Cummins CNG -> LPG / Dual Fuel 1

[cullennewsom]

Howdy,

I have a "B" series Cummins CNG fueled engine. It is the 195 HP variety with Turbo and Intercooler. It is circa 2001 vintage. It runs just fine, and seems relatively economical.
My problem is that since I am in Texas the lack of CNG filling stations limits the usefulness of the vehicle. I suppose I could completely change it over to LPG, but then maybe I want to go to Oklahoma (where CNG is super cheap). So I have got the bright idea that the vehicle could run on either CNG or LPG. Which makes the vehicle super versatile in that not only can I go into CNG and LPG territory, I would now have diversified my fuel sources, and can therefore pick and choose for the best prices.

Hopefully I can get by by adding:
a second fuel interlock
proper tank and piping
lpg vaporizer
some fancy valve (and use the same mixer for both)

The things I have the least confidence in are:
Ignition timing settings
Mixer tuning

Should I have different ignition timing for CNG vs LPG?
does the mixer need re-adjusting / parts replaced

Or is there something else that I am missing?

Thanks in advance,
Cullen

 

[Lou Scannon]

Cummins (actually Cummins-Westport now) makes both CNG and LPG versions of that engine. The major differences are:
pistons (lower compression ratio)
pressure regulator (LPG version is a totally different design, with a vaporizing stage)
gas flow sensor (significantly different calibration)
ECU program (different air/fuel ratio, spark timing & boost calibrations)

I can't imagine being able to successfully switch fuels on a single configuration.

 

[cullennewsom]

Thanks hemi,

I was afraid of something like that. So, (ignoring the pistions) for argument's sake, I would need also another gas flow sensor, and ECU / ignition system in addition to the other stuff.

Now for the real question, how does the difference in compression ratio affect the situation? Am I correct in understanding that the "CNG pistons" have a higher compression ratio in comparison with the "LPG pistons"? Is it a large difference? Could it be compensated for by changing boost and or timing? Do you have a source of info/specs on the LPG version that you're willing to share?

I can imagine doing this as it is basically what I do for a living. (Hacking COTS electronics together with custom electronics and software via CAN, RS485, RS232, Ethernet, etc, etc.)

Thanks in advance,
Cullen

 

[cullennewsom]

Update, I found it.
SPECIFICATIONS for B LPG PLUS

Advertised Horsepower 195 hp 146 kW
Peak Torque 420 lb-ft 569 N•m
Governed Speed 1600 rpm
Number of Cylinders 6
Compression Ratio 9.1:1
Oil System Capacity 4.0 U.S. gal 15 L
Coolant Capacity 10.5 U.S. qts 9.9 L
Net Weight w/ Std Accessories, Dry 1,018 lbs 462 kg
Fuel Type HD-10 and HD-5 propane

SPECIFICATIONS B GAS PLUS

Advertised Horsepower 195-230 hp 145-172 kW
Peak Torque 420-500 lb-ft 570-678 N•m
Governed Speed 2800 rpm
Clutch Engagement Torque 285 lb-ft 386 N•m
Number of Cylinders 6
Compression Ratio 10:1
Oil System Capacity 4.0 U.S. gal 15 L
Coolant Capacity 10.5 U.S. qts 9.9 L
Net Weight w/ Std Accessories, Dry 1,018 lbs 461 kg
Fuel Type CNG/LNG
Methane number 65 or greater

SPECIFICATIONS B GAS International

Advertised Horsepower 150–230 hp 112–172 kW
Peak Torque 375–500 lb-ft 508–678 N•m
Governed Speed 2500–2800 rpm
Number of Cylinders 6
Compression Ratio 10.5:1
Oil System Capacity 4.0 U.S. gal 15 L
Coolant Capacity 10.5 U.S. qts 9.9 L
Net Weight w/ Std Accessories, Dry 1,018 lbs 462 kg
Fuel Type CNG/LNG
Methane number 65 or greater

I'll have to check, I assume that I have the B GAS PLUS model (10:1 compression ratio)
-C

 

[cullennewsom]

Their marketing literature is confused (like most marketing lit) One place denotes CNG PLUS at 10.5:1 and another puts it at 10:1. Sigh.

 

[Lou Scannon]

Your 2001 vintage engine predates the B Gas Plus introduction, however, the main differences between the original B Gas and the Plus are in the electronics, so the specs you posted are probably correct.
The Governed Speed 1600 rpm for the LPG Plus is a typo. That should be 2600 rpm, I believe.

The ignition systems are the same. There may be a different recommended spark plug.

You can probably compensate for the higher compression, my recommendation would be by derating the power. One way to do that possibly would be to reduce the wastegate setpoint.
I don't recall if the original B Gas has a wastegate control system. If it does, disabling the wastegate control valve is another way to derate the power.
Other ways of derating power have more risk. For instance, if you retard timing, you may overheat the turbine, exhaust valves, or exhaust manifold. If you lean out the mixture (not easy to do, by the way, on an electronic engine), you may induce misfire.

I caution that running the higher compression ratio natural gas pistons on LPG runs the risk of detonation, in spite of efforts to retune the engine for LPG. Detonation can lead to total engine failure, originating with failure of the pistons.

Unless you can get it at much lower than list price, the LPG gas flow sensor will set you back many $100's. While getting your ECU reprogrammed at a dealer may be an option, if you want to be able to switch back & forth, you will probably want a dedicated LPG ECU, which will cost many $100's also.