re-entrant combustion chamber with tilted lip (RCTL) were innovatively designed and the effects of the throat diameter on the combustion process was obtained through experiments. When the RCSL with a diameter of 72 mm worked, target for the Brake Mean Effective Pressure (BMEP) of 2.6 MPa could be achieved under the condition of excess air coefficient of 1.5~1.7. At the same time, this had the advantages of lower fuel consumption, lower exhaust temperature, lower smoke level, and shorter combustion duration. Under the same BMEP, the RCTL had lower fuel consumption, exhaust gas temperature and maximum combustion pressure, as well as faster heat release rate (HRR) and shorter combustion duration than the RCSL.

1. Introduction

Downsizing is an important direction of engine development due to its fuel economy improvement. It promotes the continuous increase of engine output power density. In the aspect of high-speed diesel engine R&D, vehicle manufacturers and engine research consultants have done massive research work and developed quite a few high-power density diesel engines for the purpose of vehicle power improvement and fuel economy [1,2,3,4,5,6,7,8,9,10]. Volkswagen developed a 4-cylinder 2.0 L commercial TDI diesel engine in 2010, which is capable of producing 66 kW/L at 4000 r/min [11]. In the meanwhile, BMW also developed a 3.0 L TDI diesel engine for 740D series car, which reached a power density of 75 kW/L at 4500 r/min [12]. In the research stage, IFP institute achieved 90 kW/L at 4000 r/min on a single cylinder diesel in the laboratory [13,14]. FEV developed a high efficiency combustion system (HECS) in 2011, which could produce 105 kW/L at 4800 r/min [15]. Later on, the combustion system was further optimized for high power output. By the time a HECS-P combustion system was developed, the power density had reached 115 kW/L [16].

