A small, retrofitted LPG kit may make 10,000 two-stroke autos street legal but they will continue to pollute more than the four-stroke ones, warn experts.

"We saw how Bangalore remained smoky even after two-stroke autos (around 70,000 of them) switched to LPG to conform to the Bharat Stage II emission standard. Poor technology and the pathetic maintenance of the autos made conversion an exercise in futility," M.K. Chaudhari, a senior deputy director of the Automotive Research Association of India (ARAI), told Metro.

Calcutta risks facing what Bangalore did by following a 2007 notification issued by Chaudhari’s organisation in letter but not in spirit.

The Pune-based ARAI, which has the last word on automotive industry standards in the country, had said that only a two-stroke auto in good condition could become Bharat Stage II-compliant by switching to LPG. The Left Front government is using that very notification to create an escape route for all two-stroke autos manufactured after August 2000.

Anumita Roy Choudhury of the Centre for Science and Environment in New Delhi said a two-stroke engine would remain a more polluting one than a four-stroke one irrespective of the fuel being used. "Two-stroke autos have switched to CNG or LPG in several cities but it has been found that the emission advantage is minimal. The problem of incomplete combustion remains even after the vehicle is retrofitted with an LPG or CNG kit."

In Delhi, two-stroke autos were allowed to convert to CNG during the phase-out but are now banned from the streets.

With the more stringent Bharat Stage III mass-emission standard set to take effect next April, those autos in Calcutta that opt for the conversion lifeline will become even more retrograde. "The long-term solution, therefore, is to go for a four-stroke LPG auto rather than resort to conversion," Roy Choudhury said.

A senior official of Bajaj Auto Ltd, the largest manufacturer of three-wheelers, said from Pune that it was "impossible" to convert two-stroke petrol autos to "single-mode LPG ones" with the available technology.

"Disconnecting the petrol tank from the fuel pipe will not do. Dual-mode carburettors of two-stroke autos need to be replaced with LPG-specific carburettors, which our company has yet to manufacture for two-stroke engines. We explained this to the PVD (public vehicles department) inspectors when they came to our workshop."

Automobile expert S.M. Ghosh said many of the two-stroke autos stepping out of the ban boundary were unfit for conversion to LPG because their engines had been "abused" through long-term use of katatel (adulterated petrol). "Their emission standard surely won’t improve much even if they run on LPG."

According to auto retrofitters in Calcutta, the concept of a single-mode LPG engine might not work in the city given the options illegally available to unscrupulous operators.

"It just takes half an hour to make a factory-fitted four-stroke LPG auto run on petrol or katatel. Strict monitoring rather than new vehicle technology is required to ensure that the high court’s order is honoured," said Yash Khare, a retrofitter.

Transport secretary Sumantra Chowdhury said the breather for two-stroke autos - allegedly tantamount to contempt of court - was a practical solution.