Toto Wolff has hit back at criticisms of the changing engine rules for 2018.

In Abu Dhabi, amid the flurry of grid penalties seen late this year, Red Bull's Christian Horner said that moving from four engines per driver to three in 2018 is "barking mad".

"If it's barking mad, then he should not have put pressure on the power units becoming cheaper," Mercedes's Wolff hit back.

"The manufacturers were encouraged by everyone to optimise the delivery price for customers, and that's exactly what we did.

"And if I remember correctly, everyone was at the table when that was decided. Yes, it's a huge challenge to achieve this reliability, but that's what we decided to do."

FIA president Jean Todt agreed with Wolff that the 2018 rules are set in stone, arguing that the sport has always been challenging.

"I'm not happy when I hear that a driver receives 150 grid penalties," he said, "but Formula 1 is very demanding and every detail must be in place.

"This limited number of engines is nothing new - everyone knows the rules."

Todt has also insisted that there is no going back regarding the introduction of the divisive 'Halo' system.