With the above high intensified diesel engine combustion process, at the end of compression, the cylinder working medium pressure will reach 18 MPa and the density will be as high as 45 kg/m³, which means that the cylinder working medium is characterized by high pressure and high density. At this time, the combustion duration period of the mixture will be greatly shortened, and the combustion process becomes characterized by rapid mixing and concentrated heat release. In this environment of high pressure, high density and short ignition delay period, the matching law between fuel spray and combustion chamber shape will change significantly. The air-fuel ratio must be reduced, the excess air coefficient will be reduced from 1.8~2.0 to 1.5~1.6, and the combustion chamber design must be aimed at improving the air utilization rate. At this time, the in-cylinder flow and combustion chamber design must aim at improving the air utilization rate. The design of combustion chamber shape [17,18,19,20,21,22] is very important for diesel engines with diffusion combustion [23,24]. Firstly, the combustion chamber provides an extended space for the fuel spray, in which the large velocity difference between the high-speed spray and the air movement results in a strong air entrainment movement, which promotes the early evaporation and mixing of the fuel spray; the penetration distance of the spray is usually greater than that of the combustion chamber space. The shape of the lip, the landing point of the spray, and the secondary rebound of the spray after impingement have a great impact on fuel/air mixture. Secondly, the shape of the combustion chamber also has a great influence on the in-cylinder flow, which not only enhances the intensity of the compression swirl, but also forms a certain squish movement, which has a certain impact on the fuel-air mixing, especially after the fuel injection stops. The influence of in-cylinder spray momentum disappears, and these flows highlight the fuel/air mixture effect in the later stage of combustion, thus improving the combustion process in the later stage. Li et al. [25] found the best design parameters of the combustion chamber through the parametric design of the double swirl combustion chamber of a diesel engine with a cylinder diameter of 132 mm, so that the fuel/air mixture is more uniform, the air utilization rate in the cylinder is higher, and the fuel consumption, smoke and exhaust temperature are significantly reduced under the heavy load condition of 1100 r/min~2100 r/min. Wang et al. [26] conducted an experimental study on a 105-mm bore diesel engine, and carried out a study on the influence of the re-entrant combustion chamber and the straight combustion chamber on the in-cylinder air flow field at a speed of 2800 r/min. The results show that the re-entrant combustion chamber can increase the in-cylinder air flow velocity near the top dead center, which can maintain a higher air flow speed in a wider crank angle range, and that the swirl speed is increased during compression. Strong squish flow is formed during the combustion, which promotes the mixing and combustion, while the air flow in the straight combustion chamber is relatively weak. Yuan et al. [27] carried out multi-dimensional simulation on the combustion chamber of a high-speed direct injection diesel engine. In the simulation calculation, the model parameters most suitable for the diesel engine were selected, and the matching law between the combustion chamber shape and the nozzle hole structure parameters was studied. They established the intermediate characteristic parameters of in-cylinder average turbulent mixing rate and fuel evaporation mass fraction variance and decomposed the coupling mechanism between combustion chamber shape and nozzle hole structure in detail. The results show that with the decrease of the throat diameter, the squish velocity increases, the fuel/air mixture is better, and the heat release in the late stage of diffusion combustion is more uniform. Bapu et al. [28] took a study on the effect of eddy current on diesel engine combustion and emissions, and their results show that small swirl ratio matching deeper combustion chamber shape could improve combustion performance and reduce soot emissions. Karthickeyan [29] studied the influence of combustion chamber on the performance, combustion and emission characteristics of a low-speed diesel engine and proposed two new combustion chamber structures: an annular combustion chamber and a trapezoidal combustion chamber, which were compared with the standard semi-spherical combustion chamber. It was found that the trapezoidal combustor produces high swirl ratio and strong squish flow, which accelerated the fuel/air mixture process and achieved better engine performance than annular combustion chamber and semi-spherical combustion chamber. Lee et al. [30] used numerical simulation to study the influence of injection timing and the central convex of the combustion chamber on the combustion process and emission characteristics of the engine. Their results show that when fuel was injected at 8 °CABTDC or 12 °CABTDC, the swirl ratio with titled slope was the highest, and the air utilization rate was the highest. The swirl ratio with trapezoidal slope is smaller, and the fuel is oxidized more in the center of the combustion chamber. Zha et al. [31] used combustion image velocimetry to study the influence of combustion chamber geometry on afterburning in an optical diesel engine, and their results show that, compared with the re-entrant combustion chamber, the stepped lip combustion chamber had shorter combustion duration period and faster heat release rate. Tay et al. [32] studied the effects of three combustion chambers and six injection rates on the combustion characteristics of a diesel engine. Their results show that the turbulent kinetic energy of the re-entrant combustion chamber with the smallest throat diameter was the highest, and the CO emission was lower but the NOx emission was higher. The shallow combustion chamber with the largest throat diameter has the lowest turbulent kinetic energy, and the combustion is mainly controlled by the fuel injection rate.

Based on the characteristics of the combustion process of a highly intensified diesel engine, the re-entrant combustion chamber with step-typed lip (RCSL) and the re-entrant combustion chamber with tilted lip (RCTL) were innovatively designed in this paper. At the end of compression, the RCSL can produce a strong squish motion, which promotes the evaporation and mixing of the spray. The stepped lip structure can divide the spray into an upper part and a lower part The spray at the lower part is involved in the combustion chamber and mixed with air under the action of spray momentum and squish flow, and the spray at the upper part diffuses the spray to the area with more sufficient air in the top clearance of the piston through the secondary diversion of the step, so as to improve the air utilization rate of the top clearance of the piston, to prevent the spray from entering the side clearance between the piston and the cylinder liner along the top surface of the piston, to reduce the smoke emission, and to improve the combustion efficiency. The shape of the lip of the RCTL combustion chamber was partially improved and designed for the RCSL combustion chamber. As can be seen from the RCTL combustion chambers shown in Figure 1, the main difference between the RCSL and RCTL is that the lip is an inclined conical surface and directly connected with the top surface of the piston, and there is no obvious stepped structure, so as to improve the air utilization rate at the lip. The main parameters of the two combustion chambers are throat diameter D, re-entrant angle β, re-entrant h and other parameters, as shown in Figure 1 below, and the range of the test combustion chamber is shown in

Table 1. Parameter range of test combustion chamber.

