Why Diesels Make So Much Torque

[Ray N]

This article explains a lot: Banks Power | Why Diesels Make So Much Torque

Also, 222 mph in a diesel Dakota! - I wonder what it's tow rating is?

[cpaharley2008]

very interesting article, thanks

[Brian B-P]

Even without reading the article, I can answer the title question: turbocharging. Period.

They work well with any form of supercharger - better than a spark-ignition engine - and the turbo puts in more air per cycle, allowing the generation of more power per cycle, which is torque. Banks has this right in their article, but they also have some crap about stroke, and they list cylinder pressure which is a result of the boost.

The stroke thing is a fundamental misunderstanding of simple geometry, or an attempt to exploit the reader's similar misunderstanding. Cylinder pressure multiplied by cylinder displacement is torque, whether it is a long stoke multiplied by a small piston area (area times pressure equals force) or a short stroke multiplied by a larger piston area. Whatever the cause, this is a common misconception... and of course even if it mattered a spark-ignition engine can be built in long-stroke form as well, so it's not a diesel advantage.

The timing of combustion stuff is also gobblegook - the extended combustion period of a diesel is actually a disadvantage, reducing thermodynamic efficiency and thus power (and torque) production. The details of that are beyond this forum.


Turbocharging increases torque in diesels, and from what I have heard Banks is a first rate turbo diesel tuning operation... just don't uncriticall accept their explanations.

[Brian B-P]

Originally Posted by Ray N

222 mph in a diesel Dakota! - I wonder what it's tow rating is?

Zero... at 222 mph!

On the other hand, Diesel Power magazine did an article about their 141.998 mph record towing stunt.

[cpaharley2008]

Do all diesels use Turbo's? Do not the stock, non turbo also produce more torque than a similar sized engine?

[Darwin Maring]

cpaharley2008: Had a Ford heavy duty 3/4 that came without a Turbo. Had Banks install their turbo and other stuff.

[Brian B-P]

Originally Posted by cpaharley2008

Do all diesels use Turbo's?

In modern vehicles, yes. A normally aspirated (not supercharged/turbocharged) diesel engine would not make sense under current conditions. You can find some small diesel equipment without turbochargers, just to minimize cost and complexity.

Originally Posted by cpaharley2008

Do not the stock, non turbo also produce more torque than a similar sized [gasoline] engine?

(I assumed the intended insertion of "gasoline" above)
No, although this is a popular misconception. It has been a long time since non-turbo cars and trucks were available, but comparing very similar engines - such as gasoline and diesel versions of the same engine from companies such as Nissan - diesel puts out less torque (and thus less power) than gas every time, primarily because of the slow combustion effect which Banks claims is an advantage.

The best known example might be the old VW Rabbit, which was fine as a gas engine and a slug as a diesel of the same displacement.

[MC1]

Ray, that is an impressive pick up.... Banks Power | Banks Sidewinder Dakota - The World’s Fastest Diesel Truck

[Ray N]

Originally Posted by MC1

Ray, that is an impressive pick up.... Banks Power | Banks Sidewinder Dakota - The World’s Fastest Diesel Truck

I loved the caption under the photograph in the original reference, that is: "Most competitors at Bonneville aren't street legal, and most arrive on a trailer. Project Sidewinder drove in pulling a trailer!"

[George Nagel]

Interesting article, thanks.

About 30 years ago I had a truck with a non-turbo diesel. What a dog! I couldn't keep transmissions in it, kept burning out torque converters.

[Thomas G.]

Originally Posted by George Nagel

Interesting article, thanks.

About 30 years ago I had a truck with a non-turbo diesel. What a dog! I couldn't keep transmissions in it, kept burning out torque converters.

The slowest vehicle that I think that I have ever driven was a Ranger diesel, circa 1983. It seemed like it would barely make it through an intersection controlled by a traffic light with a short green.

[Ray N]

Originally Posted by Brian B-P

Even without reading the article, I can answer the title question: turbocharging. Period.

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?

[cpaharley2008]

"high compression" means stronger components that last longer, diesels are high compression motors which gives higher torque at lower rpms.

[Ray N]

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.

[Brian B-P]

Originally Posted by Ray N

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.

[Ray N]

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.)

[Brian B-P]

Originally Posted by Ray N

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.

[Brian B-P]

Originally Posted by cpaharley2008

"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.

[Brian B-P]

Originally Posted by Ray N

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.

[SilverGhost]

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