Chapter 3 — Power generation
Cycles
Seillinger cycle
Theoretical comparison cycle
- Otto
For gasoline. - Angular velocity of motor increase with torque
- Higher power density
- Homogeneous mix of fuel and air
- Low mechanical load -> cheaper
- Needs ignition for combustion
- lambida from 0.8 to 1.2
- Diesel
- High compression rate
- Self ignition of fuel
- No throtle valve
- Low angular velocity of engine at high torque
- Low CO2 emissions
- High NOx emissions
- Heterogeneous mixture of air and fuel
- High mechanical load -> expensive
- lambida from 1.3 for full load
- lambida up to 10 for partial load
Fast running
passanger vehicles
Slow running
Ships
Throtle losses:
Due to high pressure of carburator we have restriction of airflow into the engine intake manifold.
Solution:
- Dynamic supercharging (eg swinging inletpipe)
- Mechanical supercharging (eg compressor charging)
- Exhaust-gas (eg turbocharging)
Efficiencies
Thermal
Volumetric
Charging
Effective
th x Q x mech
- th = thermal
- Q = quality grade
- mechanical efficiency
Q = ge x b
- ge = efficiency index (graph/graph)
- b = rate of complete combustion (fuel conversion factor)
Fuels
Diesel
Otto
Injection of fuel, compression of mixture -> uncontrolled self ignition due to overpressure. The self ignition creates pressure peaks creating a shock wave -> knocking/rattling. This can damage the material It's more obvious in winter & low temperature, high ignition delay
Process:Intake of air and injection of fuel. Homogeneous mixture of air and fuel. Spark plug burns/ignition
Knocking
If self-ignition happens before flame front -> rapid combustion happen with new higher temp and pressure -> knocking f(fuel, form of combustion chamber)
Counter measures:
ENGINE: short flame travel, charge air cooling in turbocharged engines, spark plug in hottest place, limit compression ratio
FUEL: high ignition delay with high alkenes or benzene ratio or anti-knocking solutions
Engine Power
Work
W = integral of pdv
Indexed engine power
Pi = pmi x Vh x na
pmi = area equivalent of the cycle, but in a rectangle shape (Pumping Mean Effective Pressure)
Vh = base of rectangle shape = TD - BD
n = number of working strokes
top dead, bottom dead center
measured bc it is a f(V)
Indexed efficiency
ni = Pi / heat power of fuel
= Pi/(Bh x Hu)
Bh absolut fuel consumption
Hu specific heat value of fuel
Effective power (power on the flywheel)
Pe = pme x Vh x na
pme = effective mean pressure = mechanical efficiency times pmi
measured with a engine test bench
Alternative combustion op. machines
target: reduce consumption of fossil fuels because of emissions and limited supplies:
- mineral oil ~ 40years
- Natural gas ~ 60 years
- coal ~ 200 years
Alcohol
- Engine: similar to gasoline with high compression or diesel with spark ignition
- Most relevant fuels:
-- methanol CH4O: 37% less CO2 if produced with natural gas, 100% if with coal
--ethanol C2H6O: production from biomass, low quantities
- PROS:
-- clean cumbustion,
-- better efficiency due to high compression
- CONS:
-- low heat value, which requires more fuel to have the same output power compared to gasoline and diesel, therefore the tanks are bigger and require more space
-- Aggressive agains metal, rubber and plastic
-- Poor cold start properties
-- Availability limited by time
parameters to track
Pi
Pe
Characteristic map
- be = f(pme, n)
- pme = f(n)
Exhaust emissions
Pollutants:
- CO (directly harmless, only in high concentrations)
- CxHy (Damages assimilation organs of plants)
- NOx (Co-responsible for forest decline)
- CO2 (directly harmless, but GH effect)
- O3 (Directly damaging for leafs and needles, increases permeability of cell membranes for acid rainfalls)
- SO2 (Co-responsible for forest decline)
- Soot (diesel only - In small amounts harmless.) carbon from incomplete combustion
note:CO2 emissions in europe 23% due to transport
Measures to decrease pollution:
Euro 1 to Euro 7
Measuring emissions and fuel consumption:
- Having a empirical or physical model
- WLTP - worldwide Harmonized Light duty Test Cycle (new method compared to NEDC)
