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Practical Diesel Tuning: Making Boost Pressure Changes

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Making Boost Pressure Changes

08.41

00:00 - In the last module, we've talked about the turbocharger and the fact that we're likely to want to increase the boost pressure as part of our tuning strategy.
00:06 However we haven't really delved into how we can do this, which is the topic of this module.
00:10 While different ECUs may employ a variety of different boost control strategies, the general concept remains pretty consistent so if we understand this then we should be able to apply this knowledge to pretty much any tuning job.
00:21 The first task is to understand how the boost control system actually works.
00:24 And in this regard there are two ways we see boost control applied to diesel engines.
00:28 The first option is to incorporate a wastegate into the exhaust side of the turbocharger which is used to bypass exhaust energy around the turbine wheel.
00:36 The wastegate is opened by supplying it with the pressurised air and when it's open, less exhaust gas energy is supplied to the turbine wheel which in turn reduces its rotational speed.
00:45 Since the exhaust wheel and compressor wheel are attached on a common shaft, this of course slows down the compressor wheel.
00:50 Since boost pressure is a function of compressor wheel speed, this reduces the boost.
00:55 Typically an ECU controlled solenoid will control the supply of air pressure to the wastegate and by pulse width modulating this valve, the ECU can quite accurately control the opening of the wastegate and hence the boost pressure.
01:07 The more common technique used to control boost in modern diesel engines is the variable geometry turbocharger or VGT for short.
01:13 This may also be referred to as the variable nozzle turbine or VNT turbocharger.
01:18 They're very similar.
01:20 This type of turbocharger uses a set of vanes in the exhaust housing that can be moved to affect the way the turbo operates.
01:26 By closing these vanes the turbocharger behaves like a small turbo, offering fast response and minimum lag.
01:32 But at higher RPM the vanes can be open to provide more exhaust gas flow and less restriction.
01:37 With this type of turbocharger, the vane position is altered to control the exhaust gas energy being supplied to the turbine wheel and in turn controls the boost pressure.
01:44 The other aspect with boost control on a turbo diesel engine is that the exhaust gas energy is affected dramatically by the volume of fuel being delivered to the engine.
01:52 As we increase the fuel delivery into the engine, we create a larger combustion event which creates more exhaust gas energy which in turn drives the turbocharger harder.
02:01 What I mean here is that if we maintain a fixed vane position or a fixed wastegate duty cycle on a wastegate equipped engine, this will result in a boost pressure increase as we deliver more fuel.
02:12 For this reason, we'll see the boost control tables in the ECU will reference fuel quantity on one axis.
02:17 Regardless whether you're dealing with a wastegate or VGT turbo, the ECU will have a set of tables that define the base vane position or wastegate duty cycles required to achieve our desired boost.
02:27 These tables are also often referred to as feed forward tables.
02:31 These tables alone however aren't enough for accurate boost control as operating conditions can change.
02:37 So a closed loop boost control strategy is also used by most ECUs.
02:41 This looks at the actual boost pressure and compares it to our target and at any time there's a discrepancy, the ECU can make changes to the wastegate duty cycle or vane position to drive the boost towards the target.
02:52 This closed loop control strategy's referred to as a PID control which is an acronym that references the three components or gains used in the control algorithm.
03:00 These being proportional, integral and derivative gains.
03:04 Tuning a PID control algorithm in itself is a fairly complex task.
03:08 But the good news for us is that unless we're changing the turbocharger or making dramatic mechanical changes to the turbo configuration, the stock PID settings should work pretty well for us.
03:18 If you want to learn more about PID boost control and gain an in depth understanding of how to tune the PID gains, then our boost control tuning course is a perfect way to expand your knowledge.
03:28 With a basic understanding of diesel boost control out of the way, let's look at the typical tables we have access to in the ECU and we'll discuss what changes we may want to make.
03:36 The first place we will start is with our desired boost target table.
03:40 These are a three dimensional table relative to fuel quantity and RPM.
03:44 To make matters a little more complex, we'll typically find several tables here that are based on altitude and operating mode.
03:49 Often, we may not necessarily know what specific table the ECU is accessing here so it's good practice to apply our desired change across all the tables similarly.
03:59 This doesn't mean that we will copy and paste the same values to all the tables, what I mean is that we will make the same percentage change to each individual table.
04:07 We do need to be a little mindful here with regard to altitude as well.
