Why Diesels Make So Much Torque - Page 2 - Fiberglass RV


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Old 06-27-2013, 02:02 PM   #15
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Originally Posted by Ray N View Post
There must be more to it than that. If that is all that is involved in putting out high torque engines, then why don't the companies make high-torque turbocharged gas engines?
Of course, there are many implications of any technical decision, so even though the answer really is just tubocharging, turbocharging is not simple.

Two direct answers:
  1. They do! Ford's EcoBoost (turbocharged, direct injection gasoline) V6 has a displacement of 3.5 L and produces 420 lb-ft of torque, while their 6.7 L PowerStroke (turbodiesel) V8 produces 800 lb-ft; that's the same torque output per unit of displacement.
  2. Diesel engines inherently work better (e.g. knock less) when boosted; spark-ignition engines are inherently harder to manage (e.g. avoid knock) when boosted.
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Old 06-27-2013, 02:10 PM   #16
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So the basic reason why diesels are such torque powerhouses boils down to one word, compression? (I guess it's really two words, high compression.)
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Old 06-27-2013, 02:22 PM   #17
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So the basic reason why diesels are such torque powerhouses boils down to one word, compression? (I guess it's really two words, high compression.)
No, not at all, if you're thinking compression ratio. Supercharging (which includes turbocharging) is about packing more air into the engine in each cycle, which allows it to burn more fuel, which means more energy, which means more torque. That's true regardless of the compression ratio, and as I noted above works in engines which are not diesels, too.

My point is that diesels are not "torque powerhouses" - large engines are, and supercharged (including turbocharged) engines are. Of course, all the diesels you see are turbocharged, and most are huge as well.
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Old 06-27-2013, 02:27 PM   #18
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Originally Posted by cpaharley2008 View Post
"high compression" means stronger components that last longer, diesels are high compression motors which gives higher torque at lower rpms.
A high compression ratio help specific output (torque, and thus power) somewhat, but is not important compared to the effect of supercharging.

Engines last as long as they are designed to last. You can make a gas engine that lives forever, just as you can make a disposable diesel.
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Old 06-27-2013, 02:36 PM   #19
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So, then can I assume that there is some reason gas engines can't have higher compression? (Pardon my ignorance since I'm not an automotive engineer.)

I just couldn't see why turbocharging is the reason "period," for diesels having so much torque since many gas engines are also turbocharged.
First, high compression ratio is not a huge advantage.

In a conventional spark-ignition engine the fuel is mixed with the air before it goes into the cylinder, so trying to get the compression ratio too high makes the fuel-air mixture burn prematurely - that preignition or "knock". In a diesel the fuel is not injected until it is time for combustion, at the top of the piston stroke and directly into the cylinder (that's what "direct injection" means).

Gasoline engines such as Ford's EcoBoost (and many others) are starting to go to direct injection. Without direct injection gas engines are limited by knock in how much turbocharging boost - and how high a compression ratio - they can use, so most turbocharged as engines up until recently have not had as much boost as a typical diesel.
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Old 06-27-2013, 02:51 PM   #20
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To start with I'm talking in general principles, so let not sweat the details until the discussion goes there. Thank you.

Let's not forget that the lower horsepower of a diesel. It's more to do with the fact that diesels have a lower limit on max engines speed. And that horsepower is directly tied to engine speed. And the reason for the lower engine speed is slower burn rate of the fuel.

So really if the Diesel engine could achieve a higher engine speed then the HP numbers would be higher and look more like a gasoline engine. Resulting in them not looking like torque powerhouses.

Jason
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Old 07-03-2013, 09:59 AM   #21
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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.
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Old 07-03-2013, 12:23 PM   #22
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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.

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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.
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Old 07-03-2013, 12:27 PM   #23
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A gallon of diesel fuel contains more BTUs of energy than a gallon of gasoline.
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Old 07-03-2013, 12:51 PM   #24
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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!
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Old 07-03-2013, 01:53 PM   #25
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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.
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Old 07-03-2013, 01:58 PM   #26
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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.
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Old 07-03-2013, 02:26 PM   #27
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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"?
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Old 07-03-2013, 06:30 PM   #28
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stroke is torque, baby!

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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.
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