Natural Gas vs. Gasoline: Reduced Efficiency?

Interesting question. My comment here is more to subscribe to this than answering your question but there are two things I can provide.)

With so many different items that one can measure to make an efficiency comparison (fuel volume, fuel mass, energy consumed, power out torque out, etc.) it is very easy confuse a measurement.

My other proposal is that even though the energy consumed in burning in side a spark ignited (SI) engine can be the same the power out will be different due to the temperature of the flame produced.

This is all speculation on my part.

Take a look at the specific energy content of the fuels in question. Of the three most common fuels out there, their energy content is: deisel; highest, gasoline; next and NG; lowest.

That's true per unit volume, but per unit mass there all pretty much the same. The SGs are approx diesel 0.85, petrol (oh all right gasoline ) 0.8, LNG 0.65.

I don't think there's much difference in efficiency between gasoline and LNG, but others might know better. Somebody I knew with a dual-fuel car told me it had more power on gas than LNG, but it would use more fuel (in energy terms) to get it.

The difference of SG also needs to be taken account of when comparing prices. You get a lot less energy per gallon of CNG.

Just depends how you look at it, I guess.

But mass and volume are different, however you look at it!

And temperature dependent.

The way the OP's question is stated, with regard to BTU's in vs BTU's out misstates what he's looking for, I think.

I believe that the comment about specific heat will set him straight.

Your comment, on the other hand, is pretty much useless.

Oop! some bad habit in this forum is contagious ! I caught it without knowing !

Natural gas has a higher octane rating than conventional gasoline. An engine built for NG has a higher compression. Thus, a converted gas engine will not be as efficient.

What happens if you take a CNG engine and convert it to gasoline?

Ford (and maybe others too) make an engine specifically designed for both fuels, surely this is the way to go?

How the engine is built/tuned has no effect on the energy content of the fuel in question. You're talking about engine efficiency, a completly different issue here.

Apparently one of us is getting a whole different take on what the OP stated in his post. This what I saw as being the primary question he has.

" So, here comes the question: If you burn 100KBTU gasoline through an SI engine (with all of its attendant mixture preparation issues) and you get n kW out in the exchange, why would burning 100KBTU of NG through the same engine (with optimized spark timing) yield, say, .9n kW in the transaction?

Apart from the relatively minor inlet charge cooling benefits which are intrinsic to gasoline mixture preparation, what other rationale are posited in this type of scenario: Is there a set physical characteristic of NG which mandates an efficiency reduction without combustion chamber modification in a standard SI engine?"

Simply put why don't both fuels give the same efficiency in the same engine when compared on a energy in to energy out basis?

My simplest answer would be because the engine design and configuration is what effects the energy in to energy out conversion process. The fuel itself has little to do with it.

Is there a set physical characteristic of NG which mandates an efficiency reduction without combustion chamber modification in a standard SI engine?

Yes. The specific energy content.

A btu is a btu is a btu, regardless of the source of the energy.

You can't get as many BTU's out of LNG as you can gasoline. But now we get into the definition of other terms and it gets confusinger and confusinger.

"You can't get as many BTU's out of LNG as you can gasoline"

These values vary considerably by season, blend, source... but approx calorific values for CNG and gasoline are similar:
Wiki, Gasoline: 13 kWh/kg
Wiki, CNG: 13.5 kWh/kg

I agree with tcmtech: The inefficiencies experienced are due to using an engine with specifications for a gasoline, and converting SOME of those spec's to CNG.

MOM!

They're beatin' me up!

What?

This gets back to my original point. The proffered 0.9 efficiency loss by switching to NG is such an inexact statement that this could be due to almost anything from a NG fuel line leak to improper jetting of the NG to even an undetected erroneous power robbing change in switching between fuels.

I'm good with lower cost per mile, and specifically tuning an engine (raising the compression ratio and playing with timing) to run on LNG.

I'm still having a problem with specific energy being the same for both fuels. I can't buy into that.

