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  • O/T LPG/CNG powered vehicles

    I see that GM is about to produce duel-fuel pickup trucks that will run on either CNG or gasoline at the push of a button and this has me thinking about something I have wondered about for a while. It would seem, to me anyway, that an internal combustion engine can not be designed simply to run efficiently on both types of fuel and that such engines would from a practical standpoint have to be compromised in efficiency for either or both types of fuel. I may be wrong but wouldn't an engine designed to run specifically on Natural gas or Propane need different internal engine specs to operate at maximum efficiency? What I am thinking is that compression ratios, cam timing/lobe design and ignition timing might need to be quite a bit different for maximum efficiency when switching between these two fuels. While changing the ignition timing and curve would be very simple things like compression ratio and cam specs would not so that the compromise becomes necessary. The current thinking is that CNG costs are about $2 equivalent to a gallon of gasoline which at today's prices would be around half the price of driving on gasoline but could a CNG or LPG specific engine get even better economy? Just thinking here and I may be off the mark with my understanding but I would like some other opinions on this.

  • #2
    Real time ignition & limited cam timing are pretty much commonplace these days, commanded by ecu mapping.

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    • #3
      Well I already pointed out that ignition timing is easily changed and so could cam timing but that does nothing for the compression ratio (although it could affect cylinder pressure) or even cam lobe profiles both of which I would think could have a lot of effect on efficiency between CNG vs gasoline.

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      • #4
        I converted my 1972 Ford F series to dual fuel (propane & gasoline) about 30 years ago. This of course had a carburetor, so it required a throttle body that sat on top of the carb. Ran quite well and was much cheaper than gasoline. Many of the utility company trucks & vans in Florida ran on propane at the time, so filling up was easy. The fuel cost was lower and the life span of the drive train was considerably longer.

        I also remember from my under physics class that a gasoline internal combustion engine converted to hydrogen would produce about 80% of the power when compared to gasoline.

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        • #5
          LPG is over 100 octane ..so you can move the ignition timing ..it also has something like 20 percent less calorific content.

          Another problem with it ..is that it is an insulator ..so needs good ignition to make the spark jump the gap....HT leads ..and other ignition componants give up the ghost a long time before they would on petrol.

          it is also very dry ..so there is less lube for the upper cylinder components.

          Some cars have to use something called flash-lube..basically a bottle of special lubricant is attatched to a tube that leads into the manifold

          Modern LPG systems are designed to compensate for the loss of calories by shoving more of the stuff in .....this also keeps the o2 sensor happy and the cat happy.

          My last car..before the present one ...i had it set up with adjustable ignition timing..on the fly .......it had open loop...no cat ..no o2 probe ...you could mess a lot more with it ...........it was slower in top speed and had less acceleration ..but it did the same MPG on LPG as petrol...sometimes more.

          My present car ..has sequential injection .......it has to keep the o2 sensor happy and the cat .........it performs as on petrol .......but it does 25 percent less to the gallon ..

          I miss the first car .....mpg meant more to me than performance.

          oh yeah ..the oil stays clear for a long long time.

          I believe cng is at very high pressure ...........so no DIY job

          all the best..markj

          Comment


          • #6
            You are right, a bi-fuel gasoline/cng-lng engine is going to be inherently a compromise design due to the vastly different combustion characteristics of the two fuels. Propane/gasoline conversions equally so.

            The compression ratio required to take advantage of the 130 octane rating of natural gas is not available on engines that can operate on gasoline. So we end up with an engine that while cleaner emission wise while operating on NG, is still compromised in regards to power output due to the lack of technology required for variable compression.

            Engine management of timing, fuel injection, and even valve opening and closing events have already been addressed. Engine component selection can also be optimized for NG operation, but the key hurdle for NG fuel optimization is a reliable means of varying the compression ratio to take advantage of NG's 130 octane rating.

            Diesel/cng-lng engines can be more optimally designed to take advantage of NG's high octane rating due to those engines already having a high enough compression ratio.

            At this time Cummins/Westport is marketing a very successful cng/lng series of engines to the medium and heavy duty engine market that is serving markets that either require a very low emission profile, or markets that want take advantage of lower fuel costs.

            Large truck fleets are currently switching to NG engines in order to save on fuel costs. Federal, provincial, and state financial incentives are in place to help fleets during the conversion process, not only for the vehicles themselves, but also the infrastructure required to fuel these vehicles due to the 'green' connotation associated with NG as a vehicle fuel.

            But don't forget the motto..."if it sounds too good to be true, it probably is".
            As even now in NG's early stage of acceptance as a motor fuel, the same various federal, provincial, and state bodies that are giving financial incentives are already in the disscussion stage of legislating taxes on natural gas when used in a motor vehicle.
            Home, down in the valley behind the Red Angus
            Bad Decisions Make Good Stories​

            Location: British Columbia

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            • #7
              I run my little station wagon here on Natural Gas about 90 percent of the time. The fuel savings is tremendous! Plus, they say that it is about 80% less polluting.

              I do have to deal with a slight power drop, but I'm not in any races.

