Car Quiz — 50 Questions on How Cars Actually Work

Engine displacement, the number written as 2.0 liters, measures what?

Answer The combined swept volume of all the cylinders

It is the volume the pistons sweep through, added up across the cylinders — so it says how much air the engine can move per two revolutions, not how much power it makes. Turbocharging is precisely the trick of getting more air through the same displacement.

What spins the compressor inside a turbocharger?

Answer Exhaust gas leaving the engine

Exhaust on its way out spins a turbine, and that turbine drives the compressor on the same shaft, often past 200,000 revolutions per minute. A supercharger does the same job but is driven by the engine itself, which is why it responds instantly and costs power to run.

In a four-stroke engine, which stroke comes immediately after compression?

Answer Power

Intake, compression, power, exhaust — and only one of the four actually pushes. The other three are paid for by the momentum already stored in the rotating parts, which is the whole reason an engine idles rather than stopping between bangs.

What does a camshaft control?

Answer When the valves open and close

It is a shaft of egg-shaped lobes that press the valves open at the right moment. Because the best moment differs between low and high speeds, most modern engines can shift that timing while running, which is where a good deal of their flexibility comes from.

Besides lubricating, what else does engine oil do?

Answer Carries heat away from moving parts

It cools, cleans, seals the gap at the piston rings, and carries worn particles to the filter. That is why oil turns dark — it is doing its job — and why the change interval is about the additives wearing out rather than the oil disappearing.

What happens inside a radiator?

Answer Heat moves from the coolant to the passing air

It is a heat exchanger, so nothing is cooled by it directly except the coolant flowing through. Coolant is not just water either: the additives raise the boiling point and stop the metal corroding, which matters as much as the freezing point they are named for.

What does DOHC describe?

Answer Two camshafts in the cylinder head

Two camshafts, one for the intake valves and one for the exhaust. Separating them makes room for four valves per cylinder and allows the engine to breathe at higher speeds, which is why the layout became standard as engines got smaller and revved harder.

Why does an engine need a flywheel?

Answer To smooth the gap between power strokes

One cylinder fires once every two turns, so the drive arrives in pulses rather than a steady push. The flywheel stores enough rotation to carry the engine between them, and it is one reason engines with more cylinders feel smoother at low speeds.

What is the spark plug's job in a petrol engine?

Answer It ignites the compressed fuel and air

It fires a spark across a gap a fraction of a millimeter wide, at tens of thousands of volts, thousands of times a minute. The gap is what the voltage is for: too wide and the spark cannot cross, too narrow and it is too weak to light the mixture.

A diesel engine ignites its fuel how?

Answer By squeezing air until it is hot enough to light the fuel

There is no spark. The air is squeezed hard enough — typically around twice the compression of a gasoline engine — to reach ignition temperature on its own, and the fuel is injected into it. The glow plugs people mistake for spark plugs only help when starting cold.

A red temperature warning comes on while you are driving. What is it telling you?

Answer The engine is overheating and you should stop

Continuing can turn an inexpensive repair into a new engine, because an overheated head can warp. The other half of the advice is to wait before opening the cap: the system is pressurized so that the coolant can pass its normal boiling point.

The warning light shaped like an old-fashioned oil can means what?

Answer Oil pressure has dropped

It is measuring pressure, not level, which is why it is more urgent than a low-oil message. If it comes on while driving, oil may not be reaching the parts that need it, and the distance you can safely continue is roughly the distance to the shoulder.

What does ABS prevent?

Answer The wheels locking up under hard braking

It keeps the wheels rolling so you can still steer while braking hard, which is the point of it. It does not always shorten the stopping distance — on gravel or fresh snow a locked wheel builds a wedge and can stop shorter — but you cannot steer around anything with locked wheels.

TPMS warns you about what?

Answer Tire pressure

Some systems measure pressure in each wheel directly; others infer it from how fast each wheel is turning, since a soft tire is slightly smaller. The second kind cannot tell you that all four are equally low, so a gauge is still worth owning.

A battery-shaped light stays on while the engine runs. What does it usually mean?

Answer The charging system is not keeping the battery topped up

It is a charging fault, not a flat battery — usually the alternator or the belt driving it. The car keeps running on what is stored, so you have a limited window, and it shrinks fast once the lights and fans are drawing on it.

What sits behind a check-engine light?

Answer A fault code stored by the engine management system

The light is only the messenger; the useful part is the code stored behind it, which any basic reader can pull out through the standard port under the dashboard. Steady usually means investigate soon, flashing usually means a misfire, and that one means stop.

