Micro vs Mild vs Full Hybrid & Regenerative Braking

Micro vs Mild vs Full Hybrid & Regenerative Braking

 

Micro vs. Mild vs. Full Hybrid

 

As we have already discussed on Micro, Mild and Full Hybrids last week, lets briefly go through the simplest definition of Micro, Mild and Full hybrid

 

Micro Hybrids are the vehicles wherein the electric motor doesn't add thrust to the combustion engine or where it doesn't provide power assistance to the combustion engine.

 

Mild Hybrids are the vehicles wherein the electric motor adds thrust to the combustion engine or provides power assistance to the combustion engine; however it doesn't allows purely electric driving.

 

Full Hybrids are the vehicles wherein the electric motor not only adds thrust to the combustion engine or does power assistance to the combustion engine but also allows purely electric driving.

 

We will see some pictures to visualize the features and differences between these systems.

 

          

Source: PWC automotive Institute & Internet

 

Let's put the differences in tabular form so you can remember easily.

 

	MICRO HYBRID	MILD HYBRID	FULL HYBRID
Start Stop	Yes	Yes	Yes
Power Assist	No	Yes	Yes
Electric Engine output	0kW	5-15kW	50-70kW
Electric drive	No	No	Yes
Regenerative Braking	No*	Yes	Yes
Engine Downsizing possible1	No	Yes	Yes
Fuel economy gains	Very Low (5-8%)	Low (10-18%)	Medium (20-30%)
Cost	Low	Medium	High

 

*Some text may say Regenerative braking can also be present in Micro Hybrids.

** Fuel gains are approximate and for gasoline hybrid vehicle; actuals will vary in diesel vehicles. Also it can differ with Hybridization factor and other factors.

 

¹Engine downsizing

This power assist ability reduces the demands on the gasoline engine, allowing for the use of a smaller, more efficient gasoline engine while maintaining the same performance as a vehicle with a larger engine. This engine "downsizing" may be achieved by using physically smaller engines with less cylinders or smaller displacements, or may be achieved using more efficient combustion cycles. 

 

Regenerative Braking system

 

Introduction

About half of the vehicular energy is consumed in the form of heat generated from braking in the urban driving cycle. If this part of energy can be recycled, fuel consumption will be greatly reduced. This reduction would significantly improve vehicle fuel economy and emissions performance. The traditional vehicle poses difficulties to achieve this function, while the mild hybrid and full Hybrid systems with greater power motors and large-capacity battery can convert extra kinetic energy into electrical energy for storage.

 

In hybrid vehicles (such as mild and full hybrid) the kinetic energy of the drive wheels can converted into electrical energy by the electric motor (which is operated as a generator for this purpose). In this way some of the energy which is normally lost as frictional heat during braking is fed in the form of electric energy to the battery and then utilized. At the same time the generator operation of the electric motor brings about a vehicle-braking effect. This process is known as regenerative or recuperative braking.

 

System with a regenerative braking system 

Source: Autoshop101.com

A typical regenerative braking system is shown in the fig above.

 

Braking Force from regenerative braking

In regenerative braking the braking may not be sufficient to provide the full required brake force so generally service brakes are combined with regenerative braking system to provide the required braking force. So in regenerative braking the vehicle is not only braked by the generator braking torque of the electric motor but also it may combines the service brake's friction torque depending on the brake force requirements.

 

The actual regenerative braking force is varied with desired braking deceleration and vehicle speed, and it is calculated based on an analysis of the required deceleration, maximum braking force of ISG, engine braking force and state of charge (SOC) of battery.

 

Distribution of braking force

A braking force distribution strategy is required to find the optimal distribution ratio of the regenerative braking torque and the friction braking torque so that the regenerated energy can be maximized.

 

Actual regenerative braking force of an ISG motor is achieved based on the relationship among vehicle braking efficiency, braking force of the electromotor and engine, and charging power of the battery during the regenerative braking process.