              The only ongoing problem is that the fuel injectors tend to carbon up with disuse. I've been running it for about 6 years now.
              Vitَria, Brazil

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              • #8
                One intresting thing to note, is you can effectively change compression by mucking with the valve timing. All you need to do is leave the intake open after bottom center, Or close it before the cylinder is at bottom center. Less intake volume = Less peak pressure = Less effective compression ratio.

                Also, Its rather easy to alter the valve timing if you only have 2 'sets' of timing you wish to use, You just have 2 sets of cams on your camshaft and move the whole camshaft in/out an inch to switch between them.
                Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                • #9
                  Originally posted by Black_Moons
                  One intresting thing to note, is you can effectively change compression by mucking with the valve timing. All you need to do is leave the intake open after bottom center, Or close it before the cylinder is at bottom center. Less intake volume = Less peak pressure = Less effective compression ratio.

                  Also, Its rather easy to alter the valve timing if you only have 2 'sets' of timing you wish to use, You just have 2 sets of cams on your camshaft and move the whole camshaft in/out an inch to switch between them.
                  which isnt any good if you want to run on petrol

                  sometimes i run out because there isnt an lpg station near by.
                  or the stuff at that nearest station is too expensive ...ive seen a 25 percent difference in prices between stations ....petrol is always less than 10 percent.

                  it's half price in the uk for lpg overall........if a satation is selling it at 25 percent more ...then you add your 25 percent to the 25 {worst case} percent loss in economy ..and there isnt anything to be gained from running on lpg

                  all the best.markj

                  Comment


                  • #10
                    Originally posted by Willy
                    The compression ratio required to take advantage of the 130 octane rating of natural gas is not available on engines that can operate on gasoline. So we end up with an engine that while cleaner emission wise while operating on NG, is still compromised in regards to power output due to the lack of technology required for variable compression.

                    Diesel/cng-lng engines can be more optimally designed to take advantage of NG's high octane rating due to those engines already having a high enough compression ratio.

                    At this time Cummins/Westport is marketing a very successful cng/lng series of engines to the medium and heavy duty engine market that is serving markets that either require a very low emission profile, or markets that want take advantage of lower fuel costs.

                    But don't forget the motto..."if it sounds too good to be true, it probably is".
                    As even now in NG's early stage of acceptance as a motor fuel, the same various federal, provincial, and state bodies that are giving financial incentives are already in the disscussion stage of legislating taxes on natural gas when used in a motor vehicle.
                    To correct a few points, no offense intended in the least...

                    NG engines that are running on 100% gas are low compression, usually 8-12:1 as they are spark ignited, not compression ignition. The reason many of the heavy duty manufacturers build them based upon diesel platforms is that traditionally gas engines have gone into stationary platforms for electric power generation or petroleum pumping. In both applications these engines may run 24/7/365 for years on end (some of ours go 40k+ hrs), and thus it makes sense to build them as heavy/overbuilt as possible. These are not high compression however.

                    The Cummins/Westport high pressure direct injection (hpdi) 8.9L "ISX" engine is IMHO a POS novelty. Others have done similar systems better. The nation's largest fleet of them is based ~30 miles north of here, and being that I work in engine development I have made it my business to become rather "close" with quite a few of the mechanics, drivers, and owners. Very few of the individuals I have spoken with, to include some of my contacts inside of Westport, will give it a good review. Personally, I think their "technology" is anything but, but that is personal opinion.

                    NG already has a road tax applied to it, just not in all areas.

                    Personally I wouldnt see a gasoline/NG hybrid whether its a dual-fuel setup or straight either/or fueling setup as much of a problem. Considering the possible technologies such as fuel/timing/ignition/detonation/etc mapping available along with variable cam timing, the possibilities for some engines are basically unlimited. Even the aforementioned compression ratio is often adjusted "on the fly" in modern vehicles (you just dont know it). There are going to be compromises, but the engineering ones will be small. I cannot comment on the compromises made by the "business" folks however.
                    "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."

                    Comment


                    • #11
                      Originally posted by aboard_epsilon
                      which isnt any good if you want to run on petrol
                      ... Uh, Why not? you have one set of cam timing deticated to petrol and another deticated to NG.

                      Some engines actualy already do have 'shorter intake*' strokes then exhaust, by using valve timing, because its more efficent as you don't need a full intake stroke worth of fuel/air to produce a full stroke of hot, expanded exhaust.

                      *Effectively shorter because of valve timing. Slightly reduces pumping losses.
                      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                      Comment


                      • #12
                        Originally posted by Black_Moons
                        ... Uh, Why not? you have one set of cam timing dedicated to petrol and another dedicated to NG.

                        Some engines actual already do have 'shorter intake*' strokes then exhaust, by using valve timing, because its more efficient as you don't need a full intake stroke worth of fuel/air to produce a full stroke of hot, expanded exhaust.

                        *Effectively shorter because of valve timing. Slightly reduces pumping losses.
                        sorry never read the second line of your post

                        sounds expensive ..like in the thousands 4 figures expensive ...the payback time will lengthen a lot

                        standard LPG conversion here takes some 25,000 miles to pay for its self, if not done by yourself.

