Engine start/stop vibrations in truck

Engine start/stop vibrations in truck

Johan Darth

Master of Science Thesis MMK 2014:52 MDA 470 KTH Industrial Engineering and Management

Machine Design

SE-100 44 STOCKHOLM

Examensarbete MMK 2014:52 MDA 470

Vibrationer vid start/stopp av lastbilsmotor

Johan Darth

Godkänt

2014-mån-dag

Examinator

Lei Feng

Handledare

Mohammad Khodabakhshian

Uppdragsgivare

Scania CV AB

Kontaktperson

Per-Johan Jansson

Sammanfattning

De vibrationer som uppkommer under start och stopp av motor i en lastbil är inte ett problem för den vanlige lastbilschauffören. Men när bränslesparande system och hybrida drivlinor blir allt vanligare på marknaden kommer vibrationskomforten under dessa vibrationer bli allt viktigare för förarmiljön. Speciellt för lastbilar i distributionstrafik och bussar.

Olika parametrar som är av intresse under mätning av lastbilsmotorns start- och stopsekvens har undersökts och en mätmetod tillsammans med en analysmetod har föreslagits. En studie inkluderande 13 erfarna lastbilschaufförer och provningsingenjörer har genomförts med syftet att hitta samband mellan testpersonernas subjektiva uppfattning av dessa vibrationer och olika parametrar i mätdata.

Resultatet av studien påvisar att vibrationerna under motor avstängning upplevs som mer

besvärande än under start av motorn. Studien pekar också emot att vibrationslängd kan ha en

större inverkan på vibrationskomfort än maxacceleration. Ett förslag till acceptanskriterium

har också föreslagits.

Master of Science Thesis MMK 2014:52 MDA 470

Engine start/stop vibrations in truck

Johan Darth

Approved

2014-month-day

Examiner

Lei Feng

Supervisor

Mohammad Khodabakhshian

Commissioner

Scania CV AB

Contact person

Per-Johan Jansson

Abstract

The vibration comfort during engine start and stop in a truck is not a problem for the typical truck driver. But when fuel saving systems and hybrid power trains are becoming more common on the market, the aspect of driveability and vibration comfort regarding this load case will become more important, especially in distribution trucks and busses.

Different parameters that are of interest during measurement of engine start and stop vibrations have been investigated and a measurement method along with an analysis method has been proposed. A study including 13 experienced truck drivers and test engineers was made with the purpose to find correlation between their subjective judgment and different parameters in the measurement data regarding the vibrations during engine start and stop.

The result of the study indicates that the vibrations during engine stop perceives as worse

compared to vibrations during engine start. It was also found that the vibration duration may

have a bigger impact on subjective judgement than the peak acceleration. Based on the study,

a proposal to criterion of acceptance has been established.

NOMENCLATURE

Here are the abbreviations and names that are used in this master thesis presented.

Abbreviations

RMS Root Mean Square

PSD Power Spectral Density ESD Energy Spectral Density

VDV Vibration Dose Value

eVDV Estimated Vibration Dose Value MTVV Maximum Transient Vibration Value DFT Discrete Fourier Transform

FFT Fast Fourier Transform ECU Electronic Control Unit

°C Degree Celsius

Name Description

Power pack Power pack includes engine and gearbox

XCOM Scania developed diagnostic tool

TABLE OF CONTENTS

INTRODUCTION ...1

Background ... 1

Purpose ... 1

Definitions ... 2

Problem description... 2

Measurement...2

Analysis ...3

Delimitations ... 3

Method ... 4

FRAME OF REFERENCE ...6

Vibration analysis... 6

Transient signals...6

Experimental vibration analysis ...7

Measurable quantities... 8

RMS value ...9

Running RMS ... 10

MTVV ... 11

VDV... 11

PSD ... 12

ESD... 13

Engine start and stop sequence... 14

Power pack eigenfrequencies... 15

Previous work... 16

ISO2631-1 ... 16

Scania in-house ... 17

Human body vibrations ... 17

Filter approximation of the ISO 2631-1 ... 19

Statistics ... 19

IMPLEMENTATION ... 21

Tools and utilities ... 21

Measurement method ... 23

Acceleration vector ... 24

Trigger conditions ... 24

Averaging ... 27

Driver’s weight and seat suspension... 29

Engine temperature ... 30

Truck warm up time... 31

Repeatability ... 32

Reproducibility... 33

Analysis method ... 34

Frequency content ... 34

Frequency weighting... 35

Running RMS ... 35

ESD... 36

MTVV and VDV... 37

Extreme vibrations... 37

Engine start/stop study ... 38

Criterion of acceptance... 41

Running RMS ... 41

PSD ... 44

Area ... 46

Peak running RMS acceleration and vibration duration ... 48

RESULTS ... 54

Proposed measurement method... 54

Proposed analysis method ... 54

Criterion of acceptance... 55

DISCUSSION AND CONCLUSIONS ... 57

Discussion ... 57

Conclusions ... 59

FUTURE WORK ... 60

REFERENCES ... 61

INTRODUCTION

This chapter provides an introduction to why engine start and stop vibrations are of importance along with the scope of this master thesis.

Background

When a truck engine is started or turned off, the driver experience some level of vibrations in the truck cabin. The vibrations can be described in different ways, e.g. by its amplitude or root mean square (RMS) value in the time domain or by power spectral density (PSD) function in the frequency domain. In Figure 1, a running RMS value of the acceleration in one direction can be seen over time.

Figure 1. Running RMS of acceleration over time during an engine start sequence of a random Scania truck.

For the typical truck driver the vibrations are not a problem, since the driver turns the engine on and off rather rarely. However, due to the competitive market, which has focus on fuel consumption and the environment, fuel saving measures, like automatic start and stop of the engine and hybrid power trains, are becoming more common. This will result in that the engine start and stop frequency will increase significantly in trucks that have these features. This is especially interesting regarding city distribution trucks and busses.

