Oil & Fuel Diesel torque difference mostly myth?

[bugstruck]

Henro, You guys are entirely too smart. I think I'm with you on most of what you stated. The "constant HP" has me scratching a little. I'm trying to get my arms around that. You basically said under Constant HP the voltage and current remain constant but the speed changes? Is that "speed change" referencing just the speeds obtainable with Constant Torque, or implying the speed change varies or is proportional to the load change? Both perhaps? In effect I'm asking if a DC motor in Constant HP operation (constant current and voltage),.... and constant load, will run at one speed only? Seems it would. Vary the load and vary the speed I suppose? My layman's view would be if the voltage, current, and load are constant, the speed must be. DC or AC motor. Am I close or am I all wet?

Something that might relate back to this tread more closely, and might teach me and others somethng in the process, would be the difference between the torque capabilities of AC and DC motors. Is there any comparable difference that could be applied to the gas/diesel torque differences?

Thanks Henro!

 

[Henro]

<font color="blue"> The "constant HP" has me scratching a little. I'm trying to get my arms around that. </font>

With the DC motor, power can be calculated pretty accurately by multiplying volts times amps. P =IE. In the case of AC motors it is not so simple, because phase angle between voltage and current needs to be considered. But we are not talking AC motors...

With the DC motor once the motor is running at full rated armature voltage and with full field current, maximum speed is reached and the motor is happy.

At this point power input can be calculated as amps X volts. This give watts, but watts can be converted to HP at the rate of 746 watts = 1 HP (or something close).

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. But if the armature voltage is not changed, and if the armature current is not changed, the power input will not change, so HP does not change, and the speed range above motor base speed is therefore referred to as the constant HP area of operation. Torque will drop off as speed increases, since torque is the result of interaction of the magnetic fields, and when field strenth is reduced, torque reduces. (We could pick apart the above statement, but for this discussion it is basically true).

Motors will change speed with load...but percentage wise, within the operating range (ie, design limits) the percent load change is probably greater than the percent speed change, until the load change exceeds the system design parameters.

<font color="blue"> In effect I'm asking if a DC motor in Constant HP operation (constant current and voltage),.... and constant load, will run at one speed only? </font>

It is possible (and common) for a DC motor to run at different speeds in the constant HP mode. But I would expect that in a real world application the actual power used would change, since the rate of doing work would change. It is hard to compare these things sometimes because HP needed is a function of the load on the motor shaft, while HP available is a function of motor design...it is easiest to just look at an unloaded motor, and see how it reacts through it's entire speed range first, and to evaluate how it would react in a real world application. Constant HP mode of operation is a tool used to evaluate motor operation...things become a little more complex when the motor is actually doing some work.

<font color="blue"> Something that might relate back to this tread more closely, and might teach me and others somethng in the process, would be the difference between the torque capabilities of AC and DC motors. </font>

There was a time when AC motors were often used for constant speed applications and DC motors were frequently used for variable speed applications. But today, with electronic controls, AC motors can be used for just about anything that used to be exclusively the tuff of DC motors.

Somehow I think if one were to dig into this subject he might set a record for putting about 17,000 TBN members to sleep... /forums/images/graemlins/grin.gif

I think hydraulic motors might be another example of a motor that can produce max torque at zero speed, but this is only an assumption... /forums/images/graemlins/smile.gif

 

[getut]

</font><font color="blue" class="small">( Somehow I think if one were to dig into this subject he might set a record for putting about 17,000 TBN members to sleep... /forums/images/graemlins/grin.gif)</font>

On the contrary.. political, religious and technical discussions are the best kinds of discussions on the board. Too bad the political and religious ones disappear pretty quickly. That leaves technical all alone.<POOF> <GONE> hehehe
/forums/images/graemlins/grin.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/crazy.gif

 

[Av8r3400]

getgut: The answer to your quandry is in this post as to why the 20 hp Kohler in inferior to the 20 hp Kioti. The torque peak rating given may be close, but on the gas motor that torque peak is at the top of a very pointed sharp curve. The curve on the diesel's torque is much "rounder" and spread out. This is where the comment on "the area under the torque curve" is a better measure of work capability.

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

 

[Joe1]

The fuel consumption on the diesel should be far less that the gasoline engine, even though the diesel has a greater displacement. The diesel will also have a far heavier flywheel and has the ability to lug down with a load where the air cooled gasoline engine will stall under the same load.

 

[DirtHauler]

O.K., after all this............I want a diesel-electric tractor /forums/images/graemlins/grin.gif

 

[Boomerang1]

Take two identical vehicles, same transmission, gears and weight. A diesel engine in one and gas engine in the other. Each engine has same hp and torque. Start at the bottom of a big hill and floor the throttle. The diesel will out pull the gas because it has more compression, which pushes down harder on the piston. The harder the piston can push on the crankshaft, the easier it can maintain power.

 

[Syncro]

<font color="blue"> O.K., after all this............I want a diesel-electric tractor </font>

Me too but only if it has a steam self starter. /forums/images/graemlins/shocked.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/grin.gif

 

[getut]

</font><font color="blue" class="small">( Take two identical vehicles, same transmission, gears and weight. A diesel engine in one and gas engine in the other. Each engine has same hp and torque. Start at the bottom of a big hill and floor the throttle. The diesel will out pull the gas because it has more compression, which pushes down harder on the piston. The harder the piston can push on the crankshaft, the easier it can maintain power. )</font>

Boomerang.. you are correct in the outcome but way off in your reasoning for it. The forces you describe are torque, plain and simple. The force that pushes down on the piston and then pushes down on the cranshaft which turns it. This is crankshaft torque and is never any higher than maximum torque rating for the engine. The compression of the engine is already included in that calculation... so to say the higher compression makes it turn harder is true, but that force is NOT above and beyond the already measured torque value.

 

[getut]

Ok.. I've been thinking about this even more.
Let me bounce an even more controversial idea off of you guys. I think this will be controversial because the common replies for certain questions that occur on this board for diesels seem to contradict each other.

Let me explain:
<font color="blue"> Thought #1 </font> The nearly universal consensus of all people on this thread (myself included) are that diesels pull "slightly" harder because of the way that torque is made in a diesel. This creates the diesels characteristic torque curve and broad torque powerband. Even if the engine is lighter with a shorter stroke similar to a gas engine. Here is where it gets hairy. Due to the shape of the torque curve, a diesel handles load better. Set the throttle for a given RPM and slowly start applying load. For most diesel engines, as RPM decreases, the torque increases and resists further RPM loss far better than a gas engine.


<font color="blue">Conflicting thought </font> 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.

Synopsis: Diesel engines do more work because of the way torque is created in the engine even if max torque equal to a similar gas engine. BUT don't use that torque because you're lugging it.

Kick those ideas around for a while. I can't wait to see what you guys say. /forums/images/graemlins/grin.gif /forums/images/graemlins/laugh.gif

And for you guys that want to bash me. I have a diesel and would buy another diesel. I started this whole thread in the quest for good discussion. I am kicking these ideas around as much out of curiosity as I am just trying to create good debate. So nobody go kicking off into a corner sulking and saying "Well I know how good my Cummins is and thats all that matters." or "Getut sucks eggs for even questioning the almighty diesel". /forums/images/graemlins/cool.gif /forums/images/graemlins/tongue.gif