CNG Fuel Economy

Natural Gas, CNG, LNG, NGV, CH4
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Frank
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CNG Fuel Economy

Post by Frank » Thu Dec 23, 2010 1:41 pm

In conjunction with my previous topic about LPG Fuel Economy, I am curious to know what everyone is getting for their fuel economy. It's a bit of a challenge to compare CNG fuel economy with gasoline because CNG is sold by weight (lbs or kg) and gasoline is sold by volume (gallons or litres). It is often advertised in terms of GGE (Gasoline Gallon Equivalent) or GLE (Gasoline Liter Equivalent). As such, it is easy to convert kg of CNG to litres of gasoline with some information from the US DoE's Transportation Energy Data Book: Edition 28 (page 287). It lists natural gas as having 20,551 BTU/lb LHV and 115,400 BTU/gal LHV for conventional gasoline. That would translate to 0.1781 gallon/lb or 1.4862 litre/kg to get a GGE or GLE from the amount of CNG purchased.

Most propane conversions get much better fuel economy than the 72-73% expected by comparing energy contents. With carbureted systems, I think it is not uncommon for an open-loop propane conversion to get 75-80% of the vehicle's gasoline economy. With feedback carburetor systems, I believe the typical propane fuel economy is about 80-85% of the vehicle's gasoline economy. Injection conversions on OBDII vehicles do even better with expected fuel economies in the 85-95% range.

With the open-loop carburetion systems, my understanding that the better than expected fuel economy is mainly due to improved, leaner fuel mixture distribution and a more ideal ignition advance curve. A gasoline carburetor system can have a significant variation in fuel mixture from cylinder to cylinder so the average fuel mixture must be richer overall to ensure that each cylinder gets a combustible fuel mixture. Closed loop carburetor systems improve on this mainly by ensuring that the fuel mixture remains constant over a wide range of engine temperatures.

Injection conversions on OBDII vehicles can't improve on fuel mixture distribution because of gasoline port injection. Properly configured, the fuel trims on CNG should the same as on gasoline. Because this type of conversion works by intercepting the gasoline injector signals, modifying them, and then rerouting them to the CNG injectors, theoretically this should result in CNG fuel economy that is closer to 100% of the gasoline economy. However, they could potentially do better because the vehicle's PCM should allow more ignition advance due to CNG's higher octane number. The problem is that the increased ignition advance may not necessarily be ideal for CNG.

CNG conversions tend to have noticeably less power than what the vehicle had on gasoline or even on propane because natural gas reduces the volumetric efficiency of an engine by displacing air. However, since the energy content of CNG is advertised in terms of GGE/GLE and since CNG is a high-octane gaseous fuel like LPG, my expectation is that the vehicle should similarly have an equivalent fuel economy better than what it would have on gasoline.
  • Does anyone have any other explanations as to why CNG fuel economy should be better or worse than expected?
  • What fuel economy are you seeing on CNG and what would you expect it to be on gasoline?


Frank
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Re: CNG Fuel Economy

Post by Frank » Mon Jan 03, 2011 2:03 pm

Since no-one has posted any replies yet, I got to thinking that it might be because the calculations might be a bit challenging and/or no-one is keeping records. To make it easier, I made up some Excel spreadsheets that you might find useful. I've included some sample data so that you can see what the chart looks like. The first row of data (row 4) is just to initialize the distance calculation so it's not necessary to add fuel data (cost and amount). I've also included a column for the price of gasoline so that you can have an idea of what kind of mileage you'd be getting on gasoline.
If I've made any errors, please let me know so I can revise and re-upload them.

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Steptoe
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Re: CNG Fuel Economy

Post by Steptoe » Mon Jan 03, 2011 2:55 pm

I missedthis post..yep i had cng/ duel fuel on the camaro with orginal 350 ci engine and around 130K miles up on it.
It was nearly 30 yrs ago
Bottom line, performance sucked,
Had about a 60L tank...anyway the milage range sucked big time, from memory around 60 or 70 mile range????
Cost per mile was very cheap..and I mean VERY.
Would I do it again....no...
If I thought it would have been I would not have gone from CNG duel to LPG dedicated....which is lucky cause back then CNG filling stations where everywhere, now I dont know where one is or if there is still a automotive station left in NZ.
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Frank
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Re: CNG Fuel Economy

Post by Frank » Mon Jan 03, 2011 6:39 pm

CNG doesn't make a lot of sense for some people but it does for others. There are lot of people (like the Pickens Army) who are proponents of natural gas vehicles and, with gasoline prices rising again, interest in CNG seems to be on the rise again (judging by the emails I get). The relatively low driving range isn't problem if a CNG station is along your commute or if you have a VRA at home.

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Re: CNG Fuel Economy

Post by Frank » Wed Jan 05, 2011 1:51 pm

Having reviewed some EPA fuel economy ratings for bi-fuel CNG vehicles, it looks like there is sometimes a slight decrease in fuel economy with CNG. Typically, CNG seems to have about 90% -100% of the fuel economy of gasoline in bi-fuel vehicles. However, CNG in a bi-fuel vehicle seems to have about 80-90% of the fuel economy of the gasoline-only vehicle in the examples below.

Referencing the US DoE's Fuel Economy website and using New EPA ratings (city/highway/combined mpg):

2004 Ford F150 2004 Chevrolet Cavalier 2004 GMC Savana Passenger Van 2004 Honda Civic GX Other than the added weight of the CNG system, I'm not sure why a bi-fuel vehicle would have poorer fuel economy on gasoline than a straight gasoline vehicle. I would have expected the highway fuel economies to be much closer on gasoline. Either the engine is operating less efficiently on CNG than gasoline or the conversion from lbs to GGE is off.

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Steptoe
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Re: CNG Fuel Economy

Post by Steptoe » Wed Jan 05, 2011 2:27 pm

Back then..early 80s, I put my low econony...and power...to having a petrol engine, far lower compression, wrong cam, and wrong timing curves... the latter wa still very much a compromlese leaning towards petrol otherwise I had detonation issues at part throttle to full throttle.
Hence when I rebuilt the engine, I did so for LPG, and went dedicated.
My Spelling is Not Incorrect...It's 'Creative'

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