Therefore concerns have arisen at the vehicle dynamic group at Scania that the driver environment may be affected in a negative way. Also annoying start and stop vibrations do not comply with the idea of having a premium feeling when it comes to vibration comfort properties of a Scania truck.

Purpose

The overall purpose with this master thesis is to secure a premium feeling in Scania trucks regarding vibration comfort properties during engine start and stop for the driver and passenger.

0 0.5 1 1.5 2 2.5 3

0 0.25 0.5 0.75 1.0

Time [s]

Acceleration [m/s²]

Running RMS for engine start sequence

y-dir

The outcome of this master thesis should result in that Scania will have a standardised method to measure and evaluate a vehicle engine start and stop vibrations, including a proposal regarding criterion of acceptance.

In addition to the main objective stated above there are some tasks that have been done:

• Introduce the method into the internal Scania method description document.

• How to measure start and stop vibrations in a competitor truck.

Definitions

The outcome of this master thesis is supposed to be used during development of trucks at Scania.

The time limit is set to 20 weeks.

Deliverables:

• List/description of parameters that affects the engine start/stop vibration properties.

• Define standardised method to measure engine start/stop vibration, “raw data”.

• Define standardised method to analyse engine start/stop vibrations.

• Implement standardised analysis method in internal Scania MATLAB script.

• Proposal regarding criterion of acceptance.

Below is a list containing the requirements from Scania.

• The sampling frequency should be 1000 Hz.

• Acceleration should be measured in one point on the adapter to the driver’s seat.

• One person should be seated in the driver’s seat when measurements are conducted.

• The measurement method should not be dependent on one particular person.

Problem description

Below is the problem presented and what questions that are of importance in the scope of this master thesis.

Measurement

To measure the vibrations the driver experience during start and stop of the truck engine, accelerometers in three directions are used. The accelerometers are attached to the driver’s seat adapter next to the floor. The attachment that the accelerometers are fitted to is rigid and has an eigenfrequency which are higher than the frequencies of interest, which are not typically higher than 40 Hz (H Bodèn; U Carlsson; R Glav; H P Wallin; M Åbom, 2001).

Questions to be answered regarding the measurement of engine start and stop vibrations are:

• How to trig the measurement? (Starter key signal, rpm signals, manually or computer

• Definition of pre- and post-trigger time? (to ensure that the complete vibration duration is covered)

• Number of repeated measurements for averaging?

• Is it possible to control the start/stop sequence with computer?

Analysis

The information the measurement data contains are acceleration over time. The issue is how to analyse the data and presents the results.

Questions to answer regarding the analysis are:

• Acceleration in certain directions or is it the sum of all three directions?

• Are there special frequencies of interest?

• Is it possible to compare vehicle by a numerical value or is graphs required?

• How to handle/visualize statistical dispersion?

To correlate different vibration levels, a study will be performed where test persons is tasked to judge the engine start and stop vibration on four different trucks according to a subjective scale.

Delimitations

To measure acceleration in the driver’s seat three accelerometers in one point on the seat adapter has be used, hence translation acceleration in x-, y- and z-direction will be measured. Roll, pitch and yaw will not be measured. It is assumed that the seat is not placed in the centre of rotation for the majority of the movement of the cab, hence any roll, pitch or yaw will affect translation movement in the driver’s seat.

The location of the accelerometers is on the driver’s seat adapter, which was a criterion by Scania. It is assumed that any vibrations within the interesting frequency interval that is measured on the seat adapter will be transferred to the seat as well. The transfer function between the vibrations in the floor, and the vibrations in the seat, is assumed to be linear. An important thing to consider is that acceleration in the seat will be affected by parameters such as driver’s weight, position of the driver, position of the seat etc. (Lindh, 2013). Lindh concludes that depending on seat adjustment measurement of some load cases can show spread on up to 25 percent. This leads to issues regarding the repeatability when measuring accelerations on the seat cushion. The measurement method should be designed in such a way that it is not dependent on one particular person. It is desirable that the measurement method could be carried out on two different occasions by two different test engineers.

Delimitations such as placement of the accelerometer and not measuring roll, pitch and yaw in

the measurement method are weighted less against the advantage of having a relatively simple

and repeatable measurement method.

Vibrations in the steering wheel due to engine start and stop have most likely an impact on the driver environment but are not investigated in this thesis.

The total transfer function between the power pack and the driver’s seat regarding vibrations are dependent on e.g. the mountings of the power pack and suspension components of the cab. Due to the complex mechanical system that a truck represents there are several factors that contribute to transfer vibrations from power pack to cab e.g. electric cabling, air housing etc. These factors are not considered in this thesis.

Other parameters that may affect the start and stop vibrations are:

• Engine temperature

• Engine software controlled modes

• Ambient temperature of suspension components

Engine modes are different states (or modes) on which the Electronic Control Unit (ECU) controls the engine. Every mode has its individual set of parameter depending on the input data from different sensors on the truck. Example of information that is retrieved from sensors that the different modes are dependent on is ambient temperature, oil and coolant temperature and exhaust readings. An example of an engine mode is the mode that is active when an engine starts cold. Usually this mode has a set of parameters that will raise the engine temperature quickly and prolong the start up-time of the engine.

In this master thesis it is assumed that the engine behaves equally regardless engine mode. The impact of the ambient temperature is not tested and all measurements that have been carried out have been done with an outside temperature between 0 and 20 °C. However, it has been investigated if the engine temperature has impact on the engine start and stop vibrations.

Method

To obtain necessary knowledge of the topic at hand a literature study was performed. In-house knowledge of Scania, books, competitor reports and the SAE digital library etc. were reviewed.

Necessary information about signal analysis and vehicle dynamics was obtained in books and by in-house knowledge.

