Initial scoping studies on vehicle selection led key research group Simtars (the Queensland Government's Safety in Mines Testing and Research Station) to conclude diesel vehicles were the best choice to use until alternate power sources become commercially available.
“The best thing available now is a diesel vehicle – there are lots of them and they are at every mine, in every panel. We know them, we maintain them. We now know the operating environment,” said Simtars principal mining engineer Martin Watkinson.
At this stage in the three-year research project, the main message researchers have to deliver is simple: in an emergency, if a diesel engine will start, get in and drive out, he said.
Complicating research on defining the operating parameters for such vehicles is the fact that very little is known about the post-explosive atmosphere of a coal mine because the gas monitoring infrastructure is frequently destroyed.
“Despite many mines blowing up, we have no reliable readings of the atmosphere following such a disaster. We only have some readings from Moura, from a tube-bundle system, but we’re not sure which location they were coming from,” Watkinson said.
What is known is that when diesel engines run in methane-enriched atmospheres, they produce more power. Research has shown torque and power decrease with decreasing oxygen levels and increase in the presence of methane. But at what point such engines may cause a potential hazard has not been absolutely identified.
Tests on various diesel engines have been required to identify the operating parameters for the safe running of a diesel vehicle in potentially extreme operating environments. Currently Australian legislation dictates diesels cannot run if methane levels rise above 1.25% due to the risk of explosion and to provide an adequate margin of safety.
Testing at Simtars has now established a potential envelope for safe operation of diesel engines for methane concentrations up to 4% and oxygen levels as low as 17% in emergency situations. Supplementary tests were undertaken at Simtars to determine what would happen if a flameproof vehicle was driven into an explosive atmosphere. In atmospheres up to 10.5% CH4 (methane), engine idle speeds went from 900rpm to 1600rpm but beyond 10.5% CH4 idle speed decreased to 700rpm. The engine stalled at 18% CH4 (16.4% oxygen).
Constraints in the ability to effectively load the engine meant that it was not possible to test it thoroughly. Additional tests on engines in high methane atmospheres are scheduled to identify the operating characteristics in such atmospheres. These tests will be conducted on a range of new and pre-overhauled diesel engines in an attempt to cover all operating conditions.
Of course, because current legislation dictates much lower methane levels for normal operation, diesel vehicles are today fitted with automatic shutdown systems and air intake strangler valves. To enable diesels to run outside regulated atmospheres, an override system would need to be developed. This could be engaged in an emergency situation to allow the vehicle to be driven out of the mine.
Definitive information about how such engines perform is likely to have an impact on safety legislation though this is expected to be much further down the track.
The modified diesel vehicle has been developed during three years of research led by Simtars in conjunction with Queensland Mines Rescue and the Australian Coal Association Research Program (ACARP).