[DRAFT 1] Automotive Propulsion Systems: Theory and Efficiency Comparison

Running Head: Automotive Propulsion Systems: Theory and Efficiency Comparison

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Automotive Propulsion Systems: Theory and Efficiency Comparison Drew Prevost, 2014

This research paper is intended to educate the reader about the various types of automotive propulsion systems (especially hybrid-electric), including the components, operation, and advantages of each. The various systems will be compared on several different grounds, including efficiency limitations performance characteristics. The author assumes a primitive technical background.

Drew Prevost 11th Grade Hazel Green, Al 35750

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Table of Contents Introduction........................................................................................................................4 Major vehicle types............................................................................................................4 Internal combustion propulsion......................................................................................4 Electric propulsion.........................................................................................................5 Hybrid – electric propulsion...........................................................................................6 Series..........................................................................................................................7 Parallel........................................................................................................................8 Power split..................................................................................................................9 Efficiency comparison......................................................................................................10 Thermodynamic / electromechanical limitations.........................................................10 Battery / controller losses.............................................................................................11 Hybrid gains.................................................................................................................13 Series-hybrid............................................................................................................13 Parallel-hybrid..........................................................................................................14 Power-split hybrid....................................................................................................14 Torque distribution.......................................................................................................15 Conclusion........................................................................................................................16 Bibliography.....................................................................................................................16

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Illustration Index Illustration 1: Series-hybrid................................................................................................7 Illustration 2: Parallel-hybrid.............................................................................................8 Illustration 3: Power-split hybrid.......................................................................................9 Illustration 4: Combustion engine performance characteristics.......................................15 Illustration 5: Electric motor performance characteristics...............................................15

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Introduction Millions of people are impacted by the automotive industry everyday. In fact, according to the Bureau of Transportation Statistics, the average American drives over 12000 miles a year. Because of this fact, the type of car a person drives has a substantial impact on a person's life. Automotive manufactures are always striving to provide consumers with the most reliable, safe, comfortable, and fuel efficient vehicles. As a result, automotive technology can change our lives. Better engines mean lower shipping costs, more reliable “crush zones” keep us safe in the event of a collision, and better drivetrains keep fuel expenses down. Automotive technology is arguably one of the most important parts of today's society.

Major vehicle types Internal combustion propulsion Through the years, three major types of automotive propulsion technologies have been seen. The most common by far is the internal combustion engine. This device burns a mixture of chemical fuel (usually gasoline or diesel fuel) and air to cause a series of explosions timed to push several cylinders to turn a driveshaft. This shaft must be connected to a transmission before the wheels of a vehicle, because the engine has a limited range of speed (typically 1000 – 5000 RPM) in which it can operate. The engine also requires lubricating oil to be maintained, gaskets and fuel injectors to keep in working order, and exhaust to keep clean and quiet.

The speed of the engine is

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controlled by a technique called “drive by wire”. With this technique, the accelerator pedal is not directly connected to any part of the engine, (eg, carburetor) but instead sends a digital signal to a computer that will use the pedal position to determine the speed of the engine. Algorithms used to determine engine speed can drastically change the performance characteristics of a vehicle, and thus differ according to vehicle type. The accelerator pedal position is also used to determine the correct transmission gear. Electric propulsion The second major type of automotive propulsion technologies is the electric vehicle. This type of system uses batteries to store electrical energy, and an electric motor to drive the vehicle. The batteries are usually charged using grid power, although some solar or water power charging stations have been built. Energy flow from the batteries to the motor is regulated by a device called a motor controller. This device controls the amount of electric current that flows through the motor using several semiconductor switches based on the position of the accelerator pedal. This control method is known as “drive by wire”. [Laukkonen] The capabilities of the motor controller are often the limiting factor in the power of an electric vehicle. The same as the internal combustion vehicle, the motor cannot be connected directly to the wheels. The dynamics differ among the various motor types, but in general an electric motor has a usable speed range of 0 – 10000 RPM. This is substantially wider than the range of the internal combustion engine, but most importantly it does not have a lower limit (specifically series-wound

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DC motors – AC motors have a lower limit). As a result, no clutch is needed in an electric drivetrain. Also, because of the extremely wide speed range fewer gears are needed. Hybrid – electric propulsion A hybrid – electric propulsion system uses a combination of both the internal combustion engine and the electric drivetrain. Because of this, the hybrid – electric system is able to keep the benefits of both the internal combustion engine and electric drivetrain. For instance, an electric vehicle has a fixed range it can drive before it must recharge it's battery. An internal combustion vehicle does not have this problem, but it has the disadvantage of having very low efficiency and high emissions output.

