I am new at this but I really want to learn. I have seen a lot of references to detonation but I don't know what it means in the RX7 context. The term itself scares the hell out of me so I feel an urgent need to know what it is.

 

well someone would be better at tthis than i am .. but basically it means the engine is running very hot to the point that the air fuel mixtures it combusting before the the closing of the port/ closing of the valve on piston engines.

ps this can also happen in the intake stroke which is very bad . basic killer of any engine if sever enough or driven long enough

 

Wow, thats gotta hurt when it happens. Thanks

 

Here's a pretty good explanation. It's from a Mustang site, but still good:

http://www.mustangsandmore.com/ubb/Detonation.html

 

http://fc3spro.com/TECH/FAQ/deton.html


-Ted

 

Okay, I'm going to get a little technical and pick a couple nits here. Of the two articles linked above, the mustang one is closer to the truth, but the fc3spro one may be more useful for the typical RX-7 owner.

According to my "Advanced Combustion" class back in grad school, detonation is a flame wave travelling faster than the speed of sound. This is apposed to deflagration, which is a subsonic flame wave. "Ping" or "knock", on the other hand, are terms used to describe rapildly oscillating pressure waves during a combustion event in an internal combustion engine. When the spark plug ignites the fuel/air mixture (we'll assume one spark plug for simplicity, but the concept is the same with two), the mixture is mostly homogeneous (hopefully). The spark starts a flame wave at the plug location, which spreads towards the edges of the combustion chamber, leaving burned gases behind. Now the burned gases are at a much higher energy level than the unburned gases, so they exert pressure, causing the burned gases to be compressed against the combustion chamber walls. If this compression is sufficient to cause the unburned gases to raise in temperature sufficiently to autoignite, knock/ping occurs. If the flame gets near enough to the chamber walls to be quenched, knock does not occur. Race cars often actually use a little bit of knock to ensure that the last bit of fuel/air mixture near the walls burns instead of being quenched by wall interactions. The reason advancing the timing can lead to knock is that the combustion event can then occur closer to TDC, where the base air/fuel mixture is most compressed.

As for what knock is exactly, there is still quite a debate in the scientific community. Some people say that the highly-compressed unburned gases spontaneously combust, while others say the flame wave becomes supersonic. The supersonic theory is why many people equate detonation with knock or ping.

Pre-ignition is something else entirely. With pre-ignition, you have the air/fuel mixture being ignited by something before your intended spark event. This can often lead to knock/ping because it is similar to advancing the ignition timing.

edit: for spelling

 

Simply put: the fuel burn is supposed to push the rotor around, but if conditions are wrong, it can get hammered around.

Take a junk rotor or piston & hit it with a hammer - that's the sound..

 

That was a nice college-level disseration on detonation.
Too bad most of the people here can't appreciate that kinda information.


-Ted

 

Well...........uh.....uh......OK Thanks