IO-360 cylinder 4

[TangoWhiskey]

I noticed during my flight to Gaston's and the return flight that the #4 cylinder had a significant EGT disparity than the others in the Cessna 172S I was flying.

I was 6500' MSL, A2997, OAT 17C, FF 9.7 gph, 2600 RPM.

EGTs for cylinders 1-4, at 50 ROP, were: 1490 / 1500 / 1485 / 1350

CHTs for cylinders 1-4 were: 350 / 340 / 355 / 365

So, CHT was the hottest and EGT was 135 cooler than the rest of the cylinders. I noticed this disparity both coming and going. I haven't recorded these values before in this 172 or others, so it could be normal for all I know.

No roughness running on single mags, so I don't think it was due to one plug not firing.

Thoughts?? Is this normal for cylinder 4 in a C172S installation? And correct me if I'm wrong, but I think cylinder #4 is the one directly in front of me if I'm flying left seat, with #1 in the forward right from that same point of view.

 

[Steve]

comparing those EGTs to what I normally see on mine I'd say #4 is ROP and the rest are near peak

CHTS are only slightly higher than what I see (~30F),

Mike Busch would probably say you're ok on the CHTs and don't worry too much about the EGTs

 

[TangoWhiskey]

Thanks, Steve. Wasn't sure with #4 being so much cooler... I guess I'd need GAMI injectors, all ported to match, to get it to run at close to the same temp as the other cylinders?

 

[gismo]

Thanks, Steve. Wasn't sure with #4 being so much cooler... I guess I'd need GAMI injectors, all ported to match, to get it to run at close to the same temp as the other cylinders?

There are many things that can affect absolute EGT and I wouldn't be too concerned about a 130F delta from hottest to coldest. Much more important is the variation in degrees from peak EGT.

 

[Skylane81E]

Yeah, in reality your 360 is a group of four engines flying in loose formation

 

[TangoWhiskey]

There are many things that can affect absolute EGT and I wouldn't be too concerned about a 130F delta from hottest to coldest. Much more important is the variation in degrees from peak EGT.

Excellent. I noted peak, and enrichened the hottest cylinder to 50F rich of peak, per POH...

 

[charlie]

Excellent. I noted peak, and enrichened the hottest cylinder to 50F rich of peak, per POH...

This is correct IF the hottest cylinder is the one that peaks first. The leanest one is the one that reaches peak first and may or may not be the hottest. Charlie Melot Zephyr Engines

 

[TangoWhiskey]

This is correct IF the hottest cylinder is the one that peaks first. The leanest one is the one that reaches peak first and may or may not be the hottest. Charlie Melot Zephyr Engines

Good clarification. I was using the G1000's lean-assist mode, and it draws an open block at the top of the gauge for the cylinder that peaks first. You then note the EGT value that is displayed for that cylinder (I was seeing just over 1500 degrees). It zeros out the delta value, and you enrichen to -50F.

Am I correct about where the #4 cylinder is? From pilot's perspective, directly in front of me as seated in the airplane?

 

[Dan Thomas]

Good clarification. I was using the G1000's lean-assist mode, and it draws an open block at the top of the gauge for the cylinder that peaks first. You then note the EGT value that is displayed for that cylinder (I was seeing just over 1500 degrees). It zeros out the delta value, and you enrichen to -50F.

Am I correct about where the #4 cylinder is? From pilot's perspective, directly in front of me as seated in the airplane?

You're right about the position of number 4.

Hard to say what the difference is due to, especially if both CHT and EGT indications for #4 have wandered off at the same time. One sparkplug missing could make the readings goofy, as could a partially plugged injector. A hot spot of carbon could cause preignition and/or detonation and drive up the CHT. A leaking induction tube to #4 could lean the mix for that cylinder but that would raise the EGT, not lower it. Is the engine running smoothly otherwise?

Don't be too quick to rule out ignition (bad plug or lead). Both plugs on that engine are timed to fire at the same time, but small differences in the setting can make one plug fire just a little ahead of the other, and the pressure rise in the cylinder caused by the first plug will prevent the other plug's firing if it's already weak. Mild vibration might be felt, and if you check the mags everything will appear OK since that bad plug will be able to fire once the earlier plug is shut off. I have seen this very rarely but it can happen.

