Walter Atkinson
Well Known Member
There are two issues concnering timing that must be considered when operating your engine. The first and the one you can't do much about in flight is the mechanical timing set by the mechanic or the designer of an EI.
The issue of concern is where the thetaPP happens after TDC. There is a general consensus that the optimum EFFECTIVE timing is at 15-16 dATDC. This is the point where the peak of the pressure curve is best placed to gain maximum mechanical advantage with the crank-conrod geometry. UNless the EI system altering timing is MEASURING the thetaPP in real time, it is necessary to use a map to guestimate the optimal timing. Frankly, this is only marginally successful. There are three ways the pilot can affect the thetaPP. RPM, MP and MIXTURE.
A slower RPM moves the thetaPP closer to TDC.
A higher MP moves the thetaPP closer to TDC.
The maximum flame speed is seen at 40dF ROP. For any given MP and RPM, 40dF ROP will result int he highest pressures and CHTs. Richer slows the burn rate a little. A LOT richer slows the burn rate more significantly. This is why we want a very rich mixture during takeoff and high-power climb. LOP mixture slow the burn rate as well and lowwer CHT and ICP as a result. This is accomplished by moving the thetaPP.
***** OWT ALERT ******
Extra fuel ROP does not cool the CHT. Extra air LOP does not cool the CHT. The CHT is altered based on the movement of the thetaPP and the resultant lower ICP which affects the rate at which BTUs are transfered across the thermal boundary layer and pushed into the cylinder head metal.
Walter Atkinson
Advanced Pilot Seminars
The issue of concern is where the thetaPP happens after TDC. There is a general consensus that the optimum EFFECTIVE timing is at 15-16 dATDC. This is the point where the peak of the pressure curve is best placed to gain maximum mechanical advantage with the crank-conrod geometry. UNless the EI system altering timing is MEASURING the thetaPP in real time, it is necessary to use a map to guestimate the optimal timing. Frankly, this is only marginally successful. There are three ways the pilot can affect the thetaPP. RPM, MP and MIXTURE.
A slower RPM moves the thetaPP closer to TDC.
A higher MP moves the thetaPP closer to TDC.
The maximum flame speed is seen at 40dF ROP. For any given MP and RPM, 40dF ROP will result int he highest pressures and CHTs. Richer slows the burn rate a little. A LOT richer slows the burn rate more significantly. This is why we want a very rich mixture during takeoff and high-power climb. LOP mixture slow the burn rate as well and lowwer CHT and ICP as a result. This is accomplished by moving the thetaPP.
***** OWT ALERT ******
Extra fuel ROP does not cool the CHT. Extra air LOP does not cool the CHT. The CHT is altered based on the movement of the thetaPP and the resultant lower ICP which affects the rate at which BTUs are transfered across the thermal boundary layer and pushed into the cylinder head metal.
Walter Atkinson
Advanced Pilot Seminars