Car Adaptation For LPG Solely ( Without Gasoline Altogether) ?

There are straight lpg conversions done all the time, Ford Oz actually have an option to buy a Falcon that is just straight LPG, mainly for Taxi use. Back in the early 80's Nissan/Datsun sold a LPG only 200B sedan, they sold about 300 cars (in Oz)mainly to frugal people who didn't go far.

There are a number of options for tanks including the toroid type and "divers" tank style.

Straight gas conversions are only ideal (in a passenger car) where your travels don't take you beyond the metropolitan service stations that supply LPG. You need to do a lot of distance to make the return on investment on the installation worthwhile.

It used to be in Oz that LPG was roughly a 1/4 of the price of Petrol which made it desirable to convert to LPG. These days LPG is nearly 2/3's the price of petrol making the ROI hard to justify. That and the regulatory constraints and on costs really make it a pain (in Oz).

That said, there are performance upsides to using LPG over Ron 91 ULP. There's a few people who tinker with it but there's little economy involved .

Well, I can get baseline kit for ~€300 ( injectors, vaporizer/reducer, computer etc etc), then something like €100 x 2 for two tanks and something like that for two multivalves.

I don't think that investment would take too long to return. Liter of gasoline is ~€1.60 here while kilogram of LPG is €0,78-0,82.

Consider this in your calculations:

gasoline weighs .78 kg /liter

lp weighs .5 kg per liter

2 kg of LP = 1 liter,so 1 liter cost is double the kg cost.

Gasoline produces 110,000 BTU/Gallon

LP produces 90,000 BTU/Gallon

LP is inherently only 78.2% as efficient as gasoline.

Be prepared for less mileage per liter with lp.

considering all of these factors, it looks like a net loss to me.

IMHO: The powers-that be(IMHO=Big Oil) will not let a single BTU slip through their fingers.They are very greedy and diligent.

When price of gasoline goes up, price of lp goes up,hand in hand.Fuel is bought and sold in MegaTherms,regardless of fuel type.

If you also pay a big tax at the pump, the TaxMan will want his cut based on miles driven with LP. The world is run by an oligarchy.(properly spelled as OIL-GARCHY).

There is only one great equalizer.

You are confusing physical volume and BTU ratios for combustion efficiency.

Propane on a mass to mass equivalent fall in nearly identical to gasoline plus has an inherently more efficient combustion energy to mechanical energy conversion ratio.

On a mass to mass fuel comparison of two engines that have been optimized for gasoline and propane fuels the propane powered one will win on the mechanical power and efficiency comparisons per equal mass of fuel burned every time. Even more so the gasoline engine is set up fore emissions requirements opposed to fuel efficiency.

As far as running propane one a stock unmodified emissions compliant engine the fuel mileage and power numbers will also be nearly equal as well. My 99 Ford F250 Super duty with the emissions compliant V10 is set up with on the go fuel switch over capability and in all normal driving conditions the propane power and fuel mileage numbers Vs gasoline operation run so close to identical on the long term I can say fore certain that there is no real difference between them other than propane costs around 1/2 to 1/3 as much per gallon around my state!

1 Gallon of Gasoline=114,000 BTU's

1 Gallon of LPG= 84,300 BTU's

The octane of LP is a little higher, around 104 to 112,so an engine with higher compression can utilize the fuel more efficiently,but a standard designed-for-gasoline engine cannot increase compression ratio via the ECM , it can only advance timing and feed more fuel into the engine to produce more power.

A well designed-for-LPG engine can utilize up to a 17 to 1 compression ratio, and can achieve a slightly higher economy than a converted engine.

The thermal efficiency of the 4 stroke engine remains the same, at around 25 to 35%, regardless of the fuel type used.

Normally, one can expect a 25 to 35% percent drop in mileage when using LPG, which is similar to the same engine using E-85 ethanol/gasoline blend.

Unless you have an over-unity machine, you cannot get out more energy than you put in.

It takes 1.35 gallons of LPG to produce the same energy as 1 gallon of gasoline.It will require 1.35 times as much space to store the equivalent energy and mileage.

The reason I chose the terms that I previously used is because one normally buys LPG by the pound, in small containers, gallons in large containers.One normally buys gasoline by the gallon.Unless space is no constraint, then one cannot carry as much energy in LPG form as in gasoline.

I realize there are maintenance issues that will reduce the running cost of LPG engines,and there are environmental issues as well.

