Cummins using Natural Gas for Fuel

[Alan Reagan]

I've been noticing the Natural Gas service trucks here in Savannah are now running Ram/Cummins combinations with the big letters on the back indicating that they are burning natural gas (so is it still a diesel?). Now I know they do it on ships by burning about 10% fuel oil and the rest natural gas. I've never seen a diesel car/truck converted for natural gas so I'm curious. What ratio do they have to use and is the "mix" handled by the ECM? The engine still has to have diesel to "fire", right? That's what they do on a ship. As the ship is coming into port, they switch back over to 100% fuel oil for the trip in and out. I've also noticed that these trucks run very quiet. I'm talking quieter than a gasoline driven engine.

 

[jponder]

Originally posted by Alan Reagan

I've been noticing the Natural Gas service trucks here in Savannah are now running Ram/Cummins combinations with the big letters on the back indicating that they are burning natural gas (so is it still a diesel?). Now I know they do it on ships by burning about 10% fuel oil and the rest natural gas. I've never seen a diesel car/truck converted for natural gas so I'm curious. What ratio do they have to use and is the "mix" handled by the ECM? The engine still has to have diesel to "fire", right? That's what they do on a ship. As the ship is coming into port, they switch back over to 100% fuel oil for the trip in and out. I've also noticed that these trucks run very quiet. I'm talking quieter than a gasoline driven engine.

ALan,

The first Diesel was designed to run of coal dust. I would think a diesel would just mean introducing fuel into an enviroment where compustion will occur spontaneously ie, no spark. I have been seeing those CNG buses in Atlanta for years. It looks like they store the tanks on top of them.



So Far as mixtures. Man I dont know. Methane is always going to be a gas and it has to be harder to inject a gas into a high pressure chamber than a liquid, On the other hand they are probably doing something like Peoples butane injection where it is taken into the air intake which if you put to much wouldnt you get predetonation. Interesting!

 

[Alan Reagan]

Atlanta buses running off natural gas. I forgot, you guys have a good source for Methane with the lawmakers up there.



Come see me.

 

[RustyJC]

Large industrial dual fuel (also called gas-diesel) engines used to run on about 4% pilot oil and 96% methane. The latest "micro-pilot" engines can light off the methane/air mix with only 1% pilot oil or less. Unless the injectors are optimized for pilot oil service, I have no idea how much pilot oil a converted diesel might require - could be 8% to 10%!



Rusty

 

[Alan Reagan]

I'd be interested to see the HP numbers running gas diesel at various mixtures.

 

[jponder]

Well look at what people are doing with Butane and that should give some idea Methane is CH4 and Butane is C4H10. I mean how are the butane injections systems working? Butane is just 4 methanes linked together. Then again I think the butane injection that people use with diesels is a far cry from what those CNG busses are doing. We use a small amount of butane to a lot of diesel and they are doing the opposite

 

[RustyJC]

In industrial engines (at least the ones we build), gas-diesel engines have the same (up to around 210 BMEP) BHP as the straight diesels. As the BTUs supplied by methane go up, the BTUs supplied by diesel fuel go down. The operator is running on 96%+ methane primarily because it's cheaper and/or cleaner from an emissions standpoint.



The 250+ BMEP emergency standby generator engines in the nuclear power plants are straight diesel, primarily for reliability (on these engines, you don't want to have to depend on something outside the fence - like a natural gas pipeline - for your fuel source! If they don't start and run, it's China Syndrome time! )



Rusty

 

[jponder]

Rusty,

How is the fuel introduced? Is the diesel fuel injected seperately? They cant just be putting the metnae in with the air can they?

 

[Duluth Diesel]

I have seen catepilar diesels that run on 100% natural gas. Methane makes up about 86% of natural gas, and a engine can run on pure methane by just using larger jets than the natural gas ones. Since Cat can do it, I assume it is completely possible to make a Cummins do it. I have not had any experience with diesels running a mixture of both, though.

 

[Alan Reagan]

Must be a different type of injector if they run on 100%. Otherwise, how do they get the injectors to pop off? And what about the pump. Do they change out the pump to a compressed gas pump rather than compressed liquid? I'm so confused. :{

 

[RustyJC]

The Cooper-Bessemer LSVB (as an example) has 4-valve heads. The 2 intake valves have poppet-type gas valves on the stems. The gas valves open around 0. 050" lift after the intake valves open, and the gas flows into the cylinder with the intake air. The pilot shot of diesel fuel acts just like a sparkplug - it ignites the methane/air mix.



