Mexican GP: Fernando Alonso and Jenson Button handed penalties

Button (front) and Alonso are 16th and 17th respectively in the drivers' standings

McLaren's Fernando Alonso and Jenson Button face starting Sunday's Mexican Grand Prix from the back of the grid after their latest engine penalties.

Both will have new engines for the race, having already used well over the permitted number for the season.

Alonso will have a new version of the upgraded specification Honda unit he used at the US Grand Prix last week.

Button will use the upgraded engine for the first time - and will have two fitted over the course of the weekend.

Alonso will suffer a total grid penalty of 15 places, 10 for the new engine and five for a replacement gearbox - and Button 20, with each of the two engines costing him 10 places.

It is possible Button will have further penalties if other parts of the engine are changed, but they will be academic as the 20 places will already be enough to ensure he is last.

The changes will take the number of internal combustion engines each driver has used so far this season to 11, when they were permitted only five.

Honda have struggled with both performance and reliability with the sport's complex new turbo hybrid engines in its first season back in F1 since 2008.

Alonso's engine suffered a problem in the closing laps of last weekend's race in Austin, Texas, which was instrumental in dropping him from fifth place with seven laps to go to an eventual 11th, out of the points.

Honda said that was caused by an injector problem, which has now been fixed, but that he was being given a new engine for strategic reasons.

Honda wants both drivers to have two of the new-spec engines available for the remaining three races this season, and they know that the Mexican race will be a difficult one for them - so the team prefer to take the grid penalties at a track where they are not expecting a good result in any event.

The Autodromo Hermanos Rodriguez track has a high percentage of full throttle over the lap and the high altitude puts the engines under more strain than at any other circuit on the calendar.

"It will be a very difficult weekend for us because of the altitude," said Button. "Because of our compressor, it'll hurt us more than other teams."

The compressor has been one of the biggest weaknesses of the Honda engine.

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