lml@lancaironline.net Mailing List Archive

Bill:

I disagree, based on watching this many, many times on the fully instrumented test stand. The key is where the thetaPP occurs. As the thetaPP moves toward TDC, there is more expansion of the burned gases as the piston falls and the EGT gets lower, even with the same mixture. That's why during pre-ignition, EGT falls rapidly. We have multiple data files which prove this to be the case.

The latency period of the fuel is what delays the burning of the fuel prior to TDC in most normal cases. It is possible to advance the timing beyond the useful limit and change all of this.

Walter

On May 29, 2007, at 4:13 PM, Bill Hannahan wrote:

<RPM:

Higher RPM move the EFFECTIVE timing closer to TDC.>

TRUE

< This results in more expansion = lower EGT.>

NOT TRUE

Imagine the timing advanced to 60 deg before top dead center. This allows more time for fuel and air to burn before tdc resulting in higher cylinder temperature and pressure and a higher rate of heat transfer into the cylinder and head.

If the engine is operating at high MP the fuel air mixture may detonate, spiking temperature, pressure and heat transfer rates even higher.

After TDC the combustion products expand over the full CR and cool by Boyle’s law. The fuel releases a fixed amount of energy, and since more of it flows into the cylinder head there is less energy in the exhaust gas.

Under normal operating conditions fuel continues to burn after TDC, so pressure and temp do not drop as fast as Boyle's law predicts. More advance means less fuel available to burn after TDC.

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