A REASONABLE model for my (and maybe your) bus's fuel mileage

[JimGnitecki]

Cody: Regretably, you know too little about what you are talking about to actually contribute to this discussion, or you read my text too sloppily!

When I said that my 8V71 could not attain 80 mph in stock form, I was saying that because my stock 8V71 was governed to 2100 rpm. With the combiantion of my 3.73 gearing, Allison tranny, and 478 rev/mile tires, to hit 80 my engine would need to EXCEED that 2100 rpm governor speed! To do 80 mph, it would need to turn at 2377 rpm, and to do 85 mph, it would need to do 2526 rpm. In addition, the Caterpillar booklet, which I regard as much more credible than you, says very clearly that a coach roughly similar to mine needs more than 272 hp to hit 85 mph, if you add up the road resistance, air resistance, and fan power. I thought I made this very clear to anyone who is actually READING the text versus foaming at the mouth because someone is doing something differently than you like to do it. 

As for credentials, go take a long hike on a short pier. What are YOUR credentials, wiseguy? I don't owe you any credentials, just as you owe me none. If you like what I write, believe that my writing reflects some knowledge and/or ability, and believe I am credible, continue to read my postings. If you dislike what I write, or think I am not credible, just don't read my postings. It's that simple. But, get out of the way when intelligent people ask intelligent questions and look for better answers to old questions. If that bothers you, just stay off this thread. Then, neither of us has to listen to each other!

Jim G

[cody]

By your own statement you found yourself going 80 to 85, I have nothing more to say.  Your right I know nothing, I openly admit that, you should too.  I'm terribly sorry I offended or scared you, check your own math when you have time or read what you posted, and just where did you find diesel that weighs in at 71.5 pounds per gallon. I'm gone now, enjoy your thread, but even an unintellegent person like me can use a calculator. sorry if I bothered you.

[usbusin]

Jim G,

Thanks for the fresh approach to problem solving.  It's appreciated and stimulates the brain!  I know where you're coming from. Having been around racing and engines most of my younger life I enjoy looking at performance from a theoretical as well as an empirical standpoint.  Keep up the good work.

[RJ]

Jim -

Hmmmm. . .

Reverse engineer the numbers again with a coach weighing 20,633 lbs, 471 rev/mi tires, 4.125:1 final drive, 0.808 bevel gear ratio, & the following gear ratios: 1st = 4.32:1, 2nd = 2.50:1, 3rd = 1.50:1 and 4th 1.00:1.  Stock 8V71 w/ N60 injectors.

I'm curious.


Oh, and as a side note, looks like my "real world" fuel mileage figures that I shared with you in your other thread matched your modeling, eh?

FWIW & HTH. . .

 

[JimGnitecki]

RJ: This first early version of the model is not yet easy to quickly adapt to other buses and drivelines. To do that, I need to replace hard numbers in the model with alterable variables and tabular lookups. For eample, right now the only power curve in the model is the table for my own bus, and the only road and air resistance data is that from the Caterpillar booklet for a coach of not only my weight (about 34,000 lb) but also for only a limited rnage of road speeds (55 to 85 mph).

It takes many hours of work to build even a limited model like this one, and many, many more to make it capable of accepting other vehicles and a broader range of speeds. I also want to add the ability for it to automatically choose the best gear ratio for (a) cruising or (b) acceleration for any given speed, and to accept 4, 5, or 6 speed gearboxes. I also want to be able to model hills ranging from 1% to 6%, so that a user could compare performance on flat highways to that in hilly or mountainous areas.

Your request is a bit early! I will work on this model over time, but I have a high priority project that I am in the middle of - I am losing my house (I lost my job last fall and finding an new one in this economy has been a struggle), and have to concentrate FIRST on getting our possessions pared down to fit into the Eagle. I took some time to do this early rough model over the past couple of days because I LOVE doing this kind of stuff and I needed the break from the other somehwat depressing work!

Don't worry, I WILL take this a lot further and make it a lot more generically usable. I just need some time to do so. Once I have done that, it will be possible for me to model MANY people's buses with a LOT less time and effort being required for each.

I have corrected the typo that Cody was so sure proved I was stupid. I had misplaced the decimal point in my text, and had described the weight of diesel fuel as 71.5 pounds per gallon versus 7.15 pounds per gallon. The math within the model was of course correct all along, using the correct 7.15 pounds per gallon within the calculations (the results would have been VERY odd otherwise!).

I also apologize for the persistent and frequent transposition of letters as I type (e.g. letetrs versus letters). I continually have this problem, and these website forums do not have spell checkers built in like Microsoft Word does! I try to edit them out as I review my postings, but miss a few.

Jim G

[Len Silva]

I'm sorry, but I quit reading when you said your 8V-71 had a supercharger.

[cody]

Len that is only one of the areas I found difficult to accept,  Jim, I wish you well in your quest but more importantly I wish you better math, the weight of the diesel fuel was comical to me, nothing more than that, the math errors are a completely different matter, it appears people are blindly accepting your information as cast in stone and in time others will also double check them, thats all I had hoped for and I find that several have and have found the mistakes I did, regardless of my stupidity, I did pay close attention in college as many others here have also done, sometimes less time spend in front of a calculator and more time spent with a wench or steering wheel in your hand carrys some value too.

[JimGnitecki]

I'm sorry, but I quit reading when you said your 8V-71 had a supercharger.

Len: EVERY 8V71 has a supercharger. Some people call it a blower, some call it a supercharger. Although a 2-stroke engine can certainly function without one (like most 2-stroke motorcycle engines used to), the 2-stroke Detroit Diesel as designed won't work without it. Its prupose is to create positive pressure in the intake system so that when each piston is low enough in its cylinder to expose the intake ports cut into the cylinder walls, the pressurized intake charge is forced into the cylinder and also helps to evacuate the exhaust gases still in the cylinder.

