Discussion
I typically put 93 octane in my 87 turbo. I've heard that it will not hurt an engine to use higher octane, so I intend on using an octane booster. Is it true that low octane may hurt a veh., but higher octane will not hurt?? The booster I use may increase my octane from 93 to 103--is this ok??
I just started using Royal Purple full sythetic "Racing 51" (similar to 20W50). Anyone w/experience with this oil in esprit?
For transm., I am using Royal Purple 75-90 Max Gear. Anyone w/experience with this transm. oil in esprit??
I just started using Royal Purple full sythetic "Racing 51" (similar to 20W50). Anyone w/experience with this oil in esprit?
For transm., I am using Royal Purple 75-90 Max Gear. Anyone w/experience with this transm. oil in esprit??
judgea said:
I typically put 93 octane in my 87 turbo. I've heard that it will not hurt an engine to use higher octane, so I intend on using an octane booster. Is it true that low octane may hurt a veh., but higher octane will not hurt?? The booster I use may increase my octane from 93 to 103--is this ok??
I just started using Royal Purple full sythetic "Racing 51" (similar to 20W50). Anyone w/experience with this oil in esprit?
For transm., I am using Royal Purple 75-90 Max Gear. Anyone w/experience with this transm. oil in esprit??
Hi,
Save your money. Octane boosters are a waste of time and may actually hurt the fuel system.
Contrary to what many think, higher octane fuels do not contain any more power or energy. In fact, the higher the octane, the harder it is for the fuel to combust which is what it is designed for.
With a lower octane fuel (<88), it is possible on a high compression engine, that the A/F mixture can heat to it's combustion point merely through the adiabatic heat created when it is compressed by the piston causing it to combust prior to the spark being introduced. This is known as detonation. Detonation, in addition to causing combustion early, sends shock waves through the engines components stressing them many times their design limits with detrimental effect.(btw, the 9XX engine is actually a lower compression engine except when combined with the turbocharger)
By using a higher octane fuel, the A/F mixture will not combust prematurely, but rather the added energy of the spark is necessary to bring it to combustion. For a stock or near stock 9XX engine, 91-93 octane fuel is all that is needed to control early combustion. It is not a case of more is better.
The main ingredient of most octane boosters is Tolulene. The problem with Tolulene is that it can attack and degrade many plastics and natural rubbers used as 'O' rings, Hoses and seals in the Tank, Fuel Pump, Injectors and throughout the fuel system.
As far as Royal Purple is concerned, it's a quality product, but at a premium over Mobil 1. Plus there's little evidence available as to how it stands up to the high heat of the turbo, whereas there is ample evidence that Mobil 1 has the quality necessary in this application and it's easier to find and cheaper.
The problem with RP gear oil is that it is not sufficiently sulphanated for use in the Citroen C-35 box which needs a certain amount of sulphur content to prolong the life of the soft metal synchro rings. As a lubricant, it would be fine, but it is too compressible to preserve the synchros. The best choices here are Mobil 1 GL-5 rated or Red Line, both synthetics, but better for the job. Hope this helps. Happy Motoring!... Jim'85TE
maigret said:
While on the subject. Would it be OK to run a NA Series 2 on 91 Octane. I have been running it on the 96 available here but it is crappy by world standards with very high levels of Toluene and Benzine.
It is a British spec car if there is a difference to the exported models.
Hi,
Again, it is not necessary to run any higher octane than necessary to prevent detonation, sometimes referred to as ping or knock. Manufacturers often have to state an octane rating to cover a variety of driving and temperature ranges, so unless you're in the extreme of either of these parameters, the rating may actually be overstated for your individual driving and climate.
The N/A version of the 907 engine does posess a higher CR of 9.44:1(Euro Spec) than the Turbos. So it would be more sensitive to detonation. But, many engines here in the US w/ a higher CR than 9.4 run perfectly fine on 91 Octane (the second highest octane normally available here in the States).
Btw, if the higher rated fuel is considerably more expensive, other ways to compensate and allow for a lower octane fuel are to advance the static timing (in your case from 9° to 12°, because this causes the spark to occur earlier), and/or to switch to a colder plug (in your case from NGK BPR6ES to NGK BPR7ES).
