Oil & Fuel Diesel torque difference mostly myth?

mwood · Apr 3, 2005

What I was told is as follows. I believe it is correct and seems to be an easier way to understand all that has been said.

Take a diesel engine and a gas engine that produce the same amount of torque at rated speed. (Let's say 2200 RPM for the deisel and 5600 RPM for the gas).

IF you had the gas and the deisel at rated speed, then yes, the torque would be the same. A few points to ponder here as to why deisel is superior for work:

1. You have to get to rated speed before this works. Therefore, when you put the plow in the ground and start to move forward, the gas engine will have much less torque at the wheels since it is starting below rated speed and it has such a sharp torque curve. However, with the flatter curve of the deisel, you may not be getting max torque, but you are getting much more than the gas engine.

2. Once you achieve optimum engine speed, it's very hard to maintain it (especially when driving a truck). Look at a high HP gas engine. The torque curve drops off sharply on either side of the peak. Therefore, you usually have around a 500 RPM window when the gas engine is going to pull well. With a deisel, you may have closer to a 700 RPM window. When you factor in gears for a truck, that 700 RPM window is a much greater percentage of the max RPM and therefore, a much greater speed range. Hence why deisels don't have to downshift as much.

Just a couple of points to ponder. I tried to keep this as simple as possible for easy explanation to one who isn't an engineer... /forums/images/graemlins/tongue.gif

Dargo · Apr 3, 2005

( This is also why the Cummins motor is a better design for working than the V-8 diesels in pickups. Go ahead and flame me now, sorry. /forums/images/graemlins/grin.gif )

I am not a "Dodge" guy at all. However, after doing honest research, it is unavoidable to discover that the Cummins in line 6 is far superior to the other diesel pickup offerings. That is difficult for me to say since I really prefer the Ford pickups! If you want a quick confirmation of this, do some research on what is the longest lasting pickup diesel; Cummins. What pickup diesel engine holds the world record in each of the diesel pickup drag race categories; Cummins. What diesel engine holds the top spot in the diesel pickup truck pull categories; Cummins.

There must be something about the inline 6 stuff for diesels. As a matter of fact, the only semi engines I have personally seen have been huge inline 6 engines. Hmm, must be something with this inline 6 stuff. /forums/images/graemlins/wink.gif And, I've never seen a gas powered semi either. /forums/images/graemlins/tongue.gif

Av8r3400 · Apr 3, 2005

(quietly) I would rather have the Ford "truck" too. /forums/images/graemlins/blush.gif

Perfect world: Ford Truck, Cummins motor, Allison Tranny. When I bought mine, I bought a Cummins motor that just happened to be wrapped in a Dodge truck. /forums/images/graemlins/smirk.gif

RobertN · Apr 3, 2005

Have followed this thread, thought would add a couple comments.

Henro had the right idea. Compare the torque curves of both motors. While the gas Kohler engine might make the same torque as the Kioti, it only makes the torque in a very small region of the operating range. The diesel engine will still have a peak torque, but it will also operate close to that torque rating over a large rpm range while the gas engine will only make useable torque at a specific, and often hig rpm.

For you instance, the little lawn tractor can make big numbers in hp and torque. But, you could outpull it even at lower rpm, because there is torque spread across the rpm range on the diesel.

the diesel also does more work per unit volume than a gas engine. My Farmall-A makes great torques for a gas engine. So, why the change to diesels if a gas engine will make a wide torque band? Economy when you're out in the field plowing and mowin and baling all day, all year long.

Iterestingly, my Farmall rev limits at 1400rpm, has a 3" bore, 4" stroke(113ci).It is hand crank startable(all mine has), with about 6:1 compression. It makes about 20hp(22hp/1800rpm in the U2 power unit configuration). For something close to a diesel torque curve, check out the attachment...

My Kubota B8200, about a '86 year model, is 19hp. It is 3" bore, 2.8" stroke(56.6ci). There is no torque info in either the owners manual or service manual. They do list gross power in kW too, 14.2kW.

milkman · Apr 3, 2005

(
There must be something about the inline 6 stuff for diesels. As a matter of fact, the only semi engines I have personally seen have been huge inline 6 engines. Hmm, must be something with this inline 6 stuff. /forums/images/graemlins/wink.gif And, I've never seen a gas powered semi either. /forums/images/graemlins/tongue.gif )

The first semi that I drove was a GMC V12 gas, early '60's, the next was V8 2 stroke diesel, but the ones that just keep on going is the ones with 6cyl all in a row.

bugstruck · Apr 3, 2005

Quote "For reasons we don't need to get into, with the motor running happily with maximum voltage applied to the armature terminals, its speed can be increased by reducing the magnetic field within the motor frame. This is done by reducing the electrical current flowing through the motor field windings.

