![]() ![]() This is to assist in pulling the fresh charge from the intake manifold into the combustion cylinder just like the Siphon Effect. This is no way a manufacturing fault and the opening of the intake valve slightly before TDC and closing of the exhaust valve later after TDC is deliberately done. In the diagram, it is very prominent that when the piston reaches TDC in the exhaust stroke, both the valves, intake and exhaust, are open. If it the exhaust valve were to remain close till the BDC, some engine power had to be wasted in order to assist the piston to move from BDC to the TDC. This also confirms that there is no other pressure exerting on the piston during its motion to BDC. This releases the excess pressure from the combustion chamber. ‘Blow-down’ is the process where the exhaust valve opens before the piston reaches the BDC. Blow Downīoth the valves remain shut to perform the combustion process efficiently during the compression stroke all the way up to the power or expansion stroke. ![]() Now the question comes why isn’t the intake and exhaust valve opening and closing in sync with the pistons TDC and BDC? And why at TDC both the valves are open for some duration? The reason likes in the three crucial factors called Blow Down, Overlap, and Ram Effect. and the cycle repeats itself stroke after stroke. Now through BDC to all the way up to a few degrees more than TDC the exhaust valve is open after which it closes.Coming back to the valve timing, the exhaust valve opens way before the piston reaches the BDC after expansion stroke or power stroke.Let’s save ignition timing for some other day. This also depends on the ignition timing. If the engine is FI (Fuel Injected), the fuel is atomized (sprayed into tiny droplets) inside the combustion chamber before the piston reaches TDC and then fuel is ignited in SI engine (Spark Ignition engine). This is when both the valves are shut (inlet as well as the exhaust valve). The inlet valve closes just before the compression stroke.The piston’s movement from the BDC to the TDC creates a vacuum inside the combustion chamber forcing the air or air-fuel mixture to enter. The inlet valve closes after the piston reaches tad bit further than the BDC.This is when the engine is about to complete its exhaust stroke. This valve opens a few degrees before the piston reached TDC. Firstly, the inlet valve opens to let the air or air-fuel mixture enter the combustion chamber.BDC (Bottom-Dead Centre) is when the piston is at its lowest position. TDC (Top-Dead Centre) when the piston is at its topmost position. But before doing so, here are some technical terms used in the automotive world. Not going into the detail, here is a brief about valve timing. Valve Timing Process Valve Timing Process Diagram Defining the valve timing of an engine is one of the most typical processes.For you to know, the valve timing is measured in degrees that correspond to the piston’s position inside the combustion chamber.They ride the valves at definite intervals and with utmost extreme precision.As said earlier the cam or the lobes on the camshaft control the opening and closing of the valves. With that clear, let’s move to the valve timing.And the valves are like the doors for the ports from which the air or air-fuel mixture enters from the intake and exits the chamber from the exhaust valve (when it opens).In simple terms, camshafts are responsible for the opening and closing of the valves. To understand valve timing, let us take you back to “what are camshafts?” here. But what exactly are VVT and VVL? How does it help? and How does it work? So, without further wasting any time, let’s get going.īut without going into the detail of VVT we first need to look at what is valve timing. Many of the vehicles, some even on the Indian road come with the engine running VVT, VVL or both the techs. Variable Valve Timing | Image Source ( 1) One of the innovative advancements that brought a drastic change to the industry is the VVT (Variable Valve Timing) and VVL (Variable Valve Lift). Well, they work to make the engine refined, efficient, powerful and environmentally friendly. Engineers work their kith and kin and produce some really unique and state of the art masterpieces. But thankfully the R&D of this sector across the globe has got it covered. Without any advancements, the automotive industry will have been doomed by now. ![]()
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