My first car was a 1966 MGB roadster. IIRC it had an 8.8:1 compression ratio. Much of the time it would run on regular gas; but in the hot summer months it would knock (or is it ‘ping’?) on accelleration unless I used mid-grade gas. The same thing happens to my 1994 Yamaha Seca II. (The '03 R-1 has a placard that says it requires 91 Octane minimum, so I haven’t tried regular in it.)
What’s going on with knocking? It’s also called ‘detonation’, right? It happens when the fuel/air mixture ignites before it’s supposed to. But hot air is less dense than cold air. Wouldn’t this result in a richer mixture that would make the fuel/air mixture less volatile? Or is a richer mixture more volatile than a leaner mixture? And what’s the difference between ‘knocking’ and ‘pinging’?
Finally: As I understand it the higher the octane, the more resistant the fuel is to detonation. It seems to me that high-octane fuel is more volatile than lower octane fuel. But if high-octane fuel is less prone to detonation, doesn’t this mean that it’s less volatile? There’s a piece missing in my understanding, and I know that Rick or Una or one of you other mechanically-minded Dopers can find that missing piece!
High-octane fuel is less volatile - it takes more heat and/or pressure to ignite than low-octane fuel. You pay more for it because reducing the volatility requires extra (expensive) steps in the refining process.
Regarding why you need more octane for the summer, I believe it has more to do with air temperature - mixing hotter air with the fuel will decrease the pressure required for it to ignite, resulting in knocking. That’s just a WAG, though.
Yeah, hot summer days means a hotter engine which means easier knocking in your engine.
DO NOT use regular in your Yamaha unless you’re stranded in the middle of no-where. My 11.5:1 ratio Matrix MUST HAVE 91 octane (minimal) or I lose ~30 HP and probably wreck my engine if I push it.
Octane is a measurement of knock (detonation) resistance. Knock is the self ignition of the cyclinder from anything other than the spark plug. Your MGB probably had carbon deposits causing hot spots on the cyclinder walls that detonated the air/gas mixture. Using better gas is a stop gag fix on that sort of problem.
Yeah, use whatever’s recommended by the manual. I had the dealer for my Toyota tell me to use regular even though the manual and fuel door clearly state to use premium only (minimal 91 octane). Dealers suck.
Okay, here’s where I’m confused. I read somewhere, once, that kerosene (and jet fuel) has an equivalent octane rating of about 50. But kerosene and Jet-A are less volatile than mogas or avgas, which have higher octane ratings.
Jet fuel (diesel, too) is rated in terms of cetane, not octane. They’re completely different - in fact, lower cetane decreases the possibility of knocking, and higher cetane increases it (the reverse of what octane does).
Could that be what you heard? I believe 50 is a common cetane rating for jet fuel.
On the subject of octane, if you are at your local Barnes & Noble some time, check out the latest issue of Invention & Technology
They have a quite enjoyable feature article on this very subject in this quarter’s issue:
I’m here Johnny contrary to what Absolutesaid there are two types of “knock” pre-ignition and post ignition. Pre ignition knock is what used to happen to your MGB on regular on a warm day. When the air in the cylinder is compressed by the piston, the temp of the air goes up. On a cool day the increase is almost to the auto ignition temp of the fuel in the cylinder. On a warm day, with low octane fuel, the temp can go so high that the fuel self ignites. This is pre-ignition knock. Ever have a car that dieseled when shut off? That was pre-ignition knock.
Post ignition knock is when the fuel rather than burns detonates, and ignites uncontrollably.
I have been told that the marbles in a coffee can noise that “knock” makes is from multiple flame fronts colliding inside of the cylinder.
I’m at home, and don’t have any of my reference books here, so if Una stops by she no doubt will have a better explanation than I. Hell let me restate that, even with my books Una will be better able to explain this.
As far as what I’ve written about what knock is, I recommend my Staff Report and the side-discussions on it. Not that what I have to say is definitive or stuff, but it’s what I’ve said in the past and I have some cites, FWIW. Knock isn’t as simple as we’ve discussed here on the SDMB, however - my textbooks devote hundreds of pages collectively to its study and trying to determine what exactly is happening at the molecular level.
