Australian motorsport specialist MoTeC is at the
forefront of racing car electronics, producing digital dashboards, engine
management systems and data analysis software. This week we’ll look at how
racing car data is collected and then next week, examine MoTeC’s i2 data
analysis software.
So what sort of data is collected from a racing
car?
Engine
Collecting data on the engine status is made
simpler because the engine management system’s Electronic Control Unit (ECU)
already uses many sensors. The outputs of these sensors can be used not only by
the engine management system but also logged and then expressed in engineering
units. In addition, the ECU has available internally calculated data, such as
injector duty cycle.
On naturally aspirated race cars, load is normally
calculated by the engine management ECU looking at engine speed and throttle
position. (This is in contrast to road cars that most often use an airflow meter
to directly measure the mass of ingested air.) Forced aspirated racing cars use
a MAP sensor (like the one pictured) that measures manifold pressure. When this
is combined with measurement of engine speed, the ECU can again work out load.
So when engine load is logged, the data is in the form of either throttle
position and engine speed, or manifold pressure and engine speed.
MAP sensors are calibrated in absolute pressure
and are most commonly available in 1 Bar (suitable for naturally aspirated
engines), 2 Bar (ie suitable for 1 Bar of boost) and 3 Bar (suitable for 2 Bar
of boost) versions. Interestingly enough, there is also available a 1.05 Bar
version which takes into account the aerodynamic air pressure build-up possible
in the airbox of a fast-moving car.
Most often used are Delco MAP sensors which start
in cost at AUD$80. These conditioned sensors have a nominally 0-5V output and
are widely used in production cars.
- Coolant , Oil and Intake Air
Temperature
Coolant and oil temperatures are measured by NTC
thermistors. The Bosch 023 and 026 sensors are commonly used – at AUD$17 they
are cheap, use a near universal 12 x 1.5mm thread and are two-wire designs (ie
no chassis ground return) that uses a standard fuel injector plug.
Intake air temperature sensors comprise a similar
design but with the thermistor exposed to the passing airflow. Intake air
sensors can be used to sense air temp in an intake runner just prior to entry
into the engine (so measuring the temperature rise caused by the air compression
of a supercharger or turbo, and intake manifold heat soak) or at the airfilter.
Previously, the air/fuel ratio was measured by a
zirconia oxygen sensor such as the Bosch ‘four wire’ design. Based very much on
the technology of the oxygen sensors used in normal passenger cars, this device
outputs a voltage of 0-1V, depending on mixture strength. However, the voltage
is non-linear with respect to air/fuel ratio, with a sudden change in output
around 450 – 550mV (corresponding to the air/fuel ratio passing through
stoichiometric) and also varies with temperature. The Bosch unit has a slightly
flatter response than garden variety oxygen sensors but still has severe
limitations in accuracy, especially at the rich end of the automotive scale.
Linearising it requires accurate temperature and voltage compensation.
Replacing the Bosch “four wire” is the Bosch LSU
probe. This probe works on a completely different principle and requires its own
control circuit. In short, a zirconium-dioxide/ceramic measuring cell is used
which comprises a Nernst concentration cell and an oxygen pump cell, with a
small diffusion gap positioned between them. Two porous platinum electrodes are
placed within this gap – a Nernst measuring electrode and an oxygen pump
electrode. The gap is connected to the exhaust gas via a small passage. On the
other side, the Nernst cell is connected to the atmosphere by a reference air
passage. By applying a pump voltage across the electrodes, oxygen is pumped from
the exhaust gas into or out of the diffusion gap.
The sensor controller varies this voltage so that
the composition of the gas in the diffusion gap remains at stoichiometric. If
the exhaust gas is lean, the pump cell pumps the oxygen to the outside (positive
pump current). If the exhaust gas is rich, the oxygen is pumped from the exhaust
gas into the diffusion gap (negative pump current). The pump current therefore
reflects the actual air/fuel ratio. Again, linearising is required.
Other than the most recent M400/600/800 series
MoTeC engine management systems and the PLM air/fuel ratio meter, no MoTeC
logging device can accept a signal directly from the LSU sensor. Instead they
read the data from the ECU or PLM via a CAN bus communication, while the PLM
also has a configurable analog output voltage that can be read by the data
logger.
Exhaust gas temperature is measured with K-type
thermocouples. Again, an interface device is needed, this time to amplify and
cold junction compensate the signal. One example of such an interface is the
AUD$1045 MoTeC E888 input/output expander. Amongst other inputs and outputs,
this unit can accept eight K-type thermocouple inputs and then communicate this
data to the engine management ECU or digital dash logger by means of a CAN bus
connection.
Exhaust gas temperature is most often measured at
individual exhaust outlets near the engine, so explaining the requirement for
eight probes in many race car applications. These temperatures give a guide to
cylinder-to-cylinder mixture consistency and are most commonly used in drag
racing.
- Oil, Brake and Fuel Pressures
Two types of sensor are used in these
applications. The first is the traditional Bourdon tube based potentiometer, as
exemplified by the large canister VDO units used as oil gauge pressure sensors
on countless road cars. However, the accuracy of these sensors in race car
applications is suspect: when tested on the bench, a light finger tap can
sometimes change the measured output by 5 psi! Replacing these are Texas
Instruments sensors that use a load cell backed by a diaphragm. Available up to
2000 psi maximum pressures, these sensors have a conditioned 0-5V output and are
available in gauge and absolute pressure configurations.
