Alain Raposo, global head of powertrain engineering, will be moved to an advisory role, and a successor announced this week, four people with knowledge of the matter said.
A Renault-Nissan spokeswoman said there would be no comment on "speculation about personnel changes" from the companies or the people involved. "The alliance is on track with its overall convergence objectives, including engineering," she said.
Under Chief Executive Carlos Ghosn, who heads both Renault (>> Renault) and Nissan (>> Nissan Motor Co Ltd), the carmaking alliance created in 1999 is still moving incrementally towards common vehicle architectures and engines, in search of 5.5 billion euros (4.64 billion pound) in annual savings by 2018.
The two groups, with total sales of 145 billion euros, say 85 percent of engines are already shared in some way. But that understates inefficiencies, executives privately concede - as well as the cost of protracted bickering over whose technology becomes standard.
"It's a permanent punch-up - after 17 years we are still unable to think like a single company," said one of Raposo's management colleagues. "In powertrain it's always been hell."
A coming onslaught of emissions regulation in the wake of Volkswagen's (>> Volkswagen AG) exposure last year for cheating U.S. diesel tests has made the problem more urgent.
Independent studies have since blamed Renault for some of the highest real-world nitrogen oxide (NOx) emissions, piling pressure on engineering boss Gaspar Gascon Abellan and Thierry Bollore, Ghosn's second-in-command at the French carmaker.
Questions over whether Renault's technology breaches EU law have been referred to prosecutors for investigation. Renault has said all its engines are legal.
The weak emissions performance is inflating Renault's bill for curbing excessive on-the-road emissions from current engines and accelerating development of cleaner new ones, along with rechargeable gasoline-electric hybrids.
"We're behind on several projects - some engine development schedules are all over the place," said another manager. "The tighter standards are causing real difficulties, so we're hiring and doing everything we can, but it's not enough."
In one example, Renault realised too late that the next generation of gasoline engines was likely to require particulate filters, hitherto reserved for diesels, the manager said. "We had thought we could get by without them."
The scramble has begun to weigh on Renault's capital expenditure - focussing more attention on untapped savings opportunities with Nissan.
Renault-Nissan is set to top 10 million annual vehicle sales following Nissan's acquisition of Mitsubishi Motors, rivalling the likes of Toyota (>> Toyota Motor Corp) and GM (>> General Motors Company). But the economies of scale are further behind.
For instance, the carmakers have refused for years to pool three-cylinder gasoline engines, or to use each other's. The current Renault Clio and Nissan Micra have similarly powered - yet separately conceived - motors and transmissions.
And while Nissan developed continuous variable transmission (CVT) automatic gearboxes through its subsidiary Jatco, Renault purchased costlier dual-clutch transmissions (DCT) from Getrag, pending plans to design and produce its own.
During Raposo's tenure, however, sources said Renault recently agreed to use a Nissan three-cylinder and shelve DCT gearbox development, in favour of greater use of its partner's CVT in core models, starting with the Kadjar SUV next year.
Production of Nissan's next Micra, now ramping up at a Renault plant west of Paris, also includes versions with a Renault three-cylinder engine.
But these gains are too little, too late to alleviate the strains on Renault's engineering resources. "Top management wants to go faster," said another executive.
Raposo is the latest in a series of alliance directors to pay the price for slow progress on technical convergence that they lack the authority to impose.
Renault owns 43.4 percent of its larger partner but has refrained from exercising control under Ghosn's philosophy of consensual decision-making, which hands each company an effective veto on joint investments.
The Renault and Nissan executive committees "have never been capable of converging" technologies in a timely way, a senior manager said. "So they end up punishing the guy underneath."
(Reporting by Laurence Frost; Editing by Mark Potter)
By Laurence Frost and Gilles Guillaume