Aerodynamics of Engine Cooling – Part 2

Updates 13/07/2011 - further testing/observations
Updates 14/07/2011 - bad news

You will have probably seen the under-bodies of many supercars, and noticed that some of them are very flat. Wind tunnel data has shown that the under-body makes up to 30% of the vehicle's total drag, which is why engineers try to reduce the drag as much as possible so that air flow is fast moving and therefore low pressure. This low pressure acts a sucking force and increases overall downforce, even more so when a rear diffuser and front splitters are used. In fact, many race series ban the use of flat under-bodies because it is so effective at increasing downforce and not introducing much drag.

Read more after the break...

With emission laws getting stricter by the day, many manufacturers are now implementing more aerodynamic under-bodies as a way to increase fuel efficiency. Smoother under-bodies are a wonderful thing, better fuel economy, increased downforce, less drag, improved engine cooling!

All tests and modifications are performed on my Nissan Silvia S15, running a SR20DET with various modifications to produce 230RWKW. At the front of the car is a larger intercooler, sitting behind it are the AC condenser and radiator. Both AC condenser and radiator are cooled by a dual thermo fan setup from a Ford Falcon AU. These fans are controlled by a temperature switch which turn the fans on high speed when the ECU detects 83°C and turns them off at 78°C.

My plan is to make my engine bay's under-body as smooth as possible, by doing so I'll be able to create low pressure areas which will draw out some of that high pressure air inside the engine bay. And therefore remove some of that heat trapped under the bonnet and encourage more air flow across the radiator. Not to mention the slight increase of front downforce and improved fuel efficiency =)

So below are some photos of the under-body panel I've purchased. It's made in Japan by Beatrush, build quality is very good and they even have louvers in specific areas to draw out air. You could probably make something similar for less money but the price for this was pretty good =)

Wonder what's inside?

Build quality is amazing, and the entire panel is covered in a blue protective sheet.

Comes with Japanese instructions, but it's easy enough to understand. All fittings are supplied as well as a nice Beatrush sticker =)

Beatrush under-body panel installed, fitment was spot on.

For temperature measurements, I used a digital thermometer purchased from JayCar. It uses a common K-type thermocouple probe, also had to make up an extension lead so the probe would reach inside the engine bay.

Temperatures were measure at two different spots, first was at the intake air filter which has cold air ducted to it. The other was just in front of the engine.

Below are temperatures recorded before/after the under-body panel was installed. Ambient air temperature was 22°C, and all measurements were taken within a two hour period.

Interpretation of collected data...
With the under-body panel in place, temperature readings dropped slightly. But the real improvements aren't seen in the temperature drops but rather the increased air flow. I have no way to quantitatively measure air flow, but I noted the operation of the thermo-fans. When travelling above 60km/h, the thermo-fans didn't cycle on/off as much.  To me, this is a clear indication that more air is flowing through radiator/engine bay and therefore keeps the engine temperatures more stable at speed.

Before installing the under-body panel, travelling at 80-100km/h, the thermo-fans turned on every 4-5mins for a period of  30-45 seconds depending on the engine load. With the under-body panel installed, travelling at the same constant speed, the thermo-fans never turn on until you come to a stop.

Update 13/07/2011 -
I've been able to clock another 100kms of driving and have made further observations. As mentioned above, the thermo fans are never needed when travelling at 100km/h but in addition to this, there have been occasions where the temps have dropped from 83°C to 81°C when going down hill. More evidence of increased ram air through the radiator. Also when temperatures do rise, they increase at a slower rate then before.

Update 14/07/2011-
I finally had a chance to do some night driving, when ambient temps drop to around 8-10°C, and it looks like any night time highway driving will cause the car to over-cool! We were sitting at around 82°C doing 100km/h, and the temperature slowly dropped down to 74°C before we had to pull over and let it warm back up. If I continued driving, it was possible that temps could have dropped to 62.5°C which is when the Nismo thermostat closes. BTW, this is with both thermo fans turned of the whole time.... Nothing like this has ever happened before, so I'm just astonished by the cooling efficiency increase of installing and under-body panel. This also means that the stock thermostat is going back in to keep temperatures above 76.5°C.

Looking further into the collected data, before the under-body panel, temperatures measured at the front increased by 8°C from idle to 100km/h. This is shows how much turbulent air flow was inside and under the engine bay, as all that heat was trapped when speed increased. With the under-body panel installed, the turbulence decreased and the temperatures difference was reduced to 3°C.

Temperatures at idle increased slightly with the under-body panel. Should also note that the intake temperature increases when the thermo-fans are turned on, this increase drops as vehicle speed increases as more cold air is being forced through the cold air intake duct.

You may have noticed that engine coolant temperatures weren't recorded, this is because the thermostat regulates engine temps so the under-body panel doesn't really affect it.

So to conclude, the various temperature measurements clearly show the gains which can be had with simply having a smoother front under-body. The low pressure created by the smooth under-body extracts some of the heat through gearbox tunnel, this also means that the engine bay pressure drops and therefore increase air flow across the radiator.

Next post will hopefully be about the effects of vented bonnets, stay tuned!