Turbo-compound engines explained

This arrangement is easily confused with a twin-turbo or series-turbo arrangement, such as employed by Caterpillar on its ACERT engines and on several 4x4 ute diesels, but turbo-compounding is different.

A twin-turbo installation is usually on a V-engine, with one turbo on each cylinder bank.

In the case of series-turbocharging, a low-speed turbo is paired with a high speed turbo and both feed the engine inlet manifold, to provide optimum boost across the engine’s full operating range.

Where turbo-compounding differs is that the primary turbo is a variable-output one and the second turbo is there solely to recover residual exhaust gas energy. The second turbo is not connected to the engine inlet manifold.

Turbo-compounding isn’t new, having been tried first in the 1950s, to increase the efficiency of reciprocating aero engines. Efficiencies of up to 40 per cent were achieved and that’s excellent from a petrol engine.

A power-recovery turbine was positioned in the engine exhaust manifold and mechanically coupled to the propeller, using a continuously-variable transmission.

As these engines developed, it became apparent that the turbine section was simpler, more reliable and more efficient than the piston engine, so aeronautical engineers got rid of the reciprocating engine entirely.

Today, most commercial aircraft engines are turbine-based turboprops and turbojets. (A turboprop engine is a jet engine with a gear-train that drives a propellor.)

Turbine-powered car and truck engines were trialled in the late 1950s, most notably by General Motors, but were prevented from commercialisation by fuel consumption and exhaust heat and emission issues. (One GM test truck set off fire sprinklers during its development!)

Given that truck-engine makers couldn’t go down the aeronautical route and dispense with the internal combustion, reciprocating diesel, attention then turned to ways of recovering some exhaust gas energy, after it had driven the primary turbocharger.

Mitsubishi was one of the first engine makers to go down this route, using turbo-compounding on its V10, 10ZF tank engine, in the 1960s.

The first production truck application of turbo-compounding was in Scania’s Three-Series models, in 1989. On an Australian-journalists’ trip to Sweden in late 1988 I saw a prototype TC engine that achieved 400hp – 37hp more than the standard engine – with a claimed fuel consumption improvement of around five per cent.

On a subsequent trip to visit Cummins in the USA we journos were allowed to drive a prototype 450hp, L10TC engine in a Kenworth evaluation truck, but were not allowed to report on it, pending test results. We also viewed some military N14 and V903T engines that were fitted with turbo-compound kits, including one destined for the US Army’s Bradley Fighting Vehicle.

Scania was first into production with its turbo-compound 11-litre and I drove two TC trucks in Sweden in 1992 – one with a manual 14-speed box and the other with a prototype computer-aided-gearshift (CAG) transmission. (The CAG test box had some neutralising issues, but was the basis of all of today’s automated-manual heavy truck boxes.)

The ‘free’ horsepower and torque in the 11-litre TC engine was obvious, but when driven with enthusiasm, there didn’t appear to be any fuel consumption advantage over the 363hp version.

Scania kept a close eye on its TC engine in European fleets and didn’t appear keen to send it Down Under. That didn’t happen for 12 years, when new-generation engines were released.

The first production turbo-compound engine to reach Australia was Scania’s 470 engine that was based on the then-new 11.7-litre donk.

Maximum power was 470hp (346kW) at 1900rpm and peak torque was 2200Nm (1630lb-ft) in the band from 1050rpm to 1350rpm. In addition to the turbo-compounding module this engine also featured the latest Scania HPI high-pressure injection system, jointly developed by Scania and Cummins.

Scania engineers said that exhaust gas left the 470 combustion chambers at around 700 degrees C and, after driving the primary turbocharger, the gas flow had slowed and its temperature had dropped to 600C. This gas flow then passed into the secondary turbine where it slowed further and the temperature dropped below 500C.

The secondary turbo's turbine shaft had a small gear at its outer end, meshed to a large gear on the outside of a fluid coupling. The output shaft on this coupling also had a small gear that meshed with an idler gear, linked in turn to a gear on the crankshaft.

This three-gear step-down dropped the secondary turbine's speed to crankshaft speed, while the fluid coupling performed two important functions: absorbing speed fluctuations in the gear-train and preventing any torsional vibrations from the crankshaft passing back to the secondary turbo.

Speed fluctuations occurred when the engine was accelerated and the secondary turbo 'lagged' behind the primary turbo and the crankshaft, or when the engine was slowed suddenly and the secondary turbo oversped the gear-train.

Torsional vibrations were created by camshaft-driven, high-pressure injectors and by the combustion 'pulses' in the crankshaft.

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The complexity of the turbo-compound design hardly seemed worth the effort, given that Scania had a 480hp V8 available, but the 16-litre V8 tipped the scales some 200kg heavier than the six and the fuel consumption saving for the TC engine was a claimed 3-5 per cent.

