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Understanding AFR: Effect on Exhaust Gas Temperature (EGT)

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Effect on Exhaust Gas Temperature (EGT)


00:00 - Through the course so far we've discussed the concept of how the air-fuel ratio affects our combustion temperature.
00:06 And while we can't measure combustion temperature directly on our Chassis Dyno, what we can measure is a byproduct of that combustion, which is the exhaust gas temperature.
00:16 While the exhaust gas temperature is affected by a number of other parameters, all things being equal if we increase the combustion temperature, we'd expect to see the exhaust gas temperature also increase, and vice versa when we drop the combustion temperature.
00:33 So we're going to perform a couple of demonstrations here now.
00:37 We're using, for this demonstration, a Mazda FD RX-7, with a turbocharged 13B engine that's controlled by a Link G4 Plus so we can quickly and easily demonstrate the effect of altering our air-fuel ratio.
00:52 For the first test what we're going to do is run the engine in steady state at about 2500 RPM, and we're going to watch how the exhaust gas temperature is affected as we change the air-fuel ratio or lambda.
01:07 Let's get the engine running now in fourth gear at 2500 RPM, and we'll perform our first steady state test.
01:14 (engine hums) While we allow the engine to stabilize on the Dyno, let's have a look at what we do have on the Dyno screen.
01:30 And you'll be able to see here we have a few parameters that are of interest to us at this point.
01:36 So we have our derived RPM, our engine RPM, we have our manifold pressure, our lambda, and of course our exhaust gas temperature.
01:46 Now, one thing that's important to note when you're measuring exhaust gas temperature, it is quite a slow sensor to respond, so it responds and stabilizes much slower than the likes of our lambda reading.
02:01 So you can see now we're at a relatively steady state, and we can see we've got an exhaust gas temperature reading of around about 800 degrees centigrade, and we're running at around about 0.95 lambda.
02:14 So what I'm going to do is just richen the air-fuel ratio, and we'll see how that affects our exhaust gas temperature.
02:23 So I've just added some fuel into the main fuel table, and you can see we've gone from 0.95 to 0.81 lambda.
02:33 You can see that our exhaust gas temperature has dropped from about the 800, 810 degrees that we were running now down to 770.
02:42 So I've dropped somewhere around about 30, maybe 35 degrees centigrade from richening the air-fuel ratio.
02:50 Let's see what happens when we lean the air-fuel ratio out.
02:54 Remember, we were running at around about 0.95, 0.96.
03:01 I've now leaned the air-fuel ratio out to lambda one, and you can see our exhaust gas temperature has climbed, we're sitting now at 820, 830 degrees centigrade.
03:13 And as I said, it does take a little bit of time for that exhaust gas temperature to stabilize.
03:19 And it does move around a little, you can see we're now heading about 835, 840 degrees centigrade.
03:25 Now if I remove some more fuel again and we go leaner, we can see that with lambda 1.1, our exhaust gas temperature has actually climbed further, and we're sitting now between 850, 860 degrees centigrade.
03:47 Now, if we continue to lean out the air-fuel ratio, what we would find is that the exhaust gas temperature doesn't climb indefinitely.
03:57 What we'd find is that the exhaust gas temperature will climb, it will peak, and then it will actually drop off.
04:02 And all of that is related to the amount of fuel and air being combusted inside the engine, and obviously if we don't have enough fuel to properly combust all of the available oxygen, we're not going to get complete combustion, and hence the exhaust gas temperature and combustion temperature will both drop.
04:22 Okay, that's a look at what happens with the engine in steady state.
04:27 Now we're going to perform a similar test, and we're going to do a couple of ramp runs, and again see how the exhaust gas temperature is affected during a wide-open throttle ramp run.
04:38 So let's so that now.
04:39 (engine hums) Okay, so that's our first test complete there.
05:11 And we can see that in terms of power at the rear wheels, we've ended up with 209, 210 kilowatts, or 281 horsepower.