-- longer cycle time (50% higher)
-- longer distance to travel
-- higher top speed
-- higher avg speed
-- more dynamic
-- less resting time
-- higher power demand
-- lower ambient temperature - RDE - real driving emission test
-- first vehicle need to be allowed to go on traffic
-- duration from 1:30 to 2:00h
-- road <700 meters above sea level
-- emissions higher than the tests on lab. by law it can only be 1.5xWLTC
-- if exceed this, manufacturer needs to pay for every gram of co2 that every car produced
- Engine measures
- Development conditions:
-- CO: local air deficiency
-- CmHn: air deficiency and/or low gas temperature
-- NOx: air surplus and high gas temperature (>1300ºC)
- Dependance on lambida
-- OTTO
------ y=0.95 (full load, Pe,max, high NOx)
------ y=1.15 (partial load, be,min)
------ y=1.12 (ignition threshhol)
--DIESEL
------ 1.2 to 1.4 (full load)
------ 7 to 10 (partial load)
- Best scenarios:
------ Partial load (CO & CmHn because O2 surplus and NOx because of low temperature) keeping y<1.2 for otto
------ Full load for diesel
Gasoline engine (OTTO) MEASURES:
- optimize mixture creation
- optimize ignition
- optimize ignition timing
- reducing NOx by high air surplus (trade-off with CmHn because y=1.2 is ignition threshold). To reduce this CmHn emissions we can use Stratified charge engine -> not having homogeneous mix anymore (like in diesel)
stratified charge engine, lean-burn engine, exhaust gas recirculation not enough. Exhaust gas after-treatment with three-way catalytic converter required!
DIESEL MEASURES:
- Cumbustion methods
-- pre or vortex chamber instead of direct injection
-- injection start later
-- high injection pressure - Exhaustion gas recirculation
-- reduction of NOx when precisely dosed quantity (up to 30%) of gas is recirculated because the 02 concentration decreases and also combustion temperature decreases because of higher thermal capacity of CO2 - To deal with soot
-- piston wall distribution of the injection
-- good turbulance in combustion chamber
-- high lambida
-- high injection pressure
at present, the engine-based measures are partially still sufficient to stay below emission limits. In future, after-treatment of NOx combined with oxidation catalyst and particle filter required!
After treatment
Target: CO, CmHn, NOx --> CO2, H2O, N2
complete conversion in two steps:
- oxidation (O2 and CmHn)
- reduction (NOx)
CATALYTIC CONVERTER:
Catalyst is active substance, which increases the reaction speed of chemical processes, without being consumed.
def. High temperature resistant steel casing with carrier, interlayer on surface of carrier, and catalyst material on interlayer
- carrier: metal, wound from corrugated steel sheet, or ceramic, extruded). Important is a large inner surface.
- interlayer to increase the size of inner surface
- catalyst: contact material, usually precious metals such as platinum and rhodium
-PROBLEM: Aging - degradation of the catalyst
Gasoline engine w/ catalytic converter
3-way catalytic converter that promotes oxidation of CO and CmHn, and reduction of NOx
- y=1 (stecheometric):, ~90% conversion
- y<1 (air defficiency): increase CO & CmHn
- y>1 increase NOx due to air surplus
Uses a closed loop control with feedback of y to ECU. Exhaust gas passes through catalyst before going into the atmosphere
- Optimal temperature 300-900ºC
- using y-control up to 90% reduction
- y sthechometricsal increases be up to 5%
Diesel engine w/ catalytic converter
Conventional catalytic converter only used as oxidation catalyst because the y is from 1.2 to 10
-- CO and CmHn oxidies but NOx do not reduce
-- Using NOx adsorber - stores NOx at working temperatures > 150ºC, don't work on startup with cold engine. It also increases the consumption up to 2%. The NO2 is trapped until it is full and than regenerated into N2 + H2O with diesel injection
SCR catalyst
-- working temperature > 200ºC problem at startup cold eng
-- reduces consumption up to 5%
AdBlue is sprayed in before the SCR catalyst, first step creates ammonia in the catalyst, second step amonia and NOx converted into H2O + N2.
