But, despite this advanced equipment, many calibration engineers still demand the fail-safe method of bolting copper tubes to the engine block and running them into the control room of the test cell. Sometimes, even before a knock event is observed on the in-cylinder pressure traces, the copper tubes do an amazing job of providing a very clear audible crackle of the knock occurrence to the calibration engineer.
Calibration engineers run a grid of speed and load points to cover the whole operating range of the engine and vary spark timing, air-fuel mixture and boost pressure (in force-fed engines) to determine the onset of knock. For improved efficiency and lower exhaust temperatures, lower boost values and more advanced timing in a turbopetrol engine before the onset of knock are run. Therefore, knock calibration is a balancing act of these variables in order to find the optimal settings.
The pressure traces of each cylinder are used to determine the onset of knock. These traces are correlated to the signal received from the knock sensor (see The knock sensor on page 117), and fast Fourier transforms (FFT) are used to determine the centre frequencies of the knock events for each cylinder. These thresholds are calibrated to the ECU so that it can determine when a knock event occurs and then take the appropriate action.
Clever knock-control strategies
In the past, engines were calibrated for the lowest-grade of petrol and lowest octane number, and most stressful operating conditions. Therefore, it was likely that most engines ran inefficiently.
Today, thanks to highly advanced engine-management systems, manufacturers use clever software algorithms in combination with knock-control strategies to optimise the ignition to run close to the knock limit. Essentially, spark timing is advanced in small increments until the onset of knock is detected before the timing is retarded to a safe limit. This happens in real-time for every cylinder and firing event. On a four-cylinder engine that runs at 6 000 r/min, this occurs about 200 times a second.
Bentley Arnage example
In 2007, while working at Integral Powertrain in the UK, I was involved in the engine-development programme on Bentley’s 6,75-litre twin-turbo V8 engine. The output targets at the time were 373 kW and a massive 970 N.m while running on 95-octane fuel. Bentley faced the problem that the Arnage would be sold round the world, including regions where only 87 octane fuel was available. Therefore, we had to calibrate the Bosch knock-control strategy to retard the timing to protect the engine when a lower octane rating than 95 was used. This meant that power dropped significantly, but we were able to protect the engine and stay within exhaust-temperature limits.
Octane versus performance
Many people incorrectly associate the octane number of petrol with energy content. Rather, the octane number is a measure of the fuel’s resistance to knock, with higher numbers denoting more resistance. Modern vehicles (especially performance vehicles) produce more power when high-octane petrol is used because of adaptive knock-control strategies (see Calibrating knock-control strategies on the next page). These control algorithms advance ignition timing when higher-octane petrol is used until the new knock limit is established. By advancing the ignition timing, the power output is increased.
The knock sensor
Although in-cylinder pressure transducers are the most accurate way of determining the onset of knock, they are expensive and so unsuitable for use in mass-produced engines. Therefore, automakers rely on high-frequency accelerometers to “listen” for the onset of knock. These sensors use piezo-electric technology and are capable of measuring in the range of 5-20 kHz. Normally, a knock sensor bolts directly onto the block of an engine because the metal-alloy does a fine job of transmitting the combustion vibrations. This allows an inline engine to have only one knock sensor covering all the cylinders, whereas a V-layout engine would require a knock sensor for each bank.