**3. Numerical simulation**

The numerical model for the combustion process of Rotax 914 engine fuelled with kerosene is established in AVL Fire software based on the mathematical model. The combustion chamber is shown in **Figure 1(a)**. The original engine uses gasoline fuel, and a carburetor-type fuel supply system is installed. In order to ensure a reliable ignition, the combustion chamber employs a double spark plug arrangement. One inlet valve and one exhaust valve are used. Because kerosene is difficult to be evaporated, the fuel injection system is replaced by a port fuel injection (PFI) system in this study. The main technical parameters of the retrofitted engine are listed in **Table 1**. The 3D grid model of the combustion chamber is shown in **Figure 1(b)**. The function of automatic grid generation is adopted, and the grid size is set between 0.5 mm and 1.5 mm. The total number of grids is 363,785. After checking the quality of the grid, the numerical simulation is performed.

mesh model remains unchanged, and only stretching or compression is experienced. The mesh quality in the process of tension or compression is checked. The initial pressure in the cylinder is set as 1.52 bar, and the initial temperature is set to 320 K. The temperatures of the cylinder head, cylinder wall, and piston top surface are set as 400, 380, and 450 K, respectively. The number of orifices for the water

*Knock Suppression of a Spark-Ignition Aviation Piston Engine Fuelled with Kerosene*

In order to verify the accuracy of the numerical model, the in-cylinder pressure curve of the original engine with gasoline is simulated and compared with the test results, as shown in **Figure 2**. The engine speed is 5500 r/min and the throttle opening is 30%. The simulated in-cylinder pressure is in good agreement with the experimental one. Only the measured pressure near the TDC is slightly higher than the simulated one. The maximum relative error is 3.58%, which indicates that the numerical model has a high accuracy and can be used for the following simulation

For the SI piston engine fuelled with kerosene, the effects of spray time, initial

The results for the in-cylinder pressure at three different injection times are shown in **Figure 3**. It can be seen that the closer the injection time is to the TDC, the higher the maximum burst pressure in the cylinder is. The useful work of the cycle ascends accordingly. However, the injection time has little effect on the in-cylinder pressure curve as a whole. The maximum pressure difference between T1 and T3 does not exceed 0.5 bar. T3 is close to the TDC and the in-cylinder temperature is higher than T1 and T2. The high temperature in the cylinder makes the liquid water easy to be evaporated. Part of the heat energy of the exhaust gas can be recovered. The combustion speed in the cylinder near the TDC is very fast. The injected water reduces the combustion rate of the combustible mixture. Since the crank angle of T3

droplet size, and water quantity on knock suppression are analyzed using the established numerical model. The engine speed is set to 5500 r/min and the throttle opening is set to 30%. First, the effect of spray time on detonation suppression is studied. The injected kerosene quantity is fixed at 30 mg. The position of the injector is set in the center of the cylinder. The sprayed water temperature is 300 K and the initial droplet diameter is set to 0.1 mm. Only the starting time of water spray is changed and the other parameters are fixed. The start time of water injection is set as 120°, 80°, and 50° before TDC (BTDC) (represented by T1, T2,

injector is six, and the diameter of the orifice is 2 mm.

of the kerosene combustion process.

*DOI: http://dx.doi.org/10.5772/intechopen.91938*

and T3, respectively).

**Figure 2.**

**97**

*Comparison of in-cylinder pressure between simulation and experimental data.*

This chapter mainly studies the compression and combustion process in the cylinder. To simplify the calculation process, only the crankshaft angle ranging from 160° CA to 70° CA is simulated where both valves are closed. The piston moves between the top dead center (TDC) and the bottom dead center in the cylinder. The mesh of the volume swept by the piston is divided into five different mesh models according to the rotation angle of the crankshaft. Each mesh number is activated under a specified range of crankshaft angle. The mesh number of each

#### **Figure 1.**

*Combustion chamber of the four-stroke spark-ignition engine: (a) 3D model; (b) volume mesh.*


#### **Table 1.** *Specifications of Rotax 914 engine.*

#### *Knock Suppression of a Spark-Ignition Aviation Piston Engine Fuelled with Kerosene DOI: http://dx.doi.org/10.5772/intechopen.91938*

mesh model remains unchanged, and only stretching or compression is experienced. The mesh quality in the process of tension or compression is checked. The initial pressure in the cylinder is set as 1.52 bar, and the initial temperature is set to 320 K. The temperatures of the cylinder head, cylinder wall, and piston top surface are set as 400, 380, and 450 K, respectively. The number of orifices for the water injector is six, and the diameter of the orifice is 2 mm.

