Mistaken ideas about how a diesel engine works

[Turtle_n_Peeps]

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! 🙂

[wilber1]

ShinerBock wrote:

The two extra gears do not help in highway driving. Most of the time you are in your final gear unloaded. The Ecodiesel in that review actually had worse gear ratios in this instance. The Ecodiesel had a 3.55 rear gear ratio and the final gear of the 8HP70 is .67:1. The Cummins on the other hand had a 3.42 rear gear and the final gear of the 68RFE is .62:1.

In real world driving I think it does help some. All highway driving is not steady state on a level road. I find the 8 speed in my car will quite often drop a gear when conditions change slightly. It lessens the amount of boost required in a given situation. This is with an engine that makes peak torque at 1600 RPM so it isn't shifting down because it can't make enough power without downshifting.

I do agree with you that the big difference is in the masses involved. The larger moving engine components, their larger bearing surfaces and the bigger pumps required to lubricate and cool them. Heavier transmissions, transfer cases, axles, brakes etc will all require more power to turn. The IFS half ton also sits lower than the solid axle HD truck and will have a bit of an aerodynamic advantage.

I can't remember the cooling fan ever coming on when running empty at highway speeds in my 6.7 or the 5.9 that preceded it.

[4x4ord]

When I bought my first car I could work for 1 hour at minimum wage and buy enough fuel to drive 90 miles. Now my daughter works 1 hour at minimum wage and can drive her car 105 miles on the fuel she can buy.

[Turtle_n_Peeps]

I totally agree with your post Shiner, and like I said before, it's because of all the weight of the components, drag and other things that bring down the fuel mileage.

I have a friend that could build a BT4 to put out well over 350 HP. But, it will not have to duty cycle of the 6.7's or the 6.6. To keep the duty cycle up you need big heavy components. The 6.7 Cummins in a light duty pickup will not have the duty cycle of a Cat 400 HP 3208. The 3208 will weigh a whole bunch more than the Cummins, but the Cummins will burn a lot less fuel too because of the weight of the components.

It's a give and take world out there.

[ShinerBock]

wilber1 wrote:

ShinerBock wrote:

Turtle n Peeps wrote:
That's true Shiner but that's a % number.

IOW's the 2500 lost 13 more HP than the ED. That 13 HP has to be accounted for in the burning of 13 HP worth of fuel.

And who knows if the fan was lockup up on the dyno? Big fans take big HP away from the engine. It's a lot easier to keep 240 HP cool instead of 370 or even almost 400 in the case of the newest HP Cummins. Another problem is the rea

Look at the chart on page 10 of this Cat study. Over a 50 HP loss on a RW pusher! That's a bunch! I know I have read dyno charts on my 06 Dmax and the fan on it will take away about 27 HP when it kicks on.


All of these things that help duty cycle on the 2500 will take away from efficiency. Lighter duty parts will help the efficiency of the 1500 big time.

At would be interesting to see the BSFC figures of both engines. Just the engines. No drive train or any of that. I know of no way to get those figures though. :M

So is 13 more hp and slightly lighter parts going to account for 8.5 mpg unloaded and 3.64 mpg loaded? I think not.

Two more gears will help.

Today's hybrid Formula 1 cars are up to 50% thermally efficient, are faster than the fastest formula they replaced while burning 30% less fuel.

If you are charging your Tesla with power from an oil or coal burning plant, you have a bigger carbon footprint than a F1 car.

The two extra gears do not help in highway driving. Most of the time you are in your final gear unloaded. The Ecodiesel in that review actually had worse gear ratios in this instance. The Ecodiesel had a 3.55 rear gear ratio and the final gear of the 8HP70 is .67:1. The Cummins on the other hand had a 3.42 rear gear and the final gear of the 68RFE is .62:1.

[wilber1]

ShinerBock wrote:

Turtle n Peeps wrote:
That's true Shiner but that's a % number.

IOW's the 2500 lost 13 more HP than the ED. That 13 HP has to be accounted for in the burning of 13 HP worth of fuel.

And who knows if the fan was lockup up on the dyno? Big fans take big HP away from the engine. It's a lot easier to keep 240 HP cool instead of 370 or even almost 400 in the case of the newest HP Cummins. Another problem is the rea

Look at the chart on page 10 of this Cat study. Over a 50 HP loss on a RW pusher! That's a bunch! I know I have read dyno charts on my 06 Dmax and the fan on it will take away about 27 HP when it kicks on.


All of these things that help duty cycle on the 2500 will take away from efficiency. Lighter duty parts will help the efficiency of the 1500 big time.

At would be interesting to see the BSFC figures of both engines. Just the engines. No drive train or any of that. I know of no way to get those figures though. :M

So is 13 more hp and slightly lighter parts going to account for 8.5 mpg unloaded and 3.64 mpg loaded? I think not.