Send me the two bags. After I've consumed them, I'll send you the results.

This conversation has made me tired and thirsty.

I'm going home.

Don't forget to add in the 1/2 liter block of graham crackers weighing one pound that has 30,000 calories...

I have the graham crackers, left over from another experiment.

Sorry I cant come.

The wife says I have to go to work now and be useful.

(Well I may go to work but I don't plan on being useful once I get there.)

I am surprised K Fry hasn't popped in to bash on me for using less than perfect technical terminology and something or other by now.

Would 500 CC be more technically correct than 1/2 liter and what about using pounds and calories with that 1/2 L? Good lord I used metric and imperial in the same sentence!

Wives are like that.

Mine cut short my attempt to become a senior golf tour pro after only 18 months. I practiced diligently every day. Came home exhausted:) every day.

Then, when she found out that I wouldn't be eligible to really PLAY Sr pro golf tour for another 5 years, she "had a talk" with me.

I like my new job.

Well I am at work now and my nearest train that potentially needs fuel is 200 miles out and traveling 45 MPH at best. The next two are 300+ and 400 miles out and going 70 but have lots of oncoming track traffic ahead of them.

I also think the day shift guys are related to my wife. AC on high and the doors and windows were all open when I got here.

I like my job too!

It is great to see you guys gradually inching towards the metric system.

BAB

It is great to see you guys gradually inching towards the metric system.

Good for a laugh!

Since the Btu's in each case, not mass or volume, were specified to be equal, all comments regarding specific combustion heat are irrelevant.

A likely culprit is combustion temperature, which affects Carnot efficiency, as the OP suggested in his later post. Engine tuning for a particular fuel is also a factor, as Tcmtech mentions.

The typical gasoline engine has a low compression ratio. NG can support a much higher compression ratio, which would yield a much higher power output for the same amount of fuel consumed (increased efficiency) The fact that gasoline has a lot of BTUs per unit volume available only tells part of the story. Gasoline requires additives to work in an engine with higher compression (again, higher compression ratio yields greater power output). A lot of gasoline leaves the engine unburned. Some of it washes down the cylinder walls, stripping vital lubricant from the cylinder walls and increasing cylinder wall wear/piston ring wear, leading to premature failure. I would talk to an engine rebuilder who specializes in propane burning engines. Getting the proper compression ratio and camshaft for your application will go a long ways to increased efficiency. The ignition advance curve is different for propane/natural gas engines as well. If I remember correctly, little or no advance curve with a propane-burning engine. Again, a propane engine specialist is your best bet. There are racers who use propane instead of liquid fuels. The expertise is out there.

You can burn natural gas in a diesel engine; I imagine that would return a higher efficiency than in a gasoline engine. As a side note, adding a small amount of propane or natural gas or any gaseous fuel to a diesel engine increases efficiency dramatically because it causes fuel that normally leaves the engine unburned to be burned in the combustion chamber. Propane or natural gas also get rid of the diesel knock and clean up the exhaust.

The engine burning LNG will require (in a conversion from gasoline) larger main jets in the carburetor, almost twice the size. The real benefit of LNG (from a mechanics view) is the LNG motor will be much cleaner inside. In comparing efficiency of a gasoline motor to a LNG motor you would have to take into account the fluctuations in the price of gas vs. LNG. The stability of prices could have a dramatic effect on figures for efficiency if one was more stable in price than the other. Then after making adjustments to those figures you would have to compare emissions and then decide what is more important, power, emissions, economy, or a combination, and then throw in availability. There are a lot more gas stations than LNG outlets. Safety must also be considered, gasoline leak may cause a fire, whereas a LNG leak has the potential for a major explosion. One particular WMD disperses propane over a large area then ignites for a large explosion.

"The engine burning LNG will require (in a conversion from gasoline) larger main jets in the carburetor, almost twice the size. "

HUH????

