High Compression used with boosted applications - a clearer understanding
01-04-2003, 12:15 AM
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High Compression used with boosted applications - a clearer understanding
OK for the people who know, the people who think they know and everyone else who wants to know -- here is a slightly technical outline of exactly what happens (with pics) inside the internal combustion engine when a spark plug fires, this will also outline pre-ignition, engine knock, and the damages caused by either of these two hassards!!
I am writing this paragraph with reliable referrences listed below -- this is specially aimed at those who believe the nonsence about running stock compression ratios on a boosted BETA 2 Engine with normal fuel!
Have you ever wondered what causes an engine to knock? Or what spark advance is for? Or have you ever listened to someone discuss these things at great length, yet have no clue what they were talking about? I have been in both situations, as well as the situation of the person discussing it without the convenience of having any knowledge about it.
The "Normal" Spark-Ignition Cycle
You have to know how things are supposed to work before you can understand why they aren't working properly. The basic "ideal" spark-ignition cycle for one cylinder consists of four parts(four stroke engine). First, the Intake stroke: the fuel and air mixture is drawn into the cylinder through the intake valve as the piston goes down the cylinder. The pressure and temperature in the cylinder remain near outside conditions(IMPORTANT!). Second, the Compression stroke: the fuel and air mixture is compressed as the piston goes upwards towards the valves. The pressure in the cylinder goes up, and so does the temperature(the more compression the more temperature - making the fuel VOLATILE and prone to pre-ignition and detonation/knock). At the end of the Compression stroke, the spark plug ignites the fuel mixture and it (in the "ideal" version) combusts instantly. Third, the Power stroke: the cylinder is now full of hot gasses that are at much higher pressure and temperature than the unburned compressed mixture was before the spark plug fired. This hot burned gas mixture pushes the piston down in the cylinder, which rotates the crankshaft around, which inturn rotates the drive train and you move forward. Fourth, the Exhaust stroke: these burned gasses are forced out the exhaust valve into your exhaust manifold as the piston moves to the top of the cylinder again. The cycle then repeats - ALL 4 stroke engines work in this way.
As I said, this is a very simplified version of what happens in a real engine. The only difference that is really important as far as Detonation/knock is concerned is that the fuel does not burn anywhere near instantaneously at TDC (top dead center). It takes about 0.5 ms (around 7.5 degrees of crank angle at 2500 RPM) after the spark occurs for the fuel to start burning beyond a small ball around the spark plug. This region of burning then spreads throughout the cylinder, and the burning is mostly complete somewhere around 30 to 50 degrees of crank angle after the spark. Now, clearly, if we ignite the fuel mixture directly at top dead center, the piston will have travelled down quite a distance before a considerable part of the fuel burns. This fuel, then, is unable to help push the piston down as much, and thus the engine is less powerful. Because of this, the spark is usually set to occur before the piston hits TDC, so more fuel will be burned when the piston begins moving downward - you want as much downward force to push the piston downards as possible. If the spark is advanced too far, in other words the ignition occurs EARLIER, the burning fuel starts pushing against the piston during the compression stroke and hindering things instead of helping. Try pushing a rising pisto downwards and see what happens (bent rods and smashed pistons will result!) The optimum spark timing is obviously a TRADE-OFF between these two effects. Small changes don't make a particularly big difference in power, so usually the minimum advance needed to get the maximum torque is used depending on the type of induction.(this is denoted MBT).
Below is a picture of normal combustion in an L-head style engine (like the old Ford flatheads). This engine has the advantage of having the valves on the same side of the combustion chamber as the piston, so you can simply cap the whole thing with a piece of quartz and see what is happening inside. In these pictures, the valves are at the left and the piston is at the right. It appears to me from comparison to other pictures that the pictures have an interval of about 2.4 degrees of crank angle, and that ignition is occurring about 17 degrees before TDC. If this is true, the combustion ends about 30 degrees after TDC.
Detonation/Knock in Spark-Ignition Cycles:-
As you recall, the fuel-air mixture gets to a fairly high pressure during the Compression stroke. This, along with the heat of the engine, causes the mixture to become very hot. In a diesel engine, this temperature is high enough to cause the fuel to spontaneously ignite. In a spark-ignition engine, however, the mixture is kept somewhat below the ignition threshold - CONTROLLING the ignition of the mixture.
