Yes, Liquefied petroleum gas (LPG) can be used in petrol vehicles without any harm to the engine. On the contrary, many benefits to the engine are provided by the use of LPG. It is a clean burning gas without causing deposits to the combustion chamber. The octane rating of LPG is significantly higher than that of petrol. So it can bear higher compression ratio conducive to higher efficiency. But this can only be achieved in dedicated engines but not in converted engines as is the practice. The converted engine has to switch back to petrol when the supply of gas from the cylinder is exhausted!
Nevertheless, clean burning characteristics with lesser maintenance, absence of mal-distribution from cylinder to cylinder and cycle to cycle (unlike that encountered in petrol engines) will result in better performance both fuel economy-wise and emission-wise. There is no danger of benzene and other carcinogenic aromatics in the exhaust much less the much dreaded lead since it has naturally a high octane value.
In the U.S, where there is intensive research on alternate fuel technology, LPG or Propane, as it is called there, accounts for the largest number of vehicles working on this alternate fuel technology.
There are two types of light duty vehicles: dedicated which operates exclusively on LPG and the dual fuel, which can operate on either LPG or petrol.
Bulk of LPG is propane with varying quantities of ethane and butane; such that it can he liquefied at ambient temperatures by pressurizing it to moderate range of 25 to 125 psi, depending on temperature. These pressures can be safely contained in simple thick-wall steel fuel tanks. This is in contrast to natural gas which must be pressurized to 3000 psi and stored in reinforced tanks, or chilled to minus 260 F and stored in double-wall cryogenic tanks.
Two crucial factors in the use of LPG must be borne in mind. First: Safety. Have sufficient safety measures been taken to prevent risk involved in case of an explosion of gas cylinder consequent to a possible disastrous collision? Look at the damages to innocent road user by careless installation of the conversion kit.
Has the government given a nod to this conversion taking into account whether or not the vehicle can be insured against the third party risk. A gas cylinder explosion can be as disastrous as a bomb blast! Next a moral obligation to society: Is there any chance that the LPG used is from the highly subsidized domestic sources. Let us learn from the experiences of adulterating petrol with the subsidized kerosene.
A scientific answer to this relevant question must also be in tune with the societal benefits of science to the community. Hence this is a warning note to the otherwise a commendable fuel from a purely scientific point of view.