Oil & Fuel Diesel torque difference mostly myth?

Dargo · Apr 4, 2005

( I've used both gas and diesel tractors, and would NEVER consider owning a gas model again. Torque or not torque. I CURVE to the diesel. John )

/forums/images/graemlins/smile.gif Now, that's what I'm talking about! I've had both as well, and will definitely never go gas again. A Cummins has been mentioned, so I will tell you briefly about my Cummins. With my tractor on my new trailer, the weight is around 12,000 to 13,000 pounds (depending on the implement and if the loader is on). With the trailer applying just under 2000 pounds of tongue weight through the goose neck in the bed of my 4X4 dually, I can smoke all 4 rear tires at will at any reasonable speed in 1st gear. /forums/images/graemlins/laugh.gif

Short of trying it with a blown dragster engine, I have never seen a gas powered pickup that turn 4 rear tires at will with 2000 pounds of weight in the bed and 12,000 pounds plus of rolling weight behind it. Call it "torque", "grunt", "power", or whatever you want. They dyno calls it 1000+ ft. lbs. of torque. Whatever it is, a gas engine won't do it. And, I do have a 600+ hp 502ci big block in a car, so I'm familiar with what a "strong" gas engine will do.

Go diesel!! /forums/images/graemlins/grin.gif

getut · Apr 4, 2005

Henro... Given your example I would choose exactly as you would, but that or a similar example has already been brought up a couple of times in this thread.

Even though the displacment may be the same in the two engines in your graphs, the engine geometry (bore and stroke) is vastly different. The torque curves in those graphs are due more to the engine design than the fuel used. In all the other examples above I have been trying to limit the discussion to similar designs on the engines... similar displacement AS WELL AS similar bore and stroke. When compared in that vein, this is where we see that diesel still wins... simply due to the higher compression and slightly more efficient fuel, but the margin that the diesel wins is very small.

Ok... STILL no one has attempted to respond to my question about the torque curve of a diesel encouraging lugging. Please read the post #619286 above. I am eagerly awaiting you guys on that one.

Henro · Apr 4, 2005

Henro... Given your example I would choose exactly as you would, but that or a similar example has already been brought up a couple of times in this thread. 

Getut, it certainly has been. But the graphs of the torque curves wasn't. Furthermore, don't forget the root reason the thread was started:

I got into a discussion with a co-worker yesterday about tractors. He has quite a bit of land and mows with a standard lawn mower. I inevitably started lauding diesel CUT and SCUT tractors and how they were much more than just a lawn mower. I stated the usual about diesels and their much improved torque. I stated that my little 20HP diesel could do much more than a 20HP gas. Well he asked the simple little word "why". 

The question you posted related to existing engines available for use in tractors, not a theoretical question related to whether a gas engine could be produced to mimic the performance of existing diesel engines.

The answer to the why of your question has been repeated many times in this thread, in a qualitative manner. The graphs simply quantify things a bit. So they do have a purpose...

 In all the other examples above I have been trying to limit the discussion to similar designs on the engines... similar displacement AS WELL AS similar bore and stroke. 

All fine and well, but I doubt that kind of discussion will do much to convince your friend why a diesel tractor he might buy tomorrow will perform better than the gas tractor he has today.

IF generally a gas engine has a torque curve that falls off with RPM, and IF generally a diesel engine has a torque curve that increases when RPM decreases, then generally a diesel engine that is off-the-shelf-today will be a better engine for a tractor than a gas engine.

As for the lugging question, let's try a simple thought experiment. You are holding a axle shaft in your hand. At the end of axle shaft there is a wheel/tire. You hand locks the axle in place.

A strap is secured to the tire at the 3 o'clock position, and layed over the top of the tire, so it hangs on the other side. A weight is attached to the strap. You must apply torque to the shaft to hold the shaft (and weight) in position. You are happy, the weight is happy, the shaft is happy. Everything is in equilibrium.

Now if you pretend you are a gas engine, you will not be able to generate more torque if extra load weight is applied to the shaft. So if an extra weight is added, the shaft turns and the weight drops to the floor.

If you pretend you are a diesel engine, when the extra weight is added, the shaft may turn a little, and the weight may lower a little, but you add torque to the shaft by adding a little more muscle power, and the weight does not drop to the floor, but simply lowers a little.

Why, because under load the diesel torque increases, while the gas engine torque falls off. The same basic thing happes with a diesel engine in a truck or tractor.

