Gas vs PSD

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Jun 3, 2010 | 12:37 PM

#4591

miller_feed

Hook it, I'll pull it.

Joined: May 2007

Posts: 15,037

Likes: 4

From: Grand Lake, La.

Quote:

Originally Posted by Johnny Langton

Just to clarify for those that don't understand it-this is the direct result of forced induction, and is not an indicator of a fuel or engine's superiority.
JL

This don't make any since to me what so ever. It's like saying the V-10 is better because it has spark plugs.

. It is what it is.

Jun 3, 2010 | 01:09 PM

#4592

Spinaladjuster

New User

Joined: Jun 2010

Posts: 4

Likes: 0

I had a gas burner and now I got a PSD. I love my diesel more then that v10 in so many ways.
I haven't read the last 307 pages but I will have to go back, I just skimmed over them and got the basics.

Jun 3, 2010 | 02:33 PM

#4593

CampSpringsJohn

Hotshot

Joined: Dec 2006

Posts: 14,069

Likes: 17

From: Melbourne, Ky

Hey Spinaladjuster, welcome to FTE! You happened onto a great thread! Wouldn't you know it, 307 pages into it and we've yet to get 100% agreement! It's almost as bad as talking politics,,,

Jun 3, 2010 | 02:37 PM

#4594

Jrfish007

Elder User

Joined: May 2010

Posts: 650

Likes: 0

From: WV

Quote:

Originally Posted by Johnny Langton

Don't start tiptoeing around the facts.
Forced induction via a turbocharger is the key reason a turbodiesel is able to pull a load at 2000rpms down the highway and maintain speed without having to downshift.Increasing boost into the intake tract has the exact same result as increasing displacement by that same amount of airflow. Gasoline,Diesel,Propane,Nat Gas,etc,etc...the fuel does not matter-you'll get the same result every time. As boost increases,so does power and torque. A vehicle's ability to maintain speed under load is directly proportional to the amount of boost that the engine receives from the turbocharger, etc. If the turbo isn't the key, then why aren't there any N/A diesel tractor-trailers out there? Why don't the automakers have N/A diesels in their trucks anymore?
I know it's not a simple as slap a turbo on anything and go-the entire fuel and induction system must be designed for that purpose, and/or modified for that use.
JL

I'm not tiptoeing anything... But you just said it's not as simple as slapping a turbo on.

The ability of the vehicle to maintain speed while pulling up hill or whatever has to do with the power it can produce under those specific conditions. You CAN get a big enough N/A gasser to out pull even the 6.7L if you wanted too, it will cost you some big $$$, but it COULD be done.

I do disagree on the fact that adding boost is the same as adding displacement though. In fact most boosted engines will have (or can have if tuned for it) a much flatter torque curve which will be far better for towing. If you add displacement to the N/A engine you are more likely to get a "swelling" torque curve unless you add cylinders and adjust your cam timing. This swell will mean the engine will probably hunt gears to stay in the sweet spot of that torque curve. This is why Ford went with a V10 instead of a big V8 many years ago, to help flatten the torque curve. This is probably what you ment by the turbo being key, and I would agree to that extent.

Gas engines, however do have one problem diesel don't, and that's RPM's. Even with costly turbo's, gas engines still get most of their power at higher RPM's which is not desirable for towing (burn a lot of gas and runs your engine down faster). Diesels can produce this power at a lower RPM do to higher energy values per volume of fuel and compression ignition to help keep fuel mileage in check (in a relative way) and keep wear and tare to a minimal.

In order to make use of this though, the engine must be designed and optimized in a different fashion. The optimal engine design for a diesel (turbo or not) is not going to be anything like one for gas. For instance, try to run 30 psi boost on any gas motor with pump fuel... Much less 30 psi with gas that has an octane rating of 40 like diesel... These traits allow a diesel engine to use larger amounts of boost. As you mentioned, the turbo is key, it just not the only key. The turbo must be integrated with many other things to make use of it's full potential.

I guess my final point is that trying to point at any one specific reason why a turbo diesel will pull better than a N/A gas engine is not really a good idea. There are many design factors that go into each engine to optimize it for it's intended use, it's fuel, and most importantly it's market.

