Hi All,
Here's the final installment. My "take on all this is:
(1) 100LL has FOUR times the lead in it that 80/87 did.
(2) Lead fouling can be "controlled" with aggressive leaning techniques and proper practice.
(3) Unleaded auto fuel has no lead and so is much more like 80/87 than 100LL (no fouling).
(3) Our carburetor/induction system does not "normally" provide a balanced fuel/air mixture to each cylinder.
(4) Intelligent use of carburetor heat can increase the uniformity of the fuel/air mixture between cylinders.
(5) If you have a 4-probe CHT installation with which to implement (4), our engines can run safely LOP
(lean of peak) for considerably increased fuel economy (and range) without adverse consequence.
An auto fuel STC and the necessary instrument and probes would seem to be an intelligent investment for the pilot
willing to accept their responsibility to fully understand and appropriately use each.
Regards,
William R. Bayne
<____|-(o)-|____>
(Copyright 2004)
Begin forwarded message:
From: Walter Atkinson <[EMAIL PROTECTED]>--
Date: December 2, 2004 10:29:58 AM CST
To: "William R. Bayne" <[EMAIL PROTECTED]>
Subject: Re: [COUPERS-TECH] Mixture/EGT
The differences are interesting. The BTU output of the two fuels is very close to the same. Auto fuel has a very slightly lower BTU output than 100LL. If I run 100LL in one tank and switch to another tank with auto fuel in my Twin Beech, I see no measurable change in IAS. This indicates that the HP to the prop is the same. The BTU difference is masked by the change in the ThetaPP and an improved mechanical advantage on the connecting rod and crank. The CHTs run higher on auto fuel which indicates a smaller ThetaPP just like we measure on the test stand.
On the smaller engines with original factory set timing, running either fuel should be fine.
If you find these things interesting, you might consider joining us for a weekend! We will teach you more about piston engine operation than you can imagine possible. Check the website at <http://www.advancedpilot.com/explore_001.htm>.
Walter
On Dec 2, 2004, at 10:15 AM, William R. Bayne wrote:
Hi Again,
The information is amazing. This is all new, at least to me.
If I understand this correctly in the overall, it would seem that STC spec auto fuel is more in line with the design of these engines, and would actually have an edge in efficiency as compared to burning 100LL with insufficient compression and effectively retarded timing. Is that correct?
Under the stated conditions, would you venture a guess as to the theoretical efficiency of burning one versus the other (properly leaned)?
Regards,
WRB
--
Begin forwarded message:
From: Walter Atkinson <[EMAIL PROTECTED]>
Date: December 1, 2004 3:57:41 PM CST
To: "William R. Bayne" <[EMAIL PROTECTED]>
Subject: Re: [COUPERS-TECH] Mixture/EGT
I confess further elaboration as to the "latency period of the fuel" is needed, how field variables would be ascertained, and resulting peak pressure pulse (as compared to that for 100LL). As I understand, compensation would be via adjustment of engine timing.
The latency period is the time it takes for the flame front to become organized after the spark event occurs. The shorter it is, the more the timing needs to be retarded for the peak pressure pulse to remain in the optimal location with respect to TDC.
Continental specified use of 80/87 octane avgas in their C-75/85 engines (with a 6.3:1 compression ratios) and their C-90/0-200 (with 7.0:1 compression ratios). What would be an appropriate timing "adjustment", in your opinion, that would best represent a "real world" timing change from "spec" appropriate to an owner's alternative use of "STC-standard" unleaded autofuel in these engines?
If the engine was designed for the lower octane fuel, the timing set-up would have been for the lower octane fuels. No adjustment is required. When using 100LL in this situation, the EFFECTIVE timing will be retarded and the result will be less stress on the cylinders. I would not plan to alter the timing from one fuel to the other.
Walter
