The starting system in an automobile is the electrical circuit that turns the crankshaft fast enough for the engine to fire and keep running on its own. It is also called the cranking system. A 12 V battery feeds a high-torque DC starter motor through a solenoid switch; a small pinion on the motor meshes with the flywheel ring gear, cranks the engine for a few seconds, and disengages once the engine runs.
This page follows the engine starting system in the order current flows through it, then works through the numbers. The pinion engagement mechanism itself is covered in detail on our Bendix drive page, and the battery on our lead acid battery page.
Why does an engine need a starting system?
A piston engine cannot start from rest by itself. It needs something outside to draw in the first charge, compress it and turn the crankshaft past the first firing strokes. There is a minimum speed below which it will not fire:
- Petrol engine: the minimum cranking speed is much lower, well under 100 rpm in most engines, because the spark plug provides ignition. In practice starters crank well above this minimum.
- Diesel engine: a diesel needs roughly 150 to 250 rpm. It relies on the heat of compression to ignite the fuel; at low speed, heat leaks away through the cylinder walls and gas leaks past the rings before the piston reaches the top, so the air never gets hot enough. That is why diesels have bigger starters and batteries, and glow plugs for cold starts.
Early cars were started with a hand crank. The electric self-starter replaced it, and today every car, bike with a self-start button, bus and truck uses the same basic circuit.
Starting system components in current-flow order
| Component | Circuit | What it does |
|---|---|---|
| Battery | Power source | Supplies a large current for a few seconds; rated by cold cranking amperes (CCA) |
| Ignition / start switch | Control | Key position START or push button; carries only a few amperes |
| Neutral safety switch (automatic) or clutch switch (manual) | Control | Breaks the control circuit unless the gearbox is in P or N, or the clutch pedal is pressed, so the car cannot lurch forward on starting |
| Starter relay | Control | A small relay, often switched by the body or engine ECU, that feeds the solenoid; protects the ignition switch from solenoid current |
| Solenoid (magnetic switch) | Control and main | Pushes the pinion into mesh and closes the heavy contacts that connect the battery to the motor |
| Starter motor | Main | Series-wound (or permanent-magnet) DC motor that produces the cranking torque |
| Drive and pinion | Mechanical | Pinion with overrunning clutch meshes with the ring gear, then freewheels once the engine fires |
| Flywheel ring gear | Mechanical | Large toothed ring on the flywheel; the pinion-to-ring ratio multiplies the motor torque |
| Heavy cables and earth strap | Main | Short, thick cables from battery to solenoid and from engine to body, to keep voltage drop low at hundreds of amperes |
There are really two circuits. The control circuit (switch, safety switch, relay, solenoid windings) carries a few amperes to tens of amperes through thin wires. The main circuit (battery, heavy cable, solenoid contacts, motor, earth return) carries the cranking current, which can be several hundred amperes.
Starter motor: why series-wound DC
In a series-wound motor the field coils and the armature carry the same current. When the motor is stalled against a stationary engine, there is no back EMF, so the current is at its highest, and because torque rises with the product of field strength and armature current, torque is highest at zero speed. As the engine picks up, back EMF rises, current falls and torque drops. That is exactly the characteristic a starter needs: maximum torque to break the engine away, then less as it spins up.
Most petrol cars now use a permanent-magnet gear-reduction (PMGR) starter. Magnets replace the field coils, and a small epicyclic gear set lets a small, fast armature give high torque at the pinion. It is lighter and draws less current than a direct-drive unit. Larger diesels still use wound-field starters, often at 24 V on trucks and buses.
How the solenoid works, step by step
The solenoid sits on top of the starter motor and has two windings on one plunger:
- Key to START. Control current reaches the solenoid’s “S” terminal through the safety switch and relay.
- Both windings energise. The thick pull-in winding (typically 30 to 60 A) and the thin hold-in winding (about 5 to 10 A) together pull the plunger in hard. The pull-in winding’s current passes on through the motor, turning it slowly so the pinion teeth can find the gaps in the ring gear.
- Pinion meshes. The plunger moves the shift lever, which slides the pinion into the ring gear.
- Main contacts close. At the end of its travel the plunger’s copper disc bridges the battery and motor terminals. Full battery current flows to the motor and the engine cranks.
- Pull-in winding drops out. Both of its ends are now at battery voltage, so no current flows in it. The hold-in winding alone keeps the plunger in, which stops the solenoid overheating.
- Key released. The hold-in winding de-energises, the return spring opens the contacts and pulls the pinion out.
If the engine fires while the key is still held, the overrunning clutch in the drive freewheels so the engine cannot overspeed the armature.
