snipped-for-privacy@yahoo.com wrote in news: snipped-for-privacy@16g2000cwy.googlegroups.com:
Here is my data point of one - in my 1996 Acura Integra GS-R, I routinely get 29-31 mpg. Just as an experiment, I put in one tank of regular and got
20 mpg, much reduced performance and more engine noise. Then I tried one tank of mid-grade and got 24 mpg, still reduced performance and more noise. So premium ends up being a bargain for me.
Now if only the dealer would call me up and tell me my Subaru is finally in so I could go put premium in it!
hmmm - maybe I'm confused. I think some of the problem is also the 'flame front' or the shape of the combustion. That's why carbon buildup is a 2-fold problem, it not only raises compression, it can create hot-spots in areas other than the sparkplug.
If you want to experiment - try midgrade during cooler months and switch to premium in the summer. If you switch back from a lower grade, ideally, you should clear the ECU so it will 'learn' to put the timing back in the proper position for the premium.
If you like...but then we'll have to ask you "What's the difference between knocking and preignition?"
due to low octane. (If it would, think of the money we could save.)
But you also don't "see" any need for replacing fuses/fusible links when they blow, and advise people to bridge across them with wire, and not even try to determine the cause of the over- current that blew the fuse. Stick to rice, Ben.
Knocking is unintentional too-early ignition due to compression. Pre-ignition, according to some, is too-early ignition due to mis-set ignition device, such as a distributor set too far in advance. Either way it is too-early ignition. Let's not pick nits.
Hmmm... I think some of the problem has to do with the peak pressure from the combustion. I THINK lower octane gas burns more quickly and the pressure peak occurs too far before 'optimum' time. So MAYBE igniting it a little later helps insure the peak pressure is retarded into a better 'time window'. Preignition and detonation are different - but both undesireable.
re-reading my earlier post and I was thinking backwards I guess - sorry.
Of course it does. Because if the spark occurs too much before TDC (top dead center) the expansion of the gases tends to push the piston back the wrong way. The correction is to retard the spark.
But knocking will also occur if something else ignites the fuel/air mixture prematurely-- namely too high compression. Call it auto-ignition. It's what makes diesel engines work without any sparks at all. Even "Computer User" knows that. In diesel engines this auto-ignition occurs at just the right time, by design. In cars with ignition systems, however, it is a problem.
And retarding the spark from the electrical ignition system has no effect at all on auto-ignition. The only way to avoid auto-ignition is to either reduce the compression ratio of the engine (silly) or add something to the gasoline to make it less vulnerable to auto-ignition (tetraethyl lead or more octane).
The fuel/air mixture is normally ignited slightly before the point of maximum compression to allow a small time for the flame-front of the burning fuel to expand throughout the mixture so that maximum pressure occurs at the optimum point. The flame-front moves at roughly 33.5 m/second (110 feet/second) during normal combustion. It is only when the remaining unburned mixture is heated and pressurized by the advancing flame front for a certain length of time that the detonation occurs. It is caused by an instantaneous ignition of the remaining fuel/air mixture in the form of an explosion. The cylinder pressure rises dramatically beyond its design limits and if allowed to persist detonation will damage or destroy engine parts.
Detonation can be prevented by:
The use of a fuel with higher octane rating * The addition of octane-increasing "lead," isooctane, or other fuel additives. * Increasing the amount of fuel injected/inducted (resulting in lower Air to Fuel Ratio) * Reduction of cylinder pressure by increasing the engine revolutions (lower gear), decreasing the manifold pressure (throttle opening) or reducing the load on the engine, or any combination. * Reduction of charge (in-cylinder) temperatures (such as through cooling, water injection or compression ratio reduction). * Retardation of spark plug ignition. * Improved combustion chamber design that concentrates mixture near the spark plug and generates high turbulence to promote fast even burning. * Use of a spark plug of colder heat range in cases where the spark plug insulator has become a source of pre-ignition leading to detonation.
