Here are some additional comments to the statements above: In general, the aerodynamic drag of a car is influenced by the pressure distribution at vehicle front and rear end, the flow characteristics at side walls, roof and under the vehicle and the flow-through airstream for cooling purposes of engine and brakes as well as for climatization of the passenger compartment. All of these areas show potential for optimization:

- Front area: improved shape of the vehicle front in terms of reduction of turbulences and ram pressure spots

- Back area: low pressure areas behind the car result in a backward force that may influence the air drag significantly. Improvements of the vehicle shape at the rear end can improve this situation. Spoiler increase the down force at the rear axis, but they also increase the turbulences at the rear end, which might lead to higher air drag forces.

- Side walls and roof: avoidance of turbulences (caused e.g. by side mirrors or luggage racks)

- Air flow under the vehicle: in general, the share of air flow under the vehicle should be reduced. This can be accomplished by front spoiler (eventually adjustable) or by reduction of the ground clearance at highway. In addition, bottom covering and diffusor at the rear end may improve the air drag behavior.

- Flow-through area: an air flow through the vehicle also influences the air drag negatively. Variable air inlet for engine cooling and climate system might lead to improvements.

Optimizing the general shape of the mini van shows the highest potential for reduction air drag. In subsequent steps, detail optimization by use of CFD-simulation can reduce the effects of disturbing components.

Kind regards,

Mario

The best way to reduce fuel consumption is to reduce the drag coefficient and frontal area of the vehicle which means you are reducing the net drag force acting on the body. I assume that the latter is not possible. Yes, it is possible to increase the fuel efficiency marginally by adding spoilers and diffusers as they tend to reduce the net drag coefficient of the vehicle, but you must also consider the extra weight you will be adding. And there are other methods like varying the ground clearance of the vehicle. For starters, you can refer to S C C Bailey , G A Kopp & R J Martinuzzi (2002) Vortex shedding from a square cylinder near a wall, Journal of Turbulence, 3, N3, DOI: 10.1088/1468-5248/3/1/003. Link: http://dx.doi.org/10.1088/1468-5248/3/1/003. 

I hope my answer gives you a basic idea about the task at your hands. 

Here are some additional comments to the statements above: In general, the aerodynamic drag of a car is influenced by the pressure distribution at vehicle front and rear end, the flow characteristics at side walls, roof and under the vehicle and the flow-through airstream for cooling purposes of engine and brakes as well as for climatization of the passenger compartment. All of these areas show potential for optimization:

- Front area: improved shape of the vehicle front in terms of reduction of turbulences and ram pressure spots

- Back area: low pressure areas behind the car result in a backward force that may influence the air drag significantly. Improvements of the vehicle shape at the rear end can improve this situation. Spoiler increase the down force at the rear axis, but they also increase the turbulences at the rear end, which might lead to higher air drag forces.

- Side walls and roof: avoidance of turbulences (caused e.g. by side mirrors or luggage racks)

- Air flow under the vehicle: in general, the share of air flow under the vehicle should be reduced. This can be accomplished by front spoiler (eventually adjustable) or by reduction of the ground clearance at highway. In addition, bottom covering and diffusor at the rear end may improve the air drag behavior.

- Flow-through area: an air flow through the vehicle also influences the air drag negatively. Variable air inlet for engine cooling and climate system might lead to improvements.

Optimizing the general shape of the mini van shows the highest potential for reduction air drag. In subsequent steps, detail optimization by use of CFD-simulation can reduce the effects of disturbing components.

Kind regards,

Mario

As mentioned above, I would like to add some additional comments:

• The overall aerodynamic efficiency can be improved by considering the front to rear aerodynamic design of a vehicle rather than just using a spoiler or diffuser at the rear end
• In the frontal area use of air curtains that can be observed in the modern Audi and BMW models can reduce turbulence in the wheel arch area.This can also give better NVH results. Also some sharp edges around the lights or on the front bumper can give smooth flow(less pressure , high velocity) attached to the body.
•  The rear view mirror position/shape shall be improved and A-pillar shall be rounded to a certain degree or latest R&D shows the use aerodynamic devices such as in renault EOLAB.
• The boat tailing shape of the rear shall be optimized , as in a minivan in my opinion a rear conventional diffuser is quite odd. However , if a suction surface created on the underbody where the airflow is quite faster can provide forced airflow on the rear , reducing the energy of the wake produced at the rear. This strategy is used by ferrari x series. The spoon effect on the rear can give least turbulence and noise generation also
• The mini van having high ground clearance due to practical issues , may have lesser impact on the aerodynamics improvement at higher speeds where the airflow from sides can disturb the airflow underneath the vehicle.
• The spoiler may improve the drag coefficient quite well if used correctly and the design is quite adequate. According to my simulation on a fiat panda car model the drag coefficient was observed with and without spoiler which gave a improvement of .02 points approximately.

This was a general discussion from my point of view, I am including the simulation photos using Star CCM+ software for the simulation so that you can have idea of how does a spoiler improve the aerodynamics which in-turn may give a better fuel efficiency at high speeds.