Originally Posted By: crinkles
LPG is common here, what's the diff between LPG and CNG?
LPG is Liquified Petroleum Gas or Liquified Propane Gas. In UK, Europe and Australia, it is called Autogas.
CNG is Compressed Natural Gas.
Differences? LPG is mostly made up of Propane and some butane. When pressurized, it becomes a liquid. In Asia, it is known as cooking gas because it is delivered in cylinders to households with no piped town gas to fire up the burners for cooking.
LPG's composition can be a little tricky. In places where there is winter, you got to have a "winter blend" for a higher vapor pressure for easier starts. In terms of storage, LPG's got the advantage of using toroidal tanks that are shaped to spare tire wells. You just sacrifice your spare tire to put this tank in place. In terms of range, it is slightly better than CNG. Sold in liquid measure AKA gallons or litres.
CNG is mainly methane gas. That's it. It is pressurized and can only be stored in cylinders. Comparing it with LPG, it's range is really short. There is only this much gas you can hold in a pressurized cylinder. Sold in mass measure AKA pounds or kg.
Both have an octane level of well over a 100.
Calorific content of CNG is the lowest while LPG is just slightly higher than it. Next comes gasoline and then diesel.
OK systems are slightly different for their delivery. Since LPG is a liquid, it must be vaporized before it is delivered to the fuel-air mix system. CNG on the other hand is pressurized so it has to undergo a pressure reduction prior to having it delivered.
Two main systems again for fuel-air mixing. Mixer systems and sequential gas injection (SGI) systems. Mixer systems are just like your caburetors. The gas is mixed with the incoming air with a metering valve. SGI have solenoid valves on the intake manifold. They just open and shut since the gas is of a somewhat pressure and there is no real injection unlike petrol
The reason why conversion systems are expensive is due to the cost of the SGI systems. Most cars sport O2 sensors for accurate metering of fuel. This means you need some form of electronics as well. Engine management systems of SGI systems tap onto the O2 sensors, deliver a simulated signal to the petrol engine management system and sets the timing and duration of each injection cycle taking into account the fuel is CNG or LPG. These systems don't really have a map so they copy the gasoline system's mapping for this. Pretty dumb system but it works.
Mixer systems are cheaper and best used on carbureted cars.
Personally I drive a converted to bifuel Aveo. Idles on gasoline. Uses CNG when the RPM goes above 750RPM. Starts on gasoline as well.
Why these? AFR for CNG is a a whopping 17:1. It is a dry gas. So there is no cooling effect from the vaporization from the gasoline mist. On idle, lambda should be set to 1.0 but nobody has reported an engine being damaged idling on CNG. The main gripe is that the calorific content of CNG is really lousy. Just to move off, there is just not enough torque generated compared to petrol to get into decent speed fast. You get honked from behind.
Starting on gasoline is needed as well. The gasoline injectors are still there and they still need lube from gasoline since they have now become your secondary fuel just in case you run out of CNG. Another reason to keep some gasoline is your fuel pump. You got to keep it alive for this same purpose.
Another stupid thing about CNG.....actually the OBDII engine management unit. The CNG can run really lean but that would mean an O2 signal from the secondary O2 sensor throwing up a CEL.
Some personal modifications to run CNG or LPG for a bifuel car. You got to reduce the plug gaps. Gaseous fuels make it difficult for ionization and subsequently enable a spark. Your coils suffer from overheating and lifespan compromized. For a typical car with 1.1mm plug gap, you got to reduce it 1.0 to 0.9mm. The gaseous fuels also have this slower flame front thanks to the higher octane level. Advancing the ignition timing will help a lot in bringing about more power. There are these "timing advance processors" that can be installed for this.
Also if you are using certain GM engines, a cooler plug is needed. So if you are using NGK heat range 6, you should up it 7. So far the IL4 engines don't need this.