– Why are you supposed to put high octane gas in some (typically luxury brand) cars?
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High octane gas can resist compression better without igniting prematurely. High performance engines use high compression ratios to squeeze more power out of the fuel.
Modern engine control units back off the performance if they detect signs of premature ignition, but older cars could get engine damage from too low octane fuel.
When you compress a gas vapor, it causes heat. When the piston in an ICE compresses the fuel vapor in the cylinder too much and the temperature gets high enough, the fuel will burn of its own volition (how a diesel works). This happens as the fuel is being compressed on the upstroke (compression stroke) of the engine, so the fuel is combusting before it is supposed to which causes intense strain on all the components.
You can think of octane as an inhibitor. The higher the octane rating, the less prone the fuel is to combusting from pressure/temperature rise. Many expensive cars have high compression ratios and often require octane ratings above 90 to compensate for this. There are other ways to help avoid this, like adjusting the timing, but all of those also inhibit performance.
When petroleum is pushed through an engine, the petroleum is compressed into a tight chamber before being ignited – this is important for effective combustion, though the reason doesn’t matter here. The problem is that compressing petroleum into a small space causes it to heat up.
If the fuel gets too hot, it might spontaneously ignite and burn while still in the compression chamber. We don’t want that – not only will the fuel and energy be wasted if it ignites while still in the chamber, but it will likely damage the engine too (This is called ‘knocking’).
Higher octane fuels are fuels which have been formulated to withstand higher pressure before they self-ignite. Basically, you can compress higher octane fuels far more.
Most average modern cars are built with the idea that most people will use cheaper, lower octane fuels. Therefore, they design their compression chambers around the idea that lower-octane fuels will go through them. There’s no real reason for them to use highly compressed chambers, because they cost more money to produce and most average drivers won’t benefit from it.
Sports cars, however, use tighter compression chambers that compress fuel more than average. This means they can benefit from higher octane fuels that can benefit from those tighter compression chambers – they can create more power with less fuel that way.
You may be thinking there’s a missing puzzle piece here – surely that should mean that putting lower octane fuels into a sports car would cause knocking? Well, yes and no – most modern cars’ internal computers are smart enough to detect when knocking is about to happen, and adjust the engine to minimise the chance. This does cause a little lost performance, but it’s better than the performance lost from engine knocking. With high octane fuels, however, they don’t have to.
The ocatane number of petrol describes how resistant it is to autoignition, that is to say how much you can compress it before it gets so hot that it ignites itself (this is actually the method that Diesel engines just normally use for their engines).
The reason this is important is because higher compression makes engines more efficient (and therefore for a given size also more powerful), so you want to go as high as possible in the compression, but if you go to high your air fuel mix will just ignite by itself, before the spark plug can, and that can cause a backfire, which is when the fuek ignites before the zylinder passes the neutral point, meaning that zylinder is suddenly blasted in the opposite direction it’s supposed to be moving (hence back fire).
This is also why leaded fuel exists, because this drastically improves the octane number of petrol.
A higher octane rating prevents early ignition of fuel during compression, when fuel ignites prematurely it can cause what’s known as an engine knock which can wreck your engine. This isn’t much of a problem with regular car engines, but when an engine is very powerful it usually has a higher compression ratio and the additional power also makes it more likely that the engine will damage itself if it knocks.
The higher octane gas burns in a more controlled way, and actually produces less power than lower octane gas. But because it’s more controlled it becomes less likely to cause problems in these high power engines.
In simple terms, you should use the high octane fuel in these engines because they’re made to burn that fuel. And you should also avoid using it on cheaper, less powerful engines because there is no benefit in doing so. In fact doing so will cause the engine to produce less power.
The otto cycle engine has a compression step before ignition. The higher the compression rate is, the more performance you can take out from the engine assuming the same cylinder number, volume and everything else.
But there can be a problem, because the fuel can start burning (or detonating) without ignition, during compression. It means that the piston is still moving towards the inside of the cylinder (compressing), while the fuel is already trying to expand and forcing the piston backwards. However the piston has no chance to move backwards before it would finish the compression movement and reach the endpoint first. This is called knocking and extremely damaging to the engine.
To avoid that, you must use fuel that can tolerate the pressure, in other words it doesn’t start detonating in the compression phase. The normal 95 octane gas has a given compression tolerance so these engines can compress at around 12:1 rate or so. (In older times and even nowadays in some countries you would find 93 octane that would go with around 10:1 compression rate or so.)
If you want a stronger engine, you can either pack more cylinders in it, or make the cylinders bigger, or give them larger compression rate. Strong cars actually combine all of these. But if you compress more, you need larger octane number, such as 98 or even 100 (for a super sport car).
The engine compresses air fuel mixture in the pistons before it ignites it with the spark plug. The more the engine compress the air mixture the more efficient the detonation and the more powerful the engine for a given engine size. High performance engines therefore tend to compress the air mixture more then regular cars. However if you compress the mixture too much it will detonate by itself even before the spark can ignite it. This is called engine knocking and will cause the explosion to push the piston down the wrong direction. This both reduces performance and can damage the engine. Higher octane fuel have a harder time igniting so it will handle more compression before self-detonation. So it will not knock as easily as regular fuel. You can also put high octane fuel in a regular car but this have a normal compression ratio and does not suffer from the knocking so it will have minimal effect. In fact it can be harder for the spark plugs to ignite the high octane fuel and you may end up with reduced performance and it may damage your spark plugs and oxygen sensor over time.
High performance gasoline engines have higher compression ratios, which increases the risk of fuel autoignition and engine knocking (which, contrary to popular belief, are actually two different albeit related phenomena). To counter that, the fuel is refined to be more resistant to autoignition under compression. The measure of this resistance is called the Octane Number.
However, the fuel isn’t “better” in-and-of-itself. Putting 89 or 91 in a car that’s meant for 87 isn’t going to do anything other than burn a hole in your wallet. In the opposite direction, though; butting 87 in a car meant for 91 (or higher) will *absolutely* do damage to the engine over the long term. So basically always use the fuel rated for your engine.
There are a few situations where changing octane rating can be useful, though;
1) Older/beat up vehicles will sometimes have fueling problems that will increase knock. Thus, it used to be that once you got above 100-150k miles you might need to switch to the next highest ON of fuel if you didn’t want to actually fix the problem.
2) If you’re in a situation where you absolutely need to run the engine in conditions where it’s going to be prone to overheating, using a higher octane fuel will help mitigate knock issues.
3) If you slap an aftermarket turbo on your engine, you typically want to switch to the next highest ON rating of fuels available.
It’s necessary for engines with high compression ratios so the fuel doesn’t prematurely combust before the spark plug fires. You see this more often with these sport and luxury vehicles equipped with more powerful engines compared to typical vehicles.
Who knew luxury cars were just divas that needed fancy fuel to perform their best? 💁♂️💎
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