Propane vs. Gasoline (Long)

[Lazy JW]

Recently in another thread, Deano (Execute) recommended a maximum compression ratio around 9.5:1 for a heavy usage truck engine running on propane. Having near zero personal experience with propane I did a bit of digging in the only reference book I have that shows any data; my oft-mentioned Nebraska Tractor Tests manual.

While perusing the data, I noticed that some propane models developed slightly higher HP ratings than their gasoline fueled stablemates, and some showed slightly less power. Being an inquisitive hillbilly, I tallied a few numbers (12 pages of notes) to see if there was any significant trend.

There were 55 propane powered tractors tested that had corresponding gasoline powered versions, the only differences being the obvious fuel handling/delivery equipment, and most of the propane engines had a higher compression ratio. Other than that, the engines were identical in design, bore, stroke, and rated rpm.

The earliest tractor tested was during 1949 and the latest was in 1968. Nine different manufacturers were represented with various two-cylinder, four-cylinder, and six-cylinder engines ranging between 134 cubic inches and 425 cubes. RPM varied from a low of 975 to a high speed of 2500 revs.

I added the total horsepower and divided by 55, coming up with an average output of 55.142 HP per gasoline fueled tractor.

The propane powered machines averaged 55.205 hp, an amazingly insignificant performance difference. Frankly, I had expected to see more average power on the propane tractors because of the higher average compression ratio. So much for pre-concieved notions, eh?

The Propane engines carried an average compression ratio of 8.34:1 versus the gasoline model's paltry 6.98:1 ratio, an almost 20% increase for the propane tractors.

Fuel economy PER GALLON dropped off as expected when burning propane due to the lower BTU content. An average of 9.200 Horsepoer Hours per Gallon was registered whereas the gasoline tractors showed 11. 823 HP-Hrs per Gallon for about 28% more power per gallon of gasoline than propane.

Of the 55 tractors there were nine models that also had diesel fueled versions in addition to the propane and gasoline machines. Again, these engines carried the same basic specs as the previously mentioned machines with the necessary changes to compression and fuel delivery made to accomodate the diesel fuel.

Comparing these nine tractors independantly to each other as a separate test group from the 55 in the total group showed the following results:

Propane HP- 61.55
Gasoline HP- 62.95
Diesel HP- 60.80

Propane Economy- 9.42
Gasoline Economy- 12.12
Diesel Economy- 14.30

(Economy rated in HP-Hrs per Gallon of fuel)

Even with their much higher compression ratios the Diesel models showed slightly less horsepower but significantly better economy per gallon.

There were also 5 tractors that used propane and had a diesel powered counterpart but no gasoline version. These were not included in the 55 tractors tallied above. One of these machines was interesting though, the mighty Minneapolis-Moline G-1000. This propane fired behemoth used an inline six cylinder engine sporting 4.625" bores and a stroke of 5.000" for a displacement of 504 thumpin' cubic inches! At its rated speed of 1800 rpm it cranked out 110.76 hp and 323 lbs/ft of torque with an economy rating of 9.71 hp-hrs/gal of propane. At full boogey that monster would burn almost 10.5 gallons of fuel per hour Best have cheap fuel and a big truck to haul it!

Also among the 55 tractors were nine machines that were available as "All-Fuel" models meaning they were equipped to burn distllate or "tractor fuel". Sometimes referred to as "low-cost" fuel, it was definitely "Low-Octane" and was actually quite popular "back in the day". Visualize mixing #1 Diesel fuel with kerosene about 50-50 and that's pretty close to what distillate was with an octane rating right around 40 or so.

These nine engines averaged about 4.79:1 compression rato and while the propane/gasoline machines showed nearly identical power ratings the distillate burning tractors exhibited about 20% less horsepower per cubic inch. Again, these nine were compared as a sub-group.

