Fire Detection in Engine Compartments of Buses: A pilot study

Abstract: Every year a large number of fires occur in buses and coaches. Some of these could have hazardous consequences with fatalities as result. Most of the fires start in the engine compartment of the bus. An efficient fire detection system in the engine compartment will give an early detection which gives more time to evacuation and the fire will be easier to control. To ensure that the fire detection system for the engine compartment of the bus is efficient enough a good standard is needed.
This report is made to facilitate further work, like making a test method, for fire detection in engine compartments. The focus has been on the back mounted engine since this type is by far the most common type. The report is mainly based on information from literature but also some information has been given by interview with different Swedish bus manufacturers and fire detector manufacturers. Computer simulation in Fire dynamics simulator, FDS 5.5, has also been made.
There are four types of main groups that fire detectors can be divided into depending on which factor that will trigger them. These groups are flame detectors, gas detectors, smoke detectors and heat detectors. The detectors could also be divided into subgroups depending on how the detection will be made. These groups are point detectors, linear detectors, volume detectors and aspirated detectors.
In the engine compartment there are some factors that can affect fire detectors. These factors are geometry, airflow, heat, dust and particles, vibrations and climate factors. How much each factor affects the fire detector depends of the type of the detector. The most common detector types for fire detection in engine compartments today are without doubt heat detectors. These heat detectors are often of the type “linear” and have the advantage that it will cover a larger area than a point type detector. One disadvantage with heat detectors is that they have a relatively slow reaction time. In most cases the detection system is included in the extinguish system sold by the manufactures but could also, in some cases, be sold separately.
The standard for detectors, EN 54, which applies for buildings is an extensive standard with a lot of demands and tests that the detectors need to pass to get approved by the standard. This standard can however not be used for engine compartments of buses due to totally different environmental conditions. The standard for ships, FSS-code, and the standard for trains, CEN/TS 45545, does not include and specific demands on fire detection in engine compartments.
In the results from the FDS-simulations it can be seen that heat detection with high air flow is a lot more difficult than without forced air flow. It also shows that hot surfaces heats the detectors if they are positioned close to them and these locations should be avoided.
A lot of manufacturers and sources gave different information about the same detection type. It can of course be differences between manufacturers but the manufacturers can also have different experience and opinions. This has however been a problem when evaluations for the resistance and general properties were made for the detectors.
For suggestion of further works some real experiments should be made to confirm some theories in this report. It is also essential that a good test standard for fire detection in engine compartments is being established but before that some more research is needed.