Scania on how lower revs yield higher profit
Hauliers that specify their trucks for lower revs at cruising speed can lower their fuel bills by three to ten percent in some cases. But this requires fresh thinking from both buyers and drivers, writes 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. If the propshaft needs to rotate fewer times to cover a given stretch of road, energy losses in the powertrain 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 powertrain, 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 propshaft must rotate 3.08 times in order for the rear wheels to rotate once. 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 kilometres per hour, it means approximately 200 fewer revs per minute.
This reduces fuel consumption by three percent or more − in some applications by up to ten percent. In long-haulage, an annual mileage of 150,000 km is not uncommon. Assuming an average fuel consumption of slightly above 30 litres per 100 km and a typical diesel price of more than EUR 1 per litre, a three percent saving will lower fuel cost by close to EUR 15,000 per vehicle each year. That level is not possible in all transport applications, but for a haulier with many trucks, this adds up to major savings.
So what needs to be done to implement such a change in practice? Above all, two things: Choosing a different rear axle ratio, and persuading drivers to accept a slightly different driving experience. In some markets this is already standard on long-haul trucks, for example in Great Britain, Poland and Spain. Elsewhere this trend has only just begun. It is naturally impossible to lump together all transport applications and to uncritically choose a specification that lowers engine revs in an entire fleet of vehicles. When the engine frequently needs to operate at maximum output due to hilly terrain, or when a vehicle often carries the maximum permitted payload, there may be reason not to choose a faster rear axle ratio. The fuel savings are less, and perhaps the vehicle owner wants to prioritise gradeability in top gear or other characteristics. 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 gearchanging, 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.
A vehicle that normally maintains a cruising speed of 89 km/h on a typical European highway is using only a fraction of its maximum engine output. An engine with the most common output of 440 hp often uses thirty percent of maximum output on flat routes. Here operators can save fuel by maintaining lower engine revs, and this applies to between eighty and ninety percent of road transport in Europe.
For a driver, using lower engine revs means a quieter cab. It also means that the driving experience may be different, since taking maximum advantage of a new rear axle ratio requires that the vehicle be equipped with the Scania Opticruise automated gearchanging system. The vehicle also moves a bit faster when manoeuvring at terminals, but drivers soon become accustomed to this.
Hill climbing ability in top gear at cruising speed is admittedly somewhat lower. One noticeable difference is that the Scania Opticruise downshifts sooner up-hill. But this in itself is no disadvantage as an early downshift can result in higher speed in the hills. It is transport time that counts, and this does not increase with the new specification.
Powertrain specifications are increasingly important, and in many cases hauliers can no longer specify vehicles in the conventional way. It is necessary to optimise the powertrain for each user’s transport task, thereby minimising the environmental impact and the fuel bill.
Some hauliers also save a lot of money by applying a policy of 85 or even 80 km/h cruising speeds. Major savings are possible due to both lower engine revs and less air resistance at slower speeds.
Engine revs at cruising speed (per minute) - Engine revs per hour
- 1,300 - 78,000
- 1,100 - 66,000
Difference 12,000 revs for the same stretch of road
Fuel savings 3–10 percent (depending on application)