IF one can get the LPG and not have to pay the highway tax,such as for agriculture or industrial fork trucks, etc., it is the way to go.However, one cannot simply fill a 20lb cylinder from a standard home LP storage tank.It will only provide vapor into the tank.

You must have a siphon tube installed in the large tank in order to properly fill the small cylinders with LP. Note,I said PROPERLY fill.You can pump in vapor till the pressures equalize, but you will be disappointed in the range you get.

Large farms have these refilling stations on location,but you must be trained,and of course,registered.There are also stringent location and physical protection considerations around such refill stations. Fencing,abutments,Height above ground,explosion proof fixtures,etc.The list is extensive.But the end result is the OILARCHY can control every BTU you consume.

Bon appetit

"However, one cannot simply fill a 20lb cylinder from a standard home LP storage tank.It will only provide vapor into the tank.

You must have a siphon tube installed in the large tank in order to properly fill the small cylinders with LP. Note,I said PROPERLY fill.You can pump in vapor till the pressures equalize, but you will be disappointed in the range you get.

Large farms have these refilling stations on location,but you must be trained,and of course,registered.There are also stringent location and physical protection considerations around such refill stations. Fencing,abutments,Height above ground,explosion proof fixtures,etc.The list is extensive.But the end result is the OILARCHY can control every BTU you consume."

Huh? Not around here!

I fill small and large cylinders plus my vehicles off my 1000 gallon bulk tank all the time, as do countless others around here who also have their bulk tanks set up that way, without any of that and non of its illegal here either.

I have a liquid feed port on my bulk tank, the liquid feed valve cost me about $100, and I can either use pressure differential feed by by bleeding off the vapor from the tank being filled or I can use my transfer pump I have if I need to move a larger volume. Neither methods and none of the equipment or components are illegal or regulated here.

As far as over unity, what? No over unity is needed just basic common sense and understanding of how different fuels can burn with different levels of energy transfer efficiency. As far as IC engines are concerned very few are built to run at any where near their theoretical peak efficiencies especially emissions compliant ones. Most of those are lucky to get half their maximum capable efficiency out.

As someone who has driven around 200,000 miles on LPG fuel now covering multiple vehicle ranging from a 1974 Ford F100 up to a 199 Ford F250 super duty with a fuel injected emission compliant V10 and a number of other vehicle in between I can assure you I have studied and paid close scientific attention to how much power and fuel consumption variations go between each fuel in each vehicle I have ever ran it in and I can assure you I can reproduce my findings easily enough to anyone willing to come and look.

Pictures available if needed as well. I have nothing to hide.

Local regulations vary of course, and so do tax laws.The point I was making is that with the average home bulk tank, you only get vapor from the tank.The siphon,or wet tap as you call it is required to fill the small cylinders.

Safety regulations also vary by locality and I was simply stating Federal rules.

The more common sense you have, the less regulations are needed.

I cannot argue with your experience,because that always beats calculations hands down.

I do have some hands on with LP powered fork trucks,however, and all run hours were logged diligently.We also had electric,diesel and gasoline powered fork trucks as well.

The lowest Total Operating Costs were the electrics, followed by Diesel, followed by LPG and then gasoline engines.These machines were run 24/7, and maintained strictly on schedule.

These costs included every part used and all fuel and lubricants consumed, as well as down time for repairs,etc.

The electrics had a lot of battery problems at first because they were being run too deeply into discharge before charging.

We finally added a voltage monitor that cut off the hydraulic pump ("UP" Solenoid) when the battery got too low, forcing a battery swap.

We eventually color coded the batteries for each shift to ensure a fresh battery was installed at the beginning of every shift.

If you were caught with the wrong color battery in your forklift, it was goodbye.

I would like to see a diesel direct injection retrofitted for liquid LP injection.I bet that would be really good.

From the day I started doing serious propane conversion research it has always amazed me how many persistent myths there are about it and using it in a vehicle.

Sadly the vast majority of the info is stuff regurgitated by those who have near zero mechanical skills and even less vehicle knowledge but are all too proud to tell "I heard it from a friend who heard it from a friend who's neighbor supposedly tried doing it about 30 - 50 years ago" stories without a clue as to what they are talking about.

That and those too afraid of their government/big oil conspiracies shadow to ever try it themselves.

