Demands on Today’s Starting Systems

Today’s heavy-vehicle engines demand the most from starter motors since emission reduction strategies have increased engine cylinder pressures during cranking. Anti-idling laws, which require drivers to reduce the amount of time their engine idles, require modern starter motors to crank warm and more mechanically resistive engines many more times than before. Start-stop energy reducing drive systems and hybrid drive vehicles place even more strain on starter motors.

In spite of the increasing demands, new designs of starter motors and systems controls are enabling starters to last longer and increase output torque while substantially reducing motor weight.

A typical example of this is the improvements and changes that have occurred to Delco Remy starter motors in recent times. The Delco Remy 40MT weighs 66Ib(30kg), and the Delco Remy Starter 39MT weighs only 30Ib(14kg) yet produces mre cranking torque with much greater reliability. The Delco Remy 37MT is the latest, third-generation starter using a gear reduction drive system to multiply torque output. The 37MT weighs approximately 50 Ib(23kg). All three starters are used on engines from 0 to 15L displacement. Minimum life expectancy for a starter now is 4 years with 7,000 starter cycles.



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Heavy Vehicle Starter Motors

An engine with a capacity of more than 3 L requires a large starting motor, specifically for a compression-ignition (CI) engine. CI engines must be cranked at a speed of at least 100 rpm to initiate combustion. Due to a very high compression ratio in CI engines, the maximum torque required is greater than a petrol engine.

The high pressure in the cylinder accelerates the piston rapidly during its downward movement. This causes considerable variation of both the rotation­al speed and the resisting torque loading on the starter motor.

To achieve high starting power heavy vehicles normally use a 24 V system. For a given electrical power requirement, the doubling of the voltage compared with a 12 V systems, reduces the current by half. Without the extra 12 volts, the current load on the battery and starter circuit becomes exceptionally high, especially on a cold morning in winter. Another problem associated with large engines is the requirement of high torque to overcome breakaway and inertia of the heavy parts. It also needs a strong drive system and the pinion of this system must be fully meshed with the ring gear before the motor is fully powered. Two types of motor and drive system in use are axial (sliding armature), and coaxial (sliding gear).

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Starter Motors -Why it doesn't stay in mesh

Starter Motors -Why it doesn't stay in mesh

The starter motor engages with a gear on the flywheel of the engine, to crank the engine, in order to make it fire. 

To explain further - the gear on the starter has for instance, ten teeth. The flywheel gear has 100. The gear ratio is therefore ten turns of starter to one of the engine. 

If the gear on the starter itself turns at 2,500 rpm, this will relate to an engine speed of 250 rpm. If the starter stayed engaged once the engine had fired, the internals of the starter would not be able to cope with the centrifugal forces generated by the engine now turning the starter! 

The average engine will routinely rev to 4,000 rpm and many are capable of well in excess of this. The net result of a starter staying in mesh is that, with a gearing of 10:1 it would rev at 40,000 per minute (well in excess of the designed speed) and almost certainly cause a complete failure.






So the first requirement of the starter (apart from spinning the engine) are that it disengages. 

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