Detonation: The Engine Killer
AcesHigh
03-23-2004, 12:42 PM
I noticed a few people asking about using higher octane fuels in their Preludes in the Power thread. So I guess I'll try to explain and clarify this misunderstood topic: Detonation. What exactly is it?
To get a good understanding of detonation, you have to first understand the rough mechanics of how an engine works. In short, the power needed to push the pistons comes from mixing the right amount of air with the right amount of fuel and then igniting the mixture at the right time. As this air/fuel charge is burning, the flame heats up the gaseous byproducts of the combustion and the unreactive gases within the cylinder. The pressure of the heated gases work to push the piston down and the kinetic energy produced by the explosion is converted into mechanical energy by the piston and crankshaft (It is a common fallacy that its the actual FLAME that pushes against the pistons: it's actually the heated inert gases that do this).
While this all seems very complicated, the fundamentals of the engine is actually a very simple thing to understand. Just when you thought you'd never have to hear this science crap again after Physics, eh. :wink:
Okay, onto the topic. For starters, the common term for detonation is engine knock. Detonation is described as any uncontrolled ignition in the cylinder. This can happen when the air/fuel charge is being compressed in the cylinder and ignites before the spark plugs deliver a spark to the mixture. When it happens before your spark plugs fire, it is called <preignition>.
Detonation can also occur after the spark plugs have fired. After the spark has ignited the air and fuel mixture in the cylinder, the pistons are moved downward by the kinetic energy released by the explosion. However, the air/fuel charge is not ignited completely and the previous ignition has reacted with the unburnt portion of the charge causing another uncontrolled explosion. Depending on how ununiformed the mixture is, this can create a chain of smaller explosions that causes a series of spikes in the combustion chamber.
As Prelude owners, we have to worry more about preignition because of the higher AFR in our engines. This mostly pertain to h23a and h22a owners (92-96 Prelude Si, VTEC and 97-01 Prelude Base and SH).
Detonation is not only important for daily driving, is it crucial for tuners. The hotter that you can get the gases in the chamber to be, the harder they will push against the piston and the more power that you can get from each explosion. Detonation is the ULTIMATE determinate to how much power you can get out of an engine. It is simply a matter of physics (Holy shit, Physics ISN'T completely useless?!?).
PV = nRT
P = Pressure
V = Volume
n = number of moles (of the gas)
R = Universal Gas Constant = 8.3145J/mol K
T = Temperature
Thats pressure times volume is equal to the amount of gas multiplied by the gas constant times temperature. So what does the Ideal Gas Law have anything to do with cars? Well, everything. Let's say you've got a mole of an ideal gas (just for simplicity's sake; a 2.2 liter cylinder works out out less than a mole). Anyhow, looking above at PV = nRT, you will see that Pressure X (A constant volume, which will be fixed so we'll just consider it a 1) = (number of mols, which we've also set at 1 mol) x (The universal constant that never change, to play with the variables we'll also set this at a 1) x Temperature. Whew. If we tear down that seemingly huge equation and simplify, we get something like P=T. What this tells you is, if you increase pressure, the temperature will rise; and vice versa.
That same PV = nRT again. If that first one confused you before, here's a logical one that's easier to follow. Replace the T, R, and n again with the constant, 1. You get PV=1, which if we move V to the other side becomes P=(1/V). This one tells you if you increase the P, pressure, the volume goes down. Simple? When you squeeze a balloon, it's got less volume? When your pistons push up on the air/fuel charge, it's got less volume, too?
Well, from these fundamentals we get some pretty useful stuff. For instance, we know that raising pressure will raise temperature. If we compress the air/fuel mixture, we will raise its temperature. Here's the kicker: **IF YOU RAISE THE TEMPERATURE, YOU MAY IGNITE THE AIR/FUEL CHARGE**. This is where detonation comes back to haunt you.
Detonation literally goes off like a bomb in your chamber. It will punch holes in your piston rings, destroy your ring lands, and tear your spark plugs apart, among other long term damage to your engine. It is a major engine killer that you should be careful of, especially when thinking about modifying your car!
So now that you're experts on what detonation is, what can you do to stop it? Just think about it, if you've read this far then you can probably figure out how to prevent it yourself.
