Someone I knew when I was younger converted a car to run on compressed natural gas (CNG) because CNG was extremely cheap in comparison to petrol or diesel.
How can cars be converted to run on CNG and how do they work?
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This article pretty much covers it.
Not taking into consideration a source for CNG.
You need a lot of money. You need a new fuel tank capable of holding pressurized gas up to 3600psi. A fuel regulator and new fuel injectors. As well as a tune so that you can run the gas without destroying your engine.
Much in the same way you car runs on petrol, it can run on other types of fuel such as compressed natural gas, propane, or alcohols.
On modern cars however quite a few changes need to be made software side to support a conversion.
A few examples.
Air Fuel Ratio - On a petrol car a stoich AFR is 14.7:1, however on a car with CNG an AFR of 17.2:1 is required.
Ignition Timing & Fueling - A vehicle has multiple timing and fueling maps to take advantage of higher and lower octanes. So say you have a car with that uses a stock timing/fueling map and you are running a lower octane gas than recommended. Eventually you will encounter spark knock/detonation/pinging. Resulting in the engine computer shifting to a lower timing/fueling map to try and reduce the amount of knock to zero.
On the other hand with a higher octane fuel you can run more aggressive timing/fueling maps.
The engine computer will have a default map based on the manufacturers recommended octane level. This varies by state/country, in my region for example this can vary from 87-94 octane. Note that you can purchase higher octane fuels for racing purposes.
CNG has an octane rating of 130.
OBD2 - All US OBD2 cars have Evaporative Emissions control systems. Liquid petrol releases fuel vapors. Now this would normally go to waste, modern cars however reintroduce this fuel vapor into the engine intake.
However in the case of CNG the system is closed. EVAP systems are not needed and will need to be disabled in the engine computer. This however can cause problems with state OBD2 emissions testing.
Note that self modifying fuel systems to support CNG is prohibited under the Clean Air Act http://www3.epa.gov/otaq/consumer/fuels/altfuels/altfuels.htm.
Any change to the original configuration of a certified vehicle or engine, including alternative fuel conversion, is a potential violation of the Clean Air Act section 203(a)(3) prohibition against tampering (42 U.S.C. §7522 (a)(3)). The tampering prohibition is important because poorly designed modifications can increase emissions. However, EPA has established protocols through which conversion manufacturers may seek exemption from the tampering prohibition by demonstrating that emission controls in the converted vehicle or engine will continue to function properly and that pollution will not increase as a result of conversion. Please see the Final Rule and Information for Clean Alternative Fuel Conversion Manufacturers for detailed information about these protocols.
Various companies sell EPA certified kits that you can buy. Prices are typically in the thousands of US dollars.
You will need:
A tank capable of supporting 3600+ psi.
A heated fuel pressure regulator to reduce fuel pressure to 125 psi.
CNG Fuel Injectors.
High pressure hose & clamps.
A pressure relief valve.
Some form of tuning/piggyback software solution.
Your area may have a station that can provide this service. Below is a crowd sourced map with locations.
Home fueling stations such as BRC FuelMaker's Phill are available for purchase and will connect to an existing natural gas line.
CNG was popular in NZ during the fuel shock of the 1970s .People converted thier big gas guzzlers in a real panic while others went for small 4 cylinder cars .I saw an early conversion that resembled a bunsen burner stuck into the air filter .There was a big loss of range and boot space due to the low calorie value of CNG .There was also a HP loss .Starting was more difficult so they sold "CNG batteries that were simpley larger heavier more expensive batteries.The way the GOVT mamaged prices was to put the CNG up whenever the petrol went up anyway.There were rumours that engines didnt last as long but that may be an urban myth.The govt removed the subsidies some time in the 1980s and the "industry" fell flat .Nowdays nobody talks about it and the filling stations have become invisible or vanished.The last time I did any electronics work on it was when I had Hair .