Speakers Information- Controls, Measurement & Calibration Congress

Control strategy for Mild Hybrid Powertrain development program

Gaurav S. SadekarNew technology development, Tata Motors Ltd., Pimpri, India.

E-mail: gaurav.sadekar@tatamotors.com | Tel: +91-20-66135711

Karishma S. Patil, Chandrakant M. AwateNew technology development, Tata Motors Ltd., Pimpri, India.

ABSTRACT

The Mild hybrid powertrain emerges as a viable beginning for advanced energy management to improve fuel efficiency, performance, comfort and waste energy recovery by adding Electric machine and specially designed battery to conventional powertrain system. It controls Electric machine to ‘intelligently generate’ by dynamically loading and operating the engine at maximum efficiency region of BSFC curve. It commands Electric Machine to harness waste energy while braking and coasting down. It provides extra torque during acceleration in the form of ‘Torque boost and Passive Assist’ while ‘Stop-start’ provides comfort and fuel economy.

INTRODUCTION

Emission from automobiles is responsible for about two third of air pollution in the urban area. The major pollutants emitted by motor vehicles, including CO, NOx, sulphur oxides, (SO), HC, lead (Pb) and suspended particulate matter (SPM), have damaging effects on both human health and ecology. Environmental regulations and Co2 legislations are happening at a pace to reduce this pollution. In order to achieve long term environmentally sustainable transportation solutions, Hybrid powertrain is one of the viable options.

The mild hybrid system consists primarily of modification to FEAD system when compared to conventional vehicle. An electric motor would replace the alternator and encompass all the functions that a mild hybrid vehicle defines. It will still be connected to engine via pulley belt, thus the system is called a belted starter generator (BSG).

The increase in fuel efficiency of the vehicle was aimed to be 10 percent. Given the weight and power class of the vehicle, selecting an electric motor system to effectively function in the vehicle was an important task. Electric machine has to provide sufficient starting torque and provide good motoring and generation capacity. This electric machine is similar to conventional alternator but it has better efficiency than conventional alternator and on top of it by controlling excitation current output current of it can be controlled as well.

This two way operation of electric machine causes slag on FEAD belt. Two way tensioner is used to automatically tighten the slag due to alternating motoring and generating operation of electric machine. Another important component of mild

Figure 1: Add-on components for mild hybrid powertrain

hybrid architecture is 12V Battery. These are specially designed batteries which can accept and draw more current compare to normal lead acid batteries. This battery is fitted with intelligent battery sensor which provides battery data for control strategy.

The control strategy for Mild hybrid vehicles demonstrates advanced energy management which supervises, coordinates and directs the vehicle to deliver performance and comfort to the driver. Based on electric machine operations, mild hybrid control strategy is mainly divided into below functions:

1. Stop-Start2. Torque boost3. Passive Assist4. Recuperation and Coasting Down5. Intelligent Generation and Alternator6. Idle/Neutral mode.

Figure 2: Hybrid Modes based upon vehicle velocity

Based upon vehicle speed and torque graph, Fig 2 shows overall representation of different hybrid functions.

The detail explanation of each function is as per below.

MILD HYBRID FUNCTIONS

STOP-START

Figure 3: Mild Hybrid System Architecture

This function is mainly used as a fuel saving function. It stops engine whenever it is required. For example – stop at traffic light signal or when vehicle is about to stop. When all required conditions for Stop are met, then Hybrid Control Unit (HCU) asks engine to cut off its fuel which makes engine to stop. This action avoids unnecessary burning of fuel and helps to improve the fuel economy and reduce emission. During engine stop, HCU applies more negative torque on electric machine that helps engine to die faster. This activity is called Quick Stop. Before engine Stop, HCU ensures that is no fault in electric machine otherwise Auto Stop do not happen. Once Auto Stop happens, HCU monitor Auto Stop event duration.

Once stop has happened and all required condition for engine start are fulfilled, then engine starts again by just pressing the clutch. To avoid unnecessary fuel burning, engine is stopped whenever not needed to run. During auto stop if driver wants to start the vehicle and he presses the clutch, this action triggers HCU to send Crank commend to electric machine and during Crank commend, electric machine applies peak torque to engine for short duration, this helps engine to have smooth start. This behavior is called Auto Start.

TORQUE BOOST

When driver demands more acceleration, electric machine helps engine by providing additional positive torque to wheel. Since there is direct torque supply from electric machine during acceleration condition, this function is called as Active Assist or Torque boost. Based upon Battery SOC level, engine rpm, vehicle speed and acceleration demand, boost levels are decided.

Figure 4: Energy flow during Engine auto start

Figure 5: Energy flow during Torque boost

PASSIVE ASSIST

When driver demands acceleration, electric machine helps engine by cutting off its load. Here, electric machine does not directly supply the torque, but it reduces its generation load from engine during acceleration condition, so this is like indirectly assisting engine during acceleration hence it is called as Passive Assist.

For example, in conventional vehicle alternator always add around 40 Nm load on engine. Assume that during passive assist that engine load becomes zero. So this is like indirectly helping vehicle to accelerate by some amount.

Based upon accelerator pedal position, vehicle velocity and engine speed condition Passive assist region will be decided.

