Slipstreaming for fuel economy
On a racetrack you will see cars slipstreaming / drafting behind each other to go faster, but is it possible to save fuel by drafting? I conducted a little experiment in SolidWorks CFD to see the aerodynamic effects. Driving close to a large vehicle creates a much larger slipstream than a car, but just how close do you need to be?
The CFD model
The model is 3 dimentional constructed in Solidworks in full 1:1 scale 3d. A wind tunnel simulation of 70 miles per hour winds (UK motorway speed) shows the size of the truck’s slipstream. The small hatchback is slipstreaming the semi truck in front.
The orange zone on the image below is air moving at 20 miles per hour. instead of 70 miles per hour. The idea would be to get the car in that zone so that the car faces less wind resistance and uses less engine power to maintain speed.
In picture 1 the car is inside the 30mph zone in yellow, which is at about 2 car lengths. The car no longer faces the full wind resistance from travelling at 70 mph. This will result in:
- Reduced load on the engine
- Improved fuel economy
- Reduced stress on the engine components
In picture 2 the car is rolling in the 20 mph zone. This looks like around 1 car length and clearly is not a safe place to be! Painful death is a risk and the grim reaper has to occupy your passenger seat. Tailgating in this way could be deadly on a normal road. The risks are significant:
- Little or no visibility of the road ahead
- The driver of the truck cannot see where you are
- Little or no braking distance in the event of an accident
- Objects thrown by the truck wheels (stones or debris) will be unavoidable
Here’s a more detailed closeup.
The theory is sound – you can save fuel by drafting behind another vehicle.
However – don’t. You’ll probably die.