Episode No.127 – The Starter Motor

Episode No.127 – The Starter Motor

Welcome back, today a real
treat, the starter motor. These are all the parts of the starter and the engineering in there is so tasty that I’ve included a knife and fork. Sic. A knife and fork is perhaps
my favorite little bit of engineering on this car, elegant in its design
and extremely reliable. So today we’re going to build this starter from its parts and then you’ll
know exactly how it works. We’ll get to explore all
sorts of new mechanisms, solenoids, planetary
gears, and sprag clutches. So stick with me, we’re
covering a lot more than just starter motors here. Why do we need a starter at all? It’s because an engine can’t start itself and this applies to most engines not just car engines, you might have seen little starter trucks driving around at airports starting up planes. Or you’ve pulled the cord
to get a lawnmower started. Now all internal combustion engines need some help to get moving. And that’s because only
one of the four strokes in an engine generates
power as the third stroke, the power stroke, the
engine could have stopped at any point with its
pistons in any position, and to start again we need to get back around to a power stroke. And that is the job of the starter motor, to turn over the engine,
taking it through the unpowered exhaust, intake and compression strokes, until the power stroke is reached and the four-stroke cycle
is able to run for itself. Now the first cars were
started with a hand crank, turning a handle attached
to the crankshaft would take the engine through
those unpowered strokes. But this was hard work, and
potentially dangerous too because the handle had this
tendency to be unpredictable so broken hands, ribs, legs were all risks of starting your car. It’s a great example of what
an extreme sport motoring was back in the day, a strong likelihood of a crippling injury before
you’ve even got moving. So safety measures were added but this was far still from ideal. Various other methods of
starting were attempted, using coiled springs or compressed air. But from 1912, every Cadillac included an electric starter motor. – [Narrator] Cadillac
and a man named Kettering found a way to start a car electrically with a push of a button. – Theoretically, this damn
thing oughta work now. – The design of starters
has been fairly standard since then, and certainly
since the 60s onwards, starters were of a near universal design like this one we have here. And there’s been a bit of change lately because we have stop-start
engines which use a different type of starter, which also
functions as an alternator. I’ll make another video on
that later as soon as I can convince someone to
give me one to dismantle but back to our classic starter. The starter has a small
gear called a pinion which interlocks with the teeth on the outside of the flywheel. Inside the starter is a
very strong electric motor which turns the pinion,
which turns the flywheel that turns the engine and
hopefully gets it started. I will install this temporarily
so we can see in action and then we’ll pull it to
bits and have a closer look. (motor revving) (soft music) Normally, the starter fits
to the transmission casing on this car so we’d have
a whole bell housing here and we wouldn’t be able to see anything. So I’ve made this bracket
and I’ve also cut away a little piece of the starter housing here so that we can see inside. One thing that we can see immediately is that the pinion gear is much
smaller than the flywheel. Compared to engines, electric
motors turn very fast but they don’t generate much torque, that is, they don’t have
a lot of turning force. And to turn the engine
over takes a lot of force. And this small motor has
to move a load of metal to get the engine started,
it’s got the flywheel, the crankshaft, the harmonic
balancer, the alternator, the power steering pump,
four connecting rods, four pistons, the timing gear,
the camshafts and the valves. And all in, I reckon that’s at
least 120 pounds or 50 kilos. And, it’s compressing the air-fuel mixture which makes things even harder. And the engine needs to
turn over at a decent speed to get it started, any
less than about 100 rpm and there’s not enough
suction or compression created inside the cylinders. Now our flywheel has 112 teeth,
and this pinion has eight. This reduction in gearing
means that on this engine the pinion turns 14 times for
every turn of the flywheel or in other words, the
torque from the motor is multiplied by 14 times. Let’s take a look inside the motor itself. Even after the reduction in gearing between the starter
pinion and the flywheel, we still don’t have
enough torque to actually turn the engine, so inside the
motor is a planetary gear set which further reduces
the speed of the motor and increases its torque. Planetary gears are super cool
and this is the first time that we’ve come across
them, so let’s go on a little detour and