Dan

 

[Skylane81E]

You're right about the position of number 4.

Hard to say what the difference is due to, especially if both CHT and EGT indications for #4 have wandered off at the same time. One sparkplug missing could make the readings goofy, as could a partially plugged injector. A hot spot of carbon could cause preignition and/or detonation and drive up the CHT. A leaking induction tube to #4 could lean the mix for that cylinder but that would raise the EGT, not lower it. Is the engine running smoothly otherwise?

Don't be too quick to rule out ignition (bad plug or lead). Both plugs on that engine are timed to fire at the same time, but small differences in the setting can make one plug fire just a little ahead of the other, and the pressure rise in the cylinder caused by the first plug will prevent the other plug's firing if it's already weak. Mild vibration might be felt, and if you check the mags everything will appear OK since that bad plug will be able to fire once the earlier plug is shut off. I have seen this very rarely but it can happen.

Dan

Interesting, makes sence and I've filled it in the mental bank for later if I ever need it.

THANKS!

 

[Henning]

I noticed during my flight to Gaston's and the return flight that the #4 cylinder had a significant EGT disparity than the others in the Cessna 172S I was flying.

I was 6500' MSL, A2997, OAT 17C, FF 9.7 gph, 2600 RPM.

EGTs for cylinders 1-4, at 50 ROP, were: 1490 / 1500 / 1485 / 1350

CHTs for cylinders 1-4 were: 350 / 340 / 355 / 365

So, CHT was the hottest and EGT was 135 cooler than the rest of the cylinders. I noticed this disparity both coming and going. I haven't recorded these values before in this 172 or others, so it could be normal for all I know.

No roughness running on single mags, so I don't think it was due to one plug not firing.

Thoughts?? Is this normal for cylinder 4 in a C172S installation? And correct me if I'm wrong, but I think cylinder #4 is the one directly in front of me if I'm flying left seat, with #1 in the forward right from that same point of view.

1-3 are leaner than # 4. Why are you running 50*ROP? I also think #2 has gone to LOP.

 

[TangoWhiskey]

1-3 are leaner than # 4. Why are you running 50*ROP? I also think #2 has gone to LOP.

Henning, to answer the bold part, see this snippet from the POH. For the red quoted portion about #2 going LOP, how do you arrive at that conclusion?

 

[Ted]

Let's remember how these engines actually work and what happens.

The parallel valve 4-cylinders typically don't have great mixture distribution (injected is better than carbureted, but neither are great). So it is entirely possible for one cylinder to be rich of peak while another one is lean of peak (although I doubt that is happening in this case).

Equal EGTs don't do you any good. As has already been pointed out, what is relevant is where each cylinder is relative to peak. GAMIjectors will not get your EGTs in line more necessarily, what they are supposed to do is get it so each cylinder reaches peak EGT at closer to the same fuel flow. This allows you to run leaner without roughness, which improves LOP operation. One friend of mine has GAMIjectors in his Travel Air (parallel valve IO-360s) and they work particularly well. However, I've seen them work rather poorly in parallel valve IO-540s. I haven't bothered with them in my Aztec, as that plane runs LOP just fine. However I do want them for the 310 - which still does LOP fine, but could do better.

In this case, you could probably save a bit of fuel running at peak or LOP while also getting your CHTs a bit lower. But if it's a wet rental, you want best power to spend the least money, and those CHTs look very happy to me, so I'd just leave it be.

 

[TangoWhiskey]

In this case, you could probably save a bit of fuel running at peak or LOP while also getting your CHTs a bit lower. But if it's a wet rental, you want best power to spend the least money, and those CHTs look very happy to me, so I'd just leave it be.

Correct. Just below the chart I showed above is this paragraph:

Operation at peak EGT provides the best fuel economy. This results in approximately 4% greater range than shown in this POH accompanied by approximately a 3 knot decrease in speed.

And I rent. If it mattered on a long trip, to avoid a fuel stop while staying within comfortable reserves, I'd look at using peak anyway.

 

[Ted]

And I rent. If it mattered on a long trip, to avoid a fuel stop while staying within comfortable reserves, I'd look at using peak anyway.

