As car makers contemplate the widening gap between legislation and demand for electric cars, the spotlight has been thrown on to a petrol-electric technology that promises to bridge that gap with fewer of the downsides of plug-in hybrids.

Range-extender (REx) cars package a large battery and a hefty electric motor alongside a small petrol engine, instead of the other way around with most hybrids, in which a large petrol engine dominates.

Advocates of REx technology say that drivers are better incentivised to actually keep the battery topped up, because the electric motor is prioritised over the petrol engine.

With PHEVs coming under fire for emitting more than three times their claimed CO2 in the EU, partly because drivers have less incentive to plug them in, REx cars could prove a better technology to phase out ICE cars by 2035.

Right now, the choice of REx cars in Europe is extremely limited. With the Vauxhall Ampera E-REV and BMW i3 Range Extender long since discontinued, the only option for British buyers is the eccentric Mazda MX-30 R-EV.

This small crossover packages a 17.8 kWh battery pack with 830cc rotary petrol engine acting as a generator only.

So far, sales have been sluggish: Mazda had registered just 109 by the end of April, despite the current offer of a £6000 discount from the £32,045 starting price.

The only other way to experience a REx is go for a ride in an LEVC TX taxi around London.

The most vocal cheerleader for REx tech in the UK right now is the unlikely figure of chemicals tycoon Sir Jim Ratcliffe, owner of Ineos Automotive.

The maker of retro off-roaders has promised a REx version of its upcoming Fusilier EV, a smaller brother to the Grenadier six-cylinder 4x4.

“Why would you ever buy a fully electric car when you have this option of an electric vehicle with range extender with the complete absence of range anxiety?” Ratcliffe wrote in The Telegraph recently.

“This is not the politician’s utopian world, but it serves the consumer well and maintains a very positive trajectory towards zero emissions." 

Standing behind Ratcliffe are the seemingly unstoppable engineers of the Chinese car industry, which is taking to REx tech in a big way.

Fom January to September last year, REx sales in China – where they are known as extended range EVs, or EREVs – exploded by 157% to almost 400,000, according to the China Association of Automobile Manufacturers (CAAM).

REx cars still aren't as popular as PHEVs, of which 1.37 million examples were sold in China in the same period, but their sales are growing at twice the speed.

Leading the REx charge in China is one of the most successful ‘new-energy' brands to emerge, Li Auto, which has built its empire on elegantly anonymous SUVs that package a 1.5-litre petrol engine with an oversized battery.

Its newest and smallest, the L6, uses a 36.8kWh LFP battery from CATL for an electric-only range of 132 miles, while the flagship L9 has a 52.3 kWh pack for an official range of 149 miles.

Li recorded 106,189 wholesales (sales to dealers) in the first four months of 2024, according to data aggregator Bestsellingcarsblog.com, putting it ahead of Hyundai, Ford, Volvo, Kia and Chevrolet in China.

In April, the company sold more plug-in vehicles in the country than BMW.

Li had been on a glidepath to shift to pure EVs, starting with the newly launched eye-popping Mega MPV. However, in its first-quarter earnings call last week, founder Xiang Li confirmed that the company will postpone its EV rollout from later this year to 2025.

Ostensibly the reason given was the lack of chargers and showroom space for new models, but leaving the comfort of its successful REx niche to push into China’s cut-throat EV market can’t be appealing right now, especially when Li posted its first loss for the quarter after a profitable run.

Others in China are eyeing up REx sales as well. Premium brand Avatr, a partnership between Changan, CATL and tech giant Huawei, has just launched the 07 crossover with the option of a 39kWh battery and a 1.5-litre petrol engine.

Further down the price rankings, Neta has launched the L EREV SUV with either a 30kWh or 40kWh battery, the latter giving a claimed 193 miles before the 1.5-litre petrol engine is needed.

In China, REx cars and PHEVs are popular because they attract many of the incentives as EVs for both car makers and buyers.

In the UK, PHEVs are mainly sold to company car drivers tempted by the lower BIK tax bills resulting from low official CO2 ratings. 

However, that loophole is going to change in 2025, when the generous WLTP emissions measuring test is changed to reflect the much lower real-world battery running time of PHEVs.

That would double the official CO2 figure for the BMW X1 PHEV from 40g/km to 85g/km, the International Council of Clean Transportation (ICCT) calculated.

Car makers are increasing battery sizes in PHEVs to offset that change, but it could also be the perfect catalyst for a REx rollout in Europe.

So far, however, no major car maker in Europe has said it's working on a solution. In fact, Mercedes-Benz has ended a development programme to investigate REx tech, in which an EQS electric limousine was fitted with a turbocharged 1.0-litre two-cylinder engine, a source told Autocar.

Mercedes chief engineer Christoph Starzynski didn’t confirm that news in a recent chat with Autocar, but he did voice the German company’s preference for its current strategy. “For us, the better choice is to stay with performance hybrids,” he said.

Mercedes isn't convinced that its customers would accept what it sees as a compromise in driveability by introducing REx cars.

“My strong belief is that if you want to have a combustion engine in an E-Class or S-Class, you don’t want a four-cylinder engine which turns on when the batteries are empty,” he said.

Drivers willing to go electric are well served by the long-range options in EVs like the EQS, Mercedes believes.

BMW is also not about to return to REx, CEO Olive Zipse told investors and analysts on the company’s first-quarter earnings call. “To have a very large battery plus a combustion engine in there... There is a limit to the financial logic,” he said.

“Our PHEVs are currently around the 100km [range], which I think is the perfect point where the customer says ‘this is what I would like to pay for’.”

Given the time and cost to adapt a platform to accept a REx drivetrain, it’s probably safe to say that rivals to the MX-30 R-EV are going to be thin on the ground.

One possible contender is Leapmotor, a Chinese brand that will soon start selling cars in Europe through part-owner Stellantis.

It produces a REx version of the high-tech C10 SUV that will form part of its European launch later this year and its British launch in March 2025.

However, the decision to import the REx version along with the EV rests with individual countries, Stellantis CEO Carlos Tavares said in a press conference earlier in May to announce the Leapmotor launch plan.

“The extended-range versions will have to be homologated under the Euro 7 regulations, which of course means more lead time and more cost,” he said.

However, the broader decision on whether Leapmotor and therefore Stellantis invests in REx cars for Europe rests on the EU and whether the legislature following elections early June cranks down the steepness of the pathway to zero-emissions vehicle sales.

“Let's see what comes out of the European elections and we'll make decisions on that basis,” said Tavares.

As it stands, the timescale for rolling out the EVs that every car maker is working on doesn’t really include time to engineer a solution that will remain a stopgap only.