Pistons, valves and crankshafts do not apply to electric vehicles – welcome to terms such as battery packs, inverters and synchronous electric motors.
With the internal-combustion engine (ICE) more than 100 years old, it is fair to say that most car enthusiasts have a good idea of how the powertrain and related systems of a conventional vehicle with a petrol or diesel engine function. The same cannot be said about the new breed of electric vehicles (EV). In this article I will explain the basic components of an electric powertrain and related systems, including their functions.
1 ONBOARD BATTERY CHARGER
Most electric vehicles carry an onboard charger to cater for standard (3,3 kW) charging. The charger needs to convert the 220-240 V, 13 A alternating current (AC) supply of a typical European/South African wall socket to a direct current (DC) supply in order to charge the main battery pack. Although faster charging rates are possible, this is usually available only through an off-board charger that supplies the high DC to a special fast-charge socket in the vehicle.
2 DC-TO-DC CONVERTER
Legislation requires that occupants of EVs be protected from high-voltage sources due to the inherent safety concerns. Therefore, EVs also use 12 V systems in the cabin as on current ICE vehicles. In order to supply the cabin auxiliary equipment with 12 V power, the high voltage of the main battery pack needs to be reduced to 12 V. This is achieved by a direct-current-to-direct-current (DC-to-DC) converter. Another function of this unit is to charge the auxiliary battery.
3 AUXILIARY BATTERY
Yes, you will still find a 12 V lead-acid battery in an EV. The reason is that the main high-voltage battery is disconnected when the vehicle is switched off for safety reasons and to conserve motive energy. Therefore, to retain the cabin functionality that occupants are used to, as well as to keep the immobiliser and connected-car ability (see the December 2011 issue) active when parked, the 12 V power source is needed.
The inverter is tasked with converting the DC supply from the battery pack to a multi-phase AC supply (normally three-phase) as used by most of the electric motors currently found in EVs. It is achieved through high-frequency switching of insulated-gate-bipolar transistors (IGBT). These electric “switches” are capable of handling the high voltage and current supply needed to power an EV motor.
5 TRACTION MOTOR WITH CONTROLLER
The electric motor provides the torque moment to the drivetrain. Most passenger-oriented EVs employ a single motor that powers the wheels through a single-speed gearbox. As with different ICE layouts, there are also numerous types of electric machines of the brushless type capable of powering an EV, including:
• Brushless DC motor of the self-synchronous, variable-frequency-synchronous or permanent-magnet-synchronous type;
• Switched-reluctance motors;
• Induction motors.
Each type of motor consists of a rotor and stator element, and has its pros and cons regarding the performance, weight, control complexity and cost. For the motor controller to control the amount of torque delivered (powering the EV) and received (regenerative braking), it has to know the exact position of the rotor. This position can be measured by a Hall-effect sensor or a rotary encoder.
Although the efficiency of electric motors is in the 90 per cent region, some of the energy will be converted to heat. This heat needs to be removed by a cooling system not dissimilar to an ICE vehicle, but running at much lower temperatures (typically 40-60 degrees Celsius). Heat is easily removed from the stator but the rotor poses more of a challenge and limits the continuous performance of the unit. Most motor suppliers will supply peak and continuous-performance capability as a result.
The motor controller converts the torque demand into an actual shaft output. It also controls the regenerative-braking torque profile when kinetic energy is recovered and stored in the battery pack.
6 BATTERY PACK WITH CONTROL MODULE
The battery pack can be compared to the fuel tank in a conventional vehicle. It stores electricity as chemical potential energy until needed. Battery technology has come a long way in the last decade. Although nickel-metal-hydride (NiMH) battery technology is still used in some hybrid vehicles, the current focus is on the family of lithium-ion batteries for all EVs owing to their high specific energy. A battery pack consists of individual cells – usually of the pouch variety – that are grouped together. The cells are arranged in series to provide the necessary voltage (normally between 300-400 V) and in parallel to achieve the desired capacity. For EVs, the most important characteristics of the battery pack are:
SPECIFIC ENERGY – the amount of electrical energy stored for every kilogram of battery (Wh/kg );
ENERGY DENSITY – the amount of electrical energy stored per cubic metre of battery volume (W/m3 );
ENERGY EFFICIENCY – the ratio of energy supplied by the battery to the amount of energy needed to return the battery to its original, charged state;
SELF-DISCHARGE RATE – the loss of charge over time if the battery is not used;
LIFETIME – measured in number of charge and discharge cycles;
SAFETY – the resistance to safety concerns, such as thermal runaway;
COST – cost per energy capacity (R/Wh).
The battery-pack management system (BMS) controls the flow of electrons to and from the pack while monitoring the state of charge (SOC) and temperature of each cell. A typical lithiumpolymer cell (LiNiCoMnO2) has a nominal voltage of 3,7 V with lower and upper limits of around 3,0 and 4,1 V, respectively. The battery will be seriously damaged if those limits are exceeded during charging or discharging, and it’s the BMS’s responsibility to control the voltage range. During charging of the battery, the BMS has an important job to balance the cell voltages of the individual cells to ensure they are all charged to the same level. Battery-pack temperature severely influences the cells’ performance and the BMS is tasked to keep the pack in the operational range (typically 0 to 35 degrees Celsius). Air or water cooling/heating can be employed to control battery-pack temperature.
The high-torque output characteristics of electric motors at low speeds, combined with the high maximum-rotational-speed capability, make it possible to use only one fixed gear ratio to cover the vehicle speed range from standstill up to the national speed limit (and above). This helps to reduce the drivetrain complexity and negates the need for a clutch or reverse gear (motor can rotate in both directions).
Manufacturers are currently looking at multispeed transmissions for EVs, but with the focus on increasing the efficiency of the electric motor by running closer to the optimal speed and load region rather than achieving increased performance.
8 VEHICLE-CONTROL UNIT
The main vehicle-control unit (VCU) is responsible for controlling all the sub-systems of an EV. Some control responsibility might reside in either the motor controller or battery-management system, but the VCU still needs to be the information hub. Decisions are made by the software strategy in the VCU and calibrated by engineers to deliver the expected result. A secondary body-control unit (BCU) might control all the cabin, security and connected-car functionality, or these might be integrated in the VCU. The instrument cluster can either be a “dumb” display screen or another intelligent control unit.
(AC COMPRESSOR, WATER PUMP, VACUUM PUMP)
Auxiliary power sapping devices found on ICE vehicles are still needed in EVs to provide all the user functionality the driver expects. With the absence of available shaft power when the vehicle is stopped or excess heat, as is the case with an ICE, other solutions had to be found. Therefore, small electric motors are used to drive the water pump for the coolant system, vacuum pump for the brake system, AC compressor for the air-con and to provide power steering. The cabin heating is provided by a heat pump or positive temperature coefficient (PTC) heater.
One of the drawcards of EVs is low running costs. Electric vehicles will have extended service intervals (talks of 30 000 to 50 000 km), as there are very few serviceable items (no spark plugs, engine oil and filters, etc.). Even brake pads will wear at a much slower rate if regenerative braking is employed. This is also an economic concern for automakers due to the loss of aftermarket income. Let’s hope that OEMs won’t “engineer” service items to keep the money stream flowing.