"Safety must also be considered, gasoline leak may cause a fire, whereas a LNG leak has the potential for a major explosion. One particular WMD disperses propane over a large area then ignites for a large explosion."

WTF????

I take it you don't have a clue about LNG or propane fuels let alone the conversion process or components used?

Your entitled to your opinion. I live in a house I built myself and use propane for heat, cooking, and the refrigerator. I have been living in this house eighteen years. I have considered retrofitting my vehicles to propane but availability in this area is lacking. If you havent read of weapons of mass destruction (WMDs) maybe you should, there is a bomb developed here in the USA for the USAF that disperses a cloud of propane that when ignited.....I thought the information I provided was valid, I would think a person could disagree without making faces or judgement calls. If you prefer I will cease contributing to this forum. Your call tcmtech.

In my limited experience, engines converted to gas get a lot of extra hardware (the gas injection system) in the main air tract, when running on gas and no petrol is injected via the injectors or through a carburettor, Neither is the gas injected via these two things, they are strictly liquid only....gas comes as a liquid but must be allowed to expand to a gas at some point, before it enters the combustion chamber.

On a website I found the following comment which is a simple and good explanation I feel:-

An LPG Conversion works in exactly the same way as a normal petrol engine, just with LPG fuel. Everything about your vehicle remains the same but a separate fuelling system is added with its own piping, LPG tank, ECU and injectors.

I found that here:- http://autogassolutions.co.uk/about-lpg/how-lpg-works

Converted to gas Diesel engines on the other hand usually still need to burn diesel at every firing stroke to provide the means of igniting the gas introduced in the main air tract, in a not dissimilar way as converted petrol engines work, but as diesel engines lack a spark plug, a small amount of diesel, probably equivalent to ticking over, is still needed.

So your comments were wide of the mark and may have been seen by some here as simple guesses. That is not appreciated on CR4......try to restrain yourself and then no one will jump on you .....do a web search to find stuff out before posting is my tip.

Have a great day.

What? A gasoline carburetor and a propane or natural gas carburetor are two different animals. The man didn't ask about bomb-making

The efficiency isn't the issue - its the energy contained in the fuel. More energy contained in petrol than gas. Likewise diesel engines - they are not more efficient than petrol its just there is more stored energy in a gallon of diesel than a gallon of petrol.

I appreciate any engine can be made more efficient and who knows what the conversion is between gallon of petrol and lump of gas.

Be sure you are looking at the LHV (Low Heating Value) for Natural Gas. Because of the larger hydrogen content, there is a loss due to converting the H2O from hydrogen combustion to Steam and superheating it. These BTUs can not do work.

I am sure that you have had some good answers here already and I have no need to answer your questions myself.

But if you are considering a conversion on a gasoline engine to dual fuel, do please do some searches on the web as it seems that some conversions from many reputable companies can give serious engine problems/damage.

The original works guarantee is often not replaced by the conversion company, or its difficult to get them to do something.....that is a REAL problem here.

Some converted cars get problems here in Germany, maybe at least partly because we have far higher autobahn speeds.....thats my personal take, nothing proven....

Most conversions simply do not allow the use of gas when towing a trailer. What owner follows that fully?

The reason is that the engine runs much hotter internally on gas than gasoline. An engine designed for gas has much better cooling design in the waterjacket. A Petrol engine is not as well developed in that area.....why should it be?

My personal take (nothing proven) is that for identical petrol engines with similar usage, the gas conversion engine will wear out sooner due to higher temperatures in the engine.

Taking into account that and the conversion costs, I personaly believe that in the end a diesel engine may be a better way to go in the longterm, especially if you want to tow occasionally (or more often!)......reliability is a major theme for me personally.

Diesel engines with good lube oil and filters will usually outlive any gas or gasoline engine by several times.

Here in Germany you can only sell gas converted cars to innocent private users as normal car sales companies won't touch them with a "barge-Pole"....... Its because they know the truth behind many conversions and possible longterm engine damage......a friend of mine occasionally (when as part exchange) buys one, but at well below normal market price and he then removes the whole gas conversion before selling such cars on cheaply.