05:22 Now let's have a look at the results we've got there in a little bit more detail, though.
05:27 First of all on the top we've got our air-fuel ratio, and you can see that during this run we've ended up running through at around about 0.79, 0.80 lambda, which for a turbocharged rotary is quite a lean air-fuel ratio.
05:44 We'd normally run richer, and the reason I've chosen to run this a little leaner is just to demonstrate the effect of the air-fuel ratio on our exhaust gas temperature.
05:54 Below we've got our exhaust gas temperature, of course, and at the start of the run you can see we're sitting at about 600 degrees centigrade.
06:02 And by the end of the run we've actually peaked, you can see here, at 1001 degrees centigrade.
06:10 That's an exceptionally high exhaust gas temperature, and potentially dangerous to some of the components, particularly the turbocharger as well.
06:20 So what we're going to do now is we're going to drop into our laptop software and simply for this demonstration, all I'm going to do is highlight the entire wide-open throttle running area, and we're going to add 4% to the fuel just to richen the entire air-fuel ratio.
06:37 We'll save this particular run here, and we'll call it exhaust gas temperature, EGT1, just so we can reference it a little bit later.
06:46 And we'll have a look at our second run and see what our results are.
06:50 (engine hums) Okay, that's our second run complete there with our richer air-fuel ratio.
07:18 So let's have a look at the results we got.
07:21 So for a start we can see our power's almost identical.
07:24 We've got 207.8 kilowatts, or 278.7 horsepower.
07:29 You can see that the original run is still displayed in purple, and our current run, our second run, with the richer air-fuel ratio is overlaid almost directly on top of that.
07:42 Now, you can see first of all before we look at the exhaust gas temperature, let's talk about our lambda air-fuel ratio.
07:48 You can see our original run here in the dotted blue, and you can then see our new run here demonstrated in red, and you can see that throughout the run that additional 4% of fuel has resulted in a richer air-fuel ratio.
08:05 During the main part of the run where we're at wide-open throttle, full boost, we're running through at around about 0.75 to 0.6.
08:15 Remember our first run was around about 0.79 to 0.80.
08:19 So we've moved around about 5% rich, which is what we'd expect.
08:24 And let's look at our exhaust gas temperature.
08:26 At the start of the run, we actually had a slightly higher exhaust gas temperature.
08:32 We're sitting at around about 630, 640 degrees centigrade, and our first run was slightly cooler than that.
08:41 That starting point is probably simply due to the fact that the engine's actually been running now, it's already got some heat into the combustion chamber and into the exhaust system.
08:50 So that's to be expected.
08:52 The more interesting point, though, is you can see as we move through the run, particularly right at the top end, remember we ended up with 1001 degrees centigrade on our first run, our peak exhaust gas temperature now is 947, so 950 degrees centigrade.
09:11 So that richer air-fuel ratio has dropped a touch over 50 degrees out of our exhaust gas temperature.
09:19 And what we'll do is have a look at those runs overlaid with each other.
09:24 I'll just save that run.
09:26 And we can see we have, first of all, our power being displayed here.
09:32 Our original run, our first run with the leaner air-fuel ratio is in red, and our second run that we've just completed is in yellow.
09:41 Below that we have our exhaust gas temperature.
09:44 Remember, as I said we actually started the run here with a slightly higher exhaust gas temperature.
09:51 Despite that higher exhaust gas temperature you can see by the end of the run the exhaust gas temperature is significantly or measurably lower in our second run, and again at the top of the run we have 950, 947 versus 1001 degrees centigrade.
10:11 To the right we have our two lambda plots, and then below that we have our boost pressure.
10:17 And you can see that we were running for both of those pulls, we were running at around about 15 PSI of boost.
10:25 So that demonstration hopefully will give you a little bit more insight into how the air-fuel ratio affects the exhaust gas temperature, which obviously is a byproduct of our combustion temperature.

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