However, I should think that 91 octane would be just fine on it's own. The best bet is to try a tank. You will be able to distinguish detonation and a single tankfull won't really hurt anything. If the engine does develop a knock (especially on warmer days, or under extreme load) than keep filling the 91 octane tank with higher octane fuel, this will incrementally increase the fuel octane until it returns to the higher level, or try adavncing the timing or cooler plugs. But again, I suspect 91 octane will be perfectly fine. Happy Motoring!... Jim'85TE
lotusguy said:
Btw, if the higher rated fuel is considerably more expensive, other ways to compensate and allow for a lower octane fuel are to advance the static timing (in your case from 9° to 12°, because this causes the spark to occur earlier), and/or to switch to a colder plug (in your case from NGK BPR6ES to NGK BPR7ES).
Advancing the static timing will actually increase the likelyhood of engine knock, not decrease it.
Engine knock occurs when the fuel in the combustion chamber is ignited before it should be, creating a pressure wave that strikes the top of the piston before it has reached TDC - the knock or ping you hear is this collision. The fuel iginition can come from adiabatic heating, compression ignition, ignition from a sharp edge on something in the combustion chamber (always chamfer the edge of the combustion chamber after the head has been milled), or of course the spark plug. Advancing the ignition timing fires the spark plug earlier in the piston stroke, increasing the chance of knock.
Excessively lean fuel will also create preignition (knock), and can make a very serious mess of an engine. I did this once to a rally car of mine at about 190km/h, where we ran out of fuel at full throttle. The EFI pump started pumping a bit of air through with the fuel, we got a very lean mix and it all went very pear-shaped more or less instantly.
The octane rating of a fuel essentially relates to the propensity of a fuel preigniting through a means other than the spark plug. A higher octane fuel will not ignite as easily. There are actually high octane fuels with a higher calorific value, but they are generally expensive specialist race fuels like Elf TurboMax, which really is horsepower you can pour into the fuel tank. This fuel in Australia costs about eight times as much as regular pump fuel.
cheers,
Ian...
scoots said:
Advancing the static timing will actually increase the likelyhood of engine knock, not decrease it.
Engine knock occurs when the fuel in the combustion chamber is ignited before it should be, creating a pressure wave that strikes the top of the piston before it has reached TDC - the knock or ping you hear is this collision. The fuel iginition can come from adiabatic heating, compression ignition, ignition from a sharp edge on something in the combustion chamber (always chamfer the edge of the combustion chamber after the head has been milled), or of course the spark plug. Advancing the ignition timing fires the spark plug earlier in the piston stroke, increasing the chance of knock.
Excessively lean fuel will also create preignition (knock), and can make a very serious mess of an engine. I did this once to a rally car of mine at about 190km/h, where we ran out of fuel at full throttle. The EFI pump started pumping a bit of air through with the fuel, we got a very lean mix and it all went very pear-shaped more or less instantly.
The octane rating of a fuel essentially relates to the propensity of a fuel preigniting through a means other than the spark plug. A higher octane fuel will not ignite as easily. There are actually high octane fuels with a higher calorific value, but they are generally expensive specialist race fuels like Elf TurboMax, which really is horsepower you can pour into the fuel tank. This fuel in Australia costs about eight times as much as regular pump fuel.
cheers,
Ian...
Ian,
I do not wish to diminish your automotive knowledge in the least. However, your understanding of detonation seems lacking on a couple of very significant and fundamental points. Namely, that for detonation to occur, the fuel/air mixture explodes instantaneously in an uncontrolled explosion. It is this total, instantaneous uncontrolled explosion that produces a damaging shock/pressure wave. This type of combustion occurs without an external ignition source (spark).
In ‘normal’ or controlled combustion, the ignition starts at the spark and combustion propagates outward toward the cylinder walls and piston face along a line known as a Flame Front. This Flame Front travels at the speed of approximately 30-50 cm/s (centimeters per second). This is an estimate because due to the chaotic, fractal, dynamics of fluids, it is nearly impossible to account for all the currents and eddies, known as wrinkling in the A/F mixture.
In actuality, the true speed of the outwards propagating flame front is considerably higher due to the turbulence of the mixture. Basically, all the little eddies, swirls and flow patterns of the turbulence resident in the air-fuel mix carries the “flame” outwards. Due to the chaotic, fractal nature of these, and the ever-changing conditions, which impact them, it is impossible to determine them with great accuracy. This model of combustion is called the "eddy-burning model" (Blizzard & Keck, 1974). The energy and heat released from the mixture in such a controlled combustion is very progressive both in temperature and pressure. Remember that in gasoline engines, combustion is designed to take place before the piston reaches TDC so that the greatest amount of expanding gasses are available to thrust the piston downward once the piston is actually at TDC, unlike a diesel which is designed to have it’s fuel combust suddenly and at TDC. This results in the classic diesel ‘knocking’.
