**4. Experimental results**

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

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

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

**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

pressure appears.

**Figure 9.**

**102**

**Figure 8.**

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

*Numerical and Experimental Studies on Combustion Engines and Vehicles*

pressure. An engine speed sensor is installed at the crankshaft end. The collected data are sent to the combustion analyzer to calculate the heat release rate of the combustion process. **Table 2** lists the main equipment and relative accuracy of the

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

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

When the engine speed is 5500 r/min and the throttle opening is 30%, the duration of water injection is set to 0, 2, and 4 ms, respectively. The results of the in-cylinder pressure are shown in **Figure 13(a)**. In the case of no water spray, the maximum pressure in the cylinder reaches 33.9 bar and fluctuates apparently near the TDC with an amplitude of 7.53 bar, indicating that the unburnt end gas in the cylinder is self-ignited and the knock phenomenon occurs. With water injection of 2 ms, the knock phenomenon has been alleviated, and the maximum in-cylinder pressure is increased from 33.9 to 35.4 bar. When the duration of water injection is increased from 2 to 4 ms, the pressure fluctuation in the cylinder caused by kerosene detonation disappears completely, while the maximum burst pressure in the cylinder decreases to 32.5 bar. The corresponding IMEP values are shown in **Figure 13(b)**, which also indicates that the in-cylinder pressure decreases

Subsequently, the limit of knock suppression by water injection is studied under

**Equipment Model Accuracy** Eddy current dynamometer China Kaimai CW 160 <0.5% Fuel meter Japan Ono MF-2200 <1% Cylinder pressure sensor Kistler 6115 <0.5% Lambda sensor Bosch LSU4.9 <1% Combustion analyzer Kibox 2893 <1%

*Results for the in-cylinder pressure (a) and IMEP (b) with different water injection durations.*

three different engine speeds of 4500, 5000, and 5500 r/min. For each engine speed, the throttle opening and the kerosene injection quantity as well as water injection quantity are increased continuously. The data are recorded until the maximum water injection duration is arrived. **Figure 14(a)** shows the comparison of kerosene injection quantity. In the case of no water injection, the maximum kerosene injection quantity under the three different engine speeds is about 20 mg. If the injection quantity exceeds this value, there will be an obvious detonation. The

significantly with the increase of water injection quantity.

test system.

**Table 2.**

**Figure 13.**

**105**

*Main facilities of the experimental system.*

**Figure 11.** *Schematic of the engine test bench for the SI engine fuelled with kerosene together with water injection.*

system. Meanwhile, a water injection system is added, which includes a water injector, a water pump, a water tank, and related pipelines. The water injector is installed at the intake manifold. Since the flame propagation speed of kerosene is significantly lower than that of gasoline, the ignition system of the original engine is modified. One of the spark plugs is replaced with a jet ignition device, as shown in **Figure 12**. The jet combustion pre-chamber uses the small orifices on the surface to inject the jet flame into the main combustion chamber and forms strong turbulent flames. As a result, the flame propagation distance is shortened, and the combustion speed is effectively accelerated. The volume of the jet ignition pre-chamber is 1.5 ml, and the diameter of the six holes on the surface is 1.5 mm.

The output power of the engine is absorbed by an eddy current dynamometer. During the test, kerosene is supplied to the first and third cylinders of the engine, and the original gasoline supply system is used for the second and fourth cylinders. A pressure sensor is installed in the first cylinder to measure the in-cylinder

**Figure 12.** *Photo of the jet ignition pre-chamber.*

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

pressure. An engine speed sensor is installed at the crankshaft end. The collected data are sent to the combustion analyzer to calculate the heat release rate of the combustion process. **Table 2** lists the main equipment and relative accuracy of the test system.

When the engine speed is 5500 r/min and the throttle opening is 30%, the duration of water injection is set to 0, 2, and 4 ms, respectively. The results of the in-cylinder pressure are shown in **Figure 13(a)**. In the case of no water spray, the maximum pressure in the cylinder reaches 33.9 bar and fluctuates apparently near the TDC with an amplitude of 7.53 bar, indicating that the unburnt end gas in the cylinder is self-ignited and the knock phenomenon occurs. With water injection of 2 ms, the knock phenomenon has been alleviated, and the maximum in-cylinder pressure is increased from 33.9 to 35.4 bar. When the duration of water injection is increased from 2 to 4 ms, the pressure fluctuation in the cylinder caused by kerosene detonation disappears completely, while the maximum burst pressure in the cylinder decreases to 32.5 bar. The corresponding IMEP values are shown in **Figure 13(b)**, which also indicates that the in-cylinder pressure decreases significantly with the increase of water injection quantity.

