Anti Jerk Control Of Hybrid Electric Vehicles Engineering Essay

This interim report consists of three chapters explaining the initial study and research into anti - jerk control of hybrid electric vehicles. This project aims at electrification of anti-jerk control in hybrid vehicles by using the hybrid vehicle electric motor to damp these oscillations and to analyze its effect on fuel consumption, comfort and sportiness of the vehicle and then to compare the results with fuelling only method. This project will also include studying the impact of passive damping with DMF in these vehicles. As a part of this project, a model of a hybrid diesel engine driveline with DMF will be developed using Simulation X and an active feedback control for an electric motor will be designed to permit active damping of these oscillations.

The results obtained from simulating the drivelines modeled using Simulation X will be observed for driveline vibrations with respect to the frequency range and also the range in which the jerking frequencies affect the passengers inside the vehicle so as to concentrate on it specifically leading to maximum reduction in jerking of driveline.

The models so developed will then subjected to design modifications and investigation in accordance with the control strategies developed and affect of the same will be observed on all the drive train models with level of jerk control achieved.

Finally Suggestions will be made on effect of electric motor on hybrid drive train in damping the driveline oscillations, fuel consumption and sportiness of the vehicle also comparing it with the conventional drive train models.

Introduction

In the age of technology where everyone is concentrating on improving the products by bringing in sophisticated technologies from cars to airplanes, we as human beings neglected the environment factor over the years. 20th century was called an "industrial age" and much technological advancement have been made in this era which revolutionized the whole world.

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One of the industries which is closely related to connect our daily life are automobiles. Automobiles have grown rapidly over the years with stiff competition among automotive manufacturers across the world. As the time passed by people become more demanding with their interests widening the automotive market and also the fact that automotive majors started delivering those demands. These technological improvements along with human comfort factors all of us forgot about the effect on environment.

The number of vehicles on roads grew exponentially and consumption figures of petrol and diesel across the world increased drastically sending out alarm bells to search for alternate fuel technologies as the conventional petrol & diesel were not only running out from their existence but also affecting the environment leading to considerable amount of CO2 emissions and further contributing to global warming. This future problem opened up opportunities for research in alternative fuel technologies leading to the development of Hybrid vehicles, Fuel cell technologies & Hydrogen fuelled cars. Few of the alternate fuel vehicles are Toyota Prius hybrid, Honda Civic hybrid, Honda Insight etc which have received good response in the market in terms of performance as well as in reducing CO2 emissions and have successfully proved their presence in the market with their selling volumes and fuel savings.

On a smaller level, these developments lead to further research in areas like DC/AC motors, fuel cells, battery pack technology, advanced controller design, power optimization and new concepts which can be used as alternate fuels.

Problem Definition

Commonly in every engine driven system issue of noise, vibration and harshness (NVH) comes along with it. Automotive manufacturers have always tried to maintain healthy comfort to performance ratio so as to attain maximum customer satisfaction. With respect to comfort of driving, the manufacturer's main aims are to provide vibration free driving pleasure as much as possible and the same applied to hybrid electric vehicles. One such area of concern in drive line jerking which affects the overall drivability. Jerking is a kind of driveline oscillations which occurs due to excitation caused by sudden or huge variation in engine torque or driveline load. These excitations cause because of torsional vibration of drive train at high engine torques leading to unwanted longitudinal oscillations of passenger compartment which reduces the riding comfort and drivability. These oscillations have tendency to occur at low engine speeds which are experienced as unpleasant jerking effect by the driver. Due to the fact diesel engines produce high torque at low engine speeds, makes these engines more prone to this problem. The area of concentration under this case is vibration frequencies ranging between 2- 10 Hz as human body is sensitive towards this frequency range causing unpleasant driving experience.

These driveline oscillations majorly occur during tip in and tip out conditions. In conventional diesel engine vehicles methods like active control of fuel injection into engine and passive element such as dual mass flywheel are used to permit feed forward active as well as passive damping of these unpleasant oscillations in idle, tip-in/tip out and coast conditions. To achieve minimal amount of driveline oscillations, vehicle manufacturers have to compromise between comfort and performance of the vehicle. In diesel hybrids, the jerking can be controlled by varying fuel injection and electric motor torque control and also passively by an additional dual mass flywheel (DMF).