In contrast to the speed of the Flame Front in controlled combustion, the shock/pressure wave of spontaneous detonation travels at the speed of sound, approx. 300 cm/s. This is significant because if detonation were solely dependent on the combustion of the mixture, each ignition cycle would create this shock/pressure wave (such as the knocking of a diesel) and result in a rapidly destroyed engine. Keeping an engine’s temperature and pressures such that additional energy, in the form of a spark, is necessary to cause the mixture to combust allows for progressive combustion and relatively slowly rising pressures, yielding an engine of long life.
Essentially what keeps the engine from damage in a controlled combustion is the existence of a boundary layer of the A/F mixture that exists very close the cylinder walls and piston top. This boundary layer is a fraction of a millimeter to one millimeter thick and the fuel contained therein actually never ignites. This is due to its contact with the much cooler cylinder walls and piston top, keeping it well below its ignition threshold. In simpler terms, it provides a protective thermal barrier for the cylinders, rings and piston.
But, when detonation occurs, the greater, sudden energy released by the spontaneous combustion of the entire mixture all at once disrupts this boundary layer and exposes the engine internals to the heat of combustion some 1300+ °. Add to that the effect of exerting force against the normal momentum of the reciprocating parts, and you very quickly exceed the levels necessary to break, damage and melt engine parts.
By advancing the timing, you introduce a spark earlier and create a Flame Front that prevents the spontaneous explosion of detonation. Of course, there are limits to how far the spark can be advanced. Every type of engine is different, with different limits. Individual engines and even individual cylinders within an engine vary greatly due in part to variations in the cylinder head, combustion chamber and even the spark plug as you’ve alluded to. The 9XX engine loves lots of advance (up to 32° total adavance). Consequently, advancing the timing a mere 3° to total 28° on an engine that has a total timing advance spec of 25° does not significantly raise the detonation threshold at all.
When you describe a lean condition, again, there is no detonation here. Once the mixture leans out to a ratio less than 11:1, the heat created exceeds that which the engine was designed to endure.
So far as the Elf TurboMax fuel is concerned, I agree that there are fuels which contain more potential energy (BTUs), but the higher octane rating is a consequence and not responsible for the additional energy. This additional energy is often the result of added oxygenates that actually raise the detonation threshold by and of themselves. Other additives that prohibit uncontrolled combustion mitigate these. Sorry for the dissertation, but I felt a more thorough explanation was in order. Happy Motoring!... Jim'85TE
>> Edited by lotusguy on Wednesday 17th March 21:01
Diesel fuel has more energy per liter and much much lower octane rating. In fact, spontaneous combustion is how it works.
Don't forget, also that "octane" is merely resistance to pre-ignition, and the standard measuring technique is a variable compression ratio engine. They crank up the compression until it pings and have a quantative measure.
Our engines were designed to run on a high octane, that is what we should use. Useing less is dangerous to a boosted engine. Especially one without a knock sensor.
My Toyota 4AGE 20 valve was designed to run on 100 octane gas, as measured by the Japanese. Near as I can figger, it is about the same as our 93 (R+M)/2 octane rating, which is an average of two rating methods (R+M). It has a knock sensor and does fine on the US 93 octane stuff. Which brings up the point that US octane ratings are not the same as everyone else's. We average the two methods whereas the rest of the world, such as Japan and Oz, tends to just use the higher number.
Dr.Hess
Don't forget, also that "octane" is merely resistance to pre-ignition, and the standard measuring technique is a variable compression ratio engine. They crank up the compression until it pings and have a quantative measure.
Our engines were designed to run on a high octane, that is what we should use. Useing less is dangerous to a boosted engine. Especially one without a knock sensor.
My Toyota 4AGE 20 valve was designed to run on 100 octane gas, as measured by the Japanese. Near as I can figger, it is about the same as our 93 (R+M)/2 octane rating, which is an average of two rating methods (R+M). It has a knock sensor and does fine on the US 93 octane stuff. Which brings up the point that US octane ratings are not the same as everyone else's. We average the two methods whereas the rest of the world, such as Japan and Oz, tends to just use the higher number.