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Series

Illustration 1: Series-hybrid

Above is a diagram of a series-hybrid system. [Perry] The series hybrid uses an internal combustion engine (ICE) connected to a generator that is used to charge the battery. The battery is then used to power the electric drive motor. Essentially, a series hybrid is an electric vehicle with an on board generator. An interesting note about the series hybrid is that the ICE is not mechanically connected to the wheels. This means that the seed of the vehicle is not dependent upon the speed of the engine.

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Parallel

Illustration 2: Parallel-hybrid

Above is a diagram of a parallel-hybrid. [Perry] In this hybrid topology, the electric motor and the ICE are mechanically connected to each other. This combination of electric / combustion power is then connected to the transmission and is used to drive the vehicle. This may seem like a “bad” idea, until one considers that some electric motors convert power bidirectionally. With applied current, they spin; when spun, they induce current. This means that the electric motor can “help” the ICE during times of high load, as well as store unused power from the ICE in the battery in times of light load. Because the ICE is directly connected to the transmission, the speed of the vehicle is dependent upon the speed of the engine.

Automotive Propulsion Systems: Theory and Efficiency Comparison

Power split

Illustration 3: Power-split hybrid

Above is a diagram of a power split hybrid. [Perry] This type of hybrid is very similar to the parallel hybrid, in that the ICE is connected to both the electric motor and the transmission. However, it differs in the addition of an extra gearbox, called the “power split”. The ICE, electric drive motor, as well as a generator are connected to the power split, and the output of the power split is in turn connected to the transmission. This topology allows for the control electronics of the vehicle to gather data on the current speed, acceleration, pedal position, torque required, incline, and other information. The most efficient power split ratio is then determined based on data and information about the ICE, drive motor, and generator. During times of high load, the control electronics will take into account the current drive conditions (speed, acceleration, etc) and power output characteristics of the ICE and electric drive motor, and adjust the power split

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accordingly to optimize efficiency. During times of light load, the ICE may be the only device powering the vehicle, and any extra power is stored in the battery via the generator. Some variants of the power split hybrid do not utilize a separate motor and generator, but instead use an electric motor suitable for both modes of operation.

Efficiency comparison Thermodynamic / electromechanical limitations The efficiency differences between an internal combustion engine and an electric motor are significant. When the fuel in an internal combustion engine burns, the stored chemical energy is converted to kinetic energy of moving particles expanding, as well as heat. When gasoline and diesel burn, only 46 percent of the original energy is converted into kinetic energy while the remaining 54 percent is converted to thermal energy (heat). Because only the kinetic energy of the expanding gas exerts force on the engine's cylinders, an internal combustion engine has a maximum efficiency of 46 percent. [Spakovszky] An electric motor, on the other hand, is not limited by thermodynamics. An electric motor operates by magnetism, thus the efficiency is limited by the geometrical layout of the motor and the motor type. Permanent magnet and wound field DC motors have an efficiency of 85 – 90 percent, while AC motors will commonly offer efficiencies of 90 – 95 percent. [The Engineering Toolbox] The reason an electric motor has a higher efficiency is due to Faraday’s law. The speed of an electric motor is proportional to the applied voltage, while the torque is proportional to the current. The