Aims & Objectives

The aims and objectives of this project are:

To develop a model with three different configurations of powertrain using Simulation X those are: Conventional diesel engine, Conventional diesel engine with DMF and finally hybrid power train with DMF.

To study the effects of jerk in each individual power train configuration before developing the control strategy for anti jerk control.

To electrify the anti jerk control in hybrid vehicles by using Crankshaft integrated stator/generator to damp the oscillations.

To analyze the effect on fuel consumption, comfort and sportiness of the vehicle.

To compare the results with conventional fuel control method.

Literature Review

Hybrid Electric Vehicles

Hybrid Electric Vehicle is the one which uses two or more sources of energy to propel the vehicle which consists of an internal combustion engine and electric motor power by batteries. Hybrid vehicles are of various types such as series hybrid, parallel hybrid and combination of series parallel and based on their hybridization factor they are categorized into micro, mild and full hybrids. Figure 1, below shows the typical power split type power train. Generally, hybrid power trains are costly to build due to its complexity but the costs are paid off against the running costs due to improved fuel efficiency. Regenerative braking plays an important role in conserving energy by charging the battery pack.

Figure : Showing a Power Split Hybrid Powertrain

Types of Hybrid vehicles based on drive train structure

Parallel Hybrid

In parallel hybrid vehicles, there are two parallel ways for transmitting power to the wheels of the vehicle; those are by engine and by electric drive, as shown in below figure 2. The transmission is coupled with the motor/generator and the engine, allowing either, or both, to power the wheels. Control architecture of parallel hybrid vehicles is more complex compared to a series hybrid due to the requirement of efficient coupling of the motor/generator and engine so as to maintain optimum drivability and performance.

Figure : Parallel Hybrid Powertrain

Operating modes available in parallel hybrid vehicles are:

Engine only traction

Electric only traction

Hybrid traction

Regenerative braking

Series Hybrid

In series hybrid there is only one way for transmitting power to the wheels of the vehicle, but consists two energy sources. As shown in figure 3, the conventional engine is coupled to a generator for charging the battery pack which provides electrical energy to a motor/generator to power the wheels via transmission. The motor/generator can also be used for recharging the battery during braking and deceleration.

Figure : Series Hybrid Power train

Operating modes available in series hybrid vehicles are:

Electric only traction

Electric traction & battery charging

Battery charging and no traction

Regenerative braking

Series-Parallel (combined) Hybrid electric vehicles

A Series-Parallel (combined) hybrid vehicle has both the aspects Series and Parallel energy transfer paths. As shown in figure 5, a system consisting of motors and/or generators consisting of a gearing or power split device allows the engine to recharge the battery. Changes in this power train configuration can range from simple to very complex type depending on the number of motors /generators and their working terminology.

These configurations can be called as Complex hybrids (such as the Toyota Prius and Ford Escape Hybrids), Split-Parallel hybrids, or Power-Split hybrids.

Figure : Series-Parallel (Combined) Hybrid Power train

Driveline Jerking

Driveline jerking also called as driveline oscillations can occur due to many different factors such as load changes, gear shifting and also the condition of the road. In recent times, direct injection diesel engines with extensive refinement of components and significant weight reduction in driveline lead to the problem of driveline jerking. Also with advancement in diesel engine technology over the years, output torque of these diesel engines has risen tremendously for passenger cars effecting the comfort and drivability. The problem with high engine torque is, it causes torsion of driveline due to the gear ratios of the final drive causing the torsion at drive shafts which in turn causes whole power train to oscillate with combined effect of vehicle jerking. Figure 5, shown below show the measurements on a test car caused by tip in behavior in which driver suddenly steps on the throttle combined with high torque gradient and tip out condition, the back out maneuver.

Figure : Engine torque and engine speed for tip in and tip out maneuvers.

