I got more info on the CNG kits. It's my hope to make this thread an FAQ for others looking for information about CNG Conversions, Conversion Kits, Refueling Stations (aka CNG Compressors), and CNG tanks. This has been a learning process and I'm hoping to collect as much information about the pitfalls and whats to be expected from the choice to do a CNG conversion into a single location. It'd also be nice if it was a high hit on Google for common CNG searches.
The cheaper kits are well, you get what you pay for. They sell for about $600 or so. For specifics about the kits and some interesting FAQ reading, click around all the pages here:Energy & Water Solutions>CNG Conversions
The bottom line is they are rudimentary CNG conversions away from gasoline, but the installation of the conversion kit reduces your performance substantially. This is because the mixer you have to install into your intake track is an orifice device that is quite smaller than even the 55mm stock GT MAFs. The mixer creates a venturi effect to pull the CNG into the air much like a carburetor does. There are various size mixers, but ALL of the engine air has to flow through the center of this device for proper CNG metering:
As you can see, even the largest one still only has an opening of maybe 30-35mm if that. This gets installed right in front of your throttle body and they give rubber stuffing to go around the mixer so air doesn't flow around it. Using these kits can cut airflow by as much as 60-70% over stock which is a substantial performance reducer. Although the conversion to CNG, even if it isn't going through a reducer tends to reduce performance since NatGas has an octane rating of about 120 (cited from Wikipedia) and most gas-engines just don't have the compression to make good use of that high of an octane rated fuel. Even high compression engines (DCR in the mid 8s) are still not really taking advantage of the fuel's performance potential. My estimate is that you need an engine with a Dynamic Compression Ratio up in the 9s to run optimally with NatGas. This would explain why NatGas conversions of Diesels are much more popular. Although even on Diesel conversion, you still need a small amount of diesel always being injected just to lite-off the NatGas. But diesel consumption with NatGas conversions is still substantially reduced.
There is a computer that "influences" the NatGas flow via a stepper motor. At 1st, I thought the stepper motor was like an injector, but it isn't. In concept, you can think of it more like metering rods in a carburetor. When opened wider, it simply lets more NatGas flow into the air that is passing through the venturid mixing device. This computer taps directly into the TPS and a single HEGO's sensor lines. While at "closed loop" TPS positions, the computer modulates the stepper to maintain the mix near stoic via the HEGO feedback. With wider TPS positions, the stepper is opened up to give an enriched mix. No details are given as to how far it is opened or what the target lambda AFR is at high load/WOT. But based on the simplicity of this system, my guess is the stepper is just set wide open and whatever AFR you get is what you get. With the engine having a vast majority of its airflow potential restricted, it's not like running lean at WOT is going to hurt the engine.
There's also a device that goes between your EEC and injectors called an Injector Emulator. In reality it's nothing but a commanded series of switches...for the electricians out there, think of this device as a 4-pole, single-throw relay. So for a 6/8 cyl, you need 2 of these devices. When in CNG mode, these devices simply cut the connection between the EEC and injectors. If you manually flip back to gasoline mode or you run out of CNG, then the CNG computer reactivates the gasoline injectors. It's as simple as that. The stock computer continues believing it is in control so it'll maintain spark, EGR, Canister Purge, Smog Pump, transmission, and whatever else the EEC might normally do. The catch is there's nothing to keep the CNG system in sync with the EEC's attempts to control AFRs. So it is possible that the EEC LAMBSEs, while in Closed Loop, could hit adjustable limits and eventually activate Check Engine codes. In older EECs as long as the EEC see's some HEGO switching going on, that's not likely to happen, but in newer EECs, you may hit a limit that triggers a code that the EEC has adjusted too far rich or lean which could result in Check Engine lights and codes. Regardless of the era EEC, having the LAMBSEs at or near their limits is likely to make returning to gasoline harsh upon reactivation. Once control is given back to the EEC, Closed Loop action will adjust the LAMBSEs back to where they need to be for running gasoline. But that could take 10-30 seconds to completely correct for depending on your HEGO bias settings. EECs setup for forced Open Loop will have a flawless return to gasoline since the EEC was never aware it wasn't in control the whole time.
