Why can’t I match my plug-in hybrid’s fuel economy numbers?

The combination of petrol and electric motors in plug-in hybrid vehicles (PHEVs), which incorporate batteries big enough to provide decent zero-emissions driving range, are an excellent solution for many Australians.

In many cases their zero-emissions EV range exceed the average Australian’s daily work commute, while offering a longer range than any battery-electric EV on the weekend, as we discovered with our recent 1000km range test of the Mitsubishi Outlander PHEV.

That makes them the ideal replacement for large-capacity internal combustion engines in family cars and SUVs, which consume more fuel and emit more CO2.

But don’t get hooked by their staggering claimed fuel consumption figures. They are impossible to match in the real world. Here’s why.

First of all, a plug-in hybrid (PHEV) denotes a vehicle that has both an internal combustion engine and an electric motor, but is more advanced than a conventional hybrid system.

The latter uses the electric motor to assist the petrol engine and has a small-capacity battery pack that is only recharged via the engine or through regenerative braking while driving.

A PHEV system has a more powerful electric motor and a larger battery that allows the vehicle to be driven without the petrol engine for, in some cases, up to 100km. As the name suggests, the battery can also be replenished by plugging it in to a charging station or from any domestic household powerpoint.

PHEVs are, quite literally, both an electric car and a normal hybrid, and offer greater flexibility than both with long-distance driving range, reduced fuel consumption and little or no compromise in power and performance.

Like every car that is sold in Australia, plug-in hybrids must advertise government-mandated fuel consumption figures in all promotional material, as well as a windscreen sticker in the showroom. This denotes how much fuel it uses in urban and extra-urban conditions along with a combined average.

For PHEVs, the claimed figures are extremely low. The most efficient PHEVs available in Australia are currently the Peugeot 308 GT PHEV hatch and Lexus NX450h+ medium SUV, both of which claim to consume an average of just 1.3L/100km.

In fact, the majority of plug-in hybrids have claimed averages under 2L/100km. Even Bentley says the 2.5-tonne Flying Spur Hybrid with a twin-turbo V6 and an electric motor consumes just 2.7L/100km. Sure!

These fuel consumption figures are derived from a standardised test protocol called the World harmonised Light vehicle Testing Procedure (WLTP). In 2018, this replaced the previous New European Driving Cycle (NEDC) test that was first introduced in the 1980s.

WLTP is designed to better replicate real-world driving conditions.

The test procedure is the same for every type of passenger car regardless of whether it has just an internal combustion engine, is a hybrid, plug-in hybrid or dedicated battery-electric vehicle.

To achieve consistent and repeatable results, the driving simulation is performed on a rolling road in a laboratory with constant environmental conditions including temperature, humidity and air density.

Each car is ‘driven’ for 30 minutes through four phases – low speed, medium speed, high speed and extra high speed – that have prescribed maximum and average speeds, rates of acceleration and durations when the vehicle is stationary. This is aimed to replicate a real-world mix of urban, suburban, rural and highway conditions.

The results from the low and medium segments are combined to determine the urban fuel consumption figure, while the high and extra high segments are combined to provide the extra urban figure. And the median of the two results is used as the claimed average consumption.

All vehicles with a battery – conventional hybrids, plug-in hybrids and EVs – have to complete the test cycle twice to ascertain the amount of energy harvested through regenerative braking while decelerating.

The first starts with a fully charged battery, and the second is designed to finish with the battery almost depleted. The median results from the two tests determine the final outcomes.

While the WLTP test process does provide consumers with a way to compare fuel consumption, it cannot fully replicate the broad spectrum of variables that ultimately dictate real world conditions.

It is possible for some drivers, in ideal situations, to achieve better fuel consumption figures than those advertised. But not with plug-in hybrids because they basically cheat the system.

For starters, the test is only 30 minutes long and covers 23.26km which allows most plug-ins to complete the majority of the four driving phases on battery power alone. Even the second cycle, where the battery starts with less charge, is still covered mostly by the electric motor.

This skews the real-world relationship between the petrol and electric motors, and results in the staggering low fuel consumption figures.

Now, there is the potential for owners of plug-in hybrids to rarely use the petrol engine if their regular daily commute is, say, less than a 20km round trip at low speeds through the suburbs and the battery is recharged every day. That’s the benefit of a PHEV.

But the reality is that many of us don’t drive in a rigid pattern or have access to a charging station every day. As such, in the real world, PHEVs will run out of battery power and tap into their petrol engines more often, which subsequently increases the fuel consumption figure above the advertised number.

Adding to that is, without any assistance from the electric motor, the petrol motor is having to work harder than it would in a non-plug-in hybrid. Firstly, because most PHEVs use a smaller-capacity internal combustion engine than what would normally be used in a large vehicle like an SUV. And, secondly, the battery pack, electric motor and all the ancillaries needed to connect the system together adds significant weight to the vehicle.

However, most PHEVs have a range of settings that allow owners to optimise the relationship between the petrol and electric motors, such as a pure EV mode that only uses battery power until the energy runs out, a Save mode that only uses the petrol engine and stores the energy for when it is best needed, a Recharge mode that uses the petrol engine to both drive the car and act as an on-board generator to replenish the battery, and an Auto setting that lets the computer do all the hard work.

Essentially, a PHEV is a great solution for many. You get all the benefits of an electric car for short trips without the hassles of range anxiety and lengthy recharging stops.

Just don’t be fooled by their claimed fuel consumption figures. They’re unreal. And a direct consequence of the testing process.