Under the hood: The chemistry of cars - Cynthia Chubbuck

引擎蓋下 : 汽車中的化學概念 - 辛西婭．查巴克

譯者: Gibbs Lee 審譯者: 瑞文Eleven 林Lim

There are over one billion cars in the world today,

世界上有超過十億輛汽車

getting people where they need to go,

將人們送到需要到達的地方

but cars aren't just a mode of transportation,

但汽車自並不只是一種交通工具

they're also a chemistry lesson waiting to be taught.

也是一門等著大家去探究的化學課

The process of starting your car begins in the engine cylinders,

啟動汽車的流程始於發動機汽缸

where a spritz of gasoline from the fuel injector

自噴油器先噴出少量汽油

and a gulp of air from the intake valve

加上一點來自進氣閥的空氣

mix together before being ignited by a spark,

兩者均勻混合後再以火花點燃

forming gases that expand and push the piston.

燃燒產生的氣體會膨脹，進而推動活塞

But combustion is an exothermic reaction, meaning it releases heat.

但燃燒是一種釋出能量的放熱反應

Lots of it.

生成很多的能量

And while much of this heat escapes through the tail pipe,

所以縱使很多熱能從排氣管釋出

the heat that remains in the engine block needs to be absorbed, transported, and dissipated

還有許多停留在引擎內的熱能 需要被吸收、傳遞和消散

to protect the metal components from deforming or even melting.

如此才不會使金屬零件變形、甚至熔化

This is where the cooling system comes in.

這就是冷卻系統的作用

A liquid gets circulated throughout the engine,

我們需要液體繞著引擎循環流動吸熱

but what kind of liquid can absorb all that heat?

但是哪種液體能吸收這麼大量的熱能?

Water may seem like an obvious first choice.

水似乎很是首選

After all, its specific heat,

畢竟它的比熱

the amount of energy required to raise the temperature

也就是溫度提升1℃

of a given amount by one degree Celsius,

所需的能量

is higher than that of any other common substance.

比其他相似的物質都還多

And we have a lot of heat energy to absorb.

而我們有很大量的熱能需要被吸收

But using water can get us into deep trouble.

但是如果用水，會帶來很大的麻煩

For one thing, its freezing point is zero degrees Celsius.

例如它的凝固點是0℃

Since water expands as it freezes,

我們知道水結冰時體積會膨脹

a cold winter night could mean a cracked radiator and a damaged engine block,

在寒冷的冬天夜晚，結冰可能 就會導致散熱器破裂或引擎缸體受損

a chilling prospect.

真是令人不寒而慄！

And considering how hot car engines can get,

再者，因為汽車引擎溫度很高

the relatively low boiling point of 100 degrees Celsius

水100℃的沸點就相對太低了

can lead to a situation that would get anyone steamed.

會導致水動輒沸騰

So, instead of water, we use a solution,

所以說，我們需要一種溶液取代水

a homogeneous mixture consisting of a solute and a solvent.

那是一種含有溶質和溶劑的均勻溶液

Some of the solution's properties will differ depending on the proportion of solute present.

有些溶液的性能會隨著 溶質的比例而改變

These are called colligative properties, and as luck would have it,

這叫做「依數性質」，而幸運的是

they include freezing point depression and boiling point elevation.

它可以降低凝固點並提升沸點

So, solutions have both a lower freezing point and a higher boiling point than pure solvent,

所以，溶液有比純溶劑(水) 還低的凝固點及較高的沸點

and the more solute is present, the bigger the difference.

濃度愈高，差別就越大

So, why do these properties change?

為何這些性質會改變呢?

First of all, we need to understand that temperature is a measure

首先，我們需要知道溫度取決於

of the particle's average kinetic energy.

粒子的平均動能

The colder the liquid, the less of this energy there is,

液體溫度越低，所含的能量就越少

and the slower the molecules move.

分子的運動速度也越慢

When a liquid freezes, the molecules slow down,

當液體分子運動速度慢到一定程度時，就會凝固

enough for their attractive forces to act on each other,

慢到讓它們受到彼此引力的牽引

arranging themselves into a crystal structure.

進而形成規則的結晶構造

But the presence of solute particles gets in the way of these attractions,

但溶質分子會阻礙這些作用

requiring a solution to be cooled down further before the arrangement can occur.

所以我們需要一種在 結晶前就能被冷卻的溶液

As for the boiling point, when a liquid boils,

至於沸點，當液體沸騰時

it produces bubbles filled with its vapor,

它會產生充滿蒸氣的氣泡

but for a bubble to form, the vapor pressure must become as strong

但要生成氣泡，液體的蒸氣壓需要

as the atmosphere constantly pushing down on the surface of the liquid.

大到與當時液面的大氣壓力相同

As the liquid is heated, the vapor pressure increases,

當液體升溫時，蒸氣壓會上升

and when it becomes equal to the atmospheric pressure,

當升至與大氣壓力相等時

the bubbles form and boiling occurs.

就會形成氣泡，即是沸騰

A solution's vapor pressure is lower than that of pure solvent,

溶液的蒸氣壓比未加溶質的純溶劑低

so it must be heated to an even higher temperature

所以須被加熱到高於原沸點的溫度

before it can match the strength of the atmosphere.

蒸氣壓才能與大氣壓力一樣大

As an added bonus, the pressure in the radiator

另一項有利的因素是在散熱器中的壓力

is kept above atmospheric pressure,

會保持高於大氣壓力

raising the boiling point by another 25 degrees Celsius.

這可將沸點再提升25℃

The solution commonly used for a car's cooling system

汽車冷卻系統中常用的溶液

is a 50/50 mixture of ethylene glycol and water,

是乙二醇和水1比1比例的混合溶液

which freezes at -37 degrees Celsius and boils at 106 degrees Celsius.

它的凝固點是零下37℃，沸點為106℃

然而， 最佳比例是7比3

At the highest recommended proportion of 70 to 30,

the freezing point is even lower at -55 degrees Celsius,

凝固點甚至低至零下55℃

and the boiling point rises to 113 degrees Celsius.

沸點則高達113℃

As you can see, the more ethylene glycol you add,

由此可知，水中加入越多乙二醇，

the more protection you get, so why not go even higher?

就越能保護汽車，那為何不 多多益善呢?

Well, it turns out you can have too much of a good thing

原因是會「太補」了！

because at higher proportions,

因為在很高濃度時，

the freezing point actually starts to go back up.

凝固點可能反而會上升

The properties of the solution head towards the properties of ethylene glycol,

此特性和乙二醇的性質有關

which freezes at -12.9 degrees Celsius,

乙二醇在零下12.9℃時凝固

a higher temperature than we attained with the solution.

比最佳比例的溶液還高

The solution flows through the engine, absorbing heat along the way.

溶液流經引擎，一路上持續吸收熱能

When it reaches the radiator, it's cooled by a fan,

到達散熱器時，會被風扇

as well as air rushing through the front of the car

及吹入引擎室的空氣冷卻

之後再流經引擎

before returning to the hot engine compartment.

So, an effective and safe engine coolant

所以一種高效率且安全的引擎冷卻液

must have a high specific heat, a low freezing point, and a high boiling point.

必須具有高比熱、 低凝固點及高沸點的特性

But instead of searching all over the world for the perfect liquid to solve our problem,

但除了遍地尋找可以 解決問題的完美液體外

we can create our own solution.

我們還可以自己製造