Outline of the work flow:

1. Literature study including Scania in-house knowledge.

2. Vehicle measurements.

3. Initial analysis of measurement results.

4. Implementation of automatic analysis script in MATLAB.

5. Vehicle measurement.

6. Engine start and stop study.

7. Proposal of acceptance criterion regarding vibration comfort for the driver.

8. Presentation and report.

A study will be made with two purposes. To try to connect different vibration levels to

subjective judgment and to see if it is possible to establish a criterion of acceptance.

FRAME OF REFERENCE

This chapter contains a summary of studies and literature that touch upon the subject of how vibrations affect the human body and how to analyse data from vibrations measurements.

Vibration analysis

There are several ways vibration analysis can be conducted. An analytical approach is to simulate the vibrations using finite element method (FEM) or normal mode analysis. But sometimes a vibration problem can be too complex and measurements are needed to understand a vibration problem. (Brandt, 2011)

Vibrations can be analysed in the time domain and in the frequency domain. In the time domain the acceleration over time contains all frequencies that are present in the vibration. When looking at the frequency spectrum, the amplitude of the frequencies can be studied. Different sensors can be used and the output is a signal of some sort. There exist random, periodic and transient signals. In periodic signals there exist discrete frequencies while in random and transient signals all frequencies are present (Brandt, 2011). An example of a frequency content of a random and a periodic signal can be seen in Figure 2.

Figure 2. Frequency content of a random (left) and periodic (right) signal.

As seen in the figure above a random signal contains all frequencies and a periodic signal contains only discrete frequencies.

Transient signals

A transient signal is a non stationary signal and is considered to have a finite amount of energy.

Hence it is of interest to observe the total amount of energy in the signal rather than energy per time unit as in a stationary signal. When analyzing a relatively short transient signal, for example

0 5 10 15 20

0 0.05 0.1 0.15 0.2

Frequency [Hz]

Acceleration [m/s²]

Frequency content of a random signal

0 5 10 15 20

0 0.05 0.1 0.15 0.2

Frequency [Hz]

Acceleration [m/s²]

Frequency content of a periodic signal

a hammer blow on a solid surface, it is important that the signal starts and ends at zero to avoid leakage. (Randall, 1987)

Experimental vibration analysis

Experimental vibration analysis is usually done by collecting data of a system that have vibrations that perceives as problematic e.g. large vibrations or occurrence of motions sickness or in other ways are interesting in a vibration point of view. The data collected are mostly acceleration over time but other entities can also be measured (Brandt, 2011). The most common method to conduct vibration analysis is by frequency analysis, but in some cases it can be necessary to observe the level of vibration in the time domain as well. To compute the frequency content of a random signal, the Fourier transform is used. The Fourier transform can be defined as

𝑠(𝜔) = � 𝑠(𝑡)

∞

−∞

𝑒

𝑑𝑡 (1)

where s(t) is the time signal (H Bodèn; U Carlsson; R Glav; H P Wallin; M Åbom, 2001). The basic idea of the Fourier transform is that all signals can be described by a set of sine waves, it is Fourier series. The Fourier transform is derived from the Fourier series. To the right in Figure 3 can a periodic signal be seen, which has been divided into two frequencies components, which can be seen to the left.

Figure 3. Two signals with different amplitude and frequency (left). Signal (right) which is the sum of the two signals to the left.

Fast Fourier Transform (FFT) is the main tool for frequency analysis today (Brandt, 2011). FFT is an algorithm that computes the Discrete Fourier Transform (DFT) much faster than direct DFT. The number of computations to calculate N frequencies for DFT is proportional to N

while for the FFT algorithm it is proportional to N log

(N) (Brandt, 2011).

0 2 4 6

-1.5 -1 -0.5 0 0.5 1 1.5

Time [s]

Amplitude [-]

Two signals with different amplitude and frequency

0 2 4 6

-1.5 -1 -0.5 0 0.5 1 1.5

Time [s]

Amplitude [-]

Summation of the two signals to the left

Measurable quantities

In the analysis of vibrations, there are several parameters that may be important in terms of vibration comfort of the human body. Depending on what is most important, it is advantageous to analyze the vibration in certain ways. The most common ones are presented below.

When vibration analysis is conducted, acceleration can be measured. In Figure 4 the time signal in the y-direction of a start and stop sequence can be seen. It is clear that the levels of vibration are much higher during engine start and stop compared to idle vibrations.

Figure 4. Time signal of the acceleration during a start (left) and stop (right) sequence of a random Scania truck.

When analyzing acceleration data one can choose to make certain frequencies more important than other by frequency weighting. Frequency weighting are of interest due to the fact that the human body are more sensitive to certain frequencies (Kjaer&Bruel, 1989). To perform frequency weighting of a signal it needs to be divided into its frequency content. Once the frequency content has been calculated the amplitude can be multiplied with desirable weighting which will change the amplitude depending on the frequency.

According to ISO 2631-1 frequency weighting should be used. The weighting in the z-direction and the x- and y-direction can be seen in Figure 5.

0 0.5 1 1.5 2 2.5 3

-1.0 -0.5 0 0.5 1.0

Time [s]

Acceleration [m/s²]

Time signal for engine start

y-dir

0 0.5 1 1.5 2 2.5 3

-1.0 -0.5 0 0.5 1.0

Time [s]

Acceleration [m/s²]

Time signal for engine stop

y-dir

Idle Idle

Figure 5.Weighting curves as a function of frequency, 0.5 to 80 Hz. The x- and y-directions are dominant between 0.8 and 1.6 Hz whilst z-direction is weighted higher between 5 and 10 Hz.