There was also the noise question that was much more significant in Europe than here. A detuned version of the 470 rated four ‘Eco Points’ – a significant advantage for trucks that had to pass through Switzerland and Austria.

In Australia, the TC engine could be ordered with the standard GRS900 14-speed transmission, fitted with optional hydraulic retarder, or Scania’s Opticruise, CAG-origin, automated shifting.

My testing showed up an issue that was almost certain to negate any potential fuel consumption saving: Australia’s B-Double weights and road speeds vs European ones, plus our steeper highway gradients.

To keep the 470hp engine moving under Australian conditions it was necessary to downshift much earlier than would be the case with Europe’s roads, speeds and lighter loads.

Fleets discovered that drivers liked the TC engine’s added performance and flexibility, but used those advantages for better performance and there was little or no fuel consumption benefit. Given the additional cost and complexity of the TC system, fleets found that it just wasn’t worth it.

Volvo countered Scania’s initiative with its own TC engine: the D12D 500. It turned out to be a stop-gap engine in the Australian market, because although 500hp at 1600-1800rpm and 2400Nm in the 1000-1300rpm band worked well with Europe’s 42-tonnes GCM, it was marginal at 60+ tonnes and 100km/h in Australia.

The D12D TC engine’s best economy was at 1100-1300rpm and the engine just couldn’t do the B-Double job in that revolution band.

When the Volvo 16-litre arrived shortly afterwards the TC six went away quietly.

Experience with turbo-compounding showed that it could make a smaller engine behave like a larger one, but fuel economy wasn’t automatic in Australian conditions. Fleets could get economy benefits, but only with strict driver control.

Daimler had been working quietly on turbo-compounding for its new generation engines that were launched in the USA in 2007, before becoming globally available. At various times and in different markets the DD13, DD15 and DD16 engines have been available with and without turbo-compounding.

These engines have established a bench mark for economy in Australia, but the TC mix is different from that sold the USA and Europe.

The DD15 with turbo-compounding was introduced to Australia in October 2010, in the Coronado 122 and is still available. However, TC was discontinued on the DD15 for North America around 2015, probably because the USA’s lighter highway weights – only 36 tonnes GCM – didn’t make the 15-litre engine work hard enough to justify TC cost and complexity.

On the Mercedes-Benz side, the 16-litre OM473 with turbo compounding was introduced with new-generation Actros in 2016 and the North American-sourced DD16 keeps the TC system. The DD16 was introduced to Australia in the Cascadia at the end of 2019.

However, the shorter-haul, lighter-GCM DD13 that has TC in some markets is sold in Australia without TC.

The secret of Daimler’s now-legendary economy from its OM and DD TC engines is engine-revolution and fuel-burn control, where cruise control and automated-manual transmissions are key.

Volvo’s 2020 turbo-compound engine, the D13K500TC, shows that Volvo has learnt from its previous TC effort. Where this engine differs from its predecessors is in its powertrain packaging.

In the Australian market the engine comes with 500hp that seems light-on for 68 tonnes work, but peak torque is a hefty 2800Nm, across the very low 900-1300rpm band.

Even at top GCM weights this engine is designed to operate below 1600rpm and to make sure that happens it’s available only with I-Shift automated transmission and a very tall 2.83:1 final drive ratio.

Volvo Truck says that turbo-compounding raises the engine’s brake thermal efficiency – the amount of energy produced by a litre of diesel to power the wheels — from 43 per cent to 48 per cent, but is not expected to need additional maintenance over the expected life of the engine.

My testing of this engine showed that it tried to operate all the time in the highest possible gear and the prodigious torque at low engine revs saw the tacho showing as little as 1200rpm when highway cruising.

However, it’s early days for this package, so we’ll monitor some of the early fleet trucks, to see how their economy is working out in the real world.

All diesel engine makers are investigating ways of utilising exhaust gas energy and among these is an electric turbo-compound (ETC) system. This involves using a turbine that’s coupled to a high-speed alternator in the exhaust gas flow, to convert exhaust energy into electrical power.

An ETC is like mechanical turbo-compounding, but with an exhaust- gas-driven alternator replacing the gearing and fluid coupling of mechanical turbo-compounding.

This AC or DC electrical power can be used to power accessories and in doing so, take the load off the engine’s alternator.

Another research area is thermo-electric generation, where the temperature difference between the two surfaces of a thermo-electric module generates electricity using what is known as the Seebeck Effect. A typical difference of several hundred degrees is capable of generating electricity.

This technology is already proved in some static engine applications, but poses difficulties for truck designers. One obvious issue is that current emissions equipment relies on exhaust gas temperature to function properly, so too much exhaust gas temperature drop would have an adverse effect on the particulate filter.

Regardless of the degree of future truck electrification in distribution and ‘last-kilometre’ operations, line-haulers will rely on diesel engines for the foreseeable future. The pressure to make them more efficient just keeps increasing.