Heck I have been doing this for so long now I am even coming across local stories apparently about myself relating to things that are not even remotely true about my propane powered vehicles! Both for good and bad!

As I said, I will not argue with your experience with your conversions.

You know what you know.

I was however,stating Facts when I reported the total operating costs of the forklifts.As for mechanical experience, I have rebuilt v8 engines from the ground up, diesel engines,air cooled aircraft engines,Porsche, VW, Mercedes.I have been working on gasoline engines since Magnetos were the ignition source,then points and condenser,now solid state single coil per plug,no distributor,all timed by computer and a magnetic pick up sensor on crank or harmonic balancer for timing reference.

I have built a race car,for the 1/4 mile track and turned 11.55sec at 118 mph,all with my own experience and knowledge.I have raced Motocross in the 500 cc class,and of course, you know you must do your own maintenance if you do not have a factory ride.

I know what I know also.

I also can send pictures of a V8 Cadillac with an LP Conversion from the 1970's( Dog gone that was 40 years ago!) if you are interested.It was owned by the owner of a local LP distributor, and purchased by a friend of mine.He was well pleased with it, but due to poor health, it has been sitting for quite a while.Ran good when parked,good candidate for restoring.When he passes on, I know it will be cannibalized, and he can use the money now.It is for sale if interested.

As for paranoia about big oil,better paranoid than naive.

Tempting to say the least!

But being I have cracked the secrets and tricks to putting dual fuel LPG systems on modern EFI emission compliant vehicles I have little interest this time in a 1970's car when I can set up any newer one to do the same and have all the modern connivance and luxuries to go with it including a fully disabled emissions system as well so the engine can run and breath with the power and efficiency it should have!

With the oil boom here in the state its getting easy to pick up newer luxury cars cheaper now as well. I have been eyeing getting a big SUV for the wife in the next year or two and a newer Mercury Grand Marquis as well and may be setting them up with dual fuel systems.

Similar here as well although most of my engine work has been industrial and farm related.

I have helped build a few vehicle engines over they years and I did do a heck of a job on redoing a Ford 460 out of a 1977 Lincoln a number of years ago. I used it to replace a low mileage factory long block Ford 351 Windsor in a Ford F150 extended cab FWD I used to have. Grandpa blew up the engine in dads pickup so I sold him my low miles 351 and used the money to build up the 460.

I went with a heavy towing cam and moderate head porting work plus the whole bottom end was all redone with heavy duty performance parts all set up and intended to run with maximum fuel efficiency in mind. The machine shop that did the work I couldn't do like the over bore job and parts weight matching and rotating assembly balancing did a super job. I did the rest of the work then added a E4OD with an independent controller in place of the original C6 transmission.

When i was done it would put out right around 400 HP and 500 ft/lb of torque on propane plus averaged nearly double mileage numbers over the emission compliant 351 it replaced in the exact same driving conditions.

That pickup went from being a gutless fuel pig to a absolute thrill to drive like nut while still averaging 12 - 13 MPG fuel numbers on propane whereas the old 351 emissions compliant engine got around 6 - 8 on gas at best and still gets that in dads pickup today.

My cousin used my pickup once after I rebuilt it to go from Minot ND down to Fargo to haul stuff back when he got out of college and swears he got around 18 MPG on gas driving it half way civilized.

Then one day a little old lady took the front end off it while I was in town and its been parked behind my shop all smashed up ever since with dreams of refitting the engine and transmission into something more suitable. Now as of last year I have picked up a 97 Ford super duty flatbed truck that has a factory 460 in it and absolutely crappy fuel mileage numbers, 6 -7 conservative driving and 3-5 towing, and is grossly under powered given the size of engine it has which is most likely where it going to go some day.

A friend of mine was involved with a company doing diesel to LPG conversions in China (of all places). Converting the diesel motors in buses to spark ignition then adding the LPG system to it. Was a program to aid in reducing air pollution in the cities caused by diesel soot. Seemed that the chinese diesels run low compression (for a diesel) and made the conversion easy.

Smart Gas P/L was the company out of Brisbane Oz, but with an affiliated company in China.

I don't know what was the conclusion to the program as I lost touch with the friend.

I don't know who told you you need to run on gas first to heat up the vaporizer unit but that's wrong. I have been running propane conversions here on multiple vehicles in North Dakota for around 12 years and approaching 200K miles combined without ever needing to preheat the vehicle engines with gasoline first and we have regular winter temps down to -30 F.

