外文文献Intake and ExhaustTuned intake manifoldFirst introduced by Mercedes 300SL in 1954, tuned intake manifold is not exactly a new technology. It is discussed here just because its principle is useful to our further study of variable intake manifold.Before 1950s, engineers believed short intake manifolds were the best to engine breathing. Then they discovered that under some conditions long intake manifolds could actually improve output, thanks to a so-called supercharging effect. How is that done ? Lets see the following illustration:When fresh air is sucked into combustion chamber, it gathers speed and momentum in the intake manifold. As soon as the inlet valve is closed, the fast-moving air hits the valve and compresses, generating high pressure. With no where to go, this high pressure bounces back, travels along the intake manifold, hits the plenum at the other side and bounces back again. In this way, the high pressure bounces back and forth along the intake manifold until the inlet valve opens again, creatingpressure waves.Now the interesting thing is: if the inlet valve opens again exactly when the pressure wave comes back, the pressure wave will help charging the combustion chamber due to its high pressure. This is not unlike charging the combustion chamber with a light supercharger, thus we call this supercharging effect.In order to match the timing of valve opening, the frequency of pressure wave shall synchronize with engine rev, obviously. This frequency is dependent on the length of the intake manifold (L in the figure). The longer the length, the longer the time pressure wave takes to bounce back thus the lower frequency of pressure wave is attained. As a result, a longer intake manifold leads to supercharging effect at lower engine rev. A shorter manifold leads to supercharging effect at higher rev. By selecting a suitable manifold length, we can obtain the desired power characteristic.Calculations found in order to achieve useful supercharging effect, the intake manifold shall be unusually long. If it is too short, the pressure wave will bounce back and forth too many times in the manifold before the valve opens again, by then the high pressure is largely diminished. Therefore a tuned intake manifold shall be long.Unfortunately, tuned intake manifold works only across a narrow rev band. If the engine revs beyond that band, the pressure wave will arrive too late in the intake stroke, contributing little to charging. If the engine runs below that rev band, the pressure wave will arrive the inlet valve before it opens. In both cases, the low pressure area of pressure wave may even work against cylinder charging, hampering torque output.A sports car engine may employ a shorter tuned intake manifold to optimize its output at high rev (in the expense of low to medium rev output). On the contrary, a heavy sedan or commercial van engine may choose a longer manifold to favour low-rpm output at the price of high-rev output. As you can see, the selection of manifold length is always a compromise. Thats why many modern engines turn to variable intake manifold.Variable Intake Manifold (VIM)Variable intake manifold has been popular on naturally aspirated engines since the mid-1990s. It is primarily employed to broaden the torque curve, or in order words, improve the flexibility of engine. Conventional tuned intake manifold focuses on a narrow rev range. In contrast, VIM offers 2 or more stages of intake configuration to deal with different engine speeds. This sounds very much like variable valve timing, but VIM is generally cheaper to produce because it involves only some cast or plastic manifolds and a few electric-operated valves. That explains why it was applied to cheaper and smaller engines well before VVT became popular. Today, many engines employ both features to achieve the best results.However, variable intake manifold is rarely used on turbocharged or supercharged engines - Volkswagens 2.0 TFSI engine is one of the few exceptions. This is because forced induction offers a strong supercharging effect already. This largely reduces the benefit of VIM, thus its additional costs and weight are difficult to justify. As more and more cars are switching to turbocharged engines, the popularity of VIM is expected to cool down in the foreseeable future.There are two kinds VIM: variable length type and resonance type.Variable length intake manifold (VLIM)Variable length intake manifold is a direct answer to the shortcoming of conventional tuned intake manifold. If a fixed length intake manifold is optimized for a very narrow rev band, why not give it 2 sets of intake manifold, one with short pipes to serve high rpm while another with long pipes to serve low rpm ? By using simple butterfly valves, switching between the long and short pipes are easy.Some early VLIM systems, like this Ford 2.5 Duratec V6, used separate long and short pipes, which is easily visible