MPG on standard output "efficiency" Cummins - Dodge Cummins Diesel Forum
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post #1 of 86 (permalink) Old 07-09-2019, 11:27 AM Thread Starter
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MPG on standard output "efficiency" Cummins

It appears that Cummins is doing the same thing to their 6.7L pick up engine that they have been doing with the larger on highway engines for the past few years by making a performance version and an efficient version of the same engine. The performance versions have higher power levels at the expense of lower compression ratio to keep NOx levels within standards. The efficient versions generally have higher compression ratios at the expense of power.

For those that do not know, compression ratio has a direct correlation with an engines efficiency and how much NOx it produces at a given power output. Essentially the more power you want to produce in stock form, the more NOx you create so to combat this you have to lower compression ratio which reduces efficiency.

With the 2019 Cummins, the 400 hp "performance" version with the Aisin comes with a 16.2:1 compression ratio(down from 17.3:1 from 2007-2018) while the 370 hp "efficient" version with the 68RFE comes with a 19.0:1 compression ratio which is considerably higher than any pickup diesel engine. It appears Ram is not calling them performance or efficient version like Cummins does with their other engines.

Anyone with a standard output Cummins with the higher compression ratio see any improvement in mileage? Just curious.

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post #2 of 86 (permalink) Old 07-09-2019, 09:54 PM
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I was hoping this post would spark some discussion. I am interested in hearing more about this topic. After driving the SO and the HO I was sold on the HO option and we have been looking for a truck with all of our options locally with no luck so far. I keep following posts and discussions between the two versions looking for more info on mpg and DEF usage. I would have thought that the lower compression engine might use less DEF to combat the NOX but it seems the lower compression HO engine is thirsty on both sides DEF and MPG. The HO was almost acceptable power wise in stock form but the SO just felt a little lack luster when comparing to my 2012 with just a mild tune and VB with trans tuning. Again I'm very interested in this topic and hope to hear more real world results from current owners!

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post #3 of 86 (permalink) Old 07-09-2019, 11:58 PM Thread Starter
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If I knew I was going to eventually tune/delete it like I did with my 2014 after just 25k miles, then I would get the standard output and have the best of both worlds of power and better fuel efficiency. Higher compression ratios generally equates to more power(up to a point) if you are not trying to stay emissions legal.

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post #4 of 86 (permalink) Old 07-10-2019, 09:52 AM
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I cannot speak for the 2019 Standard Output MPG Ratings. However, the fuel efficiency ratings so far on my 2019 3500 Mega SRW HO/Aisin are 16.6 MPG overall @ 5200 miles (varied usage via the lie-o-meter), & 19.1 MPG on last weeks approx. 1800 mile all highway trip. That trip was running 75-80 average with light cargo in the bed (hand calculated average between 4 fuel stops). Those numbers are slightly less than than my previous 2015 2500 Mega under the same conditions. DEF usage does seem higher on the 2019 vs. my 2015, but I cannot be certain since I only topped off the DEF tank at my last fuel stop. I had used just the 2.5 gallon DEF jugs before. It seems that around town and towing that the HO in my 2019 does use slightly more fuel than my 2015, but it absolutely has more power and faster boost spool-up as well. On the highway, I think that most of the slight loss I see in MPG between my 2015 & 2019 are due to the Gear Ratio change (2019 3:73 vs 2015 3:42) and not the motor. Maybe someone that has had the same truck/gears between year models can provide a better analysis. I can say that it that every CTD I've owned seems to get slightly better MPG after its had some miles put on it, compared to when they are brand new. I guess time will tell.

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post #5 of 86 (permalink) Old 07-10-2019, 11:09 AM
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The high output engine has the same compression ratio as the final generation of the TDI. The final generation of the TDI is the most efficient.

The NOx isn't much of a concern for the engines since Cummins has a very efficient SCR system and EGR if necessary. The higher injection pressure over the 2018 will help give more tuning flexibility for optimizing different configurations.

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post #6 of 86 (permalink) Old 07-10-2019, 12:57 PM Thread Starter
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I thought the fact that VW allowing the later TDI's to emit more NOx when not hooked up to an emissions tester is what lead to its better fuel mileage versus previous iterations. Also wasn't all 4 cylinder TDI's had a compression ratio of 18:1 or greater?

NOx is a major concern with stock tuning even with an SCR. The higher the compression ratio at a given power level, the higher the NOx output versus the same power level with a lower compression ratio. Hence the reason why the difference in compression ratios and power levels in all Cummins efficient version and performance version engines.

We have had these two versions in multiple Cummins engines(X15, X12, L9, and B6.7) in the medium/heavy duty market since 2017 and there is a noticeable difference in fuel in the fuel economy of the two versions from the feed back I am getting from customers. Generally a 10-15% difference depending on the engine size and application.

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post #7 of 86 (permalink) Old 07-10-2019, 07:49 PM
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You guys need to talk to Cummins engineers, and if you can find one that will talk to you, they will be the only accurate source of information.

