Red Bull team principal Christian Horner says he and his team were left frustrated by Renault's false promises of progress, which ultimately contributed to their strained relationship which, at one point, looked to be over, before a rekindling of sorts late last year.
The Milton Keynes based team enjoyed huge success with Renault, taking four back-to-back drivers' and constructors' championships between 2010 and '13. However with the introduction of a new engine formula in '14, it became apparent that Renault had fallen behind its rivals.
Despite that, Red Bull still managed to secure three victories. Renault promised it would bring improvements over the winter, but things took a step back, particularly with reliability, and repeated promises of improvement failed to materialise.
"Mercedes has done a remarkable job with its powertrain, but in '14, the days when they made ​​mistakes, we were there to enjoy it," Horner told Motorsport-Total.
"Ferrari and Renault were close enough to the level of performance that year. The problem we encountered in 2015 is that after our three victories in '14, Renault had made ​​us a promise that it would catch up with Mercedes. The expectations were very high on the side of Renault.
"When you're an ambitious team like ours, we want to win, and if victory doesn't come, it's frustrating. So it was frustrating because we had no control over the engine."
Whilst Horner admits chassis problems contributed to their woes at the start of the season, he insists those were resolved, suggesting Renault's power unit was entirely to blame for their results, or lack of, in the second half of the season.
"We had a few problems on the car at the beginning of the year, but we could fix it. We had a good chassis. You could see that in the middle sector of Spa, or in Hungary, Singapore and Monaco. Wherever the engine power does not play such a big role."
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