Though the oscillations caused by engine speed are somewhat absorbed by engine mounts but the oscillations due to wheel speed is responsible for horizontal vibrations of the vehicle's longitudinal acceleration affecting the performance of the vehicle which is experienced directly by passengers. To restrain these oscillations affecting the comfort and drivabilityfor the passengers, damping of these driveline oscillations is necessary which is referred to as "Anti Jerk Control". The oscillations leading to power train jerking in conventional power train can be categorized by the Eigen frequencies of following driveline components:

Torsion of shafts causes pitching and jerking due to suspension system, mass of the vehicle and damping. The range of frequency for this cause lies between 2 - 5 Hz. Also, we know that the natural frequency of the vehicle with respect to pitch motion is lies between 1 - 2.5 Hz.

The frequency of oscillations caused by engine movement and mounts lies in the range of 15 - 200 Hz.

The vibration due to gearbox assembly lies in the frequency range of 50-80 Hz.

There are oscillation depending upon the type of engine, no. of cylinders, engine speed and also the combustion process shown in below figure

Table : showcasing different engine types and their Eigen frequencies

Engine Type

Multiples of Eigen Frequency

4-Cylinder ,R4, 4-Stroke

2,4,6,8,10

6-Cylinder,R4, 4-stroke

3,6,9

6-Cylinder,V60,4-Stroke

1,5, 3, 4,5, 6, 7,5, 9

6-Cylinder,V90,4-Stroke

1,5, 3, 4,5, 7,5, 9

12-Cylinder, V60, 4-Stroke

6,12

From the above categorization of the components and their respective Eigen frequencies, the main aim is to narrow down on the components leading to vibrations which are felt by passengers majorly and to minimize the level of vibrations by designing model based predictive control strategy while retaining the performance of the vehicle as much as possible.

Hybrid Vehicle Design

The power train which has been considered for testing the jerking behavior and the anti jerk control strategy to be designed is shown in the below figure 6. This power train is one of the possible hybrid vehicle architecture consisting of I.C engine, CISG (Crankshaft Integrated Starter/ Generator), Dual Mass Flywheel (DMF), Clutch, Gearbox assembly , High voltage battery pack and power electronics.

Figure : Hybrid Vehicle architecture

In operation, at lower rpm the vehicle acts as pure electric vehicle powered by the on board battery pack and at higher rpm, both electric drive and I.C engine together produce required power by the vehicle. The percentage of power transmission sharing to propel the vehicle plays an important role in determining fuel efficiency.

Design Considerations

The components and factors which play important role in hybrid electric vehicle design shown in figure 6 are:

Engine design and selection: The I.C engine as in conventional power train plays an important role in hybrid vehicles. Generally, engines designed for hybrid vehicles are smaller in size compared to conventional vehicles. Though the design & selection of the engine is completely based on the power requirements of the vehicle.

Crankshaft integrated starter/generator (CISG): The starter/generator is device which is controlled electronically. It combines both the functions of a conventional starter and generator into a one single unit. The motive of single CISG unit is replacing the starter as a single entity which is passive in nature; need to replace the old fashioned belt and pulley type connection between the alternator & the engine. Also to replace the modern day rotor wound alternators with slip rings and brushes. The integrated starter/generator works as a bi-directional device which converts electrical energy to mechanical work and vice versa. As a electric motor, it assists in starting the I.C engine without any noise and also much quicker than conventional starter. As a generator it produces power required for electrical components of the vehicle and is also used to charge the batteries. Commonly integrated starter/generator is placed between engine and gearbox assembly. The main operating features of ISG are it enables stat/stop, onboard power generation and acts as power assist when required.

Figure : Integrated Starter / Generator.

Dual Mass Flywheel (DMF): Dual mass flywheel consists of two flywheels connected by long arc travel springs located between I.C engine and clutch or transmission. The first DMF was introduced for the automotive industry in 1985. At that time non lubricated dampers were used consisting of heavy springs which were problematic. Then there was breakthrough in the DMF technology and arc spring type dampers was introduced in 1989 solving almost all the problems which were caused by DMF earlier. Due to the high cost of this product not everyone was interested but was used in large vehicles. Though DMF is a passive driveline element, it has been proven that DMF reduces torsional vibrations to certain extent and plays an important role in anti jerk control of hybrid vehicles also. The operating performance of DMF can be characterized by the spring rate and its damping characteristics.

