The US Department of Energy has set its sights on a dramatic increase in fuel efficiency, for heavy-duty diesel engines in particular. With the participation of Daimler, Volvo, Navistar and Cummins (working jointly with OEM Peterbilt), under the ‘SuperTruck’ banner, the DoE is striving for a quantum leap in truck and bus fuel efficiency.
Today’s most advanced automotive diesel engines achieve a fuel efficiency of around 42%. That is to say, only 42% of the energy in the fuel is converted into usable performance at the wheels. The remaining 58% is thrown away in overcoming friction losses, air drag, tyre rolling resistance and, perhaps most critically, wasted heat.
30 or so years ago it was hoped that ceramic piston crowns and valve heads would allow engines to run hotter, requiring far less heat to be dispersed
By 2016, it is hoped development programmes will raise that figure to 50%, says Patrick Davis, director of the DoE’s Vehicle Technologies Office. Part of the improvement will come from further refinements of established fuel saving measures, notably higher injection pressures, better air (i.e. turbocharger/intercooler) management, lower-friction pistons and cylinder liners and more aerodynamic vehicles. But the most innovative part of the strategy will come through the recovery of engine heat that is currently lost through the exhaust and the cooling system.
Engines must be cooled, otherwise components would fail prematurely through extremes of temperature. 30 or so years ago it was hoped that ceramic piston crowns and valve heads would allow engines to run hotter, requiring far less heat to be dispersed – and lost – through the cooling system. But emissions legislation controlling oxides of nitrogen (NOx) intervened. Higher combustion temperatures unfortunately ratchet up NOx levels.
New forms of engine heat recovery are now being looked at. But, says the DoE’s Davis, “they’ve got to be affordable and durable and must fit under the hood of a truck”. Turbocompounding, as already used on Daimler’s Detroit DD15 engine, which takes the exhaust leaving the turbocharger through a second auxiliary turbine, is a possibility, though a mechanical drive to the flywheel is effective, in fuel economy terms, only on near constant-speed long-haul operation. The DoE SuperTruck programme is tending to favour a turbocompound-like solution with hybrid overtones, the secondary turbine being harnessed to a generator charging a battery able to augment engine power.
The DoE SuperTruck programme is tending to favour a turbocompound-like solution with hybrid overtones, the secondary turbine being harnessed to a generator charging a battery able to augment engine power
Serious consideration is also being given by the DoE consortium to a more radical form of heat recovery based on the Rankine cycle, comprising a closed-loop system that captures exhaust heat and runs it through a heat exchanger that converts a liquid, of which there are several alternative formulations, into a super-heated and pressurised gas. The gas is fed through an expander turbine, whose output can again be harnessed either mechanically or electrically, before being returned into a liquid in a refrigeration-like heat-exchanging condenser.
All four participating manufacturers are expressing cautious optimism that heat recovery could be the next big step forward. But the costs involved in a Rankine cycle installation have yet to be quantified. It is likely that the first trials in two or three years’ time will be on long-distance heavy trucks where fuel costs are operationally most significant.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
Alan Bunting has a background in engineering, and has been writing on commercial vehicle and powertrain related topics since the 1960s. He has been an Automotive World contributor since 1996.
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