Now, consider what happens when the fuel starts to burn. You can see from the above picture that there is a fairly large amount of mixture that does not normally begin burning immediately. As the mixture to the left burns, it compresses and heats up the mixture to the right even more than it is already heated and compressed from the engine heat and compression. If the pressure and temperature of this unburned gas gets high enough, it can spontaneously ignite without waiting for the flame to spread to it and ignite it in a controlled fashion. This process, known as "Detonation or knock" or specifically "combustion knock" is shown in the picture below:
NOTE:- that in this case, a considerable amount of the mixture has spontaneously ignited in only about 2.4 degrees of crank angle. This same fuel normally takes almost 30 degrees of crank angle to burn. The result of this "almost instantaneous" burning is that the pressure in the right side of the cylinder is raised dramatically and suddenly. This causes a pressure wave which reverberates throughout the cylinder, causing adverse effects to the combustion cycle, high stresses on the engine, and annoying pinging noises --- when you hear this expect to BLOW up the engine! If it happens before the piston reaches TDC, it also pushes down hard against the piston on its compression stroke and considerably less on the power stroke, producing MUCH less engine output power and bucking/hesitating of the vehicle, not to mention very high stresses on the engine internals!
I think you all know that detonation/kocking is bad for ou engine --- if you want more prrof that it is VERY BAD for your engine - look here:-
Pre-Ignition in Spark-Ignition Cycles:-
Pre-ignition is term used for ignition that occurs before the spark plug is fired. I mention it here because it can lead to detonation/knock - this is primarily caused by excess cylinder compression which leads to the dramatic increase of air fuel mixtures which then combust spontaneusly without the need for the sprak plug to fire. Usually preignition also occurs because of a hot spot in the engine that causes the fuel-air mixture to be heated past its autoignition point. There are many possible hot spots in the engine. If deposits are built up on the spark plug, they will be heated to a very high temperature when the spark fires. However, they do not lose their heat quickly, and can remain hot enough to cause pre-ignition in the next compression stroke. Exhaust valves are generally quite hot under normal operation, and if something happens to over heat them or keep them from being properly cooled, they can cause pre-ignition. Finally, carbon deposits on the cylinder walls can stay hot like burning coals and cause pre-ignition. Even if pre-ignition does not cause knocking, it causes higher temperatures and pressures in the cylinder that can cause rings to fail and even cause aluminum-alloy pistons to melt. Not pretty.
Now running stock compression ratio on the BETA 2 engine WITH a large Turbocharger like the CNK kit - is not wise because ignition will need to be retarded CONSIDERABLY to prevent detonation/knock BUT in reverse what you are creating is a situation where the mixture will ignite LATER because of the retarded ignition (when the exhaust valve opens) and this heats up the exhaust valve's edges dramatically causing it to go RED HOT and pre-ignition results --- Pre-ignition is a non-audible process so don't expect to hear if anything is wrong with the engine.
Next thing you notice is smoke everywhere, especially out your tappet cover breather and exhaust pipe -- not nice I can promise you that.
What to do About detonation/knock:-
to re-cap ---- detonation/knock occurs because the pressure and temperature in the cylinder are high enough when combustion is occurring that the unburned fuel/air mixture spontaneously ignites. Thus, these temperatures and pressures must be kept below a certain level to prevent knock from occurring. I will discuss several factors that affect this temperature and pressure, and thus affect knock.
ENGINE COMPRESSION RATIO:- If the mixture is compressed more, then the pressure gets higher, and knock is more likely to occur. Thus, a higher engine compression will cause knock to be more likely. OK you have the BETA 2 Engine with stock compression, running a certain cylinder pressure under normally aspirated conditions with adequate and safe ignition timing to keep pressures and temperatures down. Now you boost that same engine with the same engine compression ratio to more than double the atmospheric pressure and you expect to have something relaible?? Not at all -- you decide to retard ignition timing by lets say 1.5 degrees per PSI - it has been said that 15PSI is doable on the BETA 2 engine with stock compression. So you need to retard the ignition by 22.5 degrees ---- I'm sure you can imagine the rest of this picture.
