Why Are Some Electric Vehicles More Efficient Than Others?

posted in: White Paper | 0

One of the key performance indicators for an electric vehicle is the achievable range on a given amount of battery capacity. The battery is the single most expensive component in the electric vehicle, representing around 60% of the total materials cost. Even at the much talked about target of $100 per kWh cell cost, which translates to about $150 per kWh, the finished pack cost of a 100 kWh battery will be $15,000.

Generally speaking, electric vehicle users don’t really care how big the battery is, what they care about is how far it will get them, getting the most range we can from the smallest possible battery capacity is the goal, as this will deliver the most competitive overall EV package. It is this challenge which brings up the problem of efficiency, as improving efficiency by just 10% could take $1,500 out of the costs of the car, which in automotive margin terms is huge. 

We have been asked now by several listeners to our podcast; “What are the main reasons for the differences in efficiency?” Some have asked; “why is there a difference in the efficiency published between Europe and the USA” and others have been interested to know; “Why there is such a large variation between seemingly similar cars?” In other words, why does a Hyundai Ioniq have a published efficiency of 11.5kWh/100km whereas the similar size Nissan Leaf has an efficiency of 15kWh/100km, meaning the Ioniq uses over 20% less electricity per 100km than the Leaf, therefore, in theory, could have a 20% smaller battery pack but deliver the same range?

 

Nissan LEAF Battery Pack

All electric passenger cars have a published efficiency figure, typically this is expressed in kWh per 100km. This represents the amount of energy used to travel 100km, in other words, with an efficiency of 20kWh/100km you will be able to travel 500km on a 100kWh battery pack. In Europe and the USA, the published figures are obtained by subjecting the car to a standardised driving test cycle. This is the reason there will be some differences between the published number and what you achieve in real life. The test cycles currently used differ between Europe and the USA. In the USA, the EPA test cycle is used, whereas in Europe until recently the relatively gentle NEDC cycle was in use, this is the same official test cycle as was being used to test the emissions and fuel consumption of internal combustion engines. This has now moved to the new and more demanding WLTP test cycles which also include the conversion efficiency of the charger, meaning values, while still different, are now closer together.

In the future, RDE or real driving conditions will be used on EVs as well as ICE vehicles which will close the gap between published figures and real-world performance. But for now, it’s worth bearing in mind that just like your petrol or diesel, achieving the same efficiency as the published figures requires careful driving.

Achieving a good result in these tests is a big deal for manufacturers and people are beginning to realise the extent of the work and optimisation required to deliver an optimal result.

There are five principle sources of efficiency gains in the electric vehicle:

  1. Battery and charging
  2. Traction motor and power electronics
  3. Ancillary systems
  4. Aerodynamics
  5. Vehicle weight

Battery and Charging

Every testing cycle takes into account the efficiency of the charging process. This will include the efficiency of the on-board battery charger and also the batteries. The on-board battery charger in an EV converts mains AC electricity into DC to charge up the battery pack, it must be able to precisely control its output DC voltage and DC current to allow for effective charging of the lithium battery pack. It must also have large filters that ensure no unwanted electrical noise gets back into the local electrical grid. This involves several conversion processes inside the battery charger. Firstly the AC is converted into DC, typically this is done using a PFC boost converter and then there is a secondary DC/DC converter this topology allows the output voltage and current to be controlled.

These converters use high voltage switching devices such as IGBTs, along with diodes, inductors, capacitors, and a transformer. All these devices have losses associated with their operation, as high-frequency operation reduces loses in inductors and the transformers but increases switching loses in the IGBT’s. So there are design tradeoffs and considerations to make.

When charging the battery we are passing current to the battery pack, during this, there are internal ohmic losses both in the battery pack due to the cell internal resistance and also in the cables and busbars used to transmit the power. Optimising the layout of the pack and charger to minimise these losses can deliver some large gains.

Traction Motor and Power Electronics

When driving the EV, the stored DC electricity in the battery is converted into AC to drive the electric traction motor. This is done with a device called an inverter, which again uses switching devices such as IGBT’s to create a sinusoidal AC electrical output with voltage and current control. This AC voltage creates torque in the motor, which drives the wheels. So the efficiency of the inverter at creating this AC output and the traction motor in converting this AC electrical input into a torque output are critical, as is the interaction between the inverter and the motor. It is possible to design a highly efficient motor that causes large losses in the inverter.