SOOT is a problem even with the catalyst, but can be overcomed with filters.
Comparison of catalysts on DIESEL and on OTTO
diesel is only better on CO2 and be. It is way worse on NOx
rich vs lean
- lambida < 1 rich
- lambida > 1 lean (less air than fuel)
- lambida =1 100% oxidation & 100%
Rapeseed oil
- PROS:
-- emissions similar to diesel, but CO2 cycle with plant growth closed, no SO2 and less particles
-- required space and efficiency similar to diesel
- CONS:
-- required cultivation areas not available
-- 8x more expensive than diesel fuel
E-Fuels
Fuels made of regenerativ electric power (climate neutral). CO2 from the atmosphere is merged to molecular chains.
- PROS:
-- existing infra and combustion engines can be used
-- useful when battery eletric drive is not possible (planes, ships)
-- generation in the desert
- CONS:
-- energy intense production
-- 1L e-fuel Diesel would cost 4,5€
Hydrogen
- Engine: modified gasoline engine
- Fuel: Production from water (by electrolysis), combustion to H2O, with low quantities of NOx.
solution: water injection, lowers combustion temp. and therefore NOx. By production with solar energy: No CO2 in entire process!
- PROS:
-- no CO2 emissions
-- no toxic emission
- CONS:
-- storage
-------- gas: pressurized cylinders which is dangerous and heavy
-------- liquid: it has higher energy density than gas, but require insulation on the tank (cryogenic tank)
-------- bound in metal hydride (eg. titanium-iron)
-------- bound in form of methanol which makes it become liquid in ambient temperature, but in this case it has CO2 emissions
-------- bound in N- ethylcarbazole, it's fluid, stored without pressure, but it's toxic
-------- Absorbed in carbon nanotubes
LNG (liquified natural gas)
LNG operations are already being carried out in isolated cases in heavy commercial vehicles.
- Egine: diesel or otto engine, with otto no need of fuel, with diesel 10% of diesel fuel
- Fuel:
-- mixture of mainly methane (98%)
-- it's possible to use biogas or RE methane in form of LNG (bio-LNG)
- PROS:
-- clean cumbustion
-- lower fuel costs
-- drastic reduction on GHG emissions
- CONS:
-- larger tanks compared to diesel and gasoline
-- refueling cumbersome - difficult to refuel
-- fuel LNG heats up in the tank over time, increasing the pressure
-- poor infra
-- security aspects
-- CAPEX
Electric Motors
Pure-electric drive
Axle drives
The axle drive is the most common type of drive for the traction drive in all-electric vehicles. Only a single motor per axle is used for driving and the power is distributed through the powertrain to the wheels. Like in a classic vehicle a differential and drive shafts are required, but the transmission (almost) always can be omitted.
*- PROS:
smaller unsprung mass compared to wheel hub motor. Less environmental influences compared to wheel hub motor. Construction of hybrid driver easier. Classic and matured design of vehicle. Low technological control effort because of single engine. Entire weight of engine smaller than of multiple single ones. Compact contruction.
- CONS: drive train required. Single wheel drive not easily realizable. More space required compared to wheel hub drive. Friction losses through power transmission
Wheel hub drives
Motors integrated into the wheels, both with and without gears. Limited to special applications such as prototype vehicles, small urban vehicles and military applications.