Dr.Hess
Doc,
You had to get me going eh..?? Ok, then, how is octane rating determined? First, you go out and get a suitable supply of the fuel that you wish to test. Then, you get yourself some heptane (made from pine sap) and some iso-octane (a petroleum derivative). Finally, you and your buddies arbitrarily, agree that iso-octane has an octane rating of 100 while heptane has an octane rating of 0.
Next, you call up Wauskeshaw Motors - www.waukeshaengine.com/internet/businessunits/waukesha/index.cfm and order yourself an ASTM-CFR Test Engine. Make sure you have about $250,000 + S/H available on your VISA card before you order it. This single-cylinder engine has a four-bowl carburetor and a movable cylinder head that can vary the compression ratio between 4:1 to 18:1 while the engine is running.
You fill your new ASTM-CFR engine full of the fuel to be tested, and, for automotive fuels, you run two test protocols using the ASTM engine. One protocol is called the ‘Motor’ protocol and the other is the ‘Research’ protocol. You vary the compression ratio until the onset of knock and write down all kinds of various scientific parameters.
Next, you run your reference fuel, made up of various proportions of heptane and iso-octane through the ASTM-CFR engine. You keep varying the proportion of heptane to iso-octane until you get a fuel that behaves just like (knock-wise) your test fuel. Once you get that, you say to yourself "How much heptane did I have to add to the iso-octane to get the mixture to knock in the ASTM-CFR just like my test fuel?" If the answer is, say, 10% heptane to 90% iso-octane, your test fuel has an octane number of 90.
How do the motor and research protocol differ? Mostly in input parameters. In the motor protocol (ASTM D2700-92), the input air temp is maintained at 38 °C, the ignition timing varies with compression ratio between 14 and 26 degrees BTDC, and the motor is run at 900 RPM. In the research protocol (ASTM D2699-92) the input air temperature varies between 20 °C and 52 °C (depending on barometric pressure), timing is fixed at 13 degrees BTDC, and the motor is run at 600RPM. The ratings are designated as MON (Motor Octane Number) and RON (Research Octane Number). In the US, MON and RON are averaged to yield PON (Pump Octane Number), in Europe and Asia, only RON is considered and published.
The motor method, developed in the 1920s, was the first octane rating method devised. After its introduction, many more methods were introduced. During the 1940s through the 1960s one of those methods, the research method, was found to more closely correlate with the fuels and vehicles then available.
However, in the early 1970s automobiles running on high-speed roads, such as the German Autobahn, started destroying themselves from high-speed knock. It was found that the difference in ratings between the research and motor method, known as the fuel's sensitivity was important as well. The greater the fuel's sensitivity, the worse it performed from a knock point of view in demanding, real-world, applications.
At the pumps, the results of the motor and research numbers are averaged together to get the value you see. The fuel's sensitivity is not published. Highly cracked fuels have high sensitivity while paraffinic fuels often show near zero difference between the two. While the fuel's sensitivity is not published at the pump it can be a valuable indicator as to the fuel's real world octane performance.
Remember, the octane tests are conducted in a lab using a special test engine; the lower the fuel's sensitivity, the more likely it is that the fuel will, indeed, behave as expected. Generally, the closer the fuel's research rating is to the published rating, the more reliable the published rating. Because the motor and research methods primarily differ in terms of input parameters (the test engine is the same for both), the greater difference that a fuel exhibits between its motor and research test will be due to differences in input parameters (intake temp, timing, etc.). A fuel that has an octane rating that varies with intake parameters is said to be more "sensitive."
Technically, there is no such thing as an octane number above 100. If you're in a crowd, avoid saying things like "110 octane gasoline" because people will get up and walk away from you. You should say, instead, "a gasoline with a performance number of 110." A fuel’s rating of more than 100 octane is done by pure extrapolation. A more reliable method, however, is through the use of so-called performance numbers. Briefly, these are arrived at by determining the instantaneous mean effective cylinder pressure (IMEP), using the fuel under test, at the highest boost that does not cause knocking. This number is then multiplied by 100 and the resultant is divided by the IMEP at the highest boost that does not cause knocking on the 100 octane equivalent fuel.
