Anybody considering CNG conversions, check what I found

[cgrey8]

With the price of gas going up, I thought I'd at least research this a little. And here's what I found:
EECTuning.org > Anybody ever used a Ford EEC with a CNG/LPG system?
This thread is heavy geared toward someone that is also a DIY tuner, but some of the info about the CNG kits could prove useful to anybody that's just wanting to know a little more about them, what the options are, typical investment/costs, and other random details.

[cgrey8]

Well, I'm going to try doing a CNG conversion. I bought the tank today. I'll need to get the CNG conversion kit next. And finally, the BIG dollar item, the CNG compressor.

I'm looking forward to being able to drive without thinking about the cost of fuel. I miss the days of $1.30/gal. If successful, this will bring those days back to me while driving the truck within the travel distance of my CNG tank which is 95% of the time.

[MalcolmV8]

What sort of mpg equivalent are you looking at getting? With the way gas prices are it should make for a fun project that's also practical.
My turbo Honda project I'm working on has an unexpected surprise. I run the hell out of this car, still n/a, and I'm averaging 35.x mpg on a tank of mixed city/highway driving. Shoot at that rate I started driving it to work too

[cgrey8]

On most vehicles, CNG gets a slightly lower MPGe, however that's generally because they are running CNG on engines with Compression Ratios designed for 87 octane pump gas. This newly build 331 is much higher compression and in light of my decision to go CNG, I told the machinist to pop the pistons out of the holes another .001" to bump the compression ratio just a tad higher. That pretty much requires me to stay on 93 octane when on gasoline, but if the CNG conversion works out, I'll be running CNG 99% of the time, and the CR will actually be a tad LOW for CNG. I could easily turbo/supercharge the engine with CNG. But then I could never run gasoline on it again or if I did, I'd have to be careful never to get in the boost...ever. Point is, with the higher compression engine that's more inline with what CNG needs, I honestly don't expect a drop. And from the beginning, I've been crossing my fingers that building the 331 mild would preserve the fuel economy I currently get or improve it. After seeing how pathetic the compression ratio of stock Explorer engines are, I'm REALLY hoping I boost fuel economy a few points while on gas. I'll need it if I'm running 93 just to maintain the same mile/$.

[MalcolmV8]

When you say the mpg equivalent is lower I think you mean the actual fuel consumed vs miles driven right? What I was asking is by cost. So say for example your truck gets 15 mpg now and fuel is $3.75 a gallon it would cost you $25 to drive 100 miles. So that's 25 cents a mile to drive. On natural gas from your home how much would it cost to drive the same 100 miles? what does it work out to per mile in cost?
I'd assume less or you wouldn't be doing it.

How did your machine shop pop the pistons out by 0.001" more? Just deck the block? does 0.001" make much difference in compression?

If you do decide to put a turbo on just make sure you run an electronic boost controller. When you put pump gas in you just set boost to zero and the wastegate will stay open and it will never go into boost. That way you can drive normal and even go WOT safely.

[Chris]

What kind of compression ratio is ideal for cng? What are we talking about 10.5:1 or something higher??

[cgrey8]

Ideal and max are fairly relative. Although lets put things into perspective. Lets talk MAX first.

Regular 87 octane rated gasoline has a max Dynamic Compression Ratio in most engines of about ~7.9 Dynamic Compression Ratio (DCR) which is ~9.0-9.5 Static Compression Ratio (SCR) depending on the cam.

Premium 93 octane rated gasoline has a max DCR of ~8.5 DCR which is between 10.25-11 SCR again depending on the cam. More radical cams will lower the DCR allowing you to run a higher SCR. That's why it isn't uncommon to hear about radical engines running 11:1 SCR on premium pump gas. Try to run 11:1 with a stock or mild cam and you'll ping the heads off even with 93 octane.

E85 ethanol has an octane rating of around 105 although that's usually the RON (Research Octane Number) number, not the AKI (Anti-Knock Index) octane rating we use here in the states. Unfortunately, I can't find what a MON (Mechanical Octane Number) value for E85. AKI for any fuel is calculated as:

Code: Select all

RON + MON
__________
    2

Look on any gas pump where you see the octane rating and you'll see that formula.

Testing has found the AKI number for E85 is ~96-98. So the highest you'd want to run a dedicated E85 engine is around 8.8-8.9 DCR which will put SCR into the 10.75-12 range depending on cam. Most people set them up for 8.5 and then have the ability to run either premium gas or E85 with only a minor change in spark map for gasoline to keep premium from detonating.

Propane has an AKI octane rating of 106. So you are talking a DCR well into the mid 9s...SCR around 12.5-13.5.

Now Methane (the primary component in Natural Gas) has an AKI octane rating of 120 which means you could have a DCR well into the 10s and I suspect 11s wouldn't be out of the question which puts SCR into the 13-14:1 range.

This is why most people that do CNG conversions on a 100% stock engine don't burn the CNG as efficiently as they could if they built an engine specifically for CNG...the compression isn't optimal and the spark is still assuming gasoline. But even with the inefficiency, it's still cheaper to burn CNG than it is to buy gasoline right now. However if you had an engine you could dedicate to CNG, then you could build it to be 13:1 SCR with a mild streetable cam and get GREAT fuel economy from it. Although your standard starter might have a hard time turning it over. You might have to upgrade the starter.