E.g. michelin Active Wheel (contains gearbox, mechanical disc brake and active suspension in each wheel. Second motor integrated as electric damper). Schaeffler W-Wheel drive, Ziehl-Abegg ZAwheel, Fraunhofer e-concepts -> wheel hub without gearbox
- PROS: saves space, torque vectoring possible. No friction losses by transmission, diferencial etc. Extreme turning angles possible
- CONS: motors are exposed to heavy environmental influence. Can be damaged by minor accidents and cause high repair costs. Higher unsprung mass compared to other applicants. Relatively low power density since the engine speed is relatively low. More expensive and heavier than a high-speed central engine. Higher costs due to the distribution of the drive power to two or four motors with a corresponding number of inverters. Higher technological control effort because of multiple motors. The integration of the mechanical brake is difficult in terms of design and leads to additional heat input and further reduction of the motor power output. In the event of a motor failure, yaw moments and thus dangerous driving conditions can occur
Parallel shaft drive
motor diameter limited for space reasons. Suitable for high-speed drives.
E.g. Tesla, Renault Zoe, BMW i3, VW ID.3 and many more
Coaxial drive
Due to the required hollow shaft, the applicable motor technology is limited. On the other hand, the space requirement is minimal. Only permanent magnet synchronous machines can be used here.
E.g. Honda (Fit EV, FCX Clarity) and the Audi e-tron
Angular drive
offers the maximum degrees of freedom in electrical design from the point of view of the electric motor
E.g. Porsche e-Ruf Greenster
Hybrid drive
A “hybrid vehicle” is a vehicle, where at least two energy converters and two energy storage systems (built in the vehicle) are present to move the vehicle
Structure: Electric drive with gasoline or diesel engine for greater distances.
Types of system
Parallel hybrid system
Operation optional or simultaneous
- The mechanical addition of both power sources is possible
- Manual gearbox needed
- closely modelled upon existing drive trains
- One electric motor needed
- Often also as a plug-in hybrid vehicle
- Implementation:
- microhybrid
- mild hybrid
- full hybrid
- Mounting positions:
- P0 Hybrid - Belt-driven starter-generator BSG
- P1 Hybrid - Transmission integrated starter-generator
- P2 & P3 Hybrids - Integral part off transmission
Serial hybrid system
Operation with motor
- No mechanical coupling between combustion engine and wheels.
- No need for manual gearbox
- Combustion engine only for generation of electricity
- Two high powered electric motors are needed
Plug-in electric vehicles
Serial-Parallel hybrid system
- Division of power between combustion engine and electric motor
- Summing transmission (Planet gears transmission) to:
-- Split the mechanical and electrical power
-- Combining the mechanical and electrical power
-- Stepless conversion of torque and rotational speed - Two high power electric motors needed (sometimes as mutual power supply)
Use cases
Full power
More power at hill and for overtaking
- The battery supplies additional energy for the system if needed
- Therefore additional power for electric drive
- By combining the gasoline engine and electric motor, we have an acceleration like a vehicle of the next higher class
Deceleration / Energy recovery
Recovering energy while braking
- Electric motor functions as generator while decelerating
- Kinetic energy of vehicle is converted into electric energy
- Storage in battery
Regular operation
Energy saving operation with gasoline engine as main power source
- Operation with gasoline engine at higher speeds
- Planetary gears as power switch:
-- Partially direct power to the wheels by gasoline engine
-- Partial operation of wheels over the path generator – control unit – electric motor - Therefore implementation of a stepless transmission.
Stand still
All drive elements are turned off
- Gasoline engine and electric motor as well as generator are all automatically turned off
- No energy loss while idling
- The gasoline engine keeps running (in some cases) to charge the battery when the battery is low on charge or the AC unit is running.
Slow to medium speed
Energy efficient operation with the electric drive
- No optimal efficiency for gasoline engine in the partial load range
- High efficiency of electric motor at low speeds
- Therefore operation with electric motor from the electric energy stored in the battery
- When the battery is low on charge, the gasoline engine kicks in and powers the generator which in turn produces electricity.
Regular operation / Battery charging
Charging of battery with spillover energy
- Optimal workload of gasoline engine
- Resulting power spillover is converted into electric power
- Storage in battery
Start
When starting the vehicle, the high torque of the electric motor at a slow speed is used
- Operation with electric motor
- Energy from battery
- Gasoline engine stays off
- Gasoline engine runs only until it is warmed up.
