THE DEFINITIVE TRUCKING SITE

July 2008

Fuel Saving Tips

Idling Engines

It is well-known that idling wastes fuel and adds to maintenance costs. Not so well known is just how much it wastes and costs. The American Trucking Association’s Technology Maintenance Council’s codes of practice suggest that large diesel engines powering typical six- and seven-axle rigs consume around four to five litres every two hours. Patrick Swan, the respected South African tribologist, says idling such engines use between 300 and 350 grams/ kW-hr. This confirms just how expensive it can be and how important it is to eliminate idling as much as possible.

There are several situations that cannot be entirely avoided. These include fridge units, air-conditioning, cab heaters, PTO and so on. A simple, hypothetical example illustrates the cost of fridge units that run while vehicles wait to load/unload, crawl in traffic or run to maintain the load’s temperature:

Assume a large fridge unit as used in typical 28-pallet Reefers. Maximum engine power is approximately 25 kW. Fridge unit idles for an average of two hours a day while working 260 days a year.

It pays to establish and implement an “idling policy” supported by the necessary training and management controls. 

Fridge units that have to run while waiting at offload points burn fuel. Ensure customers don’t keep your rigs waiting.

Aerodynamics 

Poor airflow, especially for high speed long distance vehicles often means poor fuel economy. There are many different types of air flow kits available to operators. However, choosing the solution best suited to your vehicle is not a simple matter and needs careful consideration. Appearance alone is not a guarantee that a particular kit will provide the expected result. 

Among the most popular solutions are the roof-mounted units some of which have open sides and do not extend to the rear of the cab roof. Others have closed sides and do extend to the rear of the cab roof. Depending on the kit, fuel saving can be as much as 15% for units pulling high trailers at high speed over long distances where no air flow equipment was previously fitted. Vehicles with raised roof cabs that permit drivers to stand up inside the cab can benefit by as much as 10% better fuel usage when the correct kit is fitted. The gap between trailer and truck or trucktractor can yield  nything between one and five percent improvement in fuel
usage.

It is important to remember that air flow kits are helpful when vehicles tend to travel at freeway speeds (80km/hr) and where the prime mover and trailer have similar dimensions. The wrong kit can have adverse effects on vehicle performance and will increase fuel consumption. According to US tests, dead bugs anywhere on the cab’s frontal areas indicates the airflow is on to the vehicle and not around or over it. In other words, drag is increased requiring more power to overcome the increased resistance. The evaluation and final choice of aerodynamic styling is so important if the full benefit of the potential fuel savings and improved vehicle performance is to be achieved.

It would indeed be worthwhile to obtain a copy of “Truck Aerodynamic Styling”, a book published by AEA Technology in Harwell, Oxfordshire in the UK. The book contains comprehensive coverage of aerodynamic basics, features, savings, evaluates manufacturers’ claims, gives a spread sheet and CD to gain a quantitative understanding of “real world” benefits. The contents cover rigid, articulated and drawbar vehicle configurations. The website address is www.transportenergy.org.uk/bestpractice

Air flow kits should be chosen not solely for their styling and good looks but more so for their contribution to fuel savings.