From the figure above it can be seen that the frequencies between 5 and 10 Hz in the vertical direction are of interest while other frequencies are suppressed. Regarding for-aft vibrations the most important frequencies are around 1 Hz. (ISO2631-1, 1997)

RMS value

If a single value is wanted to describe an acceleration time signal the root mean square (RMS) is the most common used (Brandt, 2011). Also ISO 2631-1 states that the primary quantity to measure vibration is the RMS value of the acceleration. RMS value is defined as

𝑎

= � 1

𝑇 �𝑎

(𝑡)𝑑𝑡

𝑇 0

�

12

(2)

where a

(t) [m/s

] is the weighted acceleration as a function of time and T is the duration of the measurement. In Figure 6 the time signal of the acceleration are presented along with the RMS value over the whole measurement time.

Figure 6. Time signal of the acceleration including corresponding RMS value during a start (left) and stop (right) sequence of a random Scania truck. Note that the RMS value is not a running RMS.

10⁰ 10¹

0 0.2 0.4 0.6 0.8 1 1.2

Frequency [Hz]

Gain [-]

Frequency weighting curves for ISO 2631-1

x- & y-dir z-dir

0 0.5 1 1.5 2 2.5 3

-1.0 -0.5 0 0.5 1.0

Time [s]

Acceleration [m/s²]

Time signal for engine start

y-dir RMS

0 0.5 1 1.5 2 2.5 3

-1.0 -0.5 0 0.5 1.0

Time [s]

Acceleration [m/s²]

Time signal for engine stop

y-dir RMS

When vibration comfort is the main objective, ISO 2631-1 states that vibration total value of weighted RMS acceleration should be used, see equation below.

𝑎

= �𝑘

𝑎

+ 𝑘

𝑎

+ 𝑘

𝑎

�

(3) where a

is the vibration total value, a

, a

and a

are the weighted RMS acceleration in x-, y- and z-directions, k

, k

and k

are multiplying factors and for the case of comfort they are equal to one.

Crest factor is defined as the measurements peak value divided by the RMS value and if the crest factor is equal to 9 or below, ISO 2631-1 states that the RMS value can be used. If the crest factor is higher than 9 or if the signal contains occasional shock or have a transient behaviour one of the following methods can be used:

• Running RMS

• Maximum Transient Vibration Value (MTVV)

• Vibration Dose Value (VDV) Running RMS

The running RMS method handles occasional shocks and transient behaviour by the use of a short integration time constant and is defined as

𝑎

(𝑡

) = � 1

𝜏 � [𝑎

(𝑡)]

𝑑𝑡

𝜔₀

𝜔₀−𝜏

�

12

(4)

where a

(t) is the instantaneous acceleration, τ is integration time for averaging, t is the time integration time (block size) and t

is time of observation. ISO 2631-1 recommends the use of frequency weighted acceleration and that τ is one second and an overlap of 50 percent is used. In Figure 7 running RMS of an engine start and stop sequence can be seen with and without frequency weighting. The running RMS is calculated with a block size of 0.25 second and an overlap of 90 percent. A discussion regarding the settings for the running RMS is presented in chapter “Implementation” under section “Running RMS”.

The running RMS is basically several RMS values of the signal unlike the previous stated RMS,

which is a single value of the whole time signal. The running RMS values in Figure 7 have a new

RMS value every 0.025 second because the RMS value is calculated with data collected during

0.25 seconds and an overlap of 90 percent is used. Compared to the RMS value the running RMS

shows the dynamics in a signal, which is interesting in the vibrations that occur during engine

start and stop.

Figure 7. Running RMS of a start (left) and stop (right) sequence of a random Scania truck with and without frequency weighting. The running RMS value is calculated with block size of 0.25 seconds and 90 % overlap.

MTVV

The Maximum transient vibration value (MTVV) is defined according to (ISO2631-1, 1997) as

𝑀𝑇𝑀𝑀 = 𝑚𝑎𝑚[𝑎

(𝑡

)] (5)

where a

(t

) is the same as in equation (4). In other words the MTVV is the maximum running RMS value during the measurement, see Figure 8. The MTVV can be used when maximum acceleration is important in a measurement.

Figure 8, Running RMS and corresponding MTTV value of start (left) and a stop (right) sequence of a random Scania truck.

VDV

The vibration dose value (VDV), defined in (ISO2631-1, 1997), is most sensitive to peaks of the above mentioned methods due to the use of the power of four instead of the power of two. It is also a cumulative method which means that the VDV increase over time. Therefore it is important to wisely choose the duration of the measurement. VDV [m/s

] is defined as

0 0.5 1 1.5 2 2.5 3

0 0.25 0.5 0.75 1.0

Time [s]

Acceleration [m/s²]

Running RMS for engine start y-dir

y-dir, frequency weighted

0 0.5 1 1.5 2 2.5 3

0 0.25 0.5 0.75 1.0

Time [s]

Acceleration [m/s²]

Running RMS for engine stop y-dir

y-dir, frequency weighted

0 0.5 1 1.5 2 2.5 3

0 0.25 0.5 0.75 1.0

Time [s]

Acceleration [m/s²]

Running RMS for engine start y-dir MTVV

0 0.5 1 1.5 2 2.5 3

0 0.25 0.5 0.75 1.0

Time [s]

Acceleration [m/s²]

Running RMS for engine stop y-dir MTVV

𝑀𝑉𝑀 = ��[𝑎

(𝑡)]

𝑑𝑡

𝑇 0

�

14

(6)

where a

(t) is the instantaneous acceleration and T is the duration of the measurement. Also here ISO 2631-1 recommend that frequency weighted acceleration should be used The VDV method are applicable to analysis where sudden changes in the signal are of interest. For a discrete version of the VDV please see Griffin (1990). A comparison of a running RMS and VDV can be seen in Figure 9.

Figure 9. Running RMS and corresponding VDV of engine sequence of a random Scania truck.

As seen in the figure above, the VDV increases drastically when there are high accelerations and that the VDV continues to increase due to the engine idle vibrations.

An estimation of the VDV can be used and is called estimated vibration dose value (eVDV) and is defined as

𝑒𝑀𝑉𝑀 = [(1.4𝑅𝑀𝑅)

𝑇

]

(7)

where RMS is the root mean square value of the time signal, T

is the length of the measurement.(Griffin, 1990).

