The greatest power demands on an engine occur during initial launch, rapid acceleration and climbing grades. As a result, most engines generate significantly more power than what is required to maintain cruising speeds on relatively level highways, says Bob Johnson, the retired director of fleet relations for NTEA, the association for the Work Truck Industry.
Advances in engine management systems, coupled with advanced transmissions, may allow engines to operate at significantly reduced rpm when operating under cruise conditions, he says, and notes that this is already done to some degree in over-the-road applications.
“But if done in advanced transmissions, this may allow the use of very low overdrive ratios – 0.50 or lower – to further reduce engine cruise rpm.
“Transmissions may use multi-step fixed overdrive gear ratios, some form of overdrive torque converter or possibly even continuous variable ratio (CVT) systems. These designs are mainstream in light vehicle applications, but can absorb limited input horsepower.”
Johnson says the challenge from an engine management point of view is to control valve timing, fuel injection rates and timing and ignition points to ensure efficient combustion at these low rpm.
“In general, these engine management technologies are already developed, but they may need some fine tuning, so transmission design is the primary challenge. At this point, I suspect this to be primarily an issue of cost as opposed to technology for multi-gear concepts.
“We already see nine- and 10-speed automatic transmissions coming to market,” he continues. “The development of high horsepower CVT systems is still in the future.”