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effect of this is that for a constant voltage, as load increases on the motor, current will rise while the speed remains steady, limited by the maximum possible torque at that speed for that particular motor. The reason for this is that when a motor is spun, a reverse EMF is produced that opposes the applied voltage (Faraday's Law). Torque increases proportional to current because the magnetic field density around a conductor is a function of the current through that conductor. [Ward] In an electric motor, only the current necessary to provide the needed torque at any given speed is drawn, so the efficiency of an electric motor is very high. This is not the case with an internal combustion engine. An engine will draw a relatively constant amount of fuel over it's entire power range, even if it is only delivering a small percentage of that power. The overall effect of the phenomena described in this section is that when an internal combustion vehicle is driven, it has a maximum thermodynamic efficiency but will only reach that peak efficiency in an extremely small range of speeds, because it is drawing fuel at a relatively constant rate. Battery / controller losses When comparing the efficiencies of internal combustion engines and electric motors, the motor controller and battery pack efficiencies cannot be ignored. Motor controllers for both DC and AC motors both work on the principle of pulse width modulation. Pulse width modulation is a technique that varies the ratio of on to off times of a square wave of constant frequency, consequently varying the average voltage output.

A

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semiconductor device called a transistor is used to create this signal. A transistor is a kind of electronic “switch” with no moving parts. It takes an input signal, and turns on or off based on the state of that signal. Common transistor types for this application are MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and IGBT (Insulated Gate Bipolar Transistor).

[McGraw-Hill]

Transistors do not turn on and off

instantaneously; they take a short amount of time to change states (usually around 500nS in motor control applications). When they are fully on, a small amount of power is lost due to internal resistances and manufacturing imperfections. When they are fully off, no power is lost. When a transistor is between these two states, however, power loss is significant. The power loss during the time it takes a transistor to change states is called “switching losses”. [ARRL] Switching losses make up for a large portion of the power loss in a motor controller. [Millett] Other losses in the electric / hybrid vehicle have to do with the battery pack. Battery cells have an internal resistance of their own, which results in some power loss. For a constant resistance, power loss is proportional to the square of the current through the resistive component. During high current times of operation (eg, acceleration) this power loss may be significant enough to overheat the battery pack. Battery balancing methods also introduce power losses of the same nature. While switching losses and battery losses are a concern, the percentage lost is still comparatively small.

These losses are significant, because if appropriate cooling

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methods are not implemented, they could cause the battery pack or motor controller to overheat. Hybrid gains Series-hybrid The series – hybrid vehicle is powered first by a combustion engine, so it may seem that it would not have an increase in efficiency. However, this is not the case. While the hybrid system does in fact begin with chemical fuel, the use of that fuel is more efficient. As stated previously, the combustion engine only operates at maximum efficiency within a very small range of speeds. As a result, a combustion powered vehicle very seldomly operates at maximum efficiency. That is the fundamental reason that highway driving generally results in better fuel economy when compared to other driving conditions, because the engine is operating closer to it's most efficient speed for a greater period of time. During the series hybrid discussion, it was noted that in the series system the ICE is not mechanically connected to the transmission, but instead a generator. This permits the ICE / generator system to operate at it's peak efficiency all the time. Another way the series hybrid conserves energy is at low speeds or idle conditions. During these times, the ICE is operating at it's optimal speed, but is outputting much higher power than is needed. The energy not used to drive the vehicle is stored in the battery. An analogy would be a hydro-electric power dam. During nighttime hours when electricity usage is low, turbines are used to pump water up into a

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holding tank, using the excess power from a power plant. During daytime hours, the water is let out of the holding tank through the turbines to generate electricity and help supplement the power plant. This increases the overall efficiency of the power plant. The series hybrid vehicle operates on the same principle. The ICE is like a power plant, the battery is like a holding tank, and the drive motor is the fluctuating load. Because of this, the series hybrid is able to maintain maximum efficiency, even while stopped at a red light, because the chemical fuel is still being stored in a usable form instead of wasted. Parallel-hybrid The parallel hybrid operates very similar to the series hybrid. During times of high load, the electric motor “helps” the ICE by using energy in the battery. During times of light or no load, the electric motor stores extra power from the ICE in the battery for use later. The parallel hybrid allows for the same energy transfer as the series hybrid, but is less efficient because the ICE is not operating at it's maximum efficiency. Power-split hybrid The power split hybrid is the most complex of all the hybrid systems. It is also theoretically the most efficient of all propulsion types presented, because all directions of energy transfer are available to and from all devices in the system. The ICE can operate at it's optimal speed, and based on torque requirements the electric motor may supplement accordingly. All the while, any excess power may be stored in the battery.