When the motor field current is weakened, the motor will speed up."

Got this one figured and can't get the blue quote on???Thanks for clearing all this up. I was guessing /forums/images/graemlins/confused.gif that varying the field current changed the speed.

Then I made the mistake of thinking about something I was taught back in High School and had forgotten along the way. I was focused on DC motor or starter operation and couldn't remember how the polarity reversed to get it past my conceptual "it's locked up" stage. This link refreshed me rather quick. http://zone.ni.com/devzone/conceptd.nsf/webmain/85399932CDCC7DCB0625683F007290A7 I couldn't remember if it was a geometric or electronic solution that reversed the polarity in a simple motor operation. The dumber I get the older I am. /forums/images/graemlins/grin.gif

Appreciate the help. You explained it just well enough that I'm not tripping all over this..... again. /forums/images/graemlins/cool.gif /forums/images/graemlins/grin.gif

Thanks,

getut · Apr 3, 2005

Wow that is some big torque.. and you're graph shows that big torque numbers are not nearly as tightly tied to the fuel used, diesel or gas, as it is to the way the engine is designed. Todays diesels seem to be sacrificing the traditional high torque engine designs.

Take a look at todays small diesels... Daedong, Yanmar... anyone in about the 20-25HP range. The peak torque is MUCH LESS than that 20HP Farmall gas engine. Most of todays small high revving diesels in that horsepower range are peaking around 40-45 lb-ft of torque. That 20HP farmall is making nearly 80 lb-ft.

Oh.. no one has any comments about my post dealing with lugging yet? /forums/images/graemlins/grin.gif

Av8r3400 · Apr 3, 2005

The only diesel that I have any experience with that had specific mention of lugging is my TDI VW. Their reason is the motor will over boost if the RPM's aren't high enough.

My Cummins pulls like a mule from idle up to 2300 RPM where the governor starts to de-fuel it. (for now /forums/images/graemlins/grin.gif )

Truck driver friends have told me that lugging is, as was stated, applying full throttle and getting no increase in RPM. Some add a modifier of LOW RPMs to where the full throttle is added. I don't know what "low RPM" means though. VW specifies the low RPM as 1500.

Egon · Apr 3, 2005

An electric motor draws the most amps at zero RPM. When its running theres something about Back EMF produced by the motor.

On a steam engine the maximum pressure exerted on the piston is at zero rpm. As soon as the piston moves this pressure will be reduced.

For torque on gasoline and diesel engines one has to take into consideration the design of the motors. The diesel is usually designed to run very efficiently in a narrower RPM range than a gasoline engine. To properly compare the two they both have to have a specific design for a specific job. When this is done there should be little or no difference.

Egon /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif as always

RobertN · Apr 3, 2005

I looked at Kubota, Deer/Yanmar, and Kioti site. Kioti showed torqu curve, and although Jehn Deere did not, it was an easy lookup at the Yanmar site, for the motor used in the JD4110.
Yanmar torque curve, jd4110 tractor Kioti used the 3C093 motor; you have to look it up in the drop down menu.

I was looking at 20hp models. Kubota did not show any info on thier 20hp offerings.

Of note, the Kioti and Yanmar engines data starts at 1600rpm. The curves are pretty flat...

Kohler offered a performance curve, on the 20hp "Command" model. Kohler page

The Kioti and Yanmar had really flat torque curve. The Kohler was kinda peaky. Only Yanmar and Kohler had torque in Newton/metres. The Kohler had a peak of about 44Nm or 32ft/lb, while the Yanmar had a peak of over 65Nm or about 48ft/lbs. The Yanmar had a flatter torque curve, and about 1.5x more torque than the Kohler. The Farmall did peak higher, at about 80ft/lbs.

Interesting to note, is the Farmall is the biggest, the Yanmar medium, and the Kohler relatively small. The Farmall is is 113ci, the Yanmar about 60ci, and the Kohler 38ci.

the 48ft/lb at 1600rpm still aint bad though(for the Yanmar). That's not revving that high to hit peak torque... It has more than 1/2 the torque of the '41 Farmall gas engine, at approximately 1/2 the displacement.