The dual mass flywheel consists of following important characteristics:

Primary and secondary inertias.

The torsion damper rate.

And the damping characteristic.

Advantages of using DMF in conventional power train are:

Segregation of torsional vibrations: we know that, torsional vibrations are caused by torque fluctuations. A vehicle is a kind of vibrating system with all the components like engine, transmission, drive shafts etc all contributing to the cause. Below figure 8, shows the simple driveline model so as to observe vibration behaviour. In this case engine and transmission are supposed as rotating inertia connected by springs. The spring C2 represents the spring damper characteristics and spring C3 showing the stiffness of the drive train.

Figure : Drive train with vibration modes.

Figure 9 shown below depicts the fluctuations in vehicle speed and in this case damped resonance occurs at around 1700 rpm. The main aim of DMF is to sideline the vibrations occurring from the engine as far as possible from the rest of the driveline components. Also the figure compares the difference between the extents of vibrations in a conventional drive train compared to one fitted with dual mass flywheel. Dual mass flywheel efficiently reduces the engine vibrations and reduces gear rattle, helps in saving the fuel consumption and also improves driving comfort.

Figure : Comparison of vibration damping in a conventional drive train to the one with DMF

Transmission assistance: As DMF is said to reduce the engine vibrations, it has positive effect on the transmission system as the stress induced will be significantly less compared to a compared drive line increasing the transmission efficiency and cycle life.

Crank shaft assistance: In a conventional power train, we know that flywheel and clutch are connected to engine crankshaft rigidly and due to the inertia of the flywheel high reaction forces are developed on the crankshaft. But in case of DMF, the secondary flywheel can be neglected for bending load case as it connected loosely to the primary flywheel by torsional damper and roller bearings which practically don't allow high transferable reactive forces. The primary flywheel is significantly lighter in weight, more elastic in nature compared to conventional flywheel.

Battery pack design and selection: The main criteria in battery design and selection depend upon the capacity, the output characteristics based on type of battery pack to be used, cycle life, cost, scope of reusability and whether recyclable on not. The size and weight of the battery pack depends on the capacity requirement. If capacity is high, more will be size and weight of the battery pack. In fact, battery pack design plays a very important role as the performance of the vehicle is dependent on the overall weight of the vehicle. In case battery pack is heavier, this would have direct impact on the power consumed to propel the vehicle which is set to rise with increase in battery weight and will also reduce the operating range of the vehicle.

Figure : Typical Battery pack arrangement

Electric Motor: Generally AC, DC and PMDC electric motors are used in hybrid vehicle depending upon the requirement and usage. In automotive industry, certain motor specifications are not acceptable due to their inability to meet the specific requirements such as power rating at peak load conditions, continuous power supply, volumetric power density, cost, efficiency, weight, lifetime, level of protection against water and dust.

Gear box assembly: In hybrid vehicles, mostly a power splitter device is used which is consists of planetary gear set allowing power flow from two power generating sources to propel the vehicle. The I.C engine is generally connected to Sun gear while electric motor is connected to the planet gear.

Clutch: It is found in every vehicle with manual gearbox. Though the use of clutch varies from one hybrid drive train to another depending the gearbox being used as most of the hybrid power trains use automatic transmission which don't require clutch. The main purpose of clutch is to assist the vehicle starting off from stand still position as it compensates for the speed difference between engine and the driveline and slips on to synchronize both. Clutch also used to engage and disengages engine from the transmission while gear shifting takes place.

Modelling Approach

Procedure of work

Power train jerking in hybrid vehicles is majorly due to engine & electric motor torque changes during tip in and tips out operations. For investigating the problem of jerking for a hybrid vehicle, certain steps have been laid out for modeling the power train in five different steps which are in below figure 9 to observe and analyze each case individually with root cause:

Figure : Modelling Procedure

Basic Driveline Equations

Engine: The engine output torque is expressed by the driving torque (Te) generated from combustion , friction torque (Tfric,e) and the external load from clutch (Tc) .By Newton's second law of motion, we can achieve the following model:

Where Je is the mass of inertia of the engine, is the crank angular acceleration.

