Simcenter Amesim in torsional vibration analysis – example and basic analysis of results

Introduction

Torsional vibration is one of topics which still has major influence on the design of the rotating machinery - starting from internal combustion engines, through classical drivelines, test benches (e.g. for gearboxes), marine propulsion and, recently the most relevant, electric drivelines. Importance of controlling torsional vibration, avoidance or dampening of resonances (resulting from overlap of excitation frequency and eigenfrequency of a system) span from need of ensuring structural integrity of a mechanical component to improvement of NVH properties of a designed machine.

One of the components, which in the past has been subject of numerous studies regarding torsional vibration, various optimization procedures and deep research is cranktrain of internal combustion engine. Constant pursuit of increased efficiency and performance leads to increase of indicated mean effective pressures (IMEP) and higher peak firing pressures. Change of those parameters lead to visible changes of torsional excitation applied to the crankshaft, thus pushing engineers to improve solutions for vibration control and consideration of this phenomenon already at a design stage.

Thanks to long history and multiphysics background Simcenter Amesim allows you to analyze torsional vibration in various systems. In this article we will check how to tackle this problem and analyze results based exactly on the cranktrain example. Moreover, in following articles, we will take a look into simplified process of damper tuning for simulation purposes and other related topics.

Simulation of torsional vibration on example of basic Simcenter Amesim model of a cranktrain

Using simple example, from one of our demos (see qthelp://lmsimagine.lab/ame_dir/demo/Libraries/TR/EngineCrankshaft.html in Simcenter Amesim help), we will modify the sketch and discuss how to understand results of torsional vibration analysis both in time and frequency domain, and how they correspond with each other.

Figure 1 - Basic crankshaft for torsional vibration analysis

Please copy on open the model from the example. The basic setup already has a torsional vibration damper (TVD) embedded into model – we can remove it, so we will see torsional mode shapes and resonances of following orders of excitation from combustion, with these mode shapes, will be better visible.

Figure 2 Layout without TVD

To observe the most critical speeds and related frequencies, which we should address in our strategy of vibration control, we should do engine run up. In order to do so, we have to control crankshaft speed. We set up N crankshaft as ramp from 1000 rev/min to 8500 rev/min, duration of run up can be set up as 24s. Not only should we modify the piecewise linear signal of speed, but also we should remember to set up duration of simulation to 24 s.

Figure 3 Setting up crankshaft velocity

 

 

Please go to simulation mode and set up following simulation settings:

Figure 4 Simulation settings for the created set up

We are interested the most in the relative angular displacement (fig. 5) of the spring-damper components at the front (between the cylinder 4 and the pulley), the end (between the cylinder 1 and the flywheel) and between cylinders – these are basically descriptions of stiffness and structural damping, in each section, and results is torsional deformation of the main bearing section of the crankshaft.

Figure 5 Variable representing torsional deflection - for MB1 it is between the pulley and first main bearing section

If we plot angular deflection between each section representing, by assumption, torisonally stiff element (pulley, crankthrows and flywheel – 5 sections in total) we get plots of torsional vibration for each main bearing section. These will be useful in comparison with linear analysis results, which we will cover in following articles.

Figure 6 Torsional deflection of each section in crankshaft

These plots are particularly interesting if we had longer crankshaft (with more sections), where more eigenfrequencies were present (or other systems, which are susceptible to torsional vibration, such as vehicle powertrain or marine propulsion system). Total deflection, between pulley and crankshaft is time summation of all signals.

To create relevant plot for our further analysis we should do the following:

1. We sum up relative angular displacement (are) from spring-damper elements into Total angular deflection post-processing variable, please use Add option
   
   

Figure 7 Creation of total torsional vibration signal

2. Create crankshaft velocity variable from control signal from UD00 element (next to N crankshaft (rev/min). To be able to create order tracking plot please set unit to rpm.
3. Drag and drop first created crankshaft velocity variable, then created Total Angular deflection and switch XY plot
4. Go for Spectral Map plot to create 3D FFT (waterfall) plot:

Figure 8 Spectral map creation

5. Right click on 3D FFT plot, select order tracking and enter desired orders:

Figure 9 Order Tracking

For purpose of the article, we select orders 1-6. Bandwidth is frequency shift between each order (which is constant at constant speed) at the beginning of the simulation. As we start our run at 1000 RPM, this value should be set up as follows:

1000 RPM → 16.66 Rev/s → 16.66 Hz

Created plot, after adjustments of style, is visible below. We can use it in out subsequent studies of our system and set up of the torsional damper.

Figure 10 Results of torsional vibration analysis in time domain and their FFT results

Based on these results we can identify natural frequency of undamped crankshaft, main orders of the excitation (resulting from combination of firing order and gas pressure curves, within cylinders) – in here e.g. 2, 4, 6 or speeds at which resonance occurs. Moreover, using these properties, we will be able to initially tune our damper.

Summary

A procedure how to analyze system, which is subjected to torsional excitation and is prone to torsional resonance, was described in the article. Example how to generate plots from time domain results, relevant for torsional vibration analysis, and how to interpret them, was shown. The user can analyze system in time domain, what gives unique opportunity in comparison to classical linear analysis – such as influence of non-stationary periods (variable load and rapidly changing speed of the engine) or non-linearities of components in a system.