KMB operations director Lui Po- chu said the first two Euro IV engine buses will be put into service next month.

The engines reduce particulate matter from 0.1 to 0.02 grams per kilowatt hour - a 80 percent reduction when compared with the Euro III engines, according to KMB. The engines also reduce nitrogen oxide emissions by 30 percent from 5 grams per kilowatt hour.

KMB has 964 buses with Euro III standard engines, Lui said, adding that 529 of those are equipped with continuous regeneration traps and an exhaust gas recirculation device that helps to achieve emission standards comparable to Euro IV engines.

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Responding to the news, environmental group Clear the Air said the new buses will not make a difference.

"They are not doing anything special. They have 1,500 really old buses on the roads today. Two buses do not even make a dent in that," said Clear the Air chairwoman Annelise Connell.

More than three-quarters of the KMB fleet has Euro I or II standard engines, or engines of an earlier standard. Euro III standard engines emit 0.36 and 0.15 grams of particulate matter per kilowatt hour, respectively.

Lui said if the buses pass the required tests, more Euro IV buses will be introduced.

"If the Euro IV buses prove effective, the fuel costs saved could offset the price of the engines," Lui said. "The result could be a cheaper fare for passengers."

But first, they will undergo tests for about a year, to evaluate if the vehicles will be suitable for Hong Kong.

Referring to the hot and humid conditions in Hong Kong, Lui said it will be some time before it is known "how they [the buses] are going to react to the weather or road conditions here."

The buses will begin carrying passengers early next month, he said.

The vehicles will be tested on main routes, for example, on Nathan Road, cross-tunnel routes and in Central.

Lui would not comment on the cost of the Euro IV engine buses relative to earlier models, but said KMB replaces about 100 to 150 buses each year.

KMB buses have a lifespan of about 17 years and the average age of the fleet is 7.9 years. It has a fleet of more than 4,000 buses, KMB said.

Clear the Air's Connell said KMB's competitor, Citybus, although a smaller operator, is the dirtiest bus company in Hong Kong.

As of November last year, Citybus had only 10 Euro III engine buses out of a fleet of 752. The remaining buses are of Euro II and I, or pre-Euro standard, according to the Web site of Clear the Air.

A Citybus spokeswoman said it plans to purchase Euro IV standard buses but that there is no timeline to indicate when.