 

When the vehicle with regenerative braking brakes, the ISG motor runs into the generation model to recover kinetic energy under certain conditions. In the process of regenerative braking, the engine and ISG motor will synchronize their rotary speed, the driven engine will produce engine braking force and the driven motor will produce electricity while regenerating braking force. The braking force of the system, including friction braking force and regenerative braking force, is equal to the required braking force less the engine braking force. The braking force of the system will come from the regenerative brake completely if the regenerative braking force is bigger than the system braking force. On the contrary, if the regenerative braking force is smaller than the system braking force, the rest of the braking force will be provided by friction braking. The regenerative braking force is restricted by both the ISG motor efficiency and battery charge performance. To obtain the actual regenerative braking force of a mild HEV or full HEV brake system, it is necessary to research relationships among required braking force, engine braking force, ISG motor regenerative braking force, and battery charge efficiency.

 

Disclaimer

The article doesn't include any confidential information. Nor does it include any references from the sources which doesn't allow taking excerpts from articles. The images have been taken from internet not necessarily from direct source. The article is only for your reference; please do not forward without permission. 

 

Working of Start-Stop Systems

Working of Start-Stop Systems

Working of a start stop system:

In automobiles, a start-stop system automatically shuts down and restarts the internal combustion engine to reduce the amount of time the engine spends idling, thereby improving fuel economy and reducing emissions.

In a typical situation the driver releases the accelerator pedal, activates the brake paddle and the vehicle comes to a halt. The driver takes the car out of the gear. The Engine ECU checks the following:

1. Engine is idling and no gear is engaged
2. The wheel speed sensor shows a zero speed
3. Electronic battery sensor shows adequate battery charge for next start operation

When all these conditions are satisfied the engine will wait for some manufacturer specific debounce time and then switches off automatically. The starter pinion engages in the ring gear¹ preparing for the next start. This enables the engine to be started quickly

As soon as the clutch is actuated the starter receives the signal to restart the engine. The engine is started quickly and quietly and is immediately ready for operation again.

The conditions in which the system will go to stop mode may differ with various customers. The above is mentioned to give a general idea

Components of a Start-Stop System

1. Start/Stop sensor motor
2. ECU with Start/Stop functionality
3. Electronic Battery Sensor EBS
4. Cycle resistant battery
5. Pedal Equipment with Sensors
6. Generator

Generally in latest Micro-hybrid systems the separate alternator and starter motor are replaced with a combined motor/generator as it reduce the start time and the noise emission²

In the start/stop system a reinforced starter³ replaces the conventional starter. The start/stop system requires an adapted engine management system, which has additional interfaces to the starter and sensors. Since the start/stop system is an emission-relevant system, it must satisfy the requirements of OBD (onboard diagnosis), i.e., it must be monitored in driving mode and exhaust-gas-relevant faults must be stored in the ECU's fault memory.

Because of the many starting processes it has to manage, the battery must be cycle-proof. It is monitored by a battery sensor, which checks the battery state of charge and signals this to the engine ECU before the internal-combustion engine is automatically switched off. Ancillaries such as the A/C compressor, which are normally driven via the internal-combustion engine and are also required during the standstill phases, must be electrically driven or replaced by other solutions. This also applies to the mild hybrid and the full hybrid, in which the start/stop function can be realized by means of the electric motor.

Notes:

¹Ring gear is a medium carbon steel ring with teeth, it transfer torque from the starter motor pinion to the flywheel to rotate the engine to begin the cycle.

The process of engaging starter pinion during the stop is called as pre-engaging.

²Noise emission is amount of noise emitted from the vehicle. Nosie emission is generally high during starting. Noise emission is one of the major criteria except start time while designing start stop systems.

³Reinforced starter is different from conventional starters to work it with start stop system. In general it has certain differences from conventional starter for better starting time, lesser noise and higher lifetime. Some of the differences in a reinforced starter are

- Electric Motor with Increased size and Performance

- Reduced Gear Ratio

- increased number of teeth in starter pinion

Questions & Answers

What will happen if the battery gets discharged due to battery loads during a long stop?