Two more gears will help.

Today's hybrid Formula 1 cars are up to 50% thermally efficient, are faster than the fastest formula they replaced while burning 30% less fuel.

If you are charging your Tesla with power from an oil or coal burning plant, you have a bigger carbon footprint than a F1 car.

[ShinerBock]

Turtle n Peeps wrote:
No, not at all Shiner.

It's that, PLUS many other things like total weight of the truck, surface area of the front end and even things like height of the truck. It's very difficult to compare one truck to another. Here is a good example. Ford changed the front air dam on the front of their F250 and picked up between 1/4 and 1/2 MPG JUST with changing the chin spoiler. That's a BUNCH! Just with a chin spoiler change on the front of the same truck.

HP is just a measurement of lifting weight in a set amount of time. That's why city mileage suffers so much. You're lifting that weight over and over and over for every stop sign or light you hit.

In any event, I started this thread to try to explain why adding power to the SAME exact diesel engine will not hurt your fuel mileage unless you use that power. This is the exact reason why tuners on diesels don't cost mileage. They add power by adding fuel and timing. If you don't use that power, there is no mileage penalty.

I am not disagreeing with you that adding more peak power to the same exact diesel engine will not hurt its fuel economy. I am however saying that a smaller displacement diesel will get better fuel economy than a larger displacement diesel due to many factors with one being less internal parasitic loss to not having to move larger engine internals.

One prime example of the is seeing the 4BT and 6BT run on the Dynos when I was at Cummins. The 3.9L 4BT is essentially a 5.9L 6BT with two cylinders cut off. It consistently got better fuel economy than the 6BT.

In another scenerio, a Cummins ISX 15L would not get the same fuel economy as a ISB 6.7L or ISF 2.8L does making 100 hp.

[Lessmore]

Turtle n Peeps wrote:
No, not at all Shiner.

It's that, PLUS many other things like total weight of the truck, surface area of the front end and even things like height of the truck. It's very difficult to compare one truck to another. Here is a good example. Ford changed the front air dam on the front of their F250 and picked up between 1/4 and 1/2 MPG JUST with changing the chin spoiler. That's a BUNCH! Just with a chin spoiler change on the front of the same truck.

HP is just a measurement of lifting weight in a set amount of time. That's why city mileage suffers so much. You're lifting that weight over and over and over for every stop sign or light you hit.

In any event, I started this thread to try to explain why adding power to the SAME exact diesel engine will not hurt your fuel mileage unless you use that power. This is the exact reason why tuners on diesels don't cost mileage. They add power by adding fuel and timing. If you don't use that power, there is no mileage penalty.

Your explanation(s) and logic seem very clear and correct to me.

[Turtle_n_Peeps]

2001400ex wrote:

deltabravo wrote:
I agree with the OP.

I ran the same route over 100 times doing RV transport between Spokane to Pendleton and back with my 06 LBZ. I played with my speed a lot. Slowing down helped my profit margin. I could easily get 22.5 mpg running empty at about 60 mph.

The OP is arguing nothing can help the mileage of a diesel because of the way they work. Which totally isn't true. As I've mentioned, gearing and a 10 speed transmission can effect and improve mileage. Same thing is the could get cylinder deactivation to work on a diesel.

The manufacturers can improve fuel mileage if they wanted to invest in it.

BTW, I am at 19.5 mpg on this tank of diesel, a few days commuting to work and a trip last night from Spokane to Pullman for the football game.

Where did I argue that nothing can help the mileage of a diesel because of the way they work? I actually stated that by adding more power to a diesel there won't be a mileage penalty. This has been proven many, many times over with people that run tuners. Diesels work by running dead lean or dead rich with no throttle plate. They can run so lean they don't much more than a few drops of fuel at idle to going so rich you can hydraulic them.

Unlike diesels, gasoline engines run right at stoichiometric conditions. Run much more than stoichiometric and the engine will burn itself up or won't run at all. Run much less and you will burn a bunch of fuel and lose a bunch of power.

As far as manufactures not doing everything they can in their power to use every single drop of fuel to make the best power and economy is just a crazy statement.
Why do you think you can get 20 MPG and have more power on tap than any big block gas engine EVER made in history. It's not because the manufactures are resting on their laurels.

As far as cylinder deactivation on diesels. There is a reason for that. This is just one (there are more) If you ever turned one cylinder off on a diesel by killing off an injector you would understand. :R

[Turtle_n_Peeps]

No, not at all Shiner.

It's that, PLUS many other things like total weight of the truck, surface area of the front end and even things like height of the truck. It's very difficult to compare one truck to another. Here is a good example. Ford changed the front air dam on the front of their F250 and picked up between 1/4 and 1/2 MPG JUST with changing the chin spoiler. That's a BUNCH! Just with a chin spoiler change on the front of the same truck.