RECUPEARTION AND COASTING DOWN

Both these functions are mainly used to charge the battery by using vehicle free energy.

Recuperation-

During driving, driver presses the brake pedal, this makes vehicle to slow down. During this braking event, HCU applies negative torque on the electric machine and used that free energy from vehicle inertia to charge the battery.

Coasting Down-

During driving, driver releases accelerator, brake and clutch pedal, and vehicle coast by itself based upon its inertia. During this event, HCU applies negative torque on the electric machine and uses this free energy of vehicle to charge the battery.

Recuperation or Coasting Down potential are determined, based upon below conditions

1) At high vehicle velocity, Recuperation or Coasting Down potential is more compare to low speed.

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Figure 8: Torque assist levels at different Battery SOC

2) At high engine rpm, Recuperation or Coasting down potential is more compare to low engine speed.3) At high battery SOC, Recuperation or Coasting down potential is less compare to low SOC.

Graphical representation of above condition is shown below.

NEUTRAL / IDLE

When battery is almost full and if recuperation event happens then we will not be able to charge the battery because there is no space available to put that free energy into the battery. Hence to use this free energy from recuperation to charge the battery, the battery has to discharge first or battery should not be completely full.

The intention of this function is ensure that battery is never 100% charge, so the free energy can be accommodate into the battery.

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Figure 10: Recuperation levels at different Battery SOC Figure 11: Recuperation levels at different Battery SOC

Figure 12: Energy flow during Neutral Mode

Figure 9: Energy flow during Recuperation and Coasting down

INTELLIGENT GENERATION AND ALTERNATOR

Intelligent Generation-

Intelligent generation is similar to normal alternator charging function but here based upon better BSFC point engine has moved to better BSFC region and at the same time equal and opposite negative torque is applied on electric machine to maintain same effective torque delivery on the wheel. This function makes engine to operate in efficient region which automatically helps to improve fuel economy numbers.

For instance, in fig 14, at 2000 rpm, engine is operating at 120NM with 190 g/kWh brake specific fuel consumption (BSFC). Now if we load the engine more, operating point gets shifted to more efficient region with lesser BSFC. Here BSG loads engine to its maximum level so that engine operates in more efficient region with 185g/kWh BSFC

Engine is loaded for 10Nm by BSG during Intelligent

generation caused engine operating point to shift from lower efficiency region to

BSFC is 190 g/kWh before regeneration

BSFC reduced to 185 g/kWh due to

intelligent regeneration

Figure 13: Shifting of engine operating point during intelligent generation showed in BSFC curve

In fig. 14, AVL simulation shows engine operating point percentage at different torque level and speeds. Fig. 14(a) shows plot before regeneration wherein engine generates at 115Nm for 4.93% of operating time. After implementing intelligent generation, engine regenerates at 125Nm for 4.72% of operating time. Thus it is shifting engine to more efficient region causing reduction in BSFC and increase in fuel efficiency.

Too much of calculation involved in this mode as machine loading is calculated from engine speed and BSFC curve. Hence look-up table approach is followed.

Alternator-

Alternator function is similar to conventional alternator. The only difference is that this electric machine has higher efficiency than conventional alternator and since we have control over excitation current which helps to produce more current at same speed by just controlling the excitation current.

Figure 14: (a) Engine operating points before intelligent Generation (b): Engine operating points during intelligent generation

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Figure 15: Generation levels at different Battery SOCFigure 16: Energy flow during Generation mode

CONCLUSION

Mild hybrid control strategy directs the vehicle to achieve most efficient modes while driving, braking and standstill conditions. Following figure shows the activation of different modes during NEDC.

Figure 17: Hybrid mode activation during NEDC: Red colour-boost and passive assist, Yellow colour- Generation, Green colour -recuperation and coasting down, Blue colour-Start-stop

Figure 18: Engine operating points (AVL results) during NEDC

As shown in above figure, engine operating points shift to lower side while boosting at lower speeds as electric motor assists engine to provide acceleration. This helps to reduce fuel consumption which in turn increases fuel efficiency. At higher speeds, engine is loaded to operate at higher torque while generating, thus operating point is shifted to higher efficiency region and harnessing the electric energy simultaneously. This control strategy helped to achieve 7% more fuel efficiency. The same control strategy improves the fuel efficiency by 11% during city drive.

REFERENCES

NEDC and MDC reports of conventional powertrain in Tata Indica, Tata motors Ltd.

CONTACT

Mr. Gaurav S. SadekarSr. Manager, New technology development, ERC, PVBU, Tata Motors Ltd., Pimpri, India.E-mail: gaurav.sadekar@tatamotors.com | Tel: +91-20-66135711

Ms. Karishma S. PatilAssistant Manager, New technology development, ERC, PVBU, Tata Motors Ltd., Pimpri, India.E-mail: ksp661932@tatamotors.com | Tel: +91-20-66135711

ABBREVIATIONS

CO- Carbon monoxideNOx- Nitrogen oxidesHC- HydrocarbonsFEAD- Front end accessory driveSOC- State of chargeHCU – Hybrid Control UnitEMS – Engine Management System