Indian Oil Corporation (IOC) on Tuesday launched the nation''s first 100 Octane petrol, helping India join a select league of nations globally that have such superior quality fuel. Launching the fuel, Oil Minister Dharmendra Pradhan said XP100 premium petrol will initially be available at select outlet of IOC in 10 cities -- Delhi, Gurgaon, Noida, Agra, Jaipur, Chandigarh, Ludhiana, Mumbai, Pune and Ahmedabad. The fuel is manufactured at IOC''s Mathura refinery in Uttar Pradesh and supplied at select petrol pumps, he said. Octane ratings are measures of fuel stability. It is a measure of a fuel''s ability to avoid knock. Knock occurs when fuel is prematurely ignited in the engine''s cylinder, which degrades efficiency and can be damaging to the engine. The higher the octane number, the more resistant the petrol mixture is to knock. Worldwide, 100 Octane petrol has a niche market for luxury vehicles that demand high performance and is available only in six countries of USA, Germany, Greece, Indonesia, Malaysia and Israel. At most retail stations, three octane grade are offered, 87 (regular), 89 (mid-grade) and 91-94 (premium). Pradhan said Hindustan Petroleum Corporation Ltd (HPCL) had recently launched Octane 99 and now IOC has come to the market with XP100.