Cut trailer drag for more mpg
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REDUCE fuel consumption by eliminating the low pressure area formed behind a truck at motorway speeds. That is the idea of aerodynamicist Mr Dan W. Wright.
Most trucks and vans are cut off square at the rear : a design which causes a low-pressure area to build up behind the vehicle on the move. This sets up a drag at the rear of the container or boxvan against the forward motion; and it is also responsible for the buildup in this area of air-and water-borne grime.
This drag component is known as "base pressure drag."
The total drag of a vehicle is made up from a number of factors. These include skin friction as the air passes close to the container surface, induced drag, profile drag and base-pressure drag. Only the latter can be altered without making major alterations to vehicle geometry.
The particular idea suggested by Mr Wright is that a duct, open at both ends be incorporated in the design of the vehicle (or semi-trailer). The standard chassis layout of a heavy vehicle could be simply adapted to this aim by boxing in the open-frame section to form two ducts, the ends of which would both be open.
To help the air flow through the frame, the forward facing ends could be fitted with scoops to direct the passage of air. The enclosed pipework and wiring in the chassis should not present too much of a problem in this respect as detachable panels could be built in for access.
In the case of a container, the structure could be fabricated with a double skin to further ducts at the rear or alternatively the back face of the box (the doors themselves if necessary) could have a perforated outer panel.
The principle of operation for an artic/semi-trailer combination can best be illustrated by reference to the diagram. The air is collected by the scoops (5) and passed through the ducts formed by the box members (3) under the body (2). The rear panel (or doors) (4) is perforated.
As the truck moves forward, the air flows through the ducts and is exhausted to the rear of the vehicle to raise the base pressure in that area. This has the effect of reducing drag.