What do you mean? Where I'm from LPG cars are extremely popular(13.4% of all cars on the road[0]), the winters are very harsh(-20C and less isn't uncommon), and all these LPG cars drive just fine. I used to have one myself, in winter you basically had to start it on petrol but you could switch to LPG 10 seconds later and it was fine.
Here in the US, the only LPG vehicles I've seen are forklifts and backyard hackjob conversions. My favorite one was a Subaru BRAT with the propane tank strapped into the bed.
In the UK the main problem was that the government went with LPG conversions as the cleaner greener way forward, then got a lot of pushback from car manufacturers who wanted to sell more cars. So in the early 2000s, they pushed diesels as cleaner and more efficient and got everyone to scrap their petrol cars and buy diesels.
And you know how that worked out.
But the long and short of it is, "what happened to them?" comes down to "they didn't sell enough debt to people" is what happened to them.
Different chemistry. Petrol must be burnt quite rich in the cylinder - remember in first year chemistry in high school learning how to use a Bunsen burner? Opening up the air valve to get a bright blue flame, closing it down to get a cooler yellow flame? Anyway if you burn hydrocarbons in a fuel-rich environment you get soot and carbon monoxide - there's too much carbon for the amount of oxygen available.
Propane wants to burn leaner, and burns with a bigger, slower, and hotter bang. You can run a far leaner mixture without pre-ignition (pinking or knocking) because it's got a higher octane number, something like 115RON. You don't get anything like as much soot, you don't get any carbon monoxide, and because of the different proportions of carbon and hydrogen you get more water vapour and less carbon dioxide per mile travelled.
Electric cars are clean though, you have massive improvement in air quality in cities alone.
And they are great economic opportunity since you're selling everyone a new car.
That's a massive economic boost.
Anything related to mining, well a lot of mining is bad and it's not like internal combustion engines have been all that great for the environment either. So moot imo.
Frankly I think the notion that EV is worse for the environment and battery technology being allegedly bad are just parroted talking points that negate to compare how EV would stack up to combustion technology if you applied the same number of engineering man hours to it but I digress.
Regardless propane from what I can gather isn't that much cheaper than gasoline at least for the U.S. so any potential savings in it being less expensive is easily lost by needing to burn more to go the same distance as gasoline.
Plus the conversion sounds expensive and warranty voiding. Most consumers wouldn't bother.
They're not very clean to make, which is the bigger problem.
If you convert a petrol car to run on propane, the exhaust emissions are water vapour and carbon dioxide, with no CO, no HC, and pretty much no nitrogen or sulphur oxides.
This is why things like warehouse forklifts run on propane - you can't die from breathing the exhaust fumes, unlike petrol or diesel.
Typically you'd convert older cars, of which we have plenty. We don't need to make new cars, because there are enough cars for everyone - we just need to make them run cleaner.
As I previously mentioned, it doesn't sell anyone any amount of new debt.
The whole point of "scrappage schemes" where you can scrap your dirty inefficient old car that only gets 40mpg and buy a new one that is much cleaner and more efficient at 41mpg for only 600 quid a month at 77% APR is to get people as deep into debt as possible.
Propane or natural gas vehicles have great emissions profiles, but unless it's a fleet operation that runs its own filling station, you're going to get range anxiety unless there's a massive uptake.
Pressurized fuel tanks have a larger safety requirement than gasoline or diesel, too; additionally, fueling accidents tend to be much higher energy. In the US, at least, the tanks expire and need to be replaced which is often uneconomical and puts a hard cap on vehicle lifetime.
LPG tanks in vehicles are far safer than petrol tanks.
Which is more crash-proof? A metal tank made of 5mm thick steel bolted to brackets welded into the frame of the vehicle, or a leaky plastic bucket tied underneath with a couple of straps?
Stress of fueling is probably a bigger deal than crash resistance. Most vehicles never get into a collision, and very few get into a collision where the integrity of fuel storage is an issue, but most are fueled often.
I've never seen a gasoline or diesel station have enforced safety requirements, but when I was looking at CNG cars, PG&E required pre-registration to access their fueling station, with a required fuel system safety inspection (which you can't pass with an expired tank) and a safety video. And it would be very inconvenient to drive through California without using their fueling stations.
Meanwhile, gasoline stations often run unattended, with no registration or safety inspection.
LPG is stored at about 8-10 bar. That's less pressure than is in an air suspension tank, and only about three or four times as much pressure as in the tyres.
In Belgium cars running on LPG cannot park in underground garages even though they go through very stringent MOT-like checks.
One of the problems is that conversions need more than just a switch of just gas to LPG. You need a tune and you need an adjustment / expansion of the cooling setup. LPG runs a lot hotter than gasoline and this needs to be managed.
Funnily enough modern gasoline engines are tuned to manage temperature by ... dumping in more fuel.
Which is why your turbocharged 3 cylinder .9L has amazing mpg while driving calmly, but once you push it it proportionally consumes way more.
You don't really need to do much. Most "modern" conversions bypass the petrol injector electrics to pick up injector pulse timing for the gas injectors, and have a separate ECU that manages the fuel mapping.
Once you've got it installed and given it an initial rough setup (not much more than programming in how many cylinders and how large an engine) it'll just trim itself. The LPG ECU maintains a map that makes the vehicle's own ECU think it's running at a perfect mixture on petrol, and is otherwise totally unaware that anything is different.
Older systems are a bit cruder, but still don't need much more than an extra lambda sensor fitted for the gas ECU.
You're still dealing with a system that expects that under certain conditions injecting MORE fuel will induce a cooling effect.
Not the case when you have an LPG system installed.
Whether the piggy-back understands this procedure and and obeys it or fakes it is irrelevant.
If your cooling system cannot handle this increased heat output you'll run into issues. Better fan will help, but again, this is more heat coming from the inside of the cylinder, so certain blocks will handle upgrades better than others.
This turns out not to be a problem, because the engine runs cooler on LPG.
The gas leaves the tank in liquid form and has to be boiled into a gas. Its boiling point is -40°C, and so the vapouriser is plumbed into the cooling system. This pulls a lot of heat out of the coolant.
I regularly tow at the maximum permitted weight and speed with my LPG-converted Range Rover, and the coolant never gets above its normal 95°C even on the hottest day.
You don't even need a catalytic converter, although it helps with the microscopic amounts of NOx that does get formed.
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