In order to verify the accuracy of the numerical model, the in-cylinder pressure curve of the original engine with gasoline is simulated and compared with the test results, as shown in **Figure 2**. The engine speed is 5500 r/min and the throttle opening is 30%. The simulated in-cylinder pressure is in good agreement with the experimental one. Only the measured pressure near the TDC is slightly higher than the simulated one. The maximum relative error is 3.58%, which indicates that the numerical model has a high accuracy and can be used for the following simulation of the kerosene combustion process.

For the SI piston engine fuelled with kerosene, the effects of spray time, initial droplet size, and water quantity on knock suppression are analyzed using the established numerical model. The engine speed is set to 5500 r/min and the throttle opening is set to 30%. First, the effect of spray time on detonation suppression is studied. The injected kerosene quantity is fixed at 30 mg. The position of the injector is set in the center of the cylinder. The sprayed water temperature is 300 K and the initial droplet diameter is set to 0.1 mm. Only the starting time of water spray is changed and the other parameters are fixed. The start time of water injection is set as 120°, 80°, and 50° before TDC (BTDC) (represented by T1, T2, and T3, respectively).

The results for the in-cylinder pressure at three different injection times are shown in **Figure 3**. It can be seen that the closer the injection time is to the TDC, the higher the maximum burst pressure in the cylinder is. The useful work of the cycle ascends accordingly. However, the injection time has little effect on the in-cylinder pressure curve as a whole. The maximum pressure difference between T1 and T3 does not exceed 0.5 bar. T3 is close to the TDC and the in-cylinder temperature is higher than T1 and T2. The high temperature in the cylinder makes the liquid water easy to be evaporated. Part of the heat energy of the exhaust gas can be recovered. The combustion speed in the cylinder near the TDC is very fast. The injected water reduces the combustion rate of the combustible mixture. Since the crank angle of T3

**Figure 2.** *Comparison of in-cylinder pressure between simulation and experimental data.*

**3. Numerical simulation**

**Figure 1.**

**Table 1.**

**96**

*Specifications of Rotax 914 engine.*

The numerical model for the combustion process of Rotax 914 engine fuelled with kerosene is established in AVL Fire software based on the mathematical model. The combustion chamber is shown in **Figure 1(a)**. The original engine uses gasoline fuel, and a carburetor-type fuel supply system is installed. In order to ensure a reliable ignition, the combustion chamber employs a double spark plug arrangement. One inlet valve and one exhaust valve are used. Because kerosene is difficult to be evaporated, the fuel injection system is replaced by a port fuel injection (PFI) system in this study. The main technical parameters of the retrofitted engine are listed in **Table 1**. The 3D grid model of the combustion chamber is shown in **Figure 1(b)**. The function of automatic grid generation is adopted, and the grid size is set between 0.5 mm and 1.5 mm. The total number of grids is 363,785. After checking the quality of the grid, the numerical simulation is performed.

*Numerical and Experimental Studies on Combustion Engines and Vehicles*

This chapter mainly studies the compression and combustion process in the cylinder. To simplify the calculation process, only the crankshaft angle ranging from 160° CA to 70° CA is simulated where both valves are closed. The piston moves between the top dead center (TDC) and the bottom dead center in the cylinder. The mesh of the volume swept by the piston is divided into five different mesh models according to the rotation angle of the crankshaft. Each mesh number is activated under a specified range of crankshaft angle. The mesh number of each

*Combustion chamber of the four-stroke spark-ignition engine: (a) 3D model; (b) volume mesh.*

Combustion chamber Bathtub Valve actuation 2 valves per cylinder Air-fuel ratio Stoichiometric

**Item Parameter Unit** Displacement 1.211 L Bore stroke 79.5 61 mm Cylinder number 4 — Rated power 75 kW Rated speed 5500 r/min Max. torque 144 N.m Speed at max. Torque 4900 r/min