Take a look at that annoying PV = nRT one more time. What factors can you manipulate?
> Pressure is one you can play with. High compression is the greatest gift and pain to our Preludes. You can reduce the chances of detonating if you lower your compression ratio. This, however, is not generally recommended as it makes for poor streetability. If it's your daily driver, skip this, but if you go on to the higher levels, this is an option to choose. If you are using some kind of forced induction device, a lower boost level also reduces the risk of detonation. One last measure you can use is to retard ignition timing. That is, make the spark plugs deliver the spark a little later down the stroke than it's supposed to. Remember that volume and pressure go hand and hand, and if your volume increases, the pressure on your AF mixture will be less.
> Temperature is another variable that you can play with. if you cool the gases, they won't be as liable to detonate on their own. A cold air intake will help you here. A ceramic coated header will help keep the heat in so that your air intake has cooler air to suck in from the engine. The air in your engine gets hot, and will only get hotter in the cylinder. If you're going FI, you've got several options before you (but these are even more important than in an NA motor). Because you *are* compressing the air, you'll get higher temperatures anyways. We've discussed this before. This is why turbocharged cars need intercoolers. An intercooler cools the air charge before it enters the turbo. Because there is less of a pressure drop and colder incoming air, it'll help out your turbo alot. Anything you can do to get cooler air to your manifold, NA or FI, will help lower chances of detonation. The side bonus is more horsepower for you! :biggrin:
> You can decrease the tendency that your air/fuel charge will ignite on its own. This is the question that got me started writing this entire article; "Should I use +92 octane fuels on my car?". If you've got a 92-96 Prelude Si or VTEC and 97-01 Prelude Base and SH, then you bet you should. It is entirely possible to use lower grade fuels on your car; you will still be able to drive it. But you'll probably experience engine knock if you push it hard or do it too often. So if you do experience engine knock, try out some of that premium fuel your local gas station has been advertising. Higher grade fuels work because they need a higher temperature to ignite. That means you can squeeze/raise the temperature of the A/F mixture more than you can with a lower grade fuel. Remember that more pressure makes more power, and as such, many high compression cars like ours must use higher octane fuels. If you raise compression any higher, or use really high boost, consider using some +100 octane race gas or your engine won't last you too long.
When shouldn't you use higher octane fuel? Well, there's never really a reason NOT to use it (unless you go out and buy obscenely high octane fuel), but there ARE times when its completely uneccessary. The older Preludes, I believe, run relatively normal air fuel ratios, so they don't need premium gas. High octane fuel burns effeciently and completely, so it may leave slightly less residue than lower grades, but its so slight that on a regular engine its not needed. Only use high octane fuel when its recommended, or when you start hitting high compression. Contrary to popular belief, 92 octane won't get you ANY more power than 87 octane. Even 100 octane race gas won't get you anything (except a fuel that's DIFFICULT FOR YOUR CAR TO IGNITE). High octane fuel is used to support more power, not create it.
> A polish of your cylinders can also help prevent detonation. If you have ragged or sharp edges on the side of your cylinder, from damage, residue, whatever... it can create a spark on its own indepedent of the spark plugs.
So, a synopsis of all this information?
*** I'll say this one last time. Detonation is a major engine killer that you need to keep in check at all times if you have modified your car in the higher levels. There are several things you can do about it. But don't worry about detonation on your stock car, unless you've got one that has high compression.
To get a good understanding of detonation, you have to first understand the rough mechanics of how an engine works. In short, the power needed to push the pistons comes from mixing the right amount of air with the right amount of fuel and then igniting the mixture at the right time. As this air/fuel charge is burning, the flame heats up the gaseous byproducts of the combustion and the unreactive gases within the cylinder. The pressure of the heated gases work to push the piston down and the kinetic energy produced by the explosion is converted into mechanical energy by the piston and crankshaft (It is a common fallacy that its the actual FLAME that pushes against the pistons: it's actually the heated inert gases that do this).
While this all seems very complicated, the fundamentals of the engine is actually a very simple thing to understand. Just when you thought you'd never have to hear this science crap again after Physics, eh. :wink:
Okay, onto the topic. For starters, the common term for detonation is engine knock. Detonation is described as any uncontrolled ignition in the cylinder. This can happen when the air/fuel charge is being compressed in the cylinder and ignites before the spark plugs deliver a spark to the mixture. When it happens before your spark plugs fire, it is called <preignition>.