PSD

Random or transient signal does not contain discrete frequencies as a periodic signal does but rather all frequencies. Hence it is not possible to describe the amplitude of discrete frequencies.

To describe the frequency content in random or a transient signal a density function is used (Brandt, 2011), see Figure 10. If the measured signal is acceleration [m/s

] the unit of the power spectral density (PSD) is [(m/s

)

/Hz]. The PSD is the average of the Fourier transform magnitude squared over a time interval and can be defined as

0 0.5 1 1.5 2 2.5 3

0 0.2 0.4 0.6 0.8 1

Acceleration [m/s2 ]

Running RMS and VDV for engine start

Time [s]

0 0.2 0.4 0.6 0.8 1

Running VDV [m/s1.75 ]

y-dir VDV

𝑃𝑅𝑉 = lim

𝑇→∞

𝐸{|𝑋(𝑓)|

}

2𝑇 (8)

where X(f) is the Fourier transform of the time signal and T is the duration of the measurement (Scaott Miller; Donald Childers, 2012). The PSD can also be defined by its auto correlation function (Brandt, 2011).

Figure 10. PSD plot of the start-up sequence of a random Scania truck, block size 0.25 second, overlap 90%. The RMS of the time signal corresponding to frequencies between 12 and 14 Hz are equal to the square root of the

highlighted area.

The PSD graph can be interpreted as follows. The square root of the area below the curve in the PSD graph between two different frequencies is equal to the RMS value in the signal in specified frequency interval. Thus following relationship

𝑎

= � � 𝑃𝑅𝑉(𝑓)𝑑𝑓

𝑓_{𝑢𝑢𝑢𝑢𝑢}

𝑓_{𝑙𝑙𝑙𝑢𝑢}

(9)

In PSD it is the area that are of interest and not the peak value because the peak value are dependent on which frequency resolution are used. If a PSD is calculated on the same signal but with two different frequency resolutions the PSD with the finer frequency resolution will have higher peaks compared to the PSD with the lower frequency resolution. This is because the area below the curve has to be constant (Brandt, 2011).

ESD

For a transient signal the energy spectral density (ESD) can be used to analyse the acceleration data. Unlike periodic signals, transient signals have a continuous spectrum and starts and ends at zero in finite time. The ESD is basically the PSD multiplied with the measurement time used for one FFT-block based on the fact that power is energy divided by time (Randall, 1987). Thus the single sided ESD can be defined as

0 2 4 6 8 10 12 14 16 18 20

0 0.2 0.4 0.6 0.8

1 PSD for engine start

Frequency [Hz]

Acceleration PSD [m/s2 ]2 /Hz y-dir

RMS²

𝐸𝑅𝑉 = 2(∆𝑡)

|𝑉𝐷𝑇{𝑚(𝑛)}|

(10) where ∆t is the time window, the factor two converts the double sided spectrum level to match a single sided spectrum and |DFT{x(n)}|

is the frequency components from the discrete Fourier transform of the time signal x. The area below the curve in the ESD spectra corresponds to the total energy in the transient signal and the area between two frequencies correspond to the energy between these frequencies. (Brandt, 2011)

Engine start and stop sequence

The main reason of the relatively high vibration levels during engine start and stop is the rotation of the crank shaft that is exciting several of the power pack eigenfrequencies when the RPM are increased to idle at start, or decreased to zero at stop. The main forces that acts upon the power pack are mass forces from the crankshaft and piston and pressure forces in the cylinder (Takayoshi Yoshioka; Hiroshi Sugiata, 2001) (Jung-Hwan Bang, Hee-Wook Yoon and Kwang- Min Won, 2007).

To be able to start an engine the fuel mixture has to be injected into the combustion chamber.

The fuel mixture has to be applied at the right moment when the engine is in compression phase.

To determine in which phase the engine is in, the crankshaft has to rotate a couple of revolutions before the fuel mixture can be applied. Therefore there exists a delay between starter motor activation and combustion of the engine. The time window when combustion should take place is a function of temperature and pressure and is reduced with decreasing temperature. To be able to have a rapid start time enough fuel has to be applied. But when the fuel is injected the pressure and temperature will decrease in the combustion chamber, and if too much fuel are injected the fuel will not be able to ignite. Therefore is the start of an engine a compromise between start time and robustness.

When the start sequence of an engine starts and the revolutions per minute (RPM) gradually increases from zero, the crankshaft will excite resonance frequencies of the power pack. The same applies for the engine stop sequence. In Figure 11 the RPM signal can be seen over time in a start and stop sequence of a random Scania truck along with the acceleration in y-direction.

When resonance frequencies of the power pack are exited, large amplitudes of acceleration can be observed. The sudden change in the RPM signal is due to that the engine control unit transmit zero on the CAN bus when the RPM drops below a certain value.

When looking at the acceleration time signal along with the RPM signal it is worth notice that

max acceleration occurs approximately at the same engine speed in both start and stop sequence,

which is in the range 200 to 300 RPM. But when comparing the time length of the ramp-up

during engine start and ramp-down during engine stop, it is worth notice that the engine stop

sequence takes longer time. This results in that the time which the power pack excites the trucks resonance frequencies is longer, which results in longer duration of high accelerations.

Figure 11. Accelerations in y-direction and RPM signal of an engine start (left) and stop (right) sequence of a random Scania truck. The sudden change in the RPM signal is due to that the engine control unit transmit zero on

the CAN bus when the RPM is below a certain value

Due to the installation of the engine and the rotation axis of the crankshaft, either vibrations in y- direction or x-direction will be dominant. If the engine is mounted in the y-direction the largest vibrations will be experienced in the x-direction and if the engine is mounted in the x-direction vibrations in y-direction will be dominant. (Karlsson, 2011).

