Why do diesels cost so much to run?

Traditionally, diesels had the reputation for being durable, with great fuel economy, but when repairs are needed, expensive to repair.

In the new world of EPA compliant diesels starting around 2003, diesels have given up a lot of their historical reliability,  and in the first generation of diesel fuel fired regeneration Diesel Particulate Filters, considerably higher fuel consumption.

But wait.... something else happened.

On the way from the 7.3 to the 6.0, 6.4, and now the 6.7, we have seen horsepower and torque rise sharply.

During the last year of the 7.3, it produced 250hp and 525ft-lbs of torque.

Lets compare that to the 6.0 that replaced it, which produced 325hp and 570ft-lbs of torque.

Or the 6.4, rated at, 350hp, 650ft-lbs.

Now, the 6.7 is pushing 400hp, 800ft-lbs of torque.


Everyone complained of the poor fuel economy of the 6.4.

No one is asking the question, are we making fair compares when the engines are so different?

What about behavioral issues?  With that much more power, are users using the power or just using a very light foot as power increased?   It is a given that if you ask an engine to produce that kind of power it is rated at, it is going to have to burn fuel to make it.

Does that  suggest that much of the loss in fuel economy is due to different driving behavior?   Highly probable.

What about the loss in fuel economy from emissions devices like the EGR recirculation and Diesel Particulate Filter?

EGR, by lowering combustion temperature, certainly do have an impact on thermodynamic efficiency --- the larger the thermal gradient, the better the efficiency is a fundamental law of thermodynamics.

How about the DPF?  Yes, it does add a bit of exhaust restriction / back pressure.   But the real issue is fuel consumption during regeneration, as the current generation of mass market DPFs all use fuel for regeneration.   

In a future post, I will look at how much fuel does the DPF really use.