24 November 2016
Fuel-consumption targets and emissions legislation, combined with the relative maturity of engine and bodyshape designs, are forcing commercial-vehicle manufacturers to find innovative new ways to improve the efficiency of their vehicles – and one of the latest technological developments in their armoury is digital simulation.
Indeed, testing systems in a virtual environment is now a key tool in the development of more fuel-efficient and eco-friendly vehicles.
Computational Fluid Dynamics (CFD) is one form of simulation in which development engineers are increasingly placing their trust. CFD solves complex equations to simulate the flow of fluids and gases across, around or inside an object. The results are then rendered in highly realistic graphics that enable engineers to visualise the flow around a vehicle body or through an internal combustion engine, for example.
CFD is now sophisticated enough to be used by development engineers as a predictive tool, rather than as a means of validating the results of laboratory tests. CONVERGE CFD software, which is used in combustion development, automates the generation of the ‘mesh’ or grid that digitally represents the surface over which the flow occurs – the inside of an intake manifold, for example. Because the mesh is always the same, the results of the simulation are repeatable and the performances of different designs can be directly compared.
As a result, engineers can evaluate the performance of perhaps 10 different combustion chamber designs or fuel-injector spray patterns in simulation, and then concentrate on the best-performing options during laboratory testing.
Ford used Convergent Science’s CONVERGE CFD during the development of its new EcoBlue diesel engine for the Transit. One of the areas where it was applied was in perfecting a new, mirror-image porting design for the integrated inlet manifold. This enabled the engineers to have closer control over the way the engine burns fuel.
Ford says that when fine-tuning the combustion process, digital experiments used measurements from more than 1,400 factors that affect performance of the valve lift and timing alone.
“CO2 reduction has become the major driver in powertrain development right across the industry and numerical simulation has been the main tool for many years now, but we’re relying on it more and more as software and computing power improve,” says Dr Werner Willems, a Ford technical specialist for combustion systems.
In aerodynamics as in powertrain development, the gains to be made are relatively small, which magnifies the need for very precise, repeatable and predictive test methods. Those qualities don’t always apply to the physical testing of prototype vehicles, where changes in climatic conditions or differences in the configuration of wind tunnels can lead to inconsistent results.
When it comes to testing the aerodynamics of heavy trucks, it can be hard to find wind tunnels large enough to accommodate a full-size articulated rig, so tests are often conducted on reduced-scale models. That can introduce further uncertainty into the results because the detailed geometry and cooling airflow of the full-size rig cannot always be accurately incorporated into a smaller model.
Once again, CFD simulation is an appealing alternative. “Commercial vehicle OEMs are more inclined to do development work virtually,” says Kevin Golsch, technical director for North America Ground Transportation at CFD specialist, Exa Corporation, whose PowerFLOW software is used by the likes of MAN and Scania. “For example, there is no facility we know of that can adequately test a full-size [heavy] commercial vehicle. If you’ve got to make the jump to a scale model, most people will make the jump to virtual development [instead] and validate on the road when they get a full-size vehicle.”
Aerodynamic development is complicated by the fact that the cab and trailer make their own contributions to the drag (air resistance) of the full rig.
“The cab splits the air apart and the trailer puts it back together,” Golsch explains. “The relationship between how efficiently you can part the air and put it back together dictates the total system drag. That means that truck makers have to keep in mind what the trailer looks like.” The fact that the two halves of the rig are usually made by different companies, and are unlikely to stay together for the life of either vehicle, is a further complication.
In the USA, Exa worked with Peterbilt on the US Department of Transportation-supported SuperTruck research programme, which almost doubled the fuel economy of an 18-wheel truck-and-trailer combination. The CFD analysis led to some surprising findings.
“We learned that you can actually improve a truck too far and pay a penalty in overall system drag,” he reveals. “You’ve got to work on the two sides together, or you’re in trouble. If you take a SuperTruck cab – designed for a highly efficient trailer – and pull a standard trailer with no skirts, then the drag in that configuration could possibly be higher than taking a current production vehicle and pulling that same trailer. The synergy between truck and trailer is a message that people are starting to get to grips with. It’s important for us to be out there spreading the word.”
PowerFLOW CFD software, which has a particular strength in accurately simulating turbulent airflow, is also used for thermal management in the engine bay, to reduce soiling and to improve brake cooling. “If you have up a 36-ton load on a 6% incline, it can be quite important to keep the brakes cool,” Golsch observes. “And when the brakes get hot for extended periods of time, they can heat the wheels up, which in turn heats the tire bead and can lead to a blowout. Commercial vehicle manufacturers therefore look very carefully at brake temperatures and cooling, perhaps even more than a passenger car OEM might.”
Exa expects simulation-led design to replace developmental testing in many areas, well within the next decade. Not all OEMs are adopting CFD as quickly as others, but at Convergent, Kaczmarek, too, believes that the trust placed in simulation tools will only increase.
“We’re at the sunrise of this predictive capability,” he assesses. “Commercial vehicle makers are increasingly looking at how carbon deposits work, for example. It’s a very hard problem to be able to predict, but we have some customers who have had good success in predicting soot levels so it’s not completely out of bounds. And as the hardware gets faster and cheaper, it opens up the door for a more predictive type of engineering.