A conventional starter motor is not able to run the engine at idle. The starter motor only rotates the crankshaft enough until the engine is able to combust, which occurs before idle speed is reached. A hybrid vehicle can use the electric motor to start the combustion engine. This can result in a much faster start sequence than a conventional starter motor can manage due to higher torque and speed output in the hybrid electric motor. During stop of the engine the electric motor could be used to break the engine to zero RPM, resulting in that the stop sequence will be shortened.

Power pack eigenfrequencies

There exist several resonance frequencies for the power pack. Six eigenfrequencies appear in translation and rotation of x-, y- and z-direction, see Figure 12. It can also have structural resonance in vertical and horizontal direction when the whole power pack deflects as a beam.

However, the power pack can be assumed to be rigid for frequencies below 30 Hz (Lee, 2007) (Jung-Hwan Bang, Hee-Wook Yoon and Kwang-Min Won, 2007). There can also be resonance in the twist movement of the power pack. However, the twist is not relevant in regarding engine start and stop vibrations because twist appears when the engine experience a torque and that is not the case thus the gearbox is in neutral during start and stop of the engine.

0 0.5 1 1.5 2 2.5 3

-1.0 -0.5 0.0 0.5 1.0

Time [s]

Acceleration [m/s²]

Acceleration and RPM for engine start

0 0.5 1 1.5 2 2.5 30

200 400 600

RPM [r/min]

y-dir RPM

0 0.5 1 1.5 2 2.5 3

-1.0 -0.5 0.0 0.5 1.0

Time [s]

Acceleration [m/s²]

Acceleration and RPM engnine stop

0 0.5 1 1.5 2 2.5 30

200 400 600

RPM [r/min]

y-dir RPM

Figure 12. Schematic of the power pack comprising engine and gearbox. The arrows indicate resonance frequencies of the power pack

Previous work

The research that has been done regarding vibrations effect on the human body mostly concerns the effects of long term exposure to random and transient vibrations and not in particularly vibration comfort. Below is a summary of known knowledge regarding vibrations effect on the human body, statistics and frequency weighting.

ISO2631-1

The ISO2631-1 standard defines quantified methods of whole-body vibrations with respect to health, motion sickness and vibration comfort. It states that frequencies between 0.5 to 80 Hz are of interest when it comes to periodic, random and transient vibrations. The vibrations shall be measured at the point where the vibrations enter the body.

Standard ISO2631-1 also states that the use of band limitations should be achieved by two pole high-pass and low-pass filter. The standard defines approximate values of maximum allowed vibrations regarding vibration comfort in public transportation, see Table

.

Table 1. Approximate values of vibration comfort reactions to vibration environment. Below levels are based upon that the measurement are performed accordingly to ISO 2631-1 i.e. an integration time of one second and an overlap

of 50 percent is used if the running RMS is used

. Overall total value Indications Less than 0,315 m/s

Not uncomfortable 0,315 m/s

to 0,63 m/s

A little uncomfortable

0,5 m/s

to 1 m/s

Fairly uncomfortable 0,8 m/s

to 1,6 m/s

Uncomfortable 1,25 m/s

to 2,5 m/s

Very uncomfortable

Greater than 2 m/s

Extremely uncomfortable

The values in Table 1 are obvious very subjective and depends on in which situation it is applied

and what activities the person has undertaken, for instance reading a paper or trying to sleep.

starting or stopping an engine but rather focus on health and vibration comfort effects measured over longer time. Also these levels are based upon that the measurement are performed accordingly to ISO 2631-1 i.e. an integration time of one second and an overlap of 50 percent is used if the running RMS method is used.

ISO2631-5 asserts the effects of whole-body vibrations on the human body. The standard assumes that the dominating health risk of long term exposure to vibrations is multiple shocks on the lumbar spine (lower back). Hence the standard focuses on the impact on the lumbar spine.

The standard assumes that the there is a health risk due to the transient pressure change arising from material fatigue process. Conclusions are made that there is a linear relationship between input shock and peak acceleration in the spine.

Scania in-house

According to (Hamache, 2013) the eigenfrequency of the cabin is around 1.8 Hz. The work of Jansson (2013) implies that the start and stop process can be smoother when a hybrid truck makes use of the electric motor as the starter motor rather than a conventionally starter motor.

Human body vibrations

Studies have been made which uses the methods described in ISO 2631-1. In the study made by Jönsson and Johansson (2005) it is concluded that there is a correlation between body length and discomfort and that pitch vibrations between 50 to 100 Hz are of importance when predicting vibration discomfort while ISO 2631-1 neglects these frequencies. It is also concluded that the evaluation method, maximum transient vibration value (MTVV) compared to the vibration dose value (VDV) method, both mentioned in ISO 2631-1, is more accurate to predict discomfort due to vibrations (P.Jönsson; Ö. Johansson, 2005). But in other studies it has been concluded that the VDV method is preferable when it comes to predict discomfort (N.J Mansfield; P.Holmlund;

R.Lundström, 2000).

To measure vibration comfort in an analytic way and get predictable results that correlate to a driver’s subjective feeling is not trivial. When looking at research that touches upon vibration comfort and drivability no comparable results are shown, but some general trends can be seen.

For instance, the human body is most sensitive to vibrations in z-directions in a range between 4 and 8 Hz. This frequency consorts to the vertical resonance frequency of the abdominal cavity.

In some studies a slightly higher sensitivity can be observed in the range from 10 to 20 Hz due to

different organ resonance. For vibrations in x- and y-direction the maximum sensitivity appears

in the range between 1 and 2 Hz. This happens to be around the resonance frequency of the

upper torso. These observations are derived from using a pure sinusoidal input which differs

rather much from the complexity of engine vibrations (Gillespie, 1992). But there exist

contradicting results where the use of frequency weighting can predict how a driver perceives

correlation to the subjective rating that will be obtained from on road survey (A.J.Healey ; E.