In the past, tetraethyl lead was added to gasoline to mitigate detonation. Tetraethyl lead raises the octane rating of a fuel not because it adds more "octanes" to the fuel, but because it makes the fuel knock at a higher compression ratio in the ASTM-CFR engine. According to the latest research, octane ratings go down with fuels comprised of long, straight, hydrocarbon chains (paraffinic fuels). Fuels with branching hydrocarbon fuels, and aromatic fuels, have a higher octane ratings. Oxygenates and alkyl lead affect the pre-flame reaction pathways by retarding branching sequences. Lead was previously believed to work by slowing the flame front, thus leading to a slower pressure rise in the cylinder. While general flame-front propagation speed does affect octane ratings, lead does not significantly affect it.
Combustion chamber design, localized hot spots, piston speed, and a host of other factors can all contribute to an engine's propensity to knock.
Additionally, altitude extremes and super/turbo charging affect octane requirements. Increased induction pressures (such as would be encountered in a turbo/supercharged engine) cause more rapid flame-front propagation. Likewise, decreased exhaust pressure also tends to increase flame-front propagation speed. Both of these effects can combine to raise octane requirements.
One the other hand, Diesel fuel, along with kerosene have, indeed, terrible octane numbers; typically about 15-25 "octane". They tend to ignite easily from high compression. Their use in a gasoline engine will quickly destroy the engine.
Diesel fuel is rated by its cetane number that is determined, like octane, by running the fuel in a test engine. Instead of heptane and iso-octane they use napthalene (cetane rating = 0) and n-cetane (cetane rating = 100). In total opposite to octane ratings, the higher the cetane rating the higher the fuel's propensity to knock!
Just as using a fuel with an octane number higher than necessary in a gas engine will gain you noting, using a fuel with a cetane number higher than necessary in a diesel engine gets you nothing. On the other hand, where using a fuel with too low an octane number in a gas engine will result in a damaged engine, using a fuel with too low a cetane number of a diesel engine will just result in a rough-running (or not running at all) engine with no damage.
Why can diesel engines tolerate a low octane fuel? In all gasoline engines, (including injected gasoline engines!) the fuel/air mixture is present in the cylinder the entire time the piston is travelling upward on its compression stroke. This means it could be ignited at any time whereas you only want it to be ignited when the spark plug fires, some time just before the very top of the stroke. Furthermore, you want a nice, even, steady, pressure rise in the cylinder as a result of ignition. This means that you want the flame-front to travel linearly from the source of ignition (the sparkplug) to the other side. We do not want combustion to occur randomly within the mixture as that may cause a too-rapid pressure rise.
In a diesel engine there is NO fuel in the combustion chamber as the piston starts up on its compression stroke. Instead, fuel is injected at high pressure (up to 3000PSI!) into the combustion chamber at the exact moment when ignition is desired. In a diesel engine with a compression ratio of around 20:1 (compared to 7:1 for many modern gas engines), the heat of compression will have raised the combustion chamber temperature to around 1000-1500F. The injection time takes about .002-.004 seconds during which the fuel spontaneously ignites from the heat of compression at just the right time. Even so, a diesel fuel with too low a cetane rating may not ignite, or may ignite poorly - especially on cold days starting a cold engine.
The second critical difference is that Diesels are set up to burn the fuel in a slightly different way. In a gas engine, you typically set it up so that the mixture is ignited before the piston hits the top of the stroke. What you're aiming for is for the mixture to be fully burned around the top of the stroke - thus combustion pressures are maximized at the top of the stroke and gradually fall off as the piston moves downward on the power stroke (and increases the volume in the cylinder).
Diesels, on the other hand, are set up to inject fuel very close to the top of the compression stroke. The fuel spontaneously ignites (auto-ignition) and, actually, knocks. The combustion pressures in the diesel increase evenly as the piston goes down. The net result is that the diesel piston "feels" a constant pressure on it as the piston travels from top dead center to bottom dead center whereas a normally operating gasoline engine piston "feels" a constantly decreasing pressure as it travels to the bottom of the stroke. The net result is that the diesel feels a lot lower PEAK pressure while the pressure is maintained over a longer period. The gasoline engine feels a much higher peak pressure which starts to fall off immediately as the piston travels downward.
The implication, for the latter, is that it periodically operates very close to the capabilities of the base metals. Anything, such as knocking, which increases those peak pressures even more is apt to push beyond the capabilities of the base metals and result in engine damage.
A high octane rating ensures that it takes a REALLY hot ignition source to ignite the fuel (such as a spark plug or the flame-front itself) and not just the rise in pressure & temperature that's a result of normal combustion. Note that the thermal rises in the cylinder are in direct proportion to the compression ratio of the engine. The higher the compression ratio, the higher the octane of the fuel that's required to prevent detonation.
