Why Diesels Make So Much Torque

[semievolved]

Brian, I don't agree with you about torque. Stroke is an absolutely key factor in the equation. If you have 2 engines with identical displacement but one has a short stroke (larger piston diameter) and the other has a long stroke, the torque will be greater in the long stroke version. Torque is the ability exert force to turn something and is a direct function of the lengthnof the torque arm, which I'd hope you agree is longer in long stoke cylinder.

[Brian B-P]

Originally Posted by semievolved

If you have 2 engines with identical displacement but one has a short stroke (larger piston diameter) and the other has a long stroke, the torque will be greater in the long stroke version.

It might, but there is no fundamental reason for the longer stroke engine to produce more torque.

Originally Posted by semievolved

Torque is the ability exert force to turn something and is a direct function of the lengthnof the torque arm, which I'd hope you agree is longer in long stoke cylinder.

Yes, torque is the product of lever arm length and force; displacement is the product of stroke length and bore/piston area.
In this comparison, the engine with the longer stroke will have proportionately smaller piston area, and thus proportionately smaller force for the same cylinder pressure, and thus the same torque.

In practice, long-stroke engines work poorly at high speed, but have other advantages (such as narrow bore spacing and small head area), so long-stroke design is used for engines intended for low-speed operation. Engines designed to run slowly are optimized for low speed operation, and thus typically produce better effective cylinder pressure (and thus torque) at low speeds than engines designed to be able to run fast... nothing like proportional to the stroke, and fundamentally unrelated to it.

[Bruce H]

A gallon of diesel fuel contains more BTUs of energy than a gallon of gasoline.

[floyd]

To paraphrase the great Ronald Reagan...
"Well, the trouble with some of our friends is not that they're ignorant; it's just that they know so much that isn't so.”
Some of it almost sounds convincing!

[Brian B-P]

Originally Posted by Bruce H

A gallon of diesel fuel contains more BTUs of energy than a gallon of gasoline.

True.

This has nothing to do with the torque production capability of the engine. The amount of energy delivered in a power stroke is limited by how much energy can be derived by burning fuel, but that in turn is limited by how much air can be packed into the cylinder. To burn a given mass of fuel takes about 15 times that mass of air. The ratio is slightly different for diesel and gasoline, but not enough to matter; the volume occupied by each fuel is different, but still irrelevant compared to the air volume.

So the torque output superiority of one engine type over the other does not depend on the fuel's energy density, but does depend on packing air in... yes, turbocharging.

[Brian B-P]

Originally Posted by floyd

To paraphrase the great Ronald Reagan...
"Well, the trouble with some of our friends is not that they're ignorant; it's just that they know so much that isn't so.”
Some of it almost sounds convincing!

Floyd, if you have something constructive to contribute, please do so. If all you want to say is that someone is full of crap, then have the guts to say that and present something intelligent to back it up... you have not done so thus far.

[floyd]

Originally Posted by Brian B-P

Floyd, if you have something constructive to contribute, please do so. If all you want to say is that someone is full of crap, then have the guts to say that and present something intelligent to back it up... you have not done so thus far.

But,but... what if that's "all I want to say"?

[semievolved]

Originally Posted by Brian B-P

It might, but there is no fundamental reason for the longer stroke engine to produce more torque.


Yes, torque is the product of lever arm length and force; displacement is the product of stroke length and bore/piston area.
In this comparison, the engine with the longer stroke will have proportionately smaller piston area, and thus proportionately smaller force for the same cylinder pressure, and thus the same torque.

Now, we're getting to the discrepancy. I thought it would be understood when I said the 2 engines had identical displacement that I should have also said the same combustion volume as well. So, the smaller piston would have more head space above it to have the same combustion volume of the larger piston. This would be necessary in any case to keep the compression ratio the same. To restate then, with equal compression volumes, compression ratios, and displacement, the longer stroke engine WILL produce higher torques, do you agree?

In practice, long-stroke engines work poorly at high speed, but have other advantages (such as narrow bore spacing and small head area), so long-stroke design is used for engines intended for low-speed operation. Engines designed to run slowly are optimized for low speed operation, and thus typically produce better effective cylinder pressure (and thus torque) at low speeds than engines designed to be able to run fast... nothing like proportional to the stroke, and fundamentally unrelated to it.

