Renault's frenetic attempts to catch up with teams using controversial rear diffusers paid instant dividends today when Fernando Alonso qualified second for the Chinese Grand Prix.

The double world champion had qualified 10th and ninth in the first two races of the season but he said the modified diffuser, declared definitively legal only this week, had made a big difference.

"We only had three laps to test the new equipment on the car," said the Spaniard, who will line up alongside Red Bull's polesitter Sebastian Vettel tomorrow.

While post-qualifying weights published by the governing body showed that Alonso would have the lightest car on the grid, with 637kg compared to Vettel's 644kg and 661kg for Brawn GP's fourth-placed Rubens Barrichello, the Renault driver was still elated. "It is unbelievable and the first time I have seen such an incredible effort. The team has been working 24 hours a day for the past few weeks to produce these parts and today was time to deliver," he said.

Renault were one of the most vocal opponents of the so-called 'double-decker' diffuser, used to great effect by the Brawn GP, Toyota and Williams teams in the first two rounds of the season.

They were one of four teams to appeal against its use, an appeal that was denied by the governing International Automobile Federation (FIA) on Wednesday.

Like many other teams, Renault had been working on their own version of the diffuser, which smoothly channels air down and out of the back of the car improving downforce, just in case.

Shortage of time, however, meant they were able to put it only on Alonso's car and not on that of his team mate Nelson Piquet, who will start 16th.

"Fernando's front row position is a tribute to the hard work from everyone back at the factory to get the new diffuser on the car following the result of our appeal earlier this week," said director of engineering Pat Symonds.

"It's working as expected ... now we must get the same equipment for Nelson and give him a chance to move up the grid."