Detonation 101
Thread Starter
Registered User
Joined: Mar 2004
Posts: 14
From: Windsor, Ontario
Detonation 101
By: Dennis Adams
Detonation
Detonation or "knocking" arises from the production of an explosion wave in the combustion chamber. This is due to rapid ignition and combustion. A portion of the unburnt air/fuel mixture becomes compressed prematurely, which in turn rapidly increases in temperature until it ignites uncontrollably and almost instantaneously. The shock wave produced has a characteristic of a metallic sound and can vary in magnitude. It can range from a mild form, occurring only at relatively low speed and wide throttle positions (typically known as "pining") to such violent effects that the engine is no longer running in a controlled manner and power output dies.
Detailed view of detonation
Now that we know the definition of detonation we need to comprehend that if the temperature of an air/fuel mixture is raised to high, the mixture will eventually explode spontaneously. This is known as spontaneous ignition temperature. But, before this explosion there is an interval called ignition time-lag. If the piston approaching tdc of the combustion stroke takes longer than this period before the spark plug ignites there will be a premature explosion. The longer the time-lag the less chance of spontaneous there is of spontaneous ignition. If the air/fuel mixture is fired at the correct "timed" point and a good flame is achieved power loss through detonation is still possible. The burnt mixture behind the flame front will be at a high temperature and pressure, which will cause compression of the unburnt gas ahead of the spontaneous ignition point. The flame front must move right through the unburnt charge before the end of the ignition time-lag period if not this can lead to overheating and excessive mechanical stressing.
Fuel choice and detonation
With any fuel the brake thermal efficiency 1 will decrease if the compression ratio is increased. A properly chosen fuel can help diminish detonation. The advances in modern day fuels have extended their ignition time-lag periods and have decreased the spontaneous ignition temperatures, which allows for the use of a higher compression ratio (safely up to 11:1) to be used in the combustion process under complete control. There are also special fuels made especially for racing applications that allow compression ratios so high that the limit is actually dependent upon mechanical aspects of the cylinder head design.
Summary
The benefit of increasing compression ratio is to increase the torque output of an engine. The harm in raising the compression ratio is that cylinder pressures increases in a linear fashion as compression ratio is increased. An 8:1 compression ratio yields compression pressures of about 275 psi, where 15:1 yields compression pressure of about 575 psi. At the time of ignition the pressures will rise 3 or 4 times the compression pressure. Also, when an engine has been modified where even a slight increase in torque is achieved it is important that detonation is not allowed. The noise of detonation of a high-compression ratio engine using the correct grade of fuel is much harder to detect than that of an engine with a much lower compression ratio burning lower octane fuels. 1 Thermal Efficiency -Thermal efficiency is the measure of the efficiency and completeness of combustion of the fuel, or, more specifically, the ratio of the output or work done by the working substance in the cylinder in a given time to the input or heat energy of the fuel supplied during the same time.
Detonation
Detonation or "knocking" arises from the production of an explosion wave in the combustion chamber. This is due to rapid ignition and combustion. A portion of the unburnt air/fuel mixture becomes compressed prematurely, which in turn rapidly increases in temperature until it ignites uncontrollably and almost instantaneously. The shock wave produced has a characteristic of a metallic sound and can vary in magnitude. It can range from a mild form, occurring only at relatively low speed and wide throttle positions (typically known as "pining") to such violent effects that the engine is no longer running in a controlled manner and power output dies.
Detailed view of detonation
Now that we know the definition of detonation we need to comprehend that if the temperature of an air/fuel mixture is raised to high, the mixture will eventually explode spontaneously. This is known as spontaneous ignition temperature. But, before this explosion there is an interval called ignition time-lag. If the piston approaching tdc of the combustion stroke takes longer than this period before the spark plug ignites there will be a premature explosion. The longer the time-lag the less chance of spontaneous there is of spontaneous ignition. If the air/fuel mixture is fired at the correct "timed" point and a good flame is achieved power loss through detonation is still possible. The burnt mixture behind the flame front will be at a high temperature and pressure, which will cause compression of the unburnt gas ahead of the spontaneous ignition point. The flame front must move right through the unburnt charge before the end of the ignition time-lag period if not this can lead to overheating and excessive mechanical stressing.
