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Thread: LPG fumigation in diesels

  1. #11
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    Re: LPG fumigation in diesels

    Some of the gas systems can use CNG, NG or LPG. The computer makes the necessary adjustments.

    I have an LPG home heater. The same model of heater also came as a unit that would use NG. I understand that the difference is largely a substitution of jet sizes.

    If such was the case, then a basic LPG system may be able to be converted to the use of CNG with a minium of work. I'm looking into that issue with this unit I am going to check out.

  2. #12
    David Guest

    Re: LPG fumigation in diesels

    Hi Tom,

    If Range and economy are your main aims, perhaps a good old fashioned WVO conversion may meet your requirements as well.

    I looked at how CNG worked some time back and from my understanding, the system at the time worked by you leasing a compressor that filled your tank from your gas at home. As I understand it, Cng has to be compressed to extremely high pressure because in a car system it can't be compressed to liquid so to get any range the gas has to be pressurised right up. I seem to recall that relatively huge tanks like 200+L were needed to get a car 500KM or so. Apparently the power draw of the compressor adds significantly to the cost of fueling the vehicle besides the gas itself because it takes a lot of power to compress the gas to such high pressures.

    As I recal, it takes something like 6-8 hours to fill a tank and range is pretty limited anyway. As far as I am aware, there are very few service stations where you can fill up on CNG, most filling points are privately owned.

    It is some years since I looked into this so maybe things have changed but this is how I understand CNG systems to work.

  3. #13
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    Re: LPG fumigation in diesels

    LPG does not give a more complete burn of the diesel fuel

    It gives more power by introducing more fuel
    Direct quote from the sales brochure left pinned to the notice board at work by owner of one of the vehicles I mentioned.

    Regardless of the sales pitch, it certainly work. Whether the net gain justifies the cost is something I would be looking at rather closely.
    Rgds

    Adam

    "Revolution never comes with a warning!"

  4. #14
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    Re: LPG fumigation in diesels

    Hi David,

    I thought about a WVO conversion some time ago when I bought the Peugeot, but from everything I've read about SVO/WVO conversions and Lucas rotary pumps, I'm not game to try it! As much as I like the idea of using WVO as a fuel, I've not really been inclined to try it. Apparently, the Lucas pump on my model Pug does not tolerate WVO well at all. The Bosch pump, on the other hand, does quite well. I guess you could always swap the pump if you wanted to do it, but I'm not too keen on the idea of it for some reason.

    Hmm, looks like the CNG is a no go! Sounds like far too much hassle and inconvenience. Looks like I might have to stick to the LPG conversion. Still, two thumbs up to those that do run CNG! I like the idea of being able to fill up at home!

  5. #15
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    Re: LPG fumigation in diesels

    Hello Adam

    Yes, there is a LOT of inaccurate information available about how things work.
    You can google Natural Rubber Biodiesel and find over 200,000 websites warning about the "Natural Rubber" fuel lines used in most pre 1993 vehicles.
    Natural Rubber was NEVER used in automotive fuel system, petroleum products degrade natural rubber the same as biodiesel does.

    You can go to many LPG web sites that will tell you that typically a Diesel engine only burns 75% of the diesel fuel injected, the rest goes out the exhaust. LPG Makes more of the diesel burn.
    What a load of bunkum!
    If a diesel only burns 75% of it's fuel then a petrol engine must be just as bad because we ALL know that diesel engines get better economy than petrol engines. That would only happen if petrol engines were pumping out lots of unburnt fuel like they claim a Diesel does.
    How long would a catalytic converter (Diesel or Petrol) last if 25% of the fuel was going out the exhaust pipe unburnt?

    In my mind, the correct answer has to do with the questions:
    Why does a diesel engine typically get much better fuel economy and produce less power than a petrol engine of the same displacement?
    Does a Diesel engine ever achieve "Stoichiometric Combustion" with injected diesel fuel or is it always running Lean?
    If the answer is that a Diesel always run lean could a gaseous fuel such as LPG be included into the air to use some of this unused Air to produce extra power?
    If you got rid of the injection system on your diesel and converted your diesel engine to run on 100% LPG (Spark plug required and probably lower compression) coming through the turbocharger mixed with air at the correct stoichiometric ratio, what do you think would happen?
    Last edited by tillyfromparadise; 21st January 2007 at 07:45 PM.

