Cng burner

So a buddy of mine I recently helped get a TDI, and he's going nuts watching YouTube videos on them. He showed me one where someone had a CNG tank with regulator, and had it injecting into a port drilled right next to the inlet of the crankcase breather. Boasted getting 150 mpg. Has anyone else heard of this? I'm calling BS on it but wouldn't hurt to ask.

Putting fuel into a CI engine intake (e.g. any diesel) that is not metered and timed to the peak of the compression stroke is asking for extraordinarily bad things to happen.

I don't know what they do to convert Diesel engines to run CNG, but Waste Management had/has a large number of their trucks converted as does the "Holiday" brand fuel/gas retailer on their bulk delivery trucks. I talked with one of the drivers about it and they are quiet like a gas engine. He had no experience with cold starting because they are either never left cold-in use or in for maintainence building.

Are we sure it wasn't propane?

Nero Morg; I'm familiar with CNG on spark-ignition engines (have had 4 trucks on CNG. Currently a Ford EcoBoost on CNG). They were all dual-fuel. Meaning they would start on gasoline then switch to CNG and run. Only switching back when either shut down or ran out of CNG.

I, too have watched the numerous video's on TDI/CNG conversions. I find the mileage claims difficult to accept. Most of those conversions are simple fumigation systems that add the CNG to the intake via a crude metering venturi system. The Natural Gas (now at very low pressure) displaces some of the air being drawn into the engine intake. Power is increased due to the fact the natural gas is a combustible product. The same amount of diesel fuel is being injected as there are no modifications made to that system. So as I see it you get more power but no increase in mileage and an increase in your cost-per-mile as you are now using both diesel and natural gas.

jettawreck; The Waste Management Garbage trucks are spark-ignition engines based on their diesel-equivalent blocks. They have added spark plugs and coil packs along with different pistons, cylinder heads, crankshaft, turbocharger, electronics etc. It's a Cummins ISL-G engine.

KLXD; This can also be done with propane. Propane has a lower octane (105 vs 140) than natural gas.

Genesis; Most diesel/CNG fumigation systems only add about 10% natural gas. Any more and bad things can happen. It's added upstream of the turbocharger.

Most of the true CNG "diesel converted" engines I've seen are diesel core engines with a cylinder head on that has drillings for spark plugs instead of injectors, and the intake is essentially a giant carburetor.

The video I saw was using diesel in conjunction with CNG, using both fuels at all times (after operating temps were reached)

From my experience of working on Waste Management trucks overhauling those CNGs, they're definitely quieter, but very gutless compared to its diesel counterpart.

Interesting topic. Some older discussion over in Alternative Fuels forum.
Believe NG and propane are much less expensive per unit of energy. No idea of the hardware or affect on the motor.

BobnOH said:

Interesting topic. Some older discussion over in Alternative Fuels forum.
Believe NG and propane are much less expensive per unit of energy. No idea of the hardware or affect on the motor.

Click to expand...

I tried looking over there first before posting here, but it looks a little dead over there.



I'm not interested in trying to do this type of mod to my car, just interested in the concept behind it and if anyone's ever seen it done.

Propane is NOT less expensive per unit of energy. Price propane by the gallon; it's ~92,000 BTUs. A gallon of diesel is 139,000 BTUs.

NatGas (Methane; NatGas is basically all CH4) is, but it's a PAIN IN THE BUTT due to storage considerations. It is not liquid at reasonable (non-cryogenic) temperatures irrespective of pressure, and thus you either need to refrigerate it as a liquid (and it boils off when not in active use) OR compress it to very high pressures (which requires expensive and large tanks to get reasonable range.) The range issue is not too bad with a truck since they have plenty of room for the tank. But in a passenger vehicle it's another story entirely.

CNG is used by some firms both to "virtue signal" *AND* because it burns very cleanly as a fuel, and thus if you don't care about the range issues due to the limited area of operation and such (e.g. a garbage truck) you can lower your fuel *and* engine maintenance cost. This makes it worth the cost of the compressor (to fuel said trucks) and tanks in that application. Retrofitting a spark-ignition engine to run on CNG is very easy but the best benefits are obtained by using a diesel block with a head designed for it as the octane of CNG is EXTREMELY high (~130!) and thus you can run much higher compression ratios and get even better efficiency. This however means you want to start with a diesel block due to the greater stress -- so they do.

I've always wondered about CNG vehicles since it is a cryogen. Didn't seem like they'd get a decent range even with a big tank. What pressure do they run in the tanks?

3-5,000psi. The tanks are hoop-wrapped (fiberglass, basically) or composite-enhanced (kevlar, carbon fiber, etc) for weight reasons but they're still quite fragile all things considered and expensive. If you don't care about weight then 3AA or 3AL can be used. 3-5kpsi is the practical limit for pressure due to the fact that various valving, hoses and compressors are available in ordinary commercial service at that working pressure; beyond that things get very exotic and expensive very fast. One BIG difference is that ALL composite tanks have a defined service life that is specified in their special exemption certificate under which they're manufactured (typically 15 years) after which they're junk -- 3AA and 3AL (steel and aluminum) cylinders can be used forever provided they continue to pass their recertification (hydro and visual inspection.) It's utterly common to see welding cylinders with original hydro dates from the 1970s and even earlier; they're perfectly safe so long as they keep passing inspection. I have steel dive tanks with 1970s date original hydros and they're safe and in active use today.

The problem remains that range is still a fraction of that obtainable from gasoline or diesel in the same volume and mass, so it's not practical for a consumer vehicle. There's no way around the physics on that. For a commercial vehicle, however, it may be both practical and worth it from a life-cycle perspective as the fuel is cheaper on a per-BTU basis and the decreased maintenance cost is quite material. If you can whack some number of cents/mile off the operating cost of a trash truck (including the cost of the compressor at the depot) then it's DEFINITELY worth doing in such an application. Another place it frequently makes sense to do this is city buses; again, limited range requirement, LARGE vehicle (so tank size and mass is not much of an issue), the depot typically has gas service available and the vehicles all return there so there's no refueling problem and the decrease in operating and service expenses is a big bonus as well.

I don't know much about the fueling storage/delivery of CNG. I did however see that the tanks on the waste management trucks were gigantic, ranging from 8-10ft in length easily and did have tags showing 3000-5000psi pressure range. Now I understand why it's very impractical to even try using it on our cars. Thank you for all the info.