I am biased against the conversions, because over the years I have seen what can happen being on the "edge" of the car world, your choice.

I still hope this helps you further.

I think the last set of comments nailed it.

You are trying to fuel an engine that has been carefully designed to run on a different type of fuel. Burn rates are different, characteristics are different so you would have to expect a shortfall. The sink temp is important and the greater the difference the greater the efficiency.

The calorific values of the fuels are different and I think it would difficult to gauge a specific amount of fuel to do the comparison you want to do.

Good Morning, Folks!

I clearly need spend more time at my "day job" so that I can more readily keep pace with threads like this :o)

Thanks again for all of the input (and humor).

Something which struck me in the back-and-forth was the mention of higher peak combustion temps with NG; and some encouragement that Mr. Carnot may be in play here as well. I've heard this time and again; and wonder if TBC on the piston crown, combustion chamber, and exhaust tract might push both durability and Carnot away from the "dark side" in a given setting with NG (I believe Swain in New York messes around with this stuff for both auto and industrial markets).

On that point, I've also heard (in the case of mature diesels) that TBCs increase efficiency, reduce efficiency, and do nothing at all; sometimes in the span of a single paper. However, meaningful information dealing with TBC/LHRE schemes for SI engines, particularly in a scenario such as this, seems to be somewhat lacking...

More chocolate and graham crackers, anyone???

From personal experience I would have to disagree about the not towing or doing heavy work with a dual fuel converted engine. I have ran the crap out of all off my pickups that I have done conversions on without fear of damage.

For long distance heavy towing running my F250 on propane is my preference at this time. I have done tens of thousands of miles of heavy towing with the stock engine running on propane without issues. The temp gage says its running slightly cooler not hotter.

The Mazda not being all that powerful to begin with usually got driven with the pedal to the floor when pulling anything and I am even running an over sized vapor carb system on it. Same with the F250 it has put on thousands of miles over the last four years running at heavy throttle pulling loads in the 10,000 - 14,000 pound range at interstate speeds of 75 MPH for 150 - 400 miles at a time without problems.

Thats been my personal experience over the last ten years while putting on over 200,000+ miles on three vehicles now just with propane and I have never babied any of them.

Many variables there and NO German Autobahns either.

Here the conversion companies stipulate NO TOWING very clearly. They obviously know why, even if many other people don't.....

What the drivers (you for example) do is up to you......you are ultimately responsible for your vehicle.

Maybe I did not make it clear that probably only a small percentage of conversions get problems, but apparently many times more than straight petrol engines of exactly the same type and usage.

Then there are the problems of guarantee as the original vehicle manufacturer nulls and voids large areas of the guarantee on such conversions, mainly anything to do with the motor of course.

The conversion companies can wriggle like eels if something goes seriously wrong, even get taken to court and get away with it.

If for example it was only 2% of the conversions that cause serious problems, still no way for me to take that risk Jose!!

WHO THE F**K NEEDS THAT?

If the OP still wants to do it, at least he has been fully informed and heard both sides of the equation.

I personally believe that its most likely that a diesel to gas conversion may be basically a better deal as diesel engines are built far more robustly in all areas.

Many diesel buses in large cities around the world are so converted and run happily for many hundreds of thousands of miles....such a conversion would interest me more.

But is it common for people to pull twice their vehicles weight on the autobahn at speeds greater than our interstate speeds?

As far as I can work the math out going 75 mph with 22,000 pounds of vehicle takes more effort than going 100 MPH with 7500 pounds.

Different country different road conditions and different alternative fuel fallacies and myths.

As far as I am concerned a 2% fail rate is very low considering our nations big auto manufactures have a hard time getting under 20% fail rates with any vehicle they build. Gasoline or Diesel most people here expect to have about 1/2 the vehicles new purchase cost burned up in warranty work before it expires.