Gasoline HP- 43.46
Distillate HP-34.50

Gasoline Economy- 11.90
Distillate Economy- 11.00

One tractor deserves special mention, the venerable John Deere 720. This was the only machine available in all four fuel variations. Its two-cylinder engine sported a 6" bore and a 6.375 stroke for a displacement of 360 cubic inches. At its rated speed of 1125 rpm it gave the following results:

Propane HP- 55.48
Gasoline HP-55.11
Distillate HP- 42.38
Diesel HP- 56.66

Propane Economy-9.10
Gasoline Economy-12.21
Distillate Economy- 10.89
Diesel Economy- 17.97

Propane Compression-7.94:1
Gasoline Compression- 6.14:1
Distillate Compression- 4.91:1
Diesel Compression- 16:1

The diesel version cheated slightly by using a 6.125" bore which increased displacement to 376 cubic inches, otherwise the machines were quite similar.

The JD 720 Diesel set a fuel economy record that stood from 1956 until 1983 when a Japanese built turbocharged JD Model 1650 finally edged it out for the all-time lead.

Conclusion: With the notable exception of the low-compression distillate burning tractors, it appears that similar engines burning different fuels will deliver similar power output.

Hope I didn't bore Ya'll too much,
Joe

Edit: Forgot to mention that the highest propane compression ratio was 9.8:1

 

[Silver Shadow]

It would seem then if you can reinvent the wheele from differnt matirial it will still go round like the old wood wheeles of the same size. So the truth is fuel is mostly a matter of choice not advantage unless you have an aboundant supply of one over the others. Vewy Vewy Vewy intrwesting.

 

[Lazy JW]

... the truth is fuel is mostly a matter of choice not advantage unless you have an aboundant supply of one over the others. Vewy Vewy Vewy intrwesting.

Yup. That's pretty much my take on it. Cost, availability, and convenience would seem to be the major detirmining factors.
Joe

 

[Luckyman]

Thank you sir, for doing all that research and sharing the results. That is very interesting, informative, and well done.

 

[Lazy JW]

You are most welcome

 

[ASMART]

It was a good read. Thanks

 

[Pinhead]

The question that I have, though, is what fuel is best suited for the internal combustion engine? If everything were optimized to the max for each fuel, which would be able to produce the lowest BSFC?

You compared engines with pretty much identical configurations. While this makes a good comparison when you're trying to figure out which fuel you should run on an engine that is already built, it doesn't necessarily tell you what fuel is the "best."

For example, if you were to take a SBC engine and run it on gasoline, propane, and diesel fuel, you will find that it will run the best on gas, then propane, and lastly diesel. This doesn't mean that diesel fuel is a terrible fuel, it just means that the engine wasn't optimized for it beforehand.

The question I'm getting to, is what optimizations would be needed to close the gap between gasoline and propane? IMHO propane should produce lower BSFC numbers than gasoline due to the fact that propane is injected in vapor form and therefore should produce a much more homogeneous mixture and should burn faster, making the engine more thermally efficient.

The only thing I've heard about propane is it is able to run on higher compression without detonation. Other than that, I've not heard about many other optimizations. I suppose the propane engine is simply suffering from a lack of development due to the development of diesel and gasoline engines and therefore doesn't get enough attention.

 

[Lazy JW]

The question that I have, though, is what fuel is best suited for the internal combustion engine? If everything were optimized to the max for each fuel, which would be able to produce the lowest BSFC?....

That's a very good question. I suppose that somewhere, someone has done some research on this but I have no idea where it might be found.

Keep in mind that this data is pretty much obsolete, the newest information being almost 40 years old and some almost 60 years. Modern fuels may well skew the numbers significantly one way or the other. At any rate, I do not know of any testing similar in scope comparing real-world engines generally available to the public.

Since you specified BSFC as a criterion for detirmining "what fuel is best" I ran the numbers on these tractors.

I looked up the specific gravity specs on the internet. If they are incorrect, someone please advise as I have no other source for reference.

Gasoline: .74 Specific Gravity ; 6.14 lb/gallon
LPG: .54 Specific Gravity ; 4.42 lb/gallon
Diesel ..84 Specific Gravity; 7.48 lb/gallon

The best performing individual tractors of the 55 machines included in this study were as follows:

Gasoline: Farmall 706 @ 12.86 hp-hrs/gal; BSFC .48
Propane: JD 520 @ 10.27 hp-hrs/gal; BSFC .43
Diesel: JD 720 @ 17.97 hp-hrs/gal; BSFC .42

Average BSFC of the 55 tractors tested
Gasoline: BSFC .52
Propane: BSFC .48

This data includes the best Propane and Diesel tractors ever tested at Nebraska but excludes the all-time best gasoline machine because it had no propane counterpart. This was the Oliver 1800 tested in 1960. It featured an inline six-cylinder engine sporting 3.75" bores and a full 4" stroke for a displacement of 265 cubic inches. At 2000 rpm it develpoed 73.92 PTO horsepower and 194 lb/ft of torque with a fuel economy rating of 13.18 hp-hrs/gallon and a BSFC of.47 lbs/hp-hr.

