Variable Cam Control Tuning: Fixed vs Variable Cam
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Fixed vs Variable Cam
05.16
| 00:00 | - Traditionally engines have used what I refer to as fixed cam timing where the cam timing is determined by the engine manufacturer and this is then set by way of the location of the cam and crank pulleys and the belt or chain that drives them. |
| 00:14 | In this sort of engine design, the cam timing as we've already discussed, is a compromise between low RPM and high RPM performance. |
| 00:23 | Once OE manufacturers developed the technology to adjust the cam timing as the engine was running, we started to see more and more engines adopt this technology. |
| 00:32 | Initially it was applied predominantly to performance engines and often only to the intake cam. |
| 00:38 | As the technology has matured and the costs of production has dropped, we're seeing it become more widespread and it's often applied to both inlet and exhaust cams as well as to engines that target efficiency and economy rather than outright power. |
| 00:53 | Every manufacturer has developed their own internal names for describing variable cam control and you'll hear acronyms such as VVT, VANOS, VarioCam and iVTEC just to name a few. |
| 01:06 | While there are a few subtle difference, essentially these systems are all describing the same continuously variable cam control system. |
| 01:15 | The key point with a continuously variable cam control system which differentiates it from other switched cam control systems we'll discuss shortly, is that this style of control allows the cam timing to be moved anywhere between the minimum and maximum limits of the cam travel. |
| 01:31 | This requires a few considerations for the system to work as intended. |
| 01:36 | Firstly we need a target cam timing table or map which defines what the cam timing should be with respect to engine RPM and load. |
| 01:45 | These maps will look a little different depending on the system you're tuning but the principle is always the same. |
| 01:51 | In order to actually move the cam timing we also need a control system which uses engine oil pressure and a specially designed cam wheel to allow the cam to be advanced or retarded. |
| 02:03 | We'll dive into the inner workings of the mechanical system shortly. |
| 02:07 | The ECU also needs a way of manipulating the oil flow to actually move the cam and this is usually achieved with solenoids that the ECU controls with a pulse width modulated signal. |
| 02:20 | The last part of the puzzle is that the ECU also needs a way of tracking what the actual cam position is and this is achieved via a sensor on the cam itself. |
| 02:29 | Often this sensor plays double duty and provides a synchronisation signal to the ECU so that it knows where abouts it is in the engine cycle too. |
| 02:39 | Now that the ECU has a target cam angle, a means of moving the cam to achieve that target and a way of measuring what the actual cam timing is, it uses a common PID control algorithm in order to accurately track the target. |
| 02:53 | PID stands for proportional integral and derivative and is a very common control strategy used in many of the control systems in our ECU. |
| 03:03 | Don't worry too much about it at this stage as we'll be covering it in detail in a separate module. |
| 03:10 | In essence though we could liken this system to an idle speed control system. |
| 03:14 | Just like with our cam timing, the ECU has a target idle speed table, a means of adjusting the idle speed, either directly by moving the throttle plate or via a separate idle air control valve and of course the ECU is always measuring engine RPM so it knows what the actual idle speed is compared to the target. |
| 03:34 | Using the exact same PID control algorithm the ECU can then manipulate the airflow into the engine in order to achieve the desired target idle speed. |
| 03:44 | Regardless of the system being conrolled, the devil as they say is in the detail and getting the PID control dialled in correctly is the key to a system that tracks the cam target quickly and accurately. |
| 03:58 | Again, you don't need to get too concerned about this for now as we'll be covering the process in detail as we go through the course. |
| 04:05 | The good news is that tuning PID control for cam control is also relatively simple and quick to achieve, particularly given the steps that you will learn in this course. |
| 04:16 | So far so good but of course there are some limitations with continuously variable cam control. |
| 04:21 | The system can move very quickly but it's a mechanical system and it can't move between the extremes of travel instantaneously. |
| 04:29 | We need to consider this with our cam target so that we can give the system the best chance possible of tracking the targets properly. |
| 04:36 | There's also a limit to how far the camshaft can be moved before we risk piston to valve or valve to valve contact. |
| 04:43 | I know that this is a concern for many who are new to cam tuning but if you're dealing with a stock unmodified cam control engine the good news is that the manufacturer has done their homework and ensured that regardless of what you do with the cam timing maps, interference between components cannot occur. |
| 05:02 | The same however can't be said for a cam control engine that has been fitted with aftermarket cams or pistons and this is something we're going to look at in detail in a coming module. |
