4WD Dyno Cell Construction Blog.
(Part 5... Airflow!)
This for me was the part I was actually looking forward to. It took many months of planning, calls, meetings and more planning to get the system specified and ordered, and the same again to get it all working. This is because airflow is crucial. It is the single most important aspect of creating a rolling road dyno cell that works properly if you want accurate, repeatable figures... and you should. Why else would you build one?
The big thing that most dyno cell set-ups fail on is just having a fan blowing air around the engine bay but not actually venting the room itself, so all they are doing is transferring the heat out of the radiator, intercooler, engine bay, exhaust etc into the room. This then heats up the room air and of course then that same warmed up air is sent back over the engine again by the fan, so the cooling gets worse and worse as the air gets warmer and warmer in there.
The air in a dyno cell must stay in motion. It should come in from the outside of the cell at one end, be forced through anything hot to remove heat, and then carry that heat to the other end of the cell and expel it outside. think of it like a car radiator... without constant fresh air going through it, it just keeps getting hotter. The cell is the same.
The first problem, is how to get the air in?
For me, blowing workshop air through an engine bay is far from ideal. To replicate road conditions we need pressurised air from the outside, ideally travelling at high speed... just like a car. With this in mind, I ordered the biggest centrifugal fan Dyno Dynamics sell and its going to be hard sited slap bang where it needs to be. You may wonder why I chose a centrifugal fan. The reason is simple, it creates high speed air and also, more importantly, it comes from a concentrated outlet and thus directs it to where it needs to be. People are amazed when they stand just one foot to the side of my engine cooling fans and cant feel them blowing much, yet in front of them it blows all your clothes off! (Female dyno operators, please send me your CV!)
This unit blows in 20'000cfm of cold, fresh outside air at around 70mph.
Our system cools only the engine and heat exchangers and passes that air under the vehicle to the rear of the cell. It does not cool wing mirrors, windscreens and dyno operators. This is an important consideration if you want to do it properly. Sitting a huge axial fan in front of an engine and 50% of its air spreading out past the car is far from ideal. Axial fans very often have a dead spot with virtually no airflow right in the middle too. Yep - right where the radiator of the car usually is! (Try it with your hand on your desk fan... ;) )
The bulk of the cooling fan unit sits in the fan control room, so we have a huge fan that doesn’t encroach into the dyno cell. I’m very happy with this. Just a shame it actually looks smaller and less impressive from within the cell.
Next comes the job of how to extract that 20'000cfm of hot air without making too much noise.
That’s where these come in. Awesome, huge fans, with a similarly sized price tag, but these are just incredible and run almost silently. For high air flow and low noise, you need size. Here’s my lovely wife modelling one of the two we plan to run.
One will do the job just fine, but we planned to install two running at half speed as they are more energy efficient at low speed and also a lot quieter, by about 9db! These fans are going to be speed controlled so it can maintain the same pressure in the cell as the pressure outside. The gubbins for this lot are Kennys department, but look something like this.
The final thing we must do now that we know we have 25'000cfm of air going OUT of the cell (wall extractors and exhaust extractors combined) and only 20'000cfm being forced in with the engine cooling fan, is to ensure that we can create a balance for that missing 5000cfm of airflow.
Why does it matter you may ask?
Failing to do so will create a negative pressure in the cell which at best, will affect engine figures, at worst it can rip the ceilings and walls down. Think of it like driving the pikes peak, you may have read that the cars lose power as they climb the mountain, that’s partially because of low air pressure. High air pressure skewing figures is easy to understand... we call it forced induction. (turbocharging... ;) ) Crucially, we also need to do this without transmitting too much noise outside, so the nice easy "Drill a hole in the wall" solution doesn’t work here.
The correct solution are acoustic louvres. I worked out we needed two of these and naturally, installing them at the front of the car means any air drawn through them will travel the correct route... in the front, past the hot car, and out at the back.
Sadly, while its a complex subject with a lot of outlay and planning, there aren’t many more images to accompany this airflow section. After all, they are just fans. Its the planning that makes them interesting.
However, I’m sure you will agree that the fan in the wall just looks awesome! It will be a real shame to fit the protection grills to these. (image from after dyno install day)
With the airflow side sorted, we got some great news - The dyno is on the boat, so we need to get the cell looking more respectable!