Forum Discussion

Turtle_n_Peeps's avatar
Turtle_n_Peeps
Explorer
Sep 28, 2016

Mistaken ideas about how a diesel engine works

I've seen a lot of grumbling as of late about the power output of new diesel engines.
These same people complain and ask why the manufactures have to run the power up so much and why don't they work on fuel economy.

The answer is simple: A diesel engines power output is in direct proportion to the amount of fuel it burns.

In other words: If you have a 500 HP diesel pickup and you only use 100 HP, it will only burn 100 HP worth of fuel. If you're WOT, you will burn 500 HP worth of fuel.

Case in point: A friend has a Cummins that is bombed. Intake, twin snails, studs, exhaust, cam and a few other goodies. Dyno'ed at 525 to the rear wheels. This truck will pull down 23 MPG on the hiway all day long at 65 MPH. Why? Because he is only running about 60 HP worth of fuel through it at that speed and load. Now if he puts a heavy trailer on the tail and tries to climb a 12% hill at 75 MPH; guess what? He will burn 525 HP worth of fuel doing it.

Another case in point: I keep books on all of my fuel stops over the years when I'm on vacation.
I have a 93 200 HP 6.5 diesel and an 06 360 HP Dmax diesel. I have pulled my TT many years with both. The 6.5 would get 12 MPG pulling the TT. My Dmax gets........are you ready..........12 MPG pulling the same trailer.

The Dmax gets a little bit better mileage because it has a 6 speed tranny where the 6.5 has a 4 speed. The 6.5 gets a little bit better fuel mileage because it can only flow 200 HP worth of fuel at WOT where the Dmax can burn 360 worth of fuel. So all in all, over all the years they both average 12 MPG in 10's of thousands of miles.

So, if you buy a new diesel stop bitchin about the HP and mileage deal. You want better mileage with your new 440 HP pickup? Use only 275 or 300 or 325 HP worth of fuel. To do this you are going to have to slow down; even on the hills.

Remember; if you only flow 300 HP worth of fuel you will only burn 300 HP worth of diesel. IOW's "you" are in control of how much fuel your new high HP diesel burns, not the manufacture! :)
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  • ShinerBock's avatar
    ShinerBock
    Explorer
    Sep 30, 2016
    Adam R wrote:


    While there are similarities between a 1500 and a 3500 and may share similar sheet metal, they are not the same truck. One weighs considerably more than the other, sits taller and is built with heavier duty components.


    I know, which is why I said "I would bet that a 240 hp Ecodiesel gets considerably better fuel economy than a 370 hp Cummins does pushing the same full size truck down the road."

    Meaning that even if they were pushing the exact same truck down the road unloaded, I still believe the 240 hp Ecodiesel still get considerably better fuel economy doing it than the 370 hp 6.7L Cummins.


    Adam R wrote:
    Yes, it can do it, but I'd be willing to bet the mileage difference between a 3.0 Eco and an early 5.9 pulling a 12,000 trailer is pretty similar.



    I don't think they would be similar at all. Due to technology like common rail, electronically controlled injectors, intercoolers, and VG turbos to name a few the 3.0L Ecodiesel is able to burn fuel much more efficiency than an early 5.9L. Basically it is able to get more out of each drop of fuel. If you put both in the same truck and slapped the same emissions devices(or no emissions devices) to them and I would bet $1000 that the Ecodiesel would get better fuel economy than the early 5.9L.
  • Me_Again's avatar
    Me_Again
    Explorer III
    Sep 30, 2016
    Adam R wrote:


    Just for fun, lets see someone put a 3.0 Eco in a 3500 MC and see how it does for mileage. That would be an interesting comparison.

    Adam R.


    Just load a 1500 to weigh the same as a 3500 Cummins to do your test. Oh, maybe that is over the GVWR of a 1500. Chris
  • Adam_R's avatar
    Adam_R
    Explorer
    Sep 30, 2016
    ShinerBock wrote:
    Adam R wrote:
    but fortunately for a diesel, the difference in mileage numbers between a 200hp engine pushing a full sized truck down the road and a 400hp engine is negligible.


    I would disagree with that. I would bet that a 240 hp Ecodiesel gets considerably better fuel economy than a 370 hp Cummins does pushing the same full size truck down the road. At least 5 mpg better, but probably more.

