Turbocompound engines: what they are and when they pay

A spec sheet that reads D13TC, or MP8HE, or “Energy Recovery Technology”, is telling you one specific thing: there is a second turbine in the exhaust stream, and it is not there to feed the intake. It is geared to the crankshaft, and its job is to take back a slice of the energy that would otherwise leave through the pipe. Whether that hardware earns its keep depends almost entirely on what the truck does between six in the morning and six at night.

What the second turbine actually does

A conventional turbocharger takes energy out of the exhaust and puts it straight back into the engine as boost pressure. Turbocompounding does something different with what is left over. Downstream of the turbocharger sits a further turbine that drives nothing pneumatic at all — Volvo returns its output to the flywheel through a train of reduction gears, and Mack describes its Energy Recovery Technology as converting exhaust heat “to mechanical energy” and delivering it “back to the crankshaft as torque”.

The reason there is anything left to recover is unflattering to the diesel engine. A modern heavy-duty six still sends more energy out of the exhaust than it delivers to the driveshaft. Speaking to Transport Topics, Volvo product marketing manager John Moore put the effect of turbocompounding at a lift in brake thermal efficiency from 43% to 48% — a large step in an industry that fights over single percentage points.

It shows up as torque, not as headline power

This is the part that confuses buyers reading two data sheets side by side. Our own record for the Volvo FH lists a standard D13 spanning 309–397 kW with 2,100–2,600 Nm, and the turbocompound D13 in the I-Save package at 309–368 kW with 2,400–2,800 Nm. The turbocompound engine tops out lower on power and higher on torque. Volvo’s own launch material puts the difference at up to 300 Nm extra.

That is the design intent, not a compromise. Volvo’s stated reason is that “less acceleration and fuel is needed to keep a steady speed in highway traffic”, and Mack says its system “enables the engine to operate at 1,000-1,100 RPM, saving fuel without sacrificing performance”. A torque surplus low in the rev range keeps the truck in top gear over a rise instead of dropping a gear and burning fuel to recover the speed it lost.

The corollary matters just as much. If the engine never settles into that band, the recovery turbine never gets the steady exhaust flow it needs, and you have bought weight.

The published figures, and what they are measured against

Volvo Trucks quotes up to 7% lower fuel costs for the D13TC in the I-Save package, measured against a D13 Euro VI Step D without those features, for long-haul customers “typically exceeding 160,000 km per annum” — and lists, in the same release, what moves the number: cruise control use, vehicle specification, load and weight, topography, driver experience and weather. Transport Topics reported 7.5% and an extra 50 hp for the North American D13TC, and up to 9.5% for the Mack Anthem with the HE+ package.

Read those carefully. None of them is an engine-only figure. Every one is a package: turbine plus pistons plus axle ratios plus predictive cruise plus, in Mack’s case, aerodynamics. Anyone quoting you turbocompounding on its own at 7% is quoting something the manufacturers did not claim.

The duty-cycle test

Turbocompounding pays when the engine spends most of its running hours at high, steady load in a narrow speed band. Concretely: international long-haul at or near maximum permitted weight, mostly motorway, cruise control genuinely in use, high annual mileage. Volvo’s North American page adds variable-load work — tanker, bulk haul, flat-bed — where the extra torque reserve is what stops the truck downshifting every time the load shifts.

It does not pay on multi-drop urban distribution, short regional stems, work at consistently light payload, or any operation with a high idle share. The device eats exhaust energy; a lightly loaded engine on a thirty-kilometre run never produces enough of it to repay the extra mass and the extra purchase price.

The practical test before you tick the box is four numbers: annual kilometres, average payload, share of hours on motorway, and the fleet’s actual cruise-control usage. If you cannot produce them, you cannot justify the option, and the salesperson’s percentage is not evidence.

When the manufacturers themselves said no

Detroit fitted turbocompounding to the DD15, then removed it after the 2014 GHG rules and replaced it with an asymmetric turbocharger that was simpler and, according to Transport Topics, 70 pounds lighter. Detroit’s current DD15 page lists a ball-bearing simple-geometry turbo and claims the engine is “up to 3% more fuel efficient than our previous model”. Turbocompounding is not mentioned on it at all.

Navistar declined the technology outright. Its marketing director Jim Nachtman told the same publication that “if you add a lot of complexity to a vehicle, there’s a higher propensity for something to fail”, and set that against fleet downtime he priced at $1,000 to $1,500 a day. That is the honest counter-case, and it comes from people who sell trucks for a living: a modest fuel saving is wiped out by a handful of unplanned days off the road.

Volvo’s answer to the durability question is that the D13TC uses a fixed-geometry turbocharger “designed to meet the 1.2 million mile B50 life of the engine, with no extra maintenance required”. Treat that as design intent rather than a warranty, and ask your dealer what the service history actually shows on trucks in your region.

Why it exists at all

The EU requires manufacturers to cut fleet-average CO2 from new heavy-duty vehicles by 45% by 2030, 65% by 2035 and 90% by 2040 against the reference period, measured with the VECTO simulation tool. Every full percentage point a manufacturer finds in the driveline is a point it does not have to find by selling electric trucks into a market that may not yet be ready to buy them.

That is why a 1990s idea came back, and it also explains what it is up against. Aerodynamics, higher compression ratios, faster-shifting gearboxes, longer final drives and map-based predictive cruise all chase the same percent, usually at lower cost and lower complexity. The flagships this truck is cross-shopped against — the Mercedes-Benz Actros L and the Scania S-series — reach comparable fuel numbers by different routes. Turbocompounding is one answer, not the answer.

What to ask when you are buying

Ask whether the specific truck is the turbocompound variant. It is usually carried by a package badge — I-Save, HE — rather than by the engine family name, and the two are easy to confuse on a used advert.

On a used tractor, ask what work it did. A turbocompound truck that spent five years on regional multi-drop was mis-specified when new, and its fuel record will say so. A turbocompound tractor coming out of a genuine long-haul fleet has been used the way it was designed to be used.

Then price the resale honestly. These variants hold a premium in markets with long-distance work and sell at a discount where the buyer’s operation would never use the torque. If your own work does not fit the profile above, the cheaper standard engine is not the compromise — it is the correct specification.