It is interesting to note the excellent performance of the two-cylinder John Deere tractors on diesel and propane fuel but only mediocre performance on gasoline. The inline six and four cylinder engines ruled the roost on gasoline, however.
Joe

 

[xctasy]

I do believe in the scientific proof that propane has real advantages, but they are mangled up with some really bad points.


Campared to gasoline, propane is just the wrong fuel as it gets blown around on a whiff, whereas gasolene has weight in the intake tract, and can produce swirl. Too little is understood about propanes flame propagation. Statified charge systems LPG systems are being experimented with, and hold a lot of promise, but with a dedicated, optimized propane engine using propane delivered as gas to the injectors as it is these days, it is still impossible to exceed gasoline performance and keep to the emissions requirements. Certainly the fuel economy is not the same as gasoline. Fords last propane BA Falcon lost 20 hp, and 5% on fuel economy, and that wasn't because Ford engineers were being lazy. Spark plug life is poor, and ignition wires take a hammering.

Propane is not the fuel for maximum power, but it is good for low end torque. Due to its difficulty in light off burning, it is not good for spark plug life, nor is it able to look after exhast valves and head gasket life the same as a gasoline engine can. It cannot be made to produce the wild 'accelerator pump' shove a good port on port triple carb Webered Triumph 250, Datsun 240Z or Aston DB6GT or aftermarket triple Weber XKE 3.8 gasoline engine can. When rich of stoich, there is no power increase, and dangerous misfire or detonation can set in.


Propane no longer has massive benifits on lower emissions over gasoline, but its PM 2.5 and 60 particulates reading is still much better than it is for diesel at the moment.

When injected as a gas, its a great fuel, when injected as a liquid it is even better than gasoline, its quite brilliant. The risks are too great for liquid injection until the systems have another 10 years development.


In my opinion, Methane is taking over propane. Its RON rating is 35% higher, its got some isssues with poor touring range, because it has to be held as compressed natural gas rather than liquid propane gas, but its a much better future fuel prospect than propane. In the USA and Australiam the dollars are bing poured into methane/CNG. Impco in the USA has gotten hold of the Italian compressed natural gas systems (methane), and it will yield the next big thing. 130 octane fuel on 14:1 compression engines will be common place, statified charge and diesel/gaseous hybrids will eventuate.

Getting back to the barbeque gas. I'd say the advances in cheap propane conversions are already there in wide lobe cam design, and getting better carbs, injection and mainfolds, and eliminating the management computer. I think that most ignition advances, especially lost spark, are already there on most emission engines, so the existing post 1975 or the better post 1995 waste spark ingitions are going to be used in most cases. Over in my neck of the woods, GRA Australia started out using US made Century propane parts, and has a very good developement profile on the staged GRA carb, 350 to 400 cfm at 2"Hg. It isn't ever going to set the world on fire, but its 500% easioer than mapping an EFI set-up, and propanes key advantages of simplicity have created a very strong following in Australia on aftermarket engine rebuilds.

I love propane. It's the fuel I like to use, my Falcon was twice the car with an 175 odd hp Impco propane setup than as a stock 2-bbl Webered 131 hp plodder. It was the perfect family car. The future for me would be in making some kind of port on port Holley 2-bbl, triple carbed version for my i6's. I think there is a gap in the market for a set-up which would instantly bolt on 25 to 50 hp more with just a multiple carb set-up. Then cam and compression changes would yield great improvements in part thtottle economy. The problem is propane really has to be optimized, and that is a very expensive prospect unless you are selling a whole 'green' engine to a customer with a gaurentee, and some proof that it does what it proports to do. Issues with legitimate engine numbers, getting an inflation free source of engines and having the capitalisation to make them to cover warranty matters is more than I'm prepared to do at this stage.