Clutch: Generally a friction type clutch is installed in vehicles with manual transmission connecting to the engine flywheel and input shaft of the transmission. During clutch engagement if we assume no internal friction, Tc = Tt can be obtained. Then the torque transmitted is function of angular difference (αcs - αc) and angular velocity difference ( ) resulting in:

Figure : vehicle driveline with corresponding angle and torque labels.

Transmission: A transmission consists of a gear set each with a different operating ratio it leading to relation between input and output torque

Where Tp is the transmission output and Tfric is the internal friction torque of the transmission.

Propeller shaft: The shaft which connects the transmissions output to the final drive unit in which no friction is assumed which gives us Tp = Tf , resulting in model for torque input to final drive

Final Drive: final drive can be considered as a final ratio if as for the transmission leading to input and output torque relation

Drive Shafts: The shafts which connect wheels to the final drive are called drive shafts. It can be assumed that both the wheels are rotating at same speed ( ). Ignoring the vehicle dynamics, the wheel speed shall be equal to speed of vehicle body's centre of gravity.

Therefore, drive shafts can be modelled as one shaft.

Wheels: The forces acting on a vehicle with mass (m) and velocity (v) the longitudinal forces (FL)acting on the vehicle gives

Rolling resistance (Rr) = f*m*g and Aerodynamic drag (Ra) =1/2*Af*Cd*ρ*V2.

Where, m=mass of the vehicle, g=gravity constant, f=0.015(co-efficient of rolling resistance), Rr=rolling resistance, Ra=aerodynamic drag, Af=cross sectional area, Cd=coefficient of drag, density (ρ), V=velocity.

The drive line model can be expressed as a system consisting of rotating lumped inertias, compliances, damping losses, input torque and loads acting due to environmental resistive forces. A simple 2 degree of freedom model (fig - 10) is sufficient to show the first torsional mode of vibrations which lead to jerking of vehicle.

Figure : Free body diagram of a conventional driveline

Where, Teng = engine torque, road load = Tload , e = engine speed, v = vehicle speed, Je = mass moment of inertia, be = viscous friction, r = final drive ratio, Ks & CS = drive shafts flexibility, Cw = damper, vehicle inertia = Jv.

Hybrid Power train Model

As discussed earlier in report, the hybrid power train consists of additional components compared to conventional type which include electric motor, batteries, power electronics which form together the electric drive unit of the vehicle.

Engine: Engine in hybrid power train plays a very important role in terms of fuel savings and vehicle assist. An engine model to be used for hybrid power train must be considered for following issues:

The engine model must produce desired torque output irrespective of operating conditions.

The engine's dynamic behaviour leading to torque fluctuations generates drive line vibrations which in turn affects the driver comfort.

As engine is not quick enough to respond to control actions implemented externally, the response delay can have significant affect on the vehicles performance.

Engine can be modeled in many different ways depending upon the level of complexity required and the availability of parameters which can later be used for validating the inputs. The basic equation of engine model has been already discussed earlier which can be applied for the hybrid power train model also.

Electric Motor/ Controller: The electric traction motor and controller consume the power from the onboard energy storage device like battery to provide the power source so as to generate the required torque for vehicle propulsion. The electric motor can also be used as starter/alternator or as a generator to recover energy during braking which can be used to charge batteries. The traction motor model can be described by following equations:

Where idr , iqr are d,q axis rotor current respectively; ids ,iqs are d, q axis primary current respectively; Lm, is mutual inductance; Lr , LJ are resolved rotor, stator inductance respectively; P are poles; Rr, Rs are resolved rotor, stator resistance respectively, ohms; Vds Vqs are d, q axis primary voltage respectively, V; λdr and λqr are d, q axis rotor equivalent flux respectively, V-sec; λds and λqs are d,q axis stator equivalent flux respectively, V-sec; ωe is synchronous frequency, rps ; ωr is rotor frequency, rps;& , is rotor acceleration, rps2; Ts is electric motor torque, Nm.

