Is this a scratch or a crack?

[Cyril Gibb]

On 5/29/2019 at 1:26 PM, PT20J said:

Great summary. 

Here's some actual APS data for one operating condition for one (unspecified) engine. It's a little hard to read, but the original wasn't all that good. The graphs are internal cylinder pressure curves for the same engine at the same power output with the only difference being mixture.

Left graph: 27.2"/2500 rpm / 75F ROP / 390F CHT / 18.3 gph / 244 bhp / 140.5 BMEP / ICP_max= 825 psi

Right graph: 29.9"/2500 rpm / 50F LOP / 365F CHT / 16.4 gph / 244 bhp / 140.5 BMEP / ICP_max= 740 psi

Note that BMEP is not a "real pressure", but an average. It is equivalent to torque times a constant. As shown below, you can achieve the same BMEP with different shaped curves. 

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One additional point that may not be as clear for those that haven't read enough about LOP operations:  The peak pressure LOP is later in the crank rotation than for ROP because the leaner mixture has a slower flame front. That shifts the whole pressure graph to the right giving two advantages:

- the pressure against rotation (pressure before TDC in the compression stroke) is lower when LOP.

- the peak pressure LOP is at a point later in the crank rotation that has more mechanical advantage

 

[carusoam]

Nice explanation, Cyril!

Had to double check who wrote that.  Could have been Ross giving that much detail regarding flame fronts and propagation...

-a-

[PT20J]

On 6/11/2019 at 6:00 AM, Cyril Gibb said:

One additional point that may not be as clear for those that haven't read enough about LOP operations:  The peak pressure LOP is later in the crank rotation than for ROP because the leaner mixture has a slower flame front. That shifts the whole pressure graph to the right giving two advantages:

- the pressure against rotation (pressure before TDC in the compression stroke) is lower when LOP.

- the peak pressure LOP is at a point later in the crank rotation that has more mechanical advantage

This is an interesting point. For an ideal Otto cycle engine, the ignition occurs at TDC and the mixture burns instantaneously at constant volume and then expands adiabatically (without loss or gain of heat) as the piston moves down on the power stroke transferring work to the crankshaft. In a real spark ignition (SI) engine, the mixture takes a finite time to burn and there is a rise in pressure to a peak and then a decline. The peak pressure position (PPP) measured in crankshaft angle (degrees after TDC) is determined by mixture strength, rpm and spark timing. There exists a PPP for each engine (based on engine geometry) that will produce the maximum brake torque (MBT). Operation at MBT produces the greatest efficiency. In automobiles, the spark timing is viable allowing PPP to achieve MBT over a range of operating conditions. Our engines have fixed timing, and the rpm is variable only over a limited range due to propeller efficiency. Thus, our primary control to affect PPP is mixture. LOP mixtures operate nearer the ideal PPP which is why LOP mixtures have the lowest brake-specific fuel consumption (BSFC). 

This all is just the physics behind why LOP is more efficient than ROP.  For the pilot, it means that if you can get the same power (meaning true airspeed since that is what the power is converted to in cruise) running LOP as you can ROP, then LOP will be the more efficient choice. This is clearly apropos to turbocharged engines. For normally aspirated engines, if you can accept the airspeed generated by operating LOP at a particular altitude, then LOP is again your most efficient option. If you need to go faster and the throttle is already wide open, you will have to run ROP and accept the lower miles per gallon.

BTW, the loss in efficiency from operating off MBT by 5 degrees or so is small, which is another way of saying the optimum operating point is broad. This is why Lycoming could change the timing from 25 deg BTDC to 20 deg BTDC without claiming a power loss -- the loss was absorbed in the tolerances permitted by certification. But the engine will produce slightly more power with the 25 deg timing.

Chapter 2 of the attached document has some interesting description as well as measured engine data.

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Maximum Brake Torque Timing.pdf