As you are possibly aware from Super Mag and others, Holley did make some protoype propane carbs back in the late 70's, but ditched them. I'd say three Holley throttle bodies which could be side draft mounted like the Screeming Eagle Harley carbs were, could be a great system for making propane the ducks gutts.

Summary is, I can assure any 1 or 2-bbl I6 can yield a 25 to 33% power boost if the propane conversion allows me to do some
1) manifold,
2) carb,
3) cam and
4) compression
5) and exhast header upgrades in one fowl swoop.

I had done that in 2000 with just one 2-bbl carbed Falcon. I'd be doing it tomorrow again if I could control those five things! The problem is I can't control that unless I make exhange engines, and I'm not gonna do that in New Zealand.


Hence propane remains a dead duck because it needs a disciplined focus on the five things above to make sure the engines give good power as well as stalwart low end torque.

 

[lyonsy]

whille on the lpg front
my nc lane iam looking at putting on gas injection got a guy who can do it and all that going to cost $3800 an impco system is $2800 iam leaning towards the injection setup as gas is getting more expensive and it uses less.
and with the injection setup's you dont get gas build up in your manifold and air box explosions do ya?
whats ya reckon of it.

 

[xctasy]

The injection set up is far superior. Only issue is getting it calibrated. Since intake manifold spacings are similar, one off an AU could be made to fit any other X-flow after 1980, and the control mechanisms can be made to run of the older cylinder block (ignition, crank trigger, sensors can be retrofitted, stroke and power potential of the earlier OHV version can be similar if a good gas cam is fitted).

Any LPG carb can potentially backfire if there is an aggressive spark and enough cc's of manifold. Most Falcons have about 2.3 liters volume, and thats a lot more than on a carby engine. GM 3800's have to be delicately pared back on advance, as they run up to 48 degrees lead at part throttle. If an LPG carb is used on a fomerly EFI engines manifold, the unburnt LPG banking up the EFI system can create quite a bang, taking the carb off. Impcos CA 300 have a huge diaphram to contain backfire just in case. It's never happended to me though!

 

[Lazy JW]

...

Propane is not the fuel for maximum power, but it is good for low end torque ..

Tell me more....

Summary is, I can assure any 1 or 2-bbl I6 can yield a 25 to 33% power boost if the propane conversion allows me to do some
1) manifold,
2) carb,
3) cam and
4) compression
5) and exhast header upgrades in one fowl swoop....

Do the same to a gasoline engine and it will give very similar results. Unless the fowl swoops down and flies off with it
Joe

 

[xctasy]

The information form Gas Research Australia in 1995 yielded some interesting points. They did a lot of development work using a 5 liter gas bottle, and a dyno. From the weight of gas used, they were able to deduce what Wade and Speco Thomas camshafts grinds made the best specific fuel consumption, and then sought to develop a rack and pinion carb which eventually resulted in the GRA 350 and 400 cfm carbs. A key point was getting junk yard engines in good shape, and then playing around with the camshafts to optimise the package (Details from Street Machines periodical Propane discussion sessions).

No matter how well the cam was optimized from the gasoline variant, and nomatter if it was a 250 Falcon, 351 F-150, 308 Holden, or 4.0 or GM 3800 V6, all engines yield less peak power on propane, but substaintially more torque. Specifically, the 351C 2V, an engine used in all local V8 F-100/Bronco/F-150's from 1977 to 1985, it was always noted as having a power 'hump' at 2800 rpm on gasoline, where the air speed and mixture modion suddenly allowed the Cleveland to make a substaintial increase in torque. This was noted in StreetMachines 1988 'Power Games' series with Lynx Engineerings Super Flow Dyno, using engines on 97 leaded. Anyway, on LPG, the details from GRA's propane and gasoline tests showed that torque skyrocketed over the gasoline variant, but peak power was less.

Same with the GM 3800/ Buick 90 degree V6. Great low end torque, up significantly on gasoline, but then a measurable in peak power.

