Rating of Plug-in Hybrid (PHEV) tests

A PHEV can be operated the following ways:

  • Charge Depleting (CD) mode: with the high voltage (HV) battery charged where the vehicle will prioritise, if the situation allows it, to be run predominantly in electric vehicle (EV) mode only activating the internal combustion engine (ICE) in exceptional cases.
  • Charge Sustaining (CS) mode: with the HV battery discharged where the vehicle acts like a NOVC-HEV (not off-vehicle charging (i.e. conventional) hybrid electric vehicle), running predominantly the ICE. The hybrid management system selects the strategy to be followed, i.e. when the power at the wheels is provided by the ICE, by the electric motor or both.

Green NCAP’s test matrix pursues the goal to provide consumption and emissions values that are more realistic than the figures stated in the official documents. This is achieved by testing under more representative conditions and postprocessing the results in a more sensible way by weighing both modes in accordance to the measured useful electric range.

The standard test stage (stage 1) for PHEVs considers the test matrix for ICE vehicles and HEV when the vehicle is working in CS (Charge Sustaining) mode with the addition of the following tests done in CD (Charge Depleting) mode: WLTC+ CD Sequence (Cold Laboratory Test), PEMS+ Cold (On-Road Drive) and the On-Road Short Trip. The WLTC+ CD Sequence consists of driving the vehicle over the WLTC+ test starting with the HV battery at 100% SoC. The requirements of the test are the same as of the WLTC+ Cold test. The vehicle is driven over multiple WLTC+ tests in a row until it reaches the CS condition. This WLTC+ CD Sequence procedure is also used to determine the available battery capacity and the EAER (Equivalent All Electric Range). EAER stands for the total driving range attributable to the use of electricity from the battery over the WLTC+ CD Sequence.

The tests driven in CD mode in the additional robustness test stage (stage 2) are PEMS+ Eco (On-Road Light Load) and PEMS+ Heavy (On-Road Heavy Load). These tests are not conducted in CS mode. For PHEV, each index scores are calculated in both CD and CS mode and combined. The contribution of results in each mode is determined by the obtained EAER. The higher the EAER, the lower the influence of the results in CS mode and the other way round, see Tabel 1.

Table 1 CS/CD determining ratings for PHEV.

At a test level, the score of a test, which is conducted in both CS and CD mode, is the result of a weighing based on the assumed ratio of usage with charged battery and as a conventional hybrid vehicle. For this reason, the PHEV result of the Cold Laboratory Test (which is performed in CS and CD mode) is displayed to be higher than the result of the Warm Laboratory test (which is conducted only in CS mode).

Limitations

Clean Air Index

The Clean Air Index is based on a score for each pollutant: maximum for very low emissions; zero (or even negative score) for high emissions.  The scale is based on values which make it challenging for modern petrol and diesel cars to achieve a top score in this part of the assessment, allowing to clearly differentiate between vehicles that just comply to the legislative requirements and such that really emit low or no pollutants.  Since battery electric vehicles do not emit any exhaust gases, they receive the full score in the Clean Air Index.

Energy Efficiency Index

The Energy Efficiency Index is calculated on a ‘tank to wheel’ basis and represents only the energy consumption needed by the vehicle to move and its auxiliaries to work.  Upstream efficiencies (vehicle assembly, sourcing of raw materials etc) and downstream energy usage (recycling, re-use) are not yet considered.

Greenhouse Gas Index

The Greenhouse Gas Index considers the sum of tailpipe and upstream emissions. The upstream emissions are related to the processes necessary to supply the energy used by the vehicles. Such processes are e.g., the extraction from raw oil and resources, the construction of refineries and renewable energy power plants, the supply and usage of resources needed for their operation, the appropriation of the necessary filling and charging infrastructure, etc. The upstream greenhouse gas emissions are determined by the method of Life Cycle Assessment (LCA) based on the average values of the 27 European countries and the United Kingdom. Green NCAP calls this approach of considering local tailpipe and upstream greenhouse gas emissions ‘Well to Wheel+‘.

Green NCAP looks at three of the most important greenhouse gases: Carbon Dioxide (CO2), Nitrous Oxide (N2O) and Methane (CH4).  Other gases emitted by the vehicle which may have a ‘greenhouse effect’ are not measured.

Laboratory Tests

For its laboratory tests, Green NCAP currently uses the same road load data used for legislative testing.  A methodology is under development to verify this data.

Measurement Technology

Green NCAP’s test laboratories measure particulates with a 23 nm (nanometer) cut-off filter, as required by the latest legislative standards.  However, such equipment cannot detect the very injurious super-tiny particles which are emitted in large numbers by some vehicles (particularly petrol-engined cars which do not have a gasoline particulate filter).  Green NCAP will use 10 nm cut-off filters in the future but, for now, the rating is based on current measurement technology.

Life Cycle

A Life Cycle Assessment (LCA) is a method to estimate the material and energy flows of a car to analyse environmental effects, like Greenhouse Gas emissions, over the entire lifetime of the car from cradle to grave. From 2022 onwards, Green NCAP also provides full LCA information on its website. However, the currently available data does not allow the integration of greenhouse gases originating from vehicle production and recycling into the rating system yet.  The LCA values may differ from country to country and the information available for the analysis is on a generic statistical level, which does not allow for an accurate enough differentiation between the production emissions of different cars.