Brian, the summary you make above is correct. 2-stroke (gasoline) engines are high revving, short stroke, low torque that get their power from their high speeds. Diesels are on the other end of the spectrum; low revving, long stroke, high torque engines that achieve their power primarily because of their long stroke. Of course 4 stroke gasoline engines fill the broad spectrum between these end points. Remember old VW engines that were surprisingly strong because they had long strokes so they could get good torque out of a small engine but it wasn't a good idea to rev them too much!

I do agree with you that the turbocharged diesels benefit from higher compression ratio and can provide a proportionately greater power boost than for a gasoline engine with lower compression ratio.

I thought this was a great article and appreciate the original poster for sharing it.

[Brian B-P]

Originally Posted by semievolved

I thought it would be understood when I said the 2 engines had identical displacement that I should have also said the same combustion volume as well. So, the smaller piston would have more head space above it to have the same combustion volume of the larger piston. This would be necessary in any case to keep the compression ratio the same.

Sure, yes.

Originally Posted by semievolved

To restate then, with equal compression volumes, compression ratios, and displacement, the longer stroke engine WILL produce higher torques, do you agree?

Still fundamentally no. Longer stroke with smaller bore still means longer lever but lower force, for the same torque... if engines are each working as effectively at the same speed. There's no point in comparing a short-stoke high-speed racing engine to a long-stroke low-speed working engine... of any fuel. Of course, long stroke tends to be chosen in the design of slow-revving high-torque engines.

Originally Posted by semievolved

2-stroke (gasoline) engines are high revving, short stroke, low torque that get their power from their high speeds.

Not really. While the 2-stroke designs we most commonly see (snowmobiles, for instance) allow very high speeds, they are usually used for simplicity and light weight, and a 2-stroke can be as slow as you want. The old Detroit Diesel engines that were common in buses - and for some of us are best known as the source of the famous 6-71 supercharging blower - were 2-strokes, and there have been many other slow diesel 2-strokes.

Originally Posted by semievolved

Diesels are on the other end of the spectrum; low revving, long stroke, high torque engines that achieve their power primarily because of their long stroke.

Typically diesels are long-stroke, but not always. Many VW diesels have shorter stroke than bore (undersquare, the opposite of long-stroke). It's a matter of designing to a purpose, and comparing long-stroke / narrow-bore diesel truck engines to large-bore / short-stroke gasoline car engines confuses diesel/gasoline features with truck/car features.

Originally Posted by semievolved

Remember old VW engines that were surprisingly strong because they had long strokes so they could get good torque out of a small engine but it wasn't a good idea to rev them too much!

That didn't sound right to me, so I did a very quick search: VW and Porsche boxer engines have tended to be oversquare (big bore), not long-stroke at all.
AirCooledTech.com

Originally Posted by semievolved

I do agree with you that the turbocharged diesels benefit from higher compression ratio and can provide a proportionately greater power boost than for a gasoline engine with lower compression ratio.

But turbocharging boost benefit isn't tied to higher compression ratio; diesels can use more boost because of their inherent direct injection and combustion-as-injected, not because of the high compression ratio that they need in order to run at all.



This is getting really mired in bore and stroke considerations of engine design. The original topic was "Why Diesels Make So Much Torque", and one suggestion was that it was due to long stroke. Whether one believes that is true or not long stroke is not a required or exclusive feature of diesels.

In the tow vehicle for your egg, you might want to choose an engine - using any fuel - which can produce the required sustained power at a moderate engine speed. That will lead to large displacement and/or supercharged (which usually means turbocharged) engines, which must have relatively high torque output so that when power=speed*torque, there's enough power.

[semievolved]

Well Brian,

It's good we can agree at least that this is not really going anywhere. In closing though, I disagree with your assertion that the article contained misinformation and implore you to contact the authors for their persepctive. It seems to me that the preponderance of information and consensus is that typical diesels are used because of the torque they develop and that torque is primarily a result of their longer stroke. I realize that different diesels are designed with different objectives and they don't all have that characteristic. Longer stroke equates to greater torque (all other things being equal and independent of the force applied at the piston, which does not enter the equation).