Spark Advance. The maximum pressure and temperature in the cylinder occur some time after the spark occurs. If the spark is delayed so that this maximum pressure occurs after TDC, the downward motion of the piston will somewhat counteract the pressure rise from combustion, causing the maximum pressure to be lower. Thus, retarding the spark causes knock to be less likely --- to an extent where the combustion is still occuring when the exhaust valve is opening - heating it tremendously causing a hot spot and resulting in pre-ignition. Not only will your engine pre-ignite, but you will loose considerable power aswell since you have much less downforce as the mixture is still burning while the piston is rising in the cylinder again.
Pre-Ignition. As mentioed Pre-ignition has the same effect as increasing the spark advance. It causes detonation/knock to be more likely.
Combustion Chamber Design. This has a fairly complicated relationship with detonation/knock. First, if the distance from the spark plug to the far corners of the combustion chamber is reduced, combustion occurs more quickly(nice!) This increases the power, and also reduces the amount of time that detonation/knock has to occur. (Minimizing this distance led to the design of near-hemispherical heads in the Ford 429 and Chrysler Hemi, among others a while ago) Second, if the exhaust valve is placed close to the spark plug it will not heat up the later unburned gas as much, and thus will reduce the likelihood of detonation/knock provided you have adequate ignition advance. Third, cooler areas such as the intake valve and areas close to both the piston and cylinder head can be placed in the parts of the cylinder that burn last, thus cooling this unburned gas and reducing detonation/knock. Finally, increasing the turbulence in the engine (up to a point) increases the burn rate and thus reduces detonation/knock (swirl polish valves, flowed cylinder heads with swirl finish) This will allow higher boost ability with less likelihood to detonate/knock. Turbulence can also be increased by valve design or by creating areas of mixture that are squeezed between the piston and head, and are forced at high velocity into the middle of the combustion chamber (as emphasized in the Chrysler Wedge engines) --- this is where the CNK pistons design was born!! The design of the dish forces the mixture into the middle of the combustion chamber.
There is once significant factor other than the cylinder pressure and temperature that affects detonation/knock. That is the temperature at which the fuel/air mixture will spontaneously ignite. Since the ratios of fuel and air are fairly well determined by other considerations, the main variable here is the fuel. If the fuel is able to withstand higher temperatures before self-igniting (higher octane thus less volatile), it will clearly be less likely to detonate/knock. The resistance of fuel to detonation/knocking is quantified as the Octane rating. The Octane rating is defined by the percentage of iso-octane in an iso-octane/n-heptane mixture that is required to match the detonation/knock behavior of the given fuel in two different tests. Both of these tests are done on a standardized variable-compression single-cylinder engine, at two different speeds and intake temperatures, and the results are averaged. Octane ratings above 100 are defined by extrapolation from the iso-octane/n-heptane mixture.
Why use High-Octane Fuel:-
A direct quote from Stone (p. 80): "The attraction of high octane fuels is that they enable high compression ratios to be used. Higher compression ratios give increased power output and improved economy [assuming the same power of engine]... The octane number requirements for a given compression ratio vary widely, but typically a compression ratio of 7.5 requires 85 octane fuel, while a compression ratio of 10.0 requires 100 octane fuel. There are even wide variations in octane number requirements between supposedly identical engines." --- makes you think doesn't it??
Raising the octane level of your fuel will primarily help prevent detonation/knock. If your engine is not currently detonating/knocking, you won't see any significant advantages of using higher octane fuel. On the other hand, if the engine is designed from the beginning for higher octane fuel, it will have considerable advantages in power and economy.
The CNK Turbo kit IS NOT DESIGNED FOR HIGH OCTANE FUEL (although it is fully compatible with it) --- Instead it is designed for EVERY DAY DRIVING ON PUMP FUEL ---- RELIABLY!!!
With high performance comes limits --- with CNK comes RELIABILITY!
--------------------------------------------------
References
Lichty, L. C., Internal-Combustion Engines, Sixth Edition, McGraw-Hill, 1951. (all pictures are from this book)
Stone, R., Introduction to Internal Combustion Engines, Second Edition, Society of Automotive Engineers, 1993.