04:11 It might seem a little unusual for the manufacturer to reference altitude and change the boost targets to suit but the reason for this is that altitude or more correctly atmospheric pressure, affects the pressure ratio the turbocharger is working at.
04:24 To illustrate this, let's assume we're operating at sea level and for simplicity we'll say atmospheric pressure is 100 kPa.
04:31 And let's say that the boost pressure in the inlet manifold of the engine is 250 kPa absolute.
04:37 We can now work out the pressure ratio which is simply the pressure at the compressor outlet divided by the pressure at the inlet.
04:42 This is 250 divided by 100, giving us a pressure ratio of 2.5.
04:47 We would plot this on our compressor map from earlier to get an idea of where we're likely to be running in the efficiency map.
04:53 Now let's assume we drive to the top of Pike's Peak at 14,000 feet and find out atmospheric pressure has now dropped to 75 kPa.
05:02 If we're still maintaining 250 kPa pressure in the manifold, then our pressure ratio has now changed.
05:09 If we divide 250 by 75 we get a pressure ratio of 3.3 and if we plot this on our map, we can see it has a dramatic effect on where abouts we're operating as far as efficiency islands on the compressor map.
05:20 In reality this is not going to be an issue for people at sea level but it's worth considering if you're expecting to see large changes or travelling at high altitude.
05:28 Essentially you may need to trim you boost targets down at higher altitudes in order to make sure you're still operating in an area of the compressor map where it's efficient.
05:36 Since the stock mapping for the boost targets will take this into account, our strategy of applying a percentage increase across all these individual maps is still valid.
05:44 We also need to consider that we don't need to change boost targets under all operating conditions.
05:49 Really we only want to produce more boost pressure when we're wanting to increase performance which is typically in the moderate to high fuel flow regions where the driver is at or close to full throttle.
05:58 We can easily find out where we are operating by logging our fuel quantity versus RPM during a full throttle ramp run on a dyno or acceleration pull on the road.
06:08 From here we can see what areas of these boost target maps we're operating in under these conditions and only make changes where required.
06:16 A little common sense is required here and it's a good idea to smooth or blend these tables so they don't contain irregular steps.
06:22 If the software you're using offers an interpolation or smoothing function, this can help speed up those processes.
06:27 Once we've made the required changes to our boost targets we also need to make sure that we're not exceeding any boost limits that may be incorporated into the factory ECU.
06:35 We want to retain boost limits for safety however they need to be raised to provide sufficient headroom above your new targets.
06:42 A good guide here is to see how far above the current boost set point the limit is set and retain a similar margin of safety for your revised tables.
06:49 You also may need to apply changes to the torque management tables too, which we'll cover in a separate module.
06:55 At this point, we're ready to actually test and see how these changes have affected our boost pressure.
07:00 Initially I'd suggest leaving your base vane position or wastegate duty cycle tables untouched and just testing the response of the system.
07:07 It's quite likely that without changing these tables, your boost response will be a little slow to hit target.
07:13 Or you may not quite reach the target.
07:15 If this is the case, then we'll need to make some adjustments to those tables.
07:18 If you have the ability to datalog the vane position or wastegate duty cycle, then this would be a big help, correctly filling out any necessary changes in those tables.
07:26 What we want to do is log the vane position or wastegate duty cycle along with our manifold pressure and RPM.
07:32 We're comparing our boost pressure to our target and if we are slow to reach the target, or are not hitting the target, we'll need to raise the values in our base position tables.
07:40 Even if you're hitting your target boost level, It's a good idea to check the actual logged vane position or duty cycle and compare this to your base table values.
07:48 It's quite possible that there will be a reasonably discrepancy which means you're relying on the closed loop control to pick up the pieces and get us to the target.
07:56 The closer we have our base table values the less work the closed loop control needs to do and the better our overall boost control will be.
08:04 Since we're likely to have a series of base vane position or duty cycle tables, we'll apply the same logic to making similar magnitude changes to all the tables.
08:12 The last consideration is that there may also be a set of tables that define maximum vane position or wastegate duty cycle.
08:18 If you're commanding more boost than stock then it's possible that you may need to modify these tables.
08:24 The giveaway here would be your boost pressure's not reaching your target in a vane position or wastegate duty cycle that remains steady rather than increasing.