appreciate their beauty. A planetary gear, or epicyclic gear train, is so called because it has planet gears orbiting around a sun. They have a couple of benefits
but one of the big benefits for us is that they’re
compact, so compact that from the outside we wouldn’t even realize that there’s a gearbox in the motor. They’re also strong, stable and efficient. In our system we have a sun gear, which is driven by the electric motor. Around the sun gear are our planet gears, we have three planet gears but
we could have more or less. Three just happens to be a nice number. The planet gears mesh on
their outside with a ring gear which in our starter is fixed. So as we turn the sun gear, the
planet gears all roll around the ring gear and orbit around the sun. To get power out of this system, we use the orbiting motion
of the planet gears. A carrier with small shafts
runs through each planet gear, linking them together, and
taking the rotation out at a greatly reduced
speed and a higher torque. One of the benefits of this
system is that we have lots of teeth meshing at once,
if this was just a pair of differently sized gears,
then just a couple of teeth would be taking all the force. This way the force is spread
across multiple points, and the gears can be made from
weaker, cheaper materials, we’ve got a nylon ring and sintered gears. Calculating the gearing
ratio of planetary gears is supposed to be complicated, tricky math but it’s actually really, really easy. Just go on Google and you
type in planetary gear ratio calculator and you’ll
find someone else has done the hard work for you. Standing on the shoulders of giants. Now from this calculation,
I can see that our planetary gear ratio is 6:1 so six
rotations of our motor gives us one rotation of the carrier. So the gearbox reduces
the speed by six times, and then we’ve got the ratio
of the pinion and flywheel which takes things down another 14 times. So spinning our motor 84 times will turn the engine once. Okay. (crashing) It’s very common to find
electric motors that incorporate a drive set like this, because motors are most
efficient at high revolutions. So you’ll find gear sets
like this inside drills, power tools and most electric motors. To turn this motor we
need a lot of electricity. If you get into a car with
even a slightly flat battery, everything will seem fine until
you go to start the engine. And that’s because the starter
motor is the single biggest user of power in the car. The radio, the headlights, the fuel pump, none of it even comes
close to the power demands of the starter motor. The label on this starter shows
that it will take a current of 400 amps, depends on the load. On a cold day, with thick
oil it might take 400 amps but normally it’s probably
100 amps, maybe 200 amps. So how does this motor
compare to, for example, this beefy mains powered compressor motor. Or to a kettle? We can’t compare the current
figure, that 400 amps because a low voltage like 12 volts requires much more current
than a higher voltage like the 240 volts that this motor takes. We need to calculate power,
and power is measured in watts. And it’s actually very
easy to work out power, it’s voltage times current,
and it’s a useful formula to know for all sorts of things around the garage or in the house. So the voltage is 12 volts,
and the current is 400 amps. So 400 times 12 gives us 4,800 watts. Or 4.8 kilowatts, and that’s a lot. A kettle takes about 1.2 kilowatts so this starter can put out
the power of four kettles. And this huge motor takes 2.2 kilowatts which is surprising, this small motor is double the power of this bigger one. And while we’re comparing
these two motors, notice the difference in design. This bigger motor is
covered in cooling fins because powerful motors get hot. And even though the starter
motor is more powerful, it doesn’t really have any cooling fins. That’s because it’s intended
to only run for a few seconds, not for hours like this beast. A starter is designed for bursts of high power and not endurance. The current requirements of
a starter are going to vary. A starter for a bigger
engine will need more power. Diesel engines have higher
compression so the motor needs to work harder there too. Starting a warm engine with
thin oil is much, much easier than starting a cold engine
on a cold day with thick oil. And a big diesel truck
might well take 1,000 amps for its starter motor. But even at 400 amps, getting
the current to the starter motor needs a big fat supply cable, the thickest cable in the car generally. A cable will run straight from
the battery to the starter, and if you want to find your starter, then just follow the thickest cable off the positive terminal,
and the return path to the battery because this is a circuit is made via the engine itself, the negative side of
our motor is connected to the metal motor body which
is connected to the engine. The battery’s negative terminal