The behavior of the system is different for various customers. In some cases the engine may start automatically without drivers input. In other cases the system may give indication to driver to start engine by a battery indication lamp.

Both systems have their own advantages and disadvantages. In one hand, if the vehicle starts automatically then the driver may panic while on the other hand if the driver doesn't start after the indication the system may have problems during next start due to low battery.

I want to implement start stop in my car. Is it possible?

Yes, it is possible. But it maybe quite cumbersome, complex and expensive for customers to retrofit the start stop system

For implementing a simple start-stop system you need to make few changes in your car.

E.g. the starter needs to be replaced by an improved starter which can handle more cycles.

You also need few additional sensors like Electronic battery sensor, neutral gear sensor and clutch paddle sensor. Also when you are implementing start stop system you will need electronic circuitry to control Start & Stop of engine. The handling is performed by the engine ECU if the start stop is fitted by OEM. But if your ECU doesn't support it then it can be implemented via an Electronic Control Unit with a small 8-bit microcontroller which communicates with your Engine ECU.

Will start stop system affect my engine performance?

Maybe Yes*, (see disclaimer)

The lubrication systems of internal combustion engines are inherently least effective immediately after the engine starts; Majority of the engine wear occurs at startup, hence the frequent starting and stopping of such systems reduce the lifespan of the engine considerably. Also, the frequent start and stop cycles may reduce the engine's ability to operate at its optimum temperature, thus reducing the engine's efficiency. 

Disclaimer:

I have received information from various reliable websites and system experts. But the questions and answers are based on hypothesis.

*Though I found this information under heading hybrid electric drive, I believe the same applies to normal start stop system. None of the manufacturer mentions it. I consulted with few start stop system experts and they too agree on that there will be some affect on performance. The affect may not be high for a well designed and properly lubricated system. 

Basics of Start-Stop system and Micro Hybrid

Basics of Start-Stop system and Micro Hybrid

 

Start-Stop System

 

The Start-stop system is based on an intelligent combination of engine, brake, and battery management, which stops the internal-combustion engine when the vehicle is at a standstill in a traffic jam or at a red light. As soon as the driver presses the clutch pedal, the system automatically restarts the engine. The electronics ensure that the driver can save fuel and cut CO₂ emissions without compromising on convenience.

 

The start-stop system is also known as Stop-Start or Idle Stop and Go system. This feature is present in hybrid electric vehicles, but has also appeared in vehicles which lack a hybrid electric powertrain. For non-electric vehicles fuel economy gains from this technology are typically in the range of 5 to 10 percent.

 

How Start-Stop System Works:

 

Note: The working of Start-stop System will be handled in the next part of the article. It requires basic understanding of automotive systems.

 

Systems with Start-Stop

 

1.) Micro Hybrid

 

A Micro Hybrid is a conventional fossil-fuel vehicles (diesel/gasoline etc.) with Start-stop system.

 

The Micro hybrids are fitted with an Internal Combustion Engine (ICE) and a large electric machine. These vehicles allow the engine to be turned off whenever the car is coasting, braking, or stopped, yet restart quickly when the clutch or accelerator is pressed. The basic propulsion is via the ICE but the electric machine will help start the vehicle.

 

A Micro Hybrid system the electric motor doesn't add thrust to the combustion engine. In other words it can not power assist to the ICE.

 

 

2.) Mild Hybrids

 

In a Mild hybrid the internal-combustion engine is assisted by an electric motor, which delivers additional drive power and braking power in different operating states. There is a small difference between Micro and Mild Hybrids. In mild Hybrids the electric motor adds thrust to the combustion engine or does power assistance to the combustion engine. However the electric motor cannot turn the drive wheels by itself. The fuel advantage for mild hybrid is slightly better then Micro Hybrid. They also implement regenerative braking to generate power.

 

3.) Full Hybrid

 

In a full hybrid internal combustion engine is combined with one (or two) electric motor(s). In additional to running on the internal-combustion engine and being assisted by the electric motor, it also allows for purely electric driving.