The perfect example of this is the late 70's/early 80's diesels truck (these were not always turbos, so comparing N/A to N/A). Diesels were basically gas engines converted to diesels, most even had the same intake manifolds. The diesel could pull like beast compared to the gas, even though they where basically the same engine, just the diesel was beefed up for the energy of the fuel. Of course, the diesels always broke down in massive ways because it wasn't designed for that type of power on a consistent basis.

Jun 3, 2010 | 07:17 PM

#4595

Sand_Man

Posting Guru

Joined: Oct 2009

Posts: 2,066

Likes: 4

From: Rogue River, Oregon

I've driven unblown diesels before just like you describe and not one "pulled like a beast" as you say. In fact, I'd be hard-pressed to even figure out how on earth they sold the second one! Terrible motors that split blocks like they were made of wood and didn't make power. They were a far cry from today's 1,000 lb, turbocharged monsters in more ways than one.

Jun 3, 2010 | 08:05 PM

#4596

bucci

Laughing Gas

Joined: Mar 2009

Posts: 843

Likes: 6

From: Jessup, PA

Quote:

Originally Posted by Sand_Man

If you didn't say it, I was going to.

Jun 3, 2010 | 08:16 PM

#4597

M88

Mountain Pass

Joined: Dec 2009

Posts: 196

Likes: 1

From: Central NY

Quote:

Originally Posted by Sand_Man

I'm thinking of the 6.9 and 6.2. Weren't those more for fuel economy anyway? Pulling was an afterthought?

Jun 3, 2010 | 08:52 PM

#4598

Sand_Man

Posting Guru

Joined: Oct 2009

Posts: 2,066

Likes: 4

From: Rogue River, Oregon

The entire engine was an afterthought! A freind of mine works in an automotive machine shop and he told me that he had never seen an early converted diesel of any kind that didn't have a cracked block. Couple that with the fact they were quite gutless and you have the worst of both worlds! BUt yes, I do beleive they got good mileage.

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Jun 3, 2010 | 09:15 PM

#4599

Lead Head

Lead Driver

Joined: Jul 2007

Posts: 7,868

Likes: 9

From: Rhode Island

N/A diesels never pulled like a beast compared to gas counterparts. Everyone I know with an N/A diesel said the gas equivalent of the same truck always pulled harder. (very few diesels as far as I know were actually just literally converted gas engines, even the Olds 5.7 is very different then a gas 350). They were somewhat more torquey, but not really any more so then an equivalent displacement big block. The main advantage was improved fuel economy.

Don't quote me for sure, but I believe the main reason why Diesel's can't make any power N/A is due to the need for a relatively flat head to maintain high compression (valve heads parallel with the piston head isn't really ideal for efficient air flow) and the fact that they don't rev.

Jun 4, 2010 | 01:16 AM

#4600

Jrfish007

Elder User

Joined: May 2010

Posts: 650

Likes: 0

From: WV

Yes... I supose pulling like a beast may have been an over statement, but it was a comparative term. Comparing them to simliar displacment gas engines with a simliar style of engine. We had one in the late 70's that cracked the block (ended up loosing 2 of the 8 cylinders) and still pulled a 12,000 lb trailer over 800 miles, I'm pretty confident the gas version of that engine couldn't have done it on 6 cylinders.

Quote:

Originally Posted by Lead Head

Don't quote me for sure, but I believe the main reason why Diesel's can't make any power N/A is due to the need for a relatively flat head to maintain high compression (valve heads parallel with the piston head isn't really ideal for efficient air flow) and the fact that they don't rev.

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ther; mso-font-pitch:variable; mso-font-signature:0 0 0 0 0 0;} @font-face {font-family:Calibri; panose-1:2 15 5 2 2 2 4 3 2 4; mso-font-charset:0; mso-generic-font-family:swiss; mso-font-pitch:variable; mso-font-signature:-520092929 1073786111 9 0 415 0;} /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-unhide:no; mso-style-qformat:yes; mso-style-parent:""; margin-top:0cm; margin-right:0cm; margin-bottom:10.0pt; margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:Calibri; mso-fareast-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-fareast-language:EN-US;} .MsoChpDefault {mso-style-type:export-only; mso-default-props:yes; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:Calibri; mso-fareast-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-fareast-language:EN-US;} .MsoPapDefault {mso-style-type:export-only; margin-bottom:10.0pt; line-height:115%;} @page Section1 {size:612.0pt 792.0pt; margin:72.0pt 72.0pt 72.0pt 72.0pt; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> </style><!--[if gte mso 10]> <style> /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority

9; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} </style> <![endif]-->Yes, most diesels cannot achieve the higher RPM's. The fuel it's self burns slower, so it's much easier to tune the engine to get peak power at lower RPMs.