The only requirement is that the vaporizer needs to plumbed into set of coolant ports that have continuous flow when ever the engine is running. That's it.

Here is my full propane only converted $200 beater as well.

http://cr4.globalspec.com/thread/59332

Yes, but LPG here means mix of butane and propane, which gets skewed toward lower boiling point at vinter time, but that varies.

Also, you might not have a problem if you are using _liquid_ phase injectors instead of ordinary system ( here sold under name "Prins" and probably others), but that's _much_ more expensive.

No fancy setups here. All of my conversions are based on good old fashioned all mechanical components based on designs from way before electronic propane systems were ever invented. FWIW the propane system on my Mazda has a vaporizer and vapor mixer units that are around 40 years old that came off old farm machinery I found at a local junk yard.

If you can get anything over a few PSI of liquid flow on the coldest day of the year mechanical vaporizers will still supply more than enough vaporized fuel to run any size of engine that they were designed for.

Honestly I have lived through enough -20 to -30F winter days while running propane on numerous vehicles to know that vaporizer freeze up is not an issue if the coolant supply lines are plumbed into the correct points on the the engine. The biggest engine I ran on propane was a built up 400+ Hp ford 460 with a heavy towing cam and if that couldn't cause a vaporizer freeze up while running dead cold at -30 F any other engine wouldn't either.

Yes, but you are talking about pure propane. Slovenia is in EU. Our LPG is propane/butane mix, which behaves differently. Butane is somewhat more energy rich, but has lower boiling point than propane.

I did a bit of reading and your coldest winter time average temps are about 20 - 30 C higher than ours with an average temp still higher than what a 80/20 mix of butane/propane boils off at.

To me this vaporizer freeze up concern is really not an issue. As far as a whole system setup on the cheap goes just find a vaporizer and vapor mixer unit for a forklift or other commercial machine with a similar sized engine and be done with it.

I forgot to mention, that there is a direct liquid LPG fuel injection system available , no vapourisor required. It is expensive though...

I've run a number of vehicles on LPG, I've only had cold start issues when the coolant system didn't have sufficient coolant, or the thermostat was faulty or the numbnut who installed the system had placed the Vapourisor too high in the engine bay, as that can allow the vapourisor to ice up due to an air lock forming as it is above the main body of coolant in the motor.

I don't run LPG fuelled cars anymore as the financial incentive is no longer there. My preference is diesel. Although there are those that do run LPG supplemented diesel, it is "problematic".

As I said, liquid phase systems are not an option for me due to stratospheric price.

Have you considered perhaps drilling and tapping the intake manifold to accept a glow plug?Do this as close to the head as possible.If you do not want to remove the intake to drill and tap, try this:Break out all of the porcelain from a spark plug,keeping the metal body.Braze a pipe nipple to the plug body.Install a male quick-release fitting to the nipple, and insert in the plug hole of the cylinder you will be working on, and apply air pressure.Rotate the engine manually until you hear air escaping through the manifold.Continue to rotate to maximize this flow. This means the intake valve is open to the cylinder.

Drill slowly into the intake manifold,and the air pressure will force any debris out of the hole and prevent it from entering the intake.I suggest wearing goggles or a face shield while doing this.

Do this for all of the cylinders.

I think this would work.

A caveat on LP gas in gasoline engines:The oil will never get dirty or black.It will, however need to be changed at the same interval as normal.Abrasives accumulate in the oil, and the additives become neutralized and diluted.

Good luck.

Nice idea ! It sounds like you already tried this - i mean heating mix with the diesel heating plug.

Did you ?

Man, I have to google this around the globe. Someone somewhere surely tried it...

One thing- wouldn't heating plug influence airflow ?

If after considering my last post on overall cost,you still want to do it:

The depth of insertion of the glow plug can be controlled by a nipple of proper length.

Yes,there will be some turbulence generated in the air flow, but the effect on performance will not be noticeable, unless the intake runners are very small and the glow plugs are very protrusive.

I have never done this, but if I were going to, that is how I would do it.

I have used the air-pressure trick to re thread spark plug holes by putting the adapter on the cylinder next to the one I was working on, and rotating the engine until air comes out of the one I was working on.This is called inter cylinder valve overlap,when the valves on adjacent cylinders are open at the same time.Not to be confused with valve overlap on the same cylinder.

Good luck with your project, and let us now the results.