  6. #16
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    Re: LPG fumigation in diesels

    Quote Originally Posted by tillyfromparadise View Post
    Hello Adam

    Yes, there is a LOT of inaccurate information available about how things work.
    You can google Natural Rubber Biodiesel and find over 200,000 websites warning about the "Natural Rubber" fuel lines used in most pre 1993 vehicles.
    Natural Rubber was NEVER used in automotive fuel system, petroleum products degrade natural rubber the same as biodiesel does.

    You can go to many LPG web sites that will tell you that typically a Diesel engine only burns 75% of the diesel fuel injected, the rest goes out the exhaust. LPG Makes more of the diesel burn.
    What a load of bunkum!
    If a diesel only burns 75% of it's fuel then a petrol engine must be just as bad because we ALL know that diesel engines get better economy than petrol engines. That would only happen if petrol engines were pumping out lots of unburnt fuel like they claim a Diesel does.
    How long would a catalytic converter (Diesel or Petrol) last if 25% of the fuel was going out the exhaust pipe unburnt?

    In my mind, the correct answer has to do with the questions:
    Why does a diesel engine typically get much better fuel economy and produce less power than a petrol engine of the same displacement?
    Does a Diesel engine ever achieve "Stoichiometric Combustion" with injected diesel fuel or is it always running Lean?
    If the answer is that a Diesel always run lean could a gaseous fuel such as LPG be included into the air to use some of this unused Air to produce extra power?
    If you got rid of the injection system on your diesel and converted your diesel engine to run on 100% LPG (Spark plug required and probably lower compression) coming through the turbocharger mixed with air at the correct stoichiometric ratio, what do you think would happen?
    Tilly,
    I'll have a go at your quiz. Is there a prize? Perhaps a 20 litre container of the liquid you left with me a year or so ago. Mmmmmm.

    1. The diesel engine has a better thermal efficiency than a SI engine. A SI engine of the same capacity can deliver a significant increase over the diesel engine because it can dump in bulk fuel and, despite the very low efficiency at that condition, it will pump out significantly more power. A petrol engine operates at a stoichiometric fuel/ air ratio. If it runs rich or lean, the efficiency drops further.

    2. The only time that a CI engine could possibly achieve a stoichiometric fuel / air ratio would be if it were adjusted to over-fuel at maximum throttle. At that condition, the engine fuel efficiency drops and partly burned fuel appears in the exhaust as soot. At ALL times, a correctly adjusted CI engine should run LEAN.

    3. Yes, that is the advantage of LPG fumigation, to provide additional fuel which does not cause soot production, resulting in increased engine power. Use of LPG does NOT increase the engine efficiency, but it can increase engine power output (more fuel is used to achieve this).

    4. Diesel engines converted with spark plugs and a gas carburettor are commonly used on gas pipeline projects. They use an engine produced as a diesel, for the strong build of the engine components and use spark plugs to ignite the stoichiometric air / fuel ratio provided by the gas carburettor. It can provide similar power levels to similarly sized engines designed as SI engines, but the fuel efficiency drops to that of SI engines. Turbo chargers allow more fuel to be introduced to the combustion chamber because there is more air in the compressed charge air, resulting in an increased engine power output.

    Did I win the prize?

    When can I expect the 20 litres of your nectar?

    Tony

  7. #17
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    Re: LPG fumigation in diesels

    So the increase in range is not due to better diesel combustion, it is because there is an extra fuel tank?
    Is adding an extra fuel tank a better proposition of getting extra range?
    I guess if you really want to get money from the government you could always join the army reserve
    cheers<BR>Chris.<BR>1990 landcruiser 80, 1HD-T two tank, copper pipe HE+ 20 plate FPHE, toyota solenoids and filters. 1978 300D, elsbett one tank system.<BR>

  8. #18
    David Guest

    Re: LPG fumigation in diesels

    Wait!
    I want a crack at this too!

    How long would a catalytic converter (Diesel or Petrol) last if 25% of the fuel was going out the exhaust pipe unburnt?

    Not very Long. Cat's run at very high tempratures and are sensitive to excessive/unburnt fuel in the exhaust gas stream. Any significant amount of unburt fuel causes them to overheat and melt. when you consider these things are are mainly made of ceramic and titanium, it would take a lot of heat to make that melt but it certainly does happen. Turbo cars can be overfueled at the top end to reduce the onset of detonation and this excess fuel can have the effect of melting the cat if the overfuel condition is held long enough.