Detonation can also occur after the spark plugs have fired. After the spark has ignited the air and fuel mixture in the cylinder, the pistons are moved downward by the kinetic energy released by the explosion. However, the air/fuel charge is not ignited completely and the previous ignition has reacted with the unburnt portion of the charge causing another uncontrolled explosion. Depending on how ununiformed the mixture is, this can create a chain of smaller explosions that causes a series of spikes in the combustion chamber.
As Prelude owners, we have to worry more about preignition because of the higher AFR in our engines. This mostly pertain to h23a and h22a owners (92-96 Prelude Si, VTEC and 97-01 Prelude Base and SH).
Detonation is not only important for daily driving, is it crucial for tuners. The hotter that you can get the gases in the chamber to be, the harder they will push against the piston and the more power that you can get from each explosion. Detonation is the ULTIMATE determinate to how much power you can get out of an engine. It is simply a matter of physics (Holy shit, Physics ISN'T completely useless?!?).
PV = nRT
P = Pressure
V = Volume
n = number of moles (of the gas)
R = Universal Gas Constant = 8.3145J/mol K
T = Temperature
Thats pressure times volume is equal to the amount of gas multiplied by the gas constant times temperature. So what does the Ideal Gas Law have anything to do with cars? Well, everything. Let's say you've got a mole of an ideal gas (just for simplicity's sake; a 2.2 liter cylinder works out out less than a mole). Anyhow, looking above at PV = nRT, you will see that Pressure X (A constant volume, which will be fixed so we'll just consider it a 1) = (number of mols, which we've also set at 1 mol) x (The universal constant that never change, to play with the variables we'll also set this at a 1) x Temperature. Whew. If we tear down that seemingly huge equation and simplify, we get something like P=T. What this tells you is, if you increase pressure, the temperature will rise; and vice versa.
That same PV = nRT again. If that first one confused you before, here's a logical one that's easier to follow. Replace the T, R, and n again with the constant, 1. You get PV=1, which if we move V to the other side becomes P=(1/V). This one tells you if you increase the P, pressure, the volume goes down. Simple? When you squeeze a balloon, it's got less volume? When your pistons push up on the air/fuel charge, it's got less volume, too?
Well, from these fundamentals we get some pretty useful stuff. For instance, we know that raising pressure will raise temperature. If we compress the air/fuel mixture, we will raise its temperature. Here's the kicker: **IF YOU RAISE THE TEMPERATURE, YOU MAY IGNITE THE AIR/FUEL CHARGE**. This is where detonation comes back to haunt you.
Detonation literally goes off like a bomb in your chamber. It will punch holes in your piston rings, destroy your ring lands, and tear your spark plugs apart, among other long term damage to your engine. It is a major engine killer that you should be careful of, especially when thinking about modifying your car!
So now that you're experts on what detonation is, what can you do to stop it? Just think about it, if you've read this far then you can probably figure out how to prevent it yourself.
Take a look at that annoying PV = nRT one more time. What factors can you manipulate?
> Pressure is one you can play with. High compression is the greatest gift and pain to our Preludes. You can reduce the chances of detonating if you lower your compression ratio. This, however, is not generally recommended as it makes for poor streetability. If it's your daily driver, skip this, but if you go on to the higher levels, this is an option to choose. If you are using some kind of forced induction device, a lower boost level also reduces the risk of detonation. One last measure you can use is to retard ignition timing. That is, make the spark plugs deliver the spark a little later down the stroke than it's supposed to. Remember that volume and pressure go hand and hand, and if your volume increases, the pressure on your AF mixture will be less.