 

Note:

- A Hybrid vehicle is a vehicle that uses two or more distinct power sources to move the vehicle. The term most commonly refers to hybrid electric vehicles, which combine an internal combustion engine and one or more electric motors. In this article hybrid vehicles term refers to hybrid electric vehicles (HEVs) unless otherwise specified.

- There are more variants in Hybrid Electric vehicle wherein Start-Stop system is used which is out of scope for this article.

 

 

Advantages of Micro Hybrid as compared to Conventional system

 

- The start-stop system reduces the amount of time the engine spends idling, thereby improving fuel economy and reducing emissions.

- Fuel economy gains from this technology are typically in the range of 5 to 10 percent

- The CO₂ reduction is almost the same as the gain from Fuel Economy.

- Implementation cost is not very high (generally in range $300-$400)

- In combination with Modern Spark ignition engine, a warm start uses as much fuel as is required for 0.7 seconds of idling. So every stop more then 0.7 seconds pays off.

- Generally it implements regenerative braking which improves the energy efficiency of vehicle.

 

Disadvantage of Micro Hybrid System

 

-         Fuel Saving is not as good as Mild or full Hybrid

-         Some vehicle functions may not run when engine is Off (Air conditioner etc)

-         Even though the implementation is cheap, the vehicle manufacturer may charge huge amount for vehicles with Start-Stop systems

 

Regenerative Braking

 

In a traditional braking system, brake pads produce friction with the brake rotors to slow or stop the vehicle. Additional friction is produced between the slowed wheels and the surface of the road. This friction turns the car's kinetic energy into heat.

In regenerative braking system, the system that drives the vehicle does the majority of the braking. When the driver steps on the brake pedal these types of brakes put the vehicle's electric motor into reverse mode (generator mode), producing electricity that is then fed into the vehicle's batteries

 

Vehicles using regenerative brakes also have a hydraulics braking system, the vehicle's electronics must decide which braking system is appropriate at which time.

 

Automobiles with Micro Hybrid System

 

Some of the earliest vehicles to use this technology were the Volkswagen Polo "Formel E" and Fiat Regata "ES" models of the 1980s. These early implementations could not get much commercial success due to their pricing.

 

Toyota has been selling cars with start-stop system for several years in Japan. Both Toyota and Mazda will be introducing stop-start technology available also outside of Japan in some of their 2009 model year vehicles, and Renault will introduce the technology in all of its European models by 2010.

 

Fiat introduced the Robert Bosch GmbH made system in the end of 2008 in Fiat 500, while Volvo introduced it in 2009 in its DRIVe models

 

Micro hybrid in India

 

Mahindra & Mahindra introduced the stop start based Micro Hybrid system to Indian Automotive market. This involved home grown technology for vehicles with mechanical fuel injection and later JV with BOSCH for common rail based vehicles. It is known as "Fuel Smart" and is implemented in its SUV's Mahindra Scorpio, Mahindra Bolero.

 

Tata Motors has later introduced this system on their LCV Tata Ace.

 

There are many others vehicles which will implement Start stop in near future.

 

 

 

Important Points to Remember:

 

1. The non-Electric vehicle with Start-stop systems is known as Micro Hybrid

2. In many texts Start-Stop system is referred as Micro hybrid. But in actual Start-Stop is a subsystem of Micro hybrid not the full system.

3. Many people have misconception that there is no difference between Micro Hybrid and Mild Hybrid but that's not true.

4. The driver can deactivate the Start-Stop system at any time, and vehicle will act as normal ICE vehicle.

 

 

Further Reading

 

I will add more details on below topics in my future articles:

- Detailed Working of Start-stop System in non Electric Vehicle (Micro Hybrid)

- Difference in Micro and Mild Hybrid such as Electrical Power difference and the features.

- Component of Micro Hybrid System

- Regenerative Braking

- Hybrid Electric Vehicle and electric drive