                        the LPG used to be 1/3rd of the price of petrol ..now its more than half.

                        and government pulled out of subsidising conversions over 5 years ago.

                        over 60 percent of the price of petrol here is now tax

                        all the best...markj
                        Last edited by aboard_epsilon; 03-05-2012, 07:29 PM.

                        Comment


                        • #13
                          Originally posted by justanengineer
                          To correct a few points, no offense intended in the least...

                          NG engines that are running on 100% gas are low compression, usually 8-12:1 as they are spark ignited, not compression ignition. The reason many of the heavy duty manufacturers build them based upon diesel platforms is that traditionally gas engines have gone into stationary platforms for electric power generation or petroleum pumping. In both applications these engines may run 24/7/365 for years on end (some of ours go 40k+ hrs), and thus it makes sense to build them as heavy/overbuilt as possible. These are not high compression however.

                          The Cummins/Westport high pressure direct injection (hpdi) 8.9L "ISX" engine is IMHO a POS novelty. Others have done similar systems better. The nation's largest fleet of them is based ~30 miles north of here, and being that I work in engine development I have made it my business to become rather "close" with quite a few of the mechanics, drivers, and owners. Very few of the individuals I have spoken with, to include some of my contacts inside of Westport, will give it a good review. Personally, I think their "technology" is anything but, but that is personal opinion.

                          NG already has a road tax applied to it, just not in all areas.

                          Personally I wouldnt see a gasoline/NG hybrid whether its a dual-fuel setup or straight either/or fueling setup as much of a problem. Considering the possible technologies such as fuel/timing/ignition/detonation/etc mapping available along with variable cam timing, the possibilities for some engines are basically unlimited. Even the aforementioned compression ratio is often adjusted "on the fly" in modern vehicles (you just dont know it). There are going to be compromises, but the engineering ones will be small. I cannot comment on the compromises made by the "business" folks however.
                          As an engineer for the competition I can certainly appreciate you being in the loop so to speak more than I, and understandably perhaps also a little biased toward the significant inroads that Cummins/Westport has made into the heavy duty engine market.

                          I also realize that the company you work for, although very active in the heavy duty engine market, and also a dominant force there, does no longer directly produce engines for the on-highway market so it is understandable that Cummins Westport is the dominant player in that segment with Kenworth/Peterbuilt both offering the Cummins Westport gaseous fueled engines.

                          I also should have qualified some of my statements by stating that for the most part I was referring to the 15L HD 400/450/475 series of compression ignition engines. These of course rely on a 5% diesel pilot injection, 95% NG. Using high pressure LNG in this application allows a significant range vs. CNG. Some very large fleets are currently in the process of utilizing this powerplant for 'B' train applications in Canada and probably the US market as well.

                          But more to the point these engines are not bi-fuel engines. I personally do not under estimate the ability of the industry to overcome the technological hurdle of variable compression if that is the direction it wants to direct it's focus on. All one has to do is look back just two decades to see how far we've come in terms of meterials, lubricants, machining precision, and engine management.

                          The spark ignited B/C/G series of engines are I agree, lower compression versions of their diesel fueled brothers.

                          The price advantage for NG will in my opinion be lost when government officials realize that there is a large enough presence of NG fueled vehicles to institute tax increases. Never under estimate the length of the governments arms when it sees a potential source of revenue.

                          It will be a cold day in hell when you see a politician with his hands in his own pockets instead of yours.
                          Home, down in the valley behind the Red Angus
                          Bad Decisions Make Good Stories​

                          Location: British Columbia

                          Comment


                          • #14
                            Thanks for remaining a gentleman Willy. I didnt intend that to be insulting in the least, and on second glance my earlier comment was rather harsh.

                            I understood the engine you meant, I simply mistook the displacement between what is known here as the "ISL-G" and the "ISX." Ive actually attended a few Paccar/Westport training sessions on the west coast on both, but simply believe that the dual injector "technology" is more of the "next step" in a natural progression and not really a "leap." I give Westport credit for producing them, but believe that a true "dual fuel" that reverts back to full diesel mode has more practical advantages, and as I said earlier the local fleet doesnt seem to like them. Granted, Im wearing the wrong color, so maybe they are simply "playing up" to me. Personally, I like the other Westport gas engines, and honestly think they saved Cummins' NG engine line, but that other one simply doesnt appeal to me.

                            You are definitely correct about the politicians' hands. This time of year especially, I get rather disappointed as I dont even get a reach around.

                            Slightly OT, but had to post after seeing this on PM.

                            http://www.youtube.com/watch?v=7qGI6...eature=related
                            Last edited by justanengineer; 03-05-2012, 08:48 PM.
                            "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."

                            Comment


                            • #15
                              Run a Truck like that in 1948.
                              Wood Gas or Propane.
                              If you don't know what wood gas is, you have 5-6 bags of wood cubes in the back witch gets burned up in some big round 6ft tall 2 ft OD kettle, (burner).
                              H12721

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