What is Octane?

조회 수 2395 추천 수 0 2012.12.04 14:28:43

Gasoline Grade에 대한 좋은 Article이네요. Basically, most modern vehicles equipped with on-board computer can adjust any grade of gasoline w/o causing much serious engine damages. 

그리고, European 차들의 Octane계산법((RON)이, 미국식의 Octane계산법((R+M)/2) 보다 5정도 높게 되어있답니다.  예로 제 Mercedes는 93 Octane(RON)이니, 미국에선 88 Octane((R+M)/2 method)정도입니다.  

AutoGuide.com에서 따온글입니다.

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Under the Hood: What is Octane?

  |  Dec 03, 4:01 PM

Anyone that’s ever topped off their tank or filled up a jerry can knows gasoline comes in different grades, from regular unleaded to pricey premium.Each “flavor” has a number that corresponds to its octane level, but what does this mean? What is octane and what role does it play? AutoGuide reached out to experts in the fuels field for answers.

To understand the function of octane, which is basically a measure of the chemical resistance of gasoline to auto ignition – the higher the number the less likely it is to explode under pressure – you first have to know the basics of how a gasoline internal-combustion engine functions. If you’re already familiar with pistons, valves and spark plugs feel free to skip the next three paragraphs, if not, please continue reading for a quick overview.

Chrysler-Pistons.jpgThe engines found in almost all vehicles today function on something known as a four-stroke cycle. Within each engine are pistons that move up and down inside cylinders. The pistons are attached to connecting rods, which spin on a crankshaft. With the help of a transmission, the force applied to these parts is what ultimately drives your Camry’s wheels and gets you to work in the morning.

What are the four strokes of this all-important process? Well, in order of operation they’re intake, compression, power and exhaust.

The cycle starts as a piston moves downward in a cylinder. This is called the intake stroke. As it drops, a precisely controlled mixture of air and fuel vapor is drawn into the cylinder through open intake valves. Next, the intake valves shut and the piston rises, beginning the compression stroke where the air-fuel mixture is squeezed. Just as the piston reaches the top of its travel the spark plug fires, which ignites the air and fuel resulting in a potent bang. The power stroke has begun. This pushes the piston down again and provides the driving force that keeps the engine running and the vehicle moving. No sooner does the piston reach the bottom of its travel than it’s time to go up again. This begins the exhaust stroke, where all of the hot, leftover gasses from the combustion process get pushed out of the now-open exhaust valves. Once the piston reaches the top the intake valves open and the whole process starts again. Imagine how fast all of this happens when you’re merging on the highway!

Octane.jpg

Hopefully that makes sense; if you’re a gearhead welcome back. Now for the main event and the reason you’re here in the first place – the role octane plays in an engine.

Stephen Russ, Technical Leader with Ford Motor Company’sengine engineering department is a fuels expert. According to him, “octane is just a measure of the chemical resistance to auto ignition.” He also said “high-octane fuel is more resistant to auto ignition.”

So what does that have to do with anything? Well remember, during the compression stroke a piston is squeezing a combination of air and fuel. If this mixture gets put under too much pressure it will spontaneously ignite, which is a serious problem should it happen before the spark plug fires.

Ford-3.5-Liter-EcoBoost-V6-Engine2.jpgAuto-ignition, more commonly referred to as “spark knock,” can result in an audible pinging noise. This potentially destructive clatter sounds a bit like pennies rattling in an empty pop can.

According to Bill Studzinski, Fuels Technical Specialist at General Motors,auto ignition can lead to “pressure waves that run into each other that cause the ringing or the knocking that you hear.”

Beyond noise, spark knock can wreak havoc on internal engine components. Surprisingly it can melt holes in pistons and even bend connecting rods. Thankfully Russ said “it’s really, really rare nowadays” because of advanced engine-control computers.

“We have knock sensors on our vehicles,” Studzinski said. These are small electronic transducers attached to an engine’s block that listen for specific sound frequencies associated with spark knock. If it’s detected the powertrain control module can do a number of things to get combustion under control again. It can lower boost levels on forced-induction engines, pull spark timing or enrich the air-fuel mixture to prevent internal damage.

High compression ratios allow engines to create more power while burning less fuel.  A compression ratio is basically how tightly the air and fuel mixture inside a cylinder gets squeezed. Modern engines typically have ratios right around 10-to-1, but they can exceed 12-to-1 or more if they have direct fuel injection. Also, forced-induction engines will be a little bit lower.

Automakers have to walk a fine line though; they can’t go overboard. This is where octane fits into the equation. Engines with higher compression ratios – often found in performance cars – almost always need higher octane fuel because it is less likely to automatically ignite. Russ said higher octane gasoline “does enable us to put more spark advance in and get more efficiency.”

To recap, higher cylinder pressures require higher octane, which prevents engine-damaging auto-ignition. But mistakes can happen, what if you put the wrong grade of fuel in your vehicle?

“If you’ve got a premium-required car and you put in 87 octane and you start hearing audible knock, you need to baby that car until you can get that fuel changed” Studzinski said, adding “even if you don’t hear the audible knock the vehicle is reacting.” It’s reducing performance, lowering fuel economy and sending more heat into the exhaust catalyst, which reduces its durability.

US-Octane-Formula.jpgSo, how is an octane rating calculated? According to Studzinski, in the United States the average of two separate tests is used to determine the number you see on a gas pump.

