Dear @Sundar, here are some facts regarding my country over LPG and LPG cars: 

78 % of LPG consumption in Serbia

App. 500,000 cars (25 % of 2 000,000 total cars) with LPG installations

End sellers (motor fuel LPG stations) are excised with 150 e/t

More than 650 motor fuel LPG stations. Last couple years companies like OMV, EKO, HELLENIC, MOL, LUKOIL, MB Gas, EUROGAS… established their own

network...

The resource is attached!

http://www.argpl.ro/congress2010/session2/session-2_butangas-serbia_goran-djurdjevic-serban-popa.pdf

What is the difference between LPG and LPS cars?

I agree with you dear Professor @Sundar that the LPG-based cars have the advantages of very low pollution and therefore, more research should be done to minimize the pollution and help protect the environment.

"On a cycle representing congested urban traffic, both LPG and CNG outperform petrol powered vehicles on emissions of carbon monoxide (CO). Indeed, emissions of CO from CNG powered vehicles are of the same order as those emitted by diesel vehicles. However, emissions of total hydrocarbons (THC) from CNG vehicles are relatively high because of methane, the major component of natural gas. Although methane is a small contributor to the formation of low level ozone it is a major factor in global warming. Emissions of NOx and particulates from both LPG and CNG powered vehicles are significantly lower than those from diesel vehicles. Moreover, emissions of NOx from CNG vehicles are half those from equivalent petrol engined vehicles. A recent study using a small delivery van fitted with a three way catalyst and capable of switching between CNG and petrol, showed that on a modified EU emission test cycle, emissions of CO, non-methane hydrocarbons (NMHC) and NOx were 76%, 88% and 83% respectively lower with CNG than with petrol. Using data from other studies CNG also compares favourably with emissions from equivalent sized diesel-engined vehicles."

http://www.air-quality.org.uk/26.php

Here are the differences between CNG and LPG.

"CNG is Compressed Natural Gas, which is mainly methane compressed at a pressure of 200 to 248 bars. LPG is Liquefied Petroleum Gas, a mixture of propane and butane liquefied at 15 °C and a pressure of 1.7 - 7.5 bar. Some variants of LPG are primarily propane so LPG is often colloquially called propane. CNG is cheaper and cleaner, but LPG has a higher calorific value. Distribution is easier for natural gas over long distances via pipelines."

http://www.diffen.com/difference/CNG_vs_LPG

Dear Nageswara, welcome back to you and your activities. In fact, I agree with the statement that "On a cycle representing congested urban traffic, both LPG and CNG outperform petrol powered vehicles on emissions of carbon monoxide (CO)". 

So far as India is concerned large scale use of LPG can pose Socioeconomics problem. Cooking gas may become costly. Subsidy will be raised and will burden exchequer

BBiodiesel may prove more fruitful for environment as weleconomiics.ic

Dear Professors  Kamal, Mahfuz & Nageswara Rao:

From the environment point of view, LPG cars may be better with less harmful emission.

However, the safety issue of LPG cars may be kindly addressed as well!

Thanks, Sundar

The pros and cons of autogas systems: See,

http://gazeo.com/up-to-date/news/2012/The-pros-and-cons-of-autogas-systems,news,6603.html

I beleive LPG cars are preferable, but hydrogen cars would be the ultimate objective!

LPG - cars may easily gets fire and on highway several accidents have occurred. So comparatively they are less safe than petrol or diesel cars.

For consumers in Poland, the most important benefit is almost twice lower price per kilometer LPG-based car  than petrol car

Low prices are certainly an effective motivation to use ecological fuels.

1. LPG can be used for CI engines either in gas or liquid phase. Liquid phase LPG has advantages over gaseous one due to its higher evaporation heat capacity and density. Higher evaporation heat leads to lower temperature of fuel-air mixture before inlet valves (in case TBI or MPI are used). Higher energy helps to eliminate the lower calorific energy of LPG with respect to petroleum fuels such as gasoline. 

2. As a gas (at atmospheric condition), LPG transport faster compared to liquid fuels and therefore it improves mixing quality => more complete combustion => less emission. On the other hand, H/C ratio of LPG is higher than gasoline => can reduce greenhouse gas emissions.

3. Price of LPG fuel in one country is incomparable with that in others because some Governments subsidize (usually via tax) for this fuel to encourage people using it and others don't do so. 

4. Regarding to safety, this is controversial but I think LPG is as safe as gasoline. LPG fuel system contains several safety valves which can isolate the fuel in the tank with the supply pipe. In case the car get a bad accident and the crash can create an explosion, I would imagine the dangerous level is similar to that of gasoline car.