is attached to the chassis, and the engine is connected to the chassis with one or more earthing
straps as we’ve seen before. Of course the earthing
straps need to be just as thick as the starter supply. At 12 volts and big currents,
there is big huge voltage drop along the wires so the
cables to the starter motor should be as short as possible. Most cars have their
battery in the engine bay, but in this car, the
battery lives in the trunk because that improves the
weight balance of this vehicle because it tends to be
front heavy with the engine and the gear box and all the
rest in the front of the car. So there’s a long thick cable running from the battery to the starter. Okay, so we have the
current into our starter, and the motor is generating enough torque to actually spin the engine. But right now we don’t have any way of turning the starter on and off. What we need is some sort of switch. There’s a challenge here
because we could haul this huge supply cable
up into the dashboard but then we’ll need a massive switch on it like something out of Frankenstein. – It’s coming up. – And big currents require big switches and chunky metal contacts. Routing this wire up into the dashboard would also double the length
of the cable that we needed. We just talked about voltage
drop and really wanting short cables between the
starter and the battery so before drivers began
worrying about their camel skin seats and hand-brushed aluminum interiors, a big clunky switch was
exactly how the starter was operated, often a
big metal foot switch. Nowadays, we want an elegant little key, or even a push button that
can activate the starter. And that’s achieved by
splitting the electrical supply into two separate circuits,
a starter motor circuit which carries the heavy
currents to drive the motor, and a control circuit
which uses lighter wires that control the heavier circuit. And to do this, we use a solenoid. The principle behind the
solenoid is that a small current is used to turn on a bigger current. And if you’re familiar with relays then it’s exactly the same idea. A solenoid is a simply
a tight coil of wire that generates a magnetic
field inside itself when it’s powered on. That magnetic force will pull a piece of iron into its center. A starter solenoid uses this exact design and here’s one that I’ve cut open. You can see that there’s a
coil of wire running around inside and inside that coil
of wire fits a plunger. And when we turn on the
current to the solenoid coil, this terminal is a
positive and like usual, the casing of the solenoid
acts as the ground, as we turn on the current,
the plunger is pulled in and it’s pulled in with
quite a lot of force there as you can see, when
we release the current, the plunger pops back out. Now at the end of the
solenoid is a push button which is just inside the
coil and when the plunger is pulled into the solenoid,
it pushes on that switch, which connects the heavy
current to the starter motor. On the back here, we have two terminals. One terminal brings power
from the battery’s positive terminal, and the other terminal
runs to the starter motor. And when the button is
pushed, this big hefty piece of copper bridges the
two copper terminals here and that takes current to the
starter, the starter runs. In some cases, the plunger
will directly bridge these terminals but having the push button that we have here and the
separate area where the contacts are made keeps any sparking or arching in its own little area here. We’ve seen the two big terminals
on the back of the solenoid and there’s a third smaller terminal here and that third, small terminal
is used to turn the solenoid on and off, it’s called the
S wire or the S terminal, the S being for signal and the
wire from this small terminal leads up to the ignition switch. And as we said, with most
electrical systems in the car, the solenoid is grounded via its casing so we only need one wire to control it, a current flows down the signal wire, in around the solenoid coil
and then back to ground through the chassis of the vehicle. It’s quite common for
there to be two coils inside a solenoid, a strong
closing coil that pulls in the plunger, and a second
weaker coil which holds it in position because it takes
a lot more energy to pull the plunger from outside the coil into it than it does to hold it in position. When the contacts for
the starter are made, a switch also de-energizes
that pull in coil so it saves a bit of
power from the battery that’s not needed when the
starter is actually cranking. But don’t worry about that
because if the solenoid coils aren’t working then you’re
replacing the whole solenoid or probably the whole starter assembly. If you’ve ever tried to start
a car with a flat battery, then you’ve probably heard
a repeated clicking sound (clicks tongue) that’s
not a clicking sound. Click, click, click,
click, click, click sound and the engine doesn’t start. That clicking is the solenoid