I will now lock this thread as being for informational purposes only.
[ January 04, 2003, 01:09 PM: Message edited by: CNK Performance ]
I am writing this paragraph with reliable referrences listed below -- this is specially aimed at those who believe the nonsence about running stock compression ratios on a boosted BETA 2 Engine with normal fuel!
Have you ever wondered what causes an engine to knock? Or what spark advance is for? Or have you ever listened to someone discuss these things at great length, yet have no clue what they were talking about? I have been in both situations, as well as the situation of the person discussing it without the convenience of having any knowledge about it.
The "Normal" Spark-Ignition Cycle
You have to know how things are supposed to work before you can understand why they aren't working properly. The basic "ideal" spark-ignition cycle for one cylinder consists of four parts(four stroke engine). First, the Intake stroke: the fuel and air mixture is drawn into the cylinder through the intake valve as the piston goes down the cylinder. The pressure and temperature in the cylinder remain near outside conditions(IMPORTANT!). Second, the Compression stroke: the fuel and air mixture is compressed as the piston goes upwards towards the valves. The pressure in the cylinder goes up, and so does the temperature(the more compression the more temperature - making the fuel VOLATILE and prone to pre-ignition and detonation/knock). At the end of the Compression stroke, the spark plug ignites the fuel mixture and it (in the "ideal" version) combusts instantly. Third, the Power stroke: the cylinder is now full of hot gasses that are at much higher pressure and temperature than the unburned compressed mixture was before the spark plug fired. This hot burned gas mixture pushes the piston down in the cylinder, which rotates the crankshaft around, which inturn rotates the drive train and you move forward. Fourth, the Exhaust stroke: these burned gasses are forced out the exhaust valve into your exhaust manifold as the piston moves to the top of the cylinder again. The cycle then repeats - ALL 4 stroke engines work in this way.
As I said, this is a very simplified version of what happens in a real engine. The only difference that is really important as far as Detonation/knock is concerned is that the fuel does not burn anywhere near instantaneously at TDC (top dead center). It takes about 0.5 ms (around 7.5 degrees of crank angle at 2500 RPM) after the spark occurs for the fuel to start burning beyond a small ball around the spark plug. This region of burning then spreads throughout the cylinder, and the burning is mostly complete somewhere around 30 to 50 degrees of crank angle after the spark. Now, clearly, if we ignite the fuel mixture directly at top dead center, the piston will have travelled down quite a distance before a considerable part of the fuel burns. This fuel, then, is unable to help push the piston down as much, and thus the engine is less powerful. Because of this, the spark is usually set to occur before the piston hits TDC, so more fuel will be burned when the piston begins moving downward - you want as much downward force to push the piston downards as possible. If the spark is advanced too far, in other words the ignition occurs EARLIER, the burning fuel starts pushing against the piston during the compression stroke and hindering things instead of helping. Try pushing a rising pisto downwards and see what happens (bent rods and smashed pistons will result!) The optimum spark timing is obviously a TRADE-OFF between these two effects. Small changes don't make a particularly big difference in power, so usually the minimum advance needed to get the maximum torque is used depending on the type of induction.(this is denoted MBT).
Below is a picture of normal combustion in an L-head style engine (like the old Ford flatheads). This engine has the advantage of having the valves on the same side of the combustion chamber as the piston, so you can simply cap the whole thing with a piece of quartz and see what is happening inside. In these pictures, the valves are at the left and the piston is at the right. It appears to me from comparison to other pictures that the pictures have an interval of about 2.4 degrees of crank angle, and that ignition is occurring about 17 degrees before TDC. If this is true, the combustion ends about 30 degrees after TDC.
Detonation/Knock in Spark-Ignition Cycles:-
As you recall, the fuel-air mixture gets to a fairly high pressure during the Compression stroke. This, along with the heat of the engine, causes the mixture to become very hot. In a diesel engine, this temperature is high enough to cause the fuel to spontaneously ignite. In a spark-ignition engine, however, the mixture is kept somewhat below the ignition threshold - CONTROLLING the ignition of the mixture.
