Vibration Reduction in Indigenous Wankel Rotary Combustion Engine with Structured Layer Damping

Abstract

Rotary engines are simpler in design and operation compared to the gas turbines. Rotary engines propel many monoplanes, power hang gliders, and unmanned aerial vehicles (UAV). In an indigenization effort, a 65 hp Wankel rotary combustion engine (WRCE) was successfully developed in the country for a wheeled version of Nishant UAV. As a part of the testing and certification process, three engines were required to be tested in the test bed for a stipulated number of hours. The engine was mounted in the test bed on two cantilever bolts in horizontal and two in the vertical direction with equi-frequency anti-shock mounts. During testing without the alternator, it was observed that the vibrations are higher with 1X amplitude of 24 g. To identify the source of vibration, a detailed modal analysis was carried out. Impact test data showed the existence of dominating frequency around 138 Hz. To reduce the vibrations, the engine mount is modified suitably, and structured layer damping is introduced between the engine mount and support structure. This modification resulted in increased damping leading to vibration reduction to the acceptable level. Testing of Wankel engine with alternator up to the required speed was completed successfully using the structured layer damping method.

1 Introduction

The lightweight aerial vehicles, mainly UAVs, have seen phenomenal growth and improvements in their performance in the past couple of decades. The UAV’s find the utility in defense and civil sector, especially for surveillance and carrying the payloads [1]. High demand for UVA’s is a driving force for the continuous efforts toward their performance improvement. One of the rationales behind the performance enhancement is the vehicle’s propulsion system. The power plant requirements for the UAVs are very demanding. Some of the major considerations in the power plant design are high specific power, low specific fuel consumption (SFC), lower frontal area, minimum noise and vibration, high reliability, low maintenance, and ease of serviceability. Wankel rotary combustion engine (WRCE) is one of the promising contenders that satisfy a large number of requirements of a small aircraft power plant. Hence, WRCE is being considered for powering many aircraft such as UAVs, microlights, and hang gliders all over the world. It is also used as APU for the aircraft.

Activities toward the development of WRCE have begun in CSIR-NAL couple of decades ago. As on today, CSIR-NAL has developed 55 and 65 hp water-cooled rotary combustion engines for UAV applications successfully. Currently, 30 hp air-cooled engine is under development. This paper deals with the case study on the vibration issues developed during the trials on the 65 hp WRCE in the testbed and methodology implemented for rectifying the high vibration problem.

2 Details of 65 hp WRCE

The 65 hp WRCE consists of the core engine, reduction drive, water pump, and fan [2]. The in-built reduction drive system has a speed reduction of 1:2 to keep the propeller speed lower. Water cooling system is used to maintain the housings operating temperature under safe limits. The air cooling system cools the rotor and also aids in the flow of lubricant to various bearings of the engine. The complete engine model indicating the critical parts is shown in Fig. 1. Detailed specifications for this engine are given in Table 1.

Fig. 1

Engine assembly indicating major parts

Table 1 Engine specifications

3 Experimentation with WRCE

The designed WRCE engine is modeled and analyzed for the thermal and structural aspects to ensure its safe working until the maximum operating speed of 8000 rpm. After the analysis, the various engine components are fabricated and assembled into a WRCE. The performance of the designed WRCE is evaluated by mounting the engine on the thrust cradle simulating the actual engine configuration as shown in Fig. 2.

Fig. 2

Engine mounted on the thrust cradle

The mounting arrangement of the engine on the vehicle is shown in Fig. 3. It consists of two horizontal and two vertical bolts projecting from the DS frame which is a part of the UAV structure. To reduce the transmission of the engine vibration to the vehicle, an equi-frequency mount is provided at the engine mounting locations. These equi-frequency mounts have same stiffness in both the radial and axial directions for the specified load range. The mounting selected in this case has a stiffness of 14 N/mm and natural frequency of 15 Hz, which provides 50% isolation [3]. Details of the equi-frequency mounting arrangement are shown in Fig. 3.

Fig. 3

Mounting of engine with frame

In WRCE performance evaluation phase, the bare engine was run up to the full speed. Vibrations of WRCE without alternator were observed to be higher at full speed. The spectrum plot of engine vibration acquired at 8000 rpm is shown in Fig. 4. Spectrum plot shows that 1X is the dominant frequency with 24 g as the amplitude of vibration. The amplitude of vibration observed is higher compared to the sponsor recommended values of 10 g.

Fig. 4

Frequency plot of the WRCE (without alternator) running at speed of 8000 rpm

In order to find the critical structural frequencies getting excited during running, an impact hammer study was carried out using OROS vibration analyzer [4]. Figure 5 shows the results from the impact hammer experiments performed at stationary conditions with the sensor mounted on the firewall and also hit on the firewall to determine the point FRF. It can be seen that a frequency of about 138.75 Hz is evident in the impact experiments. This frequency is likely to get excited when the engine operates at full speed of 8000 rpm. Hence, it was decided to alleviate this frequency by introducing structured layer damper between the engine mount and the lower bracket in the horizontal plane by replacing the shock mounts. A Turcite sheet of 150 mm × 50 mm × 4 mm is sandwiched between the engine mount and lower bracket as shown in Fig. 6. The experiments are repeated without the alternator. Figure 7 shows the results of experiments performed at full speed with structured layer damping in place. It can be seen that the magnitude of vibration has decreased from 24 g to about 14 g. This confirms that the structured layer damping is effective in reducing the vibration levels in the engine.

Fig. 5

Frequency response plot of the engine without alternator

Fig. 6

Engine firewall fitted with structured layer damper (Turcite)

Fig. 7

Frequency plot of the WRCE at running speed of 8000 rpm with structured layer damping without alternator

As per the mission requirements, the testing has to be carried out with the alternator. In the next phase of testing, the engine is fitted with an alternator and experiments are performed with structured layer damping in place. Figure 8 shows the vibration levels encountered when the engine is running at full speed. From frequency plot, it can be seen that the overall vibration levels have been reduced further with a maximum 1X amplitude of around 6 g, which is well within the prescribed limits of 10 g.

Fig. 8

Frequency plot of the WRCE at running speed of 8000 rpm with structured layer damping with alternator

4 Conclusions

The systematic approach of modal frequency identification with fundamentals of vibration analysis was followed to identify the source of vibration during the testing of indigenously developed Wankel engine. The mounting stiffness in the horizontal direction was identified as a controlling stiffness. The mounting stiffness and damping are improved by adopting the structured later damping. Demonstration of Wankel engine with the alternator is completed successfully up to full speed by adopting the structured layer damping.