Fan Braking

From Holdenpaedia
Jump to: navigation, search

Original Submission by T Dec 4th 2005:

Back to Cooling

Bosch Headlight Relays operating as Fan Relays, one for each of 2 fans, mounted on the offside guard. Each Fan has its own Relay, Fuse and Wiring to provide complete redundancy in the event of failure of either Fan, Relay, Fuse or Circuit. Click to Enlargen. Photo by T.


What is Fan-Braking?

Fan-Braking is the simple process of wiring the Vehicle's Electric Cooling Fans to the Brake Light Switch so that the fans spin whenever the Brake Pedal is applied and the Ignition is on.
Note there are Diodes to arrest the BackEMF generated when the Relays turn off, this prevents Electrical Spikes from entering the +12V line. These relays are activated by the Thermo Switch as well as the FootBrake. Click to Enlargen. Photo by T.


Foot Brake Switch


What Are the Benefits?

While this sounds an unconventional notion, it actually shows efficiency gains.
Under normal operation a car with Electric Fans sits stationary at the lights with the fans stopped and the cooling system heating up.
Somewhere before or around the time that the Traffic Lights change to green, the Thermostat turns on the Electric Fans and the Car drives away from the lights spinning the Cooling Fans.
This process reduces the pickup of the car and reduces the fuel efficiency.
It may also explain the number of broken fanbelts seen near Traffic Lights due to the increase in load on the Fan-Belt while the engine is accelerating. The increase in load when the vehicle is decelerating seems to minimise this stress.
The concept of Fan-Braking is to turn the Fans on before the intersection.
The electrical load introduced by this loads the Alternator and consequently slows the Vehicle down.
This process also takes some of the load off the Brakes.
It also makes the Cooling Fans more efficient because they are supercharged by the incoming airflow.
In a car with an Automatic Gearbox the driver naturally keeps a foot on the brake pedal the whole time the car is at the lights which keeps the fans spinning. Consequently the Cooling System does not suffer a lengthy period without airflow. There is only a brief reduction in airflow between the time the driver lifts the foot from the Brake Pedal and the vehicle makes sufficient forward speed to cool the engine without needing a fan.
A torquey Engine like a Red/Blue/Black Six might as well do something useful at idle and keeping itself cool shows a proven power advantage. (Davies-Craig published this on their website as part of an EWP promotion).
The instant the vehicle is driven away from the Traffic Lights the fans stop and the cool engine finds accelerating the car from the lights easier.

Engine Momentum:

The first phase of Engine Braking is dissipating the momentum built up in the Engine.
The startup current of the fans is heavy and places a high load on the Alternator. This
high load is transferred to the Engine, slowing it down. It is very good at reducing the Engine
RPM and aids in the Engine Braking process.
The more intense the incoming air, the harder the fans bite in, the higher the startup current, the greater the load on the engine and the better the braking effect.

Applying the Brakes at Speed:

When the brakes are applied at speed interesting things happen. If the incoming airflow spins the fans at their natural rotation speed, they show a significant reduction in their current draw.
If the incoming airflow spins the fans faster than their natural rotation speed, they take the load from the Alternator by providing a small charge to the system.
As explained earlier, if the incoming airflow spins the fans at less than their natural rotation speed, the fans are supercharged and draw heavy current slowing the vehicle down and reducing the load on the Brakes.

What's The Downside?

The Engine is likely to run cooler around town which may effect the Engine's thermal efficiency.
Because I run Trimatics those Gearboxes respond to the increase in power and the fact that the transmission oil is cooler gives them improved coupling and longer gearbox life.
The system may not suit Manual car drivers who like to down shift before Traffic Lights. The downshift overspeeds the Mechanical Fan and to some extent achieves the same result, though this practice can impact Synchro, Clutch and Ancillary Equipment.
Clearly Fan-Braking isn't for everybody but it does reduce the stress on the Cooling System by anticipating the Cooling System's greatest stress, City Idling. It also helps the Vehicle compete against late model cars when accelerating off the mark.


The easiest way to implement Fan-Braking is with a Relay.
The Relay's Coil has one end connected to the Brake Light side of the Brake Light Switch.
The other end of the Relay Coil is wired to Chassis Ground. The Relay should now click on anytime the Brake Pedal is pushed hard enough to bring the Brake Lights on.
The Relay's Power Contacts are wired across the Electrical Thermoswitch. The same switch that turns the fans on.
Now the fans will come on with the Brakes and also whenever the Thermoswitch needs them to just as per usual.


Having used Fan-Braking for five years now I've found that the Thermoswitch is only likely to turn the Fans on for a total of about 30 minutes a year and that's with daily driving.
One cause of Thermoswitch activation can be a hot tail wind while the vehicle is crawling in traffic.
Another is when driving in slow peak hour traffic through one of the long tunnels fed by an endless stream of late model cars running hot radiators and travelling below the speedlimit.
Another is when the Engine is idling and there is no Driver in the car.


Fan-Braking is not to be confused with Fan-Breaking which is something you do with a Hammer.

Compressor Braking:

Having the Air Conditioning Compressor activate under braking will show an efficiency gain too. The Compressor could be wired so that it activates under brakes only while the Air Conditioning is turned on.

Interior Fan Braking:

Wiring the Interior Fan through a Brake Light Switch Controlled Relay will turn on the Interior Fan(s) under brakes.Most useful when the vehicle comes to a stop by keeping the Cabin air flowing. The Fan(s) cut out when the Brake Pedal is released dropping the Engine load.




Back to Cooling