Subsequently, the limit of knock suppression by water injection is studied under three different engine speeds of 4500, 5000, and 5500 r/min. For each engine speed, the throttle opening and the kerosene injection quantity as well as water injection quantity are increased continuously. The data are recorded until the maximum water injection duration is arrived. **Figure 14(a)** shows the comparison of kerosene injection quantity. In the case of no water injection, the maximum kerosene injection quantity under the three different engine speeds is about 20 mg. If the injection quantity exceeds this value, there will be an obvious detonation. The


#### **Table 2.**

system. Meanwhile, a water injection system is added, which includes a water injector, a water pump, a water tank, and related pipelines. The water injector is installed at the intake manifold. Since the flame propagation speed of kerosene is significantly lower than that of gasoline, the ignition system of the original engine is modified. One of the spark plugs is replaced with a jet ignition device, as shown in **Figure 12**. The jet combustion pre-chamber uses the small orifices on the surface to inject the jet flame into the main combustion chamber and forms strong turbulent flames. As a result, the flame propagation distance is shortened, and the combustion speed is effectively accelerated. The volume of the jet ignition pre-chamber is

*Schematic of the engine test bench for the SI engine fuelled with kerosene together with water injection.*

*Numerical and Experimental Studies on Combustion Engines and Vehicles*

The output power of the engine is absorbed by an eddy current dynamometer. During the test, kerosene is supplied to the first and third cylinders of the engine, and the original gasoline supply system is used for the second and fourth cylinders. A pressure sensor is installed in the first cylinder to measure the in-cylinder

1.5 ml, and the diameter of the six holes on the surface is 1.5 mm.

**Figure 11.**

**Figure 12.**

**104**

*Photo of the jet ignition pre-chamber.*

*Main facilities of the experimental system.*

**5. Conclusions**

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

In this chapter, the feasibility of knock suppression using water injection for an

SI engine fuelled with kerosene is investigated. First, a 3D numerical model is established in AVL Fire software. The influences of water injection timing, initial droplet diameter, and water injection quantity are analyzed. Then, an engine

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

The results indicate that water injection can suppress the knock of kerosene effectively. The water injection timing should be advanced and far away from TDC because the evaporation of water droplets needs time. Accordingly, a lower pressure rise rate can be obtained, and the phase angle of the peak pressure can be postponed. The autoignition time enlarges, the flame propagation speed drops, and the intensity of the detonation decreases, due to a reduction of in-cylinder temperature. Meanwhile, a better result of knock suppression can be obtained with a smaller initial water droplet diameter, owing to a larger evaporation surface area of droplets. With the increase of the water injection amount, the reduction of in-cylinder temperature increases. The allowable throttle opening and injected kerosene amount also increase, resulting in a significant increase of the IMEP. The experimental results show that the maximum IMEP is improved from about 8 to 10 bar with an engine speed of 5500 r/ min. However, an excessive water injection will lead to a decrease of the IMEP. To evaluate the effect of water injection comprehensively, an optimization design of the water injection system and the combustion chamber geometry is required. Meanwhile, more experimental work should be carried out, especially for the practical

The authors would like to thank for the support of the National Natural Science

experiment is performed to evaluate the limit of IMEP improvement.

operation conditions of an unmanned aerial vehicle.

Foundation of China (Grant No. 51876009).

The authors declare no conflict of interest.

IMEP indicative mean effective pressure

LIF laser-induced fluorescence

**Acknowledgements**

**Conflict of interest**

3D three-dimensional SI spark ignition ATDC after top dead center BTDC before top dead center CKI combustion knock index GDI gasoline direct injection

ON octane number PFI port fuel injection PM particle material NOx nitrogen oxide TDC top dead center

**Acronyms**

**107**

**Figure 14.**

*Results for the maximum amount of injected kerosene with/without water injection under different engine speeds (a) and the mass ratio of water over kerosene (b).*

maximum kerosene injection quantity is increased by about 50% with water injection. The corresponding mass ratios of the sprayed water over kerosene are shown in **Figure 14(b)**. It can be seen that the mass ratio is close to 1:1 at three engine speeds.

**Figure 15** shows the comparison of IMEP improvement under three different engine speeds. The IMEP can be increased from less than 8 to nearly 10 with water injection for each engine speed. The results show that using water injection can improve the kerosene injection quantity, leading to an increase of the IMEP and the power output of the engine. This is because after spraying water, the liquid water evaporates and absorbs heat, and the temperature in the cylinder decreases. Accordingly, the compression and radiation effect of the burnt gas imposed on the unburnt end gas weakens, and the autoignition time becomes longer, avoiding the violent detonation. The allowable charge of fresh mixture in the cylinder increases. Thus, more fuel can be injected and the engine's output power is improved. Meanwhile, it can be seen that the IMEP of the original engine with gasoline can reach 13 bar, and the IMEP of the kerosene fuel with water spray still needs to be enhanced.

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