It's great idea, but after some googling around, there seems to be a few issues.

I managed to find someone talking about Falcon model using it ( whatever it is, USA car models are terra incognita to me - I see a few get blown up in action movies and that's it), but it uses glow plugs to heat the LPG that gets squirted on them by special nozzle, not for heating gaseous mix.

It seems that gas has to be heated at vaporizing and after that it would be of limited use.

But some other guy used glow plugs to heat water that was entering the vaporizer. He did it in awkward way, but i think it could be improved significantly.

He just put his heater with 3 glow plugs in the plumbing before waporizer, so that heater was forced to heat all the water in the engine, which is hopeless.

But with a few valves I could close the loop and have just very small qty of water within it. Then I could use one of those miniature 12V pumps and a heater to heat that water and run it through the vaporizer, so I'd get enough of warm gas at start up.

This has another advantage of less chance for bureaucratic yadda-yadda at technical check-up. Ever since of that Nazi Zyclone-B stuff that happened eon ago, everyone is nervous around gas and so are bureaucrats. One of standard jokes in LPG cars is still letting a quiet killer fart out and yelling "I SMELL GAS !", so.

All parts have to be compliant with relevant standards and homologated and car adaptation has to be done by someone with "gas diploma".

Changing something in coolant circuit, external to the engine shouldn't be scrutinized so much....

The LPG Falcon is manufactured by Ford Australia, mainly for fleet/Taxi use.

Ford falcon with straight lpg review

Your fear of having the vapourisor/Convertor freezing up is well founded if you mount it too high in the engine bay. If it is mounted at engine block height and the hoses run low and direct to the block/head without loops then you will be ok. The LPG which is a propane butane mix here in Oz too, will flow as gas even at the coldest temperatures as long as the coolant flows through the vapourisor. Which is why it has to sit at blok height not on top of the motor. If you have a block coolant heater installed (and used) as many euro cars do then winter driving on LPG should not be an issue.

The cooling systems in euro built cars are designed to come up to operational temperature in under 3 minutes, its part of the TUV regulations I beleive. So running LPG in those cars should not have freezing of the vapourisors(convertors) as an issue. Your Vectra (Opel/Vauxhall/Holden) is in that league.

One other thing, you should be using an Antifreeze/Anti boil coolant in your cooling system no straight water.

LPG being introduced to the manifold as a cold gas is beneficial as it helps to provide a denser air/fuel mix which helps with performance. If you want to put a glow plug into the system put it in to the water jacket of the vapourisor/convertor where it will do some good, putting it into the manifold will only cause a fire...

For all interested: A Calculator that lets you compare total life cycle cost of of LP,Diesel and Electric Fork lift.Gasoline is not shown, but it is the most expensive of all.

http://et.epri.com/LiftTruckComparison_with_cap.html

I took a stock 1968 302 from a wrecked Z28 Camaro,bored it .030 0ver(making it a 306),installed sodium filled exhaust valves,a hotter Isky cam,11.5 to 1 pistons,a 780 Dual line Holley,with pumper kit(no stock-dual pumpers back then),and you could get good leaded gasoline,Hooker Headers,Thrush Mufflers(With cut-outs for drag strip) a Muncie M22 (rock crusher) close ratio tranny, an 11 inch Corvette (finger type) pressure plate and clutch, a set of Zoom 4/11 gears in the rear.

It Dyno'ed at over 650 Hp at 7200 RPM.Torque and HP lines crossed at 6800 RPM.

Dropped this into a 1955 2 door Belair Coupe.Used Hurst mounts front and center,and had to chain the motor to the frame to prevent breaking bell housing mount,and the rotor button in distributor cap.

When street driving, I dropped in a 3.08 rear end, and if I kept my foot out of it(Which was hard to do at that age in my life) it would get 18 mpg on the open highway.It only turned 2000 rpm at 60 mph in high gear,3000 at 90mph, so the motor was not even in the performance area of the cam.Don't know what the top end was, because handling became Squirrelly" above 130.(I chicken-'d out.)Speedo topped out at 115,so the rest was calculation based on rpm/mph ratio.

I will admit it was low-torque at low RPM with the 3:08,and not friendly to drive due to loping of the cam,but man, did it sound great!

Wish I still had that car.

I agree, the smog controls take a lot of performance away from the engines,as does the unleaded gas,and the 85/15 regulations will take even more power in the tri-fuel cars.