By definition in physics terms, longer stroke = longer lever arm = greater torque.

Been a pleasure, over and out.

[Brian Van Snell]

Gasoline engines work on the Otto cycle, while diesel engines work on the Diesel cycle -- they are fundamentally different. See this reference:

Mister Rudolph Diesel was aware of the gasoline engine ([Otto cycle - Wikipedia, the free encyclopedia Otto cycle]) problems and wanted to improve it. The gasoline engine inherently has problems with efficiency and/or fuel. In order to improve the efficiency one must increase the compression ratio of an internal-combustion engine (see the bonus section at bottom of this article). However, in the gasoline engine there is a limit - the gasoline-air mixture will self ignite once the compression gets too high (because every compression drives temperature increase). So, either you can have a low-efficient, low-compression engine that uses a cheap fuel, or you can have a high-efficient, high-compression engine that uses expensive, high-refined fuel that wont self-ignite even at high compression levels (a 120 octane gasoline?). In [Diesel engine - Wikipedia, the free encyclopedia diesel engine] this problem is solved. The diesel engine can use much higher compression levels than the gasoline engine reaching higher efficiency. In addition, the diesel engine can use fuel that is not nearly as refined as the high-octane gasoline fuel (thus cheaper). To make this possible, Rudolph changed the Otto cycle and created the diesel cycle. The difference is that during compression phase, no fuel is present in the cylinder and thus no self-ignition can happen. The fuel is only injected at the moment the ignition is wanted - when injected into the hot pressurized air the diesel fuel self-ignites immediately (the diesel-air mixture, as we said already, is happy to ignite even at relatively low temperatures).

What are the differences between otto cycle and diesel cycle

Brian

[Darwin Maring]

This thread is exhausting.

[semievolved]

please clarify if it is gasoline exhausting or diesel exhausting.

thanks for that explanatory section, Brian, it was great!

[Brian B-P]

Originally Posted by Brian Van Snell

See this reference:
What are the differences between otto cycle and diesel cycle

This isn't a terrible article, but it deviates from the reality of modern engines when it gets to the fuel issue. It also doesn't in any way address the original topic of this discussion thread, which it torque production. The fact that it does not attribute any torque advantage to diesels is interesting, especially if it is such a good explanation.

the diesel engine can use fuel that is not nearly as refined as the high-octane gasoline fuel (thus cheaper)

Diesel fuel is more dense (longer-chain hydrocarbons), was at some points in history less refined than gasoline, and was even cheaper when demand was lower. In fact, modern diesel fuel is a highly refined and critically spec'd product like gasoline, and is about as expensive. Don't believe me? Put home heating oil in your 2013 diesel truck's tank, and let us know how well it works and how many thousands that costs you!

It also glosses over some important information. Yes, explanation of how a diesel avoids knock and thus allows high compression ratios is correct:

The difference is that during compression phase, no fuel is present in the cylinder and thus no self-ignition can happen.

... but it fails to mention that the delayed combustion fundamentally reduces efficiency, or that the efficiency benefit of higher compression ratio falls off rapidly above the ratios commonly used in spark-ignition engines. The net effect of higher compression but longer burning time still typically comes out significantly in favour of diesel, but it is not nearly so simple as the article suggests.

Generally, I find that wiki Answers content is poor and superficial compared to other sources, although better than junk such as answers.yahoo.com.

[Brian B-P]

I dropped responses to this thread, because I realized a major difficulty in this discussion. We started with a conversation about distinctions between engine designs, in some cases relatively subtle; however, any such discussion is futile if participants are not starting with fundamental understanding of how an internal combustion engine works. This would include understanding that the pressure in the cylinder drives the piston.

The Wikipedia Internal Combustion Engine page isn't bad. I would be interested if anyone were to find an even more clear - and correct - fundamental description.