    Also, I think the argument here (TnP, please correct me if I am wrong) is the fact that people saying they want more fuel economy than the power of today diesels. Basically instead of the high powered apples we are getting, they want fuel economy oranges while keeping the same power. What should be stated is that with a diesel if you increase the engines ability to use up more of the fuel's energy giving you better fuel economy, more power is also created given that the displacement is the same. Well, unless you inject less fuel per injection event. At that point, if you want to have better fuel economy while not increasing peak power then you have to decrease displacement.

    I will use the Ecodiesel again. Due to the manufacturers using technology like common rail, VG turbos, electronic injectors, and more valves the 3.0L Ecodiesel is able to have the same power output of an early 5.9L 12v Cummins while having much better fuel economy. The Ecodiesel is able to burn fuel more efficiently than the old 5.9L therefore it is getting more out of each drop of fuel.

    However, if you are talking about the exact same engines using the exact same technology that only gets X% of energy out of a drop of fuel, then TnP is correct that the fuel economy will not be effected negatively if peak power output is increased.


    While there are similarities between a 1500 and a 3500 and may share similar sheet metal, they are not the same truck. One weighs considerably more than the other, sits taller and is built with heavier duty components. In my original comparison, of the Hellcat and Corvette, the vehicles are nearly identical but you can get a better optimized engine that will give you up to 7 mpg better mileage in each. While a 3.0 Eco Diesel can generate 240 hp, you are running it at 83% duty cycle when asking it to produce 200 hp. Yes, it can do it, but I'd be willing to bet the mileage difference between a 3.0 Eco and an early 5.9 pulling a 12,000 trailer is pretty similar. The early 5.9 is far from being overtaxed and is capably of 600 hp with a few mods. Just like the towing mileage of the 3.5 Ecoboost is apparently nothing to write home about, it's fairly well optimized for daily commuting duties. The whole reason manufacturers are going to 7, 8 , 9 and 10 speed transmissions to keep the engine in it's most optimal rpm range and hoping to eek out another 1 or 2 mpg. It's hard to compare apples to apples in these examples, but I too get tired of guys griping about "too much hp, give me mileage." If that is the case, go buy an Eco, but forget towing 18,000 lbs with it. If you want to play with the big loads, you need to buy the truck that has the grunt and chassis that has the components that will properly handled those loads with relative ease.

    Just for fun, lets see someone put a 3.0 Eco in a 3500 MC and see how it does for mileage. That would be an interesting comparison.

    Adam R.
  • Turtle_n_Peeps's avatar
    Turtle_n_Peeps
    Explorer
    Sep 30, 2016
    Me Again wrote:
    Grit dog wrote:
    And the eco will get about the same mileage as a BT4 or a 3liter Perkins or any other similar size diesel. So what's your point?
    I just can't leave! Watching these discussions is like a train wreck, just gotta see what happens next!


    So why are the old two cycle Detroits so fuel hungry? They are noted fuel hogs in boats and on the road. Chris


    Me Again, as was said above, the two stroke diesel has the highest BSFC figures EVER! In other words, they put out the most power for the fuel burned of any ICE that I know of. The bigger they get the better the BSFC gets.

    An old two stroke made in the 30's is only 1% lower in BSFC figures than that of a modern common rail. Link Think about that! :E
  • Turtle_n_Peeps's avatar
    Turtle_n_Peeps
    Explorer
    Sep 30, 2016
    ShinerBock wrote:
    Adam R wrote:
    but fortunately for a diesel, the difference in mileage numbers between a 200hp engine pushing a full sized truck down the road and a 400hp engine is negligible.


    I would disagree with that. I would bet that a 240 hp Ecodiesel gets considerably better fuel economy than a 370 hp Cummins does pushing the same full size truck down the road. At least 5 mpg better, but probably more.

    Also, I think the argument here (TnP, please correct me if I am wrong) is the fact that people saying they want more fuel economy than the power of today diesels. Basically instead of the high powered apples we are getting, they want fuel economy oranges while keeping the same power. What should be stated is that with a diesel if you increase the engines ability to use up more of the fuel's energy giving you better fuel economy, more power is also created given that the displacement is the same. Well, unless you inject less fuel per injection event. At that point, if you want to have better fuel economy while not increasing peak power then you have to decrease displacement.