5. One of the disadvantages of LPG fuel is problems related to storage (in both stations and in cars) and refueling.

The following literature may be useful for your research

The advantages of LPG include:

Because LPG vaporizes when released from the tank and is not water soluble, LPG does not pollute underground water sources.

Power, acceleration, payload and cruise speed are comparable to those of an equivalent vehicle fueled on gasoline. Propane has a high octane rating of 104, in-between Compressed Natural Gas (CNG) (130) and regular unleaded gasoline (87).

Refueling a propane vehicle is similar to filling a gas grill tank; the time it takes is comparable with that needed to fill a CNG, gasoline or diesel fuel tank.

Its high octane rating enables it to mix better with air and to burn more completely than does gasoline, generating less carbon. With less carbon buildup, spark plugs often last longer and oil changes are needed less frequently.

Because it burns in the engine in the gaseous phase, propane results in less corrosion and engine wear than does gasoline.

The drawbacks of LPG include:

In cold conditions, below 32 degrees Fahrenheit, starting could be a problem because of the low vapor pressure of propane at low temperatures.

One gallon of LPG contains less energy than a gallon of gasoline. The driving range of a propane vehicle is about 14 percent lower than a comparable gasoline-powered vehicle.

LPG is generally higher priced than other fuel alternatives such as CNG and gasoline.

There are over 4,000 LPG refueling sites in the US, more than all of the other alternative fuels combined. Most of these stations, however, are not readily available to consumers on a 24/7 basis. This is one of the reasons why most on-road applications are bi-fuel vehicles, which burn LPG and gasoline.

Govt should encourage in form of giving subscidies, concession in manufacturing etc., so as to minimize pollution levels iwth gasoline, which are causing health hazards and environmental impact.  

Gasoline-fueled light-duty vehicles in the U.S. are among the lowest polluting vehicles in the world and are now nearing zero emissions of criteria pollutants.  There are numerous vehicles certified as LEV-II SULEV or Tier 2 Bin 2 that achieve combined NMOG+NOx emissions of less than 30 mg/mi and PM emissions of less than 1 mg/mi.  This is roughly equivalent to the lowest emitting natural gas fueled vehicles (Honda Civic GX) or better and is significantly lower than spark-ignition vehicles converted to LPG.  Instead of subsidizing a particular fuel, environmental goals may be better served by focusing on stringent standards for NOx, NMOG, PM and CO2 and by reducing sulfur levels in all fuels (gasoline, diesel, NG, LPG, including mercaptans added as odorants to gaseous fuels) to enable the use of very high efficiency catalytic emissions controls.

Driving LPG car or any light vehicle is cost effective and ecofriendly in terms of being non-polluting. However, safety of such vehicles is an issue of  concern that need to be looked into.

I wouldn't consider LPG to necessarily be ecofriendly or non-polluting.  This is still an internal combustion engine and thus LPG SI engines still require the same exhaust emissions control systems as other SI engines in order to control NOx, CO, semi-volatile organic particulate matter from lubricating oil consumption, aldehyde and ketone emissions, etc.  This would generally be through stoichiometric A/F control and use of a 3-way catalyst, just like with other SI engines.  Given a similar level of such emissions control technology, there is no clear advantage of LPG with respect to emissions of air pollutants.  LPG is less carbon-dense than gasoline but more carbon dense than methane, so CO2 emissions would be intermediate between petrol and natural gas at comparable thermal efficiency.  If mixture preparation is via an upstream gaseous fuel mixer, fuel control would be less precise than with port fuel injection of gasoline and emissions of some pollutants could actually be worse than for gasoline-SI engines.

Gas LPG is popular in these countries where is relatively cheap in relation to the petrol. This problem is multidimensional: for the consumers only the price is important, for politicians - economic factors, for the scientist.... everyone will find field for himself for research

LPG is no more than the liquified propane gas which we see in cigarette lighters. On the face of it, it is cheaper than gasoline & burns more cleanly but I do not think that  switching into liquefied petroleum gas car is a big deal. Firstly, computerization of modern cars was built for gasoline-fueled cars & not for LPG-fueled cars. The "re-computerization"  will add cost to the cost of good conversions in the internal combustion engine "which was optimized for gasoline", and this makes the whole process EXPENSIVE. When talking about cheapness, many forget that the heat energy of propane/ liter is less than that of gasoline which means that you have to fill more LPG in the tank for the same distance. 

Add to all of the above, that spare parts are not available everywhere for different car models as well as that the fuel LPG cannot be found in many gas stations worldwide. Gasoline & gasoline-run cars are universal.