Oil & Fuel Diesel vs Gas Engines

[Charliebrn]

In looking at various engines in various tractors and mowers, it seems strange to me that some much larger engines (gas, 25hp) are in very small garden tractors such as MTDs. Is there a difference in engines (besides longevity) between a diesel 22 hp and a 22hp gas? For example, my neighbor has a 25 hp craftsmen riding mower, and i have the much smaller 22 HP BX. I know the machine is a thousand times more useful in the Bota, but would that 25 HP gas engine in that thing out-perform the 22 in the Bota? Just looking at the sheer size difference of the engine block, the Bota D900 dwarfs the B&S 25...so what gives??? What piece of the puzzle am I missing? Are they rated differently? Please forgive my ignorance, I'm new to diesels. But I am learning to love the bucket of bolts sound... /w3tcompact/icons/smile.gif

 

[Roy]

So much has been discussed, sometimes producing hot arguments (not here, at another discussion group) over this subject: When are any two different engines actually equivalent, even when both are the same fuel-type? I cannot list all the factors (nor can I claim to know all of them), but a quick summary:

+ How horsepower is measured for small tractors is apples and oranges. At what RPM? Peak or continuous?
+ Just because an engine is rated at x-horsepower, doesn't mean it is actually delivered to where it is actually used (wheels, PTO).
+Torque does your work. Diesel engines tend to produce high torque at a lower RPM than gasoline. On the other extreme, electric engines can produce high horsepower (equivalent to wattage), but stink at torque (have an inability to overcome resistance).

The author of the Small Tractor FAQ has been sorting through this problem for some time. His analysis is currently in progress. You can read what has been written so far: http://www.andrew.cmu.edu/user/kb13/TF_home.htm.

I have attached the curves for the BX2200 engine (from the Kubota website). Have your neighbor get the graph for his engine, and then compare!

 

[Thomas]

Also compare diesel vs gas engine in fuel consumption per gallon per hour.

Diesel use to be a cheap fuel for the framers etc...not anymore or at least not in this area $1.69 per gallon. /w3tcompact/icons/frown.gif
I believe it was the JD 1010 tractor w/ the backhoe which started on gas than switch to diesel which help in a longer engine life.

Some mechanics say diesel motor run cooler than gas engines...kinda like AC vs DC current.

"Ignorance" no such thing!!
For if one doesn't ask questions than how can one learn and pass on the knowledge...consider it a tool.

Have a pleasant day.


Thomas..NH /w3tcompact/icons/wink.gif

 

[Anonymous Poster]

Hi Roy,

The "Andrew" e-address failed repeatedly for me.
Does it work for you, as given here?

Thanks,
Larry

 

[TresCrows]

Surely your 'bota BX dwarfs your neighbors Sears. I have two neighbors with Sears 25 hydros and one with a Johndeere 25(?) with a Kawasaki engine. My BX is quite a bit larger and heavier than any of those machines with a flat floor vs. a step over so I am not sure which Sears tractor you mean? Anyway, I purchased a tine harrow, about 6 feet by 8 feet and used it to shread up my ground for seeding. They all wanted to use it and of course I said yes. Well, none of them were able to pull it effectively, the green machine managed but barely and over heated and neither Sears tractor was able to pull it effectively mostly spinning tires, slipping belts and bogging down when the harrow dug in to soft ground. I even flipped the harrow around so it was not as aggresive and still they had problems so I unhooked one section and then they were able to pull it fairly easily. I also have a nice little hydro 16 horse B&S industrial my wife likes. It cannot even move it. The BX pulls it easily and effortlessly at any speed needed. Those 25 horse industrial engines are good engines, they seem to make their power at about the same RPM as the 'bota 18 and 22 horse engines but I doubt they have anywhere near the torque or sustainable power. I could not get that address to work, is it spelled out wrong? Oh, I borrowed a fairly large disc which my BX easily pulled also, one of the guys has a harware single gang disc for his Sears that I can pick up in my arms, he gets stuck pulling it. There really is no point in comapring a BX to any garden tractor by Sears or Johndeere. At the moment they have nothing that can compare. Their machines are nice, I do like that Sears hydro for some reason, manuverable and light on the grass.JR

 

[caddison]

Charlie,

I had a White 1855 w/18 h.p. Briggs industrial. It was on of the largest "garden tractors" you could buy.
Problem was you didn't have enough weight. Generally power wasn't an issue because the tires would break loose before I would run out of power. The BX out weighs the others by quite a bit so you are getting more power or torque to the ground. Diesels also operate under much higher compression than their gas counterparts. Aids in providing more torque at lower RPM's.