    I will use the Ecodiesel again. Due to the manufacturers using technology like common rail, VG turbos, electronic injectors, and more valves the 3.0L Ecodiesel is able to have the same power output of an early 5.9L 12v Cummins while having much better fuel economy. The Ecodiesel is able to burn fuel more efficiently than the old 5.9L therefore it is getting more out of each drop of fuel.

    However, if you are talking about the exact same engines using the exact same technology that only gets X% of energy out of a drop of fuel, then TnP is correct that the fuel economy will not be effected negatively if peak power output is increased.


    Yes, that's what I'm saying.

    A Dmax or Cummins or Ford or whatever that is rated for 400 HP now is not going to get worse mileage if you jack the fuel up to a 450 HP rating..........unless you use that extra 50 HP.

    As you know, that's how a diesel engine works. More fuel = more RPM and more power. The more fuel you add the more power you get; until the engine hydraulics or melts down.

    Here is a great chart for diesel BSFC.

    Some of you need to read the top 4 bullets that "engineers" wrote about diesel engines on the above website.
  • Me_Again's avatar
    Me_Again
    Explorer III
    Sep 30, 2016
    dodge guy wrote:
    I think the big thing that some of us are saying is too stop with the HP wars, hold them where they are at and work on mileage instead. I see no reason why the new crop of diesels can't get at least 3+ more MPG! If they they put the effort into that instead of power lots more people would sit up and take notice.

    Years ago they said a gas V-8 would never get better than 20 mpg. Guess what mid 20's is what they are now getting!


    HP and Torque may continue to go up, however torque management will be used to only apply the power at very specific areas of RPM and speed.

    Chris
  • sparechange's avatar
    sparechange
    Explorer
    Sep 30, 2016
    dodge guy wrote:
    I think the big thing that some of us are saying is too stop with the HP wars, hold them where they are at and work on mileage instead. I see no reason why the new crop of diesels can't get at least 3+ more MPG! If they they put the effort into that instead of power lots more people would sit up and take notice.

    Years ago they said a gas V-8 would never get better than 20 mpg. Guess what mid 20's is what they are now getting!


    I agree with most everything you said accept the modern v-8 getting over 20. There's a website that tracks user uploaded mpg by vehicle and year (fuelly.com) and from what I've seen most on average are worse than diesels. Usually around 15-16 combined. I could be wrong but I spent a lot of time looking around on there and talking to guys at work with halftons a couple years ago when I was truck shopping
  • ShinerBock's avatar
    ShinerBock
    Explorer
    Sep 29, 2016
    Grit dog wrote:
    ShinerBock wrote:
    Grit dog wrote:
    And the eco will get about the same mileage as a BT4 or a 3liter Perkins or any other similar size diesel. So what's your point?
    I just can't leave! Watching these discussions is like a train wreck, just gotta see what happens next!



    I am unsure who you are referring to in this. If it is me, then I don't understand what you are trying to say in relation to what I did.


    You. 3litre engine vs 6litre engine is apples vs basketballs.


    I guess you didn't read what I was quoting. He was comparing a 200 hp engine versus a 400 hp engines saying the fuel mileage between them is negligible so I used an example of two modern engines around those horsepower ratings showing that their fuel mileage difference is more than negligible.
  • Grit_dog's avatar
    Grit_dog
    Navigator
    Sep 29, 2016
    ShinerBock wrote:
    Grit dog wrote:
    And the eco will get about the same mileage as a BT4 or a 3liter Perkins or any other similar size diesel. So what's your point?
    I just can't leave! Watching these discussions is like a train wreck, just gotta see what happens next!



    I am unsure who you are referring to in this. If it is me, then I don't understand what you are trying to say in relation to what I did.


    You. 3litre engine vs 6litre engine is apples vs basketballs.
  • Grit_dog's avatar
    Grit_dog
    Navigator
    Sep 29, 2016
    Me Again wrote:
    Grit dog wrote:
    And the eco will get about the same mileage as a BT4 or a 3liter Perkins or any other similar size diesel. So what's your point?
    I just can't leave! Watching these discussions is like a train wreck, just gotta see what happens next!


    So why are the old two cycle Detroits so fuel hungry? They are noted fuel hogs in boats and on the road. Chris


    I rest my case.

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