They are not my source for advice on engine operation. They have too many OWT and many do not have a clue about the topic.
So, which EGT is hotter, 50 ROP or 50 LOP? Duh, that is like saying 1 out of 10 people are in the top 10 percent. Ten percent of what?
At 50 ROP, the CHT will be the hottest. At 50 LOP, the CHT will be up to 30 F lower.
Yes – spot on.
NCYankee wrote:
I just set the fuel flow to 12.5 GPH on my IO520BA engine. That provides me with about 65% power, almost totally independent of the MP and RPM.
That works great as long as you (1) always cruise at 65% power, and (2) have a table to hand showing MP and RPM for 65% for every altitude at which you cruise.
Silvaire wrote:
That works great as long as you (1) always cruise at 65% power, and (2) have a table to hand showing MP and RPM for 65% for every altitude at which you cruise.
Not really because LOP power is a function of fuel flow. 12.5 gph gives 186 HP which is 65% or 285 HP.
Power is always a function of fuel flow, plus air flow. You can’t burn fuel without air. EGT is a function of the proportion of fuel flow to air flow, even with excess air (i.e. lean of peak). You want the right amount of excess air to achieve optimum lean of peak EGT at a given fuel flow. Air flow is varied by changing MP and RPM.
How do you find a way around that?
Stating the obvious, but if those 12.5 gph put you on the rich side of the peak then the whole logic doesn’t work any more. The only way to get to the rich side would be to restrict the amount of air, by using a low MP (e.g. in a normally aspirated plane at altitude, or by closing the throttle, or icing up your air filter ;) ).
You wouldn’t break anything since you’ll never get a higher power ROP than LOP from your 12.5 gph, but you could further reduce fuel flow in those cases with the benefits that entails.
Which is what @Silvaire wrote, put differently, I suppose?
And conversely, if you pump more air through the engine while holding 12.5 GPM, lean of peak, at some point it has to start misfiring, no?
Yes, one cylinder would start to no longer have enough fuel to ignite the mixture, leaking the remaining fuel out to the exhaust, where it will then heat up and ignite in an uncontrolled way in the exhaust, thus backfiring. As I understand it.
Presumably misfire is what’s happening when the specific fuel consumption starts to rise again when you get way LOP (see chart on first page of thread, post #3)
Very interesting discussion. As is often the case, I think differing terminology and assumptions are half the barrier to understanding!
The best way to run LOP is wide open throttle. That gives the best possible airflow. Then pull back the mixture to 60 – 65% power (LOP fuel flow is proportional to power)
As long as you don’t exceed circa 60% power you can’t generate sufficient heat to damage the engine (on the SR20 it would equate to around 8.5 gph for approx 63%) then you are safe to run LOP with just a basic fuel flow instrument.
For operations LOP above about 65% you need full multi-cylinder engine monitoring, and balanced injectors:
