What Do Octane Numbers Mean?
At the gas pump, you have a choice of octane numbers. 87? 93? What’s the difference?
To explain octane numbers, let’s take an imaginative leap into the internal combustion engine of a car.
Inside the cylinders are combustion chambers where a mixture of gasoline and air burns. Normally, the spark plug ignites the mixture and the pressure from the resulting explosion pushes the piston down. But, if the fuel is not matched to the engine, the heat in the cylinder can ignite the fuel even before the spark plug fires.
This early ignition causes pressure waves that rattle the piston, making a knocking sound. It can also overheat the spark plugs, erode the combustion chamber, and make the car run inefficiently and roughly.
Octane numbers, also known as antiknock ratings, tell you how resistant gasoline is to knocking. Gas at the pump has been tested in a lab, where it’s compared to two reference fuels. The first is very knock resistant, rating 100 on the knock resistance scale.
The second knocks easily and rates a lowly zero. The two are combined to make comparison mixtures. Gas with an octane number of 87, when tested, acted just like the comparison mixture with 87% knock resistant fuel. So,the higher the octane number, the more resistant to knocking.
If your engine doesn’t knock with lower octane gas, there’s no reason to spend more on higher octane. Because knocking depends partly on the engine’s compression ratio, some engines need higher octanes to prevent it. Owner’s manuals often specify the right octane for your engine, and your ears are a good measure, too!
氣泵上的辛烷值是什么意思?
氣泵上有兩個辛烷值,87和93。你會選擇哪個?這兩個數(shù)字有什么不同呢?
為了解釋辛烷值,讓我們富于想象力的思維先跳躍到汽車的內(nèi)燃機(jī)上面。
氣缸內(nèi)部是供汽油和空氣的混合物燃燒的內(nèi)燃室。正常情況下,火花塞會點(diǎn)燃這種混合物,爆炸產(chǎn)生的壓力會推下活塞。但如果燃料和發(fā)動機(jī)不匹配,氣缸的熱量在火花塞點(diǎn)火前就會點(diǎn)著燃料。
提前點(diǎn)著燃料會引發(fā)壓力流,使活塞咔噠咔噠作響,發(fā)出爆震音。同時,會使火花塞過熱、燃燒室產(chǎn)生磨損,使得汽車在行駛過程中效率低下并且無法平穩(wěn)運(yùn)行。
辛烷值也稱為抗爆性指標(biāo),表示汽油的抗爆性。泵里的汽油是經(jīng)過實(shí)驗(yàn)室檢測的,另兩種參考燃料會在實(shí)驗(yàn)室內(nèi)與其進(jìn)行對比。第一種抗爆性極好,辛烷值達(dá)到100。
第二種易產(chǎn)生爆震,辛烷值很低,為零。我們把兩種燃料混合起來做成用于對比的混合油,發(fā)現(xiàn)在試驗(yàn)過程中,辛烷值為87的汽油和含有87%抗爆劑的混合油效果一樣。因此,辛烷值越高,抗爆性就越強(qiáng)。
如果發(fā)動機(jī)在使用較低辛烷值汽油的情況下不發(fā)生爆震,那就沒有必要在較高辛烷值的汽油上花更多的錢。因?yàn)楸瓞F(xiàn)象部分取決于發(fā)動機(jī)的壓縮比,一些發(fā)動機(jī)會需要較高辛烷來防止爆震。車主的實(shí)用手冊上通常會寫明適合發(fā)動機(jī)的辛烷,而你的耳朵同樣也是檢測爆震的好工具。