Aerodynamics of Engine Cooling – Part 1

After installing dual thermo fans into my S15, I've been addicted to the topic of engine cooling. Although it seems like simple system, consisting of a radiator - water pump - thermostat - cooling fan, it can get quite complicated once your car starts producing a lot more power than the cooling system was designed for. This is especially true when it comes to SR20DETs, the factory cooling system doesn't allow for much headroom and temps will quickly rise once you constantly stay on boost. Not only will the cooling system be insufficient, it'll also be inefficient with a stock clutch fan.

Read more after the break...

Note : most of this post will be covering core concepts of an engine cooling system and aerodynamics. Stay tuned for my next post which will use all this knowledge to modify my Nissan Silvia S15's cooling system.

The function of the radiator and cooling fan is to cool down the coolant as it flows through the radiator, it does not directly cool the engine. A thermostat's job is to regulate engine temps - if engine temps are cold then it'll stay closed to allow the temps to rise, if engine temps are too hot then it'll open up so cold coolant can circulate around the engine. A very simple design!

My last post briefly talked about upgrading the thermostat to a lower temperature unit. Basically this means the thermostat will open earlier and therefore will be fully open earlier and allow a higher flow of water around the system. I really only recommend doing this if you have temperature switched thermo fans or it's a dedicated track car. A stock clutch fan is ALWAYS cooling down your radiator, so it'll start cooling once the thermostat opens which will be too cold if you run a lower temp unit.

Ideally, the SR20DET should be operating between 78-83°C. Now this much lower than many modern engines, as newer cars have to meet lower emissions so they need to run at high temperatures, around 100°C. But at 90°C, the SR20DET ECU goes into "overheating procedures" and activates the extra cooling fan to help.

So my goal is not to over-cool the engine, but to make the cooling system as efficient as possible so that engine temps can stay stable no matter what the conditions are. On the track heavy on the boost, blazing 40°C summer heat with air con blasting, etc...

OK now onto the topic of aerodynamics... most of the ramblings about aerodynamics are simply that! You can never be certain of the outcomes unless you're able to do wind tunnel testing or have access to Computational Fluid Dynamics and 3D modelling software. But let's talk about some basic principles.

Low pressure and high pressure areas on a car's surface are caused by the varying velocity of air particles, very simplified explanation. When air is moving fast it creates a low pressure area, laminar flow. High pressure areas are caused by slow moving air particles, so turbulent air flow and stagnation points. Stagnation is where air velocity is zero, these areas can be found right at the front of the car, base of the windscreen, and sloping edge of the rear window, just to name a few. Air flows from a high pressure area to low pressure and will always take the easiest path.

Red to yellow areas indicate high pressures and blue areas being low pressure.

Now that we've got that out of the way, we can talk designing a more efficient engine cooling system! YAY!!!

When air hits the front of a car it causes a stagnation point, high pressure. So for air to flow across the radiator, there must be an area of relative low pressure on the engine side. The larger the difference in pressures, the more air will flow across. To increase this movement of air across the radiator, it's very important to shroud all edges so that air MUST flow through the radiator instead of around it.

After air passes through the radiator, what happens to it all? Well some of it quickly exits through the under-body, but not much because air flow under a car is slow from all the drag caused by the gearbox, exhaust, suspension, etc... The rest of it stays inside the engine bay and creates a lot of turbulence as it collides with the engine and other components. This is therefore the limiting factor as it creates a high pressure, slow moving air (turbulent), so now the difference between pressures in front and behind the radiator is dramatically reduced. Air flow across the radiator decreases as a result...

So we now need to remove all that high pressure build up inside the engine bay, and the most common way to do this is by using cooling fans to forcefully push air out through the under-body. Although this works for 99% of road cars, it is sometimes inefficient in a high performance car. So below are two alternatives that I'll be experimenting with but will go into more detail about these components in the next post.

• bonnet/hood venting
• under-body aerodynamics

I have a vented bonnet on order, and that should be arriving next weekend. I also have an under-body panel arriving on Monday that I plan to modify and test. Various temperatures will be taken before and after the components are installed, I hope to collect as much data as possible. The under-body panel will be going on first, then I'll be modifying it with a goal to further improve the efficiency. And finally the vented bonnet will go on for further temperature readings.