Fuel choice and detonation
With any fuel the brake thermal efficiency 1 will decrease if the compression ratio is increased. A properly chosen fuel can help diminish detonation. The advances in modern day fuels have extended their ignition time-lag periods and have decreased the spontaneous ignition temperatures, which allows for the use of a higher compression ratio (safely up to 11:1) to be used in the combustion process under complete control. There are also special fuels made especially for racing applications that allow compression ratios so high that the limit is actually dependent upon mechanical aspects of the cylinder head design.
Summary
The benefit of increasing compression ratio is to increase the torque output of an engine. The harm in raising the compression ratio is that cylinder pressures increases in a linear fashion as compression ratio is increased. An 8:1 compression ratio yields compression pressures of about 275 psi, where 15:1 yields compression pressure of about 575 psi. At the time of ignition the pressures will rise 3 or 4 times the compression pressure. Also, when an engine has been modified where even a slight increase in torque is achieved it is important that detonation is not allowed. The noise of detonation of a high-compression ratio engine using the correct grade of fuel is much harder to detect than that of an engine with a much lower compression ratio burning lower octane fuels. 1 Thermal Efficiency -Thermal efficiency is the measure of the efficiency and completeness of combustion of the fuel, or, more specifically, the ratio of the output or work done by the working substance in the cylinder in a given time to the input or heat energy of the fuel supplied during the same time.
Contributing Member
Joined: Mar 2004
Posts: 573
From: Washington State
I have to add to this-call it simplify. Sharp corners and edges on valve reliefs, combustion chamber edges, and pistons will get hot and rob power. Most of the time your ECU will sense this and back off the timing/add more fuel. You can run more boost and a leaner mix and make more power without detonation by simply smoothing edges on everything. My last car was a Civic with a boosted D16. It detonated and blew the head gasket right away when I added the turbo kit. I removed the head and found that the shop that surfaced the head left the edges sharp, and they were inside the firing ring of the the head gasket. I used a machinist debur tool and shaved off the sharp edge using the gasket as a template to make sure I didn't take off too much. Carbon can also cause problems smooth the chamber and top of the piston while your in there. My valve reliefs were fine but others may have sharp edges.
Registered User
Joined: May 2004
Posts: 88
From: N.Y. just for a year then going back to FL
help me out a bit, does time-lag= retarding your timing of your ignition, causing your stroke to reach tdc and avoiding detonation
the higher the octane the less chance of detonation?
Damn i learned something new thanks alot even though i was alittle unclear about a few things i just want to make sure i didnt misinterprit <(however you spell it) anything
Man this is really execellent, Good job bro
the higher the octane the less chance of detonation?
Damn i learned something new thanks alot even though i was alittle unclear about a few things i just want to make sure i didnt misinterprit <(however you spell it) anything
Man this is really execellent, Good job bro
Registered User
Joined: May 2004
Posts: 142
From: STOCKTON, CA
Originally posted by BoogiepopJDM
the higher the octane the less chance of detonation?
the higher the octane the less chance of detonation?
SUMMARY
hi temp+low octane= good chance of detonation
hi temp+high octane=greatly reduced chance of detonation
OH YEAH!!...and running high boost along W/ high compression=detonation
Contributing Member
Joined: Mar 2004
Posts: 573
From: Washington State
A higher octane fuel is HARDER to burn, and will help resist pre-ignition. Basically part of the engine gets hot enough to ignite the fuel early instead of the spark plug. Get your beavis and butt head mentality on and try to light some octane booster on fire, you'll see what I mean. Timing can be backed off to compensate, but this is a "duct tape and bailing wire" way of fixing things. Add more fuel, smooth sharp edges, and some colder plugs can help.