The brake warning light comes on and the pedal feels soft. What should you do?

Answer Stop somewhere safe and have it checked before driving further

A soft pedal suggests fluid has escaped or air has got into the lines, and the next application may be worse than this one. This is one of the few dashboard warnings where continuing to the next town is not a reasonable choice.

What does a timing belt do?

Answer Keeps the crankshaft and camshaft turning in step

It ties the valve timing to the piston movement. In an interference engine — most modern ones — a snapped belt lets valves meet pistons at speed, which is why it is replaced on a schedule rather than when it fails.

Why is a clogged air filter a problem?

Answer It restricts the airflow the engine needs

An engine is an air pump, and choking the intake makes it work harder for the same result. Modern management adjusts to hide the symptom, so the first thing you notice is often fuel consumption rather than a loss of power.

Which complaint does a wheel alignment usually fix?

Answer Uneven tire wear and a car that pulls to one side

Alignment and balancing get mixed up constantly. Alignment sets the angles at which the wheels meet the road and fixes pulling and uneven wear; balancing corrects small weight differences and fixes vibration at speed.

On a tire marked 205/55 R16, what is the 205?

Answer The tire's section width in millimeters

The three numbers use three different systems. The width is in millimeters, the next number is a percentage of that width rather than a measurement, and the last is the wheel diameter in inches — a metric, ratio and imperial figure in one code.

A tire run below its correct pressure tends to wear where?

Answer On both shoulders, at the edges

Too little pressure lets the middle collapse slightly so the edges take the load; too much does the opposite and wears the center. Reading a worn tire is a reasonably reliable way to learn what the pressure has been doing.

Why does braking distance grow so sharply with speed?

Answer Kinetic energy rises with the square of speed

Doubling your speed roughly quadruples the distance needed to shed that energy, because it goes up with the square. Reaction distance rises only in proportion, which is why the gap between a moderate and a high speed is far larger than it feels.

What is happening when a car aquaplanes?

Answer The tires ride up on a film of water and lose contact

The grooves can only pump so much water out of the way, and past a certain speed the tire climbs onto what it cannot clear. Steering and braking both stop working at once, and the response is to lift off gently rather than brake or turn.

Tread depth matters most in which condition?

Answer Wet roads, where the grooves clear water

On a dry road a bald tire actually grips well, which is why racing slicks exist. In the wet the grooves are what give the water somewhere to go, and wet stopping distance starts growing long before a tire reaches the legal limit.

Apart from the tread pattern, what makes a winter tire different?

Answer Its rubber compound stays flexible in the cold

The compound stays pliable when it is cold, while a summer tire hardens and loses grip below roughly seven degrees. That is why winter tires help on a cold dry road too, and why the change is about temperature rather than about snow.

Disc brakes turn the car's movement into what?

Answer Heat

Braking does not make energy disappear; it converts motion into heat and throws it into the air. Discs are drilled or vented for exactly that reason, and it is also why an electric car can recover part of that energy instead of wasting it.

What causes brake fade on a long downhill run?

Answer Brakes that have grown too hot to work properly

Repeated braking on a descent puts more heat in than can escape, and pads lose bite or the fluid boils and the pedal goes long. The fix is to use a lower gear so the engine does the slowing, which costs nothing and cannot overheat.

Why should tire pressure be checked when the tires are cold?

Answer Driving heats the air inside and raises the reading

Driving warms the air inside and the reading rises by roughly a tenth of a bar for every ten degrees. Check a warm tire and you will conclude it is fine, then find it low in the morning.

The four-digit code stamped on a tire sidewall tells you what?

Answer The week and year it was manufactured

It is the week and the year the tire left the factory — the first two digits are the week. Rubber ages whether or not it is used, so a tire with plenty of tread can still be too old, and a spare that has never touched the road is the usual case.

A fuel's octane rating measures what?

Answer How well it resists igniting before the spark

It measures resistance to igniting on its own before the spark arrives, which is what knocking is. It is not a measure of energy or cleanliness, and in an engine not designed for it a higher grade buys nothing at all.

What does regenerative braking do?

Answer Runs the electric motor as a generator to recover energy

Slowing down turns the motor into a generator, so energy that would have become brake heat goes back into the battery instead. It is why hybrids do best in city traffic and why their friction brakes often last remarkably long.

What makes a car a hybrid?

Answer It carries both a combustion engine and an electric motor

The two can be arranged in several ways — the engine may drive the wheels, or only generate electricity, or both. What they share is that the combination lets the engine spend more of its time near its efficient range and be switched off the rest.

