VIEWS: 10 PAGES: 4 POSTED ON: 9/15/2012
Camshafts and another interesting idea! • This discussion, and ALL such discussions, always consider an "ideal" engine! One where the valves open and close instantly and completely, to allow each of the four Otto engine cycles to occur very distinctly and separately. However REAL engine valves are always operated by a camshaft which has lobes shaped to push each valve open, against very strong spring pressure, and then where those springs cause the valves to close after the camshaft lobe has passed. The point being, each valve TAKES TIME to open and then to close! The engine designer CHOSE a particular size and shape for the camshaft lobe for each specific engine. • It turns out that since the valves take that much time to open and to close, the "valve timing" meaning the shape and timing of the camshaft lobe shape and position OVERLAPS. • For example, the EXHAUST valve is ALWAYS designed to begin to open FAR BEFORE BDC (bottom dead center) so it necessarily RELEASES the productive pressure inside the cylinder during the POWER stroke. Even worse, the exhaust valve CONTINUES TO STAY OPEN beyond the end of the exhaust stroke and it is still open well into the INTAKE stroke! With BOTH valves then open, some of the fresh gas-air mixture being sent INTO the cylinder goes completely through and OUT THE EXHAUST! In fact, much of the rich sound of a high performance engine is due to this, where raw gas-air mixture goes through the engine and is then ignited by the extremely hot metal surfaces of the exhaust manifolds! • But this is clearly extremely wasteful of the precious gas-air mixture that drivers pay for at the gas pump! If you think about it, if an engine was IDEAL, there would be virtually NO exhaust sound at all, even without any muffler. The already completely burned up end products of the cylinder combustion would simply be squeezed out of the cylinder as the piston rose. • So an engine's overall efficiency is also affected by the valve timing and duration. This subject is very complex because both sides of the street are involved. IF an engine is to be designed to produce maximum power, then it is important to get rid of as much of the old exhaust gases in order to get more fresh gas-air mixture into the cylinder to burn. This can be done by greatly INCREASING the length of time that the valves are open. It is essentially conceded that a significant amount of unused fresh gas-air mixture goes through the engine unused, in order to be able to create the absolute maximum amount of power. That means that an engine that is set up for extra power is also WORSE on fuel efficiency. It might have seemed that the opposite should be true, but the REASON that the engine creates more power is because a LOT more fuel-air mixture goes through the cylinders, and the fact that a good deal of that is lost is ignored! • Manufacturers therefore design very conservative camshafts for their vehicles to be sold, but for their racecars that look the same, they have very different camshafts in them! Gearheads know that there are STOCK camshafts, STREET camshafts, and various levels of RACING camshafts. When the engine has a STOCK camshaft, it idles smoothly and starts easily. With the most extreme racing camshafts (such as used in Dragsters), as peculiar as it sounds, the valves virtually never close! Both valves are ONLY closed for a very brief time during the early part of the POWER stroke, in order to use the generated power to drive the piston downward. Beyond that, one or the other or both valves are at least partially open at all other times! • Anyone can instantly HEAR the effects of an engine with any exotic camshaft, because of that effect mentioned above regarding the rich sounds of exhaust when a lot of fuel is burning IN THE EXHAUST HEADERS! Such engines are also nearly impossible to cause to idle (with the valves rarely both being closed!) and so such engines tend to need to spin at 2,000 rpm or more to keep running (rather than the 550 rpm common in conventional cars with stock camshafts). Finally, a normal starter motor is only able to spin an engine a little faster than the needed 550 rpm for stable idling, so when you have an engine that cannot idle at below 2,000 rpm, starting it is a real problem. Around 40 years ago, some creative drag-racers discovered that they could cause a standard starter to spin fast enough if it was powered by two or three or even four batteries in series, instead of the standard one battery. • In any case, the central point is that all camshafts have shapes which were developed by experimental results! Thousands of failed designs eventually narrowed it down to cam lobe shapes that are now used. Amazingly enough, there is virtually NO theoretical basis for almost anything about a camshaft! It was all Trial and Error! And after a hundred years or so, they have found cam lobe shapes that seem to be as good as they can be, whether for economy or for power or anywhere in between. A Physicist goes crazy when some technology advanced simply by an endless number of bad guesses! We prefer that there is actually some REASON and LOGIC behind trying new variants!
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