May 2004 Articles & Features
By Mike Dale | May 2004
Conventional wisdom suggests that dropping or removing a tailgate saves gas. But scientific investigation reveals the behavior of wind over pickup bodies isn't so predictable.
When talking about vehicles and electronics, it's easy to forget the electronics that doesn't wind up as a sensor, a module or a computer. In today's vehicles, significant electronic content is baked into the product during development. Computer modeling and its simulation of mechanical, electrical and aerodynamic systems have become the norm. These models help determine performance, function and material requirements before any product is even built.
Understanding how this works can help you learn more about the automotive development process. Besides, sometimes these computer models produce answers that really surprise you. Recently, Kevin Cooper of the National Research Council of Canada published a paper through the SAE which proved that removing the tailgate from a pickup truck not only doesn't help gas mileage, it actually makes it worse. We can all visualize the drag caused by that tailgate. We've all seen the plastic mesh replacements and the ads that claimed higher fuel mileage when the mesh is used. Removing the tailgate looks like it should re- duce drag, but a number of processes, including wind tunnel testing and computer modeling, proved it doesn't.
When it comes to the design of a new component, there are two basic approaches. One is the so-called Thomas Edison method. The Wizard of Menlo Park would test his way through an entire list of possible materials until he found something that would work for the filament of his light bulb. Thousands of materials were used and thousands of tests run until he finally hit on tungsten. You can imagine what efficiency experts might think of this approach.
The second approach is to start by making a list of all the product requirements. Then you determine what materials and what possible designs would have the properties needed to meet those requirements. Then a design proposal is drawn up. In the past, the next step would have been to go back to the Edison method. Prototype parts would be made and then tested until they failed. Design changes would be made, and then it was back to testing.
In today's automotive environment, this approach is simply unaffordable. A full design validation (DV) test can take six months or more and eat up enormous amounts of test equipment and man-hours of testing time. What's needed is a way to check the performance of a part even before it's built to make sure it will perform as intended. When that's complete-theoretically, at least-the DV test can be run once, and then it's on to production.
When a structure like a steel bridge is built, the design can be verified by using mathematics to determine the forces that will act upon the structure. By knowing the properties of the steel, the length of the beams and the forces that will be applied to them, it's possible to verify that the structure will be strong enough. The "strength of materials" equations used can be solved using relatively simple computing tools.
The problem in automotive parts is that the needed components are much more complicated than the beams of a bridge. An electronic sensor, for example, is made of multiple materials shaped into complex forms and configurations. In actual use it will be exposed to vibration, shock, temperature extremes, impact and corrosive fluids.
Two types of computer modeling programs are currently being used: finite element analysis (FEA) and computational fluid dynamics (CFD). FEA is used to model mechanical strains and stresses in solid structures such as axles and frames. CFD is used to solve problems where the forces are fluid or changeable, such as air, electricity or magnetism.
These modeling processes have been around for quite a while. But the programs and the computers that run them are much more sophisticated today than they were even five years ago. It's this increasing sophistication of software and computers, combined with the need to reduce development times and costs, that has made modeling such a basic process in automotive engineering.
The two basic modeling methods are based on different scientific principles. Finite element analysis was first introduced in the 1950s. Prior to the development of FEA and CFD software, engineers employed integral and differential calculus to solve analysis problems. This method worked by breaking the problem into an infinite number of elements.
FEA works by breaking down the structure into a fixed, or finite, number of elements. It's based on Hooke's Law (introduced by Robert Hooke in 1678), which says that an elastic body will stretch in proportion to the force applied to it. The formula F = kx states that the Force is equal to the proportional constant (k) times the distance of stretching (x). What this means is that if you start with one element and calculate the forces applied to it, you can determine what forces will be applied to the adjacent elements. If you then calculate the forces on the surrounding elements, you can determine the forces applied to the next layer of surrounding elements. In this way, you can eventually calculate the forces on all the elements in the product to be analyzed.
The foundation of CFD analysis is based on the mathematical equations of Navier and Stokes, which describe how fluids flow. With CFD, the areas of interest are subdivided into a large number of cells, or control volumes. The Navier-Stokes partial differential equations can be rewritten as algebraic equations that relate the velocity, temperature and pressure in each cell to the same types of values in neighboring cells. These equations are then solved numerically, yielding a complete picture of the airflow down to the accuracy or resolution of the cell grids. The resulting set of equations can be further processed until a complete description of the flow throughout the whole domain being studied emerges.
Several methods-including CFD flow analysis-were used to reach the conclusions presented in Kevin Cooper's SAE paper. Using a variety of popular make full-size trucks and a 75-mph wind speed, the vehicles were tested in a wind tunnel with the tailgate up, down and removed. One had a mesh gate made of duct tape. One had an adjustable tonneau cover that could cover various portions of the truck bed.
The wind tunnel balance-measured all six components of aerodynamic force and moment. Drag coefficients (Cd) for the trucks were normalized so the focus could be kept on box aerodynamics. All the measurements showed the same basic results-a small increase in drag with the tailgate down and a larger increase with the tailgate off. These measurements were made with the vehicle directly aligned with the stream of air.
Another interesting result was that lowering or removing the tailgate caused a larger increase in drag on 61/2-ft. boxes than on 8-ft. boxes. Installing the mesh gate was shown to yield the worst results of all. In one example, the Cd increased from .48 to .53 with the mesh gate in place of the solid tailgate.
What does work is a tonneau cover. Interestingly, a partial 20-in. cover placed nearest to the tailgate did 88% as well as a full tonneau cover. On an 8-ft. box, the estimate is that the improvement in fuel economy with a full tonneau could be 1.35 mpg, or a saving of 243 gallons over 100,000 miles of driving. Conversely, the same vehicle without the cover and with a mesh tailgate got 1.28 mpg worse mileage than a vehicle with the tailgate up and no cover.
The wind tunnel and road tests answer the question of whether removing the tailgate is a good idea. What it doesn't answer is why. For this, Cooper and his group used CFD software to model the flow of air in and around the box of a pickup truck. They found that with the tailgate up, a bubble of air forms in the truck bed that allows air flowing from the front to the rear of truck to ride up and over the vehicle. When the tailgate is removed, there's a stronger downwash that forces some of the air to flow over the rear bumper. This stronger downwash behind the cab is what's responsible for the increased aerodynamic drag.
Among the conclusions in his paper, Cooper points out that for an average pickup, the fuel saved by leaving the tailgate up is likely to be on the order of 122 gallons over 100,000 miles of freeway driving. On an individual basis, this is not a lot of fuel or a lot of money. However, when you apply this to the millions of vehicles out there, this really is large.
And from a safety standpoint, having the tailgate down makes it and not the bumper the first point of contact in a rear-end collision. Based upon the data-and the cost of gasoline-having a full or partial tonneau might be worth the investment.