As well as the ability to efficiently accelerate the vehicle, the ability of the motor and inverter to efficiently slow the vehicle down and convert this movement back to energy is also critical. Some of this efficiency is related to the function of the devices but also particularly from a layout point of view. If the EV is rear wheel drive only it is likely that regenerative braking will be compromised by trade-offs for vehicle dynamics – having strong regen braking effect on the rear wheels only can have a very negative effect on handling.

Ancillary Systems

All vehicles have a number of ancillary systems such as powertrain cooling, infotainment, lights, heating, ventilation and air conditioning that create an electrical load from the battery. These loads are typically referred to as parasitic loads, and can be responsible for a large proportion of the drive cycle energy consumption. Furthermore, an electric car or truck moving in slow traffic but in hot or cold weather could use as much power for HVAC as for traction. A poorly optimised thermal management system for the vehicle powertrain could have a high power consumption requiring as much as 15% of the energy required to propel the vehicle. There is also a feed in here from some other elements of the vehicle design, for example really efficient motors and inverters will need less cooling, giving a knock on benefit in reduced parasitic power consumption.

 

AVID Electric Vehicle Ancillary Systems

Some motors require special oil cooling systems for cooling and lubrication which whilst this results in a really efficient motor gives a much larger parasitic load on the vehicle. Some manufacturers are moving to heat pumps instead of conventional immersion heaters to provide warm air for the occupants and even simple things like automated control of the HVAC system to a set temperature rather than just turn to red for hot and blue for cold simple controls can make a big difference. There will also be an impact from the body design of the vehicle, thermal insulation is not needed in ICE vehicles due to the abundance of waste heat but in an EV properly insulating the vehicle to hold in heat when it’s cold outside and to limit solar gain when its sunny is really important.

Aerodynamics

We have talked previously, on one of our podcast episodes, about the potential for much more aerodynamically efficient trucks with electric powertrains compared to conventional engine vehicles, because of the greatly reduced powertrain thermal management requirements and the ability to change the cab position as a large ICE engine did not have to be worked around. It is a similar story in passenger cars and depending on the origins of the vehicle platform some vehicles have more potential than others.

From a styling point of view, for years now vehicles that resemble racing cars have been popular with aggressive large open grills and vents to the front profile, even having ‘fake’ grills and ducts on many road cars, in a racing car this is needed for cooling the engine, brakes and generating downforce but this comes at the expense of aerodynamic drag. In an EV if the thermal system is correctly designed and optimised there is a greatly reduced need for airflow through the heat exchangers at the front of the vehicle. There is also no combustion engine to work around at the front of the car, meaning a much more aerodynamic profile can be created that slips through the air more efficiently. In an EV that was originally designed to be an ICE the opportunity to modify the aerodynamics of the vehicle to improve efficiency may be more limited than in a dedicated EV platform depending on how much consideration was originally given.

Vehicle Weight

The final area surrounding efficiency is vehicle weight; in some ways, this is a combination of all of the other areas. Put simply, a heavy vehicle and battery require more energy to move around than a light one. Optimising all of the vehicle components to reduce powertrain weight and overall vehicle weight is critical and has some big pay-backs. This means thinking at a system level as to how components can be optimised for performance and weight. This is not only for the devices but also for the cables and interconnection systems, as good component layout optimisation and device integration can give weight savings approaching 25kg in the wiring harness of a mid-size EV.

The opportunity to reduce battery pack weight by having a more efficient overall powertrain system is significant. The opportunity to improve the weight of the battery pack itself is also significant, as there is currently up to 25% variation between the best and worst battery weights. There are some developments on the horizon that could halve this weight, which, with a typical EV passenger car battery pack weighing in at around 300 to 500kg is a huge saving in weight.

In summary, these five reasons are the most common factors surrounding the efficiency of electric vehicles, all five of these factors have a huge impact on a vehicle’s range, and therefore its usability. With more and more electric vehicles coming to the market every year, we will most definitely be seeing great advances in EV efficiency in the near future.

AVID Learning Electric Vehicle Technology Podcast Cover Art

Additional Information

To learn more about electric vehicle technology download and listen to the popular ‘AVID Learning: EV Technology’ podcast to hear more:

iTunes

Stitcher

Spotify

Youtube