HP is just a measurement of lifting weight in a set amount of time. That's why city mileage suffers so much. You're lifting that weight over and over and over for every stop sign or light you hit.

In any event, I started this thread to try to explain why adding power to the SAME exact diesel engine will not hurt your fuel mileage unless you use that power. This is the exact reason why tuners on diesels don't cost mileage. They add power by adding fuel and timing. If you don't use that power, there is no mileage penalty.

[wnjj]

2001400ex wrote:

deltabravo wrote:
I agree with the OP.

I ran the same route over 100 times doing RV transport between Spokane to Pendleton and back with my 06 LBZ. I played with my speed a lot. Slowing down helped my profit margin. I could easily get 22.5 mpg running empty at about 60 mph.

The OP is arguing nothing can help the mileage of a diesel because of the way they work. Which totally isn't true. As I've mentioned, gearing and a 10 speed transmission can effect and improve mileage. Same thing is the could get cylinder deactivation to work on a diesel.

The manufacturers can improve fuel mileage if they wanted to invest in it.

BTW, I am at 19.5 mpg on this tank of diesel, a few days commuting to work and a trip last night from Spokane to Pullman for the football game.

I don't think cylinder deactivation applies to a diesel because they can simply decrease the amount of fuel for all of the cylinders as power demand drops. The air/fuel mixture doesn't need to stay constant like it does on a gas engine. Now if there is a point at which burning too little fuel per cylinder is actually less fuel efficient than burning less cylinders, there may be a point to it.

[2001400ex]

deltabravo wrote:
I agree with the OP.

I ran the same route over 100 times doing RV transport between Spokane to Pendleton and back with my 06 LBZ. I played with my speed a lot. Slowing down helped my profit margin. I could easily get 22.5 mpg running empty at about 60 mph.

The OP is arguing nothing can help the mileage of a diesel because of the way they work. Which totally isn't true. As I've mentioned, gearing and a 10 speed transmission can effect and improve mileage. Same thing is the could get cylinder deactivation to work on a diesel.

The manufacturers can improve fuel mileage if they wanted to invest in it.

BTW, I am at 19.5 mpg on this tank of diesel, a few days commuting to work and a trip last night from Spokane to Pullman for the football game.

[ShinerBock]

Adam R wrote:
I still agree with the OP. Mileage is directly related to how hard you push an engine

And I never debated the fact that mileage is directly related to how much horsepower you demand from the engine.

Adam R wrote:
and the 1500 to 2500 fuel mileage comparison was still far from apples to apples. Did you notice the 1960lb curb weight difference? The 2500 sits taller and is less "flat" on the underside so it's cD is not the same as the 1500. Going down a level/flat road, the vast majority of fuel used is in overcoming wind resistance. Mechanical losses are actually pretty miniscule. That's way it takes 20 hp to go 60 mph in a sedan and over 600hp to go 200 mph

A gasoline 3.5L V6 Honda Pilot is further from apples to apples to a gasoline 5.7L V8 Toyota Sequoia yet you still said that was a good comparison for your argument and was more inline to what you were trying to say. I bring up a comparison between a V6 diesel 3.0L Ram 1500 to say I6 diesel 6.7L Ram 2500 and all the sudden these aren't valid comparisons per the topic yet a V6 Honda Pilot versus a V8 Toyota Sequoia is to you. I don't understand that one.

Adam R wrote:
Regardless, life, like engines, is a bunch of compromises. I'll take the big engine that returns 20 mpg and be just fine with it. If someone wants a 28 mpg truck, then they will need to accept the limitations that brings.

I am with you on that one and is exactly what I have said to many small diesel owners. They bought them for fuel economy so don't expect or try to say that it pulls like a big diesel. On the flip side, big diesel owners bought their diesels for pulling power so don't try to say you can get small diesel fuel economy.

[Adam_R]

I still agree with the OP. Mileage is directly related to how hard you push an engine and the 1500 to 2500 fuel mileage comparison was still far from apples to apples. Did you notice the 1960lb curb weight difference? The 2500 sits taller and is less "flat" on the underside so it's cD is not the same as the 1500. Going down a level/flat road, the vast majority of fuel used is in overcoming wind resistance. Mechanical losses are actually pretty miniscule. That's way it takes 20 hp to go 60 mph in a sedan and over 600hp to go 200 mph.

Regardless, life, like engines, is a bunch of compromises. I'll take the big engine that returns 20 mpg and be just fine with it. If someone wants a 28 mpg truck, then they will need to accept the limitations that brings.

With that, I'm out of this one.

Adam

[deltabravo]

I agree with the OP.

I ran the same route over 100 times doing RV transport between Spokane to Pendleton and back with my 06 LBZ. I played with my speed a lot. Slowing down helped my profit margin. I could easily get 22.5 mpg running empty at about 60 mph.