Does Clean Really = Reduced Economy?

[GaryCarter]

Not looking for a flame war here, but I want to challenge the assumption that the third gen engines have reduced fuel mileage due to the changes made to supposedly reduce emissions. Here's why:

In my personal experience- I have owned a second gen CTD, a 7. 3, and a 6. 0. Family members own 6. 6's. I keep careful track of mileage (hand calculated) and my current truck is the best mileage of the bunch, whether empty or towing. All of these trucks are 4 door 4x4's. My weight slips show this one is the heaviest of the bunch, also.

I get around 10 mpg at 17,000 gvw & 70 mph, 17 mpg around town, slightly under 20 running solo if I drive it right.



I want to ask for a couple of inputs- does anyone have the data on BSFC for this generation vs. previous generations? Cummins used to have this on their web site but I can't find it now. (Brake Specific Fuel Consumption is the fuel burn rate at a particular RPM and HP rating)

-If you've owned previous generations, has your own personal experience been that the new truck has better or worse mileage?



We all hear a lot about mileage issues, but as we all know, a dissatisfied owner will be a lot more vocal than a satisfied owner. I would like to take the pulse of the group.



Gary



PS- and let's not forget the effect of increased power; today's stock trucks are equal to a seriously bombed previous generation truck, so your right foot can spend a lot more fuel.

 

[klenger]

I get the same or better milage with my 04. 5 than I did with my 01. 5 HO and have more weight and 90 HP more power. There are plenty of reports of 305/555 owners with low milage too. I think the early low milage reports from the 600 owners was due to a batch of bad injectors. Don't hear too many reports of poor milage from the 05 and 06's.

 

[RankRam]

I get the same or better milage with my 04. 5 than I did with my 01. 5 HO and have more weight and 90 HP more power. There are plenty of reports of 305/555 owners with low milage too. I think the early low milage reports from the 600 owners was due to a batch of bad injectors. Don't hear too many reports of poor milage from the 05 and 06's.

I agree with ya Ken, my 05 gets the same mileage as my 03 and 99 did. If I stay below 70 I get between 20 and 21 mpg. Towing I have always gotten around 10 to 11. (10k trailers towing 65-70 mph)

 

[Vaughn MacKenzie]

I've been on TDR for 6 years and MPG gripe threads go back to the beginning. . . back then people complained about how rotten their mileage was compared to the 12-valve, but when you really read into it the 12v only gets maybe a skosh better mileage overall IMO.



Vaughn

 

[BHolm]

I had an '03 and '05 around at the same time for a while. In the same conditions, same gear ratios(3. 73), transmission(48re) and 285 BFG tires my '03 easily knocked down 3-4 mpg better than my '05. The '03 is slightly louder without the extra injection events but without a doubt was capable of better milage than the '05 consistantly. Both trucks did much better than my previous '03 Chev D/A. I could only muster 13-15 empty with the Chev. My '03 QC short box was getting 20-22 empty and the '05 QC LB was getting 16-18. My 30 mile drive home from the dealer last night in the '06 Mega showed 14 mpg on the overhead. I need to get it calibrated for the 285 tires though and check the overheads accuracy with some more fill ups.



Personally I think the '03-04 trucks are the best balance of economy and performance. My '03 went like stink(stock) and returned great milage for a 7000 lb 4x4 truck.

 

[jwduke]

I have to agree with you too, Ken. My '04. 5 w/automatic is getting 19. 4 mpg, and I have only 11,300 miles on the clock.

 

[robert4570]

I kept handwritten fuel/mileage logs for my 99 drw 3500 ctd auto since day one .

I am currently doing the same for my 05' 6spd drw . The mileage is very close for both on the highway averaging 19 mpg .

I did a empty test run with my 99 at 55 mph all highway at got 24 mpg hand calculated ! Not convinced thinking i somehow recorded something wrong ,did the same run and got 23 mpg . Obviously as your speed increases your mileage will suffer .



In conclusion ,both are gettin similar mpg's . However if you drive at 75 - 80 mph or higher ,dont expect great mileage .

 

[Sled Dog]

I get the same or better milage with my 04. 5 than I did with my 01. 5 HO and have more weight and 90 HP more power. There are plenty of reports of 305/555 owners with low milage too. I think the early low milage reports from the 600 owners was due to a batch of bad injectors. Don't hear too many reports of poor milage from the 05 and 06's.

What's your build date?

 

[EBottema]

Common sense says that when you have double or triple injection phases you will use MORE fuel. That is the most simple reason for lower MPG. The use of these phases were required by new emission standards. ANYTHING like this makes the vehicle more complicated is a BAD thing...

 

[rbattelle]

I want to ask for a couple of inputs- does anyone have the data on BSFC for this generation vs. previous generations?

This thread is the only one in the 3rd gen forums I'm aware of that addressed the issue. As you can see, it fizzled out.



-Ryan

 

[Hohn]

There are a couple things at work, some with negative effects on mpg, other with positive effects-- where they balance is up to each driver.



First, the trucks themselves have become bigger and heavier. mpg: down



Then, the engine changes. Technology has advanced partly BECAUSE of emissions. If it weren't for the emissions legislation we all hate, the HPCR would probably not exist-- nor would many other wonderful technologies.



Emissions legislation has driven engineering that had been both good and bad for MPG. Since NOx is a major smog gas (and targeted by EPA), Cummins has had to focus on reducing peak combustion temps, which is a big negative for overall thermal efficiency. As you probably know, ideal (or Carnot) efficiency is maximized when there is the largest difference between temps in the cumbustion process.



