Direct injection is an interesting concept that might work well with CNG since the pressure is quite high
LP gas is stored as a liquid at around 300PSI
The pressure for CNG in the tank is about 200 atm (14*200 = 2800 PSI).
Usuall pressure in engine is about 12-16 atm (160-260 PSI).
So I don't beleive that direct injection of CNG is practical. However make use of the higher pressure for gas enabled (LPG & CNG) vehicales could be the option.
The technical challenge in this case is how to combile the gas (higher pressure) and gasoline injection in single engine. Most of the cars has two tanks: one for usuall gasoline and another for CNG/LPG.
OK, so I may have been a bit off about needing to heat the CNG before using it. The busses have a real advantage in that they have a bus-barn to refuel at and a special person, or crew, to do the refueling. Do you have any idea about the capacity of those fuel tanks?
The busses in Fort Worth run on CNG. They do have a heat exchanger mounted on the side of the buss to re-heat the CNG before feeding it to the engine. They have huge tanks on top of the buss and have to fill up every 2 hours. It's still economical compared to gasoline and the required maintenance is somewhat less due to the cleaner fuel.
Right now the big advantage of CNG is cost. Smog emission are 20% to 45% lower and Green House gas emissions are 5% to 9% lower on an equivalent basis. The equivalent cost per gallon is about $1.00+ less.
CNG (Compressed Natural Gas) can be successful in small vehicles. Honda has a CNG version of the Civic. However it's only available in California. The driving range is about 190 miles.
The overall operation is similar to LPG even though LPG contains 2.44 times as much energy per cubic foot. Since much of LPG is made from Natural Gas, there is no cost advantage. Even though LPG is a liquid, reasonable range is still provided by CNG due to the higher storage pressures used.
An alternative is to use LNG (Liquefied Natural Gas) This improves the range issue compared to CNG. The availability of this technology has helped drive the recent move to Natural Gas as a vehicle fuel. However the big deal is still cost.
There are quite a few differences betwen using Liquid Propane (LP), and using natural gas (CNG). LP gas is stored as a liquid at around 300PSI, depending on the ambient temperature, while CNG must be kept at a much higher pressure since it is a gas, not a liquid. Both are used in the engine at a quite low pressure, so there is a lot of cooling as the pressure drops. LP regulators often include an arrangement to use hot engine coolant to prevent them from freezing, but I don't think that is needed for CNG regulators.
Direct injection is an interesting concept that might work well with CNG since the pressure is quite high, but manifold injection is probably the only other option with LP gas, due to it's lower pressure.
The real challenge would be in the logistics of refueling, since we have gasoline stations on half of our street corners, while LP refueling stations are much less common. Of course the 25 pound propane tanks do seem to be available at most corner stores, but it is not clear to me that a tank that size would provide enogh miles to make it a good choice. Also, I don't know if any of those stores accept trades, or do they just sell filled tanks?
The other very real challenge is the competence level of many people as far as being able to connect the tank without allowing a major leak. There is a whole class of folks who cross-thread lightbulbs and garden hoses, and I doubt that they would be able to connect a filling hose without allowing a lot of leakage. So there is a challenge that would need some real innovation to solve.
I have travelled to Korea (south of course) fairly often and they stand out as an example of how this fuel integrates into a country. They have widespread fueling stations for both gasoline and LP (not sure if LP or CNG). To the average consumer, using either is not an issue. However, it appears that the use of LP is more widespread in taxis than regular drivers.
That being said, they have a whole different dynamic than here in the US. Korea is much smaller than the US (about 300 miles will get you the whole way across the country). People don't have to drive as far or as much. Public transportation (train, bus, taxi, etc...) is widely utilized.
I found out on one trip that there are considerations when in a place that LP is used. A co-worker and I could not fit ourselves and our luggage into a standard sized taxi that ran LP. The driver had us use another taxi that looked the same, but was gasoline powered. Everything fit fine in it.
@Charles - There is definitely performance degradation on natural gas, however there is increased efficiency due to cheaper cost. Another downside though, is that, there is a gas cylinder which takes up a lot of space when natural gas is used as a secondary fuel source.
@naperlou - I'm from South Asia so I can affirm to that. Most vehicles ranging from trucks down to the conventional taxi cab runs on natural gas over here and even though we have a rich natural gas resource over here, the overwhelming number of vehicles has caused a supply shortage resulting in crises.
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