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Truck hybrids herald ‘electric ptos’

Previously unforeseen spin-off benefits from diesel-electric truck and bus hybrid drives are beginning to emerge. Eaton in the US has just announced the development of what it calls auxiliary power generator (APG) options for its hybrid drivelines. The corporation says the technology will “allow customers to use vital equipment without requiring the engine to idle for power generation”.

Drives for all-manner of auxiliary equipment on trucks and buses have long relied on mechanical power-take-off (pto) installations, enabling the vehicle’s diesel engine to be directly harnessed – often with the vehicle stationary – to power hydraulic tipping gears, refuse collector compaction gear and other on-board systems. Different pto configurations are available to suit diverse applications, via outlets from either the transmission or directly from the front or back of the engine. When fuel was relatively inexpensive, few qualms were expressed about pto drives, though there were always some environmental objections to the noise of engines being revved during on- or off-loading.

The traction batteries in a series or even a parallel hybrid heavy-duty diesel-electric driveline are a potential source of far more power for auxiliary equipment drives than has hitherto been available from regular lead-acid batteries.

Coincident with, but till now quite separate from, hybrid developments over the past decade or so, have been the initiatives made towards reduction of power (and therefore fuel) draining losses associated with traditional engine-driven auxiliaries. Brake air system compressors, for instance, now typically ‘freewheel’ when reservoirs are up to pressure. There is also, more pertinently, an ongoing trend to ‘demand actuated’ electric motor drives for power-steering, cab (and bus/coach) air-conditioning systems and other auxiliaries for which regular vehicle batteries and electrical controls – if necessary marginally upgraded – can provide sufficient power.

But it is clear from Eaton’s announcement that the two strands of commercial vehicle powertrain development are ready to start coming together. Crucially, the traction batteries in a series or even a parallel hybrid heavy-duty diesel-electric driveline are a potential source of far more power for auxiliary equipment drives than has hitherto been available from regular lead-acid batteries, whose most taxing requirement is arguably to provide a short-burst of power to get the engine started on a cold winter’s morning.

Eaton’s APG development and other similar projects that are sure to follow will bring worthwhile fuel savings, together with the twin environmental gains of emissions and noise reduction.

Eaton says the APG options on vehicles incorporating its diesel-electric hybrid systems will handle 115 volt single-phase and 208 volt three-phase loads. That implies ample – and silent – power for tipper or refuse compactor hydraulics, for example: applications in which there has till now, at least on heavy-duty chassis of say 10 tonnes gvw and above, been no alternative to an engine or transmission-based pto.

It is not quite a win-win situation, however, because discharging a hybrid’s traction batteries, by extracting power for the secondary purpose of driving auxiliaries, will necessarily require the engine to cut-in more frequently to restore the state of charge needed to fulfil the batteries’ prime role of vehicle propulsion. But the moves already well under way, from mechanical to electric power, for those lighter-duty truck and bus engine auxiliary drives, indicate that Eaton’s APG development and other similar projects that are sure to follow will, largely through reduced accumulated mechanical friction losses, bring worthwhile fuel savings, together with the twin environmental gains of emissions and noise reduction.

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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