Additionally, if you look at the chemistry of diesel, it is a longer carbon chain (i.e. more carbons per a molecule). Diesel is generally around 20 carbons per molecule compared to gas which is about 7-9 carbons. When burned, these carbons turn into CO2. So we are taking a liquid fuel and turning it into a gas, this creates lot of pressure. This pressure pushes down on the cylinder creating force which turn the crankshaft and so on. Obviously the more of this CO2 you can produce from a single gallon of fuel, the more power you can produce, hence the reason diesel has more power per volume. This fact that when gas burn it expands about 8 times and diesel expands about 20 times it's initial volume is the reason diesel engines have to built so much tougher, there is quite a bit more force be produced.

This longer carbon chain is also what allows it to burn slower which makes it hard to rev very high. Think of the big log on a fire versus the little twig. If you take 10 lbs of twigs and toss it on a fire, you get a lot of action quickly and it dies down shortly after. Take a 10lb log and it burns slowly for a long time. The burn rate is similar between gas and diesel, hence high RPM's just aren't feasible, nor desirable.

Coming back to the heads you mentioned, yes, different style of heads give different gas/fuel mixing characteristics. Since few diesels are designed to rev high, the heads are generally designed for lower flow. Gas engines on the other hand need to rev higher to make their peak power so they need heads designed for higher flowing and mixing.

Back to the previous post:
This ability to generate more power at low RPM's is why an apples to apples comparison between a diesel and a gas engine for towing will favor a diesel. Just those comparisons are not easy to find, as pointed out

Adding a turbo, as mentined before, will further maginfy this difference because the gas engine will have to produce more RPM's, consume more fuel, and cause more wear to maintain near peak power when compared to a diesel.

Jun 4, 2010 | 01:22 AM

#4601

Jrfish007

Elder User

Joined: May 2010

Posts: 650

Likes: 0

From: WV

Quote:

Originally Posted by Sand_Man

BTW, I didn't mean pull like muscle car pull, I was refering to towing ability. As for speed, they were beyond pathetic with their tiny amounts of HP. Hook a trailer up to an old diesel though and it would out tow its gas conterpart, at least in my families experience. The gassers could do it but were always hunting gears and revving like mad.

Jun 4, 2010 | 07:39 AM

#4602

Sand_Man

Posting Guru

Joined: Oct 2009

Posts: 2,066

Likes: 4

From: Rogue River, Oregon

Sorry, but again, my experience is wildly different. My father-in-law's GMC diesel (early '80's) couldn't pull or haul any better than my old Mazda B2200. Loaded or unloaded, it was a huge dog at any RPM or any amount of load.

Jun 4, 2010 | 07:49 AM

#4603

krewat

FTE Leadership Emeritus

Joined: Jul 2002

Posts: 42,561

Likes: 423

From: Long Island USA

Quote:

Originally Posted by Jrfish007

This longer carbon chain is also what allows it to burn slower which makes it hard to rev very high. Think of the big log on a fire versus the little twig. If you take 10 lbs of twigs and toss it on a fire, you get a lot of action quickly and it dies down shortly after. Take a 10lb log and it burns slowly for a long time. The burn rate is similar between gas and diesel, hence high RPM's just aren't feasible, nor desirable.

FINALLY, you hit the nail on the head. Without saying "BTUs"

Jun 4, 2010 | 12:21 PM

#4604

Jrfish007

Elder User

Joined: May 2010

Posts: 650

Likes: 0

From: WV

Quote:

Originally Posted by Sand_Man

given thier reailabilty, I bet their actual power ratings came with a 50% error.

So it doesn't surprise me. But my experience was primarily a 454 versus the GM "big" diesel, Iforget the size, I would have to look it up and am about to leave on a trip for the weekend so I don't have time. Might have been the 6.2, but I forget.