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! 🙂

[Grit_dog]

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!

[ShinerBock]

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.

[Adam_R]

Lots of competing ideas here, but the OP is pretty correct in his original assertion. Just because an engine is "able" to get massive amounts of HP, does not necessarily mean it gets really poor mileage. Case in point is the Hellcat or Corvette. 600-700hp, but still able to get 22 mpg. Yes, they could do better with smaller, more optimized engines and net 29 mpg, but you can't always make things apples to apples. Life is made up of compromises, 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. The real difference is how the driver drives the vehicle. If you use more hp, you are going to get poorer mileage, it's physics pure and simple. As for massive mpg gains to be made in the future from an internal combustion engine, not likely or it would have happened long ago. That thing called physics once again gets in the way and a certain amount of heat (btu's) is going to be wasted through the radiator or the tailpipe no matter what.

Adam

[Me_Again]

Fordlover wrote:


Hey I worked with that guy about 10 years ago. His 05 Dodge Cummins never dipped below 28MPG, no matter what he was pulling. And he drove like a bat out of hell.

Hey, I met him in a bar once!

[BenK]

Suggest should'a included: "All other things being equal"...as so many folks take
things as an absolute

There are many, many other variables

From technology to frictional losses and many of them factor each other to either
cancel out or multiply those losses

Hybrid and all electric will some day over take/replace ICE's, but technology is
not there yet. Mainly for the power storage (AKA battery). Saline is looking very
good, but they are not ready for vehicles just yet...too big

Next gen hybrid will NOT have an alternator...nor starter...the electric half
of the hybrid will be all that and more...

On why diesel is more efficient than a gasser...the main reason is that on a gasser,
it is throttled...the most efficient throttle position on a gasser is WOT...and
diesel's are WOT all the time...then factor in more BTU's per gallon and then
of course diesel has higher MPG's

Plus diesel's have much higher compression ratio's...therefore higher PSI on the
piston tops

Also lost is the duty cycle rating of all of these in discussion. Even
hybrid or all electric. There is a guy in San Louse Ca who gets over
200 MPG with his Prius and key is the extra wet/lead-acid batteries and
the custom controller he built

Know...guessing...what Turtle is getting at...is that these hugmongo HP's/torque
numbers are more for acceleration and ability to go faster up hill than ever before

That once up to speed (from slow poke to racer) and at steady speed...the HP
required to maintain that speed is way below the new ICE's max ratings...

[ShinerBock]

2001400ex wrote:


Any Prius that gets 17 mpg isn't running right.

Actually that is about right for how hard it was being driven in the video I linked.

[2001400ex]

ShinerBock wrote:

Bedlam wrote:

That's been my experience too. When I had a little 4-cylinder gasser pickup, I could get 25 mpg when empty while driving over the mountains but was in single digits when towing 5000 lbs. This was an engine with 121 hp and 149 lb-ft which by your explanation should have been getting better mileage while towing because of the limited power it created.

That reminds me of the Top Gear UK mpg test they did a while back where they put a 1.4L 4 cylinder Prius versus 4.0L V8 BMW M3. They ran a Prius around a road track 10 times at the best it could run and followed it with a BMW M3 essentially using the same hp for their weight and drag. The BMW got more than 2 mpg better than the Prius The track was not a high speed track so the BMW did not benefit from any drafting.

BMW M3 vs Toyota Prius

Any Prius that gets 17 mpg isn't running right.

[ShinerBock]

Bedlam wrote:

That's been my experience too. When I had a little 4-cylinder gasser pickup, I could get 25 mpg when empty while driving over the mountains but was in single digits when towing 5000 lbs. This was an engine with 121 hp and 149 lb-ft which by your explanation should have been getting better mileage while towing because of the limited power it created.

That reminds me of the Top Gear UK mpg test they did a while back where they put a 1.4L 4 cylinder Prius versus 4.0L V8 BMW M3. They ran a Prius around a road track 10 times at the best it could run and followed it with a BMW M3 essentially using the same hp for their weight and drag. The BMW got more than 2 mpg better than the Prius. The track was not a high speed track so the BMW did not benefit from any drafting.

BMW M3 vs Toyota Prius

[Bedlam]

DownTheAvenue wrote:

Turtle n Peeps wrote:


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.

This is an over simplification and actually not correct. Anyone with a degree in automotive engineering will tell you you that fuel consumption is only one component of power output of any internal combustion engine, regardless of fuel type, method of fuel delivery, or combustion attribute (spark, internal or external heat).

That's been my experience too. When I had a little 4-cylinder gasser pickup, I could get 25 mpg when empty while driving over the mountains but was in single digits when towing 5000 lbs. This was an engine with 121 hp and 149 lb-ft which by your explanation should have been getting better mileage while towing because of the limited power it created.

[Fordlover]

Farmerjon wrote:
It hurts it big time, because the ones I see with the big stacks are blowing black smoke all the time even in town between traffic lights and he will be the same guy claiming 30 mpg while pulling 30,000 lbs at 70 mph. LOL

Hey I worked with that guy about 10 years ago. His 05 Dodge Cummins never dipped below 28MPG, no matter what he was pulling. And he drove like a bat out of hell.

[Dadoffourgirls]

ktmrfs wrote:
...I think one of the best ways to improve mileage would be to add start stop technology to pickups along with a mild hybrid system. Battery bank big enough to absorb the kenetic energy from say a 50mph stop, with maybe an option for a big enough bank to absorb kenetic energy for a 20,000lb combined weight down a 4 percent, 1 mile grade.



the first would be a big help on unloaded in town economy, the second a help when towing over up and down hills.

If you notice, the 2017 Colorado Crew Cab gets a new RPO V6 and 8 speed transmission. This RPO 3.6 V6 engine has start/stop in the Cadillac's.

So the real fuel economy should be going up in these trucks.

[Cummins12V98]

The 2019 6.7 Cummins will have improved fuel economy over current models because the engine will have no emissions components. All exhaust scrubbing will be down stream.

Good post op!

[MVH1]

Great discussion, thanks for the post TnP! Now what I have a hard time understanding is the $50,000-$70,000 price tags on these machines ??

[DownTheAvenue]

Turtle n Peeps wrote:


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.

This is an over simplification and actually not correct. Anyone with a degree in automotive engineering will tell you you that fuel consumption is only one component of power output of any internal combustion engine, regardless of fuel type, method of fuel delivery, or combustion attribute (spark, internal or external heat).

[2001400ex]

After this many posts, and no one acknowledges that gearing and transmissions (10 speed for instance) can improve mileage. Some people mentioned technology, which can improve mileage as well. Even a good tune can improve mileage without taking off the emissions stuff. I know one reason for better mileage only through a tune is timing of fuel delivery. One simple example of this is my Duramax used to lug up hills in 6th, which sucks up fuel. If you manually downshift to increase the RPMs (assuming stock), it burns less fuel because it takes less fuel to push the same rpms. On the tune I have, it downshifts earlier without going into manual to save fuel.

So I reject the notion that huge car manufacturers with billions of dollars in R&D can't come up with more fuel efficient options.