There’s two parts to it according to my refs, which is the complexity and geometry of the hydrocarbon chains, and the volatility as measured by the boiling point. I mean, it’s fairly well accepted that longer and more complicated hyrdrocarbons (longer chains, more complicated chains, and more complicated cyclic structures) do have a greater tendency to knock. However, if the hydrocarbon chains are less “dense” along the main trunk (have more branching to them…I see the gasoline FAQ backs me up on that, which is good since all my other refs are hardcopy and I can’t link to them…) then the tendency to knock decreases.
And fuels which boil at lower temperatures tend to have less tendency for knock, so there’s definitely a connection there as well. However, I have forgotten enough chemistry to state definitively whether a hydrocarbon chain with more branching has a lower boiling point than the same compound (same number of C and H and other molecules) with less branching.
Going back to your original question - knock tendency is influenced by the inlet air temperature, which directly influences the charge temperature entering the engine. In addition, your engine could be operating at a higher temperature than normal during the Summer, and this will also impact the tendency to knock. The gasoline FAQ has a quick note that says “Increasing the water jacket temperature from 71C to 82C, increases the (average octane number requirement) by two.” I wish I could write more but I think we’ve covered this exact question in the past here, so maybe there’s more info if you search…it’s midnight now here…
Seems like ping from hot air is more an issue of state of tune and/or condition of heads/valves and pistons. Sometimes higher octane does not help it there is a chunk of carbon glowing in there.
With a blown engine (supercharger/turbocharger) after spark knock or detonation gets dangerous in the cold air - the denser air charge seems to lean things out a bit and increase pressure - the KR (Knock Retard) on the computer is watched more carefully by me in the winter time - got to keep them head gaskets in place!
forgot this link about gas, don’t think I’ve seen this one posted yet:
Been a long time since I read that Sunoco web page but I think they have a good FAQ section that deals with some myths about energy potential and it burns slower I tells ya
I am no chemist or petroleum engineer, but I own a CNG-burning Honda and what information I’ve been able to glean bears this out. Natural gas is mostly methane with a smattering of heavier fractions and has an octane rating of 130. Can’t get much simpler then CH[sub]4[/sub] and its boiling point is really low. And, as has been pointed out by others, the octane rating has nothing to do with how much energy is obtained when the fuel is burned. NG gives less energy per unit than gasoline does. Bi-fuel vehicles’ performance suffers when using NG as a consequence of this. My Honda is ‘straight’ though (burns nothing but NG) and Honda took advantage of NG’s high octane rating and put a 16:1 ratio engine in it, as well as having no-compromise software for timing and the like. Performance is quite good.
Your engine shouldn’t wreck itself by using lower grade gas. The engine managment system in a car with fuel injection will have a knock detector which will adjust timing and fuel/air ratio. Plus a O2 detector and air mass meter to help compute the best air/fuel ratio.
The older cars knocked becasue they typically had fixed timing. Then came vacuum advance in conjunction with some spinning weights in the distributor. Amazingly complex if you were to read into it. Plus, the carb’s mixed fuel via vacuum pressure developed via the restrictor plate/intake manifold. The methods to adjust timing helped prevent knock but it still exists in the older engines under certain conditions. So, with my dad’s 1966 Buick Riviera GS (after lengthy arguements and a lot of proof --what an ass he can be-- ) he now runs 91 or higher and uses stabilizers in the summer. But, the older leaded gas contained a a glycerin compund with lead in it (I forgot the actual name) which was a stabilizer and internal component lubricant.
Any ways, if you really want to, you can run on any octane rating that you want to. When a manufacturer reccomends a certain ictane that might be for stated performance or effeciency. If you use lower octane rating and your engine actually pings the engine managment system will detect this and adjust accordingly. Of course, there will be a decrease in performance. A 30HP drop followed by a wrecked engine is doubtful.