You’ve probably noticed the formula (R+M)/2 before. A lot of times it’s printed right on fuel pumps. The “R” stands for “research octane” and the “M” for “motor octane.” Each of these is one of the tests just mentioned. The average is designed to give an acceptable rating for all drivers in all different conditions. The fuel needs of someone towing a trailer up a mountain pass in the desert are quite different from someone driving a small passenger car at sea level.

According to Studzinski the research-octane test favors larger, older vehicles – cars and trucks typically powered by big-bore, naturally aspirated engines.  It features higher in-cylinder temperatures than the motor-octane test, which is geared more toward small, efficient, turbocharged engines.

In America the average of two tests determines octane, but Studzinski said “we don’t have a harmonized global octane standard,” and “the rest of the world only uses research octane numbers.”

If you’ve ever been to Europe for instance you may have noticed something like “95 RON” on their fuel pumps. RON, of course, stands for “research octane number.”

GM-2.0-Liter-Turbo-Engine.jpgInterestingly, Studzinski said Europe’s regular-grade gasoline, which is rated at 95 RON equates to about 90 in the (R+M)/2 test. That means European vehicles have to be recalibrated to run on America’s regular-grade fuel, which is only 87 octane.

One thing Studzinski asked was “is (R+M)/2 an appropriate rating for modern engines?” a question many fuels experts and oil refiners are no doubt asking themselves. For the average driver though, it’s best to follow what’s in their vehicle’s owner’s manual by running the recommended grade of gasoline for the best performance and highest efficiency.




메이저

2012.12.04 21:02:05
*.237.181.128

잘 읽었습니다. 선배님!

샤인

2012.12.05 00:49:57
*.214.248.163

 'Octane'에 대한 설명이 참 좋습니다. 좋은 정보 고맙습니다.

Han Kim

2012.12.05 02:18:19
*.162.211.116

혼돈을 피하기 위해서는 하루 빨리 "harmonized global octane standard"를 만들어야겠네요~~

옥탄가가 높을수록 폭발 에너지도 크고, 폭발 시간도 앞당겨지는 것으로 이해하여도 맞는지요??

삿갓

2012.12.05 03:06:06
*.95.1.53

제가 다니는 Mercedes Dealer에서는 이용하는 모든 차량들(Mercedes Model)에 Regular (87 Octane)을 쓴다고 하더군요. 예전부터 전 저희집 모든차량에 Regular grade만 써왔다가, 최근 저희집 Boss의 성화에 High Octane으로 바꾸었습니다. Engineering을 이해못하는 Boss(wife)와 싱갱이하기보단 몇센트 더 쓰기로했습니다. 하지만, 가끔씩 Regular Grade도 넣는답니다.

Scott Kim

2012.12.05 03:36:55
*.95.1.53

Knocking은 흡-압-폭-배 의 4행정에서 폭발이 완전압축되기전에 일어나서 엔진 부품에 기계적무리를 주는것이므로, Higher Octane 연료는 폭발을 완전 압축될때까지 일어나지않도록 postpone시키는 역활을 하는것으로 이해됩니다.

차량을 설계하는 단계에서 이미   Regular Octane로 디자인된 차량은 higher octane fuel를 넣더라도 기계적(knocking)으로나 연료효율(MPG)면에서나 큰 잇점이없을것같습니다. 

반대로 Higher Octane 차량에 lower octane fuel than required (after consideration of different Octane grading method)쓰시게되면, On-board computer의 도움으로 knocking은 생기지 않겠지만, 엔진 출력이 몇% 떨어질수가 있을것 같네요.

실제로, 어떤 Hyundai model들 (Genesis Sedan or Coup)은,  아예  Manual에 Higher Octane fuel 이나 Regular Octane fuel을 쓸수있다고 명시되어있고, 또 그에따른 출력을 기표해놓았다고 읽었습니다.   

3.8 V6 for Hyundai Genesis
  • 314 hp (234 kW) (premium fuel) / 306 hp (228 kW) (regular) (2009–2012)
    348 hp (260 kW) (premium fuel) / 344 hp (257 kW) (regular) (2013– )[1]
이경우엔 1%의 출력차이가있네요. 이정도는 운전자가 느끼기 쉽지않은 힘의 차이입니다.

Scott Kim

2012.12.05 03:49:49
*.95.1.53

Regular Octane fuel용 차량에 Higher Octane fuel을 넣더라도 흡-압-폭-배 의 압 cycle이 Higher Octane fuel이 성능을 발휘할 기회(higher pressure(compression) and/or higher temperature)가 주어지지 않기 때문에 덕이 없다는 뜻입니다.

제 wife가 옆에서 질문해서 답을 써보았습니다.

백운대

2012.12.05 23:04:26
*.162.211.116

자세한 설명을 읽고나니 어느 정도 윤곽이 잡힙니다...

예로서, 100미터 가속시 premium fuel을 사용한 차가 일반 연료를 사용한 차보다 1미터 정도 앞선다는 것이네요.

그러므로 일상적인 드라이브에서는 큰 차이가 없는 것으로 간주됩니다(자동차 경주가 아니므로).

다만 추가로 고려할 사항은 엔진수명과 연비 등도 대상이 되겠지요...

premium fuel을 사용시 가속반응느낌은 좋아지겠지만, 이를 위해서 추가지불을 감수해야만 할지...(작은 고민?)

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