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Variable Cam Control Tuning: Cam Timing Fundamentals

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Cam Timing Fundamentals

07.04

00:00 - Before we can get into the details of optimising cam timing, it's important to spend a little time understanding the fundamentals of how the valve timing events effect the operation of the engine as well as a more in depth discussion about the various methods of adjusting the cam timing that will come into play as we move through this course.
00:18 While it's not absolutely essential to understand this in order to be able to tune a cam control engine, I'm a firm believer that a solid understanding of what's happening inside our engines will allow you to do a better job in less time.
00:33 We're going to start with this module where we'll discuss the basics of cam timing, what the terms mean and how it affects the cylinder filling.
00:41 This isn't intended to be an all encompassing discussion on cam timing or fluid dynamics but rather it's intended to give you a basic overview.
00:51 It's easiest to visualise the valve timing events if we start by drawing out the full engine cycle and then we can plot the valve opening and closing on top of this to make it really clear what's happening and when.
01:03 When we do this, we're going to plot the crankshaft rotation on the horizontal axis and valve lift on the vertical axis.
01:10 There's 720° of crankshaft rotation in a single engine cycle so we'll mark out the horizontal axis from 0 to 720.
01:20 Now we can draw in each of the 4 engine strokes starting with the power stroke.
01:24 Remembering that each stroke takes 180° of crankshaft rotation.
01:29 We can also note the piston location on this drawing with TDC standing for top dead centre and BDC standing for bottom dead centre.
01:39 Lastly, we can plot the valve opening, starting with the exhaust valve and then the intake valve as we move through the engine cycle.
01:47 It should be reasonably straightforward to understand that the exhaust valve needs to be open during the exhaust stroke so that the combustion gases can be evacuated into the exhaust system.
01:57 However we can see from our plot that the exhaust valves actually begin to open while the piston is still moving down the bore, on the power stroke.
02:05 We can also see the exhaust valve closes well after the piston is moved past top dead centre on the exhaust stroke and begun the intake stroke.
02:14 Likewise, when it comes to the intake valve we can see that this begins to open near the end of the exhaust stroke and closes after the piston has begun the compression stroke.
02:25 On face value, this might not make much sense and you may have assumed that for example the exhaust valves would only be open during the actual exhaust stroke and the intake valves only open during the intake stroke.
02:37 The reason the cam designer will extend the opening range of both valves is due to the nature of the airflow in and out of the engine.
02:45 What we're trying to do, if performance is our key aim is to force as much air into the cylinder as possible during the intake stroke.
02:54 Likewise we want to get rid of all of the combustion by products during the exhaust stroke so that the cylinder is empty and ready for the next intake stroke.
03:04 The reason we won't achieve these aims if the valves were only open during their respective strokes is due to the nature of air and airflow.
03:12 Air has a density which means that a given volume of air also has a specific mass.
03:19 This also means that air has inertia and it takes time for it to speed up and to slow down.
03:26 Let's take the inlet stroke for an example and assume that the valves are currently closed and there's a column of air in the inlet port sitting against the inlet valve.
03:35 When the valve opens there will be a pressure differential between the inlet port and the cylinder.
03:40 In simple terms, the pressure in the cylinder will be lower than the pressure in the inlet port which causes the air to want to flow from the high pressure region to the low pressure region to achieve equilibrium.
03:52 This is one of the fundamental principles of engine operation however remember that inertia we talked about before, this means that when the valve first opens it takes time to get that column of air to start moving and accelerate into the cylinder.
04:05 This is why we want to open the valve a little bit before the piston reaches top dead centre and begins moving down the cylinder on the intake stroke.
04:14 So that by the time the piston starts moving the column of air is already starting to move.
04:21 The fact that the exhaust valve is also open at this time plays into this too but we'll get into that in a second.
04:28 Likewise, at the end of the intake stroke, the air in the inlet port is still moving towards the cylinder at high speed and due to the intertia of the air, we can take advantage of a ram effect if we leave the valve open a little way into the compression stroke.
04:42 This is a fine like though because as the piston starts moving up the bore, the pressure in the cylinder starts to increase and if we hold the valve open for too long, we'll start to force that fresh inlet charge back out of the cylinder.
04:56 Timing of the exhaust valve events follows similar considerations.
05:00 As the piston nears bottom dead centre on the power stroke, we can gain an advantage in evacuating the exhaust gas by opening the exhaust valve before the piston reaches BDC while there's still some pressure in the cylinder to achieve what is referred to as blow down.
05:15 If the valves didn't open until the piston actually began the exhaust stroke, there'd still be residual pressure in the cylinder that would actually make it harder for the piston to move up the cylinder reducing our engine power.
05:27 Of course this is also a compromise because if we open the valves too soon, then we allow blow down of the pressure that we want acting on the top of the piston to create torque at the crankshaft.
05:39 The last aspect we'll discuss here is the overlap period where both the inlet and the exhaust valves are open simultaneously.
05:47 We've already mentioned one of the reasons we open the inlet valves before the piston reaches top dead centre but by also delaying the exhaust valve closing we can achieve a scavenge effect where the velocity of the exhaust gas exiting the engine can actually help draw the fresh inlet charge into the cylinder.
06:05 The tricky part with the valve timing events is that what works well at low RPM where air velocity is low, won't work well at high RPM where the air velocity and inertia is much higher and vice versa.
06:18 This is all a delicate balancing act and particularly if the cam timing is fixed the engine designer must decide on what compromise to make with the valve opening and closing events.
06:29 A subtle aspect that's also worth mentioning in this module is that the camshaft rotates at half engine speed which means that the camshaft rotates a full 360° for each engine cycle.
06:42 Remember an engine cycle in a 4 stroke engine takes 720° of crankshaft rotation.