Discussing truck aerodynamics with Brian McAuliffe, Senior Research Officer at the National Research Council
Q: Let’s focus on aerodynamics for most heavy-duty vehicles? Why is it important to reduce drag in trucks and what are some of the basic principles to consider?
A: Aerodynamics is important because any body moving through the air will have a drag force generated for it. It’s something that resists its motion through the air.
Essentially on a truck – as with any vehicle – that drag force has an impact on the vehicle performance and in particular for heavy trucks where fuel consumption is very important. So we want to try to minimize the aerodynamic drag as much as possible.
It’s a little complicated when it comes to heavy trucks because there are multiple bodies. You have the tractor and the trailer, which can make it difficult to find the appropriate pairing of technologies for both.
Often times, the tractor may be owned and operated by someone different from the trailer and each tractor might be pulling different trailers every day and in general there are probably two to three trailers out there for every tractor. It’s not as a trivial task as it would be for, let’s say, a passenger vehicle where you can optimize a shape because it stays that way, always.
With a heavy-duty truck, it’s always paired differently and there are a lot of operational considerations with the setup of the truck. For example, things as simple as the tractor-trailer gap. When that’s changed, it can change the aerodynamic performance.
It’s really a task of trying to find the right technologies that will work together in unison for the entire vehicle and for all of its possible options. It makes it a little tricky, however, there are families of technologies that tend to work better than others.
Q: What are some of the most cost-effective aerodynamic modifications for freight? How do they work?
We’re essentially at a state where most of those general technologies have been proven.
For the tractors themselves, the roof fairing is one of those really important contributors to reducing the drag. Things like chassis or tank fairings alongside the tractor also do have an impact; they will save fuel over time.
With roof fairings, one of the main things is the importance of the proper height matching. If you’re pulling a dry van trailer or reefer, a high roof fairing on your tractor is good because you’re matching the height of the tractor with the trailer.
However if you’re pulling a flatbed with very little cargo on it, the high roof fairing isn’t very good for you. It would be better to have a lower roof fairing. Of course this complicates the logistics of shipping if you’re switching tractors and trailers regularly. But in general, you’ll get the best benefit by trying to match the height of the tractor and the trailer with its load.
Side extenders that close up the aerodynamic gap between the tractor and the trailer where turbulences occur are really important and many manufacturers have a lot of good options for those.
Also, a lot of manufacturers now have aerodynamic mirrors that really help because the mirrors can contribute to upwards of one percent of the total fuel use of the vehicles. Over time, that adds up.
Some of the more nuanced features these days on tractors are things like what we call air dams along the bottom of the bumper. These downward extensions really prevent air from getting sucked in the under body and interacting with the drivetrain components. If you can push the air around the vehicle, around the smooth sides of it, then you can save fuel.
Side skirts on trailers are a given. I’m surprised when I still see trailers without them. They’re probably the most cost-effective devices on the market. They will make their money back very quickly for most long haul operations. Probably a year and you’ll make your money back on those.
Then, if every truck puts aerodynamic mud flaps or wheel covers, each truck may only see a small benefit, but overall the aggregate will be large. Any technology that helps fuel savings – especially simple ones that are inexpensive – will benefit the operators economically and the environment in the end.
One of the technologies that hasn’t taken hold as strongly as I might have expected are things like the trailer tails. Those are fairly effective for long haul operations but I still see a lot of them closed up on trailers on the highway.
I know they tend to have some operational issues. They’re mechanical components that move, drivers have to remember to open them up and sometimes they’re not as reliable as they could be. But they’re getting more dependable.
However what we started to see, sort of as a replacement for tails, are smaller fairings that sit on the back vertical edges of the trailer and also along the top, just above the rain gutter.
These curved panels essentially redirect the air inwards behind the truck, in a similar manner to a tail. Several manufacturers are starting to make them. They won’t have the same magnitude of performance as the boat tail but they do have a measurable fuel savings associated with them. They’re cheaper to fabricate to begin with and maintenance will be a lot easier to deal with. And also they would work on roll-up doors. A lot of the trailer tails are designed for swing doors.
The performance of trailer tail devices is fairly sensitive to the location of the trailer bogie. In order to optimize your performance with the tail, you want to have your bogie in as forward a position as possible. The further back the bogie is, the less performance you’ll get from the tail.
Q: What are the benefits of a more aerodynamic vehicle?
There are really two key outcomes from aerodynamic trucks. One is the economic benefit, which is really important for the operators and, in the end, consumers and the price they pay for products on the shelf. Fuel use is still a large contributing factor to the overall operational costs of any good’s transportation.
You also have the environmental benefits, where by saving fuel, inherently you’re reducing greenhouse gas emissions from the vehicle.
Aerodynamics allows you to tackle both problems with the same technology because you’re saving fuel. So you have the economic benefits and you have the societal benefits. They’re important these days with our attempts to reduce our carbon footprint in the world.
Q: What kind of advancements are there recently in aerodynamics?
Most of the technologies on the market are essentially improvements or variations on what we’ve seen for the last decade.
But some of the things we’re going to see moving forward are some more advanced technologies associated with connected and automated vehicles.
We hear about platooning a lot these days for trucking. The foundation of platooning and the foundation of the fuel savings benefits from it is essentially an aerodynamic benefit.
When two vehicles get very close together, the net overall aerodynamic drag on the two bodies is reduced.
It’s definitely a phenomenon that you can take advantage of. The big question now is whether or not it will be implemented strictly like an SAE Level 1 type of environment, which is a very simple level of automation, strictly about the speed control. However the benefits of platooning will translate into all aspects of higher level automation in trucking.
There’s still a lot of discussion about the feasibility and the long-term outcomes. I personally believe that platooning is an effective technology and will result in long-term benefits.
That’s probably where we’re likely going to see some of the next generation aerodynamic benefits in the future.
Q: Many people think that, in order to be more fuel-efficient, a company needs to buy new trucks. Can a company, with the appropriate aerodynamic modifications, significantly improve the fuel efficiency of their fleet?
Older trucks on which aerodynamic technologies are added can be great. Whether it’s an old truck or a new truck, the more streamlined the vehicle is, the more it will save fuel.
Given the low price points we see on some of the current aerodynamic technologies for trucks, often times they’re a really good choice for retrofit.
Natural Resources Canada offers three programs that can help freight carriers improve performance whether they are looking to benchmark their fuel use, train their drivers, or upgrade their fleet – the SmartWay Transport Partnership, SmartDriver, and the Green Freight Assessment Program work to help carriers save fuel and cut costs. For more information on these programs, please visit:
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