**Figure 3.** *Results of in-cylinder pressure with different water injection times.*

is later than that of T1 and T2, the amount of water vapor in the cylinder is less, and the effect on the reaction rate reduction is the least significant. Therefore, the incylinder pressure of T3 is the highest. On the other hand, the closer the injection time is to the TDC, the earlier the phase of peak pressure appears. This is because the injected water vaporizes and absorbs heat in the cylinder and the oxygen concentration in the gas decreases, leading to a decrease of the reaction rate of the combustible mixture. The required compression work also decreases. The farther the injection time is away from the TDC, the longer the duration time of the combustion delay time in the cylinder. Therefore, the peak pressure with water injection time of T1 appears the maximum phase delay. The highest in-cylinder temperature for T1 is about 2300 K, while the highest temperature for T3 reaches 2500 K. The later the start time of water injection, the higher the temperature in the cylinder at the same crankshaft angle (**Figure 4**).

**Figure 4.**

**Figure 5.**

**99**

*Results of in-cylinder temperature with different water injection times.*

*Knock Suppression of a Spark-Ignition Aviation Piston Engine Fuelled with Kerosene*

*DOI: http://dx.doi.org/10.5772/intechopen.91938*

*Results of flame surface density with different water injection times.*

The influence of water injection time on the flame surface density is shown in **Figure 5**. The right spark plug starts to ignite at 52° CA BTDC, and the left spark plug ignites at 40° CA BTDC. The development of the double flames is observed. It can be seen that the flame surface density is greater at the same crank angle if the water injection time is closer to the TDC. The phase for the peak of the secondary heat release rate and the pressure rise rate is advanced accordingly. The peak phase of T3, T2, and T1 is 10°, 8°, and 4° CA BTDC, respectively.

In Fire, an index called combustion knock index (CKI) is used to label the knock intensity inside the cylinder. Generally, an obvious knock happens if the CKI is greater than 20. The results for the CKI under different injection times are shown in **Figure 6**. The CKI values for all the three injection times are maximized and exceed 20 at TDC. The CKI is the smallest for T1 and the strongest for T3. Meanwhile, the phase that the knock starts moves forward if the injection time is late. It can be seen that the knock happens at the position near the cylinder wall far away from the two spark plugs. The detonation phenomenon is reduced after the water injection. With the water injection time advanced from the TDC, the area where the detonation occurs tends to shrink, and the detonation intensity is weakened. This is due to the rapid vaporization of the water droplets after being sprayed into the cylinder, which absorbs a lot of heat in the cylinder. As a result, the in-cylinder temperature decreases. The effects of the compression and radiation of the burnt gas to the end gas reduce accordingly. Therefore, the autoignition time becomes longer than the

*Knock Suppression of a Spark-Ignition Aviation Piston Engine Fuelled with Kerosene DOI: http://dx.doi.org/10.5772/intechopen.91938*

**Figure 5.** *Results of flame surface density with different water injection times.*

is later than that of T1 and T2, the amount of water vapor in the cylinder is less, and the effect on the reaction rate reduction is the least significant. Therefore, the incylinder pressure of T3 is the highest. On the other hand, the closer the injection time is to the TDC, the earlier the phase of peak pressure appears. This is because the injected water vaporizes and absorbs heat in the cylinder and the oxygen concentration in the gas decreases, leading to a decrease of the reaction rate of the combustible mixture. The required compression work also decreases. The farther the injection time is away from the TDC, the longer the duration time of the combustion delay time in the cylinder. Therefore, the peak pressure with water injection time of T1 appears the maximum phase delay. The highest in-cylinder temperature for T1 is about 2300 K, while the highest temperature for T3 reaches 2500 K. The later the start time of water injection, the higher the temperature in the

The influence of water injection time on the flame surface density is shown in **Figure 5**. The right spark plug starts to ignite at 52° CA BTDC, and the left spark plug ignites at 40° CA BTDC. The development of the double flames is observed. It can be seen that the flame surface density is greater at the same crank angle if the water injection time is closer to the TDC. The phase for the peak of the secondary heat release rate and the pressure rise rate is advanced accordingly. The peak phase