Motor types
DC with brushes
Using commutator and brushes. Not used for traction, only for accessories such as window control and windshield wipers
Alternating field machines
an alternating current flows through the windings. Due to the series connection of rotor and stator winding, both fields change their direction in the same direction, so that a uniform rotation of the rotor still results.
Rotary field machines
Stator and rotor field rotate with constant frequency. The fields have a constant amplitude. Due to magnetic attraction, the rotor follows the stator field, resulting in a rotary motion. Rotary field machines have high power density and good efficiency. With today's power electronics, these machines can be controlled very well, which is why this type of machine is nowadays exclusively installed in vehicles as a traction drive
- Permanent magnet synchronous
- Separately exited synchronous
- Asynchronous
- Switched reluctance machine
PM syncronous
In a synchronous machine, the rotor always follows the stator field. The speed of the rotor is therefore the same as that of the rotating field, i.e. the rotor and stator field run synchronously
- The higher the number of grooves, the smoother the torque (no ripples) because there are more sinosoids to sum up.
- Serial or parallel connection of the coils of each phase possible, depending on the available battery voltage.
- Small system voltages (48 V, 120 V)
-- Parallel connection preferred
-- Higher currents (for the same machine power) - High system voltages (400 V, 800 V)
-- Serial connection preferred
-- Lower currents (at same machine power) - 2 magnets in the rotor premagnetized in direction S → N
Even if the rotor follows the stator at the same speed, there is an angular offset depending on the load, the so-called pole wheel angle. In the unloaded state (no-load operation), the north pole of the rotor and the south pole of the stator field are exactly aligned with each other. If one increases the load on the machine, the increasing stator current generates a counter field, which leads to a rotation of the rotor relative to the stator field. The rotor thus follows the stator field with a load-dependent pole wheel angle.
- Pole wheel angle 0° to < 90° (motor operation, increases with load)
- Pole wheel angle 0° to < -90° (regenerative operation, increases with load - in direction to -90)
- Pole wheel angle = 90° or = -90°.(breakdown point of the motor, maximum power)
- Pole wheel angle < -90° or > 90°. (motor out off step, rotor can no longer follow the stator, strong torque pulsations which can damage the motor and the gearbox, must never occur.)
Separately excited synchronous machines (SESM)
The difference between a separately excited synchronous machine, also called electrically excited synchronous machine EESM, and a permanent magnet synchronous machine is the design of the rotor. The stators of both machine types do not differ. In the stator, as in the PMSM, a uniform rotating magnetic field is generated.
- Replacement of magnets by excitation coils
- Rotor field is generated by direct current (DC) in the excitation coils
- PROS:
- Exciter field can be adjusted independently of load and speed
- Thus, depending on the operating point, better efficiency than with PMSM possible
- Good field weakening capability → high speeds (wide speed range) possible
- CONS:
- Additional losses due to current in the rotor
- Thus, depending on the operating point, poorer efficiency than with PMSM is also possible
- Complicated transmission of electrical energy to the rotating rotor
Transverse flux machine
The transverse flux motor is actually an outer pole synchronous motor, but the rotating field spools are placed in peripheral direction. The magnetic flux proceeds in a meandering pattern through the stator and rotor. The drive power is generated like in every electromagnetic machine through the change of the energy density in the air gap (dW/dx).
Asynchronous machines (ASM)
The asynchronous machine also uses the same stator design as the PMSM. However, the rotor functions fundamentally differently from the PMSM or SESM.
- A large number of bars (yellow) made of a conductive material (usually aluminum or copper) are embedded in the laminated core of the rotor
- At the axial ends, the bars are connected to each other by a ring (see Fig. 3-79)
- Operating principle
- The rotating stator field induces a voltage in the rotor bars
- By connecting the bars at the ends, these voltages lead to currents
- The currents now generate their own magnetic field, the rotor field
- The rotor field has the same number of poles as the stator field, since it is generated by the stator field.