Name	Range
D (mm)	70, 72, 74
Β (°)	72
H (mm)	4.5

.

In this paper, the effects of combustion chamber structure on the combustion process of a highly intensified diesel engine were investigated experimentally. Experiments on the effects of combustion chambers with different throat diameters on the combustion process were carried out based on three kinds of RCSL with different throat diameters. And the comparative experiment between optimized RCSL and RCTL with the same throat diameter was carried out. The first purpose of the experiments was to analyze the influence of throat diameters on the combustion process, and the second purpose was to compare different types of combustion chamber in order to obtain the optimal comprehensive performance and achieve the highly intensified BMEP of 2.6 MPa.

2. Experimental Method

2.1. Experimental Device

The test was carried out on a highly intensified single cylinder diesel engine, which is shown in Figure 2. The main structural parameters are shown in

Table 2. Boosted diesel engine test system.

Engine Type	Single Engine
Bore X Stroke	110 × 110 mm
Compression ratio/L	14.3
Combustion chamber	ω type
Fuel Injection	Common Rail Direct Injection
Injection Pressure	1800 bar
Test conditions	3800 r/m; 2.6 MPa BMEP
Excess air ratio	1.5~1.7

. Engine cylinder head adopts a 4-valve structure, where the injector is vertically installed in the center of the cylinder head and two overhead camshafts individually drive the intake valve and exhaust valve. The high-pressure fuel pump of the common rail fuel system is driven by a separate motor, the common rail pressure is 180 MPa, and the fuel flow rate is measured by a fuel flowmeter installed between the high-pressure fuel pump and the diesel tank. The high-pressure air was provided by an external air compressor, which was introduced into the stable pressure box of the single-cylinder diesel engine laboratory through the pipeline. The pressure sensor and the pressure closed-loop control system were installed in the stable pressure box to adjust the intake pressure required by the engine. A heater and a temperature closed-loop control system were also provided in the stable pressure box to adjust the intake temperature. A conical valve was installed in the engine exhaust pipe to adjust the exhaust back pressure. The inlet pressure of the single cylinder engine was realized by compressed air. In order to simulate the exhaust process, the exhaust back pressure was set according to the exhaust boundary conditions of the multi-cylinder engine with supercharger. In this way, the inlet and exhaust boundary conditions of the single cylinder engine and the multi-cylinder engine were consistent, and the combustion state in the cylinder was consistent with the multi-cylinder engine [33].The inlet flow rate was measured by ABB vortex flowmeter, and the inlet and exhaust pipe instantaneous pressure was measured by piezoresistive pressure sensor. Cylinder pressure was measured by kistler piezoelectric pressure sensors. The crankshaft angle sampling resolution was 0.5 °CA. Combustion characteristics such as cylinder pressure, rate of pressure increment combustion duration, and heat release rate are displayed online using the labview data acquisition and combustion analysis system [34]. An electric dynamometer was used to measure the engine torque.

Table 3. Range and accuracy of sensor on the test bench.

Number	Name	Performance	Range	Accuracy
1	ABB Flowmeter	Intake flow measurement	1400 kg/h	1%
2	Temperature Sensor	Intake Temperature Measurement	0~300 °C	1%
3	Pressure Sensor	Intake Pressure Measurement	800 kPa	0.5%
4	Fuel Consumption Meter	Fuel Consumption Measurement	250 kg/h	0.5%
5	Temperature Sensor	Exhaust Temperature Measurement	0~1200 °C	1%
6	Pressure Sensor	Exhaust Pressure Measurement	800 kpa	0.5%
7	Combustion Analyzer	Cylinder Pressure Measurement	25 MPa	0.5%
8	Angle of the instrument	Crank Shaft Angle Calibration	0~720 °CA	0.5%
9	Speed Sensor	Diesel Engine Speed Measurement	8000 r/min	±1 r/min
10	Pressure Sensor	Cylinder Pressure Measurement	25 MPa	0.5%
11	Dynamometer	Output Power Measurement	160 kW	0.1%

shows the range and accuracy of sensor used on the test bench.