Registered User
Joined: May 2004
Posts: 142
From: STOCKTON, CA
Originally posted by rkcarguy
Get your beavis and butt head mentality on and try to light some octane booster on fire, you'll see what I mean. Timing can be backed off to compensate, but this is a "duct tape and bailing wire" way of fixing things. Add more fuel, smooth sharp edges, and some colder plugs can help.
Get your beavis and butt head mentality on and try to light some octane booster on fire, you'll see what I mean. Timing can be backed off to compensate, but this is a "duct tape and bailing wire" way of fixing things. Add more fuel, smooth sharp edges, and some colder plugs can help.
Registered User
Joined: Aug 2004
Posts: 267
From: Orlando FL.
I was watching the History Channel and they explained the octane in gas.
Now I don't remeber the names exactly but a quick explanation would be something like this:
The gas is composed of two parts, the part that makes it explode and the rest that expands and burn along with aditives. (lets say octane). Octane doesn't explode, but burns. When you buy gas with more octane, you have more to burn ,but less explosive tendency. A higher octanage fuel has more of the "burnign part" and less of the "explosive" part. So higuer octane is harder to make ignite but it tend to be more powerful since once it does, there's more fuel to expand.
Now thats a very broad generalization but it should give you an idea of WHY it doesn't explode, and not just say "It won't iginite just b/c it has a higher number".
Now I don't remeber the names exactly but a quick explanation would be something like this:
The gas is composed of two parts, the part that makes it explode and the rest that expands and burn along with aditives. (lets say octane). Octane doesn't explode, but burns. When you buy gas with more octane, you have more to burn ,but less explosive tendency. A higher octanage fuel has more of the "burnign part" and less of the "explosive" part. So higuer octane is harder to make ignite but it tend to be more powerful since once it does, there's more fuel to expand.
Now thats a very broad generalization but it should give you an idea of WHY it doesn't explode, and not just say "It won't iginite just b/c it has a higher number".
Registered User
Joined: Aug 2004
Posts: 267
From: Orlando FL.
Now I actually did my homework and found out this
From How Stuff Works:
thast the "explosive and burn part" thingy I was trying to explain earlier. That should give you a mental idea. So the botom line is, lower octane will explode more, thus giving more HP to a stock car?
From How Stuff Works:
The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.
It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.
It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.
Registered User
Joined: May 2004
Posts: 11
From: Melbourne
No, lower octane is more unstable under compression. You want a controlled explosion under compression (explosion at maximum compression which is started by the spark plug firing)
Higher octane allows you to compress an air fuel mixture more before it becomes unstable. This is why you need to use high octane fuel in cars with high cylinder compression ratios and cars that use forced induction.
The actual strength of the explosion is related to the amount of compression and some other factors like composition of the fuel etc
Higher octane allows you to compress an air fuel mixture more before it becomes unstable. This is why you need to use high octane fuel in cars with high cylinder compression ratios and cars that use forced induction.
The actual strength of the explosion is related to the amount of compression and some other factors like composition of the fuel etc
Registered User
Joined: Aug 2004
Posts: 267
From: Orlando FL.
im 14 pwease dont wape me
My mother used to say:
"Create fame, then go to sleep"
Registered User
Joined: Mar 2003
Posts: 81
From: NEW YORK CITY QUEENS
Originally posted by Joel SX
Now I actually did my homework and found out this
From How Stuff Works:
thast the "explosive and burn part" thingy I was trying to explain earlier. That should give you a mental idea. So the botom line is, lower octane will explode more, thus giving more HP to a stock car?
Now I actually did my homework and found out this
From How Stuff Works:
thast the "explosive and burn part" thingy I was trying to explain earlier. That should give you a mental idea. So the botom line is, lower octane will explode more, thus giving more HP to a stock car?