Same with my mates FE 390 fiited to his mongred LandRover/Landcruiser/Bronco hybrid. On gasoline with a 465 cfm Holley, his LandRover gave great peak power, with fair low end torque. With LPG and the large CA 425 Impco (460 cfm at 1.5"Hg), it produced astounding torque, but less wide open throttle power. Around his local 4 by 4 track, his machine was great ambling along in low speed undulations, but it was much less powerfull when doing mud work.

My Falcon was much stronger on propane everywhere when I modified the carb and intake and exhast, but granted, the same mods on a gasoline engine would have yielded much, much better power.

The point I'd make is that a propane engine needs modification to work with no power loss on an I6. Gasoline makes up for a lot of the compromises that become compound problems when propane is used. A 5% power loss is very noticable if your driving up a 11% grade, and so is the increase in fuel consumption. The only way to mask that is do do other modifications on a propane engine to offset the loss.

Holden had the best idea for the LPG police cars....it circulated gasoline through the duel fuel propane engines at wide open throttle, to get the torque boost at low speed, and the power boost at wide open throttle. That's the greatest admission of propanes shortcommings.

Incidently, when turbocharged, propane comes into its own, and is about 15% better for power than the gasoline variant.

Futher reading is Jay Storers Propane Performance and Hugh McInnes turbo book, and David Vizards Reworking Fords SOHC engine. All intimated certain aspects of the simplicity of propane and turbos.

Yet again, its not apples matching apples and its sort of like the V8 guys feature creap, where they won't admit there V8 won't match another brands V8 for power. I'm saying propane is lousy for maximum power compared to gasoline if no mods are done.

 

[Lazy JW]

...Yet again, its not apples matching apples .....

.....I'm saying propane is lousy for maximum power compared to gasoline if no mods are done.

I'll agree with that based on the tractor data. Most of the tractors had compression increased with no other mods (besides the obvious fuel delivery).

One shortcoming of the tractor tests is they did not rev above "rated" rpm so no data was collected there.

Also, the book I have is merely a summary intended mostly for pleasure reading; it does not include the full dyno test data so does not show torque readings at lower rpm. That info is available from the University but they charge money for each test; I can't afford that.

The main thing I learned from this is probably obvious to a mechanical engineer, but it seems that a given engine will flow about X amount of air and it doesn't seem to make a lot of difference which fuel is used to mix with the available air. Diesel seems to prefer slow speeds, gasoline works better at high speeds than diesel, and propane needs to be inexpensive to be worth the bother.
Joe

 

[Fiorelli]

One of these machines was interesting though, the mighty Minneapolis-Moline G-1000. This propane fired behemoth used an inline six cylinder engine sporting 4.625" bores and a stroke of 5.000" for a displacement of 504 thumpin' cubic inches! At its rated speed of 1800 rpm it cranked out 110.76 hp and 323 lbs/ft of torque with an economy rating of 9.71 hp-hrs/gal of propane. At full boogey that monster would burn almost 10.5 gallons of fuel per hour Best have cheap fuel and a big truck to haul it!

The 504's accually arn't to bad at all on fuel under full load pulling the chisel or disk if you lug one down to 1600 they use alot less fuel and didn't give up any power, may accually gain a little we could run about 60 gal of fuel through one in a day and under heavey loads and under light loads they easily would run all day or two on 36 gal. The LP460 we have on a well uses more than that. The 504 in the lp 1355 isn't that way they are unbleavelably hard on fuel and terable lame.

The 504 was like the 300 very choked down in the lp version I think the carb was only about 100 CFM and the cam isn't very great either get the intake off an lp G900 tractor with the 425 CID and gain 10 to 15 hp. There is lots of play room with these. The four cylinders around 440 cid several pullers clame with camming and a little porting to get 125 hp at 1275 RPM naturally asprated of course.

They didn't have any specks for theG1000 or G1350 or G1355 or A4T 1600 with the lp HD800 5.319 X 6 in it did they. That engine was 250HP @ 1800 RPM and 712lbs/ft of torque at 1100 RPM that should be continus figures at the flywheel I'd love to have one of those. I don't think any were tested at Nebraska.

I'm saying propane is lousy for maximum power compared to gasoline if no mods are done.

If you use a stock gas engine and put it on Lp you will have very poor results!!!!!!

