Oil & Fuel Diesel torque difference mostly myth?

   / Diesel torque difference mostly myth? #21  
The diesel engine is limited by mean piston speed since there is only a specific time frame when the valves are closed and the right crank angle degree is available for fuel injection once the combustion chamber has been filled. So the limiting factor is the injection event itself. Top mean piston speed is around 13-15 meters per second or 5,000 or so rpms.

However, a gasoline engine already has the fuel in the combustion chamber so can reach much higher piston speeds over 22 meters per second and 18,000 rpms.


Which one do you think wears out first......?
 
   / Diesel torque difference mostly myth? #22  
<font color="blue"> As I recall, one way to do the comparison was to compare the area of the graph under the torque curves -- or was it the HP curves. </font>
The area under the torque curve will be much more with the diesel, because the torque curve of a diesel is much flatter. The numbers quoted by manufactures are peak numbers. Peak means very little in the real world.
 
   / Diesel torque difference mostly myth? #23  
We are going to do some simple internal combustion engine mechanical engineering course work here! I don't have my texts in front of me but lets give this a shot!

Torque is a function of the brake mean effective pressure (BMEP) and the displacment of the engine. NOT THE STROKE.

HP is a function of the torque and speed.

BMEP is the average pressure exerted on the top of a piston and is governed by several things:
#1 Compression Ratio
#2 Fuel Charge (and how well it burns when in there)

Diesels by nature have a much higher compression ratio so the peak pressures and temperatures are higher. They also have a larger window to inject fuel.

Both diesels and gas have a limited piston speed. Gasoline air mixtures in an engine actually burn slowly. The flame front moves at speeds reasonably expressed in inches per second (think about that!) but the turbulence folds the flame fronts around and causes the distance to be covered to be very short.
 
   / Diesel torque difference mostly myth?
  • Thread Starter
#24  
Yup.. but stroke is involved in the brake calculation. Stroke plays a huge role in that. Work per revolution is a function of stroke and displacement so it comes full circle. It is not direct corrolation but very close.

2 engines of equal displacement. The shorter stroke of the two will have less torque. So again... all the statements hold up.

Manufacturers are making less expensive higher RPM engines with shorter strokes, but the performance characteristics are getting **closer** to gasoline engines. Less torque vs an identical displacement diesel with longer stroke.
 
   / Diesel torque difference mostly myth? #25  
The effect of stroke is not always to increase torque. An engine with large pistons and a higher BMEP can have higher torque than an engine with smaller piston area and longer stroke.

Plus the the longer stroke engine may be limited to lower rpms and greater friction loss to the piston skirt side loads.
 
   / Diesel torque difference mostly myth? #26  
The greater the compression ratio, the greater the piston stroke.

The greater the piston stroke, the greater the crankshaft throw.

Crankshaft throw is a simple lever.

The longer the lever, the greater the torque (for a given force).

Diesel engines have higher compression ratios than gas engines.

Displacement for displacement, diesel engines have more torque then gas engines at the same RPM (assume each is operating at nominal efficiency).

Simple mechanics.

Jim
 
   / Diesel torque difference mostly myth? #27  
I didn't read through everyone elses replies, and I HOPE I'm not repeating someone else, but here goes.

You will see only a marginal difference when comparing a small gas vs. a small diesel engine. It's a percentage issue.

When you compare BIG cu. in. engines is when the difference is obvious.

The term that I like was one my father used to describe a diesels "advantage" TORQUE RESERVE....

It's there. It might be less obvious on a15 or 20 HP comparison, but line up a 500 HP gasser and a 500 HP diesel, and the comparison won't even be close.
 
   / Diesel torque difference mostly myth?
  • Thread Starter
#28  
I think you guys see what I am talking about now... still not saying diesel has LESS... it has MORE torque. I know that and even stated it in the first thread.

Why are manufacturers making the small diesels with short stroke that run higher RPM similar to gasoline engines (and create less torque than their longer stroked brethren of similar displacement)? Why not save gasoline engines for that task? Keep making the diesels with their trademark big torque, slow RPM and super long life.

Rhetorical question: If the tractor manufacturers are using these lower torque, higher RPM diesels, why didn't they just save some money and use a gasoline engine?
I still would have bought diesel because I like the fuel economy... but the thing that started this whole thread kind of disappointed me.

Because of these little high revving diesels, A Kohler 20HP gas engine is putting out nearly the same torque as a Daedong 21HP, a Yanmar 23HP, etc, etc. etc.
 
   / Diesel torque difference mostly myth? #29  
<font color="blue"> Why are manufacturers making the small diesels with short stroke that run higher RPM similar to gasoline engines (and create less torque than their longer stroked brethren of similar displacement)? </font>
A small stroke engine fits in a smaller package. By going to higher RPMs, the manufacture can built the engine lighter, which is less expensive.
 
   / Diesel torque difference mostly myth? #30  
No this is absolutely positively wrong ok?

All you need to do is reduce combustion chamber volume.

</font><font color="blue" class="small">( The greater the compression ratio, the greater the piston stroke.

Jim )</font>
 

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