moving or the plunger moving inside the solenoid,
now with a good battery, the solenoid moves once
to start the motor, so it moves inside, clicks, the motor runs and if you listen carefully
you’ll hear that click before the starter motor starts running and it drowns it out
with a whirring sound. But when the battery is discharged
there isn’t enough power to supply the massive current
that the starter motor needs. And the solenoid doesn’t
need much power so it still plunges, and that turns
on the starter motor. As soon as the motor’s turned
on, it takes all the current from the battery in an attempt to move, but there isn’t enough
current to power the motor and there isn’t enough current
left or isn’t enough power left in the battery to
hold the solenoid close so it’s forced back open by the spring and the motor is disconnected. But now with the motor
disconnected, there’s enough power for the solenoid so it plunges
back in, energizes the motor, the motor can’t run but
it takes all the power, solenoid pops out and this
process just continues. And that’s the clicking that you hear as you attempt to start the
car with a flat battery. So if you’re sitting in a car with a pack of velociraptors bearing
down on you and the car won’t start, you just
hear this clicking noise, just think of the beauty
of this solenoid mechanism. So far, we have a starter
motor that can be switched on and off, and through
the reduction in gearing it has the torque to turn the engine over. And with this design we
can start the engine. But the engine runs much
faster than the starter motor and with the setup that
we have already discussed, the starter motor would
be permanently meshed with the flywheel and after
the engine has started it would be driving the motor around, in a situation called backdrive where the flywheel is
driving the starter motor. Now driving the starter motor all the time would be a total waste of
energy but more importantly it would tear this motor to
pieces because it’s designed to do the driving, and not to be driven. Take a look at a video of the starter in action from earlier. You can see the pinion
engages the flywheel, then spins, and then
disengages the flywheel when we release the ignition. The pinion moves in and out when the ignition key
is turned on and off. And fortunately for us, we
already have something moving in and out when the starter is engaged. That’s the plunger inside the solenoid. You might have noticed that
there’s a small white pin on the plunger and it
hooks to a nylon lever which I need, I’m going
to find this nylon lever. This pin on a plunger
hooks to a nylon lever that pulls the pinion gear in and out. And this lever is a fork
that fits over a ring on the shaft of the starter’s drive gear. And the layout of the
plunger and the fork mean that the pinion is moved
into mesh with the flywheel just before the motor is turned on. This is called a pre-engagement system because the gears are engaged
before the motor runs. As the pinion moves,
sometimes we’ll be lucky and the pinion teeth will fit
into gaps on the flywheel, the ring teeth here but we
can equally be in a situation where the two gears clash and the teeth bang into each other. There are three ways this
situation is handled, the first is with a small
bevel on the pinion gear here. Which means that as it clashes here, it will tend to be guided
just a slight rotation to bring it into alignment so
if we’re slightly misaligned here then that small bevel
will hopefully correct things. The second way that this
clash of gears is handled is through a slight
twisting in the pinion. Let’s look again at the starter
engaging in slow motion. Can you see there’s a slight
rotation of the pinion as it moves along the shaft. That’s achieved with this
helical twist on the splines here on the armature, as the fork
pushes the pinion assembly along, it rotates
slightly on these splines. And combined with the bevel on the teeth, it’s quite rare that
the pinion won’t engage, but it is still possible. And to handle this situation, the pin on the solenoid plunger has a spring. When the gears have collided,
and they’re not interlocking, the plunger isn’t going
to make it all the way to the motor contacts, so
the solenoid keeps pulling and this spring is overcome
and it’s a very strong spring allowing the plunger to
make it to the contacts and switch on the starter. As soon as the motor spins,
our gears will pop into mesh and the flywheel can spin, it’s brilliant. Now we’re almost there, but
there’s one final challenge that the starter needs to engineer around. What would would happen if
someone keeps the ignition key turned after the engine has started? At the moment, the starter
would stay connected to the flywheel and it would
forced around by the engine until we release the ignition. I’m sure you’re all saying oh
I’ve got super fast reflexes, I release the key immediately
when the engine fires up. Well what about your gran? She’s probably got super
