I've been looking over recent posts and i believe that there's been a shortage of Technical Information and basic questions. Coupled with the excessive amount of free time i have these days, I thought a tachnical session was in order. Being "motorheads" i assume that we are all familiar, some more so than others about engine, transmission and suspension basics. Hence i've decided to start from some rather advanced engine operation and in succeeding order after opening the hood, i've chosen "the valve train" ( Camshafts,valves, terms , Variable valve timing mechanisms and such.
Engines put simply, are sophisticated Air Pumps. A combination of air and fuel is sucked into the cylinder, compressed,ignited and expelled. So the four cycles can be aptly called, suck squish, bang, blow. Keeping this in mind if the cylinders are the heart then Valve train (contained in the cylinderhead) is the brain sunchronising these actions.
A valve train consists of valves, camshafts with their lobes that open and close them and other smaller components which you're all familiar with. Cam operation is detrimental to the operation of the engine as explained.
Like i mentioned earlier, imagine the engine as a complex air pump. The flow of gases in and out of the engine is controlled by valves. Imagine a single cylinder engine in slow motion. The intake valve would open as the piston starts moving from TDC to BDC letting in the air/fuel mixture. Close for the compression stroke as soon as the piston reaches BDC and starts back to TDC. Ignition at TDC (Power stroke) and then back to BDC where the Exhaust valve begines opening on the journey back from BDC to TDC.
Have you visualised the above?
Now speed it up.
When you increase the RPM, the slow RPM configuration for the camshaft will not work very well. Just say if the engine is running at 4,000 RPM, the valves are opening and closing 2,000 times every minute, or 33 times every second. At these speeds, the piston is moving very quickly, so the air and fuel mixture rushing into the cylinder will also be moving at a very quick rate.
When the piston starts its intake stroke and the intake valve opens, the air/fuel mixture in the intake runner starts to accelerate into the cylinder. By the time the piston reaches the bottom of its intake stroke, the air/fuel mixture is moving at a pretty high speed and if the valves go shut at this moment, the mixture comes to a sudden stop.The piston completes the remaining three cycles and it goes on all over again.
To optimise A/F dlivery into the cylinder at higher rpm, manufacturers have devised "variable Cam timing" but thats another topic. To make a long story short, the faster the engine goes, the faster the air/fuel mixture flows and the longer we would want the intake valve to stay open. We would also want the valve to open wider at higher speeds. Also affecting the cams performance is lift, the duration, overlap and timing.
Given below are various terminologies for your information.
VALVE LIFT:
The distance the Valve Lifts or travels away from the valve seat.
CAM DURATION:
Duration is the angle in crankshaft degrees that the valve stays off its seat during the lifting cycle of the cam lobe.
CAM TIMING:
Cam timing as the name implies is the opeing and closeing of the valves relative to cranskhaft postion ot timed with the cranckshaft.
For timing take this as an example
22-62, 62-22.
The first 22 shows how many degrees the inlet valve opens before top dead center,
The first 62 shows how many degrees the inlet valve closes after bottom dead center,
The second 62 shows how many degrees the exhaust valve opens before bottom dead center,
The second 22 shows how many degrees the exhaust valve closes after top dead center.
The duration and overlap is calculated by using these values.
VALVE OVERLAP:
Overlap is the angle in crankshaft degrees that both the intake and exhaust valves are open. This occurs at the end of the exhaust stroke and the beginning of the intake stroke. Increasing lift duration and/or decreasing lobe separation increases overlap. At high engine speeds, overlap allows the rush of exhaust gasses out the exhaust valve to help pull the fresh air/fuel mixture into the cylinder through the intake valve. Increased engine speed enhances the effect. Therefore increasing overlap, increases top-end power and reduces low-speed power and idle quality.