Types of starter drive
| Drive type | How the pinion engages | How it disengages | Where used |
|---|---|---|---|
| Bendix (inertia) drive | Pinion’s own inertia screws it along a coarse thread once the armature spins | Engine overruns the pinion and the thread throws it back | Older cars, small engines; historical today |
| Pre-engaged with overrunning clutch | Solenoid and shift lever push the pinion in before full current flows | One-way roller clutch freewheels; solenoid withdraws pinion on key release | Standard on modern cars, vans, trucks and motorcycles |
| Axial (sliding armature) | Whole armature is offset in the field and is pulled axially into the poles when current flows, carrying the pinion into mesh | Spring returns the armature when current stops; plate clutch limits torque | Older heavy diesel engines |
The inertia and pre-engaged drives, the overrunning clutch and the gear-reduction starter are compared in full on the Bendix drive page.
Worked example: starter torque, speed, power and battery current
Data: an engine needs a drag torque of 100 N m to crank at 200 rpm. Pinion-to-ring-gear ratio 1:12. Take the gearing as ideal, overall efficiency from battery to crankshaft 60%, and battery voltage under load 10 V. Find the motor torque and speed, the cranking power and the battery current.
- Motor (pinion) speed = 200 x 12 = 2400 rpm.
- Motor torque = 100 / 12 = 8.33 N m. The ring gear multiplies torque by the same ratio it divides speed.
- Crankshaft angular speed ω = 2π x 200 / 60 = 20.94 rad/s.
- Cranking power P = Tω = 100 x 20.94 = 2094 W, about 2.1 kW. Check at the motor: 8.33 x (2π x 2400 / 60) = 8.33 x 251.3 = 2094 W, the same, as it must be with ideal gears.
- Electrical input = 2094 / 0.60 = 3491 W.
- Battery current I = 3491 / 10 = about 349 A.
Two lessons follow. First, the gear ratio is what makes a small motor possible: 8.33 N m is modest, but the ring gear turns it into 100 N m. Second, 349 A is more than the 310 CCA rating of a typical small-car battery (the INT 60 on our battery page). An engine with this much drag, such as a cold diesel, needs a larger battery. Cranking for 3 s at 349 A uses 349 x 3 / 3600 = 0.29 Ah, which the alternator puts back within a few minutes, as shown in our page on battery charging methods.
Start-stop systems
A start-stop system switches the engine off when the car is stationary, for example at a red light, and restarts it when the driver releases the brake or presses the clutch. It saves the fuel otherwise burned idling in traffic. The ECU only allows a stop when conditions are right: engine warm, battery charge high enough and air conditioning demand not too high.
Because the engine may restart dozens of times on one trip, these cars use a reinforced starter with a longer-life pinion and brushes, or a belt-driven starter-generator that replaces the pinion altogether. They also need an EFB or AGM battery and a battery sensor, because the battery is discharged every time the engine stops with the lights and fan running.
Starting system troubleshooting
| Symptom | Likely causes | First checks |
|---|---|---|
| Nothing at all when key turned | Flat battery, open ignition switch, gear not in P/N or clutch not pressed, failed relay or fuse | Battery voltage; try starting in N; listen for relay click |
| Single click, no crank | Solenoid pulls in but main contacts are burnt, or voltage collapses under load from a weak battery or loose, corroded terminal | Clean and tighten terminals; measure battery voltage while cranking |
| Rapid clicking or chattering | Battery too weak to hold the solenoid in; open hold-in winding | Charge or test the battery; check hold-in winding |
| Slow crank | Low battery charge, high resistance in cables or earth strap, worn starter brushes or bushes, thick oil in cold weather | Voltage drop across each cable during cranking; battery load test |
| Grinding noise | Worn pinion or ring gear teeth, loose starter mounting, pinion not fully in mesh | Inspect ring gear through inspection hole; check mounting bolts |
| Starter spins but engine does not turn | Slipping overrunning clutch, broken Bendix spring, pinion not moving into mesh | Replace the drive assembly |
| Starter keeps running after engine starts | Solenoid contacts welded shut, ignition switch stuck in START | Disconnect battery immediately; replace solenoid or switch |
A useful rule on any starting complaint: test the battery first. Most “starter faults” turn out to be a discharged battery or a corroded terminal. Crank in short bursts and let the motor cool between attempts; a starter is built for short-time duty and overheats if held on. The alternator that recharges the battery afterwards is explained on our alternator page. For lecture material on automotive electrical systems, see NPTEL.
FAQs
What is the starting system in an automobile?
It is the circuit that cranks the engine until it fires: battery, ignition switch, neutral safety or clutch switch, starter relay, solenoid, starter motor and pinion drive meshing with the flywheel ring gear.
Is the cranking system the same as the starting system?
Yes. Cranking system and starting system are two names for the same circuit. “Cranking” describes what it does: it turns the crankshaft.
Why is a series-wound DC motor used as a starter motor?
A series motor gives its highest torque at zero speed, when current is highest and there is no back EMF. That is when the engine is hardest to turn, so it suits starting.
What is the minimum cranking speed of an engine?
A petrol engine can fire at well under 100 rpm; a diesel needs about 150 to 250 rpm, because it depends on the heat of compression to ignite the fuel.
Why does the starter only click and not crank?
Usually the battery is too weak or a terminal is loose or corroded, so the voltage collapses when the motor draws current. If the battery is good, the solenoid’s main contacts may be burnt.