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TURN, BABY, TURN: CRANKING SYSTEMS

Posted: August 1, 2014 by Rolf Lockwood

On days when even the mercury is having trouble getting up in the morning, your engine should be rarin’ to go in no time. The electric cranking systems on diesels today — the starter motor, the batteries, and the cables that connect them — are generally reliable and simple in theory: a motor engages a gear with the engine’s flywheel, moved by a solenoid’s magnetic force. Combustion follows and the engine fires. Hot-damn, you’re on your way.



If not, the starter motor has to work longer and harder to keep the engine turning until combustion conditions improve. In very cold weather, it can take a lot of cranking before the pistons can compress and heat cylinder air enough to promote burning of fuel. Turning over a big, high-compression diesel takes up to 25 horsepower. The motor may be capable of several minutes of cranking a balky engine, but the power supply may not. That’s when the jumper cables come out. The key to avoiding that scenario starts with spec’ing a robust cranking system in the first place, starting with, fittingly, a starter motor.



Your starter motor has to be strong enough to produce rated power for 20 to 30 seconds without overheating or burning out. Then, after a pause long enough to mutter ³C’mon, c’mon,² it has to be able to do it again. The tendency of a really frustrated driver is to crank continuously without giving the motor a chance to rest. So spec a motor with overheat protection: a circuit breaker or similar device that pops when the starter gets too hot. It will reset itself when things cool down.



The batteries, meanwhile, need enough reserve capacity to pump out the required number of amps over a similar length of time. These amps are measured in the hundreds, so electrical cables and battery connections must be stout enough to carry the load without melting or deteriorating or causing excessive resistance.



Battery Basics



Today’s storage batteries still use the lead-acid principle, but advances in construction and materials make them less trouble-prone than they used to be. Many of today’s batteries are so-called maintenance-free, where the battery case is virtually sealed against the escape of gases (which otherwise would carry off moisture with them). So the unit should need little or no water added during its life.



New compounds within the battery, like calcium paste on the plates instead of antimony, also reduce deterioration and add to life. Reputable battery manufacturers put their products through a lot of testing to ensure they meet industry standards. The standards include minimums for:



CCAs: Cold cranking amps are the measurement of amperes available at 0 F. A heavy-duty battery is usually rated at 625 to 1,000 CCAs, and it takes 1,200 to 1,800 CCAs to crank a big-bore diesel. Simple addition says we need two to four batteries to supply those amperes. And the greater the number of batteries, the less the failure of one battery will hurt the system. This is why class-8 trucks, and many class-6 and class-7 vehicles, have a bank of three or four batteries rather than just one.



The Technology and Maintenance Council of the ATA recommends four batteries for big-bore diesels requiring more than 1,800 CCAs. This includes most models with displacements of 10.6 to 14.75 litres. Whether you go with four batteries or try to save weight by cutting down to three, be sure the total CCAs exceed the engine builder’s requirements.



Charge acceptance: The battery is discharged to 20 per cent of capacity and then recharged with 14.4 volts (the usual voltage for an alternator) at 30 F. After 10 minutes, the ampere reading should be within 2 per cent of the battery’s CCA rating. Some batteries have city operation in mind and are made to cycle often. Others, made for highway service, will last longer if they don’t see much discharging.



Connections



TMC has specific recommendations on cable size, insulation, location, and connector and battery post design for various applications. Among other things, they take into account the length of cables in specifying maximum allowable voltage drop, and demand crimp- or screw-type connections at various points in the system. Location is important — the closer to the cranking motor, the better.



Because vibration is a battery’s worst enemy, TMC recommends that the battery box be mounted at the rear of the cab between the frame rails, where road vibration is the least severe. Most battery boxes are on the frame rails. That’s OK if they’re right on the rails; mounting them cantilever-style away from the frame rails causes a jarring ride, so avoid it. Another enemy of batteries is heat, which can cause a battery to be overcharged. So keep batteries away from the exhaust system.



Starting Aids



Though not related to the cranking system, starting aids definitely affect its performance. An easy engine to fire means less work for the cranking system. For example, glow plugs on some imported engines warm the air in the cylinders before ignition, which helps initial combustion.



Ether-injection systems can help stoke non-glow-plug diesels on very cold mornings. The latest ether systems are activated automatically when the starter is engaged and when temperature sensors indicate the need. Electronic controls ensure injection of the correct amount of fluid. Such systems are options on new trucks and are not hard to retrofit.



All of these ideas will help get you moving, but they’re no substitute for a properly spec’d and maintained system. Check the batteries and cables regularly, keep all fittings tight and bright, and listen to the starter motor for deterioration. If it’s conking out, replace it before it fails out on the road.



And when you do replace any of the starting system’s components, ask for the capacity your engine calls for and make sure you get it.

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