In Fire, an index called combustion knock index (CKI) is used to label the knock

intensity inside the cylinder. Generally, an obvious knock happens if the CKI is greater than 20. The results for the CKI under different injection times are shown in **Figure 6**. The CKI values for all the three injection times are maximized and exceed 20 at TDC. The CKI is the smallest for T1 and the strongest for T3. Meanwhile, the phase that the knock starts moves forward if the injection time is late. It can be seen that the knock happens at the position near the cylinder wall far away from the two spark plugs. The detonation phenomenon is reduced after the water injection. With the water injection time advanced from the TDC, the area where the detonation occurs tends to shrink, and the detonation intensity is weakened. This is due to the rapid vaporization of the water droplets after being sprayed into the cylinder, which absorbs a lot of heat in the cylinder. As a result, the in-cylinder temperature decreases. The effects of the compression and radiation of the burnt gas to the end gas reduce accordingly. Therefore, the autoignition time becomes longer than the

cylinder at the same crankshaft angle (**Figure 4**).

*Results of in-cylinder pressure with different water injection times.*

*Numerical and Experimental Studies on Combustion Engines and Vehicles*

**Figure 3.**

**98**

of T3, T2, and T1 is 10°, 8°, and 4° CA BTDC, respectively.

area between gas and liquid and accelerates the heat transfer rate. When the initial particle size decreases to 0.01 mm, the smaller droplet diameter makes the boundary conditions required for the fragmentation of the droplet smaller, and the critical Weber number becomes smaller. The required energy for the fragmentation and deformation of the droplets is lower. Therefore, the droplet is easier to be broken at this time. The increase of the surface area makes almost all the droplets evaporated, absorbing a lot of heat in the cylinder and reducing the heat release rate and the

*Results of the in-cylinder pressure (a) and temperature (b) with different initial water droplet diameters.*

*Knock Suppression of a Spark-Ignition Aviation Piston Engine Fuelled with Kerosene*

The results for the in-cylinder temperature are shown in **Figure 7(b)**. When the initial droplet diameter is between 0.05 and 0.2 mm, the in-cylinder temperature changes slightly with a maximum temperature decrease up to 100 K. However, when the initial particle size is reduced to 0.01 mm, the in-cylinder temperature decreases greatly, and the maximum temperature reduction reaches nearly 500 K. The results of the cloud chart for knock intensity with different initial droplet diameters are shown in **Figure 8**. The crank angle corresponding to the maximum knock intensity is different. The larger the initial droplet diameter is, the earlier the maximum detonation time will appear. The detonation intensity of the four particle sizes is also different. The larger the initial particle size is, the greater the CKI value of detonation is, indicating that the detonation is more dramatic. The knock phenomenon at the TDC with an initial particle size of 0.2 mm is the most severe, whose CKI value is as high as 30. When the initial droplet diameter is 0.1 mm, the detonation intensity maximizes at 1° CA ATDC, and the occurrence area is smaller. When the initial diameter is 0.05 mm, the maximum detonation happens at 2° CA ATDC, and the occurrence area is reduced apparently. The CKI value is decreased to around 20. When the initial droplet diameter is 0.01 mm, the detonation takes place at 10° CA ATDC. The corresponding CKI value drops to 12, which is reduced

The main purpose of spraying water into the cylinder is to reduce the temperature in the cylinder to suppress the knock. The amount of water injection directly affects the temperature reduction in the cylinder. Finally, the influence of water

**Figure 9(a)** shows the results of the in-cylinder pressure under different water injection quantities. The in-cylinder pressure generally decreases with the increase of water spray quantity. The maximum burst pressure is 5.4 MPa when no water is sprayed. When the water injection quantity is 20 mg, the maximum burst pressure is 5.1 MPa, which is 5% lower than that without water injection. Furthermore, the

peak pressure of the combustion process.

*DOI: http://dx.doi.org/10.5772/intechopen.91938*

**Figure 7.**

by 60% compared with an initial particle size of 0.2 mm.

injection quantity is analyzed.

**101**

**Figure 6.** *Results for CKI with different water injection times.*

flame propagation time, and the intensity of the detonation drops. Meanwhile, the thermal efficiency reduction caused by the heat loss of the knock phenomenon alleviates. For the aviation kerosene with an octane number of only 40–50, water injection can make the SI engine operate normally without power loss due to the reduced possibility of detonation. Meanwhile, the water injection time must be far enough away from TDC to suppress the knock effectively.