When operating as a diesel engine, the air/fuel ratio controls are disabled - the engine runs on maximum air. When operating in a dual fuel mode, the air/fuel ratio controls come into play. They keep the methane/air mixture on the lean side of stoichiometric, so it doesn't tend to detonate or pre-ignite during compression - it needs the energy of the pilot oil injection to light it off.



And yes, the LSVB can be built as a tri-fuel engine. The diesel injector can be pulled and replaced by an adaptor with a spark plug. That way, the engine can be run on straight natural gas, gas/diesel or straight diesel.



This particular engine has a 15. 5" bore, a 22" stroke and a compression ratio of only around 11. 1:1 - if you'll note, compression ratios of diesel engines go down as their size goes up. This is a function of the swept volume to combustion chamber surface area ratio. The LSVB-20-GDT (the V-20 version of this engine) is rated at 6300 kWe @ 400 RPM - at 98% generator efficiency, this translates to about 8620 BHP. It's primarily used for municipal or industrial power generation



Rusty

 

[RustyJC]

The injectors (Bosch, Bendix or L'Orange, typically) use a separate jerk pump driven by an individual fuel cam for each cylinder. The jerk pumps are connected to each other and the governor by a fuel rack on each bank. Each jerk pump is controlled by a helix shaft that's controlled by the rack. The rack moves to a 100% fuel position at full diesel rated load and a 4% pilot fuel position at gas/diesel rated load - almost like the engine is idling at no load insofar as the diesel fuel system is concerned. When in gas/diesel operation, the governor controls engine speed by manipulating a gas valve that controls the amount of methane admitted to the gas manifolds. The pilot fuel setting of the diesel injection jerk pumps remains fairly constant.



These are DEMA-rated engines, by the way. That means they can operate at 110% of nameplate BHP for 2 hours out of every 24 hour period, so the fuel injection system and rack calibration has to take that 110% operation into account.



Rusty

 

[jponder]

Rusty> All I can say is WOW! Man you know some stuff.

 

[RustyJC]

I'm just a pore ole country boy from West Texas. Aside from the years spent in engineering college, any practical product knowledge I have comes from 30+ years experience working at Cooper.



Rusty

 

[Rman]

Rusty's more full of that knowledge stuff than a Thanksgiving turkey!

 

[jponder]

Rusty>

Here is one thing about it that perplexes me. If diesel is the "Pilot fuel" then it would seem it would reach ignition easier than the Methane. I mean the Methane is in the cylinder and it hasnt detonated yet. Yet if I had to choose what ws more flamable, or easily ignited. I would definately choose methane over diesel. I dont understand that

 

[RustyJC]

The methane is in a lean fuel/air mixture - lean enough that it is under its autoignition point at the time of fuel injection. When the atomized diesel fuel is injected, it starts a flame front that raises the heat and pressure within the combustion chamber. The heat and pressure accompanying this flame front have enough energy to ignite the lean fuel (methane)/air mixture.



If the diesel fuel were in its raw liquid state, you would be right - the methane is more flammable. However, in its finely atomized state, the diesel is ready to light off at the pressures and temperatures it experiences when injected. Therefore the diesel (pilot fuel) ignites the gas.



Think of it this way - for those running propane in their ISB's (which has some real dangers!), why doesn't the propane ignite before the diesel? Because (hopefully) only enough propane is being added to make an extremely lean mixture in the cylinder such that the diesel injection event is required to light the propane off. Same principle with methane.



Rusty

 

[Rman]

So rusty, the diesel essentially becomes a catalyst instead of a typical "fuel". It's not providing the heat and power of combustion, just starting it so the methane can take over?

 

[RustyJC]

The small amount of diesel injected starts the combustion process - like a spark plug. It's really not a catalyst since it's chemically involved in the combustion process - it's a source of ignition energy. Once combustion is initiated in the methane/air mixture, the resulting increase in temperature and pressure make it self-sustaining.



Rusty

 

[Rman]

this is why I chose electronics over chemistry... i suck pretty bad at chemistry.