Models of the V71 series of engines that have a "T" in their designation (e.g. 8V71T) ALSO have a turbocharger, to force even more air into the intake system. Models with "TT" in their designaiton have TWO turbochargers.

Models that have "TA" or TTA" in their designaitons have aftercoolers as well as turbochargers, in order to cool the compressed intake charge (the process of compressing the intake charge heats it up, and hot air makes less power than cold air, so it is very beneficial to cool it back down as much as possible).

Where's the problem here???

Jim G

[bobofthenorth]

I'm sorry, but I quit reading when you said your 8V-71 had a supercharger.

As far as I knew Len every 2 stroke diesel has a blower.  Yours is different?

[cody]

A quote from DD,, There are NO superchargers on the 8V-71 engine. ALL 2 stroke series engines from DDA have blowers. These are either gear or shaft/gear driven. This includes the series 53,71,92,110,149.  But what do they know, this is from one of their own service papers.  I won't be responding to this thread anymore, I learned a long time ago not to pee into the wind.

[Len Silva]

All right, first of all, I apologize, that was smarmy of me and not my style.

However, when someone refers to the blower on a DD two stroke as a supercharger, my perception is that they probably have never turned a wrench or gotten their hands dirty.  It's just a question of semantics but DD refers to it as a blower or scavenger even though the 6-71 blower is often used as a supercharger on gas engines.

I slink away now.

[JimGnitecki]

Len that is only one of the areas I found difficult to accept,  Jim, I wish you well in your quest but more importantly I wish you better math, the weight of the diesel fuel was comical to me, nothing more than that, the math errors are a completely different matter, it appears people are blindly accepting your information as cast in stone and in time others will also double check them, thats all I had hoped for and I find that several have and have found the mistakes I did, regardless of my stupidity, I did pay close attention in college as many others here have also done, sometimes less time spend in front of a calculator and more time spent with a wench or steering wheel in your hand carrys some value too.

Cody, I thought you said you were "leaving". Is this your ghost, or do you simply love being crotchety?

If there are math errors in my model, I would not be offended if someone pointed them out, but rather would aprpeciate it, as I want as accurate a model as it is reasonable to build. As I pointed out in my text, there are big gaps in the available data in both literature and on The Web, and I did say I had to make some assumptions (one was how an 8V71 performs when revved past its factory governor speed), and I did point out exactly where I did make those kinds of assumptions. We are not planning a space flight here, so having ALL the data EXACT is not that critical, but obviously the more accurate it is, the more accurate the calculated results will be. Since my predicted results so closely match the actual results obtained (basically, "right on"), I suspect I am not very far off anywhere.

If you see specific errors, please do us all a favor and point them out. If you are correct, I'll correct the model.

Please do note that virtually everything in the model is from sources like Detroit Diesel itself or Caterpillar, so before you throw too many stones make sure you yourself are right. I think the engineers who designed and built the engines, and the well resourced companies they are part of, probably know more about them than either you or me. I'm simply presenting the information in a way that makes it easier to understand and to use in a practical way.

I agree with your comment that spending time with a "wench" (your spelling) is more fun than with a calculator.

Jim G

[JackConrad]

I also apologize for the persistent and frequent transposition of letters as I type (e.g. letetrs versus letters). I continually have this problem, and these website forums do not have spell checkers built in like Microsoft Word does! I try to edit them out as I review my postings, but miss a few.
Jim G

Jim,
    I have the smae probleme!  I think it has smoething to do with my gettin' older?? LOL When you write a post look to the right of the box that you click on to post your message. There is a box that says Spell Check.  A former moderator, who passed away last year, was always a stickler for propper spelling and grammar. Are you watching over us Richard?  Jack
Errors intentionally left in this post

[Len Silva]

 

[JimGnitecki]

All right, first of all, I apologize, that was smarmy of me and not my style.

However, when someone refers to the blower on a DD two stroke as a supercharger, my perception is that they probably have never turned a wrench or gotten their hands dirty.  It's just a question of semantics but DD refers to it as a blower or scavenger even though the 6-71 blower is often used as a supercharger on gas engines.

I slink away now.

Len, please do NOT "slink away". I confess that the use of the term "sueprcharger" is a giveaway on my 42 years spent hotrodding cars, bikes, and pcikup trucks versus buses. All of us that have done that have got too used to calling the Roots type blowers "superchargers". They are indeed actually "blowers". The difference is sometimes subtle, but you are nevertheless correct on this fine point (and so is Cody therefore, despite his rudeness throughout this thread).

The actual technical definition of a supercharger is that it compresses the air INTERNALLY. A Roots type blower does not actually compress the air INTERNAL to itself. It instead pushes enough air into the intake tract to create pressurization THERE versus inside the blower housing. This is ebcause the Roots blower design was originally invented NOT to pressurize intake tarcts on engines, but rather to circulate fresh air into underground mine shafts. It was subsequently recognized as a great way to pressurize the intake tract of an engine. It is particularly attractive in an application like the Detroit Diesel 2-stroke engine where its combiantion of large volume and modest pressure is pretty ideal for a large displacement, slow revving engine.

Its design is also inherently "modular", just as the Detroit Diesel engine itself is. For the larger versions of the engines, simply make the blower larger. This is why these blowers come in a rnage of sizes (which the car hotrodders really appreciate!).

Most hotrodders don't care about this subtle distinction, as the end result is the same: a pressurized intake tract. But, you and Cody are correct in that I should have said "blower" versus "supercharger".

This terminology distinction has NO impact on the math.

Jim G