Rather than go on in ever greater detail, if you have any specific questions regarding octane ratings, methods or application, just chime in, I’ll be happy to try and supply an answer. Happy Motoring! … Jim’85TE
>> Edited by lotusguy on Wednesday 17th March 23:12
You had to get me going eh..?? Ok, then, how is octane rating determined? First, you go out and get a suitable supply of the fuel that you wish to test. Then, you get yourself some heptane (made from pine sap) and some iso-octane (a petroleum derivative). Finally, you and your buddies arbitrarily, agree that iso-octane has an octane rating of 100 while heptane has an octane rating of 0.
Next, you call up Wauskeshaw Motors - www.waukeshaengine.com/internet/businessunits/waukesha/index.cfm and order yourself an ASTM-CFR Test Engine. Make sure you have about $250,000 + S/H available on your VISA card before you order it. This single-cylinder engine has a four-bowl carburetor and a movable cylinder head that can vary the compression ratio between 4:1 to 18:1 while the engine is running.
You fill your new ASTM-CFR engine full of the fuel to be tested, and, for automotive fuels, you run two test protocols using the ASTM engine. One protocol is called the ‘Motor’ protocol and the other is the ‘Research’ protocol. You vary the compression ratio until the onset of knock and write down all kinds of various scientific parameters.
Next, you run your reference fuel, made up of various proportions of heptane and iso-octane through the ASTM-CFR engine. You keep varying the proportion of heptane to iso-octane until you get a fuel that behaves just like (knock-wise) your test fuel. Once you get that, you say to yourself "How much heptane did I have to add to the iso-octane to get the mixture to knock in the ASTM-CFR just like my test fuel?" If the answer is, say, 10% heptane to 90% iso-octane, your test fuel has an octane number of 90.
How do the motor and research protocol differ? Mostly in input parameters. In the motor protocol (ASTM D2700-92), the input air temp is maintained at 38 °C, the ignition timing varies with compression ratio between 14 and 26 degrees BTDC, and the motor is run at 900 RPM. In the research protocol (ASTM D2699-92) the input air temperature varies between 20 °C and 52 °C (depending on barometric pressure), timing is fixed at 13 degrees BTDC, and the motor is run at 600RPM. The ratings are designated as MON (Motor Octane Number) and RON (Research Octane Number). In the US, MON and RON are averaged to yield PON (Pump Octane Number), in Europe and Asia, only RON is considered and published.
The motor method, developed in the 1920s, was the first octane rating method devised. After its introduction, many more methods were introduced. During the 1940s through the 1960s one of those methods, the research method, was found to more closely correlate with the fuels and vehicles then available.
However, in the early 1970s automobiles running on high-speed roads, such as the German Autobahn, started destroying themselves from high-speed knock. It was found that the difference in ratings between the research and motor method, known as the fuel's sensitivity was important as well. The greater the fuel's sensitivity, the worse it performed from a knock point of view in demanding, real-world, applications.
At the pumps, the results of the motor and research numbers are averaged together to get the value you see. The fuel's sensitivity is not published. Highly cracked fuels have high sensitivity while paraffinic fuels often show near zero difference between the two. While the fuel's sensitivity is not published at the pump it can be a valuable indicator as to the fuel's real world octane performance.
Remember, the octane tests are conducted in a lab using a special test engine; the lower the fuel's sensitivity, the more likely it is that the fuel will, indeed, behave as expected. Generally, the closer the fuel's research rating is to the published rating, the more reliable the published rating. Because the motor and research methods primarily differ in terms of input parameters (the test engine is the same for both), the greater difference that a fuel exhibits between its motor and research test will be due to differences in input parameters (intake temp, timing, etc.). A fuel that has an octane rating that varies with intake parameters is said to be more "sensitive."
Technically, there is no such thing as an octane number above 100. If you're in a crowd, avoid saying things like "110 octane gasoline" because people will get up and walk away from you. You should say, instead, "a gasoline with a performance number of 110." A fuel’s rating of more than 100 octane is done by pure extrapolation. A more reliable method, however, is through the use of so-called performance numbers. Briefly, these are arrived at by determining the instantaneous mean effective cylinder pressure (IMEP), using the fuel under test, at the highest boost that does not cause knocking. This number is then multiplied by 100 and the resultant is divided by the IMEP at the highest boost that does not cause knocking on the 100 octane equivalent fuel.