The initial droplet diameter of water spray will affect the subsequent droplet fragmentation and evaporation process. Therefore, it is necessary to study the effect of initial droplet diameter. Based on the above results, the water injection time is set to 120° CA BTDC. The water spray amount is set at 20 mg and the water temperature is set at 300 K. The initial diameters of the water droplets are set to 0.2, 0.1, 0.05, and 0.01 mm, respectively.

**Figure 7** shows the influence of the initial droplet diameter on the in-cylinder pressure. The in-cylinder pressure becomes lower if the initial particle size is smaller. When the initial particle size is 0.01 mm, the in-cylinder pressure drops significantly. The in-cylinder peak pressure is about 5.5 MPa for an initial droplet diameter of 0.2 mm. However, when the initial particle size is 0.01 mm, the incylinder pressure has been reduced to 4.3 MPa, 21.8% lower than that with an initial droplet diameter of 0.2 mm. This is because the required energy acting on the water droplets to make them deformed and broken is weak for a small droplet diameter if the Weber number and the surface tension coefficient of the water droplet are fixed. When the initial diameter is too large, the energy is not enough to break the droplet, and only the deformation of the droplet occurs. Therefore, only a small part of the droplets with initial droplet diameters of 0.2 and 0.1 mm are broken. Accordingly, the amounts of water vapor are small. Therefore, their influence on the in-cylinder pressure is very limited.

For droplets with an initial diameter of 0.05 mm, there are many modes of fragmentation in the process of atomization. This effectively increases the contact *Knock Suppression of a Spark-Ignition Aviation Piston Engine Fuelled with Kerosene DOI: http://dx.doi.org/10.5772/intechopen.91938*

**Figure 7.** *Results of the in-cylinder pressure (a) and temperature (b) with different initial water droplet diameters.*

area between gas and liquid and accelerates the heat transfer rate. When the initial particle size decreases to 0.01 mm, the smaller droplet diameter makes the boundary conditions required for the fragmentation of the droplet smaller, and the critical Weber number becomes smaller. The required energy for the fragmentation and deformation of the droplets is lower. Therefore, the droplet is easier to be broken at this time. The increase of the surface area makes almost all the droplets evaporated, absorbing a lot of heat in the cylinder and reducing the heat release rate and the peak pressure of the combustion process.

The results for the in-cylinder temperature are shown in **Figure 7(b)**. When the initial droplet diameter is between 0.05 and 0.2 mm, the in-cylinder temperature changes slightly with a maximum temperature decrease up to 100 K. However, when the initial particle size is reduced to 0.01 mm, the in-cylinder temperature decreases greatly, and the maximum temperature reduction reaches nearly 500 K.

The results of the cloud chart for knock intensity with different initial droplet diameters are shown in **Figure 8**. The crank angle corresponding to the maximum knock intensity is different. The larger the initial droplet diameter is, the earlier the maximum detonation time will appear. The detonation intensity of the four particle sizes is also different. The larger the initial particle size is, the greater the CKI value of detonation is, indicating that the detonation is more dramatic. The knock phenomenon at the TDC with an initial particle size of 0.2 mm is the most severe, whose CKI value is as high as 30. When the initial droplet diameter is 0.1 mm, the detonation intensity maximizes at 1° CA ATDC, and the occurrence area is smaller. When the initial diameter is 0.05 mm, the maximum detonation happens at 2° CA ATDC, and the occurrence area is reduced apparently. The CKI value is decreased to around 20. When the initial droplet diameter is 0.01 mm, the detonation takes place at 10° CA ATDC. The corresponding CKI value drops to 12, which is reduced by 60% compared with an initial particle size of 0.2 mm.

The main purpose of spraying water into the cylinder is to reduce the temperature in the cylinder to suppress the knock. The amount of water injection directly affects the temperature reduction in the cylinder. Finally, the influence of water injection quantity is analyzed.

