In a message dated 6/28/2006 6:40:55 P.M. Eastern Daylight Time, lendich@optusnet.com.au writes:

Lynn,

I have a question - a guide would be appreciated, a rule of thumb even!

 

Understanding that heated air ( caused one way or another) has a contributing factor to detonation at what sort of compression would you expect to see detonation starting to occur on an average day ( temperature wise) at sea level.

Secondly, given a 10:1 compression at what temperature would you expect to see detonation, on an average day, at sea level.

Is there any graph that shows the relationship to compression, inlet temp, engine temp and altitude as a guide to possible detonation events.

I would think something like this would be handy to have available to everyone, with perhaps an adjustment value for different fuels.

George (down under)

Wow, not on the spot or anything.

Think about this:

Even though it isn't a cylinder the term still applies. Cylinder filling in a stock ported engine is better at idle than anywhere else in the RPM band. This is not true exactly but just pretend for a moment.

So the evil sounding 10:1 VS 9.7 or 9.5:1 compressions make a real big difference at low RPM where you need very little extra compression and it only adds danger. High up at cruise, it helps quite a bit because of poor cylinder filling.

 In a NA engine cylinder filling goes into the toilet as the revs come up.

Simple?

There is less open time at the intake port for flow into the chamber so each event gets less total mixture. So, the effect is that since there is less stuff in the cylinder and the head space (compression volume) is the same, the compression ratio is decreasing with increased RPM.

So, the NA engine is less prone to detonation as the RPM goes up. And since the cruise RPM is high 6,000 to 6,500 RPM the possibility of detonation is much reduced even with higher compression rotors.

This is not the case for turbo charged engines.

As the RPM increase the boost pressure goes up and cylinder filling exceeds the calculated displacement of the engine.

So, the effect is that the engine is getting bigger (that is what you wanted, right? More power) however the head space is still the same. So, the effective compression ratio is going up with RPM. So, turbocharged engines tend to use lower compression rotors so as not to destroy themselves soon after starting down the runway. And when they die it is far more spactacular because they can do it from cruise RPM instead of a fast idle.

There is little danger in turbo normalizing an engine where just enough boost to mimic sea level is used. It is the same as (well) sea level. Good reliability and great performance at 12,000 feet. Even if high compression rotors are used.

My driver learned how to detonate a 12A by not listening to the owner. (Me). The race engine has very little rotating mass, and a grabby metallic clutch. More like a switch than a street clutch. So rather than slip the clutch a bit to get the car rolling up to some walking speed, (expensive and requiring skill) he would just rev it up a bit and pop the clutch, moving away at far below idle speed (2,200 RPM) maybe 200 or 300 RPM. The engine is very unhappy about this as the porting allows much of the mixture back into the intake manifold, and produces like 2 HP when it needs 3 HP to move along at a fast walk. And yes he could detonate it at a walk on a cool morning with very low intake air temps. This is all possible because the race car has a tall first gear. To be useful on the track it is the same as a short third gear in a street car. The ratio is 1.94:1 or 1.96:1 depending on the rest of the gear set. Try leaving a light in third gear, and you too can play “race car”.

We tow the car to the false grid, so as to avoid this problem.

Hanover just told me intake air temp dependent, then he says cold morning? Yes that is correct.

 

Even the heat of compression would be well below operating temps due to poor cylinder filling. However, at cruise RPM the 24-27 (degrees (for a 12A) of advance is required to get the highest torque at the correct crank angle (the whole reason for the advance) so what is the correct advance for 5 MPH 300 RPM and high load?

 

Well its about 10 degrees after TDC (or 42 degrees too much advance) so long as there is little throttle applied. And to a driver, more throttle is the answer to nearly everything.

 

Since there is a time factor involved, and it is predictable for each situation, you can expect some of the outcomes just from common sense. Note that the controller in the car takes out ignition advance when the Lambda sensor hears detonation. So the Mazda people know it is advance that can be a big (and easy to control) feature in engine damage. Also in full authority systems, (where the throttle lever is your suggestion of how much power we need right now) and how much you are allowed to have is determined by the computer, you will not be permitted (actual) full throttle right off idle as it would be pointless, and damage the engine.

 

Without regard for your authority as PIC being usurped by a little chip and some geek programmer half a world away, detonating a good airplane (piston) engine or even a rotary into scrap iron in 4 revolutions will be of no value to you or your passengers. Engine failure after a refused landing sounds all too familiar. So if you forget to return the O-550 to full rich coming down the pipe, and then after floating passed the intersection notice the Piper that seems to have taxied onto your runway facing you, because you are landing DOWNWIND, idiot............will you now remember to enrich the mixture

before slamming the throttle through the panel?

You people (rotary lovers) being near genius pilots in the Hoover tradition would remember, and have never had a refused landing for any reason. And certainly have never landed downwind.

But many lesser pilots would not. The full authority computer might piss away three quarters of a second getting back to the full throttle stop, but it will get there at 23 GPH and will start at less than full advance so as to produce the maximum available engine acceleration possible. It will do this with usable power coming on much sooner than it could have with a fixed advance, the wrong mixture and manual throttle manipulation.

 

So it is difficult to say that this or that intake air temp caused the detonation at cruise when it can be done on the ground with ease. Rotary or piston. If the damage is not bad enough to cause an obvious performance loss, then when did it happen?    

 

While there are certainly scales and tables to account for detonation probabilities I have never seen one.

Anything that alters intake air temperature (in the end its charge temperature because what happens during compression adds heat) can remove the danger of detonation.

Reduced ignition advance.

Reduced throttle setting.

Reduced coolant temperature.

Reduced oil temperature.

Increased fuel octane number.

Reduced compression ratio.

Reduced turbo boost. (same as increased compression ratio)

Reduced intake air temperature. (worse for turbos, requiring inter-cooling)

Limit rate of throttle increase.

In very general terms, NA engines can be detonated, but is not likely.

In very low boost (normalized) engines detonation is somewhat more of a problem.

In high boost engines, detonation is a very big problem, and can destroy an engine in a few revolutions.

Lynn E. Hanover