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While this is impressive, power split hybrids are few and far between because of the complexity. Another limitation to the power split hybrid is size and weight issues. An automotive propulsion system must be able to fit in the vehicle in the first place. Torque distribution It is said that one electric horsepower is equivalent to two combustion horsepower. This is because of the torque distribution of an electric motor compared to that of a combustion engine.

Illustration 4: Combustion engine performance characteristics

Illustration 5: Electric motor performance characteristics

The graph to the left represents the torque of a combustion engine over speed, and the graph to the right represents the torque of a series wound stator DC electric motor over speed. We see that the torque peaks at extreme opposite ends of the speed axis for the different devices. A combustion engine has torque at a high RPM, while an electric motor has torque at a low RPM.

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Consider the dynamics of a vehicle accelerating from a stop to cruise speed. First, the vehicle will need a great amount of torque, but low speed. As the vehicle nears cruise speed, it requires less torque but more speed from the drive device. With this in mind, we see that a combustion engine performs exactly opposite from what a vehicle needs! It is the torque requirements that allows a lower power electric motor to be used in a vehicle for the same performance as a combustion engine twice its power.

Conclusion Automotive technology is a very rapidly expanding field. It impacts the lives of millions of people all over the world. With the development of new automotive technology, a myriad of advantages are brought about. Higher efficiency means less emissions and less oil use, as well as greater comfort and safety. Automotive society is arguably one of the most important parts of today's society.

Bibliography “DC Motors and Generators” Electro-Craft Corporation. DC Motors, Speed Controls, Servo Systems, Third Edition, Ch. 2 pp. 1-14 Electro-Craft Corporation, 1975. Print.

Automotive Propulsion Systems: Theory and Efficiency Comparison

“Power Semiconductors for Motor Drives” McGraw-Hill Handbooks. Motor Control Electronics Handbook, Ch. 13 pp. 435-484 McGraw-Hill Companies, 1998. Print.

“The Laws of the Electric Circuit” Robert P. Ward. Introduction to Electrical Engineering, Ch. 2 pp. 16-35 Prentice-Hall Inc, 1960. Print.

“Electrical Fundamentals” American Radio Relay League. The ARRL Handbook 2014, Ch. 2 pp. 2.1 – 2.71 American Radio Relay League, 2013. Print.

“Calculating Motor Driver Power Dissipation” Peter Millett. Texas Instruments, Motor Drive Business Unit, 2012 http://www.ti.com/lit/an/slva504/slva504.pdf

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“A General Scheme for Calculating Switching- and Conduction-Losses of Power Semiconductors in Numerical Circuit Simulations of Power Electronic Systems” Uwe DROFENIK and Johann W. KOLAR, Power Electronic Systems Laboratory (PES) http://www.pes.ee.ethz.ch/uploads/tx_ethpublications/drofenik_id356_IPEC05.pdf

“Hybrid Electric Vehicles” US. Department of Energy, Energy Efficiency and Renewable Energy, Autonomie, 2010 http://www.autonomie.net/references/hev_26d.html

“What are the different types of hybrid cars?” Alison Kim Perry, howstuffworks.com, 2010 http://auto.howstuffworks.com/different-types-of-hybrid-cars1.htm

“3.5 The Internal combustion engine (Otto Cycle)” Prof. Z. S. Spakovszky, Massachusetts Institute of Technology, 2008 http://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node25.html

Automotive Propulsion Systems: Theory and Efficiency Comparison

“Calculating Electric Motor Efficiency” The Engineering Toolbox, accessed 2014 http://www.engineeringtoolbox.com/electrical-motor-efficiency-d_655.html

“Module 9: DC Machines” Kharagpur, National Programme on Technology Enhanced Learning, accessed 2014 http://nptel.ac.in/courses/108105053/pdf/L-40(TB)(ET)%20((EE)NPTEL).pdf

“What is Drive-By-Wire Technology?” Jeremy Laukkonen, About Autos, accessed 2014 http://cartech.about.com/od/Safety/a/What-Is-Drive-By-Wire-Technology.htm

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