Hello everyone and welcome in this video
we're going to be talking about the difference between octane numbers and cetane numbers both of which are used as
rating scales for fuels so octane numbers are used as a rating scale for
gasoline fuels and cetane numbers are used as a rating scale for diesel fuels so first. Let's get into the basic definition
will go through octane
and then we'll talk about what cetane numbers are
so the octane number is the measurement of a fuels ability to resist knock so what that means is you know a
fuel with a high octane number can be compressed more without that fuel mixture igniting
so it's a measurement of how much can you compress that air fuel mixture
before it ignites the higher the number the more you can compress it the lower the number the more likely that fuel is to ignite
all on its own from compression now of course in gasoline engines. We want to use a spark plug to start ignition
we don't want to use compression to do so
unlike diesel engines and so a high octane rating is a good thing and so our rating scale goes from 0 to 100
and you can go over 100 there are fuels with octane ratings greater than 100
but generally we're talking about this range with heptane being zero and octane being 100 so both of these are hydrocarbons
heptane C7 H16, seven carbons, 16 hydrogens and octane C8 H18 and
so octane has the rating of 100 heptane has the rating of 0 that means heptane is very likely to ignite from compression
versus octane isn't very likely to ignite from compression, so why would you want that higher number?
Well that greater knock resistance of course knock is a bad thing you want to time it with the spark plug not through compression
but using a higher octane you can use higher compression ratios and higher compression ratios lead to more torque and greater efficiency
they also allow you to advance the timing so you can fire the spark plug sooner and get peak torque to occur when that piston
is closer to top dead center create more torque and have better efficiency and are also useful for forced induction
applications turbochargers and superchargers
when you have higher pressures within that cylinder
and you want to make sure you don't run into any knock issues, so those lower knock rating fuels
those lower octane
less resistance to knock, lower compression ratios used and you know not quite as much advanced timing used
and you'll see this more with naturally aspirated engines
because of you know you don't run into those higher pressures. You don't necessarily have to worry about running into knock
so what is a fuel for example that has a ninety as its octane number?
well what that tells you is it's kind of an equivalence of if it were made up of heptane and octane
so a ninety would be
equivalent to a fuel that was 90 percent octane and
ten percent heptane that doesn't mean that's what the fuel is made of but it would behave
like a fuel that that was that blend of octane and heptane
one of the things that always comes up whenever talking about octane ratings everyone in Europe says you know like the u.s.
fuel quality is terrible our octane ratings are super
low we use different scales and so I have a separate video
which goes all into that which I'll include a link to but generally speaking the u.s.
uses AKI versus Europe uses RON and RON tends to be a higher number
even if the fuels are the exact same
so I'll have a separate link which you can check out if you're curious about the different rating scales that different countries use.
Okay so now let's talk about the cetane number and the cetane number is a measure of a diesel fuels ignition delay
so what does that mean?
Well, it's a measurement of the time from when fuel is first
injected into the cylinder and from when that fuel first starts to combust so it's a measurement of time and
you're looking at so a high cetane number
means that fuel ignites very quickly after it's injected in a low cetane number means it takes a while for that fuel to start
igniting after it's injected into the cylinder and so your rating scale at once again
we're using two different hydrocarbons for a 0 and 100 rating scale so cetane just like octane is 100 cetane number
cetane is 100 C16 H34 and our 0 is
1-methylnaphthalene however, that's pronounced C11 H10 and actually they've since switched to isocetane which has the same chemical
kind of composition
C16 H34 as cetane has a cetane rating of 15 the reason this is used
rather than methylnaphthalene is that it's a bit cheaper easier to work with
and so these may look exactly the same C16 H34
but the structure of them is very different how the atoms are actually connected to one another is very different
which gives this the lower cetane number so these numbers take a long time to ignite,
you're going to have longer combustion these numbers don't take very long to ignite combustion is shorter and
so the advantage of having that faster combustion is that you can create more torque, and you can perform
you can have engines
performing at higher rpm because the combustion doesn't take as long you have less time for combustion to occur as you get into those higher
RPM and so it's important that that combustion speed that that ignition delay is very short
it's also ideal for timing when peak torque occurs so by having quicker a combustion
you know you can alter the timing of when you inject the fuel, but ultimately by having that shorter ignition delay
you can time that more perfectly with when you want that peak torque to occur within the cylinder so you have the maximum amount of work done
so it's ideal for timing because it burns very quickly it ignites very quickly and also you get less
hydrocarbon emissions there have been some studies that have shown that you can get a more complete burn with higher cetane numbers
but however because this burns faster it tends to burn a bit hotter and as a result
you tend to get more particulate matter emissions
i've seen some studies showing this
whereas the lower combustion temperatures that result with these lower cetane numbers can result in lower particulate matter emissions
but overall you're gonna have longer combustion and less torque produced
so you know the the good on both of them you want to be kind of on the higher end and
gasolines you tend to be 85 plus diesel numbers
they'll tend to be between about 40 and 60 for the cetane number, so hopefully this clears up the difference
what each of these two numbers mean when you're sitting at the pump and
trying to fill up your car of course
I would just recommend going with whatever the
manufacturer of your car recommends you use if you guys have any questions or comments, please feel free to leave those below, thanks for watching.