Interesting EGT.....

[Brantel]

Originally Posted by humptybump

Noob here.

I now know what "Dow #4" is.

What does "burp the plug wire" mean?

Thanks.

Let the air out of the cap so it does not try to push the whole mess off the plug or coil post.

[scsmith]

Originally Posted by DanH

Typical complete combustion (90 to 100% mass fraction burned) for a relatively non-turbulent four stroke (read "Lycoming") would be about 40 ATDC, well before exhaust valve opening. The delay due to single plug ignition is unlikely to be an additional 60 to 80 degrees.

I agree. But the delay IS why the EGT is higher was my point.

[1:1 Scale]

Originally Posted by scsmith

I agree. But the delay IS why the EGT is higher was my point.

Exactly. Anyone watch their EGT's when they do a mag check?

[lr172]

Originally Posted by RV6_flyer

That is NOT what I was taught. The Lycoming cylinder diameter is so large that complete combustion does not occur when only one spark is used for ignition.

If this were the case, you would be dumping unburnt fuel in the exhaust and the next combustion cycle would ignite it, creating after-fires (i.e. pops in your exhaust). This doesn't happen when running on one mag, proving that the charge is fully combusting.

Larry

[scsmith]

Originally Posted by lr172

If this were the case, you would be dumping unburnt fuel in the exhaust and the next combustion cycle would ignite it, creating after-fires (i.e. pops in your exhaust). This doesn't happen when running on one mag, proving that the charge is fully combusting.

Larry

Larry, it kind of does, if you are at altitude, and way LOP, and you shut off a mag. It really pops and caughs and bangs. How much is mis-fires and how much is after-fires, I don't know.

[lr172]

Originally Posted by scsmith

Larry, it kind of does, if you are at altitude, and way LOP, and you shut off a mag. It really pops and caughs and bangs. How much is mis-fires and how much is after-fires, I don't know.

but it doesn't do that on the ground during a mag check. The pops and coughs can be lots of different things. I drop out a mag in the air all of the time. I have never heard an afterfire, disproving the cyl too big theory. If I drop my mag and leave the EI running, you can't even tell that I did it. This is because the timing on the EI is much more advanced and keeps the peak pressure point closer to optimum than with only the mag.

popping and coughing when very lean of peak is altogther a different animal and is not related to this theory that the cylinder is too big for one plug. The leaner you go, the harder it is to light the charge and ignition weakness become more apparent. Altitude also hurts a mags performance. THis is why they have pressurized mags for high flying planes.
Larry

[RV10inOz]

Guys, let me clear up WHY the EGT goes higher with one plug, or whole magneto is offline.

As DanH has already pointed out combustion is over way before the valve opens, and this is 100% true.

The answer lies in Boyles law, Pv=nRT which means that when you expand a gas from a small volume and high pressure to a lower pressure you get a temperature drop. Think a BBQ gas bottle being opened, it gets frosty!

So with one plug not firing, the peak pressure in the cylinder happens later, and the piston has travelled further down the cylinder, thus in a larger volume. Being in a larger volume the pressure is less.

Given the atmosphere where the exhaust is going remains the same, we have a lesser change in pressure and less expansion. If we balance up the equation the only other variable is T and with less expansion we have to have less temperature drop. Thus the EGT is higher!

So there ya go! Hope that helps.

[Ironflight]

I knew this would generate a lively discussion!

[DanH]

Originally Posted by RV10inOz

Guys, let me clear up WHY the EGT goes higher with one plug, or whole magneto is offline.

As DanH has already pointed out combustion is over way before the valve opens, and this is 100% true.

The answer lies in Boyles law, Pv=nRT which means that when you expand a gas from a small volume and high pressure to a lower pressure you get a temperature drop. Think a BBQ gas bottle being opened, it gets frosty!

So with one plug not firing, the peak pressure in the cylinder happens later, and the piston has travelled further down the cylinder, thus in a larger volume. Being in a larger volume the pressure is less.

Given the atmosphere where the exhaust is going remains the same, we have a lesser change in pressure and less expansion. If we balance up the equation the only other variable is T and with less expansion we have to have less temperature drop. Thus the EGT is higher!

So there ya go! Hope that helps.

Certainly a function of basic gas law. But...

This is a p-V (pressure -volume) diagram for a research engine at 1200 RPM, borrowed from Taylor's Internal Combustion... The X-axis is cylinder volume, with TDC on the left. The Y-axis is cylinder pressure. I've added some notes; the only variable is ignition timing.





Note that when ignition is delayed, peak pressure is indeed reduced, as David notes. However, pressure later in the stroke, around exhaust valve opening, is higher. Cylinder volume is the same at exhaust valve opening for any ignition timing, so higher pressure simply means higher gas temperature.

Pressure and temperature are proportional when temperature is expressed in degrees Kelvin. Play with the numbers. In the first post, Paul wrote that his errant EGT jumped almost 100 degrees. Assume the jump was 1400F to 1500F, or from 1033K to 1083K. The required pressure increase at valve opening is only about 5%.

[Pat Stewart]

Great post, last week I had the exact same problem Paul described only I did not read this thread before attempting to correct the problem. I spent yesterday cleaning all the injectors, seemed likely to me that the cylinder was getting too lean so there must being something in the injector. Today after the dinner and everyone down for a nap I went out back to the hangar and rolled the airplane out for an engine run. So I started the engine and after it decided to stop shaking from purging all the fuel system I let it warm up. This engine is a 100 SNEW IO360. CHTs have always been very good, EGTs always great with low spreads. After warm up at idle CHTs normal and all very tightly grouped. EGTs spread out but consistent with low idle warmup. Unfortunately after bringing the RPMs up to 1600 I was able to duplicate the problem again. EGTs normal with reasonable grouping but when I leaned it up number 1 took off again, adding mixture would bring it back down. CHTs remained good and tightly grouped at all times. After reading this thread starting to think I should change the plugs on that cylinder.