So ideally, you want to have the hottest peak combustion temps with the coolest EGT. That's the recipe for max thermal efficiency. It's also a recipe for very high NOx emissions, which the EPA says you can't have.



So to reduce NOx, they have to reduce the peak combustion temps. This is done by retarding injection and cam timing. Compression ratio is raised to somewhat compensate for this (notice how each generation of CTD has had higher compression?).



Cummins is also forced to design in higher EGTs to help the exhaust catalyst light off faster. This further hurts overall thermal efficiency (remember, ideal is hot combustion and cold exhaust, while emissions considerations push us towards colder combustion and hotter exhaust). Thus, the third injection event on the HPCR engines.



Another piece of the puzzle as far as reducing peak combustion temps is rate-of-injection. In other words, if we slow down the rate that the injector flows, the given quantity of fuel will take longer to inject, drawing the combustion event out over a longer period of time and reducing peak combustion temps.



This is why VP44 HO trucks have SMALLER injectors than the SO versions, and why they have higher compression ratios! It's also why the HO vp is higher injection pressure-- it has to be to use the smaller injector!



So here's the general trend you will see as emissions standards toughen:

-- higher compression ratio

-- retarded injection/cam timing

-- higher boost pressures

-- smaller injectors

-- higher injection pressure



Higher injection pressure is a good thing, and by itself would be capable of significant mpg and overall efficiency improvements.



By when combine with all the other design elements, higher injection pressure is NOT enough to increase MPG overall.





If Cummins was free to ignore all emissions aspects and was free to engineer an engine for max MPG, the HPCR could easily deliver 25-28 mpg hwy. In fact, I'd think you could get some pretty amazing mileage numbers out of an 03-04 HPCR truck with bigger injectors, advanced timing, proper cam timing, etc...



Justin

 

[wxman]

Caterpillar has identified "The Path to a 50% Thermal Efficient Engine", the presentation of which is available here. This is while meeting the 2010 HD emission regs (0. 2 g/bhp-hr NOx; 0. 01 g/bhp-hr PM).



Cummins is also working on a 50% BTE HD engine meeting the 2010 regs ( http://www.greencarcongress.com/2005/10/cummins_targeti.html#more ).

 

[klenger]

What's your build date?

1st week of 2004. :-laf





I think another factor that skews the milage results for members who are trading up is that their old trucks (what ever gen/engine) were well broken in. The new ones always seem kind of sluggish and have bad milage in comparison. Then we hear "my new truck doesn't get near as good of milage as my old one". After 10k or 20k, the new one is nicely broken in and getting respectable milage, but we don't hear from the owner any more because it's not new and the milage improved slowly. That's my story and I'm sticking with it.

 

[jimnance]

Hohn, I think that is one of the best explainations of the dynamics of engine efficiency I've read here. I wasn't aware that the C/R if the '05's was higher than earlier engines. Great information. Filled in some holes in my non-engineer brain. <grin>

 

[GaryCarter]

Thanks for all the informative replies. I'll check back again after we're finished doing the Wilma thing.

I expect to be in the dark for a while.



Gary.

 

[klenger]

You're going to let a little bit of bad weather keep you off the TDR? Where are your priorties? Be safe.

 

[robert4570]

Thanks for all the informative replies. I'll check back again after we're finished doing the Wilma thing.

I expect to be in the dark for a while.



Gary.

Same here , tired of waitin for this damn storm to arrive . At least we've had plenty of time to prep . Hope all goes well .

Matter of time before power goes out . :{

 

[GaryCarter]

Welllllll... that was an interesting ten days. No damage here but trees. No power for a few days either.

Tooooooo much work.



Back on topic, I believe the changes over time have just about made it a wash. For example more valves means better air flow but more parasitic drag. Lower combustion temps generally coincide with best economy, but if you're burning hydrocarbons in the catalytic converter, they're not helping propel the vehicle.

I think if we could get the BSFC figures out of Cummins they would be pretty close to earlier generations.

One of the things big brother could help us out with would be to require vehicles in this class to publish a meaningful MPG rating. For example with just passengers and also towing a standard 10k pound load.

Don't hold your breath though...



Gary

 

[Hohn]

Welllllll... that was an interesting ten days. No damage here but trees. No power for a few days either.

Tooooooo much work.



Back on topic, I believe the changes over time have just about made it a wash. For example more valves means better air flow but more parasitic drag. Lower combustion temps generally coincide with best economy, but if you're burning hydrocarbons in the catalytic converter, they're not helping propel the vehicle.

I think if we could get the BSFC figures out of Cummins they would be pretty close to earlier generations.

One of the things big brother could help us out with would be to require vehicles in this class to publish a meaningful MPG rating. For example with just passengers and also towing a standard 10k pound load.

Don't hold your breath though...



Gary

More valves don't necessarily mean more parasitic drag. The 24 Valves are lighter, and this means you can use a lighter spring. I can't say if this offsets, but it might be closer than you think.



Best BSFC number i have seen belong to the P7100 trucks. Better than 1st gen VE pumps, better than VP44, and better than CP3 HPCR.



If HPCR was optimized for BSFC above all else, no doubt it should have the best numbers because of it's higher pressure and more sophisticated injection system.

 

[JStraw]

I've been on TDR for 6 years and MPG gripe threads go back to the beginning. . . back then people complained about how rotten their mileage was compared to the 12-valve, but when you really read into it the 12v only gets maybe a skosh better mileage overall IMO.



Vaughn

I can attest to your input. My '93 and '04. 5 get identical mileage.