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post #8 of 86 (permalink) Old 07-10-2019, 08:39 PM
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Quote:
Originally Posted by BigBlueMega View Post
I cannot speak for the 2019 Standard Output MPG Ratings. However, the fuel efficiency ratings so far on my 2019 3500 Mega SRW HO/Aisin are 16.6 MPG overall @ 5200 miles (varied usage via the lie-o-meter), & 19.1 MPG on last weeks approx. 1800 mile all highway trip. That trip was running 75-80 average with light cargo in the bed (hand calculated average between 4 fuel stops). Those numbers are slightly less than than my previous 2015 2500 Mega under the same conditions. DEF usage does seem higher on the 2019 vs. my 2015, but I cannot be certain since I only topped off the DEF tank at my last fuel stop. I had used just the 2.5 gallon DEF jugs before. It seems that around town and towing that the HO in my 2019 does use slightly more fuel than my 2015, but it absolutely has more power and faster boost spool-up as well. On the highway, I think that most of the slight loss I see in MPG between my 2015 & 2019 are due to the Gear Ratio change (2019 3:73 vs 2015 3:42) and not the motor. Maybe someone that has had the same truck/gears between year models can provide a better analysis. I can say that it that every CTD I've owned seems to get slightly better MPG after its had some miles put on it, compared to when they are brand new. I guess time will tell.
Unless you prove it by hand calculating, the overhead lie o meter is worthless. Mine is reading 6 mpg too high.
around town, hand calc is 13.5, ovrhead is 20 to 21.
I have read the fix is to update the fuel map in the computer. Its not actually measuring fuel use, it is assuming fuel use off its internal fuel map. Same dumb thing is done with oil pressure gauge, not actually measuring the pressure.
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post #9 of 86 (permalink) Old 07-10-2019, 09:16 PM
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Quote:
Originally Posted by 7492 View Post
You guys need to talk to Cummins engineers, and if you can find one that will talk to you, they will be the only accurate source of information.
Are you insinuating that they'd be more truthful and knowledgeable than what's on the internet?!?
And sdowney717 strongly indicates that my Lie-O-Meter isn't accurate.

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post #10 of 86 (permalink) Old 07-11-2019, 09:38 AM Thread Starter
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Originally Posted by 7492 View Post
You guys need to talk to Cummins engineers, and if you can find one that will talk to you, they will be the only accurate source of information.
Actually I used to work at Cummins years ago on the power generation and later filtration side. Some of the engineers that I used to know may still work there, but I haven't talked to them in ages.

I now work for the largest medium and heavy duty dealership group in NA, and we have had these performance/efficiency engines in the medium/heavy duty trucks since 2017. In every case, the efficiency engine had a noticeable increase in fuel mileage over the the performance engine. As I said earlier, our customer fleets are reporting a difference of 10-15% depending on engine and application based off the data I have seen.

I know the standard output version will be more efficient in the pickups. There is no doubt about that. I am just curious as to how much.

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post #11 of 86 (permalink) Old 07-11-2019, 10:43 AM
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Quote:
Originally Posted by alacey View Post
I thought the fact that VW allowing the later TDI's to emit more NOx when not hooked up to an emissions tester is what lead to its better fuel mileage versus previous iterations. Also wasn't all 4 cylinder TDI's had a compression ratio of 18:1 or greater?

NOx is a major concern with stock tuning even with an SCR. The higher the compression ratio at a given power level, the higher the NOx output versus the same power level with a lower compression ratio. Hence the reason why the difference in compression ratios and power levels in all Cummins efficient version and performance version engines.

We have had these two versions in multiple Cummins engines(X15, X12, L9, and B6.7) in the medium/heavy duty market since 2017 and there is a noticeable difference in fuel in the fuel economy of the two versions from the feed back I am getting from customers. Generally a 10-15% difference depending on the engine size and application.
On the gen 3 EA288 engine with SCR, the cheating was there to reduce urea consumption. Urea consumption went way up to where it should have been, fuel economy didn't change much if any. The cheating improved the fuel economy on the gen 1 EA189 that used lean NOx traps instead of urea. It also kept the dpf from filling early, using EGR and reduced timing to meet the emission would have resulted in DPFs filling prematurely.

The 2015 US version of the 4 banger TDI had 16.2:1. 2009-2014 had 16.5:1. Correct in the rest of the older US 4 banger TDI engines having 18:1 and higher.

The Passat in Europe has an optional 2.0 4 banger with 15.5:1, compound turbos and 240 hp(makes 300 hp with just a tune). Fuel economy for that large sedan is still rated for 43 mpg (us gallon) combined. It has its poor CP4 cranked to 36,000 or 37,000 psi.

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post #12 of 86 (permalink) Old 07-11-2019, 11:31 AM Thread Starter
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Originally Posted by crazyrunner33 View Post
The Passat in Europe has an optional 2.0 4 banger with 15.5:1, compound turbos and 240 hp(makes 300 hp with just a tune). Fuel economy for that large sedan is still rated for 43 mpg (us gallon) combined. It has its poor CP4 cranked to 36,000 or 37,000 psi.

Ahh, well that is where it is getting its fuel economy increase from. While compression ratio does play a significant role in any engines efficiency, so does fuel pressure in a diesel. Generally the higher the fuel pressure, the higher the efficiency due to better fuel atomization. However, if they would have kept compression ratio the same as well as increase fuel pressure(which is impossible due to emission requirements) then there is no doubt that the efficiency of the engine would be even higher.

Drag coefficient may have also played a big role in that as well. That is actually one of the reason why my 2014 BMW 328d diesel with a .28 drag coefficient is rated so high. I know the 2014 Jetta had a drag coefficient of .30 and the new 2018 has a .27, but I am not sure about other year models.

Although I don't go based on EPA or EU fuel mileage tests. Working for various engine and vehicles manufacturers I have seen vehicles tuned specifically to attain the highest ratings possible on the test cycle, but that very same tuning doesn't net the same results in the real world. Hence the reason I go by real world result like Fuelly.com and not based off of EPA numbers.

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