1. a way to tell how fast a car can go
   a way to tell how much the fuel can be compressed
2. a way to tell how much the engine will wear out

The octane rating relates to how much the fuel can be compressed before it spontaneously ignites. A rating that’s too low can cause the engine to run poorly.

1. It’ll make it hard for the engine to burn the fuel.
   It’ll help the car get better gas mileage.
2. It’ll make the engine blow up.

Water will prevent the gasoline in the system from burning properly, which will cause combustion problems in the engine, which will prevent the car from starting or cause it to run poorly.

1. They set off too many smoke alarms.
   They smelled too bad.
2. They aren’t as efficient.

Internal combustion engines are a lot more efficient than external combustion engines, which is why steam engines fell out of favor for cars.

1. motorcycles
   hybrid cars
2. Oscar Mayer Wienermobiles

Many motorcycles are powered by air-cooled engines.

1. how much nitrous oxide you can use
   an engine’s maximum RPM
2. an equation that determines when the turbo kicks in

An engine’s redline, which is shown on the tachometer, determines its maximum revolutions per minute, or RPM.

1. camshaft
   timing belt
2. distributor

A <a href='http://auto.howstuffworks.com/camshaft.htm' style="font-weight: bold;">camshaft</a> is a rotating rod that uses a series of lobes to open and close the valves. Most modern engines have overhead cams -- the camshaft is directly above the valves.

1. carburetion, hybridization, electrical charging
   fuel injection, infusion, circulation
2. carburetion, port fuel injection, direct fuel injection

Carburetion, port fuel injection and direct fuel injection all get the fuel where it needs to go.

1. intake, compression, combustion and exhaust
   injection, rotation, combustion and exhaust
2. injection, carburetion, rotation and exhaust

In the intake step, a cylinder gets some fuel and air to burn. Then, it compresses the fuel to make the combustion step (the explosion) more powerful. In the last step, the cylinder releases its exhaust.

1. engine control module, which is the engine’s computer
   engine catheter manager, which controls the oil coming out of the engine
2. European custom mechanical, which helps race car owners keep track of their modifications

ECM is the engine control module, and it’s the computer that manages the engine’s functions.

1. lack of exhaust
   lack of water
2. lack of spark

Lots of things can go wrong under the hood, but the big three in the engine involve fuel, compression and <a href='http://science.howstuffworks.com/fire.htm' style="font-weight: bold;">fire</a> .

1. The engine’s movement compresses too much oil.
   It gets too dirty to work.
2. The oil evaporates in the summer.

Because engine oil cleans the engine, it picks up dirt and debris as it circulates, which gradually reduces its efficiency.

1. convert fuel into heat
   convert fuel into motion
2. convert fuel into exhaust

There's a lot of heat in a car's engine -- and a lot of exhaust coming out of it -- but these are byproducts of the engine's true purpose. An engine has to use fuel to create motion to move the <a href='http://auto.howstuffworks.com/tire.htm' style="font-weight: bold;">tires</a>.

1. lubrication, cooling, cleaning
   lubrication, getting rid of exhaust smell, making gas more powerful
2. cooling, cleaning, making more money for Jiffy Lube

Most people know that engine oil lubricates to help with metal-on-metal contact, but it also helps cool and clean the engine as it circulates.

1. a set of gears that makes the wheels turn faster
   a turbine that compresses the air traveling into the engine
2. an injector that delivers fuel to the engine faster

<a href='http://auto.howstuffworks.com/turbo.htm' style="font-weight: bold;">Turbochargers</a> and <a href='http://auto.howstuffworks.com/supercharger.htm' style="font-weight: bold;">superchargers</a> are both air compression systems. They pressurize the air moving into the engine, increasing the engine's power.

1. steam engine
   firecracker engine
2. hydroponic engine

Steam engines were most commonly used in trains and boats, but did power some types of cars.

1. It’s efficient and compact.
   It’s faster and makes less exhaust.
2. It’s cheaper and can use watered-down gas.

The 4-cylinder engine did not become popular in the United States until the 1980s, when Japanese manufacturers demonstrated how small and efficient it could be.

1. deliver fuel to the cylinders
   change linear motion into rotational motion
2. keep cylinder heads in place

When <a href='http://science.howstuffworks.com/gasoline.htm' style="font-weight: bold;">gasoline</a> ignites in a cylinder, the piston moves in a straight line. Your wheels, on the other hand, need to turn. The piston moves a connecting rod, which turns the crankshaft, producing the rotational motion.

1. the Tesla Model S
   the Volkswagen Beetle
2. the Ford Model T

The Volkswagen Beetle was designed with an air-cooled engine specifically for its simplicity.

1. air intake and turbocharger
   battery and fan
2. radiator and water pump

With a water-cooled engine, which is the most common type of cooling system, the radiator and water pump circulate water to absorb heat and move it away from the engine.

1. It’ll start an oil fire.
   The engine will seize.
2. It’ll cause oil poisoning.

Oil lubricates the pistons so they can move smoothly inside the engine, and without oil, the engine will seize.

1. sends electricity to the spark plugs, starting ignition
   sends electricity to the fuel pump, forcing the car to start
2. spins the engine a few revolutions, starting the combustion process

The starter motor has a big job -- it has to overcome friction, cold weather and other factors to get your engine started. If it fails, nothing happens in the engine when you turn the key.

1. the flywheel to the camshaft
   the crankshaft to the camshaft
2. the cylinder head to the camshaft

A timing belt or timing chain links the crankshaft to the <a href='http://auto.howstuffworks.com/camshaft.htm' style="font-weight: bold;">camshaft</a> so that the valves are in sync with the pistons.

1. banging and clunking
   clicking and clacking
2. pinging and detonation

Pinging is another way to describe the sound, and detonation describes what’s causing it.

1. V-6, V-8, HEMI
   inline, V, rotary
2. inline, flat, rotary

An engine’s cylinders are most often arranged in a line (inline), in two rows set at an angle (V) or in two opposite banks (flat).

1. It’s easy to fix.
   It can use almost any combustible material for fuel.
2. Steam pollutes less.

A steam engine just needs to create steam, so if it can burn, it can be fuel. That's why in those old silent movies, when the hero runs out of coal to power the train, he starts burning his coat and hat.

1. a nuclear submarine
   a giant robot
2. a spud gun

In a both <a href='http://science.howstuffworks.com/spud-gun.htm' style="font-weight: bold;">spud gun</a> and a car engine, you ignite a small amount of fuel, releasing a large amount of energy. (And it's equally unwise to get in front of a moving car or a loaded spud gun.)

1. the wrong spark plugs
   driving too fast
2. being low on oil

There are several causes of engine knock, and experts say that installing the wrong spark plugs is a common problem.

1. because of coal rolling
   because it contains more energy
2. Actually, it doesn’t.

Diesel has a higher energy density than gasoline, meaning that each gallon burned makes more power.

1. alternator
   battery
2. spark plugs

A large battery is used to start the engine when you turn the key and run the vehicle's electrical system. The alternator is connected to the engine by a belt and generates electricity to recharge the battery.

1. Otto cycle
   Tesla cycle
2. compression cycle

The Otto cycle is named after its inventor, Nikolaus Otto.