Please note, due to there being many firms here doing conversions, there are no reliable or exact statistics, even if it was less than 1 in 1000, nobody with any sense wants to take such risks....

Also, if such conversions are so good, why do transport firms stick with diesel?

Depending upon the country in Europe, you can tow at between 80 and 130 KMH legally, of course many tow at anything up to 20 KMH on top of that.....

Also here, we are talking generally of motors of 2 liters or less.

My caravan weighs only about 1500Kg., but presents a big wind load that also needs to be overcome, it is not unusual to have to drive with the accelerator on the metal when strong head winds are encountered, but as I drive diesel, who cares!!

You also ignored the serious loss in value of any converted car here when reselling, that is sometimes half the normal value of the car. Years of saving money thrown away....a bad surprise for people here....

You are still at the "I" or "me me" stage. You must think in a more general way as what you do only applies to you, not most of the people on this blog and around the world....

I am not saying that you are the only one doing it this way, but that such people are few and far between.....so it is still interesting to read about, but it applies to very few...

Also do not forget, that CR4 is not just "US Only", it is a world wide blog with people from all walks of life, language and religion......we must try and cover all bases....

Have a great day.

Back again.

Well, it looks as though the thread may be winding down for now, so I thought I'd summarize what I've gleaned here:

1. Natural Gas has a comparatively poorer LHV when compared to gasoline
2. NG burns with higher peak combustion temps than gasoline
3. Purpose-built NG SI engines have more robust cooling systems than their gasoline-fuelled counterparts
4. Practical experience shows varied durability limits across a spectrum of converted gasoline engines
5. NG conversion "kits" are of widely varying quality; leading to mixture-side inefficiencies in any particular setting
6. A perfectly-prepared NG mixture may or may not burn as efficiently as a perfectly-prepared gasoline mixture in any converted engine, unless static compression pressures are raised

Is this mostly right?

If so, what are the durability issues associated with raising the static compression pressures in a given engine when converted over to NG? If the stock power levels/curve for the engine are rigidly retained, it would seem as though durability would be similar for both; particularly when one considers the improved combustion parameters for NG.

Is this mostly wrong?

Thanx again --

My take is that its mostly right.

Engines need to be designed for better cooling internally, its not just a case of a larger radiator and a better water pump, though this may also need to be done.

Engines specifically designed for gas are no more of a problem than any other normal car engine, you also have the support/guarantee of the original manufacturer, not a split guarantee between 2 or more manufacturers....

I think it all comes down to these points.

Is resale value more important than fuel/money savings?

Are you conservative and easily put off by experimenting or more open to experimenting to see what you can gain and learn even if it does not always financially pan out favorably the first time?

If "What if" has ruled your life then no its probably not for you but if you are more like me following the heard always makes me fell like I am just traveling to the same places the big bunch of a-holes I see in front of me has already tread.

Ultimately it comes down to you. I for one don't regret my choices to experiment with different fuels I gain bunches learned even more and found a lot of the horror stories to be untrue or at best very weakly founded in reality.

If (as can happen here) your cars cvalue after 3 years is 25% of the new price, also the installation cost here will be between $3,500 to $5,000, the savings made on the fuel pale into insignificance in comparison.

Add an often "dodgy" conversion guarantee and there you are.......

I let the suckers test such things out, but I am not impressed by anything I have seen heard or read up to now......

I am sure that most work fine, otherwise they would have died out years ago, but they simply don't work as well as the petrol engine on petrol in all cases.....

There is nothing about a LNG powered vehicle that would convince me, at 65, living where I do now, to switch to it.

I think it's great that we have the luxury of deciding which is best for ourselves.

I'll look at all three in another few years, when I'm ready to turn in the old Ford.

Cheers.

Well put!

The oil companies will look at LNG as competition with gasoline. They would have to raise the cost of LNG to gasoline prices in order to maintain profits. The government gets it's highway taxes from gasoline and diesel. A switch to LNG would get taxes slapped on. In the long run, whether we burn gasoline, diesel, LNG, propane or battery power, the bottom line to the consumer will be cost per mile will be pretty much the same, but new technology will make that cost greater, not less.

