enginehistory.org

jjuutinen · Joined: 13 Jul 2003 Posts: 180

Many German aircraft had a possibility to clean spark plugs by retarding the ignition with a cockpit mounted lever. Why wasn´t this used in US&UK aircraft? I was just reading Zichek´s book on the XF8B-1 and it mentions how long range cruising tests had to be aborted due to spark plug fouling. It is also interesting that during cruising the lowest CHT´s measured was 130 deg C that was considered as overcooling. However, a P&W rep told the attendees at a wartime conference that arctic tests have revealed satisfactory, no harm for the engines, operation with CHTs down 20 deg C!

dbirch · Guest

What was the lead (TEL) content of the fuel in these engines? Rolls-Royce had to incorporate an afterheater in some of its later Merlins because at temps below 40 degrees the lead was separating from the fuel and fouling the plugs. Suggest that the P&W engines concerned didn't have much lead in their fuel.

Dave Birch

jwells · Joined: 16 Sep 2003 Posts: 57 Location: Victoria, AUSTRALIA

There is some comment on the fuel used by the German Airforce in the SAE paper by W.H.Sprenkle of Pratt & Whitney.(1941).
In doing a tear-down inspection of a DB 601A engine, the P&W engineers were able to collect enough fuel to carry out some analysis.
They found the TEL content to be 6.89cc/gallon and they reckoned there was some benzol in the petrol as well.
That's quite a bit higher than the levels used by the British.

jjuutinen · Joined: 13 Jul 2003 Posts: 180

Best analysis of German fuels can be found here: www.fischer-tropsch.org

jjuutinen · Joined: 13 Jul 2003 Posts: 180

Here is one extract from the above source:

TABLE II

RLM Specifications for B-4 and C-3 Gasolines.

Blue Grade
B-4

Green Grade
C-3

Density at 59° F

0.710-0.760

0.760-0.795

Distillation °F., IBP

104 min.

104 min.

10 percent

167 max.

176 max.

50 percent

221 max.

230 max.

90 percent

320 max.

320 max.

E.P.

338 max.

356 max.

Recovery, percent volume

98 min.

98 min.

Reid Vapor Pressure lbs.

7.0 max.

6.3 max.

Aromatic Content percent volume

25 max.

45 max.

Tetraethyl Lead Content percent volume

0.115-0.120

0.115-0.120

Ethylene Dibraoxide Content percent volume

0.050-0.053

0.050-0.053

Melting Point, °F.

-76 max

-76 max

Leaded Octane Number (Motor Method)

89 min

95 min

Note-The mixture response curve for each gasoline shall at least equal that of a standard reference fuel, supplied by the R.L.M, at all air-fuel ratios between 0.75 and 1.3

szielinski · Joined: 13 Jul 2003 Posts: 94 Location: Canberra, Australia

How would retarding the ignition clean the plugs?
Some sudden temperature variation that would crack the lead off??
_________________
“Make everything as simple as possible - but not simpler” – Einstein.

jschauer · Joined: 19 May 2004 Posts: 93 Location: Justin, Texas

The radials in the transports would clean the lead off about every hour by going to AUTO RICH and actuate the primer causing cold fuel to break the lead off of the spark plug electrodes.

szielinski · Joined: 13 Jul 2003 Posts: 94 Location: Canberra, Australia

had heard about that one, didn't think auto-rich retarded the timing, though.
_________________
“Make everything as simple as possible - but not simpler” – Einstein.

jschauer · Joined: 19 May 2004 Posts: 93 Location: Justin, Texas

It doesn't. The KC-97 had a two position switch to control spark advance. It was disabled later in the 97s career. I'm guessing here but maybe by changing the spark timing it causes the pressure to change in the cylinder enough to cause the lead to flake off?

szielinski · Joined: 13 Jul 2003 Posts: 94 Location: Canberra, Australia

The emissions system on my '81 auto has a thermocouple that applies full vacuum advance to the distributor when the engine overheats - maybe retarding the timing does the opposite??
_________________
“Make everything as simple as possible - but not simpler” – Einstein.