- The rotor is now rotated by the stator and rotor field due to the principle of magnetic attraction.
The voltage induction only works as long as the stator field and the rotor have different speeds. If the rotor turns slower, the rotor sees a changing stator field. Only then does the voltage induction work. If the rotor were rotating at the same speed as the stator field, an observer on the rotor would always see the same stator field. However, since no voltage is induced without a change in the field and thus no current can flow in the rotor, the rotor would lose its magnetic field and consenquentlu could not be pulled along further by the stator field. Therefore, the rotor must always turn a little slower than the stator fifeld. This speed difference is called slip s. It indicates the percentage of deviation of the rotor speed n from the speed of the stator field n1
- Idle speed - slip-s ~= 0
- With load - slip increases successively with load. Continuous operation with slip from 3 to 10%. Max torque (breakdown torque) s~=20%. A further increase in torque over the breakdown torque causes the machine to stop. However, this is prevented in controlled traction drives by the power electronics.
Since an asynchronous machine does not contain any magnets, no voltage is generated at open terminals when it is rotated from the outside. In order to operate the machine as a generator nevertheless, a rotary voltage must always be applied. Only then a field is also generated and a current can be driven back into the battery. Recuperation is therefore only possible with an external voltage source. Since this is always provided in the car by the battery, there are no disadvantages here compared with the other machine types.
Switched reluctance machine
The reluctance machines have different numbers of teeth on rotor and stator. The rotor consists of a sheet metal package that forms some kind of gear. The teeth of the stator are winded with spools and carry a current. Through the current, a cog of the rotor is pulled to a cog of the stator which carries the current. By switching certain teeth on and off, the rotor starts to turn.
Structural design
- Rotor
- Cylindrical, partially hollow design (weight reduction)
- Consists of layered sheets (0.2 to 1.0 mm thick) electrically insulated from each other
- Depending on the machine, provided with grooves for wire winding or pockets for magnets
- The distance between stator and rotor is called air gap (from 0.5 to 2mm)
- Bearing arrangement
- Usually fixed-loose bearing
- Fixed bearing on the output side (A-side)
- Floating bearing on the rear side (B-side)
- Sensor (position encoder) on the B-side, for detection of the angular position of the rotor by the power electronics
- Depending on requirements, shaft seals to protect against dirt or gearbox oil
Windings
- Stator
- Designed as a hollow cylinder (stator yoke with teeth)
- Consists of laminated, electrically insulated sheets (usually 0.2 to 1.0 mm thick)
- The winding of copper wire is located in the grooves
- Surrounded by a housing
- Housing takes over the cooling
- Air cooling
- Liquid cooling (mostly spiral cooling channels)
- Distributed windings
- The coil sides of the three phases are evenly distributed over the slots
- The windings of one coil exit at the end of the stack of laminations and reenter on the
opposite side - The coil width depends on the number of poles
- PROS:
- Magnetic field in air gap is sinusoidal
- Number of poles freely selectable = independent of the number of grooves
- CONS:
- Complex manufacturing → higher costs
- Very difficult to automate depending on the design
- Large winding heads (= a lot of installation space)
- Concentrated windings
- Each coil is wound around one tooth at a time
- Also called single tooth winding
- PROS:
- Compact winding head, space saving
- Simple construction
- Simple manufacturing
- Cost effective
- CONS
- Coil width = distance between two grooves
- For electromagnetic reasons, the coil width should correspond approximately to the pole
width- Larger machine diameter = more teeth
- More magnetic poles necessary
- Speed and thus power density decreases
- Magnetic field in the air gap is strongly distorted (= many harmonics)
- Harmonics on torque more pronounced
- Stronger noise excitation, machine becomes louder
Formed coil windings
In traction drives, so-called shaped coil windings have been used more and more frequently in recent years. These represent a compromise between distributed windings and single-tooth windings and attempt to combine the advantages of both types:
STRUCTURE: Winding consists of individual, preformed rectangular conductors. Preformed wire pieces are pushed into the grooves from one side. Bending of the wires on the other side of the stator. Welding (= interconnection to form coils). Between 2 and 8 conductors per groove. 4 variants: Hairpin, D-pin, I-pin and wave winding.