[semievolved]

now we know, that among those contributing to this thread, only brian b-p understands the principles of internal combustion engines and that the authors of the original article were all wrong about diesels, stroke, and torque.

brian b-p, if you are so much more knowledgeable about these topics than the rest of us, why don't you take some time to air your novel perspective by publishing your "work" in peer-reviewed journals and then please share those with us.

in the meantime, rather than accusing the authors of ignorance and misinformation, perhaps you should contact them publicly with your objections to their article and also post publicly their response.

that would be more honorable and illuminating than your taking potshots at them without their opportunity for rebuttal.

try to refrain from disparaging anyone who disagrees with your assertions.

[gmw photos]

It's probably not useful in these threads to launch personal attacks. I think most here are trying to explain a complex subject in a way that the average user can grasp.
This is in fact a very complex subject, and is therefore likely beyond the scope of a simple discussion on an internet forum.

Okay, having vented my thoughts on THAT

....here is a site that I have spent a fair amount of time reading. There is lots of good info here, and while it may not spell out direct differences between gasoline and diesel, it does provide a good foundation of understanding many of the finer details of the theory.

Mechanical Basics: Quick Review of the Fundamental Concepts, by EPI Inc.

To the original point of the thread, Brian is certainly spot on regarding the idea that the bottom line of power production of an internal combustion engine is largely dictated by how much oxygen can be made present during the combustion process. Forcing air with a blower, whether "mechanical" ( commonly called a supercharger ) or exhaust gas driven ( turbocharger ) is the common, expedient route to that. There are other methods, such as using oxygen bearing compounds ( nitrous oxide, and to a lesser extent, MTBE compounds ), but these introduce other issues.

Anyway, back to your regular programming.

[semievolved]

agree with what you write here. but the basic issue was not so much about power generation, which clearly is a function of how much oxygen can be burned. brian b-p had issues with the author's statement that longer stroke is important in generating more torque.

i believe much of the confusion in the discussion came from glossing the distinction between power and torque.

[Brian B-P]

Originally Posted by semievolved

now we know, that among those contributing to this thread, only brian b-p understands the principles of internal combustion engines and that the authors of the original article were all wrong about diesels, stroke, and torque.

I did not say any of that, nor do I believe any of that to be true, as I suspect everyone involved now realizes.

Originally Posted by semievolved

... air your novel perspective by publishing your "work" in peer-reviewed journals ...

... contact them publicly ...

... your taking potshots at them without their opportunity for rebuttal.

I'm not sure why my contribution to the discussion should be restricted only to peer-reviewed journals. Certainly no one else is subject to this restriction, and the Banks article is neither peer-reviewed nor published in any journal; it is essentially marketing material, although at a deeper technical level than usual, posted on their website. There are other articles there as well (some are linked on the right hand side of the page of the original article), with some excellent content. As with the original article, there is no suggestion that these have been validated by anyone outside of Banks, but I think they're still pretty good.

I don't see an invitation on the Banks site to comment on their material, and I would not expect any. Banks staff are free to join this forum and this discussion if they want. If discussion of - or at least disagreement with - their material is not allowed without their participation, then perhaps all discussion forums should be shut down

Originally Posted by semievolved

try to refrain from disparaging anyone who disagrees with your assertions.

It was not my intention to disparage anyone; my comments were sincere. We all start from different levels and extents of knowledge. If the discussion were on a different topic with which I were not familiar, I would need a fundamental reference to get me up to speed.

Originally Posted by gmw photos

....here is a site that I have spent a fair amount of time reading. There is lots of good info here, and while it may not spell out direct differences between gasoline and diesel, it does provide a good foundation of understanding many of the finer details of the theory.

Mechanical Basics: Quick Review of the Fundamental Concepts, by EPI Inc.

Thank you, George, for your insightful and well-stated remarks.

I have only started to look at the very extensive EPI site, but it looks very promising Unfortunately, it is more like an encyclopedia of engine design than an easy-to-understand fundamentals primer.

Originally Posted by semievolved

agree with what you write here. but the basic issue was not so much about power generation, which clearly is a function of how much oxygen can be burned. brian b-p had issues with the author's statement that longer stroke is important in generating more torque.

i believe much of the confusion in the discussion came from glossing the distinction between power and torque.

I agree that such fundamental distinctions are at the root of much confusion. That point comes up more than once in our discussion. I won't try to explain the difference, which would lead us back to my earlier remark which prompted such reaction.