fast reflexes as well. But anyway we only want the
starter motor to be driven one way, the motor should turn the pinion, but the pinion should not turn the motor. Actually what we want to
happen is exactly the same as on a bicycle, you know
when you’re on a bicycle, power from your feet
should turn the wheel, but the wheel should not turn your feet. This is achieved with
a freehub on a bicycle and we don’t even think
about it but it’s a type of one-way clutch, force only goes one way from the pedals to the wheels. The starter motor also
uses a one-way clutch, a design called a sprag clutch. It’s this cylinder that
looks like a bearing. It looks like a bearing because
basically that’s what it is. There are a few different
types of one-way clutch, but they all work on
pretty much the same idea. The clutch is made up of an
inner race, and an outer race. The inner race is connected to the pinion and the outer race to the motor. The two races are separated
with little wedges, called cams, or sprags. Turning the bearing one way
causes these little sprags to rotate and get jammed,
locking the inner race and the outer race together. So the outer race is now
propelled around by the inner one. This is how the motor turns the pinion. But turning in the opposite
direction makes the sprags rotate in the opposite direction too and they don’t bind up. The outer race is free to rotate and since it isn’t
locked to the inner race it can just freewheel. So when the pinion is running
faster than the motor, this clutch stops the
force being sent back to the starter motor. That’s not quite everything,
every car built since about 2005, and many built before then, will have what’s called
a starter safety switch, or a neutral safety switch. This switch prevents
the starter from running when the vehicle is in gear
and the idea behind that is that accidentally starting
the car in gear would cause the vehicle to jump forwards and potentially set
off driving on its own. There’s loads of reasons why this is bad, you might lurch forward a couple of feet and hit a car in front, or a garage wall or a kid left in the car could
start the vehicle driving by messing around with the key. The safety switch sits
between the ignition switch and the solenoid, keeping the circuit open unless the safety switch is engaged. On an automatic vehicle, this
switch is on the transmission and it’s closed only if the transmission is in neutral or it’s in park. Mechanical switches for
automatic vehicles also stop the ignition key from
actually turning in the barrel and those are called interlock switches. With a manual vehicle like this one, the switch will be on the clutch
pedal and it only actuates when the pedal is fully depressed. There is often another
switch on the clutch pedal for the ECU and that
ECU switch makes contact as soon as the clutch
is pressed even slightly so if you’re rummaging
around down in the footwell checking the clutch safety switch, just be sure you’ve got the right switch. Let’s take a look at the
starter system as a whole. When you turn the ignition
key, that acts like a switch. And current flows through
that switch, to some sort of safety switch either
on the clutch pedal or on the transmission, the
safety switch stops the car being started in gear, and
with the ignition and safety switch closed, power goes to the solenoid. When the solenoid is
engaged, it pulls a plunger. The plunger uses a lever which
pushes the pinion into mesh with the flywheel, and
when the pinion is engaged, the plunger moves a bit
further and switches on the current to the starter motor. The starter motor runs,
starting the engine. And when the ignition key is released, a strong spring in the solenoid
pushes the plunger back out turning off the starter motor
and retracting the pinion from the flywheel. So we got through a lot there. We’ll come across planetary
gears again on the differential. We’ll meet solenoids on
various valves and actuators elsewhere around the car,
although if someone talks about a solenoid in relation
to a car they’ll almost certainly going to mean
the starter solenoid. And one-way clutches,
well they’re just cool but they’re also found in
automatic transmissions. And of course you know
absolutely everything about starter motors. Now the starter uses a load
of power from the battery and we need some way to
recharge that battery once the engine is
running, so next we’ll talk about the alternator and how
it generates electricity. Guys if you like detailed videos that go into the nitty gritty Of how mechanical things work. Then jump over to howacarworks.com Where we’ve got a full video course that takes you from A to Z. From starters… no… that’s S. From alternators to…. something that begins with Z. But basically we teach you everything that you need to know. In mad detail, just like this episode about the starter. So if that’s what you enjoy, you’ll learn it all. Come visit us over there.

