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Variable Valve Timing and Lift Electronic Control |
 A man more educated than I once said life is full of compromises, well someone at Honda happened to hear that. He decided to play god for a bit and change the situation for blokes like you and I. He said I, creator of Vtec bless thee with one less compromise in life. The writing on the wall couldn't have been clearer, when Honda proved to one and all that one can achieve great economy, even with powerful engines. This feature makes Honda engines equipped thus to return phenomenal efficiency in city conditions, with no compromise on performance.
Honda has in effect deterred a design dilemma of either providing 'power' or 'efficiency', and has provided the world with engines that can adapt to either demand. Vtec, as the heading indicates, stands for variable valve timing and lift electronic control. Not very self explanatory, is it but because its such a great piece of kit, we thought that it would be worth spending some time on.
Honda devised this ingenious idea as a result of many years of research and development. There are two ways in which an engine may be designed. Keeping in mind the buyer. One configuration is designed to give maximum efficiency, at the expense of horse power and other characteristics generally associated with powerful engines and another configuration which will give great performance at high Rpms but poor overall efficiency
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Ideally, in a four stroke cycle, for the first 360 degrees of crankshaft rotation, the inlet valve is kept open, as piston moves down this is called the intake or inlet stroke. For the next 360 degrees of revolution, both inlet and outlet valves are kept closed and the charge is compressed also known as the compression stroke. At the end of compression stroke, a spark is ignited to fire the mixture. An increase in pressure is caused by the explosion and thus pushes the piston downwards, since the power required for the running of the engine is developed in this stroke , this stroke is termed as power stroke and lasts 360 degrees of crank revolution. During the final 360 degrees of crank revolution, exhaust valve is opened as the piston begins its upward stroke, all the expanded gases, mainly CO and Co2 escapes at relatively lower pressures, this is known as the exhaust stroke.
As engine speeds increase, these four strokes begin to overlap each other and thus, the intake valve might be open even before the exhaust stroke is completed. This hinders efficiency but, this also makes the engine much smoother and is desirable in high revving engines. Further, in low revving engines, the amount by which the valves are opened(lift) is kept low in order to facilitate better sealing of the combustion chamber, but in the case of high revving engines, lift is high to allow more fuel and air to enter into the chamber.
So whenever designer embarked upon engine design, there was always a stark compromise in engineering to achieve either result. An engine was either designed to be driver friendly in the city and pocket friendly to its owner or it was designed to tear the highway apart, blowing a hole in presumably big pockets!!! Jokes apart, what exactly is this Honda Vtec and what does it mean to us?
 Image here shows a schematic representation of a much more complicated system. Here, the picture on the left shows the arrangement of cam and follower/valve during low rpm cycles, the one on the right shows the arrangement during high rpm. Note that we have a small pin that passes through the two valves. This happens to be the the core of the Vtec design.
Since an engine works between two basic modes, either at low rpm during city runs and higher rpm during highway runs and boy racer times.. So, during low rpm runs, the engine is desired to be smooth and torquey and most importantly, efficient. To achieve this, the valves are operated on low lift, thus better sealing is achieved during combustion and engine maybe operated in the most efficient manner. A Vtec equipped engine is capable of giving efficiencies comparable to much smaller and less powerful engines, in this rpm range.
That raises the big question. If These Vtec engines can be as frugal as much smaller engines, what about the power and the way its delivered. The answer is found in each and every Honda City 1.5 Vtec, Accord Vtec and so on. They have power to move planets. Well, as mentioned before in the article, that little pin forms the crux of this technology.
Whenever revs increase, oil pressure inside the engine rises and triggering the pin to displace and lock itself between the follower and the two valves. Note that the high rpm cam has a much larger profile when compared to the low rpm cam, thus the valves are now lifted higher, allowing the engine to breathe better. This results in excellent power delivery because there is much vale overlap, the power delivery is seamless and acceleration, effortless.
Once the oil pressure drops, the pin replaces itself in the original position and so are valve settings. When Vtec is combined with electronic engine management chips, the results are stunning. They result in engines that have a "Jekyll and Hyde" nature.
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