**Figure 9(a)** shows the results of the in-cylinder pressure under different water injection quantities. The in-cylinder pressure generally decreases with the increase of water spray quantity. The maximum burst pressure is 5.4 MPa when no water is sprayed. When the water injection quantity is 20 mg, the maximum burst pressure is 5.1 MPa, which is 5% lower than that without water injection. Furthermore, the

flame propagation time, and the intensity of the detonation drops. Meanwhile, the thermal efficiency reduction caused by the heat loss of the knock phenomenon alleviates. For the aviation kerosene with an octane number of only 40–50, water injection can make the SI engine operate normally without power loss due to the reduced possibility of detonation. Meanwhile, the water injection time must be far

The initial droplet diameter of water spray will affect the subsequent droplet fragmentation and evaporation process. Therefore, it is necessary to study the effect of initial droplet diameter. Based on the above results, the water injection time is set to 120° CA BTDC. The water spray amount is set at 20 mg and the water temperature is set at 300 K. The initial diameters of the water droplets are set to 0.2, 0.1,

**Figure 7** shows the influence of the initial droplet diameter on the in-cylinder

pressure. The in-cylinder pressure becomes lower if the initial particle size is smaller. When the initial particle size is 0.01 mm, the in-cylinder pressure drops significantly. The in-cylinder peak pressure is about 5.5 MPa for an initial droplet diameter of 0.2 mm. However, when the initial particle size is 0.01 mm, the incylinder pressure has been reduced to 4.3 MPa, 21.8% lower than that with an initial droplet diameter of 0.2 mm. This is because the required energy acting on the water droplets to make them deformed and broken is weak for a small droplet diameter if the Weber number and the surface tension coefficient of the water droplet are fixed. When the initial diameter is too large, the energy is not enough to break the droplet, and only the deformation of the droplet occurs. Therefore, only a small part

of the droplets with initial droplet diameters of 0.2 and 0.1 mm are broken. Accordingly, the amounts of water vapor are small. Therefore, their influence on

For droplets with an initial diameter of 0.05 mm, there are many modes of fragmentation in the process of atomization. This effectively increases the contact

enough away from TDC to suppress the knock effectively.

*Numerical and Experimental Studies on Combustion Engines and Vehicles*

0.05, and 0.01 mm, respectively.

*Results for CKI with different water injection times.*

**Figure 6.**

**100**

the in-cylinder pressure is very limited.

**Figure 8.** *Results of CKI with different initial water droplet diameters.*

rate of the in-cylinder temperature slows down obviously. This is because after injection, water absorbs the heat in the cylinder and reduces the temperature. The results for the cloud charts of CKI value with different water amounts are shown in **Figure 10**. The maximum CKI value decreases with the increase of water spray. The CKI value without water injection is as high as 26. When the water injection amount is 20 mg, the CKI value is reduced to 18, which is 31% lower than

*Knock Suppression of a Spark-Ignition Aviation Piston Engine Fuelled with Kerosene*

*DOI: http://dx.doi.org/10.5772/intechopen.91938*

To explore the effect of water injection on knock suppression of SI engine fuelled with kerosene, an engine experiment is carried out [23]. First, the original Rotax 914 engine was modified, and the engine test rig was built, as shown in **Figure 11**. The fuel supply system is first transformed into a PFI kerosene supply

that without water injection.

*Results of CKI with different initial water droplet diameters.*

**Figure 10.**

**103**

**4. Experimental results**

**Figure 9.** *Results of the in-cylinder pressure (a) and temperature (b) with different amounts of water injection.*

larger the amount of water injection is, the later the phase of the maximum burst pressure appears.

**Figure 9(b)** gives a comparison of the in-cylinder temperature. When there is no water spray, the maximum average temperature in the cylinder is 2408 K. The temperature reduction in the cylinder is increased as the amount of water injection rises. When the water spray is 10 mg, the maximum temperature in the cylinder is 2365 K, which is 43 K lower than that without water spray. When the amount of the water spray is 25 mg, the maximum in-cylinder temperature drops to 2297 K, which is 111 K lower than that without water spray. In the case of water injection, the rise

*Knock Suppression of a Spark-Ignition Aviation Piston Engine Fuelled with Kerosene DOI: http://dx.doi.org/10.5772/intechopen.91938*

**Figure 10.** *Results of CKI with different initial water droplet diameters.*

rate of the in-cylinder temperature slows down obviously. This is because after injection, water absorbs the heat in the cylinder and reduces the temperature.

The results for the cloud charts of CKI value with different water amounts are shown in **Figure 10**. The maximum CKI value decreases with the increase of water spray. The CKI value without water injection is as high as 26. When the water injection amount is 20 mg, the CKI value is reduced to 18, which is 31% lower than that without water injection.
