Mechanical turbocompound systems position a second turbine downstream of the exhaust side of the first turbocharger. In the Volvo application, an axial-flow turbine returns mechanical energy to the crankshaft via a train of reduction gears.
The manufacturer announced the return of turbocompounding in 2013 with the new shape FH truck and Euro 6 driveline, but the first of the new units did not appear until last year, and were confined to the North American market. Scania dabbled with the technology with a radial-flow turbine and mechanical drive around the turn of the century, but the only current system in trucking is to be found on the Detroit Diesel DD15 engine, which is sold in Europe as the Mercedes OM472.
While the original Volvo turbocompound was used primarily as a means of increasing power output (500 hp was considered a lot from a 12-litre engine back in 2002), its primary purpose now is to contribute to the claimed seven per cent improvement in fuel economy from the I-Save powertrain by boosting torque. The D13TC is available with two outputs: 460 or 500 hp. Other manufacturers, notably DAF, can trump the 500 hp figure using a 13-litre straight-six engine, without recourse to the technology.
What the turbocompounding does do is increase torque: 300 Nm extra, according to Volvo.
Other aids to fuel economy include the fitment of a patented new-shape piston, which Volvo claims “improves combustion and increases efficiency by guiding heat and energy to the centre of each cylinder,” and single-reduction, higher-ratio drive axles to better utilise the improved torque output of the TC engine, while reducing parasitic drag.
The trucks are also fitted with Volvo’s ‘Long Haul Fuel Package’ which includes: an updated, map-based I-See predictive cruise control, updated I-Shift transmission with long-haul optimized software, I-Cruise cruise control, with I-Roll freewheeling feature, a variable-output power-steering pump to reduce parasitic losses, and automatic engine idle shutdown. These features combine to produce claimed fuel savings of up to seven per cent on long-haul operations when compared to the ‘standard’ Volvo D13 Euro 6 D powertrain.
Mats Franzén, powertrain strategy director at Volvo Trucks, said: “We have tailored every aspect of I-Save to suit long-haul operators, especially those that typically drive more than 160,000 km per year.
“The longer they drive, the more they can potentially save. It is a powerful engine which delivers a highly fuel-efficient and smooth driving experience.”
Turbocompounding is expected to be introduced by other manufacturers in due course as they come under increasing pressure to reduce CO2 output, although perhaps not as an all-mechanical system.
One application is to use the second turbine to drive an alternator which can either be used to boost charge for an electric motor on a hybrid driveline, or to power systems such as steering-pumps and air-compressors independently of the diesel driveline. However the technology is used, the goal remains the same: to harvest energy that would otherwise be wasted in the exhaust stream.