2.2. Testing Scheme

The test scheme of the effects of different combustion chambers on combustion process are shown in

Table 4. Experimental schedule for different combustion chambers.

Combustion Chamber Type	Throat Diameter (mm)	Speed (r/min)	Inlet Pressure (kPa)	Inlet Temperature (°C)	Rail Pressure (MPa)	Fuel Spray Nozzle (mm)	Protruding Height (mm)	Start of Injection Timing (°CA)	Injection Duration Period (°CA)
RCSL A1	74	3800	420	60	180	10 × 0.22	2.5	−19	34
RCSL A2	72		440					−19	34
RCSL A3	70		460					−19	34
RCTL	72		460					−19	34

. The RCSL with throat diameters of 70 mm, 72 mm and 74 mm and the RCTL with throat diameter of 72 mm were selected, as shown in Figure 3. The combustion system was composed of a swirl ratio 0 inlet port and an injector with nozzle number of 10. The test was carried out at the operating point of 3800 r/min and the BMEP target of 2.6 MPa. The experiment was carried out at 3800 r/min and BMEP target 2.6 MPa. The same inlet pressure, injection pressure and injection quantity were kept during the test, and the inlet temperature was 60 °C. The maximum combustion pressure in the test was controlled at 22 MPa, and the start of injection timing was adjusted according to the principle of maximum BMEP. However, when the cylinder pressure exceeds 22 MPa, the injection timing should be delayed appropriately.

3. Results and Discussions

3.1. Effects of RCSL on Macro Performance Parameters

The comparison of BMEP, brake specific fuel consumption(BSFC), maximum combustion pressure, exhaust temperature and smoke emission of three combustion chambers with different throat diameters are shown in Figure 4, Figure 5, Figure 6 and Figure 7, respectively. As can be seen from Figure 4, when the engine speed is 3800 r/min, the BMEP of three combustion chambers with different throat diameters basically increases with the increase of intake pressure, and the excess air coefficients corresponding to the experimental intake pressures of 420 kPa, 440 kPa and 460 kPa are 1.53, 1.6 and 1.71, respectively, thus basically conforming to the basic law that BMEP increases with the increase of excess air coefficient. When the maximum combustion pressure is limited to 22 MPa, the A2 combustion chamber with diameter of 72 mm achieves the target BMEP of 2.6 MPa under three excess air coefficients, and the BMEP reaches the maximum of 2.63 MPa when the intake pressure is 460 kPa (excess air coefficient is 1.71).

As can be seen from Figure 5, the BSFC of the combustion chamber A1 with the diameter of 74 mm is higher than the other two combustion chambers with other diameters. For each combustion chamber with a certain diameter, the BSFC at the intake pressure of 460 kPa (excess air coefficient of 1.71) is lower than that at the other two intake pressures (excess air coefficient of 1.6 and 1.53 respectively), in which the lowest BSFC of A2 combustion chamber with diameter of 72 mm is 254 g/kWh, and that of A3 combustion chamber with diameter of 70 mm is 250.8 g/kWh.

It can be seen from Figure 6 that the exhaust temperature of the three combustion chambers are less than 750 °C, which meets the limit requirements of the highly intensified diesel engine for exhaust temperature. For each combustion chamber diameter, the exhaust temperature increases monotonically with the decrease of intake pressure (and the decrease of excess air coefficient), which is caused by the increase of after-burning with the decrease of excess air coefficient. At each inlet pressure, the exhaust temperature decreases monotonically with the increase of the combustion chamber diameter. The exhaust temperatures of A2 and A3 combustion chambers are lower, and the average exhaust temperatures at three intake pressures are 14 °C and 26 °C lower than those of the A1 combustion chamber.