> Temperature is another variable that you can play with. if you cool the gases, they won't be as liable to detonate on their own. A cold air intake will help you here. A ceramic coated header will help keep the heat in so that your air intake has cooler air to suck in from the engine. The air in your engine gets hot, and will only get hotter in the cylinder. If you're going FI, you've got several options before you (but these are even more important than in an NA motor). Because you *are* compressing the air, you'll get higher temperatures anyways. We've discussed this before. This is why turbocharged cars need intercoolers. An intercooler cools the air charge before it enters the turbo. Because there is less of a pressure drop and colder incoming air, it'll help out your turbo alot. Anything you can do to get cooler air to your manifold, NA or FI, will help lower chances of detonation. The side bonus is more horsepower for you! :biggrin:
> You can decrease the tendency that your air/fuel charge will ignite on its own. This is the question that got me started writing this entire article; "Should I use +92 octane fuels on my car?". If you've got a 92-96 Prelude Si or VTEC and 97-01 Prelude Base and SH, then you bet you should. It is entirely possible to use lower grade fuels on your car; you will still be able to drive it. But you'll probably experience engine knock if you push it hard or do it too often. So if you do experience engine knock, try out some of that premium fuel your local gas station has been advertising. Higher grade fuels work because they need a higher temperature to ignite. That means you can squeeze/raise the temperature of the A/F mixture more than you can with a lower grade fuel. Remember that more pressure makes more power, and as such, many high compression cars like ours must use higher octane fuels. If you raise compression any higher, or use really high boost, consider using some +100 octane race gas or your engine won't last you too long.
When shouldn't you use higher octane fuel? Well, there's never really a reason NOT to use it (unless you go out and buy obscenely high octane fuel), but there ARE times when its completely uneccessary. The older Preludes, I believe, run relatively normal air fuel ratios, so they don't need premium gas. High octane fuel burns effeciently and completely, so it may leave slightly less residue than lower grades, but its so slight that on a regular engine its not needed. Only use high octane fuel when its recommended, or when you start hitting high compression. Contrary to popular belief, 92 octane won't get you ANY more power than 87 octane. Even 100 octane race gas won't get you anything (except a fuel that's DIFFICULT FOR YOUR CAR TO IGNITE). High octane fuel is used to support more power, not create it.
> A polish of your cylinders can also help prevent detonation. If you have ragged or sharp edges on the side of your cylinder, from damage, residue, whatever... it can create a spark on its own indepedent of the spark plugs.
So, a synopsis of all this information?
*** I'll say this one last time. Detonation is a major engine killer that you need to keep in check at all times if you have modified your car in the higher levels. There are several things you can do about it. But don't worry about detonation on your stock car, unless you've got one that has high compression.
rubix777
03-23-2004, 09:43 PM
Contrary to popular belief, 92 octane won't get you ANY more power than 87 octane. Even 100 octane race gas won't get you anything (except a fuel that's DIFFICULT FOR YOUR CAR TO IGNITE). High octane fuel is used to support more power, not create it.
I have also heard the rumor that Honda engines were designed to run optimally with 87 octane fuel. Since octane just dilutes the fuel, to help prevent detonation(pre-ignition), a tank of 87 octane gas contains more fuel than a tank of 91 octane gas. So you'd think that more fuel means bigger explosion right?
I'm not saying that if you use 91 you won't see any improvement, I wouldn't be caught dead using anything lower than 91 myself, but the only reason I do is because it burns cleaner(less emissions than if you burned 87), and has more cleaners(Techron, cleantech,bleh) than 87.
From my experience, stay away from Costco and ARCO. It makes my engine sound weird.
I like 76, Mobil, and Chevron.
I have also heard the rumor that Honda engines were designed to run optimally with 87 octane fuel. Since octane just dilutes the fuel, to help prevent detonation(pre-ignition), a tank of 87 octane gas contains more fuel than a tank of 91 octane gas. So you'd think that more fuel means bigger explosion right?
I'm not saying that if you use 91 you won't see any improvement, I wouldn't be caught dead using anything lower than 91 myself, but the only reason I do is because it burns cleaner(less emissions than if you burned 87), and has more cleaners(Techron, cleantech,bleh) than 87.
From my experience, stay away from Costco and ARCO. It makes my engine sound weird.
I like 76, Mobil, and Chevron.
plum
03-23-2004, 10:21 PM
thanks a lot aces, appreciate the info =), cheers
del
03-23-2004, 11:09 PM
good stuff aces, i'm copying this into the AF technical forum.
SiGNAL748
03-23-2004, 11:45 PM
congratz on the new mod spot aces :smokin:
VietBoy
03-24-2004, 01:08 AM
your too cool in prelude auto forum man!!!
info is great keep up the good work
:D
info is great keep up the good work
:D
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