The limiting torque of the motor can be expressed by:

Where ωb, is motor base speed, rpm; ωm is mechanical motor speed, rpm; Prated is the rater motor power, hp; Trated is the rated motor torque, Nm; Tm is mechanical motor torque, Nm.

In addition to this, equations of motion for DC electric machine are:

Where is the torque output of the machine proportional to the armature current, e is the back emf of the machine is proportional to the velocity of the rotor

Drive Shaft: The drive shaft plays very important role in the drive train for transmitting the torque and rotation. They are subjected high torsion and shear stresses and therefore must have must have high stiffness. Below shown in the drive modelled using lumped inertias and their corresponding basic equations

TDS =KDS (θT - θW) + CDS (T - W)

Wheels: The forces acting on a vehicle with mass (m) and velocity (v) the longitudinal forces (Fe)acting on the vehicle gives

Stationary model of the vehicle assuming all forces act through the centre of gravity and only in longitudinal direction.

Gears: Assuming no losses in transmission the basic equation for gearing is

But in reality there are viscous losses, sealing & bearings drag contact friction. Losses vary with gear change but can be modelled as a loss torque tl acting on the transmission input shaft there can be shown as:

Capacitor: It is device which stores energy an can be modelled by current equation

Battery: Batteries are characterized by energy density, C-rating, cycle life, thermal run away and variations in temperature. Table below shows the comparison among different types of batteries with their technical specifications available in the market for HEV applications

Property

Lead Acid

NiMH

Lithium

Cell Voltage Volts

2

1.2

3.6

Energy Density WH/Kg

30-40

50-80

100-200

Power Density W/Kg

100-200

100-500

500-8000

Maximum Discharge Rate

6 -10C

15C

100C

Useful Capacity DOD%

50

50-80

80

Charge Efficiency %

60-80

70-90

~100

Self Discharge %/Month

3-4

30

2-10

Temperature Range °C

-40 +60

-30 +60

-40 +60

Cycle Life Cycles

600-900

>1000

>2000

Micro-cycle Tolerant

Deteriorates

Yes

Yes

Robust (Over/Under Voltage)

Yes

Yes

Needs BMS

Cost per kWH

£100

£170

£150 Target

A basic battery model might include an open circuit voltage and internal resistance Rc and Rd

SOC Calculation:

SOCic = initial SOC of the battery (assume 1; fully charged)

Ibat = battery current that can be both positive and negative

Qbat = battery capacity (Ah), needs converting to A.s for the simulation

Simulation Tool

Simulation X

Simulation X is a multi domain program for modeling and simulation created by ITI GmbH. Some of its main features and capabilities are:

Enables high level modelling platform for complex systems.

Integrated with CAE design tool

Over 30 standard industry specific libraries including automotive.

Development of user specific libraries based on standard ones.

Object oriented modelling language for simple and efficient modelling.

Implementation of user based C code through external sources.

Provides interface between various software available in market such as MATLAB/Simulink etc.

Analysis tools available in Simulation X are:

Transient Simulation: Capable of computing linear and non-linear models in the time domain.

Steady State Simulation: Analyzing the models in periodic steady state condition dependent on a specific reference value.

Linear System Analysis (Natural frequencies and mode shapes) : Efficiently generates damped and un damped natural frequencies of the complete system , time constants , eigen vectors, oscillations of state variables related to particular eigen frequencies.

Linear System Analysis (Input - Output Analysis): Enables linearization in the current operating point , analysis, export of state space matrices

Modelling in Simulation X

The vehicle type selected for the analysis of jerking and its effects is a sport utility vehicle (SUV) powered by a diesel engine shown earlier in figure 6 consisting a conventional diesel engine with crankshaft integrated starter generator (CISG), a DMF, a transmission gear box, a clutch between the transmission and the DMF, differential as a part of conventional power train and high voltage battery pack with power electronics on board as electric drive unit for the vehicle. Creating simulation models for hybrid power train requires various components of power train such as mechanics, hydraulics, electronics and pneumatics including the control aspects involved.