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Practical Diesel Tuning: Fuel Quantity

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Fuel Quantity

10.28

00:00 - When it comes to tuning diesel engines, and improving their performance, there are a range of concepts that we're going to need to understand.
00:06 And in this set of modules, we'll cover the major concepts we feel are important.
00:11 Since one of the key changes we need to make in order to improve diesel engine performance is to add more fuel, this is arguably the most important topic.
00:18 There are however a couple of factors that dictate the amount of fuel that's delivered to the engine.
00:23 So we'll break this up into injector pulse width and fuel pressure.
00:26 Let's start by discussing the injector pulse width which is simply the amount of time that the fuel injectors are energised by the ECU.
00:33 When the injector is open it delivers multiple jets of fuel atomised at high pressure directly into the cylinder.
00:39 With the diesel engine, the timing of when this injection event occurs as well as the duration of the injection are both critical to performance in the engine.
00:47 To make matters a little more complex, modern diesel engines are likely to break the fuel delivery up into multiple injection events that span the engine cycle.
00:54 These include pilot injection events, a main injection event where the majority of fuel is delivered and potentially a post injection event which happens late in the power stroke.
01:03 We will deal with the pilot injection separately but our primary focus here is the main injection event.
01:09 Since the injection events in the diesel engine are so critical to the engine performance and are much shorter than what we may see in a petrol engine, the injection duration is typically represented in microseconds instead of milliseconds which you may be more familiar with.
01:22 This simply gives us a little more precision with which to work and it's easy enough to understand if you just keep in mind that injection duration of 1000 microseconds is the same as one millisecond.
01:31 Within the ECU there are typically two tables that are of interest to us here.
01:35 The first of these tables will define the required injector pulse width in order to deliver a specific quantity of fuel.
01:41 This will be a three dimensional table with fuel quantity on one axis and fuel pressure on the other.
01:46 I'd like you think of this table as the injector characterisation table as this is an aspect of the fuel injector's design.
01:53 This table is developed by the manufacturer by physically testing the injector at a variety of pressures and pulse widths.
01:58 A common technique when it comes to tuning diesel engines is to simply head to this table and increase the values by 10 - 20% which will result in the ECU delivering a longer pulse width to the injectors and hence more fuel will be delivered.
02:12 On face value, this might seem like a good idea however the reality is that doing this skews the ECU's operation and may result in some unintended outcomes like loss of smoke control, loss of shift quality and misrepresented miles per gallon statistics on the driver information centre.
02:26 In particular on more modern diesel ECUs, this can result in issues with the torque calculation as the engine will be producing more torque than the ECU has calculated.
02:34 Which could lead to an inappropriate transmission line pressure setting which leads to poor shifting.
02:39 Furthermore, as the actual torque diverges from the anticipated torque, the ECU may implement a derate or limp home mode as it tries to protect the vehicle from what it perceives as loss of throttle control.
02:50 Manipulating the pulse width table will also affect the calculations the ECU does with regard to lambda limiting and can quickly accelerate soot accumulation of the DPF if your engine's fitted with one of these.
03:00 Ideally the only time we touch this table is when we make changes to the injectors.
03:04 If we've actually fitted a new larger set of injectors to the engine then we'd want to change the table vales to reflect those, the actual fuel flow of those injectors.
03:14 As we'll find out though, when it comes to diesel tuning, we don't always work in an ideal world.
03:19 Sometimes we're going to end up manipulating this injector pulse width table, it's just unavoidable.
03:24 The second table we want to focus on or more like set of tables convert driver demand into requested fuel volume.
03:30 This can be a simple table using the accelerator pedal position versus RPM as an axis or requested torque versus RPM as an axis.
03:37 This table simply defines the specific fuel quantity to be delivered for each point on the table.
03:43 For example, if we look at these tables on a 2018 L5P engine control module, we can see that there are a variety of torque to fuel mass tables based on altitude and operating mode.
03:53 If we look at the table for low altitude mode one, we can see that with a requested torque value of 1000 newton metres and an RPM of 2800, that the table is requesting a fuel volume of 86.5 mm³.
04:05 To convert this into a pulse width that is delivered to the injector, we need to head across to the main injector pulse width table which we can see here.
04:13 To actually decide on the final pulse width, we need to know what the actual fuel pressure is at this operating point.
04:19 We're going to deal with fuel pressure in an upcoming module so for now, let's assume we're operating at a fuel pressure of 180 mPa.
04:26 We can see that we're actually operating somewhere between two of the cells on this table since we don't have a fuel volume break point at 86.5 mm³.
04:33 We can see that at 75 mm³ the pulse width would be 1055 microseconds.
04:39 And at 95 mm³ it would be 1274 microseconds.
04:43 We can apply a little simple math here and interpolate the result the same way the ECU does.
04:48 In this instance, let's start by taking the difference between these two cells which is 1274 minus 1055.