    Why does a diesel engine typically get much better fuel economy and produce less power than a petrol engine of the same displacement?

    I believe this has a lot to do with the way the engines are set up and the actual engines in question. Typically Diesel engines have been much simpler designs than petrol engines. Pretty much all the technology that has gone into diesel for years is Turbo'ing. Petrol engine design on the other hand has given them multiple valves, multiple cams, variable timing of ignition and cams, all sorts of electronic management and every other benifit to make power and economy possible. Diesels typically have been set up for economy or longevity. A lot of the efficency of a diesel comes from the timing of when the fuel is injected and the compression ratio of the engine.

    When we now look at the latest developments of the Diesel where they are incorporating these same technologyslike used in spark engines into CI engines ( like the latest BMW's) Diesels can make MORE power and economy than petrol engines of the same capacity. If one looks at what they are doing in the European hot rod scene with Diesels, even basic design 1.6 & 2.0L cars are giving similar capacity but much more developled petrol engines a real run for their money. In the states, Plenty of mods are available for "Pickup" trucks which will enable them to produce 500+ Hp with nothing more than a plug in engine management controller and a Bigger exhaust. When these guys get serious, they can easily run 1000Hp trucks as every day drivers and without laughing gas.

    You can get big numbers out of petrol engines, but not with petrol. Once you get to a certain HP per cubic inch, you have to start going to alcohol or other fuels because petrol will only allow you to go so far.
    If one looks at drag racing for instance, Petrol cars, even when bottle fed, only go so fast and that so fast is pretty well down the line of overall speed. Alky cars are much faster and run similar capacitys and equipment to cars limited to petrol. With diesel, you can get much higher specific power outputs on " Pump fuel" where the same limitation is quickly exceeded with even high octane petrol. The fastest Diesel engined car still only runs regular Biodiesel not some exotic racing fuel.

    When taken to the limit, Diesels can make more power than a petrol engine because the final limitation is the fuel. You can only put a certain amount of fuel in a given capacity engine. Because diesel has a higher energy content and can run higher compression, at the razors edge, diesel wins.


    Does a Diesel engine ever achieve "Stoichiometric Combustion" with injected diesel fuel or is it always running Lean?

    Again, I believe it depends on the engine.
    Some Diesels are set up so they never recieve the maximum amount of fuel they are capeable of burning. Other engines may be set up to produce maximum power at the cost of some efficency.

    If an engine is set up so it can achieve an over fueled condition, somewhere in the fuel air curve between lean and over fueled there must be a perfect Stoichiometric ratio achieved if only momentarily in practical use. In this case, the answer would be yes, it can achieve a perfect fuel air ratio and NO, it is not ALWAYS running lean.

    If you got rid of the injection system on your diesel and converted your diesel engine to run on 100% LPG (Spark plug required and probably lower compression) coming through the turbocharger mixed with air at the correct stoichiometric ratio, what do you think would happen?

    You'd be wasteing a perfectly good Diesel engine??
    Aside from this, I can only see that the engine would run the same as a petrol engine. The design of the engines proper isn't any different, its just the fuel, and the way it is fed.

    Turbo'ing gas engines works well for the opposite reason it works on diesels. LPG has a higher calorific value but a lower energy rating. As such, you need to put more LPG into an engine to get the same amount of energy as petrol BUT, gas can run at higher compression ratios which can lessen the power deficit through efficency.

    While I believe that turning the pump up on a diesel would produce more power than fumigating it, in this case, that does not meet Toms requirement of running a cheaper fuel. In the economy stakes, making bio and having the wick turned up on the engine would be the soloution but if memory serves correct, I believe Tom has space restrictions at his abode so again this isn't practical in his circumstance.

    OK, now I'll wait to see if I get a good mark and extra credits or a smack over the kunckles with a ruler.
    Last edited by David; 22nd January 2007 at 01:45 AM.

  9. #19
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    Re: LPG fumigation in diesels

    Tony from West Oz is the Grand Prize first place Winner!

