Compounding a turbo comeback

By: Steve Brooks

The introduction of a 13 litre turbo-compound Euro 6 engine in Volvo’s new model range comes as no surprise. After all, it was more than a year ago we first asked if Volvo Group Australia was testing a turbo-compound engine. The answer was a typical ‘no comment’ but it appears our speculative report in late 2019 was right on the money.

Compounding a turbo comeback
Side view of Volvo D13TC turbo-compound Euro 6 engine. Certainly more complexity but the end result, says Volvo, is significantly improved fuel efficiency.


The following is an abridged version of a Steve Brooks feature story titled ‘Volvo Tests Turbo Compound Down Under’ published in November 2019. It may help to understand how turbo-compounding works and Volvo’s motives for resurrecting the technology despite the negligible success of earlier attempts.

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Every two years on the eve of the Brisbane Truck Show, Volvo Group Australia (VGA) holds a media conference to discuss, among other things, the state of play of each of its three brands and more broadly, deliver an overview of the business and fiscal factors affecting Australian road transport.

After the presentations comes the Q&A session where the media mass breaks into small groups for a few minutes of question time with the leader of each brand as well as a face-to-face chat with the boss. These are sometimes good sessions. Other times not. It all depends on the question.

If, however, you’re hoping for even a vague answer to any question on upcoming product developments, well, you might as well park the pen because all you’re likely to get is a repeat of the tired response that goes along the lines of ‘we never talk about future product.’

The thing is, though, in this age of information overload, instant communications, and the obvious amortisation that drives every corporate giant in every corner of the globe, it is an increasingly lame response.

Take VGA, for instance, and the question of whether or not the 13 litre D13TC turbo-compound engine now available in America and Europe is being tested here in preparation for our market.

Following the TC engine’s release in America and more recently Europe, it doesn’t take the nous of Nostradamus to speculate that it is most likely being trialled here, largely to improve the efficiency, performance and even lifespan of Volvo’s (and possibly Mack’s) versatile 13 litre engine in the hard and fast world of linehaul B-doubles.

When it comes to turbo-compounding anyone with their ear close to the ground knows only too well that Volvo Group is revisiting the technology in a big way despite much earlier experiences which were memorable for all the wrong reasons.

This time around, however, turbo-compounding is being applied in the pursuit of greater efficiency rather than to simply fill a performance gap; a pursuit that suits Volvo’s local ambitions right down to the ground.

Some Background

In basic terms, a turbo-compound system employs a second turbine driven by engine exhaust which captures a significant proportion of the heat energy that would otherwise be lost through the tail pipe and, by channelling it through a gear train and clever fluid coupling, punches more power into the crankshaft via the flywheel. In principle and in practice, turbo-compounding is an effective waste recovery system.

Complex and costly as it may appear though, there is nothing particularly new about the technology. In fact, ‘the other Swede’ Scania was first of all the world’s truck makers to offer a turbo-compound production model when it introduced a 400 hp, 11 litre lump around 30 years ago. Cummins, Daimler, Iveco and obviously Volvo are among others who over several decades have also spent plenty on development and testing of turbo-compound systems, with varying levels of success.

Volvo’s first use of turbo-compounding in Australia came soon after the arrival of the new century when, in desperate need of higher horsepower to meet increasing B-double demands, a 500 hp (2400 Nm/1770 lb ft) turbo-compound version of its former D12D 12 litre engine was released here with some highly hopeful fanfare.

However, in both durability and efficiency, it did not do particularly well and the subsequent arrival of a reborn 16 litre engine followed two years later by an entirely new 13 litre displacement saw turbo-compound technology abruptly shelved, seemingly forever.

But as the saying goes, ‘never say never’.

Just when it seemed turbo-compounding had slipped off Volvo’s radar for good, Volvo Trucks North America (VTNA) late in 2016 announced the arrival of the D13TC engine, along with the claim of ‘a 6.5 percent improvement in fuel efficiency compared with previous engine models’. Similarly impressive was a 50 hp gain in output thanks to the captured energy of exhaust gases.

Mack followed suit in 2017 with its version of the same engine, the MP8-TC.

Delivering the holy grail of strong low-speed performance and appreciably better fuel economy required a new approach, with VTNA product marketing manager John Moore stating that the D13TC, "…. is not the turbo-compound engine of the past. It has been completely redesigned to work at low engine rpm and provide maximum fuel efficiency through extreme (engine) down-speeding.

"We’ve taken the technology of the past and we’ve tweaked it with our own innovations in our integrated driveline, and we’re making it work.

"In the past, turbo-compound designs were set up to run on performance only. You really didn’t see much in fuel efficiency (but) this engine is designed to do both. We designed the D13TC to run at low rpm for maximum fuel efficiency, and to be able to run at 1400 to 1500 rpm to give us the performance we need when we do find ourselves in more aggressive terrain," said John Moore.

Subsequently, with US experience supporting the case for turbo-compounding, Volvo added the D13TC to its European stable in March this year (2019), specifically to partner and derive maximum effect from the I-Save drivetrain package developed for its flagship FH range.

Volvo is claiming a seven percent improvement in fuel economy from the I-Save powertrain, utilising (among other things) tall rear axle ratios and a 300 Nm boost in peak torque output provided by the turbo-compound system. (In the 500 hp D13TC engine for Australia, peak torque is out to a gritty 2800 Nm/2065 lb ft).

I-Save is the platform for what Volvo describes as a ‘long haul fuel package’ consisting of tailored cruise control and transmission functions, the I-Roll freewheeling feature, a variable output power steering pump to reduce parasitic losses, and automatic engine idle shutdown.

Predictably, the 13 litre TC engine comes with a number of innovations including wave-shaped piston crowns that guide heat and energy into the centre of the cylinders to enhance combustion efficiency. On the scales, the system is said to add around 100 kg to the weight of a D13 in Euro 6 guise.

Respected British trucking scribe Brian Weatherley, writing in leading UK publication Commercial Motor, ironically likened Volvo’s return to turbo-compounding as something akin to a boomerang; ‘And just like the curved stick that circles back, compound turbocharging returns otherwise wasted exhaust gas energy back into an engine, boosting its power and torque, all for free.’

With the engine designed for the Euro 6 emissions standard, Weatherley noted, ‘… in addition to its SCR emissions control system, the D13TC also features cooled exhaust gas recirculation (EGR) which not only further reduces the amount of NOx created during combustion but reduces AdBlue costs too.’

But above all else, "The reason for using the turbo-compound unit is simply that it saves fuel," says Mats Franzen, Volvo Trucks’ director of powertrain strategy who adds that, unlike earlier generation turbo-compound engines, the D13TC and I-Save are at their most effective between 1000 and 1100 rpm.

"The improved torque level allows the use of a faster rear axle (2.83:1) while still maintaining driveability and traction," Franzen continued. "The savings created by the I-Save package balance well with the additional cost, resulting in a positive business case for long-haul operators."

Still, when it comes to developments for the Australian market, the whole turbo-compound topic remains rooted in rumour and speculation, and it’ll probably stay that way for at least as long as the corporate doctrine  continues to be ‘no comment’.

Or, of course, until speculation morphs into fact.

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That speculation has indeed now morphed into fact following the recent announcement of an entirely new Volvo model range.

See how Volvo announced its biggest ever new model range, here.

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