Race Logic Traction Control
How It Works
The system works by monitoring the speed of all four wheels using the ABS system or
specially fitted sensors. When wheelspin is detected the engine power is reduced, by
cutting a single injector pulse or a spark, until grip is resumed. This occurs in a
thousandth of a second, and appears to the driver as a slight miss-fire with no loss
in acceleration.
Maximum acceleration is achieved by limiting the slip between the tire and the road.
The point at which a tire is just beginning to slip against the road gives the maximum
coefficient of friction value.
From the graph above it can be seen the maximum coefficient of friction (µ) occurs at a
slip between tire and road of 10% when dry, and around 5% when wet.
Maintaining this level of slip is inherently difficult, as the grip levels drop off
significantly above these levels, meaning the balance between too much wheelspin and not
enough power is very fine.
To drive the car and search for these levels of slip is very difficult, the moment the
wheels start spinning too much (and how do you feel what is too much?) the power has to be
reduced (by what amount?).
Top rally drivers have a good feeling for this limit, but they still tend to stay on the side of caution,
and modulate the wheelspin between 10-20%, as this will still achieve 90% of the available traction. The
closer to 5 or 10% slip, the higher the chance of reducing the power too much, and hindering acceleration,
but also the closer you are to using 100% of the available traction.
The main reason for this is the response time of a human being. The fastest human reaction to a sense
stimulus is 1/10th of a second, and the fastest acting throttle reacts in around the same time. This
means there is a 2/10ths of a second lag between the wheel reaching a critical slip level, and the driver
being able to change the amount of power being applied. This is why really good drivers tend to drive
between 10 and 20% slip, to give a margin of safety should the tires suddenly find a little more grip,
causing the wheel to stop spinning completely.
Less experienced drivers will tend to allow 20-30% or more slip, again to maintain wheelspin rather then
let the car 'bog' down, thus limiting their grip levels to around 85% of their maximum.
With the advent of fast reacting electronics on cars, this problem has been tackled with Traction Control
systems. In race cars, Traction Control Systems have two functions, number one is to maintain the precise
level of slip that will give close to 100% of the available grip, and number two is to maintain stable
cornering. These two functions are linked, but require slightly different approaches.
The speed of reaction of a race Traction Control System is critical in maintaining a precise level of slip.
The electronics themselves can react within a thousandth of a second, but to remain effective the engine
power has to be quickly, and precisely controlled.
In road cars Traction Control normally relies on two methods of reducing the speed of the spinning wheel,
brake application and throttle intervention. Brake application is a very effective and quick way of
reducing the speed of a spinning wheel (almost unusable in a race situation - more later) but the
accompanying throttle intervention is mechanically slow, and will also only reduce the airflow, which
takes some time to become effective. On a road car the Traction Control System plays a third role, one
of safety, in this role the level of slip is reduced to zero, and held there. This results in a very
stable car, but one which will not accelerate at it's maximum potential at all times.
Race Traction Control Systems rely on much more precise, and faster acting ways of reducing power. The
first method is shutting off fuel to the engine, and the other is cutting out the spark. Both methods
have exactly the same high speed modulation ability, but the spark cutting system will happen potentially
one cycle earlier. The magnitude of difference in reaction times between spark cut and fuel cut is
negligible compared with the difference between throttle actuation and spark/fuel cut. (See
fuel cut and
spark cut below)
The Traction Control System then comes down to the interaction between the information from the wheel
speed sensors and the level of power reduction applied. A good system would be capable of maintaining a
level of slip that is adjustable depending on conditions.
Many factors affect the ideal level of slip, wet / dry conditions, speed of the vehicle, lateral g-force
(cornering), tire compound, tire pressures etc. Ideally the driver should be able to dial in a base level
of slip that takes into account weather and tires, and the system should adjust automatically for speed
of the vehicle and lateral g-force.
When cornering, the system should reduce the amount of slip available, to prevent lateral slip from occurring,
and vary this amount depending on the speed of the vehicle. At high speed, low grip situations, this slip
should be around 1-2% to maintain forward momentum, and at low speed high grip situations, this can be much
higher.
Fuel Cut
The idea of cutting fuel to an engine sets alarm bells ringing in engine builders, as they all know of
the potential disaster of a high revving race engine running lean. Running in a lean combustion mode will
elevate in-cylinder temperatures very rapidly, the denser the air/fuel charge, the more heat the lean burn
can generate. Therefore it is vital that a fuel cut system will not cause a lean burn.
The simplest way of preventing a lean burn is to remove more than 50% of the fuel from the pulsed delivery.
A mixture will only ignite if the air/fuel ratio is within a tightly defined window, look at the efforts
being put into making lean burn engines fire on very low air/fuel ratios (1:20 or more). Removing more
than 50% of the fuel will cause an air fuel ratio of over 1:25 and will result in a complete miss-fire,
with the unburned fuel passing out through the exhaust valve. Even if a high air/fuel ratio did manage
to ignite, the energy available from the amount of petrol injected wouldn't be enough to elevate
temperatures significantly. Of course the ideal system will remove 100% of the pulsed fuel delivery,
allowing the cylinder to take a gulp of fresh air, and the in-cylinder temperature would remain virtually
unaffected. Racelogic Traction Control operates in this manner - the complete injector pulse is removed so
no possibility of lean burn can exist.
Prolonged fuel cut on one particular cylinder would cause
scavenging of the petrol lining, the inlet tracts, and when
the next full fuel pulse arrived, it would be partially
reduced in quantity by the re-wetting of these tracts. Therefore
it is often important to manage a rotation of the cylinder
cutting to prevent this situation from occurring.
Spark Cut
Cutting the spark to an engine will stop any chances of a weak mixture occurring, but it carries it's
own potential problems due to a large quantity of unburned fuel travelling through the cylinder and out
of the exhaust. This petrol can remove some of the oil lining the inside of the cylinder, and pass it
thorough the exhaust, again this only becomes a problem if the fuel to one particular cylinder is cut for
an extended time. The best way to overcome this is to rotate the order in which the cylinders are cut.
The unburned fuel in the exhaust will have a catastrophic affect if there is a catalytic converter in the
exhaust, as it will try to convert the unburned fuel to harmless elements, effectively burning the mixture.
This causes the catalytic converter to heat up very rapidly, reaching temperatures in excess of 1000°C,
and possibly melting down completely. Thus prolonged spark cut is not recommended for catalytic equipped
cars.
