Lower revs yield higher performance

Reducing revolutions at cruising speed can significantly diminish fuel consumption. “A three percent saving will lower fuel cost by close to EUR 15,000 ($19,500) per vehicle each year,” states Hamish Bennett, product manager at Scania.

“It is possible to save fuel by choosing a faster rear axle ratio, thus lowering the number of engine revs per minute at cruising speed.” In detail: If the prop shaft needs to rotate less to cover a given stretch of road, energy losses in the power train will be correspondingly less. And so will fuel consumption. Aside from propelling the vehicle and its cargo, a truck engine must also overcome resistance in the power train, such as friction and other losses. It also has to drive all auxiliary systems, for example the alternator and water pump. At cruising speed on flat ground, this is about 15 percent of total losses, also including air and rolling resistance.

“Today an overall gear ratio of 3.08 is common, which means that the prop shaft must rotate 3.08 times in order for the rear wheels to rotate once,” Hamish explains. “If we choose a ratio of 2.59, this lowers the number of engine revs at a given speed. In top gear at a cruising speed of 89 km/h, it means approximately 200 fewer revs per minute.” This would reduce fuel consumption by three percent or more ? in some applications by up to ten percent. That level is not possible in all transport applications, but it will add up to major savings the more vehicles are involved.

Mr Bennett recommends a bidirectional approach to implement such a change in practice, suggesting to choose a different rear axle ratio and to persuade drivers to accept a different driving experience.
“When we choose a ratio of 2.59 instead of 3.08, many people believe this makes a truck feel less powerful. But the gearbox takes care of this difference in rear axle ratio, and with automated gear changing, the driver can relax. This means that using a 2.59 rear axle ratio, a vehicle may even have more power at the wheels since it is operating in a lower gear.”

For a driver, using lower engine revs will mean a quieter cab and a different driving experience, since taking maximum advantage of a new rear axle ratio requires that the vehicle be equipped with an automated gear changing system such as the Scania Opticruise. The vehicle also moves a bit faster when manoeuvring at terminals, whilst hill-climbing ability in top gear at cruising speed is reduced.