Throttle position sensors comprise rotary
potentiometers mounted on the throttle shaft. They are available in a wide range
of physical designs to match various shafts but a common one accepts a D-shaped
shaft.
Engine speed is sensed from the crankshaft
position sensor. This normally comprises an inductive sensor mounted on
flywheel, although in engines not specifically built for racing but instead
adapted from road cars, the sensor can alternatively be optical or use a Hall
Effect device.
Other sensors that are sometimes uses on the
engine include infra-red thermometers measuring block temperature and pressure
sensors in the coolant system, the latter used primarily to sense a catastrophic
loss of coolant.
Chassis and Suspension
Suspension data requires the installation of
specific sensors. Where the behaviour of individual wheels needs to be
monitored, this involves four sets of sensors.
Damper movement is sensed by linear
potentiometers. These are available with different stroke lengths (for example:
75, 100, 125, 150 and 200mm) and are mounted such that they move over as much of
their range as possible as the suspension moves from full bump to full droop.
These sensors cost about AUD$400 each but they are fully rebuildable, something
often required as their vulnerable positioning results in frequent damage in
racing incidents. Finding space for the sensors and mounting them so that no
bending loads are placed on them can be difficult; however, the logging software
can be easily configured to show actual suspension deflection even when the
sensor is angled from the vertical or is subjected to a non-linear motion
ratio.
The temperature of the oil within the dampers is
sensed indirectly, either by the use of stick-on thermocouples or, less
commonly, by infra-red temperature sensors.
- Lateral , Longitudinal and Vertical
Acceleration
One, two and three axis accelerometers are used to
sense accelerations. These sensors are conditioned with a 0-5V linear output and
can be specified to have maximum acceleration of 10g. (In Top Fuel drag cars the
previous 4g maxima were being exceeded in longitudinal acceleration!) However,
in circuit racing cars, two-axis accelerometers with a maximum acceleration of
4g are more normally used. Cost varies from AUD$360 for a single axis 4g
accelerometer to AUD$688 for a 3-axis 4g sensor. As we will cover next week, the
outputs of this sensor can be used by the data analysis software to
automatically construct a track map.
The accelerometer is normally mounted at the roll
and pitch centre of the car, however two accelerometers can be individually
mounted on the front and rear axle lines and when their outputs are compared to
steering angle, be used to assess oversteer and understeer.
Yaw is sensed by a Bosch yaw sensor, as normally
fitted to the Subaru STi WRX that uses an active centre differential as part of
its four wheel drive system. In addition to a yaw rate signal output, this
sensor also contains a lateral accelerometer. Cost is AUD$1014.
- Tyre and Brake Temperatures
Tyre and brake temperatures are monitored by
infrared thermometers aimed appropriately. In the case of Le Mans racing cars,
no less than three infrared sensors are used per tyre – quite a cost at AUD$480
each sensor! Tyre temperatures are amongst the most useful of data in setting-up
a car for optimal lap times as the temperature distribution shows how hard each
tyre – and each part of the tyre – is working. The infrared thermometers have a
conditioned 0-5V output and are available in 100, 200 and 1000 degree C ranges –
the latter being used to measure brake temps.
Steering angle is normally sensed by a multi-turn
rotary potentiometer driven by a toothed rubber belt from a pulley mounted on
the steering shaft.
In road cars adapted for racing, the ABS system is
usually disconnected. In these cases, one of the inductive wheel speed sensors
can then be used for measuring road speed. The logging software is configured
for the AC voltage levels of the sensor and the frequency/speed relationship. In
purpose-built race cars, a new inductive sensor is fitted behind a wheel.
Lap Time
Car racing is about going faster than anyone else
and so lap speed is a critically important parameter. MoTeC use a trackside
mounted infrared transmitting beacon and a car mounted receiver. A configurable
frequency signal is emitted by the beacon and the car’s system is programmed to
respond to only this signal. Lap times are logged and also displayed to the
driver in terms of laps to go or lap number. In addition, split times can be
gained by the use of extra trackside beacons programmed appropriately.
Logging and Displaying the Data
Given the number of channels and the frequency at
which many are collected, most teams choose to use in-car logging rather than
real time telemetry. (Telemetry is still used, but for slow-changing factors
like fuel levels and monitoring engine health.) It is useful if the device that
stores the data can also display some of it for the driver and so a common
approach is to use a customizable digital dashboard that can perform both
functions. MoTeC’s Advanced Data Logger (ADL2) is such a unit.
The ADL2 can read 28 analog voltage inputs, 12
digital inputs and two Bosch ‘four wire’ air/fuel ratio sensor inputs. And, if
even more logging capability is required, another 22 inputs can be added by
means of an expansion unit! The unit will also accept data communicated to it in
RS232 (eg from a GPS unit) and CAN formats. A 16Mb internal memory is
incorporated and the microprocessor is 32-bit. The fully configurable backlit
LCD can display any of these inputs, shown in user-selectable engineering
units.
Conclusion
As we’ve seen, literally anything that can be
sensed on a race car is capable of being logged. However, all the information in
the world is of little use if no sense can be made of it. Next week we’ll take
a look at the MoTeC i2 data analysis software which has mind-boggling
capabilities – not only can it display the data in many different forms but it
can also make mathematical calculations based on that data and then display
those calculations in relation to the collected data!
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