- PROS
- Manufacturing can be fully automated
- Same good electromagnetic properties as distributed windings
- CONS
- Only economical from higher quantities (costly tools for forming the wires)
- Low flexibility with regard to changing the number of windings
- Only practicable for mass production in automotive segment
- Separately excited synchronous machine
- No magnets
- Rotor with grooves for excitation winding
- Transmission of excitation current via slip rings
- Asynchronous machine
- Internal bars are inserted into the rotor by die casting and directly connected by shorting ring (especially with aluminum).
- Other possibility: hammer in copper bars and weld them with attached end rings (e.g. Tesla Motors)
- Permanent magnet synchronous machines
TYPE I
- Mostly used in starter generators and transmission-integrated machines
- Primarily used in hybrid vehicles
- Cost effective
- Plenty of space inside the rotor (for clutches, etc.)
TYPE V
- Application in powerful main drives
- Variants with multiple layers of magnets possible
TYPE U
- Similar to V-arrangement, with further magnet in the middle
- Same points as V
Wheel drives
With wheel drives, each wheel is driven by a separate electric motor. The motors are located inside the vehicle and act on the wheels via drive shafts. There is usually a gearbox between the motor and the transmission.
E.g. mercedes-benz SLS AMG E-cell
- PROS: lower unsprung mass compared to wheel hub motors. Best weight distribution in the vehicle possible. All-wheel drive easily possible when using four motors. Less environmental influences compared to wheel hub motors. Demand-actuated torque distribution (torque vectoring). High speed engines with high power density can be used.
- CONS: higher costs: 2 motors, power electronics and gearboxes per axle. Increased control effort. More space required than with the wheel hub motors. In the event of failure of one motor, yaw moments occur and thus dangerous driving conditions
- PROS:
- Compared to conventional vehicles: - reduced consumption
- low emissions or in parts emission free
- reducing noise levels
- increasing function comfort
- increasing drive power
- elimination or downsizing of single conventional components
- Compared to pure electric vehicles:
- existing infrastructure is sufficient
- always available through quick refueling
- unlimited range through quick refueling and existing dense gas station grid
- high drive power by combining combustion engine and electric motor
- only little rang reduction by use of air conditioning and heating
- CONS
- higher weight
- higher degree of complexity
- higher energy consumption during manufacturing process
- higher costs
Operating behavior of the three-phase machine (ASM, PMSM, SESM)
Basic speed range
- Constant magnetic flux range 𝜙
- Magnetic flux = ratio of applied voltage 𝑈 and rotational frequency 𝑓
- Constant 𝑘1 is given by the construction of the machine
(𝜙 = k1 x U/ƒ) - Torque is given by the product of flux and coil current
- k2 is given by the design of the machine (M = k2 x 𝜙 x I)
GOAL: keep magnetic flux constant
- Increase voltage proportional to speed
- Optimal magnetic utilization
- Torque is proportional to current
- Max. torque (at max. current) is constant over speed
- Power increases linearly
Field weakening range
- Power electronics generates AC voltage for the motor from the battery voltage
- Rotation frequency (=speed) is freely adjustable
- Amplitude of output voltage limited by max. battery voltage
- Speed limit is reached when the inverter must set the maximum available output voltage to keep the magnetic flux constant
- limit is called corner speed n[eck]
- Further increase in speed at constant voltage leads to:
- Decrease of magnetic flux inversely proportional to speed
- 𝜙 ~ 1/ƒ
- Drop of flux = flux weakening
- Torque also falls inversely proportional to speed
- M ~ 1/ƒ
- Power remains constant
- Field weakening range = constant power range
- Current limit given by inverter and not speed-dependent
- Current I[max] remains constant
Comparison with conventional drive train
- Base speed range (= constant torque) corresponds approximately to 1st gear.
