Plug-in Hybrids without Behavioral Compliance Risk Failure

11.04.19 | Blog | By:

When the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) commenced in September 2017, it replaced the New European Driving Cycle (NEDC) with a more realistic, ‘real-world’ approach to emissions testing. Following this switch, several models of plug-in hybrid car (PHEV) were withdrawn from sale in Europe as their emissions ‘rose’ sharply under the new test, disallowing them from various subsidies and benefits. Yet interestingly a wide range of new PHEVs are now being launched, two years later.

The new crop of PHEVs are likely to have been optimised to the WLTP emissions test, and come with larger batteries in the range of 10-30kWh instead of previously 3-6kWh. This ensures that they achieve super-credit status, or sub-50g CO2/km emissions ratings, which initially allows them to be counted twice in the fleet average CO2 calculation. This is vital for manufacturers who have to meet impending fleet average emissions targets of 95g/km from 2020 or face large fines in Europe.

This strong incentive from the EU level directly clashes with some member state policies: national governments that have cancelled generous subsidies for all PHEVs. This group of policy makers are suspicious of PHEVs.

The Dutch government, followed by the British, in late 2018, withdrew previously generous PHEV subsidies. They cited evidence suggesting that many owners, attracted by a subsidy, rarely plugged in their PHEVs.

So what is going on and who is right? If a PHEV is not plugged in, it typically drives around on a smaller than optimal internal combustion engine and achieves poor real-world results.

Of all PHEVs tested by Emissions Analytics, which includes petrol and diesel versions, the average performance in this condition is 37.2 mpg (7.6 l/100km) and CO2 emissions of 193.3g/km, which is 62.5% worse than the official NEDC results.

By no stretch of the imagination are these compelling figures if, as EU policy makers would claim, the purpose is to reduce real greenhouse gases as quickly as possible.

That is not to say that PHEVs have no claim to virtue. Their primary strength is offering electrification without range anxiety, since an internal combustion engine remains present, whether as a part of the drivetrain or as a ‘range-extending’ battery generator.

However, one of the challenges for PHEVs is that by the nature of the technology, their performance cannot be properly encapsulated and articulated by the standard, cycle-based rating. Rather, the real-world performance of PHEVs rests to an unusually large degree on user behaviour and journey length, rather than instantaneous combustion performance.

Research studies have shown that some duty cycles – for example commuting to and from work every day but charging overnight and avoiding long distances – can result in virtually no use of the ICE. The consumer has in this case had an EV ‘on the cheap’, without the weight and cost of a large battery pack. This is a PHEV at its best.1

At the other end of the spectrum, a PHEV might be deployed on long journeys and never plugged in. This results in a significant disbenefit, the vehicle typically offering worse fuel consumption and emissions than a conventional ICE-only drivetrain. This is a PHEV at its worst.

While we can recognise that many current or potential PHEV owners understand that the electric driving share of a PHEV, expressed as its utility factor (UF), is the key to its fuel economy rating and emissions, nonetheless the Dutch data, based on fuel card usage, included a significant business user fleet where there was evidently no fiscal incentive to save fuel. These owners were hardly plugging in.2

In the study referenced in footnote 1, based on 1831 Chevrolet Volts in the US, the authors found generally excellent utility factors, the average being 78%. In the Dutch data, which included smaller-battery PHEVs and owners who typically didn’t bother to plug in, the average utility factor was 24%.

If we take this spread, 24-78%, as the real range of utility factor, and return to the Emissions Analytics average PHEV performance of 193g/km CO2, it can be re-expressed as spanning 151g/km CO2 (24% UF), to 46g/km CO2 (78% UF).

The effect of the WLTP has been to force model overhauls, leading to larger internal combustion engines, and larger batteries to achieve longer electric range.

This is precisely what happened with the Mitsubishi Outlander PHEV, at different times and places the leading PHEV in Europe. To achieve a sub-50g/km result under WLTP the manufacturer fitted a 2.4 litre petrol engine, replacing a 2.0 litre unit, and increased battery size from 12kWh to 13.8kWh. EV range fell from 33 miles under the NEDC to 28 miles under the WLTP, but crucially it allowed the SUV to retain a sub-50g rating (46g/km) as a category 2 Ultra Low Emission Vehicle.

The warning to policy makers is that current and future PHEVs offer most of the same strengths and weaknesses of previous models, and that car makers are optimising their products to achieve the sub-50g result under WLTP but without guaranteeing any actual reduction in emissions.

In a very recent instance one OEM, Peugeot, boasted of an SUV featuring 4WD and 300hp yet 29g/km CO2, premised on an electric range of 36 miles and a 13.2kWh battery. The 3008 SUV GT Hybrid4 qualifies in the UK for the lowest Benefit in Kind (BIK) tax rating of 10%, re-attracting a subsidy.

In another 2019 product launch, the Volvo XC40 T5 Twin Engine claims 262hp and a preliminary WLTP rating of 38g/km. In this instance some but not all variants of this model offer a ‘hybrid’ setting that tries to optimise overall efficiency, except that whether it is deployed or not sits with the owner. Such an innovation is likely to confuse regulators and consumers alike, even if it may also work well in practice.

Our position is that on reasonable assumptions PHEVs will deliver less and less certain reductions in CO2 than non-plug-in hybrids. In other words, that they are ineffective without behavioural compliance, and that such compliance is politically infeasible in most democracies where it would be considered an intrusion on privacy.

The case for future PHEVs may lie principally in the light to medium commercial fleet, where the advent of zero-emission city centres may force dual-drivetrain approaches, the pure electric drive share being saved for last mile delivery and the ICE (diesel as well as petrol) permitting long highway distances, refrigeration units and so forth.

Geo-fencing is also strongly foreshadowed in current fleet management, from public bus fleets to Uber’s app, and suggests a straightforward way to ‘enforce’ the correct use of a PHEV, thus compelling fleet operators to plug them in.

This would go a long way to addressing the ongoing weakness of the PHEV, its drivetrain sleight-of-hand that courts generous tax-payer subsidy but delivers poor real-world performance.

In the realm of private passenger cars, however, we have shown in a previous newsletter how by comparison non-plug-in, full hybrids offer much faster and more certain emissions reductions of up to 30%.  Given the importance of reducing CO2 emissions agressively and quickly, the lower risk option may be preferable.

Republished with permission by Emissions Analytics


1 Patrick Plotz, Simon Arpad Funke, Patrick Jochem. ‘The impact of daily and annual driving on fuel economy and CO2 emissions of plug-in hybrid electric vehicles’ in Transportation Research Part A, 118 (2018) 331-340.
2 Ligterink, N.E., Eijk, A.R.A, 2014. ‘Update Analysis of real-world fuel consumption of business passenger cars based on Travelcard Nederland fuelpass data’, TNO Report TNO 2014 R11063.


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