Please excuse if posted at wrong place. Does running car on CNG wears out engine? I am planning to buy used CNG car (India) but many mechanic advise against it for any CNG car than has run more than 35000 km but have no problem if it has 100000 km on petrol. I will be travelling 50 km daily, so CNG option seems best suited for me.
CNG conversions were very popular in New Zealand in the 1970s after the oil shock.As the conversion industry matured during the early 1980s some technical issues were discovered .The conversion people recommended that you recondition the engine before the conversion .This had serious cost ramifications and put many people off.The reasoning at the time was that the CNG does not have the same cylinder lubricating properties as the petrol it replaced .There were issues of engines not lasting as long as they were supposed to when converted to CNG .Many people who were contemplating CNG conversions were running up big miles like taxi drivers .People who did low milages generally did not bother converting because the prospective savings were not there for them.For you it depends on the price of an engine rebuild in India because you may be up for one down the track.
Autistic is right. Cars that are retro-fitted with CNG systems tend to wear out the engine. This is because, they do not have the lubricating properties as petrol does. CNG also burns differently from Petrol, in the sense that the flame propogation, fuel burn properties are different. Since, the car was not designed keeping CNG in mind, you are eventually killing the car.
However, if the car came from the factory with a CNG kit installed, then its a different matter, because the manufacturers will account for the difference. Your mechanic's suggestion is correct (sort of). Its not that cars that have run a long time on petrol will do better with CNG but that the cars that can run so much tend to be better built.
For example, if you had one of those Premier Padminis or the really old Maruti 800s then you should still be okay, because those cars used to be "hardier". Be warned though, cars that have a CNG kit fitted on don't run as smoothly. If it came from the factory, that's a different case altogther.
From my experience with aircraft engines, it will eventually damage the the engine if you run it continually at peak optimal lean fuel mixture. The peak lean mixture is obtained by adjusting the mixture setting until you get the hottest avg cylinder temperature.
This optimal mixture means the fuel burns much more efficiently and thoroughly since the fuel droplets are fully atomized or vaporized for the oxygen volume in the cylinder.
While the peak lean setting is great for performance and fuel economy, the resulting cylinder temperature is too hot for perpetual runs on certain engine components like valves and can damage them. Running the engine too hot will also break down the oil faster.
You don't get to manually adjust your fuel mixtures in automobiles, and most cars are liquid cooled in contrast to air cooled aircraft engines so you can avoid the really hot extremes.
The reason I bring up the aircraft running on peak lean is because a fuel that is already in a gaseous state like CNG or propane is exactly how the gasoline is when optimally lean. Your engine will run hotter on CNG because the oxidation is much more complete. The continous hotter cylinder temperature will tend to wear out the valves and engine oil faster.
As the other answers have indicated, liquid gasoline also would play a role in distributing the liquid oil throughout the cylinder since they are miscible.
I haven't seen the conversion kits but I assume they are different valves and gaskets.