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How are fuel economy figures calculated for a PHEV?

Boost Auto Explainer

Have you ever rented a trailer only to discover that your new vehicle can't actually tow the rented trailer? If the answer is yes, you'll know having multiple standards is a pain.

The WLTP “Worldwide harmonized Light-duty vehicles Test Procedure” standard has created a better (i.e. more realistic) fuel economy testing programme for passenger vehicles. It replaced the NEDC (New European Driving Cycle) test that has been around virtually unchanged since 1992.

NEDC was open to abuse. Increasingly vehicles were being optimised (we mean, the system was being gamed) by manufacturers to get ‘better’ results, which meant the real world fuel economy figures became further away from the test results. A testing regime is a useful basis for comparison, and WLTP is meant to be more realistic, and therefore more useful.

The main difference is that WLTP is real world tested as well as bench tested. Importantly there is a BEV fuel economy testing regime as well as a PHEV testing regime. Plug In Hybrids are unusual, in that part of their appeal is that they don’t come with the range anxiety of a normal EV; conversely they have a modest EV only range.

One of the interesting and common questions that is asked in dealership and online is, "What is the fuel economy when the PHEV is in electric model and what is the fuel economy when its it EV mode?".

Cleverly the rule-makers thought of exactly that when developing the test procedure. The unique characteristic of a Plug In Hybrid is that not everyone will charge it every time, that the usage might be beyond the range and therefore during use the pure EV range will run out, requiring the vehicle to be used in mild hybrid mode.

PHEVs complete the WLTP test several times. They start up with a full battery, and then the cycle is repeated until the battery is empty. That means that the combustion engine operates for a longer time each cycle, until drive originates solely from the combustion engine and regenerative braking. So you get a repeated cycles using full BEV mode, then hybrid mode and full combustion mode. The data is then averaged.

Emissions and fuel economy are measured with each cycle and this allows fuel consumption and CO2 emissions to be measured more precisely (as well as the electrical range and total range as well). The CO2 value to be determined is then calculated as the ratio of the electrical range to the total range.

Unfortunately, the one part of the jigsaw missing here is that New Zealand hasn’t agreed on a single testing regime standard; instead we take the smorgasbord approach. We allow NEDC, WLTP, ADR and Japanese technical standards (METI Classifications and Target Standard Values). So, despite the benefits of WLTP, being more reflective of what consumers will actually achieve, it's not as useful locally as in Europe. Here there is no one standard for vehicle economy and emissions for New Zealand. And thus, consumers can’t confidently compare or select the most efficient vehicles because there is no single standard for comparison. It's like tow-balls all over again*.

Ever wondered why a fuel economy of 1.7L/100 km can be achieved? Now you know.

Further reading:

Boost Auto is an automotive consultancy working in five main areas.

  1. Sales and Marketing effectiveness for brands and dealers

  2. Green fleet facilitation for large corporates

  3. Go To Market strategies for emerging brands

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*For our international readers, NZ has a tow-ball standard of 50mm and also 1 7/8 inch. These are not interchangeable (but just to add to the fun, they look the same).

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