So for dual fuel applications where CNG must work with a gasoline-centric compression (including my high compression 331), it is not a high performance fuel per se. It's only high performance in a high compression engine that can utilize it. In lower compression engines, it just burns too slow to work well at high RPMs and if your spark advance isn't updated accordingly, you can burn exhaust valves if you delay spark so badly that the CNG is still burning when the exhaust valve opens. In low compression engines, what you find is that low RPM torque is much better than gasoline because the fuel continues to burn so much longer in the power stroke giving lower RPMs noticeably more umph which is EXACTLY what I've been looking for since the beginning of this 331 project. Since I have access to the tune to run a much higher spark advance, I can avoid some of the negatives, gain some additional HP & RPM, and get more more efficiency from the fuel while cruising. I suspect I'll also be able to completely disable the EGR while in CNG since CNG really doesn't need EGR to prevent detonation. But I'll experiment around and see if the presence of EGR still has any positive benefit to fuel economy as I found it did with E85. Having EGR improved my E85 MPGs noticeably...far more than I notice with gasoline.

Because of CNG's high octane rating, it is probably most efficiently used in CNG conversions on diesel engines which are 20-21:1 SCR. But since diesels don't have spark plugs to ignite the NatGas, you still need a small amount of diesel (roughly 20-30% of full amount) to act as the ignition source for the NatGas. So CNG converted diesels will still burn a small amount of diesel, but a tank of diesel will last many more miles than it normally would since you are primarily burning NatGas.

Now as said above all those things mentioned above are max compressions. But most engines don't run the ragged-edge. I calculated the stock Explorer engine that I did the 331 conversion on to be a DCR of about 6.6 in stock form (low 8s SCR)...far lower than the 7.9 DCR it could be with tight quench & debur sharp edges of the combustion chamber while still burning 87 octane. Just as gasoline can be run far lower than its max without too much trouble, so can CNG. The bump up in compression on the 331 will help, but it is getting no where near the max potential compression of the NatGas. To do that, I'd have to add a turbo or supercharger. And even then, I could probably run 25 PSI with the NatGas.

[MalcolmV8]

Very interesting Chris. I hate reading your research and posts cause now I'm thinking about what I can convert to CNG just cause it's cool lol I always get side tracked with projects like this. I currently have an extra turbo at home too I'm now looking to stick on something. My son wants me to put it on his quad but I told him its way over sized and wouldn't do much of anything lol.

I don't have personal experience yet with E85 because there's just not enough stations nearby. I have one near my work and another that's about 20 minutes drive north of where I live so it's not practical for me to switch to it yet but once a station opens up near my house the Cobra's going E85 for sure. From the cobra forums it appears guys are getting equivalent of 110 octane race gas performance out of it. They are rebuilding their engines and going from the stock 8.5:1 SC to 10.1:1 and using twin screw blowers such as Whipple and Kenne Bell and making some monstrous power.

Once I get this Honda turbo'd in the next few weeks my plan is to try out E85 for the fun of it and crank the boost up and see what it'll do.

Do keep us updated on the gas conversion you're doing and some pics of the parts when you get them. Most interesting.

[cgrey8]

I'm posting MOST of my findings over on the EECTuning.org thread. So periodically check it out or register on that forum and subscribe to the thread so you get notifications of updates to the thread. But there are pics and other findings not mentioned here talked about over there.

[MalcolmV8]

I saw, I was looking at it earlier. I see you even purchased the tank yet. Very cool so the project is actually on the way.

[cgrey8]

I called the guy that sells the kit, but he didn't answer. I would've gotten it today.

I also bought the 8ohm resistors to wire inline with the low impedance 4ohm CNG injectors. I actually bought 16 of them. They were only $1 each and didn't add but a $1 to the shipping so I figured why not?

Eight of the 16 will be used inline with each injector to act as a current-limit so the EEC doesn't burn itself out. However if I find that the injectors just aren't opening consistently or reliably at lower pulse-widths like at idle and decel conditions, I will have the other eight resistors to combine in series with some capacitors. The resistor/capacitor pair wired in parallel with the 1st resistor on each injector would give a slightly higher, but still suppressed, current flow when each injector initially turns ON. As the capacitor charges over time, the current it flows to the 2nd resistor will be gradually reduced. This means the initial current flow when the injector turns ON will be through both resistors putting total impedance in the 7-8 ohm range...which if sustained is a strain on the EEC's FETs, but for a quick burst, it shouldn't be too detrimental since it is so close to the typical 11 ohm resistance of high impedance injectors. The heightened current doesn't need to last long...just long enough to get the injector physically opening. As the capacitor charges, the current through the 2nd injector will be gradually reduced. Once the injector is opening, the lower current flow of only 1 injector can easily maintain the movement and hold it open. But I didn't want to just ASSUME that this would be necessary.

[MalcolmV8]

Similar issue with my turbo project. I have these 450CC DSM injectors that are low impedance so I got a resister box to wire in with them. I believe it came from an Eagle Talon but I could be wrong.

[cgrey8]

I found that some Hondas also came with resistor boxes from the factory. Although theirs look far more sophisticated since they are encased in an aluminum housing shaped like a heat-sink. So there's likely more to them than just a resistor bank...or maybe not.

The good news is it sounds like the added complexity won't be necessary.

[Soul]

I've been reading on this as well, I was wondering if I could build a v6 or inline 4 with high compression and get one of the Holley 4 barrel adapters for propane and run it, what kind of power and gas mileage would we see sense the efficiency would be way higher? A 12:1 v6 running off these in my 87 ranger. The kit I found is about 1k and is just to bolt to a carb intake, I could source an engine pretty cheap (free) and shave the head to get the compression up. Curious if this would be worth playing with? Could we get 200 + hp out of a high compression v6 and a ton of timing and make it drivable on propane?

http://www.gotpropane.com/p4.html

Here is the place where I was looking before.

Btw forgive me if I'm rambling and not really knowing what I'm talking about, just think this is a cool idea and would like to explore it.

[cgrey8]

If you modify the engine for the fuel, the efficiency should be massively improved.

Google for ace cng conversions. Their prices for all kits were far cheaper than anywhere else I found.