The basic summary is that if you're renting wet, you want best power. If you're paying for all the costs yourself, economy cruise is probably going to cost you the least $/mile. If you're renting dry, then it depends on the specifics of the plane, but typically an economy cruise will still save you money because the amount of fuel you save is significant.

When I rented the Mooney (wet) I ran it at 80 ROP. When renting it dry, I still ran it at the same, but I never really calculated the difference in speed and fuel burn for peak vs. ROP.

Now, flying the Aztec and 310, I run them LOP because the fuel savings is more than enough to offset the costs, plus the engines I believe will last longer as the CHTs are cooler and I'm running lower peak pressures. When I fly the Navajo, I run it ROP because the owner tells me to, but I still run economy cruise.

 

[jesse]

Henning, to answer the bold part, see this snippet from the POH. For the red quoted portion about #2 going LOP, how do you arrive at that conclusion?

Because #2's EGT is the highest but the CHT is the lowest.

 

[Ted]

Because #2's EGT is the highest but the CHT is the lowest.

Correct. My guess is that cylinder is closest to peak. By peak EGT your CHTs have already started to drop off.

 

[TangoWhiskey]

Because #2's EGT is the highest but the CHT is the lowest.

Still don't see how #2 could be LOP, when it peaked first (at 1550) and got enrichened to bring it down to 1500.

 

[jesse]

Still don't see how #2 could be LOP, when it peaked first (at 1550) and got enrichened to bring it down to 1500.

Not sure. Not my area of expertise, was just guessing that's why Henning said it.

I would but more faith in what Ted just wrote above.

 

[Henning]

Henning, to answer the bold part, see this snippet from the POH. For the red quoted portion about #2 going LOP, how do you arrive at that conclusion?

Well, it is in a zone that is in that "conducive to detonation" area which is alsopretty close to max power. Why I think the #2 went LOP is CHT and EGT comparison to the other cylinders. The EGT was higher with the CHT being lower. The drop in CHT while keeping the lead and carbon as hot as possible on the way out is one of the key benefits to LOP operations.

 

[Ted]

The EGT of one cylinder does not necessarily mean much about the AFR in relation to other cylinders. Same goes for CHT. Your EGT readings are impacted by a lot of factors, including insertion depth of the probe. The important part there is the peak. Your CHTs are impacted by a number of factors as well, most noticeably the differences in cooling from cylinder to cylinder.

While I agree that 50 ROP is a bad operating point as a rule, given the low CHTs and the fact that this is a low power engine, detonation is not a concern on 100LL.

 

[TangoWhiskey]

The EGT of one cylinder does not necessarily mean much about the AFR in relation to other cylinders. Same goes for CHT. Your EGT readings are impacted by a lot of factors, including insertion depth of the probe. The important part there is the peak. Your CHTs are impacted by a number of factors as well, most noticeably the differences in cooling from cylinder to cylinder.

While I agree that 50 ROP is a bad operating point as a rule, given the low CHTs and the fact that this is a low power engine, detonation is not a concern on 100LL.

And then Cessna tells me that's where they recommend I run it! Like you said, maybe it's "in this application". I noticed no engine roughness; kept an eye on oil temps and pressures, the engine was happy.

 

[Ted]

And then Cessna tells me that's where they recommend I run it! Like you said, maybe it's "in this application". I noticed no engine roughness; kept an eye on oil temps and pressures, the engine was happy.

Let's think about the reasons why 50 ROP is a bad operating point:

- Highest CHTs
- Highest cylinder pressures
- Most prone to detonation

Well, your CHTs were low. So high CHTs are an issue. Yes, you'll still have the highest cylinder pressures, but on a parallel valve 360, they're always going to be pretty low no matter what you do because the engine makes such a small amount of power. And detonation on that engine is also really not a concern when running 100LL, especially with your CHTs that low.

So basically, none of the reasons apply in your case.

You won't get roughness at 50 ROP, unless you have a severely imbalanced engine. The roughness comes in once you hit LOP if your cylinders aren't properly balanced. One of my customers has an A36 Bonanza with an IO-550 in it, equipped with GAMIjectors. It's smooth right up until it stops running. The IO-520s in the 310 I fly start to get rough after about 20 LOP or so (at which point some cylinders are ROP and some are LOP).