The pre-engaged air starter motor is ideal for environments with explosive atmospheres (such as underground mines) as it is ATEX compliant. On starting the engine, the pinion fully engages with the ring gear before cranking the diesel engine. This removes the risk of sparks that could be caused by traditional inertia starting.
The key difference between inertia and a pre-engaged air starter motor is that on a pre-engaged version the high-speed pinion rotation that cranks the engine only starts after the pinion is fully engaged with the ring-gear. There is no collision between a rapidly rotating pinion and a stationary ring-gear.
Often a relay valve is fitted to control the movement of the pinion, moving it up the shaft until it connects with the ring-gear. Once engaged, the full pneumatic (air) or hydraulic pressure of the starting system is released to crank the engine.
ATEX is a set of European Union regulations that ensure products used in explosive environments are safe. Through a cast iron construction and by using pre-engagement IPU’s pre-engaged air starter motor conform to ATEX EN1834-1 and 2. This means they are safe to be used in flammable gas atmospheres as well as underground environments vulnerable to combustible dust.