- Drop in torque in the field weakening range (= from 2nd gear)
PROS
- Wide speed range with constant power
- Advantageous for the dimensioning of battery, cables, connectors and power electronics
- In contrast to the combustion engine, this characteristic is achieved without a manual
gearbox
Maximum motor voltage
- Peak motor voltage = battery voltage
- With empty battery: 10% to 20% lower voltage
- Peak speed is reached earlier
Continuous operation
- Maximum torque limited by permissible continuous current
- Machine is in thermal equilibrium
Power electronics control
- Works via frequency converter
- Direct current (DC) of the battery is converted into three-phase current (AC)
- Frequency and effective value of output voltage and current freely adjustable
Short operation
- Maximum torque limited by permissible maximum current of the inverter
- Use of the heat capacity of the machine, especially from the cold state for a few seconds possible
Batteries
Assessment criteria
- Power rating
- Storage duration
- Cycling
- Self discharge
- Energy density
- Power density
- Efficiency
- Response time
- Charge duration
- Cost
- Range
- Safety
- Toxicity
Requirements
Crash safety:
- Corrosion resistant, crash proof battery housing
- System to discharge reaction gases in case of malfunction
- Controlled discharge reaction of cell for the case of a destroyed separator (nail test)
Operation safety:
- Cell monitoring, automatic disable before crossing critical threshold value
- Thermo management (cold start properties)
- Safety against overcharging
- Cell balancing
Service safety:
- Cable labeling
- Contact protection (isolation, plug)
- Partition in partial batteries with connecting safety switches
Function
Electrons flux from anode (-) to cathode (+) making the ions of lithium to flow through the electrolyte towards the graphine
Fuel cells
Electrochemical energy converter, chemical energy is converted to electric energy in electrolytic cell.
Constant energy supply, energy carrier liquid or gas, also
- No discharge (like battery),
- No time intensive charge (like accumulator).
Fuel cell drives are a promising alternative drive especially for commercial vehicles (long range or heavy duty).
Structure
Example PEM – cell (Proton Exchange Membrane or Polymer Electrolyte Membrane)
Sandwich contruction:
- 1 - Electrolyte: 0,1 mm thick plastic membrane, proton permeable, located in center of cell. On both sides catalytic coating (Platinum).
- 2 - Electrodes: Graphite paper, on both sides of electrolytes, permeable to gas.
- 3 - Bipolar plates: Graphite, outer layer of cell. Channels milled in, on one side loaded with hydrogen, on other side with oxygen (air).
Function
Cell reaction -> 1H2 + O2 —> 2H2O
- H2: Ionized on platinum catalyst, protons develop (cores of hydrogen atoms)
and electrons.- Protons travel through PEM – layer to opposite side.
- Protons charge electrode on the oxygen side positively (electric plus pole)
- Electrons charge electrode on the hydrogen side negatively (electric minus pole).
- O2: O2 absorbs electrons from H2, the following appear
- Negatively charged oxygen ions,
- H2O: H2 protons and negatively charged O2-Ions bond to electric neutral H2O molecules.
- Consumer: Proton migration creates a difference in voltage between the electrodes. Pooling of cells to stacks
Fuel
- Hydrogen, production from H2O: through electrolysis, energy therefore from wind, solar or nuclear energy. Problems with refueling
- Hydrogen, Production from Methanol (CH4O) in reformer: Reformer can be placed in vehicle, Refueling (liquid at normal temperature) uncritical,Energy requirement for reformer from fuel cell. Problems: CH4O from fossil energy sources (natural gases) or biomass (closed cycle) or from hydrogen + CO2 (under high pressure and high temperature, CH4O is created as H2 carrier)
- Hydrogen, production from gasification of biomass (efficiency 69-78%). Good because there no toxic emissions and it has better efficiencies than current combustion engines
Sollar cell
Potential
- Sunshines⊥, skyclear, Irradiation 1000W/m2
- Efficiency of normal solar cells, depending on type: 17-25%
- Power output ~20%
- Daily driving distance around 4km and up to 24 with additional battery
Production of solar cell energy intensive, high "energy payback time"