EV Range and Fuel Efficiency

 

Lithium battery technology developed from the IT sector is the fundamental technical advance that has made EVs a practical transport option for Consumer today. Lithium batteries store 3 times more energy per kg than conventional Lead acid batteries and are considered capable of operating reliably for the expected service life of an average passenger vehicle. This allows manufacturers to make more regular sized electric vehicles with driving performance which can meet the expectations of modern day motorists.

In Ireland, 60% of round trip daily commutes to work are less than 30km (see figure 1 below). All of the EV types qualifying for the EV Grant Scheme must have a range of at least 100km following a single overnight charge.

Graph of Average Round trip daily distance travelled to work 2006 in ireland
Figure 1  Average Daily Distances Travelled to/from Work in Ireland (km)
 
 

Calculation of EV Efficiency, CO2 and Range – New European Driving Cycle

As part of the Certification process, the CO2 and Fuel Efficiency performance of all passenger vehicles (either Diesel, Petrol, Hybrd or EV) in Europe are assessed according to UN ECE 101 (see http://www.unece.org/trans/main/wp29/wp29regs/r101r2e.pdf for more info). Specified within this standard is a driving cycle which comprises 4 urban start-stop type cycles with one extra-urban cycle intended to represent typical driving patterns for passenger vehicles.  This combined cycle is also referred to as the New European Drive Cycle (NEDC). Figure 2 indicates the driving patterns involved. The overall distance of the test is 11km which reaches a peak speed of 50kph in the urban segment and a peak speed of 120kph on the extra-urban segment of the test.

The vehicle must have a minimum of 300km run-in driving completed before being acceptable for testing. The vehicle is left overnight and must be at room temperature before testing begins. No heating or air conditioning systems are active before or during the test. Vehicles are operated on this test cycle at an indoors test facility at fixed air temperatures on a dynamometer with track roller resistance set in order to simulate engine load at that air speed. When this test is completed, the fuel efficiency and CO2 figures (eg for a Petrol/Diesel/Hybrid or PHEV) are recorded and become the numbers provided to the Consumer on the Vehicle Information Label see the How Clean is Your Car section of our website for more detail on Labelling. 

The vehicle is fully charged before the test, then run through two complete NEDC cycles. The vehicle is fully charged again with the amount of electricity (Wh) supplied to achieve full charge being measured via the plug. This amount of electricity is then divided by the total distance travelled during the test and the “direct” fuel efficiency for the EV is measured as Watt-Hours/km (Wh/km).

All vehicle types (petrol/diesel or electric) are allowed to deviate slightly from the NEDC curve during the trial. For instance, no electric vehicles are eligible for the SEAI EV Grant Scheme if they are out of synchronisation with this curve by over 30seconds unless they are using regenerative breaking. 

The NEDC was originally intended for vehicles without regenerative breaking, therefore the operation of EVs with highly energy efficient regenerative breaking means that the EVs full efficiency may not be fairly reflected in the NEDC cycle. So although an EV can break at the same rate as a conventional car if required by the driver, in the case where regenerative breaks are used for maximum energy recovery, a reduced rate of deceleration may be more appropriate for the EV. Therefore an EV may vary from the above curve during deceleration portion only in order to reach maximum fuel efficiency. 

It must be noted that the NEDC is used for calculating fuel efficiency, CO2 and range only. Requirements which govern vehicle breaking safety, strength and stopping distance are dealt with under separate EU Regulations which all EVs must comply with.

The CO2 and Combined Fuel Efficiency figures are therefore calculated from this overall combined drive cycle. The Urban and Extra-Urban Fuel Efficiency figures are then separately derived from the Urban and Extra-Urban sections of the UN ECE 101/NEDC drive cycle.

Compare the Combined Fuel Efficiency figures for a list of Eligible EVs which have been approved for the EV Grant Scheme < link to list of Registered Vehicle types>.

The range of the EV is also determined by running it through continuous cycles of the NEDC. The vehicle is charged fully the night before, then operated repeatedly on the NEDC until the maximum speed which the vehicle can reach under full throttle falls below 50kph at which point the test is stopped and thekm distance travelled by the EV is reported as the maximum Range. Note that in real world applications this range value will vary depending on driver behaviour and style, charging rate (Domestic/On-Street/Fast Charge) and weather conditions with very cold temperatures effecting range by up to 10%.

Driving Cycle UN ECE 101 applicable to European Vehicles – New European Drive Cycle
Figure 2 Driving Cycle UN ECE 101 applicable to European Vehicles – New European DriveCycle
 
 
 
     

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