With with LPG, you should be able to run as high as 17 to 1 compression with no problem,and this will allow an increase in timing,and increase the power quite a bit.

The high compression ratios available is why I speculated that a direct-injection Diesel running Propane would be awesome in durability and performance.However, that is just speculation on my part.

Perhaps it might be something you would like to play around with.

Good luck.

I did my old Mazda B2600 as a propane only engine and it ran well for its size and levels of previous abuse, two teenagers owned before me, up until it spun a rod bearing last spring.

I had the head milled for 12:1 compression and would have liked to have put a performance cam in it but unfortunately I have never been able to find one so I was stuck with tweaking the stock emission cams timing a bit to bring the peak torque into the normal daily driving range.

The other issue with going with any compression ratios higher than around 12:1 or so with propane is that most gasoline engines start having trouble with head gaskets, crankshaft and rod bearings, plus outright rods and piston failures as the compression ratios go up. Most were never designed to run at that high of compression ratios and just cant handle the physical stress reliably. I am already suspecting the 12:1 ratio my Mazda was running at was enough to push a weak bearing over the edge.

I have been interested in doing diesel to propane with spark ignition conversion for some time but as of yet I have not found any reasonable priced diesel engines that would fit into any vehicles I have.

I do have a Allis Chalmers industrial 2900T diesel that came from a combine that I have been really thinking about refitting into a second 99 F250 super duty I picked up last summer that has a bad V10 engine but my plans for that are to leave it as a diesel and modify the pump and injection systems to run on a 80/20 mix of used oil and E85.

The thought is to put a performance high boost turbo on it and rework the injection pump to get it into the 200+ Hp range by 2400 RPM! Right now I am looking for a good machinist that can make the custom bell housing adapter plate and crankshaft bushing adapter to fit the original E4OD transmission behind it.

I cant find and CAD files of either bell housing to go by for references so that I could use my brothers CNC router table to mark out my bolt patterns for the adapter plate and bushing and do it myself.

I had a 1959 ford f100 that I wanted to put a sychromesh late model transmission in.Everyone said it could not be done.

The old bell housing had 2 sets of holes, and when I checked, they were a match for the new tranny,however, the old tranny had a very large front bearing and input shaft,as well as a longer input shaft.I took the new tranny front bearing housing, turned it down to the same dimensions as the old one, cut the input shaft collar off off the old one, and bored out the inside reverse threads of the old collar.This gave a nice slide fit over the new collar, and I brazed it in place.

By doing this I was able to use the old release bearing fork,etc. with not modification.

I made a pilot bushing for the flywheel, because the new tranny shaft was a little short.Cut the driveshaft to accommodate the longer tranny.

Everything fit like a glove,and I had a fully sychromesh transmission in a 59 f100.

No more double-clutching or manual synching of tranny first gear.

One of the best knock around, hunting, fishing, hard work trucks I ever had.Could not kill it.It was dented, rusty,had a hood like the front of a barge that weighed about 150 lbs,and was made from real metal.

If anyone bumped into it in the mall, no problem.Just another battle scar.Their vehicles always fared worse.Usually, they gave me a wide berth when parking,a full space on each side.I guess they thought the rust might be contagious or something.

I had a lot of fun with that truck, and went a lot of places that were usually the domain of 4WD trucks and came out ok.Plowed thru thick brush and small trees to get to my hunting and fishing spots.

Finally sold it to my cousin, and he sold it to someone else.I don't know where it is, but I bet it is still running somewhere.

Must have been made of a Krypotonite meter.

I miss that truck!

A method used during the 70's for high compression engines was to have a groove machined into the head and block, each comprising 1/2 of a circle.A copper o-ring was inserted into this groove,with precision gap overlap and precision milling, this made a virtually fool proof head sealing assembly,when used with a slightly modified standard head gasket.I am not sure, but it seems like a gasket was offered for high performance that had this incorporated into it.

The direct injection I was referring to is not vapor, but liquid injection, as in a diesel engine.VW had a diesel in the 70's or 80's that had a low-compression(for a diesel) engine that i think ran about an 18 or so to 1 compression.

Perhaps a standard diesel with custom lower compression pistons would fit the bill.

This would give the indestructible mechanics and bottom end of a diesel with the efficiency and cleanliness of propane.A little lube injection would be required however before the injectors for longevity of he injectors and cylinders.

Good luck with your ventures.