04:54 This gives us an answer of 219 microseconds.
04:57 Now let's look at the difference between the two break points which is 95 minus 75, gives us 20 mm³.
05:03 This means that our two cells span a range of 20 mm³ and a difference of 219 microseconds.
05:09 If we divide 219 by 20, we get an answer of 10.95 which means that each one step of mm³ in fuel quantity requires an additional 10.95 microseconds of a pulse width of that given fuel pressure.
05:23 Remember that our requested fuel volume was 86.5 mm³ and we need to find out how much we need to increase pulse width to get this fuel volume compared to our first break point which was 75 mm³.
05:36 To do this, we can subtract 75 from 86.5, gives us 11.5 mm³ greater than our first break point.
05:44 Remember for each additional mm³ of fuel needed, we need to increase our injector pulse width by 10.95 microseconds.
05:51 So if we multiple 10.95 by 11.5, we get 125.93 microseconds.
05:58 To get the final pulse width, we need to now add this pulse width to the required fuel delivered at 75 mm³ which remember was So we get a total of 1180.93 microseconds.
06:11 And this would just be rounded to 1181.
06:14 Now that we understand what these tables mean and how they interact, we can talk about what we need to do to them.
06:19 The majority of these tables can actually be left alone as we really want to focus our efforts on the part of the table we're operating in when the driver's demanding high power.
06:27 If we're dealing with a table where accelerator pedal position is the load axis then this is quite straightforward and we can make our changes only under moderate to high pedal inputs.
06:36 On face value, you might think that we only need to change the commanded fuelling under wide open throttle.
06:42 However by increasing fuelling at part throttle, we can have the effect of making the engine produce more torque at part throttle and this can have a significant impact on the feel and response of the engine.
06:52 This of course comes down in large part to personal preference so some experimentation is required to tailor the fuelling curve to your liking.
07:00 Regardless of where you make those changes in this table, it's important to make sure the end result is smooth, otherwise you'll risk introducing driveability problems.
07:08 For example it might be that you want 20% of the commanded fuel above 70% throttle, if you make only that change, you're going to end up with a large step in fuelling as you move from 60% throttle to 70% throttle.
07:21 And this will result in a large torque increase over a narrow throttle range.
07:25 For this reason we always want to smooth or blend our result to avoid any sudden jumps.
07:30 It's usually easiest to see if you've got any problem areas by viewing the table graphically rather than numerically as these sort of issues become more obvious.
07:39 The majority of modern diesel ECUs are torque based which we'll cover in more detail in a later module.
07:44 However this adds one more layer of complexity because in order to know where you make your fuelling changes, we need to know what the torque request is for a given accelerator pedal position and engine RPM.
07:54 We'll cover this process in more detail shortly however once we understand what the requested torque is, the process of making modifications to fuel delivery is largely the same.
08:03 As you've seen here already, it's common for there to be multiple maps based on operating mode, as well as altitude.
08:09 This can be daunting, unless we can be very specific about exactly which map is being referenced, it can be hard to know where to make your changes.
08:17 Even if we know for example which map is being used while we're on a dyno, it's likely that other maps are going to be used from time to time based on conditions such as atmospheric conditions, that means a lot of our hard work may not end up being used or might be undone as the truck is operated at different altitudes or different temperatures.
08:34 We can deal with this situation by making a similar magnitude change to the relative areas of each table.
08:40 I'm not suggesting that you copy and paste your modified table to all the other tables, but rather if we're making a 20% change in blending to an area of one map, we would also apply the same 20% in blend to the other maps.
08:52 This keeps the relative values of all the tables comparable.
08:55 As I've mentioned already, the common technique of just manipulating the injection pulse width map isn't the technically correct way of addressing fuel delivery and may have some unintended consequences.
09:05 We don't live in a perfect world though and in many instances it's not going to be possible to achieve the required fuel delivery directly within the requested fuel tables.
09:15 For example, it might not be possible to scale 100 mm³ stock file out to 275 mm³ that we may need for a performance tune.
09:24 Some level of compromise is going to be necessary here.
09:27 We'll address this more in the practical section of the course.
09:30 One last aspect we need to watch out for is any limit tables that may cap the maximum fuel quantity.
09:35 These can be particularly frustrating as we may be making all the right changes in the tables but these can be overridden by a simple fuel quantity limiter.
09:43 These limit tables may be presented in the form of a smoke limit table which defines the minimum lambda value or in other words the maximum fuel mass that can be delivered for a given air mass.
09:53 While this table presents a minimum lambda value relative to engine speed and load, it isn't typically measured by an air/fuel ratio sensor in the exhaust, rather it's calculated by the ECU based on the output of the mass airflow meter and the calculated fuel rate.
10:08 Other fuel quantity or torque limit tables will exist to protect drive line parts, adjust maximum output based on elevation or to define the maximum output or load curve of the engine.