Oil & Fuel Diesel torque difference mostly myth?

[SkyPup]

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......?

 

[JerryG]

<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.

 

[slowzuki]

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.

 

[getut]

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.

 

[slowzuki]

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.

 

[JTC]

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

 

[Indydirtfarmer]

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.

 

[getut]

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.

 

[JerryG]

<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.

 

[slowzuki]

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>

 

[slowzuki]

The huge difference here is old and large diesels of a low hp rating are not designed for high rpms and therefore will never develop big hp numbers.

This is the same as big old gas engines. An old 6 cly gas tractor motor rated at 50 hp has huge displacement and torque at low rpm. It is choked by a tiny single barrel carb that causes the volumetric efficency to drop like a rock after 2000 rpm.

Same problem as a huge diesel, if not designed to spin fast it will not develop big hp numbers.

 

[JerryG]

<font color="blue"> All you need to do is reduce combustion chamber volume. </font>
That is correct.
<font color="blue">Same problem as a huge diesel, if not designed to spin fast it will not develop big hp numbers.</font>
That is also correct. IMHO, most are to hung up on Hp. The marketers have drilled Hp into everyone, for so long. The most important thing in tractors is torque and how flat the curve is.

 

[getut]

JerryG: That is EXACTLY why I was so surprised when I actually started researching this yesterday and finding torque numbers only very slightly better than a similar sized Kohler gas engine.

Again... a rhetorical question that I already know the answer to: If torque is so important why are they building diesel engines that minimize the very strongpoint that cause people to go to diesels in the first place (torque and longevity) by making them spin faster with shorter strokes?

Edit (answer): 1) to save money. 2) to meet emissions requirements by burning fuel more efficiently 3) because like JerryG said. they (consumers) are hung on HP numbers when it is actually torque that is important... faster RPM = higher HP rating

I would actually like a MUCH slower turning engine if for nothing other than the noise. 2800 RPM makes my head buzz. Put a beefier engine with longer stroke in it.

 

[JerryG]

Like I said before: 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.
A smaller engine to fit into small tractors and equipment, which also saves them money. Greed vs longevity. Everything is a trade off.

 

[slowrev]

Been reading posts...head starting to hurt....

All I know is my uncle had 2 identical IH tractors except for the engines. One was gas the other was diesel. Both were within I believe 3 hp or so of each other. However the diesel tractor would flat outwork gasser. Of course he used the gasser exclusively in winter /forums/images/graemlins/smile.gif

Ben

 

[Henro]

<font color="blue"> JerryG: That is EXACTLY why I was so surprised when I actually started researching this yesterday and finding torque numbers only very slightly better than a similar sized Kohler gas engine. </font>

I'm with JerryG...

Here is how I would look at it. Forget what is inside the engine and just look at the performance CURVES for the comparison.

The performance curves tell the story. Once you can agree on the fact that one is better than the other, then you can dig into the why it is better. But I would not waste time on the "why", until I concluded the "which is better" part.

Now I am pretty sure like JerryG said, the torque curve of the diesel is flatter and would have more area under it. As a matter if fact, I think the last time I looked at a diesel touque curve it increased for a while as engine speed dropped. Exactly what one would want if the tractor came under load...

Peak numbers almost mean nothing in real world application, unless one also knows how the numbers change with RPM. The performance curves tell the story. Peak numbers do not.

All this has been said before in this thread...sorry... /forums/images/graemlins/blush.gif

One other advantage of the diesel is that it uses a less costly fuel, which also has more BTUs per pound (lliter, gallor, whatever). Less costly because it does not need to be refined to the extent that gasoline does. Taxes in this country cause it to be higher than gas here (I believe this is true, someone please correct me if this is wrong).

My guess is that if you got the curves for the gas and diesel engines, and maybe used a theoretical transmission to reduce the higher RPM gas engine down to where the output shaft would have the same RPM range as the diesel, that you would still find that the diesel had a flatter torque curve than the gas engine does.

Without performance curves, and without looking at both engines over the entire operating RPM range, I doubt you will be able to come to a certain conclusion.

To come to a conclusion as to which is better, you don't really need to know what is inside the box...you just need to know how the shaft coming out of the box will perform wihen a load is connected to it.

That is the way I see it anyhow... /forums/images/graemlins/crazy.gif

I may have just repeated most of what was said above, in a different way... /forums/images/graemlins/tongue.gif

'Better get back outside nwo and enjoy the remaining part of the last day of three nice warm ones... /forums/images/graemlins/grin.gif

 

[JTC]

There is a lower limit to the compression ratio (stroke length) of a compression fired engine (diesel) where the ignition source is the thermal energy increase as a gas (air) is compressed (Boyle's gas law PV=nRT?).

There is an upper limit to the compression ratio of an ignition fired engine (gas) to prevent pre-ignition (dieseling). At best, pre-ignition results in pinging and at worst would prevent a piston from moving past TDC (a one stroke engine? /forums/images/graemlins/crazy.gif).

The compression ratio parameter range for the two engine technologies don't come close to overlapping so I don't think it is accurate to say smaller diesel engine design is converging to be more small gas engine like.

Two factors are contributing to diesel engines becoming more popular.
1. Reserve Torque - This is how fast the torque curve falls off from peak torque. Put a load on the Kohler gas engine so that the RPM's drop and the available torque falls off much quicker then a comparable diesel even though they might be rated at the same peak torque.
2. Efficiency - More work is delivered per BTU of fuel (not necessarily volume). The longer stroke allows more time in the cylinder for more complete combustion and transfer of thermal energy to mechanical.

I don't think manufactures are pushing diesels on us but are responding to the market demand.

Jim

 

[Dargo]

After having decades experience with both, it would be easier to prove that the world is flat than to tell me that similar HP gas and diesel engines have the same torque. I towed with a high HP 454ci big block for years. I thought that every engine downshifted on every incline when towing on the highway.

Hmm, several years ago I towed with a "weak" 6 cylinder Cummins diesel in a pickup that had not even half the HP that my 454 had. Amazing! It never downshifted or dropped off in speed when driving up inclines on the interstates. Also, the trailer I frequently towed with the 454 weighed just over 5000 loaded. I just got back from another trip in my Cummins Diesel pickup towing 12,000 pounds. Even with 12,000 pounds behind me my truck has never (not even once) downshifted on the highway. I have less than half the HP, but can tow nearly 3 times the weight without any problem. Oh yeah, forget the "gearing" excuse. The 454 had lower gearing than the little 6 cyl. Cummins. Therefore, the gearing would benefit the gas engine. Let's see, less HP, higher gearing, and less displacement in the diesel but it pulls more than 3 times as hard. I need no more proof. /forums/images/graemlins/wink.gif

 

[getut]

JTC... Agreed. I would still buy diesel again. My whole point in starting this thread was just because of how surprised I was that it was even close and the research that showed why it was close.

Dargo... You're right. But you've got a 6 cylinder with lots of mass, low RPM, long stroke and high torque. Which is not what we've been discussing. Rearrange the geometry (reduce the stroke and increase the bore) of that Cummins diesel so that it turns approximately the same RPM as that 454 gasser and that huge torque lead narrows VERY substantially. The diesel will still win out for all the reasons everyone has stated above (compression, shape of the torque curve, etc), but by not NEARLY as big a margin as it does currently with its huge stroke.

 

[getut]

Bah.. that example came out all goofed up Dargo...
Let me try again.

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.