Advanced simulation aids heavy-truck aerodynamics

  • 26-Aug-2015 12:43 EDT
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External aerodynamic simulations of complex, fully detailed tractor and trailer combinations can provide fleet owners insight into ways to achieve maximum fuel efficiency. Exa’s simulations can include complex real-world effects such as rotating tires, moving ground, and realistic wind conditions. (Image courtesy of Exa/Peterbilt.)

There is more pressure than ever on heavy-truck engineers to find and create significant improvements in fuel economy and reduce tailpipe greenhouse gas (GHG) emissions. This is being driven primarily by two factors. First, customers are looking for ways to improve their business model by reducing fuel consumption. Diesel fuel prices in the past few years have been volatile, ranging from $2.931/gal (Sept. 6, 2010) up to $4.159 (Feb. 25, 2013) and back down to $2.561 (Aug. 24, 2015), according to U.S. Retail Diesel Price data from ycharts (http://ycharts.com/indicators/us_diesel_price), showing the need to be prepared for possible upswings in fuel costs of 50% or more. Additionally, regulations are driving fuel-economy improvements and GHG reduction to unprecedented levels. 

To compound the challenge of these customer and regulatory pressures, the low-hanging fruit for these gains is already long gone. Aerodynamic enhancements have historically offered the biggest opportunity, but finding significant improvements today requires a shift in testing and development to be efficient and successful, and new advancements in CFD may be the key. The aerodynamic challenge with heavy trucks is that they’re pulling a brick-like shape that has a flat wall at the end. The closing of the airflow at the end of that brick creates a wake, and inside that area is extremely low pressure that pulls on the surface, creating about 40% of the pressure drag of the entire tractor and trailer. Pressure drag is about 90% of total drag on a truck and trailer combination, thus, a key target for refinement.

Exa Corp. has been working with several truck and component manufacturers in the heavy-truck industry, and has shown how its CFD software, PowerFLOW, can evaluate and quantify small changes without expensive and time-consuming prototype builds or wind tunnel testing. PowerFLOW also offers an Optimization Solution, which uses algorithms to test hundreds and even thousands of variations, literally overnight, between a baseline design and end parameters set by the engineering team.

Designs can be morphed quickly using CFD, and unlike physical testing where the design has to be modified for the consecutive tests, users can easily and accurately return to the baseline. Additionally, wind tunnels cannot accurately address how the moving road surface and spinning tires affects airflow; this is critical now as many opportunities for aero improvements—skirts, under-vehicle panels, and wheel covers—are parts of the tractor and trailer that are close to the pavement.

Exa recently worked with Laydon Composites on a trailer skirt solution that explored which design would provide optimum results. In addition to looking for the best aerodynamics to enhance fuel economy, the team also found that with the right design they could achieve U.S. EPA SmartWay Elite status without a boat tail. This can have large operational and cost advantages for a fleet.

“Anyone can put a fairing under the trailer and get a 2-5% improvement,” said Doug Hatfield, Managing Director, Heavy Vehicles, Americas, at Exa. “We developed one with extensive simulation and achieved a 9.3% improvement in fuel economy.”

Even with improvements in fuel economy through the optimized design of a trailer fairing, Exa believes there are more opportunities. Simulation provides insight into the airflow so engineers have more control to influence it. For example, through simulation and analysis, better integration of the airflow under the truck and trailer combination can be achieved to maximize the fuel-economy gain. Already popular in Europe, there is more talk in the U.S. about matching truck and trailer systems for better aerodynamics, but it would require fleet homologation to make it more realistic for the U.S. business model.

There are also aerodynamic opportunities with active grille shutters, which are becoming common in passenger vehicles. Active grille shutters allow more airflow for cooling, but can close (forcing airflow around the engine compartment instead of through it) when not needed. Exa has found that closing off the grille to airflow results in about a 1% improvement in fuel economy overall with current design of tractors, and that there’s potentially a 3-5% gain on the table if the tractor design is optimized to make use of active grille shutters. This would require a paradigm shift in tractor design and manufacturing. Vehicle manufacturers may be more willing to embrace these shutters as pressure for improvement from customers and regulations increase.

Even without active shutters, with simulation software it is possible to explore the front-end design to reduce the frontal area but still provide what’s needed for cooling. There are opportunities with trucks designed for specific routes to optimize the front-end design to balance airflow for cooling and reduced frontal area for cooling. For example, today all trucks are designed for worst-case heat scenarios: pulling full CGVWR (combined gross vehicle weight rating) at a grade at ambient temperatures above 100°F (38°C). But if the customer will never operate the vehicle in those extreme conditions, then it will never need that much cooling capacity, and the front-end design could be swayed to favor aerodynamics.

It also will be interesting to watch as technology for self-driving trucks evolves. Truck platooning, for example, presents an interesting mix of opportunity for fuel-economy improvement and unique cooling challenges. The rear trucks in a draft will have better fuel economy, but it will have increased cooling needs. Exa has already done simulation work on platooning and found significant gains in fuel economy with 50 ft (15 m) of following distance even without unique aerodynamic design.

While the future has many variables, one certain thing is that utilizing advanced simulation tools for development and testing will be critical to efficiently identifying the next wave of fuel-economy improvements.

Cole Quinnell on behalf of Exa Corp. wrote this article for SAE Off-Highway Engineering.

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