The point being that under equal conditions, the gas engine will tend to lug more all else being the same. I'll leave it to the smarter guys to prove it mathematically. Others here in this thread have proven it with their own experiences.

Now as you have pointed out, this is only true of existing engines as we might find produced today. There is no reason why a gas engine (maybe many practical reasons, but likely no logical theoretical reason) should not be able to be built with an increasing torque curve with decreasing RPMs.

Gasoline, diesel fuel, veggie oil, natural gas...they are simple energy sources...I doubt torque characteristics are dependent on the energy source. It would probably be fair to say that the torque characteristic is a function of mechanical design, and the torque being produced is dependent upon having some energy source to turn the mechanics...

But all of this is a side issue to the original question. Why is the diesel engine on a small cut probably better than a gas engine on a garden tractor? Because of torque rise the diesel offers...and also don't forget to add that you seem to get a lot more work-per-gallon of fuel out of the diesel! /forums/images/graemlins/smile.gif

cowboydoc · Apr 5, 2005

Redesign a new diesel engine, with similar horsepower as your Cummins... but it would generate the horsepower with a physically smaller engine that turns a higher RPM (an RPM similar to what the 454 gasser is turning) and that engine will not deliver much more torque than the gasser will.


Your argument does not hold water getut. You need to look at a V-8 diesel. The V-8 in gas and diesel using your analogy does not hold water. You produce 30% more torque with the diesel than you do the gas motor at the same hp. They are also not much higher revving either. My 7.3 and 6.0 both go down the road at about the same rpms.

With regard to your smaller higher revving diesels you are 100% correct. They are produced much cheaper and aren't alot better than gas. But an uneducated public that will never tear into one will never know the difference. And most people buying them will never do the work to ever test one to the limits or put enough hours on one to require a rebuild and then find out the engine is junk and can't be rebuilt. Now you know why the off-brands can sell a tractor cheaper than the name brands. They save thousands in the engines by producing a cheaper motor. They don't have to do any testing because they don't produce tractors in the hp required. The tractors that are produced in that category they refuse to have tested. Therefore they can say whatever they want about the torque and the hp. Ask them sometime for the independent tests on their hp and torque #'s. For most of them they won't give them to you.

Regarding lugging a diesel I'm not even sure what you're talking about. Both a gas and diesel are going to lug down. You want to keep your rpm's in the max. torque range. A diesel is going to have a much broader range for this. With a gas motor you hit the torque and it's gone. Thus the need to shift gears more often. With a diesel you hit the torque curve. A torque curve is virtually non-existent on gas engines. Your better diesel engines will also have a torque reserve. When you hit that point of lugging they will produce more torque to keep you going. You never lug either motor, you just to understand where your engine produces it's torque and keep your rpms in that range.

SkyPup · Apr 5, 2005

Is there some attorney that will handle a class action suit against all the diesel engine manufactures for falsely promoting this "diesel torque myth" and ripping off hundreds of millions of honest hardworking tractor operators, crane operators, loader operators, bull dozer operators, combine operators, train operators, marine and ship operators, stationary generators, truck drivers? /forums/images/graemlins/crazy.gif

Bob_Skurka · Apr 5, 2005

They save thousands in the engines by producing a cheaper motor. They don't have to do any testing because they don't produce tractors in the hp required. The tractors that are produced in that category they refuse to have tested. Therefore they can say whatever they want about the torque and the hp.

Now I don't know if any of that is true or not, but I have not been able to find indepentant data on the engines of many brands of tractors. I recently decided to look into it. I can't prove that Cowboydoc is 100% accurate, but so far as I can tell, I am inclined to go along with the statements simply because of the lack of independant confirmation.

 Ask them sometime for the independent tests on their hp and torque #'s. For most of them they won't give them to you. 

This is interesting because I have tried to get some data and as of today, I am STILL waiting. It does seem that the major players have their engines tested for power independantly and are willing to provide the information, although you might have to dig for it.

I'm not going to throw any stones at any particular brands, but if the engines are not submitted to the Nebraska type tests, then what proof is there that the tractor is what it claims to be? Personally I could not tell the difference between a 33hp and a 35hp engine from the seat of any brand of tractor.

