At the gas pump, you have a choice of octane numbers.87? 93?
气泵上有两个辛烷值，87和93。你会选择哪个？
What's the difference?
这两个数字有什么不同呢？
To explain octane numbers,let's take an imaginative leap into the internal combustion engine of a car.
为了解释辛烷值，让我们富于想象力的思维先跳跃到汽车的内燃机上面。
Inside the cylinders are combustion chambers where a mixture of gasoline and air burns.
气缸内部是供汽油和空气的混合物燃烧的内燃室。

Normally, the spark plug ignites the mixture and the pressure from the resulting explosion pushes the piston down.
正常情况下，火花塞会点燃这种混合物，爆炸产生的压力会推下活塞。
But, if the fuel is not matched to the engine, the heat in the cylinder can ignitethe fuel even before the spark plug fires.
但如果燃料和发动机不匹配，气缸的热量在火花塞点火前就会点着燃料。
This early ignition causes pressure waves that rattle the piston, making a knocking sound.
提前点着燃料会引发压力流，使活塞咔哒咔哒作响，发出爆震音。
It can also overheat the spark plugs, erode the combustion chamber, and make the car run inefficiently and roughly.
同时，会使火花塞过热、燃烧室产生磨损，使得汽车在行驶过程中效率低下并且无法平稳运行。
Octane numbers, also known as antiknock ratings, tell you how resistant gasoline is to knocking.
辛烷值也称为抗爆性指标，表示汽油的抗爆性。
Gas at the pump has been tested in a lab, where it's compared to two reference fuels.
泵里的汽油是经过实验室检测的，另两种参考燃料会在实验室内与其进行对比。
The first isvery knock resistant, rating 100 on the knock resistance scale.
第一种抗爆性极好，辛烷值达到100。
The second knocks easily and rates a lowly zero.
第二种易产生爆震，辛烷值很低，为零。
The two are combined to make comparison mixtures.
我们把两种燃料混合起来做成用于对比的混合油，
Gas with an octane number of 87, when tested, acted just like the comparison mixturewith 87% knock resistant fuel.
发现在试验过程中，辛烷值为87的汽油和含有87%抗爆剂的混合油效果一样。
So,the higher the octane number, the more resistant to knocking.
因此，辛烷值越高，抗爆性就越强。
If your engine doesn't knock with lower octane gas, there's no reason to spend more on higheroctane.
如果发动机在使用较低辛烷值汽油的情况下不发生爆震，那就没有必要在较高辛烷值的汽油上花更多的钱。
Because knocking depends partly on the engine's compression ratio, some engines need higher octanes to prevent it.
因为爆震现象部分取决于发动机的压缩比，一些发动机会需要较高辛烷来防止爆震。
Owner's manuals often specify the right octane for your engine, andyour ears are a good measure, too!
车主的实用手册上通常会写明适合发动机的辛烷，而你的耳朵同样也是检测爆震的好工具。