1. the intake stroke
   the compression stroke
2. the exhaust stroke

To make the best use of the fuel, the spark should occur before the piston reaches the top of the compression stroke. So by the time the piston starts down into its power stroke, the pressures are high enough to start producing useful work.

1. earlier
   later
2. No change is necessary.

The time that the fuel takes to burn is roughly constant. But the speed of the pistons increases as the engine speed increases. This means that the faster the engine goes, the earlier the spark has to occur. This is called spark advance: The faster the engine speed, the more advance is required.

1. retarding the spark
   advancing the spark
2. promoting the spark

Goals like minimizing emissions can be achieved when maximum power is not required. For instance, by retarding the spark timing (moving the spark closer to the top of the compression stroke), maximum cylinder pressures and temperatures can be reduced. Lowering temperatures helps reduce the formation of nitrogen oxides (NOx), which are a regulated pollutant.

1. compression
   knocking
2. ignition

Retarding the timing may also eliminate knocking; some cars that have knock sensors will do this automatically.

1. the lack of a copper gasket
   the length of the center electrode
2. the shape of the ceramic tip

Some cars require a hot spark plug. This type of plug is designed with a ceramic insert that has a smaller contact area with the metal part of the plug. This reduces the heat transfer from the ceramic, making it run hotter and thus burn away more deposits. Cold spark plugs are designed with more contact area, so they run cooler.

1. a single coil
   two coils
2. three coils

The coil is a simple device -- essentially a high-voltage transformer made up of two coils of wire. One coil of wire is called the primary coil. Wrapped around it is the secondary coil. The secondary coil normally has hundreds of times more turns of wire than the primary coil.

1. spark plug
   piston
2. coil

The distributor handles several jobs. Its first job is to distribute the high voltage from the coil to the correct cylinder. This is done by the cap and rotor. The coil is connected to the rotor, which spins inside the cap. The rotor spins past a series of contacts, one contact per cylinder. As the tip of the rotor passes each contact, a high-voltage pulse comes from the coil. The pulse arcs across the small gap between the rotor and the contact (they don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder.

1. a ceramic insert
   insulated wires
2. a copper gasket

Spark plugs use a ceramic insert to isolate the high voltage at the electrode, ensuring that the spark happens at the tip of the electrode and not anywhere else on the plug; this insert does double-duty by helping to burn off deposits. Ceramic is a fairly poor heat conductor, so the material gets quite hot during operation. This heat helps to burn off deposits from the electrode.

1. the exact temperature of the spark plugs
   the exact position of the pistons
2. the exact location of the coil

Spark timing is so critical to an engine's performance that most cars don't use points. Instead, they use a sensor that tells the engine control unit (ECU) the exact position of the pistons. The engine computer then controls a transistor that opens and closes the current to the coil.

1. the spark control unit (SCU)
   the engine control unit (ECU)
2. the coil control unit (CCU)

线圈在这种类型的系统中以同样的方式工作as the larger, centrally-located coils. The engine control unit controls the transistors that break the ground side of the circuit, which generates the spark. This gives the ECU total control over spark timing.