Nathman; C.C. smith, 1977).

The human body can be seen as a complex mechanical system. It is complex in many ways e.g.

that each part of the body has its own natural frequency, the human body is not symmetrical and that individuals will react different on vibrations. To get an overview of the eigenfrequencies that exist in a human body see Figure 13. Due to the strongly damped system that a human body represents, conclusions can be made regarding the sensitivity to certain frequencies (Kjaer&Bruel, 1989).

Figure 13. Simple schematic picture of eigenfrequencies of the human body. (Provided by Bruel & Kjaer)

The human body can roughly be considered as a rigid body at vibrations below 2 Hz, when the

person is in a standing position. Depending on if the person is seated or in a standing position,

the person will experience vibrations in different ways. One frequency that is of importance in

both cases is the resonance frequency of the abdomen, which is in the range of 3 to 12 Hz (H

Bodèn; U Carlsson; R Glav; H P Wallin; M Åbom, 2001).

Leatherwood, Dempsey and Clevenson (1980) concluded that discomfort in different frequencies has a relation to the amplitude of the vibration. It seems that vibrations with low amplitude are equally disturbing regarding the frequency.

Filter approximation of the ISO 2631-1

A complement to the transfer function suggested in the ISO2631-1standard is the low order filter approximation developed by Zuo and Nayfeh (2003). The low order approximation is preferred in practical embedded applications and the fifth order variant, W

, of the transfer function in z- direction is defined as

𝑊

(𝑠) = 87.72𝑠

+ 1138𝑠

+ 11336𝑠

+ 5453𝑠 + 5509

𝑠

+ 92.6854𝑠

+ 2549.83𝑠

+ 25969𝑠

+ 81057𝑠 + 79783 (11) where k stands for vibrations in z-direction and s is frequency. Corresponding fourth order transfer function, W

, in x- and y-directions is defined as

𝑊

(𝑠) = 12.66𝑠

+ 163.7𝑠

+ 60.64𝑠 + 12.79

𝑠

+ 23.77𝑠

+ 236.1𝑠

+ 692.8𝑠 + 983.4 (12) where d stands for x- and y-direction and s is frequency. Comparison between the filter approximating and the ISO 2631-1 weighting can be seen in Figure 14.

Figure 14. ISO 2631-1 frequency weighting curves Wk (circles) and Wd (square) compared to fifth order filter for z- direction and fourth order for x- and y-direction.

Statistics

Lundgren (2008) shows that if there is a big variation between measurements that should be compared, the use of a coefficient of variations can be used. The coefficient of variations is used as a measure of statistical dispersion of different measurements and is defined as

𝑀𝑉 = 𝜏

𝑚 ∙ 100 [%] (13)

10⁰ 10¹

0 0.2 0.4 0.6 0.8 1 1.2

Frequency [Hz]

Gain [-]

2631-1 frequency weighting compared to filter approximation

ISO 2631 z-dir ISO 2631 x&y-dir Filter approximation

where τ

is the standard deviation and x is the mean value of the measurements. The variation coefficient is expressed in percent and are described in other words as the standard deviation normalized over the mean value. The standard deviation is according to (Gunnar Blom et al, 2005) defined as

𝜏

= � 1

𝑛 ��𝑚

− 𝑚�

𝑛 𝑗=1

(14)

where n is number of measurements, x

is the measurements. The mean value is defined as

x = 1 𝑛 �x

𝑛 𝑗=0

(15) Were n is number of measurements and x

is the measurements. The use of the variation coefficient makes it possible to compare statistical dispersion between different measurements when the difference in amplitude of the measurement makes it impossible to compare the standard deviation. Lundgren also mentions the use of standard deviation as a way to visualize the dissipation of measurement data. For example if one wants to show the amplitude of the acceleration over time and conduct repeated measurements, one can simply calculate the standard deviation for each time stamp and add and subtract the standard deviation in the graph.

In this way one can get more information in the plot than just the mean value, the standard deviation is also visualised in every point. An example can be seen in Figure 15.

Figure 15. Running RMS averaged with 20 measurements, plus, minus, one standard deviation.

0 0.5 1 1.5 2 2.5 3

0 0.25 0.5 0.75 1.0

Time [s]

Acceleration [m/s²]

Running RMS for engine start

y-dir, mean value +/- σ y-dir, mean value

IMPLEMENTATION

In this chapter the measurement and analysis method are discussed and data are presented to confirm a proposed method to measure and analyse the vibration that occur during engine starts and stops. Tools and utilities that were used during the master thesis are also presented along with how the measurements were conducted during the investigation.

Tools and utilities

To measure the acceleration that occurs during start and stop of the engine, 10g accelerometers from Microtron were used with part number 7290A-10. These particular accelerometers were selected because they are used in the department where this master thesis was conducted. One accelerometer in direction x, y and z were used respectively. The accelerometers were attached to an aluminium bracket with a resonance frequency far higher than the frequencies of interest and the bracket were connected to the driver’s seat adapter with magnets, see Figure 16. The magnet equipped bracket is preferable instead of a bracket which is fasten with a screw because it simplifies the measurement procedure, which was a condition given from the start.

Figure 16. Installation of accelerometers during measurements to the left. Bracket including magnets and attached accelerometers to the right.

The measurement system used was a DEWE-43-A from DEWESoft with eight analogue

channels and CAN interfaces together with the software DEWESoft X. The sampling frequency

used was 1000 Hz which was a condition from the start due to making the data compatible with

existing analysis tools. To analyse the data, MATALB R2012b was used along with a self-

developed analysis script. Details about the trucks that were used to collect acceleration data can

be seen in Table 2.

Table 2. Specification of the trucks used to collect acceleration data from.