As can be seen from Figure 7, for each combustion chamber, the smoke emission increases monotonically with the decrease of intake pressure. For each intake pressure (excess air coefficient), the smoke number decreases monotonically with the increase of combustion chamber diameter. Compared with the A1 combustion chamber, the average smoke emission of A2 and A3 combustion chambers at three intake pressures are reduced by 1.09 FSN and 1.44 FSN respectively.

According to the above analysis results, the BMEP of A2 RSCL combustion chamber with diameter of 72 mm is the largest, and BSFC, exhaust temperature, and smoke emission are also at good levels, therefore the A2 RSCL combustion chamber has the best comprehensive performance.

3.2. Effects of the Diameter of the RCSL on Combustion Characteristics

The cylinder pressure and HRR curves of the three combustion chambers under the intake pressure of 460 kPa are shown in Figure 8. During the test, the crankshaft angle resolution is 0.1 °CA, the TDC test accuracy is 0.01 °CA, and the measurement error of the pressure in the cylinder is 0.5%, that is, the measurement error of the pressure in the cylinder is within 0.1 Mpa, and the analysis error of the heat release rate curve is within 0.1 °CA. As can be seen from the figure, the cylinder pressure and HRR of the A3 combustion chamber rose the fastest at the initial stage of combustion but declined rapidly after reaching the maximum value. Combustion chamber A2 has the highest HRR. The HRR phase characteristics of the three combustion chambers at the intake pressure of 460 kPa are shown in Figure 9. In the figure, CA5, CA50 and CA90 represent the crankshaft angles corresponding to 5%, 50% and 90% of the cumulative HRR, respectively. Further, the combustion duration refers to the crankshaft angle corresponding to CA5 to CA90. It can be seen that the initial heat release point (CA5) of the A1 combustion chamber is about 1 °CA behind the other combustion chamber, and the heat release center (CA50) and the end point of heat release (CA90) are also about 2 °CA behind, indicating that the heat release of the A1 combustion chamber is slow. The center of heat release and end point of combustion in A2 and A3 are in the front, and the main combustion duration is 1.9 °CA and 0.5 °CA shorter than that in A1, respectively. The A2 combustion chamber has a more ideal HRR, and the combustion duration is 1.4 °CA shorter than that of A3 combustion chamber.

3.3. Adaptability Analysis of Combustion Chamber to Injector Protrusion

In the engineering development of the engine combustion system, it is hoped that the combustion chamber is less sensitive to the protruding height of the injector. This can reduce the deterioration of the combustion system which caused by the assembly error of the injector. Therefore, the sensitivity of the combustion chamber to the protruding height of the injector was determined by a comparison test of three combustion chambers matching different injector protrusion heights. The test results are shown in Figure 10. It can be seen from the figure that with the change of the protruding height of the injector, the main performance parameters such as BSFC, exhaust temperature and smoke emission level of the A1 combustion chamber have a relatively large change range, the change range of the BSFC is 16 g/kWh and the variation range of smoke emission reaches 2.8 FSN. The variation range of each parameter of the A2 and A3 combustion chambers is relatively small, and the value of each parameter is lower when the injector protruding height is 2.0~2.5 mm. Therefore, the A1 combustion chamber is highly sensitive to the injector protruding height, while the A2 and A3 are less sensitive to the injector protruding height.

3.4. Comparative Test of RCSL and RCTL Combustion Chambers

A RCTL combustion chamber with a throat diameter of 72 mm was selected, and the combustion test was carried out under the conditions of rated speed of 3800 r/min and target BMEP of 2.6 MPa. The same intake pressure and fuel injection settings were used (as shown in Table 3). The limit of the maximum combustion pressure was 22 MPa, and the test results were compared with the RCSL combustion chamber (A2) of the same diameter.