    Everything Tony posted is correct according to my understanding. The only way you will get a normal Diesel to run at stoichiometric is to over-fuel it so it is belching lots of black smoke.
    I will e-mail your prize within the fortnight


    Captain Echidna is the first runner-up
    You are correct, there is no free lunch. LPG is NOT a catalyst and does not make the other 25% of the fuel burn. you get more power with LPG because with LPG you can put more total useable fuel into the engine

    David is the second Runner-up
    A Catalytic converter would not last very long if any substantial amount of raw fuel were going out the tailpipe.
    Turning the pump delivery up will typically result in an increase in power. The new Mussos can achieve a 13% power increase with no visable smoke.

    The following information is from this very informative article
    How a Diesel Engine Works - Diesel Motor Basics Diesel Power Magazine

    When the displacement is the same, the diesel engine produces only about two-thirds the horsepower of the gasoline engine. This poses a question: Why is the power output lower even though the combustion pressure is higher?
    Since combustion is an oxidation reaction, there is a specific weight of air that will completely oxidize one gram of fuel without leaving excess oxygen. This weight of air is called the stoichiometric air/fuel ratio. The gasoline engine operates with an air/fuel mixture very near stoichiometric.
    This is due to the fact that a mixture much leaner than stoichiometric is difficult to ignite in a gasoline engine with a spark plug, and an extremely rich ratio is very inefficient. The mixture is supplied to the gasoline engine by a carburetor or fuel injectors in the manifold and is well mixed and nearly homogeneous.

    In the diesel engine, the fuel is injected into the combustion chamber near the end of the compression stroke and ignites spontaneously. This is responsible for the combustion sound that a diesel engine generates that is music to the ears of everyone who reads this magazine. As mixing between the fuel and air occurs, burning continues. This process is very heterogeneous (since the fuel and air are mixed in a combustion chamber it is not as uniform as in a gas engine that has the mixture created prior to entering the cylinder head).
    Soot is formed during combustion because some of the fuel burns with insufficient oxygen, and the combustion of the fuel is not completed. As additional fuel is injected, more and more soot is produced. Therefore, the air/fuel ratio of the diesel engine must always be leaner than stoichiometric to prevent excessive amounts of smoke.
    For this reason, a modified, high-output diesel will blow black smoke because it is fueled for power alone with no concern for soot generation. The smoke-free diesel has less fuel present in the cylinder than in the cylinder of the gasoline engine, and diesel power is therefore reduced in comparison. Older, naturally aspirated diesel engines can use only about 70-80 percent of the fuel employed by a gasoline engine with the same swept volume and still avoid black smoke."
    Last edited by tillyfromparadise; 22nd January 2007 at 10:26 AM.

  10. #20
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    Re: LPG fumigation in diesels

    Why does a diesel engine typically get much better fuel economy and produce less power than a petrol engine of the same displacement?

    I thought it was compression ratio. By injecting the fuel at top dead centre and not compressing it, this means a higher compression ratio is possible. (If you run a high compression ratio in a petrol engine you risk the fuel catching alight while the piston is going up, rather than waiting for the spark. burning fuel pushing the piston down while it needs to go up is not good for power or much else)

    So there is a lot of excess air in the combustion chamber. When the fire is lit, it burns, gets bigger and pushes the piston down, same as a petrol engine. There is also a lot of excess air in there (at low throttle) which a petrol engine does not have, which gets hot, causing the air to expand, and push the piston down, the heat of the combustion is also contributing to power, not just the burning of the fuel, so there is power coming from 2 sources as such, not just one.

    Mind you I guess this is what Tony refers to as "The diesel engine has a better thermal efficiency than a SI engine"

    As for less power, Power is determined by tourque and RPM. as the air heating it is a slower process, it means a diesel cant rev like a petrol engine, so has less "power events" per minute, does not burn the same amount of fuel, cant take in as much air to burn with the fuel, so less power.

    AS for gas engines having less power than petrol ones it comes down to 2 factors.
    Petrol enters the combustion chamber as a liquid, not taking up much space. (it turns to a gas in the compression stroke as things the air gets hot due to compression) Gas takes a bigger volume, so displaces air on its way in. Less air to burn means less power.
    Lpg burns more slowly. Unless you are going to make up some special igniton system (which will mean the car cant run on petrol again) it is dificult to get gas ignition spot on.
    There are ways to get this power back, but it will mean the car cant run petrol again.

    My 2c
    cheers<BR>Chris.<BR>1990 landcruiser 80, 1HD-T two tank, copper pipe HE+ 20 plate FPHE, toyota solenoids and filters. 1978 300D, elsbett one tank system.<BR>

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