Gone is the myth that energy sources are limited. It is becoming common knowledge that there are unlimited sources of energy, and that it can be used in a variety of engines. Electricity will be cheap due to natural gas. People who live in Oklahoma, Texas, California, and Utah have been using cheap natural gas for quite awhile. Word gets around quickly. Companies want to make profits, and are going to want low priced energy. The majority of people are slow to wake up and smell the coffee. There are always plenty of nay-sayers. Overcoming the mental and emotional inertia is the job of those who see a better future.

Cost is always dependent on scarcity. There is no scarcity. A few years ago, the oil and gas companies had us convinced there was. Now the various companies will be trying all the strategies they can to raise prices of natural gas. Right now it is priced too low to store, they are running out of space. Japan is already rushing to figure out how to tap methane hydrates, as are many others. There are estimates that methane hydrates equal up to ten times what is on land worldwide.

Natural gas is all over the world. Nations are busily buying up our expertise and equipment. We will have many competitors when we start exporting LNG. This will limit our domestic price for natural gas.

Natural gas can also be pumped right from your home lines. Right now the pump is close to $5,000 but efforts are being made to get the pump price down to $1,500. A GGE (gas gallon equivalent) is about 80 cents if pumped at home.

You can help, stand, in the way, or get out of the way. Our economies, and our soldiers are in danger because of military actions in the Middle East over oil.

Sources:https://docs.google.com/document/d/1NbaKYme3bqOw0b6KMxXSjOLHLNeflalPy9gIAiTYFMQ/edit

Cost is always dependent on scarcity.

This might seem logical based on what we've learned in economics, when it comes to energy however, it is far from the truth. During the spike in natural gas prices leading up to the winter of 2005 the storage facilities in North America were already close to being maxed out. During this time I read an article from the CEO of one of the major natural gas companies who said, 'if the temperatures this winter are higher than we predict, demand will go down, we will have to cut production, and thus have to raise prices. If the temperatures this winter are lower than we predict, demand will be higher than expected, thus we will have to raise prices."

For those of you who didn't catch all that, it means 'Even though we have more than we know what to do with, we're going to raise prices anyway.'

I didn't like the smell of it, so I took the situation up with FERC. The response I received was "we cannot comment on whether or not we have an investigation into this matter".

Based on those recent past events normal economic logic doesn't seem to be of any help in predicting pricing when it comes to natural gas.

Retail price is presently as low as $1.07 per GGE ranging up to double that for those retailers are really making a killing. Guess what wholesale is per GGE? $.28. At your own pipeline, in your house it is about $.80. Then they have the nerve to tell you they are selling it at their cost, and charge you an equal rate for delivery! There is so much retail profit to be made that it is incredible. I am thinking of buying an old closed gas station and setting it up for natural gas and propane only. I will only think about it though because I want to enjoy my retirement , and don't need the money.

There are many companies that can sell natural gas. Far more than there are those that can sell refined petrol. That will make a big difference. Plus there will be more profit in retailing natural gas because the wholesale price is so much lower than gasoline. which is $2.93.

I can see a behind the scene war to prevent natural gas from being used, but too oil companies natural gas is a real conundrum because they can sell it too. They would just like to somehow keep small companies from competing at low prices.

The tax on gasoline could easily be paid by natural gas customers as well. It is a small percentage of the overall price of gasoline, but is more than the cost of the natural gas! Here is a state by state table:http://www.gaspricewatch.com/web_gas_taxes.php

New Argonne National Laboratory study on the subject: http://cngchat.com/forum/showthread.php?9511-Argonne-study-identifying-fuel-consumption-penalties-for-CNG-use

I would love for you guys to take a look at my question about using CNG with light aircraft engines! See: http://cr4.globalspec.com/thread/83632