Being able to accurately analyse combustion pressures and other combustion chamber conditions is very useful in gaining a thorough understanding of a race engine. To achieve this, various dedicated sensors are needed. Some of the most informative are those used for measuring in-cylinder combustion pressures; however, combustion pressure does not often tell the whole story so, if resources allow, further investigation into the combustion process can be beneficial.
For example, during the development of its Formula One V8 engines, the Honda team wanted to improve the driveability of its engine through closer control of the air-to-fuel ratio (AFR) entering the combustion chamber. The demands placed on an engine such as those found in the previous generation of Formula One car are considerable, not least the need to be able to move from fully closed throttle to wide-open throttle almost instantly. This requires exact control of the fuel injection events, and if this is not achieved then the AFR – particularly in the immediate vicinity of the spark plug – may end up rich or lean, hindering flame front propagation and impacting driveability.
By measuring in-cylinder pressures, Honda had found that misfires were occurring under certain transient throttle conditions, which it thought was down to an incorrect AFR. Naturally it wanted to find out why these misfires were occurring and take steps to improve the situation. To do so, the team used a micro-Cassegrain sensor incorporated into a spark plug, which was able to measure the chemi-luminescence of different elements in the flame front inside the combustion chamber. By studying the intensity of this luminescence, the rate of flame propagation and the AFR could be established.
To ensure that the sensor system was not interfering with the normal operation of the spark plug, Honda ran sensors in only four of the eight cylinders and compared the variation in combustion pressures between instrumented and non-instrumented cylinders. Also, the micro-Cassegrain sensing elements were placed behind sapphire glass shields to protect them from the extreme heat and pressure in the combustion chamber. By coupling the sensors to a high-speed data acquisition system, the team was able to record data at high resolution, with measurements accurate to within 1º of crank angle at 18,000 rpm.
The results Honda obtained from the measurement system allowed the team to gain considerable insight into the combustion behaviour of individual cylinders. For example, it was found that variations in the design of the air inlet scoop had a considerable impact on the AFR around the plugs in particular cylinders, something that had not been apparent before.
However, it was the results obtained during transient testing that were most revealing. As mentioned, the team had already found that certain cylinders were misfiring, but could not confirm why. The sensor provided the insight needed for this confirmation, and it was found that the AFR around the plugs from cylinder to cylinder varied considerably under transient throttle conditions, from rich to lean, causing the misfires that had been detected.
As with any form of testing, it is often the case that being able to record something that was previously un-recordable leads to more questions than answers. In Honda’s case, it discovered the reason behind its engines’ driveability issues, but finding a solution that would allow sufficiently precise mixture control to combat it was a different matter. However, without the ability to study exactly what was occurring in the cylinders, any solutions the team may have devised would have been mere guesses, as the exact problem it was trying to address would not have been clear to see.
Written by Lawrence Butcher