When most people think of their car’s fuel tank, it’s a simple thought about keeping it from running dry. For engineers, designing an automotive fuel tank is a complicated (and sometimes costly) endeavor. Engineers invest lots of time studying how fuel will behave inside a tank that contains gasoline or diesel fuel to accelerate, decelerate, corner, descend, or climb.
To a layman, the fuel tank seems simple enough. But the tank’s design depends on many parts to function. Early in the design phase, engineers must test the pumps for drawing fuel from the tank. Level meters assess the amount of fuel in the tank, and valves make sure that air is coming into the tank. Air must also leave the tank via a membrane that separates it from the fuel, so it doesn’t pollute the environment. Introduce velocity, speed, and temperature, and designing the tank becomes a complex challenge. Plus, there’s often limited time to do this work, especially when a carmaker is trying to introduce a new model to the market. If a prototype for a fuel tank fails during the latter stages of the car-making process, it can be a significant cost to redesign.
Moog Fuel Tank Test System (patent pending U.K. Patent Application No. 1300552.5).
So far, automakers have relied on test tracks to design a fuel tank. But taking a vehicle to a certified track requires a prototype. And the prototype tank must be connected to a fully outfitted, operational prototype car. Prototypes are expensive to make, and the carmaker has to get its prototype and measurement equipment to the track, which incurs labor and logistics costs. Safely navigating the course with the prototype is also a concern both for the driver and the designers.
To overcome some of these hurdles, automakers turn to simulation. The simulation table design has two equilateral triangular frames set one above the other, offset at 60 degrees. Each apex of the top triangle is connected to the two apexes below it on the lower triangle via actuators. These systems have six degrees of freedom (6DoF) wherein the table moves in the X, Y, and Z axes, with pitch, roll, and yaw. Engineers mount a test specimen, such as a fuel tank, atop the simulation table, or shaker table, to mimic the twists and turns of the road. Software connected to the table simulates road conditions and records how the tank responds.
However, even today’s 6DoF electric simulation tables have limitations when it comes to testing fuel tanks. For example, with a 6DoF simulation table, it’s difficult to replicate a combined pitch and roll angle of more than 25 degrees, which is commonly experienced with mountain driving or very fast stops. In addition, mimicking continual acceleration with the 6DoF generally tops out at approximately 4 m/s².
Using a universal joint to connect a 6DoF electromechanical simulation table to a 2DoF tilt table on top for increased pitch and roll motion, it’s possible to reproduce higher frequency road profiles, where the tilt table simulates the cornering and acceleration behavior of the vehicle. It can even be thought of as an 8DoF test system. Such a test system enables carmakers to include different conditions during driving that lead to extreme fuel-sloshing effects, such as mountain driving, instant braking, or sharp cornering maneuvers. Software linked to these systems replicates and plays out time history drive files that were recorded at the test track or perform resonant-frequency research. With an 8DoF system, carmakers can achieve total pitch and roll angles of more than 50 degrees and continual acceleration of up to 12 m/s².
Being able to simulate the sharper cornering and higher acceleration found in real-life driving increases the speed of testing and saves cost, because it reduces the use of outdoor test tracks. And simulations in the laboratory give engineers a way to set up tests faster and run test programs whenever they need to, under repeatable conditions. It’s also possible to add a climate chamber to the test system to simulate how the fuel tank and fuel behave across a range of temperatures. This is another way to eliminate the need for outdoor testing.
Fons Hoeberichts is a program manager for test systems and Ashgard Weterings is a system and software engineer at Moog.