Mack’s turbo-compound fuel saver

The prospect of ‘free’ power, using waste exhaust-gas heat and pressure has long proved irresistible, and the Volvo Group seemingly overcame issues that forced most other diesel engine makers away from turbo compound engines. The fact that the Group has made its 13-litre TC engines standard across most of its Mack and Volvo USA highway truck models says a great deal about confidence in this fuel-efficiency route.

The first-generation Volvo Group turbo-compound engine was introduced in North America in 2017. Following the next-generation release of the 13-litre TC engine in 2020, orders for the engine increased in North America, to the point where it became a standard offering. The TC engine has also proved popular in Europe, with a 2024 order for 1000 trucks from one fleet being an example.

Depending on the operation, the current-generation 13-litre TC engine is said to provide fuel savings up to six per cent, compared with the non-TC, variable-geometry turbo (VGT) engine, reducing emissions while still maintaining high average road speeds.

We won’t go into the niceties of TC engines here, because we’ve already traced their history.

The common complaint about previous TC-engine powertrains was the difficulty of achieving real-word economy benefits. I remember driving a Scania turbo-compound engine in Sweden 30 years ago and loved it.

However, subsequent testing by fleets and magazines didn’t discover much fuel consumption improvement: just smiling faces on drivers who liked the performance buzz.

Turbo compound made a smaller engine behave like a larger one, but fuel economy wasn’t automatic. There were potential economy benefits, but only with educated driver input and modern enhancements, including automated-shift transmissions and predictive cruise control. Even with these driving aids, economy savings proved elusive for some operators.

The Volvo Group’s post-2021, turbo-compound 13-litre engine is a Euro 6 engine that comes with wave-top pistons and new injectors, for optimised combustion efficiency, as well as a fixed-geometry turbocharger, engine and exhaust braking and a rear PTO option.

It has a familiar turbo-compound exhaust-gas-driven gear-train at the rear of the engine, powered by waste exhaust gas from the primary turbocharger – potential power that would be wasted in a conventional engine as exhaust pressure and heat.

In the Australian market, the engine comes as a 500hp unit only, but there is a 460hp version for Europe and 445hp in the USA. Here, it’s rated at 70 tonnes GCM for B-Double work, so the 500hp setting makes sense.

If 500hp seems light-on for multi-combination work, note that peak torque is a hefty 2800Nm, across the 900-1300rpm band.

Looking at the chopped-off power and torque curves it’s obvious that this engine could produce at least 600hp, but the power curve has been trimmed to maintain peak power from 1250rpm up to 1600rpm.

The designers intended this engine to operate below 1600rpm and to make sure that happened, it was available initially with a direct-top-gear automated transmission and a very tall 2.83:1 final drive ratio, relying on a 14.94:1 first gear for lift-off gradeability.

Three years ago, we checked out this powertrain in a Volvo, hauling a lightly-loaded single trailer and confirmed that it operated all the time in the highest possible gear, with prodigious torque at low engine revs seeing the tacho showing as little as 1200rpm when highway cruising.

However, in-service drivability at 60+ tonnes in the fleet world must have been less than optimal, because the 2024 iteration of the TC powertrain sees the same engine coupled to an overdrive version of mDrive, with a first gear ratio of 11.73:1 (the direct-drive box’s second gear ratio) and 0.78:1 top gear. The final drive ratio is now a more practical 3.09:1.

We pestered Mack for a TC-engine drive, so our test mount was brand new, very ‘green’ truck that hadn’t even been given its final pre-delivery. There were a few electronic issues that needed calibration and the steering hadn’t been optimised.

The new Anthem was coupled to Mack’s B-Double curtain-side, demo trailer set and was loaded to 60 tonnes. Our aim was to see if this new truck could achieve the magic 2km/litre target, while maintaining real-world cruising speeds.

I won’t go into the details of the integrated-sleeper Anthem model, because Middo’s already done that, but we’ll stick to the differences between his test and mine.

The first thing I noted, when getting comfortable behind the wheel, was the addition of stalk-mounted spotter mirrors, on the front mudguards. That looked like an afterthought, but I soon came to appreciate them, because the traditional west coast door mirrors were mounted a tad too far aft for my liking, requiring a deliberate head movement to scan them. The spotters proved invaluable when manoeuvring the B-Double in tight spaces and for checking traffic in adjacent lanes.

Ergonomics of the frequently used controls were very good, except for the pre-WWII red and yellow air brake knobs. C’mon USA, the rest of the heavy truck world moved to graduated parking-brake levers many decades ago. (Oh, plus 24-volt electrics and standard disc brakes…)

I used the cruise control function where possible, but the predictive bit wasn’t operating fully, so it didn’t initiate an early downshift on highway grades, letting road speed decay more than necessary. A manual shift intervention soon made up for that, via large ‘+’ and ‘-‘ dashboard buttons.

Those same buttons were handy for trimming downhill speed, but alternatives were using the engine brake wand, or the cruise control speed decrease function. I found lots of things to play with!

All the while, the 13-litre M8 hummed away, only slightly audibly and never feeling stressed, even on steep grades. The truck climbed all the hills on the Hume Freeway in the top half of the box.

I watched the fuel flow readout occasionally and was pleased to see it scroll from an initial 1.8km/litre to 1.9 and then 2.0. With more klicks on the clock, this ‘green’ truck would obviously have done better than 2km/litre at this weight.

In theory, the slipperier shape of the Anthem vs the streamlined-brick shape of the Volvo FH should see the Mack doing better than its stablemate in the economy stakes. Time will tell.

Specifications:
Mack Anthem 36-inch stand-up sleeper
Model: Anthem 6x4 13L Air
Cab: 36" Stand-up sleeper
Frame: Mack 8mm
Engine: Mack 13-litre MP8TC (turbo-compound) 500hp Euro 6 MP8HE
Torque: 2800Nm (900-1300rpm)
Engine brake: Mack Powerleash
Transmission: Mack mDRIVE HD 13-speed overdrive with multispeed reverse
Wheelbase: 4785mm
Front axle: Mack FXL 14.6 single-steer axle
Front tyres: 295/80R22.5 Bridgestone M840
Front suspension: Parabolic spring
Rear suspension: Mack Air Ride eight-air-spring 21.0T
Rear axle: Mack 2370B single reduction 23.0T
Axle ratio: 3.09
Rear drive tyres: 11R22.5 Bridgestone M766
Fuel tanks: D-shaped polished aluminium 760 litres (L) and 500 litres (R)
AdBlue: Polished aluminium 150 litres