Preventing Bus and Coach Fires

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Bus and coach fires are a surprisingly common occurrence in the UK and further afield. With official UK Office for National Statistics showing that there are around 500 to 600 fires involving a Bus or Coach every year [1], this number is very high considering the relatively low number of these vehicles on the road. More concerning is that in a typical bus or coach there will be a high density of passengers, often much higher than would be permitted by building regulations, and relatively poor means of escape from the vehicle.

A striking feature of many Bus and Coach fires is the relatively high level of severity, when compared to other vehicles. Total loss fires are incredibly common in buses in particular; this means that the vehicle is destroyed beyond economic repair. A total loss fire in an urban area can also cause a lot of damage to surrounding vehicles and property as well as posing a public safety hazard. This is despite many buses being fitted with fire suppression equipment. A quick Google search for bus fires will bring up some very dramatic pictures!

The purpose of this whitepaper is to discuss some of the common causes of bus fires, and what can be done to reduce the risk of a fire occurring and then reduce the severity.

Bus and Coach Operating Conditions and Duty Cycle

Buses typically operate in urban or semi urban environments transporting passengers through our congested cities commuting to work or study or for pleasure. Due to the high capital cost of the vehicle, it is important that the operator fully utilises or “sweats” their asset. This means that in many operations it is common for vehicles to be working around the clock for up to 20 hours or more per day. Drivers are changed over to accommodate breaks and shift patterns, but the bus never stops working. Added to these long hours are highly transient or stop/start driving conditions caused by urban traffic congestion and the need to make frequent stops to allow passengers to get on and off.

This contrasts to the typical long haul truck operating pattern which sees the vehicle operating long distances away from its base, it is not practical to swap drivers to accommodate breaks and shifts, although sometimes 2 drivers will man the vehicle this is not too common due to the increased labour costs. Meaning that the truck will typically rest when the driver rests. The truck will stop less frequently and typically for longer periods as large volumes of goods are unloaded at warehouses. The truck is travelling at higher speeds so can cover greater distances but with lower transient start/stop this places less stress on the drive train. The operating cycle for a typical coach is a hybrid between the truck and bus with frequent stops in for passengers and traffic in urban centres punctuated by higher speed more steady state operation.

This highly transient driving cycle for the bus puts a strain on the drive train, coupled with very high operating hours means that mechanical wear and tear can be very high when compared to other heavy duty vehicle applications.

There is also a further characteristic of the bus industry that should not be overlooked. Buses are manufactured in much lower volumes than long haul trucks or passenger cars. With even the most popular bus models made in low thousands per year compared to many tens of thousands for a truck or hundreds of thousands for a passenger car. As a result buses will use commercially available off the shelf components that were typically originally designed for the truck application and the lower volume nature of production means that the absolute overall investment in product development, testing and the use of automated production systems is much less than a typical truck or passenger car (although this may be a relatively high per unit volume of manufacture).

A typical bus maintenance regime would include an inspection every 28 to 30 days of operation. This inspection will be carried out by a skilled technician following an established check-list and process for that particular vehicle. It will involve checking for damage and condition of key components such as braking system, fluids such as oil and coolant. The inspections will be carried out in a busy workshop often under some time pressure to return the vehicle to service.

With the exception of supplier enforced maintenance such as engine oil changes, there is not a strong culture of preventative maintenance in the industry with the typical approach being to run till component failure and then replace, rather than carry out preventative maintenance before catastrophic component failure based on gathered operating data.

Key Causes of Bus Fires

It has been identified by several reports into the topic that there are several common causes for bus fires.

The most common being engine bay fires caused by oil leaks from either the hydraulic drive commonly used for engine fans or from the engine itself due to component failure e.g. turbo charger seals.

Then the next most common cause being engine bay fires caused by electrical system failures such as alternator failure, high power cables wearing through or breaking and then causing an electrical short circuit.

Drive line fires involving sticking brake systems causing brake system components to overheat leading to fires of carried over oil in the brake pneumatic system and around the brake system.

There is a serious problem with fires that start which should be should be controllable and not lead to total loss events becoming major total loss events, this is typically due to the secondary ignition of hydraulic oils or oil soaked soundproofing in the engine bay area.

A report by the US National Renewable Energy Laboratory (NREL) in September 2010 reviewing the performance of CNG buses found that there had been a significant number of fires and the leading cause was hydraulic system leaks in the engine bay area. [2]

A report by the SP Technical Research Institute in Sweden [3] found that a significant contributing factor in the severity of bus fires was the reduced effectiveness of the extinguishing system due to; “the high ventilation rate [from the cooling fan system] in the engine makes the fire harder to extinguish is that the extinguishing medium can be transported away giving it less time to work.

The NSW Office of Transport Safety Investigations report into a bus fire in July 2011 and a subsequent incident in 2015 in both cases found that leakage and ignition of the hydraulic oil for the fan drive and power steering systems had led to the fire becoming a major incident

Actions to Reduce Incidence and Severity of Bus Fires

There are several key actions that can be taken to reduce the incidence and severity of bus fires:

  1. Inspections of the hydraulic system pipework condition as part of the 28 day service
  2. Maintaining engine bay cleanliness to avoid the build up of residual oil and other flammable material in soundproofing etc
  3. Regular cleaning of the engine cooling system to reduce the demand on the cooling fan system
  4. Regular inspection of the high power cables and grounding straps in the engine bay to check for damage and corrosion

The installation as a retrofit or specification on new build of an electrically powered cooling fan system such as AVID’s eFan Micro Hybrid System addresses these key issues due to:

  1. Elimination of the hydraulically driven cooling fan system and the associated risk of hydraulic leaks and associated fire
  2. Can be linked into engine bay fire suppression systems to shut the fan system down if the suppression system is activated to improve suppression system effectiveness
  3. Facilitates simpler and faster cleaning of the cooling system meaning it is more likely to be done

The earlier mentioned NREL report [2] said in relation to fitting eFan systems to replace the conventional hydraulic systems as a means of improving the performance and safety of the vehicle that; “All the agencies surveyed were positive to enthusiastic in their assessment of such a system, which is reported to improve fuel economy in excess of 5%, improve battery life (by keeping batteries charged more fully), and significantly reduce the possibility of an engine-compartment fire by eliminating the hydraulic hoses to the fan. All the agencies that are testing these systems indicated that they will specify this system on future bus orders.”

References

[1] UK Government Official Statistics Site

[2] NREL/SR-7A2-48814 September 2010 Adams et al

[3] SP Report 2008:41 Hammarström et al

Further Information:

Bus fire attributed to hydraulic system leak May 2014

Toronto Transit Commission found 3 fires directly related to leaking hydraulic systems 

Official recall of new bus model to correct potential fire risk from hydraulic fan drive system leak