Advice

Are plug-in hybrid fuel consumption claims accurate in the real world?

Putting plug-in hybrid economy claims to the test with the BMW 530e.
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Some big claims are made by car manufacturers when marketing their electric and plug-in hybrid cars, but how often are these marketing boasts actually accurate in the real world?

For instance, BMW says its 530e plug-in hybrid electric (PHEV) sedan will do 54km on full battery electric power and record a 2.3L/100km fuel consumption when using both electric and petrol power.

But, taking BMW at its word, if it can only go 54km before running out of electric power, then BMW’s claim of 2.3L/100km is rather generous – isn’t it?

You see, our main gripe is the fact that, yes PHEVs will regularly record those astonishingly low fuel consumption numbers. However, consider the fact that the electric battery won’t last the entire 100km run, and you’re then left with a far higher fuel consumption than prescribed.

BMW is far from the sole culprit here but seeing as we had a BMW 530e PHEV in the CarAdvice garage during the week, we were keen to put various claims to the test.

Specifically, after recharging its 12kWh battery to full, we set out to see what fuel consumption the 530e would do on hybrid power up until its battery is depleted. After that, we compare against what fuel consumption it returns on petrol power, in order to get it the remaining distance to a 100km total.

Knowing these figures helps to explain more accurately how much fuel a plug-in hybrid will use over a 100km distance, instead of the often misleading 2.3L/100km claim.

Additionally, we were also keen to record the fuel consumption, and time taken, to charge the battery using the car’s generator.

But first things first, let’s explain what the BMW 530e PHEV system can do.

There are several ways to deploy the 530e’s electric power, but most people will leave the system in AUTO eDRIVE which automatically switches between electric and combustion power sources (a 135kW/290Nm 2.0-litre turbocharged petrol engine) depending on load capacity. It can then be switched to MAX eDRIVE, which solely uses electric power until the battery is depleted.

Drivers also have the option of BATTERY CONTROL mode which maintains a specific preselected battery level (or charge the car up to that point) while driving. For example, with BATTERY CONTROL SET to 100 per cent, the car will use the combustion engine to charge the battery to full.

For the first test of retrieving a fuel consumption figure for the hybrid power portion of the first 100km, we used AUTO eDRIVE. It preferences electric power where possible, so out of the 35km we did until the battery ran out, just under 30 of them were completed under electric power.

On a loop encompassing a mix of urban and highway driving, the 530e recorded a 2.6L/100km fuel consumption. That’s pretty accurate to what BMW says: 2.3L/100km. However, we still had another 65km to travel before we even hit 100km.

The remaining 65km were spent under full combustion power, having run out of electric battery charge. This netted us a 6.6L/100km fuel consumption, or near enough what you’d expect from an internal combustion engine car.

It seems like we’re picking on BMW here but they’re not the only culprits. The Mitsubishi Outlander PHEV quotes an identical 54km range, so it’d likely be the same scenario in that car.

For interest’s sake, the combined fuel consumption encompassing both electric- and petrol-powered portions of the 100km was 4.9L/100km – that is a far more accurate representation of what a plug-in hybrid vehicle will do.

With that, next up came time to recharge the 12kWh battery. There are a few ways to recharge the 530e including recuperation of braking energy, and plugging the 530e into an A/C power point, though we were most interested in the time taken, fuel used and distanced travelled to charge the battery using the combustion engine. The latter method is arguably the most energy inefficient, as you’re using petrol power to generate and store electricity, though it is one of the most convenient ways.

Unsurprisingly, after turning on BATTERY CONTROL mode to recharge the battery, fuel consumption skyrocketed up to an instantaneous 20+L/100km. After an hour’s worth of driving around and a 60km distance travelled, the fuel consumption settled down to 11.4L/100km. That is the fuel consumption required in order to charge the 530e’s battery using petrol power.

As an aside, we also tested BMW’s electric-only claim which posits that you can travel 54km on electric-only power. In reality, through a series of suburban streets, the 530e could only muster just under half BMW’s claim, travelling 25.6km before exhausting the battery charge.

Rob Margeit recently trialled Volvo’s XC40 Recharge PHEV on a road trip and could only muster 40km on full-electric power before having to revert to the petrol engine. He explains that the best way to run a PHEV was to leave it in its regeneration mode to harvest enough electricity in order to deploy in short, meaningful bursts. In that way, he was able to average 5.1L/100km on an 800km trip.

A far cry from Volvo’s touted 2.2L/100km claim, though far superior to petrol-only variants in the XC40 range.

Of course, if you can travel less than 30km or so to your destination and put the car on charge again for the return trip, more power to you. But for those who read the impressive claims by manufacturers and get confused about why it’s not as economical as first thought, this is a more accurate portrayal of a plug-in hybrid’s ability.