Now, consider what happens when the fuel starts to burn. You can see from the above picture that there is a fairly large amount of mixture that does not normally begin burning immediately. As the mixture to the left burns, it compresses and heats up the mixture to the right even more than it is already heated and compressed from the engine heat and compression. If the pressure and temperature of this unburned gas gets high enough, it can spontaneously ignite without waiting for the flame to spread to it and ignite it in a controlled fashion. This process, known as "Detonation or knock" or specifically "combustion knock" is shown in the picture below:
NOTE:- that in this case, a considerable amount of the mixture has spontaneously ignited in only about 2.4 degrees of crank angle. This same fuel normally takes almost 30 degrees of crank angle to burn. The result of this "almost instantaneous" burning is that the pressure in the right side of the cylinder is raised dramatically and suddenly. This causes a pressure wave which reverberates throughout the cylinder, causing adverse effects to the combustion cycle, high stresses on the engine, and annoying pinging noises --- when you hear this expect to BLOW up the engine! If it happens before the piston reaches TDC, it also pushes down hard against the piston on its compression stroke and considerably less on the power stroke, producing MUCH less engine output power and bucking/hesitating of the vehicle, not to mention very high stresses on the engine internals!
I think you all know that detonation/kocking is bad for ou engine --- if you want more prrof that it is VERY BAD for your engine - look here:-
Pre-Ignition in Spark-Ignition Cycles:-
Pre-ignition is term used for ignition that occurs before the spark plug is fired. I mention it here because it can lead to detonation/knock - this is primarily caused by excess cylinder compression which leads to the dramatic increase of air fuel mixtures which then combust spontaneusly without the need for the sprak plug to fire. Usually preignition also occurs because of a hot spot in the engine that causes the fuel-air mixture to be heated past its autoignition point. There are many possible hot spots in the engine. If deposits are built up on the spark plug, they will be heated to a very high temperature when the spark fires. However, they do not lose their heat quickly, and can remain hot enough to cause pre-ignition in the next compression stroke. Exhaust valves are generally quite hot under normal operation, and if something happens to over heat them or keep them from being properly cooled, they can cause pre-ignition. Finally, carbon deposits on the cylinder walls can stay hot like burning coals and cause pre-ignition. Even if pre-ignition does not cause knocking, it causes higher temperatures and pressures in the cylinder that can cause rings to fail and even cause aluminum-alloy pistons to melt. Not pretty.
Now running stock compression ratio on the BETA 2 engine WITH a large Turbocharger like the CNK kit - is not wise because ignition will need to be retarded CONSIDERABLY to prevent detonation/knock BUT in reverse what you are creating is a situation where the mixture will ignite LATER because of the retarded ignition (when the exhaust valve opens) and this heats up the exhaust valve's edges dramatically causing it to go RED HOT and pre-ignition results --- Pre-ignition is a non-audible process so don't expect to hear if anything is wrong with the engine.
Next thing you notice is smoke everywhere, especially out your tappet cover breather and exhaust pipe -- not nice I can promise you that.
What to do About detonation/knock:-
to re-cap ---- detonation/knock occurs because the pressure and temperature in the cylinder are high enough when combustion is occurring that the unburned fuel/air mixture spontaneously ignites. Thus, these temperatures and pressures must be kept below a certain level to prevent knock from occurring. I will discuss several factors that affect this temperature and pressure, and thus affect knock.
ENGINE COMPRESSION RATIO:- If the mixture is compressed more, then the pressure gets higher, and knock is more likely to occur. Thus, a higher engine compression will cause knock to be more likely. OK you have the BETA 2 Engine with stock compression, running a certain cylinder pressure under normally aspirated conditions with adequate and safe ignition timing to keep pressures and temperatures down. Now you boost that same engine with the same engine compression ratio to more than double the atmospheric pressure and you expect to have something relaible?? Not at all -- you decide to retard ignition timing by lets say 1.5 degrees per PSI - it has been said that 15PSI is doable on the BETA 2 engine with stock compression. So you need to retard the ignition by 22.5 degrees ---- I'm sure you can imagine the rest of this picture.