In the past, tetraethyl lead was added to gasoline to mitigate detonation. Tetraethyl lead raises the octane rating of a fuel not because it adds more "octanes" to the fuel, but because it makes the fuel knock at a higher compression ratio in the ASTM-CFR engine. According to the latest research, octane ratings go down with fuels comprised of long, straight, hydrocarbon chains (paraffinic fuels). Fuels with branching hydrocarbon fuels, and aromatic fuels, have a higher octane ratings. Oxygenates and alkyl lead affect the pre-flame reaction pathways by retarding branching sequences. Lead was previously believed to work by slowing the flame front, thus leading to a slower pressure rise in the cylinder. While general flame-front propagation speed does affect octane ratings, lead does not significantly affect it.
Combustion chamber design, localized hot spots, piston speed, and a host of other factors can all contribute to an engine's propensity to knock.
Additionally, altitude extremes and super/turbo charging affect octane requirements. Increased induction pressures (such as would be encountered in a turbo/supercharged engine) cause more rapid flame-front propagation. Likewise, decreased exhaust pressure also tends to increase flame-front propagation speed. Both of these effects can combine to raise octane requirements.
One the other hand, Diesel fuel, along with kerosene have, indeed, terrible octane numbers; typically about 15-25 "octane". They tend to ignite easily from high compression. Their use in a gasoline engine will quickly destroy the engine.
Diesel fuel is rated by its cetane number that is determined, like octane, by running the fuel in a test engine. Instead of heptane and iso-octane they use napthalene (cetane rating = 0) and n-cetane (cetane rating = 100). In total opposite to octane ratings, the higher the cetane rating the higher the fuel's propensity to knock!
Just as using a fuel with an octane number higher than necessary in a gas engine will gain you noting, using a fuel with a cetane number higher than necessary in a diesel engine gets you nothing. On the other hand, where using a fuel with too low an octane number in a gas engine will result in a damaged engine, using a fuel with too low a cetane number of a diesel engine will just result in a rough-running (or not running at all) engine with no damage.
Why can diesel engines tolerate a low octane fuel? In all gasoline engines, (including injected gasoline engines!) the fuel/air mixture is present in the cylinder the entire time the piston is travelling upward on its compression stroke. This means it could be ignited at any time whereas you only want it to be ignited when the spark plug fires, some time just before the very top of the stroke. Furthermore, you want a nice, even, steady, pressure rise in the cylinder as a result of ignition. This means that you want the flame-front to travel linearly from the source of ignition (the sparkplug) to the other side. We do not want combustion to occur randomly within the mixture as that may cause a too-rapid pressure rise.
In a diesel engine there is NO fuel in the combustion chamber as the piston starts up on its compression stroke. Instead, fuel is injected at high pressure (up to 3000PSI!) into the combustion chamber at the exact moment when ignition is desired. In a diesel engine with a compression ratio of around 20:1 (compared to 7:1 for many modern gas engines), the heat of compression will have raised the combustion chamber temperature to around 1000-1500F. The injection time takes about .002-.004 seconds during which the fuel spontaneously ignites from the heat of compression at just the right time. Even so, a diesel fuel with too low a cetane rating may not ignite, or may ignite poorly - especially on cold days starting a cold engine.
The second critical difference is that Diesels are set up to burn the fuel in a slightly different way. In a gas engine, you typically set it up so that the mixture is ignited before the piston hits the top of the stroke. What you're aiming for is for the mixture to be fully burned around the top of the stroke - thus combustion pressures are maximized at the top of the stroke and gradually fall off as the piston moves downward on the power stroke (and increases the volume in the cylinder).
Diesels, on the other hand, are set up to inject fuel very close to the top of the compression stroke. The fuel spontaneously ignites (auto-ignition) and, actually, knocks. The combustion pressures in the diesel increase evenly as the piston goes down. The net result is that the diesel piston "feels" a constant pressure on it as the piston travels from top dead center to bottom dead center whereas a normally operating gasoline engine piston "feels" a constantly decreasing pressure as it travels to the bottom of the stroke. The net result is that the diesel feels a lot lower PEAK pressure while the pressure is maintained over a longer period. The gasoline engine feels a much higher peak pressure which starts to fall off immediately as the piston travels downward.
The implication, for the latter, is that it periodically operates very close to the capabilities of the base metals. Anything, such as knocking, which increases those peak pressures even more is apt to push beyond the capabilities of the base metals and result in engine damage.