Truck name

Engine Displacement Type designation

A V8 16 litre R730LA4X2MNA

B In-line 6 13 litre R480LA4x2MNA

C In-line 5 9 litre P310LA4x2 MNA

D In-line 6 13 litre G400LA4x2MEB

E In-line 6 13 litre P400LA4x2MLA

F In-line 5 (gas)

9 litre P310LB6x2*4HNB

G V8 16 litre R730LB6x4MNB

Today Scania uses a self-developed diagnostic tool called XCOM to read and set parameter of the trucks ECU. With XCOM it is possible to read the engine speed of the truck which may be used when measurements are triggered. It is also possible to set the engine speed. However, there are no protocols available to start and stop the engine.

A MATLAB script was also developed in order to facilitate the analysis of the start and stop vibrations. Below is a list of the functionalities of the script:

• Comparison between multiple trucks.

• Analyse single measurements or average.

• Analyse single directions or acceleration vector.

• View time signal.

• View PSD-levels.

• View running RMS including standard deviation and frequency weighted acceleration data.

Procedure of measurements

Before every set of measurement the clearance between the cab anti-roll bar and the bump stop was investigated to ensure enough clearance. The temperature during the measurements was in the range of 0 to 10 °C. The placement of the accelerometer adapter can be seen in Figure 16. A criterion from the start was that when measurements are conducted a person must be seated in the driver’s seat. Adjustment of the driver’s seat was decided to be the same in every measurement as follows:

1. Fore aft position adjusted to middle.

3. Tilt position set at 0°.

4. Height adjustment set to middle.

5. Air suspension on.

To clarify the seat adjustment Figure 17 displays the seat adjustment levers with corresponding number.

Figure 17. The driver’s seat adjustment during measurements.

Measurement method

In the following section measurements and analysis are presented which will result in a proposed method to measure engine start and stop vibrations in the driver’s seat adapter. Different conditions that may affect the measurement are tested and analysed. The parameters of the running RMS method can be seen in Table 3.

Table 3. Parameter setting for running RMS.

Block size 0.25 s

Overlap 90 %

Frequency range 4 – 20 Hz

The orientation of the coordinate system, which has been used in this master thesis, can be seen in Figure 18.

1

5

4

3

2

Figure 18. Coordinate system relative to the truck. (Provided by Scania CV AB)

Acceleration vector

During this master thesis no correlation between vibration discomfort and the three single directions has been found. Also, standard ISO 2631-1 states that the accelerations in x-, y- and z- directions should be considered when vibration comfort is measured. Therefore, the conclusion is drawn that there is no correlation between a particular direction and vibration comfort and the following equation based upon the accelerations in each direction can be obtained

𝑎 = �𝑚

+ 𝑦

+ 𝑧

(16)

where a is the acceleration vector, x, y and z are the running RMS in each direction, see Figure 19.

Figure 19. Schematic picture of the acceleration vector and its components in each direction.

Trigger conditions

When measuring acceleration, some sort of automatic trigger is necessary to make the method user friendly and repeatable. The alternative is to trig the measurement by hand which can be time consuming and are more likely to introduce error when collecting data. The trigger alternatives that were investigated were trigger by acceleration level or by engine speed.

When a trigger on the engine speed is used the RPM signal from the CAN bus is monitored and

the measurement system starts to measure on a positive edge and during the stop sequence the trigger is set on negative edge. The engine speeds that were investigated can be seen in Table 4.

Table 4. Trigger conditions tested for RPM signal.

Start [RPM] Stop [RPM]

300 300

400 400

550 200

The starter motor spins the crankshaft at approximately 200 RPM and therefore trigger conditions during the start sequence cannot be around 200 RPM. Due to the fact that the idle RPM for the trucks tested were 500 and 600 RPM the trigger condition for the stop sequence cannot be close to 500 or 600 RPM. In this section the measurements is presented in one direction only because the interesting thing is to see how well the individual measurements agree in time. The level of acceleration is not important.

Measurements of the start and stop sequence with the different trigger conditions are presented as running RMS in Figure 20 and Figure 21. Ten measurements per trigger condition are presented.

Figure 20. Comparison between running RMS curves in the y-direction for different trigger conditions of the RPM signal during engine start sequence.

0.5 1 1.5 2 2.5 3

0 0.17 0.33 0.50

Time [s]

Normalized acceleration [-]

Running RMS during start for trigger signal 550, 400 and 300 RPM

300RPM 400 RPM

550 RPM

Figure 21. Comparison between running RMS curves in the y-direction for different trigger conditions of the RPM signal during engine stop sequence.

From the graphs above it can be seen that each measurement agrees well in time - independent on trigger condition - although the blue alternative have somewhat more dispersion after the peak acceleration. However, it is preferable to have 300 RPM as trigger condition to make the method more robust regarding possible future reduction of the idle speed. To make the method more user friendly, trigger conditions on both the start and stop sequence should be the same.

An alternative to engine speed as trigger signal is to trigger the measurement on acceleration levels. One positive aspect to this type of trigger conditions is that the method becomes simpler because the CAN interface does not need to be handled. Also, it can be hard to get access to the CAN signals when measuring on competitor trucks. Running RMS curves of the acceleration in the y-direction with trigger conditions 1.0 (red), 0.75 (blue) and 0.6 (green) [m/s²] can be seen in Figure 22 and Figure 23. Five measurements per trigger condition are presented.

Figure 22. Comparison between running RMS curves in the y-direction for different trigger conditions of the acceleration during start sequence.

0.5 1 1.5 2 2.5 3

0 0.17 0.33 0.50

Time [s]

Normalized acceleration [-]

Running RMS during stop for trigger signal 200, 300 and 400 RPM

0.5 1 1.5 2 2.5 3

0 0.17 0.33 0.50

Time [s]

Normalized acceleration [-]

Running RMS during start for trigger signal 1, 0,75 and 0,6 [m/s²]

400 RPM 300 RPM

200 RPM

1.0 [m/s²]

0.75 [m/s²]

0.6 [m/s²]