PDM Installation & Configuration: Backup Fuel Pump
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Backup Fuel Pump
06.05
| 00:00 | - In the technical background section of the course, we talked about a PMU being a great way to decouple our system inputs from our system outputs. |
| 00:08 | What that means is that we've now removed the physical hardwired connection between flicking a switch or pressing a button and the device in the vehicle turning on or off. |
| 00:17 | Instead, this is all handled by configurable software which opens up a massive realm of possibilities. |
| 00:24 | In this course section we're going to discuss some of the more advanced functionality that can be achieved when using a PMU. |
| 00:31 | The aim of this is to show how relatively complex control strategies can be built up from simple building blocks and give you some ideas around how a PMU can be so much more than just a replacement for relays and fuses. |
| 00:44 | We'll look at these advanced functions in a structured manner, first discussing the overall design outcome, then the inputs and outputs required, followed by how we build the required control strategy using logic terms we've previously introduced. |
| 00:58 | We won't discuss specifics of configuring these functions on any particular PMU platform as it's a different task for each PMU on the market but you'll be able to see similar functions being set up in the worked examples section of the course. |
| 01:11 | The first of these we'll have a look at is a backup fuel pump system. |
| 01:14 | The goal of this function is to maintain fuel rail pressure on a port injected motor in times of high demand or in the event of a fuel pump failure. |
| 01:24 | The way we're going to implement this is to install an auxiliary fuel pump in the vehicle which is selectively turned on as needed. |
| 01:31 | We need to clearly define the situations under which this auxiliary fuel pump should run. |
| 01:36 | Because this lets us determine the required inputs, outputs and control strategy. |
| 01:40 | As mentioned, we'll want to run this pump if there is a problem detected with the primary fuel pump or if the fuel rail pressure is below a lower limit. |
| 01:49 | However, obvious once it's thought of but easily overlooked is that we only want this fuel pump to run if the ECU is requesting that a fuel pump be turned on at all. |
| 01:59 | A manual override input would also be excellent as this can be used for testing that back up system. |
| 02:05 | Thinking about these scenarios, we can generate a list of the required inputs and outputs, being a fuel pump turn on request, fuel rail pressure, primary fuel pump status and auxiliary fuel pump manual override. |
| 02:18 | There's only one output required which is the channel that will supply power to the auxiliary fuel pump. |
| 02:24 | Designing the control strategy for this boils down to determining the priorities of the inputs and which combinations of them should switch that auxiliary fuel pump on. |
| 02:33 | Starting with the manual override request, this can be treated almost as a single input by itself as if it is turned on, the auxiliary fuel pump should be powered irrespective of what any of the other inputs are doing. |
| 02:48 | The next input to consider is the fuel pump turn on request from the ECU. |
| 02:53 | If the ECU isn't requesting that a fuel pump be turned on then the state of the primary fuel pump output and fuel rail pressure is not important and the auxiliary pump should be off. |
| 03:05 | The flipside to this is that if the ECU is requesting that a fuel pump be turned on, we now need to look at the state of those other 2 inputs. |
| 03:13 | In this case, either 1 or both of those other inputs should be able to control whether the auxiliary pump should be turned on. |
| 03:21 | With all this determined, we can put it in terms of the logical functions that we've introduced earlier in the course. |
| 03:27 | This function can be constructed using only "and" and "or" functions. |
| 03:32 | We'll show this on screen with a flow chart form while we're discussing it as functions like this can be misinterpreted pretty easily when just being listened to them explained to you in a spoken form. |
| 03:44 | The flow chart form uses a couple of graphics to represent the logical functions. |
| 03:48 | These graphics take the form of a central shape which defines the function type with inputs on one side and outputs on the other. |
| 03:57 | You can see here an "and" function "or" function, "not" function and "XOR" function. |
| 04:04 | These will be clearer once you see them used in practice. |
| 04:07 | There is a copy of this chart below the course module that you can refer back to. |
| 04:11 | For our fuel pump backup function, the primary element is an "or" function, one of the inputs to which is the manual override command and the other the output of an "and" function. |
| 04:23 | The inputs to this "and" function are the fuel pump command from the ECU and the output of another "or" function. |
| 04:32 | The inputs to this "or" function are the fuel rail pressure being low and a primary fuel pump failure event. |
| 04:39 | Visualising the function design like this lets us easily follow the input conditions through see how the output will be determined. |
| 04:47 | We can see here that if our manual override is switched on, the state of the rest of the inputs is irrelevant and the secondary pump will be switched on which is the functionality that we want. |
| 04:58 | In the case that the manual override is switched off, we need there to be both a fuel pump request signal from the ECU and 1 or both of the fault conditions detected to turn on that secondary pump. |
| 05:10 | This sort of design process might seem a little long winded for what is a relatively simply function but I'd suggest you practice designing some functions like this as it's the correct way to think about them. |
| 05:21 | Much of the work in designing functions like this comes from making sure unexpected inputs are dealt with as opposed to how the function would work under normal circumstances. |
| 05:30 | Once you have this general function design nailed down, you'll need to implement it in your PMU configuration software. |
| 05:36 | This is a different process for all the brands of PMU on the market and we do show some advanced functions being configured in the worked examples section of the course. |
| 05:46 | In this module we've looked at the design process used to generate an advanced PMU control strategy, being the implementation of a back up fuel pump. |
| 05:54 | The operation of which is based on fuel rail pressure and detected fault with the primary pump with also a manual override system. |