Spark Advance. The maximum pressure and temperature in the cylinder occur some time after the spark occurs. If the spark is delayed so that this maximum pressure occurs after TDC, the downward motion of the piston will somewhat counteract the pressure rise from combustion, causing the maximum pressure to be lower. Thus, retarding the spark causes knock to be less likely --- to an extent where the combustion is still occuring when the exhaust valve is opening - heating it tremendously causing a hot spot and resulting in pre-ignition. Not only will your engine pre-ignite, but you will loose considerable power aswell since you have much less downforce as the mixture is still burning while the piston is rising in the cylinder again.
Pre-Ignition. As mentioed Pre-ignition has the same effect as increasing the spark advance. It causes detonation/knock to be more likely.
Combustion Chamber Design. This has a fairly complicated relationship with detonation/knock. First, if the distance from the spark plug to the far corners of the combustion chamber is reduced, combustion occurs more quickly(nice!) This increases the power, and also reduces the amount of time that detonation/knock has to occur. (Minimizing this distance led to the design of near-hemispherical heads in the Ford 429 and Chrysler Hemi, among others a while ago) Second, if the exhaust valve is placed close to the spark plug it will not heat up the later unburned gas as much, and thus will reduce the likelihood of detonation/knock provided you have adequate ignition advance. Third, cooler areas such as the intake valve and areas close to both the piston and cylinder head can be placed in the parts of the cylinder that burn last, thus cooling this unburned gas and reducing detonation/knock. Finally, increasing the turbulence in the engine (up to a point) increases the burn rate and thus reduces detonation/knock (swirl polish valves, flowed cylinder heads with swirl finish) This will allow higher boost ability with less likelihood to detonate/knock. Turbulence can also be increased by valve design or by creating areas of mixture that are squeezed between the piston and head, and are forced at high velocity into the middle of the combustion chamber (as emphasized in the Chrysler Wedge engines) --- this is where the CNK pistons design was born!! The design of the dish forces the mixture into the middle of the combustion chamber.
There is once significant factor other than the cylinder pressure and temperature that affects detonation/knock. That is the temperature at which the fuel/air mixture will spontaneously ignite. Since the ratios of fuel and air are fairly well determined by other considerations, the main variable here is the fuel. If the fuel is able to withstand higher temperatures before self-igniting (higher octane thus less volatile), it will clearly be less likely to detonate/knock. The resistance of fuel to detonation/knocking is quantified as the Octane rating. The Octane rating is defined by the percentage of iso-octane in an iso-octane/n-heptane mixture that is required to match the detonation/knock behavior of the given fuel in two different tests. Both of these tests are done on a standardized variable-compression single-cylinder engine, at two different speeds and intake temperatures, and the results are averaged. Octane ratings above 100 are defined by extrapolation from the iso-octane/n-heptane mixture.
Why use High-Octane Fuel:-
A direct quote from Stone (p. 80): "The attraction of high octane fuels is that they enable high compression ratios to be used. Higher compression ratios give increased power output and improved economy [assuming the same power of engine]... The octane number requirements for a given compression ratio vary widely, but typically a compression ratio of 7.5 requires 85 octane fuel, while a compression ratio of 10.0 requires 100 octane fuel. There are even wide variations in octane number requirements between supposedly identical engines." --- makes you think doesn't it??
Raising the octane level of your fuel will primarily help prevent detonation/knock. If your engine is not currently detonating/knocking, you won't see any significant advantages of using higher octane fuel. On the other hand, if the engine is designed from the beginning for higher octane fuel, it will have considerable advantages in power and economy.
The CNK Turbo kit IS NOT DESIGNED FOR HIGH OCTANE FUEL (although it is fully compatible with it) --- Instead it is designed for EVERY DAY DRIVING ON PUMP FUEL ---- RELIABLY!!!
With high performance comes limits --- with CNK comes RELIABILITY!
--------------------------------------------------
References
Lichty, L. C., Internal-Combustion Engines, Sixth Edition, McGraw-Hill, 1951. (all pictures are from this book)
Stone, R., Introduction to Internal Combustion Engines, Second Edition, Society of Automotive Engineers, 1993.
I will now lock this thread as being for informational purposes only.
[ January 04, 2003, 01:09 PM: Message edited by: CNK Performance ]
