Fuel mileage....without the sugar coating.

[cleatus12r]

Ok guys and gals. You may not have specifically ASKED for this, but you have been asking about it for a long time.
"My diesel truck gets XX miles per gallon. What's wrong with it?"

Short answer is "Probably nothing......" if it runs good, that is. That being said, there are a lot of truths out there and there are also a lot of false claims that don't hold any water whatsoever. Stories like, "My dealer says that there is a new flash coming out to address mileage issues". Yeah right.

I have people come in and talk to me all the time about their Duramax truck that only gets 15 MPG and their friend (who drives the exact same truck) gets 35 MPH loaded with 80K lbs. in his toyhauler trailer going uphill both ways at 80 miles per hour....yadda yadda. My inital response is usually something along these lines:
First off, your friend is a LIAR. Second, I'd pay good money to see that Dmax pull 80K uphill....even on level ground at 80 MPH. Third, I don't know of any geographic location where you can drive uphill to and from where you're going to or coming from. Lastly, 15 MPG is really good for your 300+ HP brick that you drive around town to get groceries with.
With a deer-in-the-headlights look, they spout off about someone's uncle (or maybe it's uncle dad) that says they heard from their engineer buddy that there will be a reflash to fix the mileage and get it "where it should be".

Where should the mileage be? In short, it takes a SPECIFIC amount of fuel to maintain a SPECIFIC engine RPM at a SPECIFIC load. This is true with gasoline engines, diesel engines, and possibly even search engines (I threw that in to see if you were paying attention). Any alteration of the load or RPM will affect the amount of fuel needed to sustain either one. I will throw out some arbitrary numbers for you and I so we can keep it simple (I get confused easily).


We'll use a 5.0L naturally aspirated diesel engine (for simplicity).
This engine at 100% volumetric efficiency has the capability to move 2.5L of air for every ONE revoloution of the crankshaft. It doesn't matter how many cylinders or valves per cylinder, it has the capability of moving 2.5L of air at 100% VE. Take the diesel's wide air/fuel mixture operating characteristics and input an AFR of 25:1.....a little math later and we find that for every revoloution at 25:1 AFR uses .1L of fuel. On the other hand, diesels in modern light duty trucks do not live their lives at a steady 25:1. Some run a lot at idle (40+:1 AFR) and some people like to run near 15:1 (got smoke?). Obviously, there is a LARGE range of fuel that a diesel engine can use at any time. And also, since we have a naturally aspirated engine here, volumetric efficiency will not be 100%, but at the most 85-90%.

There are a lot of variables, but let's say that the injection rate for our particular engine is 20mm3 at 30% load, 2300 RPM, 70 MPH (Arbitrary numbers). Regardless of outside influences, this set of variables will ALWAYS require 20mm3 of fuel. A SPECIFIC load and a SPECIFIC RPM equals a SPECIFIC amount of fuel. Ok, yes, air temperature does play a role.....but it's not earth-shattering. Let's increase the RPM of the engine a little to 2700 RPM. Ok, now our speed has increased as well, so we need to increase the engine load due to wind resistance, rolling resistance, etc. 40% load at 2700 RPM equals 27mm3 of fuel per injection now. Over the long haul, 20mm3 vs. 27mm3 will net a significant fuel mileage decrease.


People ask me if putting on a "chip" (or something similar) will help mileage. I say sure. If you drive slower, don't idle the engine anymore, and if you NEVER exceed 10% throttle at ANY time. Guess what, save for some slightly higher injection pressure that may atomize fuel more effectively, the SAME amount of fuel is required to make the same load/RPM equation possible. I suppose if you towed your truck behind a Honda Civic you might realize some SUBSTANTIAL mileage increase. Buying $1000+ worth of parts to make your truck go from 15 to 15 (intentional) MPG is pointless. If you want increased performance, oh yeah, buy away!! But buying for mileage is, at best, a side benefit from increased performance.....as long as you NEVER accelerate quickly.
EGR (exhaust gas recirculation) takes away from combustion efficiency greatly. The whole point of this.....pointless.....system is to cool the combustion event and lower the amount of NOx (oxides of nitrogen) emissions from the tailpipe. Guess what....diesel engines require heat to ignite the air fuel mixture. Also, anything that gets in the way of flame front propagation in the combustion chamber KILLS engine efficiency. Oh, and have you ever seen the nasty soot that comes from diesel combustion that is cooled too quickly (EGR cooler ring a bell 6.0 guys??)? It's not good for anything besides plugging up tubing and introducing abrasives into the cylinders. So, we're adding more fuel to make up for our inert gas that is slowing down our flame propagation........ makes sense to me. NOT.


Diesel Particulate Filters and the related programming/fuel injection that goes along with it introduces us to a new set of variables.....dumping raw fuel into the exhaust to superheat and "burn off" soot. This also makes a lot of sense.....let's burn way more fuel to reduce the amount of emissions. Huh? Something is backwards.
The ECM is requesting the injectors to fire once on the exhaust stroke to introduce raw fuel into the exhaust stream. The DOC (diesel oxidizing catalyst) collects this fuel and begins to glow due to the combustion that is now taking place inside. This superheated exhaust is directed into the DPF (particulate filter) in flame form, burning off the soot accumulation. Once the ECM is happy with a certain amount of backpressure between the DPF inlet and outlet, regen stops and we're back to making 350 HP. 350 HP out of a 6.4L diesel with no black smoke!!?!?!? That's what the filter does!!!
This won't be the last bit you hear from me.......It's been building inside ever since the LB7 Duramax was replaced with the LLY and the customers that "traded up" would always come in with this one; "I used to get 17-19 with my old truck. This one only gets 15." My response...... "You're paying for the increased HP."