About the Author: Michael Flood


  1. This is qualitiy content. Thatnk you for this video. Hower i would like to know how bad is it to turn starter motor on while engine is running. It happened a few times. Can you damage flywheel or starter gear?

  2. What? How has this channel never been recommended before? Instant sub. Episode 127? Have to stop this one and catch up!

  3. Oh god, this is gold man, you explanations are so clear makes me want to learn more about mechanics. first video I watch of you bot be sure that Im gonna literally dive into the others right away! Well done, seriously 🙂

  4. Not all starter motors have the plunger/lever mentioned at @19:10, I have a 30 year old car that I've replaced the starter on a few times before I learned about the solenoid, but I'm good with electronics so I bypassed the worn out solenoid with a high amp relay but it works exactly the same with no wires going to the solenoid, I'm glad I did it that way cos I can start the car now with a much lower battery power, which is super useful since I have a habit of leaving the stereo on too long without the car running

  5. I'm super surprised as to why this channel is so underrated. I think the video title plays a role in that. Please use titles that would make it more visible when searching. I suggest you hire an expert to do that. The content is top-notch and the viewers should be there, I'm sure of it.

  6. Wow! This explanation is just fantastic. I can't wait to see more content. I hope this channel will picked up by the masses and we will see a lot more stuff. Subscribed and shared.

  7. There's a youtube about swapping super-capacitors for the battery for extra cold climates, like Canada/Sweden in the winter. He used a clamp-meter to test the amps at the starter cable. On start-up, it drew 200A for a moment to get the engine spinning, and then it drew 100A to continue spinning. He had disconnected the spark and fuel relay to prevent the engine starting during the experiment, and the engine was a common 2L gasoline 4-cylinder of normal compression. A diesel or a V8 would draw more.

  8. This video is amazing! terminologies are a little different but any mech nerd can easily get the gist of things. great work

  9. Truly phenomenal video and explanation. I'll be using this series for all my home car repairs now and sharing it with friends who DIY as well

  10. Very instructive and still quite simple thanks to your explanations. I'm rebuilding an Austin mini 1098cc engine, and after watching your video i can tell you that my solenoid is dead !

  11. What a great channel!!! Never knew it existed till it popped up on my recommended list. Except for the one way clutch (video didn't identify pinion, outer race, …), great explanations. Loved it and subscribed.

  12. Very well done, very informative and right to the point. Waiting for your video on another type of starters for start stop engines.

  13. Insane video quality, I haven't seen anything better. Just a bit of animation, mostly physical descriptions, as it should be.

  14. Thanks for a great video the only downside to having a modern manual car is that if the battery does go flat there is no way to push start it because of the safety mechanism that you mentioned with the clutch having to be fully depressed which is a major bummer.

  15. 15:15 That is a good (Wire) save my friend. And managed to keep that segment on till you pulled it back and attached it. 😀

  16. Actually, most planes have a smaller turbine engine which starts the main engines. These planes do not require a starter cart. Small turbine engines can be started relatively easily.

  17. These are phenomenally good videos, thank you! Lots of detail, extremely clear, and engaging delivery. Channel's popularity should be blowing up any day now…

  18. This is an awesome video I am already subscribed I need to check the playlist to understand all things about the car

  19. Your work and the level of detail you go into is astounding! I look forward to watching the whole 20 hours and can't wait for you to go into transmissions.

  20. How do you only have 65.7K subs? The production value here is terrific. I'm 25 and would love to learn more about Cars and Engines. Smashed that Sub button and Rang the bell. Nice Work!

  21. When I started this video I didn't I'd make it to the end, but this was one very educative video 👏🏿👏🏿👏🏿👏🏿 this video deserves a thumbs up and this channel has earned by subscription… keep making more!

  22. As everyone else has said phenomenal content. Like all your best school teachers rolled into one. Can’t help thinking that with a less generic channel name and more focus on the name of the guy presenting , this channel wld go through the roof…. but what do I know???

  23. this is flippin better than school :)) surprised that it got so few likes and i am guessing the unlike are for people getting bored?

  24. ICE engines are way too complicated and wasteful and so many tiny little parts might break at anytime – they need to be changed with something else.

  25. …and a really handsome guy as well! I can't tell what's wrong with my starter though. Only the clicking sound and a lot of stress on the indicator lights. Seems like something is stuck and the gear won't rotate at all. Guess I'll be needing a car electrician to check it out.

  26. I teach motor vehicle for a living in the UK. For the level you're aiming it at, this is as lovely an introduction to starters as I've seen, and lord knows I've given the exact same lesson enough. great stuff 🙂

  27. What a combination of information and clickbait. I mean, where's the other 120 sum odd videos? If they existed to this quality, I'd be very interested.

  28. Clear, concise, but with just enough detail to answer questions. Brilliant videos, ill be preordering if this is what i should expect from the videos

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