The larger engines in the garden tractors help sell them. My father-in-law has an "old" JD with a 54" deck and HST. It is powered by a 12h.p. Kohler and does a great job of mowing grass, just what it was intended for. Now they are putting 25hp engines in garden tractors that never see dirt. All that HP is useless if you can't get it to the ground.

 

[Roy]

Larry; It did not respond this morning when I posted (server was down), so I cut and pasted from my Bookmarks. Just checked it and they are back in action now. (It was a strange morning on the 'ole net. Even the tractorbynet.com name was undefined for a while). The direct path to the horsepower work is http://www.andrew.cmu.edu/user/kb13/ans_hp.htm (you can scroll down past his 'Stay Tuned' to see the work so far).

 

[Rick Jay]

Hi,

Since diesel vs. gas comparisons are probably useless, I'll add one more. My Honda Accord has a 4 cylinder 2000 cu. in. displacement gas engine rated at approximately 120 HP and 120 lb. ft. of torque. My Kubota B2910 has a 4 cylinder 1500 cu. in. diesel rated at about 1/4 the HP and 1/2 the torque, and these ratings are at about 1/2 to 1/3 the RPM of the gas engine. With the loader, the vehicle weights are about the same. I'm reasonably sure that if I connected them both with a chain in a great "tug-of-war", the Kubota would pull my Accord all over the place. Mostly because of the traction and the torque benefits of the lower gearing and larger tires on the tractor.


The diesel engines, as they require a higher compression ratio, also have longer strokes than gas engines, and this results in the higher torque at lower RPM figures.

Can you compare them?...naw. But if any of your neighbors start "bragging" about their higher HP tractors, go get a good tow chain and I think the winner will be obvious. (Ummm...be prepared though. You might have to help them pick up all the pieces of their tractor after it blows apart /w3tcompact/icons/smile.gif. You may want to do this on their property.)

Have fun,

Rick

 

[patton]

A couple of points:

The kubota (or Deere, Yanmar, Cat, Ford, etc) engine is designed to deliver it's hp all day long. Very few gas engines are designed to do this. As an analogy, the chevy 454 or ford 460 will run circles arround a diesel (more torque, more power, more speed), but people who are serious about towing will go with the diesel. It won't wear out from delivering it's rated load. OTOH, if you buy a 200 hp diesel, just to deliver that burst of power when you need it you are wasting your money.

Diesels will typically burn half the fuel of a gas engine at rated load- I'm not sure if this still holds at the low end of the size range we are discussing. In an over the road vehicle, the weight of the diesel will hurt fuel economy, in a tractor it's an asset.

Compairing a hyped up lawnmower to a toy size tractor isn't a fair compairson. A gas ford 1000 or 8N would be a better reference point.

When compairing tractor horse power, you have to look at how it was tested- PTO dynos are a bit hard to cheat, assuming it is run at rated PTO speed. Drawbar hp is meaningless unless a standard such as the nebraska tractor test is used. Engine hp number are a bit meaningless and easy to cheat with. Gasoline engine hp is generally 50% higher then an electric motor for the same load (assuming it was designed by a engineer.)

If you want to talk about pulling power, I used to operate a tractor that would pull any ag tractor under 100 hp backward- It was a 20 hp case stream traction engine. A steam engine will deliver max torque at 0 rpm. A large (say 80 hp) HST might move it, but I don't think such a critter exists. Then there is the traction factor, your HST has 60 lbs of diesel, mine had 4000 lbs of water, and 100 lbs of coal. Your body might be 14 ga steel, mine was 3/8" steel plate.

Pat

 

[TresCrows]

A couple more points;
The reason some of us buy and or prefer "toy tractors" or "lawnmowers" or even ZTR's instead of more manly sized machines is because guess what?, we just want to mow a little grass without knocking the house over or smashing the grass not to mention mashing the daisys to China or running over the neighbors children and dogs. I would look rediculous more than cool if I bought a Kubota L size machine to mow my yard. I will not try to convince my wife I need one because I do not nor will I try to convince anyone else they do not need one, maybe they do. I think comparing the "toy tractor" (assume meant BX et.al.) to it's typical suburbon competition is fair. I have worn out a flock of various discount brand tractors, shovels and wheelbarrows before deciding to purchase something that might last the rest of my life vs something I have to replace every few years--the old pay now or pay later argument. This is the very argument that goes on in the heads and households of many small machine buyers, should they buy a quality Johndeere or Kubota product once or a dozen Murrays, one every year. Given the extra flexibility of the sub compact utility platform revolutionized by the Kubota BX tractor vs some of the other larger garden units and comparing the value and usability of small gas engines vs small diesal engines, the question asked by the original thread, is entirely valid and I have learned somethings from the discussion. Thanks in advance for the education, what a great site. JR