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Natural Gas, CNG, LNG, NGV, CH4
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Post by Steptoe » Tue Mar 06, 2007 1:54 pm

In the practical world...yep lets accept low emissions, low power (or expense of engine rebuild) tank milage sucks
The camaro I have no idea what it runs on petrol as it hasnt seen a drop in over 20 yrs,
LPG 60l tank SB chev I run about 130 miles
CNG on a 6cylinder 80L refill at 60 miles
That is not a very good marketable feather
Then there is the consideration to fit tanks large enough..and the space it takes up....and the weight of CNG tank(s)
Add to that the yime taken to fill / miles speed of filling(slower) + more fills /milage.
Hence why even in NZ where we had some of the most taxed and expensive gas thru the late 70s and 80s in the world..as a vechilce fuel, it started with a bang and died.
And believe me in that time NZ was hard out research on alternative fuels as we have no crude here, thu large reserves of off shore gas

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CNG mileage

Post by utahguy » Tue Mar 06, 2007 6:15 pm

Steptoe: if what I understand you're saying, you are getting terrible mileage with your CNG. You only can go 60 miles on 80 (eighty) Litres of CNG in your 6-cylinder? Are you sure that isn't 8 litres? I can go 130-150 U.S. miles on 19 (nineteen) Liters of CNG (5 U.S. gallons) in my 4 cylinder (2 liter) engine, 4-door Ford Contour. Perhaps you are comparing 20-year old technology and tanks to today's. My CNG tank does take up a little more room than a LPG tank and it is heavier. It does hold less CNG than an equivalent-sized LPG tank. But, its at 3000 p.s.i. in the CNG tank. What's the p.s.i. in your CNG tank that you're trying to compare? You make mention of abundant NG reserves in NZ but short oil supplies - there is a refining technology available today to refine NG into a synthetic liquid fuel that would go into a gasoline tank as a liquid and burn like gasoline. The company that makes it is called Syntroleum and they've built pilot refinery plants only. Their fuel has been run in buses, jet engines, and other vehicles in National Parks in Alaska and Washington, D.C. For awhile, they had a contract to build a production plant in Australia, but it got canceled, for whatever reasons I don't know. I don't know if the cost of this synthetic fuel is cost effective compared to the refining cost of a gallon of petrol.

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CNG with diesel

Post by franz » Wed Mar 07, 2007 9:01 am

Your question about CNG and LPG use with diesel is timely, especially since there are websites advertising the use of.

Using a gaseous fuel with diesel is not new, Rudolf Diesel suggested using "Erd-Gaz" (earth gas) with diesel to increase the power and decrease fuel usage. Several OEM's even use Natural Gas with diesel, for example, both Detroit and Catepillar used NG.

Natural Gas has a very good antiknock rating making it ideal for use in a diesel engine, but the fuel has to be ignited somehow. By decreasing diesel fuel flow rates and increasing NG flow rates, a consistent power level can be maintained (the real secret here) until finally just a small amount of diesel is used as a fuel igniter. This is called the "PING" system, Pilot Ignited Natural Gas. It is important that in all cases of an OEM using the PING, existing power levels were not exceeded, this keeps the driveline intact and minimizes cooling system overload.

The substitution rates for a PING engine are in the 80 to 90% range at light load but it drops off considerably at load and lower speeds. At idle, the engine is usually exclusively on diesel. At rates higher than that, its hard to consistintly ignite the fuel mixture.

The drawback is the amount of fuel that can be stored onboard. CNG is relatively easy to work with but the amount of fuel that can be conveniently stored is not that great (sure, one can stack 10 to 20 cylinders on board, but, why?) LNG has reduced that problem, but creates another one, where does one obtain LNG while traveling?

When using LPG with diesel, a different approach must be used. LPG has a lower CCR (Critical Combustion Ratio) than CNG, often in the regions equal to or lower than a diesel engine static CR. The PIP (Pilot Ignited Propane) system was never adopted by any OEM since it is VERY easy to overfuel a diesel engine and develop SERIOUS internal engine damage. Substitution rates should never exceed about 20%, depending on the static CR and engine speed.

