Practical Standalone Tuning: Step 8-A: MBT Testing
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Step 8-A: MBT Testing
07.59
| 00:00 | - For the next step of our process, we're going to essentially repeat the process of steady state tuning, only this time instead of fuel, we're of course going to be addressing the ignition timing. |
| 00:10 | And this is an area which can be a little worrying for tuners that are new to the rotary engine. |
| 00:17 | As I've already discussed through this worked example, the rotary engine is very intolerant of almost any level of detonation so we need to be very mindful of this when we are choosing our ignition timing. |
| 00:27 | This obviously begs the question well how do we go about optimising our ignition timing? And this is an area where we aren't necessarily going to want to chase after MBT or in other words the ignition timing that delivers the absolute optimal torque. |
| 00:45 | This can potentially be dangerous and it can potentially end up resulting in an engine failure. |
| 00:50 | Having said that though, we do have an advantage with the rotary engine when we are tuning our ignition timing because the rotary is not as sensitive to changes in ignition timing as a comparable piston engine. |
| 01:02 | What I mean by this is we don't tend to suffer with a sharp drop off in our engine torque. |
| 01:09 | If we're a few degrees retarded from MBT timing on a rotary engine, compared to the sort of comparable torque drop off we would see with a piston engine. |
| 01:18 | And this allows us to be a little bit more conservative with our ignition timing without leaving huge amounts of power and torque potentially on the table. |
| 01:26 | Before we actually get started with the steady state ignition tuning, we're just going to go through a quick demonstration using the torque optimisation function on our Mainline dyno to show how this works. |
| 01:38 | Let's just first of all into our Adaptronic software. |
| 01:43 | And what we're going to do is look at steady state tuning a particular cell in our ignition table and we're going to choose 3000 RPM and about 50 kPa. |
| 01:51 | At the moment you can see I've got all of the cells surrounding that cell highlighted and we're going to start by setting all of those to 10°. |
| 01:58 | The reason that I'm highlighting all of those cells is just simply so that if our engine RPM or our manifold pressure varies slightly, we're not going to end up with our results being affected by interpolation. |
| 02:12 | Let's head across to our dyno screen and we'll have a quick look at what we've got going on there. |
| 02:17 | So on our dyno screen, on the vertical axis we have our torque in pound foot. |
| 02:21 | This is being measured by the dyno itself. |
| 02:24 | On the horizontal axis we've got our ignition timing coming in through from the ECU via a CAN communication bus. |
| 02:32 | So what we're going to do is start with our 10° timing, we're going to establish steady state conditions and then we're going to ramp the timing up from 10° through to about 45° in one degree increments per second. |
| 02:44 | The dyno's then going to plot the relationship between power and torque so we can really understand what's going on inside the combustion chamber. |
| 02:52 | So let's get our engine up and running and we'll start our test. |
| 02:56 | Alright we're established here at 50 kPa, 3000 RPM. |
| 02:59 | And I just want to mention as well before we start the test, we can see that currently our EGT's sitting at about 950°C so a result of that retarded timing. |
| 03:09 | So we'll get our test started, we'll clear our existing results and now we're going to just start stepping that timing through in one degree per second increments and we'll see the dyno draws us a nice plot of that relationship between our ignition timing and our engine torque. |
| 03:28 | Even now with our timing coming up through 20°, if we look on our laptop screen, we can see our EGT's already dropped from 950 where we started down to about 920, 915°. |
| 03:39 | Just showing as we advance that timing, our EGT does drop. |
| 03:45 | So let's continue with our test. |
| 04:20 | Alright we're through to 45° there so we're just going to back off the throttle, just before we do that though I'll just point out that now, our exhaust gas temperature has dropped down to 940° so 110°C drop in our EGT just based on advancing that timing. |
| 04:37 | Alright let's have a look at the graph on our dyno screen and we'll see what we can learn. |
| 04:43 | First of all, important to note here, the dyno has shown us our point where we have made peak torque, MBT. |
| 04:50 | You can see there, 32° for that particular cell, given us 144 pound foot of torque. |
| 04:56 | Interestingly at the start of that test we were at 90 so we've picked up about 50 pound foot so basically 50% give or take gain in our torque by optimising our timing. |
| 05:07 | However if we look at this broadly, what we can also see is that we saw a reasonably sharp increase from 10° through to about 25°. |
| 05:16 | Then from about 25° all the way through to about 45° we've only really seen a change of about five pound foot of torque. |
| 05:24 | So when we're looking at a change that's that small, it's not really that significant. |
| 05:29 | So my aim when I am optimising the ignition timing on the rotary engine, we're not aiming for MBT timing, particularly under boost conditions this can be dangerous. |
| 05:39 | So instead what we're aiming for is to be somewhere to the left of our MBT, at the point where we're within probably about five pound foot or a few percent of our peak torque value. |
| 05:52 | Now of course, that begs the question, how do we know how close we are to MBT if we haven't tuned to MBT? Fair enough, what we're looking for here is the trend. |
| 06:02 | So what we can see is as we come up through 10° through to about 20°, we've got a pretty sharp increase, from 10° through to 20° we've gone from 90 pound foot to 130 pound foot so quite a sharp increase. |
| 06:15 | From 20° through to 25° we've only picked up around about five to eight pound foot of torque. |
| 06:22 | So it's at this point where we start to see the gains per degree of timing start to drop away and become relatively insignificant. |
| 06:29 | This is about the point where I will tend to set my ignition timing. |
| 06:34 | This does become more important when we are however looking at our timing on boost. |
| 06:39 | Under vacuum, which is exactly where we were there as you can see, even with 45° we haven't got our engine suffering from knock, even though we are past MBT. |
| 06:50 | So a little bit of common sense applies here, the more load we've got, the more cautious we want to be. |
| 06:57 | Under vacuum conditions, under cruise conditions, we can be at or very close to MBT with relative immunity from knock. |
| 07:06 | The advantage for this as well is as we've just seen, it will help our exhaust gas temperatures. |
| 07:11 | Now on that note as well, you'll remember from the last module when we were doing our steady state fuel tuning, I purposefully richened the air/fuel ratio targets under the cruise conditions above 4000 RPM to help combat that higher EGT. |
| 07:26 | But it is a combination there, the lean target of lambda one but coupled also with the fact that at that point we were running retarded ignition timing, those both add up to cause those high exhaust gas temperatures. |
| 07:40 | Now that we've got a bit of an understanding of what's going on in the rotary engine, the relationship between our ignition timing and our torque and how that's affected, what we're actually aiming for, we can move on and actually begin the steady state ignition tuning. |