There are higher performance CNG conversion kits and as you would expect, they are more expensive. These kits are in the $1500 range. These include solenoids that get mounted in the intake near the ports like injectors. This of course, requires that if you are maintaining your existing gasoline tank and gasoline fuel system, that you drill your lower intake for these CNG solenoids. For these kits, you can think of the CNG solenoids as injectors. I haven't seen them, but I wouldn't be surprised if they literally were injectors. The solenoids get fired by a Sequential Fire Computer which is more complex than the CNG computer in the above kit, but still not nearly as complex as Ford EEC. I believe these CNG computers are Speed-Density based. They evidently do have a fuel rail and a regulator that monitors vacuum just like gasoline FPRs do. But I haven't gotten nearly as much info about them yet...still inquiring. My question to the guy that sells these is can I buy just the CNG solenoids, fuel rail, and other supporting hardware without the computer. I'd use those "Injector Emulators" and flip between CNG and gasoline tunes while also flipping between CNG and gasoline injectors but control it all from my Ford EEC. The catch is these solenoids are 3 ohm impedance. So just like running low impedance injectors, you'd need low impedance injector drivers to control the CNG solenoids. The upside to these kits is you don't have the vast restriction of air and you have true port injection of CNG. On a moderately high compression engine or blown engine, I expect you could get substantially higher performance and fuel economy than you could with gasoline since you could run a 120 octane rated fuel right on up as high as you could boost. The limits would be your booster/turbo/block, not the fuel's ability to resist detonation. If I were to do a CNG conversion, it would be with one of these kits. Although for a stock beater car, the above kits might still be compelling particularly if you've already invested in a home refueling station.
Keep in mind, these are just the kits. These are not the CNG cylinders (fuel tank) nor the home refueling stations. CNG cylinders will run anywhere from $400 to $1500 shipped depending on new or used, how large, and how close to their expiration date they are if used. CNG cylinders are often sized in Gasoline Gallon Equivalent (GGE) values. The larger cylinders I've found are capable of holding 10GGEs@3000PSI and are upwards of 150lbs. They do have smaller "trunk size" tanks, but they have much lower GGE ratings...some as low as 2-3GGEs. If you have tanks and CNG conversion kits that can handle pressures up to 3600PSI, then that GGE value increases by 10-12%. You can combine cylinders together to get longer driving distances.
Home refueling stations are nothing but multi-stage high PSI compressors. They take in NatGas from your house's gas supply, and compress it up to 3000/3600PSI depending on your setup. They wire up to 240v service and cost around $4500 shipped. I can't remember exact values, but I believe a 10GGE tank takes about 6-7hrs for a home refueling station to refill. If you have public CNG refueling where the CNG is already compressed, refueling can be done in about the same time it takes to fill up a normal gasoline tank.
Now for the real numbers.
$700-1500 Kit (depends on supplier, hardware supplied, and how extravagant the kit is)
$200 extra crap to get it installed and getting lower intake drilled/machined
$1500 used 10GGE@3000PSI tank
$4500 Home Refueling station & shipping
$100 for parts and wire to get it installed in your house yourself
$7800 total. That sounds like a lot and to be honest, it is. But lets see how long it'd take to pay off.
With gasoline at $3.50/gal, getting 22.7MPG, and driving 150 miles/week, lets see how many weeks or years it'll take to pay off...and this assumes NatGas doesn't get more expensive and gasoline doesn't get cheaper.
------------------- = 6.48 Miles/$
----------------------- = $23.15/week in gasoline I spend
If NatGas is something like $1.80/gal equivalent cost to gasoline when you consider its cost, the electricity for running the compressor, and the difference in fuel economy for running CNG vs gasoline. I haven't seen a number on the Internet this high for a $/gal gas equivalent so I believe it is actually a lot lower than this, but I'm going conservative here to get a worst case payback time.
---------------------- = 12.61 miles/$
---------------------- = $11.9/week in CNG
How long before I "save" enough in CNG to pay for the $7800 investment:
------------------- = 693 weeks or 13.3 years
That's a long time for the gas and NatGas prices to stay stable AND for my vehicle to stay running. Doing a CNG conversion for the fuel economy alone just doesn't make sense. You have to really WANT to do the conversion or have a delivery job that causes you to drive a lot of miles a day. And in those cases, your payoff time would be a lot sooner. Lets say I drove 150 miles/day, not 150 miles/week, that would have the CNG system paid off in less than 2 years. Now that might be worth it.
Now, what if your state has an alternatives conversion incentive or tax rebate. If you can deduct these costs off on taxes OR get an out-right subsidy reimbursement for the conversion, then obviously this payoff time comes down. But often these deals from the gov't are only on EPA-approved conversion kits. And none of the kits I looked at are EPA-approved. Those kits are WAY more expensive and targeted for fleet vehicles and diesel vehicles.
For an easier formula to plug into to see what your CNG payoff time would be with your estimate of CNG installation cost and tax/state savings, use this formula:
$1.8 X $3.50 X
------- + CNG Install Cost - Tax/Subsidy Saving = --------
MPG = Your vehicle's gasoline fuel economy
Update the $1.8 if you find the CNG gas equivalent cost has changed (CNG cost goes up substantially)
Update the $3.50 for going rate for a gallon of the grade gasoline you use.
Solve for X where X will be total number of miles you'd have to drive to pay for the investment. Divide X by the number of miles you drive in a day/week/year to determine how much time it'll take to pay off.
89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, 1.6RRs, FMS Explorer (GT40p) headers, Slot Style MAF, aftermarket T5 'Z-Spec', 8.8" rear w/3.27s, Powertrax Locker, Innovate LC-1, GUFB, Moates QuarterHorse tuned using BE&EA