My problem with the websites is their marketing strategy. They claim "Increased efficiency due to the more complete burning of the diesel fuel" (Huh?) and that propane "Acts as a fuel catalyst to promote more complete fuel burning". (Hmm, I guess I missed that portion of combustion theory.)

Plain and simple: Adding fuel to a diesel engine increases power. It doesnt matter which hydrocarbon fuel is used, they almost all increase power. Just ask any diesel mechanic who has washed an air filter out in solvent and didnt dry it completely, or any accident investigator where a diesel engine ran out of control due to a gasoline or kerosene fuel spill and the vapors were ingested by the diesel engine.

One thing else, LNG transport ships that use bunker or diesel fuel will recover the boiling Natural Gas vapors from the tanks and use it as a supplemental fuel. This reduces the fuel costs during trans-atlantic travel.


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PING Engines

Post by Frank » Wed Mar 07, 2007 9:45 am

The PING engine that Franz mentions is also known as Homogeneous Charge Compression Ignition (HCCI). Dual fuel Diesel engines running on natural gas are commonly found in stationary industrial and power generation applications because natural gas can easily be supplied from a pipeline.

It's a bit challenging to keep discussions well organized on a forum because ideas easily overlap. I think a continued discussion of PING/HCCI deserves its own thread.

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Post by alehander » Wed Mar 07, 2007 12:50 pm

I never thought I'd see an HCCI thread here. If you're going to relocate this portion of the existing thread, please include this link: http://c-5.org/Storage/HCCI_Thesis_AAB.pdf

This document is my thesis work at Cal Poly. The goal of the thesis was to establish control over (auto) ignition timing of a VW TDI engine running on gasoline... by using water injection at the intake manifold. Gasoline was probably the wrong fuel for PING/HCCI, in retrospect. With a much lower octane rating, the autoignition timing advanced wildly; CNG or LPG would do much better - I believe Stanford had a team of MSME students working on CNG HCCI at the same time.

Lots of fun, but I liquefied the pistons before any appreciable amount of data could be gathered.


ED: This post is continued on a new topic: Alehander's HCCI Thesis Project
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Post by utahguy » Wed Mar 07, 2007 4:09 pm

Were any of these PING or HCCI diesel engines turbocharged? How does CNG take to turbocharging or does it introduce a new set of complexities? :idea: I would imagine that if you were going to use both gasoline and CNG in an engine where you're switching back and forth between fuels, that a flexible boost system might address the compromise of the CR.

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Post by franz » Wed Mar 07, 2007 11:36 pm

Almost all of them were. These days, its rare to see a diesel without a turbocharger. The efficiency is increased tremendously with little measurable lost power to drive the turbo.

A CNG-Gasoline bi-fuel engine (switchable between the two fuels, not using both at once) does not adapt especially well to turbo. You would have to develop two totally different boost and wastegate profiles, not impossible, but a chore none the less.

Most aftermarket conversions use ignition timing to develop sufficient combustion pressures to help overcome the difference in thermal values between CNG and gasoline (and to some extent, propane). Since there are so many different engines, there is no one timing spec to fit all applications.


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Post by utahguy » Wed Mar 14, 2007 2:44 pm

I'm curious about your comment about LPG's lower CCR (critical combustion ratio) relative to CNG's. Does CCR measure the specific fuel's ability to be compressed before it ignites? Is LPG's CCR lower than diesel fuel's CCR, while CNG's is closer to diesel's? And how does the CR of the engine interrelate to that - were you talking about the compression ratio in the diesel engine being to high to handle the CCR of LPG & therefore the LPG would ignite prematurely? Why wouldn't decreasing the amount of diesel fuel below 80% as you raise the LPG content above 20% work? And doesn't the fact that LPG has a lower BTU rating than diesel mitigate the extra destructive forces on the diesel engine if you are reducing the amount of diesel fuel (using PIP)?
2000 Ford Contour w/factory GFI

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