I have a friend that tried to put his lp pulling tractor on pump gas expecting great gains, with the higher compression he thought he would really run but it didn't run very well at all put some racing fuel in it and it was fine. That was at 9.1:1. We have some lp tractors at 10.4:1 very nice.

I really have to say you can get as much or more power out of LP as pump gas remember you can run a higher compression ratio with lp than gas.

This is the only four cylinder tractor pulling in the nebraska bush pullers. For a while he was on LP but now he is alchol. He did very well on LP!!! I barrowed this picture from the nebraska bush pullers so I probably should leave a link. http://www.nebraskabushpullers.com/

 

[Pinhead]

Does anyone have the cam specs comparing the diesel/gasoline/propane engines? I may venture a guess that since the camshafts were "optimized" for the engines, that much of the "peak" horsepower is lost due to camshaft profile, especially since horsepower is dependent on RPM, which is controlled by your camshaft (peak torque vs RPM). This would also explain why propane runs so much better with a turbo, just like late-model turbo diesels.

 

[Lazy JW]

Does anyone have the cam specs comparing the diesel/gasoline/propane engines? ....

All the information I have available says that the cams were identical on the spark ignition engines. Dunno about the diesels but I'm guessing they weren't significantly different considering the rpm range was the same.
Joe

 

[Pinhead]

But if you compare the profiles of highly developed gas vs highly developed diesel engines, you'll see a SUBSTANTIAL difference. And according to above, the profiles of highly developed propane engines would also be different.

My guess is propane needs much higher cylinder pressure in order to reliably fire (hence the higher compression ratios). However, even with a higher cr, with the wrong cam, the cylinder pressure won't be enough to make propane run to its potential. This is the same reason non-turbo diesels have an extremely low-RPM power band.

The camshaft profile has to be set up in order to create a high enough dynamic compression ratio to ignite the diesel when cranking at a low RPM and at idle. With the "small" camshaft that is inherent with a diesel, the VE falls off rather quickly as the RPM's rise. This is why turbo diesels have such an RPM and therefore horsepower advantage over their NA counterparts.

I would guess that the same principals should be taken into account when building a propane fueled engine as you would with a diesel engine, though not to such extremes. This principle is strengthened by the comment from above, where a turbo propane engine has about a 15% advantage over its gasoline counterpart.

 

[Lazy JW]

But if you compare the profiles of highly developed gas vs highly developed diesel engines, you'll see a SUBSTANTIAL difference....

Agreed. Pretty tough to visualize my 1941 John Deere as "highly developed".

But when comparing two engines used for identical practical purposes at identical speeds burning readily available farm fuel the differences won't be as significant.

Cylinder filling/evacuation on a 975 rpm two-cylinder Johnny Popper will require about the same valve events regardless of fuel type. On a low-speed N/A engine, cylinder filling is going to be the primary goal for maximum power, and since the highest rpm reported was 2500 I think this holds true throughout the study. As mentioned above, this is pretty old data. Modern fuels/engines may vary.

The fuel type will dictate how much compression should be used.

Reckon it will take someone a LOT smarter than I to decide which fuel is "best". Much depends on the goal, also what is available.
Joe

 

[Fiorelli]

To answer your cam question Lazy Jw
On the MM's the gas, diesel, LP, and natural gas, all had the same cam there were different cams available but they were put in all of the engines (Except the 585 and HD800). The only difference was the compression ratio, the diesels had a differen't head,very sumular, heaver piston and a heaver conecting rod with a bigger piston pin dia.
Some where I have cam specks.

But if you compare the profiles of highly developed gas vs highly developed diesel engines, you'll see a SUBSTANTIAL difference. And according to above, the profiles of highly developed propane engines would also be different.

I do have to agree and dissagree all at the same time. The engines refferenced here for the most part were all naturally asprated all running at the same RPM the fuel probably isn't an issue. Add a turbo and you cam requirements would probably change a little. Change the rpm range greatly and the cam would need to change greatly.

Most diesels are turboed but not running at the RPM's to get there peek torque and peek Horse power that the gas engines are. I dobut that if you go buy an industrial ford or chevy that the cam is any different from the car engines wether its gas or LP.

I would think a cam for a turboed diesel in a semi tractor would be a better cam for a gas tractor than the cam for a car engine.