The 3% thermal efficiency difference sold me on turbo compounding

I always thought turbos were just for more power. Then I read a SAE paper from 2018 that said adding a power turbine to a heavy duty diesel can bump thermal efficiency from 42% to 45%. That 3% adds up to about a 7% fuel savings on a long haul truck. Ran the numbers on our fleet of 12 trucks doing 120k miles a year each at 6.5 mpg. Works out to roughly 15,000 gallons saved annually. That convinced me to spec the next engine order with the option. Anyone else looked into this for their rigs?

3 comments

3 Comments

harperg761mo ago

12 trucks at 120k miles each, I can barely keep my one old Dodge running past 90k without something falling off. That math sounds solid though, I'd probably screw up the fuel savings and spend it on coffee and donuts anyway.

noaht151mo ago

Close on the fuel savings math. 3% efficiency gain doesn't translate to a 7% fuel savings that cleanly. You're going from 42% to 45% which is about a 7% relative improvement in efficiency, but that means you're burning about 7% less fuel for the same work output only if you're running at peak efficiency the whole time. Real world driving with varying loads and speeds knocks that down quite a bit. Plus the added weight and complexity of the turbo compounding system eats into some of those gains. You're probably looking more at 4-5% real world fuel savings on average, maybe 6% on a perfect highway run. Still worth it for a fleet, but 15,000 gallons is optimistic by about a third.

claire_gibson1mo ago

Not sure I follow your logic on the "7% relative improvement in efficiency" part though. If you're going from 42% to 45%, that's a 3 percentage point bump, not a 7% relative gain. Like 3/42 is about 7%, sure, but fuel savings aren't calculated that way in real world fleet data. The engineers at Cummins and Detroit have been publishing real world numbers from their turbo compounding tests and they're seeing more like 4-6% fuel reduction on highway routes, not the 7% you're saying is possible at peak. Where did you get the 3 percentage point efficiency gain from anyway? I've only seen claims of 2% for the basic system. The weight penalty alone is around 200 pounds for the turbine and gearing, and that's going to hurt you on any route with hills or stop and go. Even on a perfect flat highway run at 65 mph, you're still dealing with parasitic losses from the gearing that eat into that hypothetical 7% number pretty quick.