Low BTU Fuel

[bowtie8]

Go easy on me (new)

I have a GM 8.1L on NG and its currently Naturaly Asperated. I am putting my hooks into developing it to burn 500-600BTU digester gas from dairy farms. In my lab I can simulate this by injecting CO2 for developing and tuning of my engine controller. My question is two fold.
I am going to be fitting it with a Holsett Turbo and am unsure if I need to get the piston domes increased to get the compression up to at least 11:1 (currently 9.1:1 w/hyperutectic)or more boost and keep the 9.1:1 in place.
Also I have found that anything more than a flat top is a custom piston.
As you know typically when adding boost we want to lower compression to minimize knock, but I believe when my BTUs are cut in about 1/2 of that of methane I have to stuff twice the A/F and my efficiency will be in the toilet if I don't get the comp. up to 11 or 12:1. What do you think?

Best Regards
John

 

[Lou Scannon]

Don't get low BTU mixed up with anti-knock qualities.
You are on the right track as far as restoring power via boost - since so much of the air in the charge is being displaced by inerts in the low BTU fuel, you need to boost to get air flow back to what it was in order to maintain power.
Antiknock characteristics will be determined mainly by the composition of the combustible compounds in the digester gas. You should have a good handle on this composition before you proceed. If it is materially different from the composition of the fuel the engine was designed for, the compression ratio may need to be adjusted upward or downward to optimize the engine. That said, it may be far more cost effective to leave the CR alone and adjust the spark timing with a less than optimum CR.
As long as the composition of the combustible portion of the gas is similar to your natural gas, and you are not seeking to increase power above stock, the existing CR is probably fine. Bear in mind turbocharging will raise the charge temperature, so if you do not intercool some retard may be needed to avoid knock.

 

[1Bluegrass]

Very well said by Hemi.
The fuel is the final determinant of were you end up with any combination by it's limits of detonation vs load.
This limit is of course determined by CR, intake temp, chamber design, timing, fuel and other parameters.
Getting as many of these finalized is the key.
Research takes a lot of time and money.
Good luck.

 

[CCycle]

Oh Boy! You are in for a major controller design. The problem with digester gas in addition to being low Btu is that it's quality fluctuates all over. In addition your digester will never be producing just exactly the amount of gas your engine needs. This necessitates gas storage which is large, complicated and expensive.

You might consider a different approach. It is quite common in Waste Water Treatment plants to use CI engines to burn off the "digester gas". In most cases this gas is a nuisance since venting it produces odor problems. Flaring the gas is not ideal because its low Btu value. It does not want to burn without adding more fuel. Waste Water Treatment plants have considerable horse power requirements to produce pressurized air for aerobic digesters and for pumping. They use a so called oil pilot diesel which draws its combustion air across the digester and in some cases through sewer piping. This takes care of the odor problem and any fuel value of the gas is a bonus. The CI engine is actually just a governed diesel. The control problem is simple. The diesel has better fuel efficiency and reliability.

 

[crysta1c1ear]

Ok, I may be wrong here as I'm disagreeing with more knowledgable posters, but here goes.
Your low BTU fuel is low energy because it is a mixture of flammable gases and something more inert? The flammable parts burn much like methane, the rest is garbage and you simulate this in the lab with CO². To get the engine to behave as if it were running on methane only, you want to roughly double the fuel and leave the air flow almost the same. (Almost, because as hemi said, the extra fuel is displacing some air.) Your engine then has two times half = normal BTU of fuel, normal amount of air, and two times half = a good dose of something inert too.

Doubling fuel supply is surely the solution to your problem.

From there, the rest is tweaking and tuning and the previous arguments apply. But basically, its not twice the air to fuel ratio. You want about twice the fuel to air you normally have and should be thinking about the fuel supply side of things rather than the air supply side!

 

[Lou Scannon]

All very true, crystalclear but I assumed bowtie8 was past that issue as he mentioned that "In my lab I can simulate this by injecting CO2 for developing and tuning of my engine controller".

 

[bowtie8]

Thanks for the info

I am concerned that I will need to still raise the compression in the chamber via a piston change in order to maintain effenciency. I have had conversation with two of the large Engine Companies that are currently equipped to run digester gas and both of which are running 11:1 and 12:1 in the chamber.
Any thoughts on this? I believe its about effeciency and/or so I don't have to run 50 degrees of timing to acheive respectable effeciencies.

Thanks
Bowtie8

 

[dwedel]

With the higher compression ratio, you will have higher Nox when you have a hotter flame and longer dwell time at temperature. You might get away with this since your application is displacing pollution.

I would guess that they are trying to maintain their engine horsepower ratings, and that in their case the turbos only partially compensated for the low BTU fuel. Turbos are often used to compensate for altitude de-rates for industrial engines, so they maybe ran out of air, and decided to use higher compression ratios to maintain the horsepower ratings.

Clark Bros did the high compression ratio upgrade on their NG two strokes to maintain HP ratings in 1950. I had nice discussion with some Dresser-Rand people, when we had some bearing problems and were worried about detonation and allowable peak-firing pressures. It turns out we have high compression heads and the bearings should live with the higher peak firing pressurs.

There are several aftermarket A/F controllers for industrial engines. Altronic, Woodward, Murphy, are three that come to mind quickly. Not sure about the size of their specific applications.

The tricky part is gas BTU value. Anderson Consulting, Training & Testing, has done a test with a coriolis(sp?) flow meter, but I'm not sure of the results. Maybe store the gas, test it, and then burn it? You are now in la-la land as my engines are strictly production machines and people get pissed if they don't run, because some engineer is testing a new idea.

From an operations standpoint, I must say that I like CCycle's idea.

dwedel
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