Renault's Director of Operations Remi Taffin says the French manufacturer 'genuinely thought we had done a good job' as it started the 2015 Formula 1 season and has explained in detail the cause of its frustrations.

Renault suffered a difficult campaign as supplier to the Red Bull and Toro Rosso squads which had a detrimental impact on its relationship with the teams.

Toro Rosso has split with Renault altogether and will use 2015 Ferrari power units next year, while Red Bull keeps Renault as it's engine supplier but has taken the development in-house and rebranded it TAG-Heuer.

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Taffin has openly admitted Renault did not meet any of its goals this season and the performance of its Renault Energy F1-2015 came as a shock compared to its developments produced over the winter.

"We arrived at the first winter test believing that we had made a big step forward between seasons," Taffin said. "We genuinely thought that we had done a good job. It was the case in certain areas, but we were caught out in three principal ways.

"The first was a significant reliability problem with the pistons. We had not seen the particular issue in any of the testing conducted and all the parts were signed off in the usual way. The other issue was that we had another part inside the PU electronics that we could not put our finger on and it continued to be a source of problems. The final element is that we braked so late for 2015, taking decisions until the last possible moment."

Taffin has also explained these eleventh-hour decisions meant Renault was powerless to make any changes to its engine specification for the season-opening flyaway races in Australia, Malaysia, China and Bahrain which was the reason behind the frequent and repeated reliability failures.

"The specification of PU for the first race was determined very late in the day and quite simply we braked too late, and we weren't ready," he said. "We wanted to make up the gap so badly that we pulled the rug out from under our own feet... But this particular cocktail of problems did allow us to learn a lot and make sure that the same issues will not reoccur in the future."

Despite changes and gradual improvements from the Spanish Grand Prix, Renault never recovered against the dominance of Ferrari and Mercedes and failed to secure a single race win in 2015.

Taffin hopes his team has learnt from its mistakes and plans to fight back next season having recently acquired the Lotus F1 team after it fell into financial troubles. The final configuration of the Louts/Renault team is yet to be confirmed and whether it will run under the Renault banner, but Taffin is determined to produce a worthy challenger in 2016.

"We will try to go through things in the right order and not make the same mistakes again," he explained. "In December, the 2016 PU we will use in Melbourne will be running in the dyno and doing endurance testing, mapping and calibration so at the end of January we can go to winter testing without any issues. The spec for the first race has been fixed and seems to be generally reliable, as it has run several thousands of kilometres in the dynos."