Correct ignition timing is essential for optimum engine performance and fuel efficiency. Modern automotive and small-boat engines have sensors that can detect knock and retard (delay) the ignition (spark plug firing) to prevent it, allowing engines to safely use petrol of below-design octane rating, with the consequence of reduced power and efficiency.
A knock sensor consists of a small piezoelectric microphone, on the engine block, connected to the engine's ECU. Spectral analysis is used to detect the trademark frequency produced by detonation at various RPM. When detonation is detected the ignition timing is retarded, reducing the knocking and protecting the engine. See also Automatic Performance Control (APC).
[edit] Pre-ignition
Pre-ignition is a different phenomenon from detonation, explained above, and occurs when the air/fuel mixture in the cylinder (or even just entering the cylinder) ignites before the spark plug fires. Pre-ignition is caused by an ignition source other than the spark. Heat or hot spots can buildup in engine intake or cylinder components due to improper design, for example, spark plugs with heat range too hot for the conditions, or due to carbon deposits in the combustion chamber. Spark plugs with a high heat range will run hot enough to burn off deposits that lead to plug fouling in a worn engine, but the electrode of the plug itself can occasionally heat soak, and begin glowing hot enough to become an uncontrolled ignition source on its own. Bits of carbon that build up in a combustion chamber can also heat soak to the point where they also are glowing hot and ignite the air-fuel mixture before the proper time.
Pre-ignition and "dieseling" or "run on" are the same phenomenon, except in the latter case the engine continues to run after the ignition is shut off with a hot spot as an ignition source. Pre-ignition might cause rough running due to the advanced and erratic effective igniton timing and may cause noise if it leads to detonation. It may also cause "rumble" which is fast and premature but not detonating combustion.
This heat buildup can only be prevented by eliminating the overheating (through redesign or cleaning) or the compression effects (by reducing the load on the engine or temperature of intake air). As such, if pre-ignition is allowed to continue for any length of time, power output and fuel economy is reduced and engine damage may result.
Pre-ignition may lead to detonation and detonation may lead to pre-ignition or either may exist separately.
From reading, it appears 'peak pressure' SHOULD occur 14-18 degrees AFTER TDC. To acheve that - due to the speed of the flame front with different gasolines and at different rpms - it may be required to have spark 15-40 degrees BEFORE TDC.
Another point to keep in mind with the analysis, is that although the bore & stroke are the same for the 2.5 and the 2.5 Turbo the compression ratios are different as are the performance & requirements.
Specification* 2.5X 2.5XT Bore & Stroke 99.5 x 79mm 99.5 x 79mm Compression 10.0:1 8.4:1 HP 173 @ 6000 RPM 224 @ 5600 RPM Torque ft lb 166 @ 4400 RPM 226 @ 3600 RPM Fuel 87 octane 91 octane Fuel Economy 22/29 M 20/27 M 23/28 A 21/26 A
Even tho it used to be popularly mis-named "dieseling", it's not simple compression that's causing knock (else it would happen ALL the time, continuously, if it ever happened at all. Keep on reading.
Ahhh...so we'd prefer to over-heat (before failure) an extra ten feet or so, of automotive wiring, before that other fuse opens? And lose the thermal acting feature of the socket fuse?
Knocking, AKA detonation, is caused by an alternative ignition source in the combustion chamber, which is lighting off a second (or more) flame front that collides with the flame front propagating from the spark plug. It's happening simultaneously or after the spark ignition.
No, my friendly rice-burner. See above, and:
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...and no, it's not COMPRESSION...
Can an engine be damaged by knock/ping/detonation? Hint: modern engines have knock sensors.
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Some would believe par-boiled equals the real- deal. Pre-ignition is early ignition, from an alternative heat source, WAY before the spark ignition, while the piston is still being mechanically driven to compress the fuel-air charge. Visualize the piston being pushed down, while the burning charge is trying to push it up.
How much of THAT do you think an engine can stand?
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