If you think I'm some eco-nazi tree-hugger, you're mistaken. I am the proud owner of a 2001 7.3 Powerstroke. Would I "trade up", not on your life. I haven't left it stock, and I haven't lost mileage. But then again, I BOUGHT A TRUCK!!! I use it a lot to pull my wife's horse trailer (6 horse) and livestock. If I didn't need a truck, I wouldn't own it....I'd have a Pinto.
Peace.
Oh, and stay tuned for the next installment.

[versitek]

That is hard truth good post!

[JJPMach1]

I love it! Good post

[150ford]

Outstanding post Cody. I bet you get tired off hearing about mileage complaints day in an day out. with the high price off diesel its probably getting worse. Thats why Im keeping my 2003 7.3 for as long as possible. It gets good mileage for a big truck an no worries about regeneration like the new generation diesels. Rep points for you Cody. This one off the best informitive posts Ive seen on FTE. It really says it all. You should get a printout off what you just posted an hand them out to everyone that complains about fuel mileage. Nah dont do that. the truth hurts. People want to have there cake an it eat it too. Sorry things dont work that way.

[mnmwhit]

Cody is way smarter than me. WOW.
NOw my head hurts.

[fuzzy1626]

Thank you Cody. Nice read. Watching for next installment.

[scottman70]

Nice post! Or should I say novel? Holy crap that took me 27 minutes to read!

[framerboy]

That was a really excellent and well written post. I will look forward to more installments.

[IDMooseMan]

Thank you, Cody. Sometimes it is plain NICE to hear the truth rather than the, "My truck used to get 900 MPG and had 5,000 HP. I installed a HARDASUKAW chip and now I get 1200 MPG and have 8,000 HP." crap.

Like you said; I, too, bought a truck. I use it to haul / pull stuff. I get an average of 17.25 MPG. A bit less when I'm pulling the travel trailer. If a 1975 4-door Maverick could do that, I'd have kept the one my mom let me use. Since that car can't do what I need, (BTW...on it's best day, it never saw 17.25 MPG), I use a truck.

I'm subscribing to this thread and am waiting for your next installment. Reps sent.

[cleatus12r]

Ok, the next installment in the series is up.

[fordfish]

In your example, Doing 2300rpm at 70 = 4.10 gears, 2300 in 3.73 gears is what 80ish?
so you have added 10+ miles , gearing is a big factor.....don't forget.
what you say is true- so much volume take so much mixture, but finding the optimum volumes and mixture with the proper timing is how mpg's increase and decrease. I am not dissing your statement, I think there are many factors, but the rants on the 08's are that they suck fuel worse than the trucks we had....I made 350hp with my trade in and Avg. 16, I think the consensus is...."what the hell happened to my mpg's, thanks to the EPA they are lower".

[cleatus12r]

fordfish,

I think you missed my point. There is not a specific example (or existing vehicle) that I am using as an example. My numbers are purely pulled out of the air. If you read my second book, I go into a little detail about gearing.

I am not trying to use specific examples of why the mileage is worse on a new '07 and up truck vs. an older one. I am simply getting into the mechanics a little of what affects mileage on ALL diesel vehicles since I constantly have to hear the complaints. Like what was said earlier.....if the price of fuel wasn't so high, there wouldn't be so many complaints.

[fordfish]

Originally Posted by cleatus12r

fordfish,

I think you missed my point. There is not a specific example (or existing vehicle) that I am using as an example. My numbers are purely pulled out of the air. If you read my second book, I go into a little detail about gearing.

I am not trying to use specific examples of why the mileage is worse on a new '07 and up truck vs. an older one. I am simply getting into the mechanics a little of what affects mileage on ALL diesel vehicles since I constantly have to hear the complaints. Like what was said earlier.....if the price of fuel wasn't so high, there wouldn't be so many complaints.

That's not my view on the price being high...I ran 460's back in the day, I paid alot for fuel, but all the 460's got the same...10mpg. I just like to be effecient...I don't think these newer diesels are peak effecient...I gues my angst is Gov't Conspirecy, there is plenty of oil, we all now it, but yet, we are told of shortages, can't build more refineries,bla bla bla. Now these new diesel motors record fewer mpg's than the previous ones all in the name of record profits and taxes. sorry, went ranting... What effects mpg's on diesels effects mpg's on everything that uses a fuel ( cars, vans, trucks, jets
etc...) for power. I know what effects mileage, and I think most of us do too.

[DJR96]

Originally Posted by fordfish

"what the hell happened to my mpg's, thanks to the EPA they are lower".

The engines are fine, they are making more hp per unit of fuel. That's technology for you and there's still room for a lot of improvement yet too.

But the EPA must be owned by the oil companies.
I've said much the same as you Cody about using more fuel to improve emissions. Just crazy???

I believe if the engine designers could just concentrate on improving efficiency and mpg, emissions would be much lower by default as well as using less fuel.

What was the EPA thinking???

[iwannaford]

diesels don't get less fuel mileage with horsepower increase, because diesel fuel has more energy in it. as long as you drive some what decent.