Figure 11 shows the comparison of BSFC and indicated thermal efficiency under the two types of combustion chambers. It can be seen that the BSFC of the RCTL combustion chamber reaches 250.1 g/kWh, which is 0.7 g/kWh lower than that of the RCSL combustion chamber. Correspondingly, the indicated thermal efficiency of the RCTL combustion chamber is also increased from 40.1% to 40.2%.

Figure 12 shows the pressure curves in cylinder of the two combustion chambers. It can be seen from the figure that the pressure of the RCSL combustion chamber rises rapidly in the early stage of combustion, but the in-cylinder pressure is relatively high in the middle and late stages of the combustion process (40~80 °CA). Figure 13 shows the changes of the maximum combustion pressure and exhaust temperature of the two combustion chambers. It can be seen that the maximum combustion pressure of the RCTL combustion chamber is 21.3 MPa, which is 0.4 MPa lower than that of the RCSL combustion chamber; the exhaust temperature of the RCTL combustion chamber is 601 °C, which is 130 °C lower than that of the stepped combustion chamber.

Figure 14 shows the changes of the instantaneous HRR and the cumulative HRR of the two types of combustion chambers. It can be seen from the figure that the instantaneous HRR of the two types of combustion chambers has little difference in the rising section, indicating that the two types of combustion chambers with different lip shapes have little effect on the initial combustion heat release stage (−5 °CA~10 °CA interval) from the fuel injection into the cylinder to the spray hitting the wall. In the maximum HRR stage (10 °CA~20 °CA interval, corresponding to the spray hitting the lip of the combustion chamber, and rebounding to form two high-speed spray motions up and down), the instantaneous HRR of the RCTL combustion chamber is significantly faster than that of the RSCL combustion chamber, so it reaches the peak instantaneous HRR first, and the rising rate of its cumulative HRR curve is obviously higher than that of the RSCL. In the decreasing stage of the HRR (25 °CA~60 °CA), the instantaneous HRR of the RCTL drops rapidly, the rising rate of the cumulative HRR curve slows down, the instantaneous HRR of the RCSL combustion chamber decreases slowly, and the after-burning is more serious. Figure 15 shows the changes of the main combustion period and the CA50 of the two combustion chambers. It can be seen that the CA50 of the RCTL combustion chamber is 21.7 °CA, which is 1.2 °CA earlier than that of the RCTL combustion chamber; the main combustion duration is shorter at 56.7 °CA, which is 0.8 °CA shorter than that of the RCSL combustion chamber.

4. Conclusions

In the paper, based on the bench test of a highly intensified single-cylinder engine, the influence of different RCSL combustion chambers and optimal RCTL combustion chambers on the combustion performance of a highly intensified diesel engine was studied. The main conclusions are as follows:

• The test study on the influence of different throat diameter RCSL combustion chambers on the combustion process was carried out at the rated speed of 3800 r/min. It was found that the RCSL combustion chambers with diameters of 72 mm and 74 mm had lower fuel consumption, exhaust temperature, and smoke emission; shorter combustion duration; and less sensitivity to the protruding height of fuel injector under the condition of excess air coefficient of 1.53~1.71 than those with diameters of 70 mm; however, only the RSCL combustion chamber with 72 mm diameter achieved the BMEP of 2.6 MPa under the condition of excess air coefficient of 1.53~1.71.
• A comparative test was carried out between the RCSL combustion chamber and the RCTL combustion chambers at 3800 r/min. It was found that the RCTL combustion chamber had lower BSFC, lower exhaust temperature, maximum combustion pressure, shorter combustion duration, higher thermal efficiency and HRR. Compared with the RCSL combustion chambers, the RCTL combustion chambers BSFC was 250.1 g/kWh, which was 0.7 g/kWh lower; the maximum combustion pressure was 21.3 MPa, which was 0.4 MPa lower; and the indicated thermal efficiency had reached 40.2%. The exhaust temperature of the RCTL combustion chamber was 601 °C, which is 130 °C lower than that of the stepped combustion chamber. This shows that the RCTL combustion chamber can significantly improve the combustion process of a highly intensified diesel engine.