A high octane rating ensures that it takes a REALLY hot ignition source to ignite the fuel (such as a spark plug or the flame-front itself) and not just the rise in pressure & temperature that's a result of normal combustion. Note that the thermal rises in the cylinder are in direct proportion to the compression ratio of the engine. The higher the compression ratio, the higher the octane of the fuel that's required to prevent detonation.
Rather than go on in ever greater detail, if you have any specific questions regarding octane ratings, methods or application, just chime in, I’ll be happy to try and supply an answer. Happy Motoring! … Jim’85TE
>> Edited by lotusguy on Wednesday 17th March 23:12
Wow:
Does this waukesha engine come with a warranty? It's probably time to put the squabbling to rest and start using some common sense about choosing fuels. Those with serious engineering questions about how octane ratings are derived can source them privately on the net if the urge is strong.
Gasoline and air being sqeezed into a tube and compressed generates heat all by itself. The more volatile the fuel (ie: lower octane) the easier it is for it to go boom when we don't want it to. So, the more it's squeezed, the hotter it gets the easier it is to go boom. More compression means we go looking for higher octane fuel. Other factors that affect combustion chamber temperatures are plug type and rating, deposits, and ignition timing. All of these can cause the air/fuel mixture to spontaneously combust. Overadvancing timing to try and 'catch' the mixture before spontaneous detonation doesn't work. The reality of having plug temperatures increasing exponentially with each degree of timing overadvance offsets any gain.
Bottom line; Use the best grade pump gas you can get. Some of those octane boosters really murder seals. Ensure that your engine is tuned properly with the recommended components and that it is relatively free of combustion chamber deposits.
And always listen to what your car is telling you.
Does this waukesha engine come with a warranty? It's probably time to put the squabbling to rest and start using some common sense about choosing fuels. Those with serious engineering questions about how octane ratings are derived can source them privately on the net if the urge is strong.
Gasoline and air being sqeezed into a tube and compressed generates heat all by itself. The more volatile the fuel (ie: lower octane) the easier it is for it to go boom when we don't want it to. So, the more it's squeezed, the hotter it gets the easier it is to go boom. More compression means we go looking for higher octane fuel. Other factors that affect combustion chamber temperatures are plug type and rating, deposits, and ignition timing. All of these can cause the air/fuel mixture to spontaneously combust. Overadvancing timing to try and 'catch' the mixture before spontaneous detonation doesn't work. The reality of having plug temperatures increasing exponentially with each degree of timing overadvance offsets any gain.
Bottom line; Use the best grade pump gas you can get. Some of those octane boosters really murder seals. Ensure that your engine is tuned properly with the recommended components and that it is relatively free of combustion chamber deposits.
And always listen to what your car is telling you.
lotusguy said:
I do not wish to diminish your automotive knowledge in the least. However, your understanding of detonation seems lacking on a couple of very significant and fundamental points. Namely, that for detonation to occur, the fuel/air mixture explodes instantaneously in an uncontrolled explosion. It is this total, instantaneous uncontrolled explosion that produces a damaging shock/pressure wave. This type of combustion occurs without an external ignition source (spark).
In ‘normal’ or controlled combustion, the ignition starts at the spark and combustion propagates outward toward the cylinder walls and piston face along a line known as a Flame Front. This Flame Front travels at the speed of approximately 30-50 cm/s (centimeters per second). This is an estimate because due to the chaotic, fractal, dynamics of fluids, it is nearly impossible to account for all the currents and eddies, known as wrinkling in the A/F mixture.
Jim,
Sorry for oversimplifying it. While some of us may grasp the intricacies of fractal maths and the application of chaos theory to hydrodynamics, it's not necessary to illustrate the practical point. My point was that no pre-ignition or detonation is good news, whatever the cause, and irrespective of whether the ignition source is the spark plug or something else. It is extremely likely that advancing the timing with not solve it, and possible that it will make the problems worse.
I was tardily using the term "knock" to include both pre-igintion and detonation, which I know are quite similar but still two different things.
cheers,
Ian...
If high levels of toluene are detrimental to the life of seals and gaskets, then I would urge those of you running Avgas to do some further research on its components. Based on the odor I would say that avgas definitely contains toluene. Unfortunately ten minutes on Google failed to find a definitive answer as to the concentration. Since I'm no longer with Conoco I don't have a ready source to find out the levels. As an aside, Conoco quit using engines to determine octane value in the early 90's. They switched to an infrared analysis device.
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