A battery rated at 60 kWh is telling you what?

Answer How much energy it can store

It is capacity, the size of the tank rather than the strength of the motor. Power is quoted separately in kilowatts, and the pair explains both figures on a charger too: how big the battery is and how fast it can be filled.

Why does an electric car's range usually fall in cold weather?

Answer The battery works less efficiently and heating the cabin takes energy

Two things happen at once. Chemistry slows in the cold so less of the stored energy is available, and unlike a combustion car there is no waste engine heat, so warming the cabin comes out of the same battery.

What does a DC fast charger do that a home charger does not?

Answer It skips the car's onboard converter and feeds the battery directly

A home charger delivers alternating current and the car's own converter turns it into direct current, which is the bottleneck. A fast charger does that conversion outside the car and feeds the battery directly, which is why it can be many times quicker.

Which uses more fuel over the same distance?

Answer Stop-start city traffic

Every stop throws away the energy it took to get moving, and it is thrown away as brake heat. Steady travel spends most of its fuel on air resistance instead, which is at least doing something, and it is precisely the city case that hybrids were built for.

As speed rises, aerodynamic drag grows how?

Answer Roughly with the square of speed

Drag rises with the square of speed, but the power needed to push through it rises with the cube. That is why a small increase at highway speed costs so much more fuel than the same increase in town.

What is idling?

Answer Leaving the engine running while the car is stationary

A modern engine does not need to be idled to warm up; it warms faster under gentle load, and idling wears it while doing no work. This is what automatic stop-start is for, and the fuel saved is largest exactly where traffic is worst.

What is the main idea behind a continuously variable transmission?

Answer It holds the engine near its efficient speed instead of stepping through gears

A conventional transmission gives you a handful of fixed ratios and the engine has to work at whatever speed they impose. A continuously variable one can pick any ratio, so it can hold the engine where it is most efficient and let road speed change around it.

Which company introduced the moving assembly line for cars in 1913?

Answer Ford

Moving the work to the worker rather than the worker to the work cut the assembly of a Model T from something like twelve hours to about ninety minutes. The car itself was not new; the method was, and it changed what a car cost.

The three-point seat belt was developed at which carmaker, which then let others use the patent?

Answer Volvo

Volvo's engineer patented the three-point belt in 1959, and the company made the patent freely available so that other manufacturers could use it. It is usually described as one of the most consequential decisions any carmaker has made.

Which reached production cars first?

Answer The automatic transmission

The automatic transmission reached mass-produced cars around 1940, decades ahead of antilock brakes or airbags, which arrived on production cars in the late 1970s and early 1980s. Convenience was solved well before safety was.

A saloon, or sedan, describes what shape?

Answer A three-box body with a separate boot

Three boxes: engine, cabin, and a luggage compartment sealed off from the passengers. What separates it from a hatchback is not the shape of the tail but whether the load area is part of the same space as the people.

What is a kei car?

Answer A Japanese class of car with strict limits on size and engine capacity

A legal category rather than a style, defined by maximum length, width, height and engine size, with lower tax and insurance in return. Those limits are the reason a whole class of Japanese cars looks the way it does.

What does all-wheel drive change?

Answer It sends drive to all four wheels rather than two

It helps you get moving on a loose or slippery surface, and it does nothing at all for stopping or turning — those depend on the tires. Confusing the two is behind a good share of winter accidents involving confident drivers.

Why does a driven axle need a differential?

Answer To let the wheels turn at different speeds through a corner

Through a corner the outer wheel travels farther than the inner one, and a solid axle would force them to fight each other. The trade-off in a simple differential is that it sends power to whichever wheel has less grip, which is why one spinning wheel can strand a car.

On a manual car, what is the standard pedal order from the left?

Answer Clutch, brake, accelerator

Clutch, brake, accelerator, and the order does not change with the side of the road the country drives on. Standardizing it means a driver moving between countries or cars does not have to relearn the one thing that must be automatic.

What was the catalytic converter introduced to do?

Answer Cut the harmful gases leaving the exhaust

It converts the harmful products of combustion into less harmful ones as the exhaust passes through. Because lead destroys it, its arrival is the reason leaded gasoline was phased out — one regulation forced the other.

What does a VIN identify?

Answer One individual vehicle

Seventeen characters that identify a single vehicle rather than a model. The letters I, O and Q are deliberately excluded so they cannot be mistaken for one and zero, which tells you the code was designed to be read aloud and copied by hand.