Charlie_in_TX · Apr 5, 2005

(
Conflicting thought In the example above, this is the very definition of lugging. The general consensus is not to lug a diesel (or really any engine). In a thread way way back, it seemed that the best way to determine if the engine was in the state called "lugging" is if it is unable to increase RPM if more throttle is applied. The torque curve of a diesel seems to EASILY put it in a state of being lugged. In fact, by the very definition (shape) of that torque curve, it is impossible to reach the maximum work level for a diesel without the engine lugging (no longer able to respond to more throttle to increase RPM). In a diesel, set a given RPM that is higher than that at which maximum torque is produced. If any load is applied to that engine that causes the RPM to drop anywhere near the RPM where that maximum torque is, then guess what. It is being lugged. A gas engines torque curve is usually sloped the opposite direction... which creates the following scenario. As more throttle is applied to a gas engine, the RPM and torque both increase together until maximum torque is reached. Much more difficult to lug. It seems a gas engine will equalize to one of three states according to load. 1) Less load than can be handled... the engine will still be able to accelerate. 2) More load that can be handled... dead stall will occur. or 3) an approximate even match.. probably the closest to lugging that a gas engine can achieve. In a car for example.. usually evident by spark knock and an unwillingness to accelerate.. too low a gear.
)
Quite a long thread, started giving me a head ache and couldn't continue reading. A lot of people throwing around theory and examples but do not seem to answer the question. Given 2 otherwise idintical motors, which fuel type will give you a more appropreat tractor motor, gas or deisel, and why?

On fella gave an apples to apples comparison in his IH. 2 identical motors, bore x stroke. Fuel delivery the only real difference. Ford (and I am sure others) used to offer the same block with different heads for different fuels. JD used to offer 2 garden tractors, 322 and 332, with the same engine block with different heads. This is were your answer will be. I don't know it.

You made one post that hit the nail on the head. In your Kioti diesel motor as speed decreases torque increases. In your buddys Koyler, as speed decreases torque decreases.

But is that a function of fuel or engine design?

I will tell you part of engine design that makes it so. Gas engines use a carb with a valve (throttle). The intent of the valve is to limit the air/fuel mixture intering the cylinder. Say you have a 1 liter gas motor lightly loaded. The valve may limit the cylinder to 1/4 liter of air/fuel mixture. An increase in the load slows the motor which causes the governer, to open the throttle plate, to allow more air/fuel in. A diesel has no throttle. It will take in a full charge of air in each revolution. The amount of fuel is not metered until the air is compressed. In short, a thottle plate in a gasser slows its resonce to load changes.

Gas motor designers are throwing a wrench into that, direct injection gas motors.

Highbeam · Apr 5, 2005

"In your Kioti diesel motor as speed decreases torque increases. In your buddys Koyler, as speed decreases torque decreases."

This is correct while running a gas engine below its powerband.

As shown in the torque curves posted, the gas engine has a torque peak. Once above this torque peak but below the power peak you are into the sweet spot. While in this zone, as the engine slows down the torque goes up. So it reacts like the slow revving diesel right? You just need to be able to tolerate running a gas motor at high RPM if you desire a "torque rise" situation.

From my days in thermodynamics classes. A higher compression ratio makes for a more efficient cycle. It is a given that the diesel engine has a higher CR than a gas engine so it is inherently more efficient. If you use the same high CR for a gas engine, then the difference is the BTU content of the fuel and how the intake is throttled on the gasser. The only way to do have equal CRs on a gasser without preignition is with direct injection of gasoline. Oops, now you have a diesel burning light oil, and throttled to keep the mixture right. Sorry about the rambling. I see no reason that the manufacturers haven't come up with a direct injected gas engine other than the high heat and nitrogen emmisions that come with a super lean burning gas engine.

To answer the original question. Look at the GM 8.1 and compare it to the duramax diesel. They make about the same horsepower and almost the same torque. Peaks of course, but the curves are both pretty dang similar with the diesel having slightly more area under the curve. Folks in the RV world that tow with both report that the 8.1 is a very similar running and performing engine, though thirstier.

Dargo · Apr 5, 2005

Hmm. I towed about two weeks ago with a dealer's 8.1 GM. It feels extremely weak compared to my Cummins. I mean there is a huge difference. Now, I've never towed with a Duramax, but if it pulls like the 8.1, GM better go back to the drawing board big time. /forums/images/graemlins/shocked.gif

JDFANATIC · Apr 5, 2005

Today I put the MMM on my 2210. For whatever reason, I decided to test the PTO at idle. The diesel didn't even strain to turn the 62" deck. Now I know my last tractor, a JD425 with a 20 hp gas V-2 and a 54" deck would have stalled if I had tried that. I guess that's why they call it torque. /forums/images/graemlins/tongue.gif