The result sounds like variable valve timing, but variable intake manifold benefits more low-speed torque than high-end power. Therefore it is very useful for sedans, which are heavier and heavier these days. For better drivability, there are also increasingly more sports cars feature variable intake manifold alongside VVT, these including Ferrari 360 M and 550M.
Compare with VVT, variable intake manifold is cheaper. What it needs are just some cast manifolds and a few electric-operated valves. In contrast, VVT need some elegant and precise hydraulic actuators, or even some special cam followers and camshafts.
There are two kinds of variable intake manifolds: variable length intake manifolds and resonance intake. Both of them make use of the geometry of intake manifolds to reach the same goal.
Variable length intake manifolds
is commonly used in sedans. Most designs employ 2 intake manifolds with
different length to serve each cylinder. The longer one is for
use. The shorter one is for high rev. It is easy to understand why high
speed need a short manifold, because it enables freer and
breathing. But why does it need longer pipe for low speed ? because
pipe results in lower frequency of air mass reaching the cylinder, thus
matches the lower rev of engine very much. This provide better cylinder
filling, thus improves torque output. Besides, longer intake manifold
to slower air flow, hence better mixing between air and fuel.
clearly see the
manifolds of Ford's Duratec 2.5 litres V6 engine. Each cylinder has a
pipe and a short pipe.
|Toyota's 2 litres Variable Intake engine also has a manifold longer than another|
Resonance intake system
Boxer engines and
engines (but not inline engines) may employ resonance intake manifold
boost mid to high rev efficiency. Each bank of cylinders are fed by a
plenum chamber through separate pipes. The two plenum chambers are
by two pipes of different diameters. One of the pipes can be closed by
a valve controlled by engine management system. The firing order is
such that the cylinders breath alternately from each chamber, creating
pressure wave between them. If the frequency of pressure wave matches
rev, it can help filling the cylinders, thus improved breathing
As the frequency depends on the cross-sectional area of the
pipes, by closing one of them at low rev, the area as well as frequency
reduce, thus enhance mid-rev output. At high rpm, the valve is opened
improves high-speed cylinder filling.
|Porsche 996 GT3's resonance intake system. Note that 2 pipes connect between the 2 plenums.|
5,000 rpm (left A and top right) : long pipes; resonance intake
5,000-5,800 rpm (left B and middle right) : long pipes plus short-pipe resonance intake, with one of the interconnected pipes of the resonance intake closed.
Above 5,800 rpm (left C and bottom right): long pipes plus short-pipe resonance intake, with both interconnected pipes of the resonance intake opened.
|Advantage:||Improves torque delivery at low speed without hurting high speed power; Cheaper than variable valve timing.|
|Disadvantage:||A bit space engaging; no much benefit to high speed output.|
|Who use it ?||
More supercars now employ variable back-pressure exhaust. It is somewhat like the variable intake manifold, just locate at the exhaust. Normal exhaust pipes for sports cars collect exhaust pulse from individual cylinders and combine them to a larger pulse, with a corresponding lower pressure behind the pulse. This low pressure actually helps drawing more air / fuel mixture into the cylinder from intake manifolds. This is so-called "reverse supercharging".
The reverse supercharging work best at a certain engine rev which is determined by the length of the exhaust pipe. The shorter the pipe, the lower rpm the reverse supercharging works. Of course, for any fixed exhaust pipes, the choose of working rpm is always a compromise.
usually provides 2 different lengths of exhaust pipes. The switching
them is via opening and closing of valves. Therefore it satisfy both
requirements of high speed and low speed output. Moreover, it helps
EU’s noise regulations, which set upper limits according to speed.
|Advantage:||Optimize high and low speed output; reduce noise at low speed.|
|Who use it ?||
Multi-valve engines started life in 1912, adopted by a Peugeot GP racing car. It was briefly used by the pre-war Bentley and Bugatti. However, it was not applied to production cars until the 60s - Honda S600 was probably the earliest production road-going 4-valve car. In the 70s, there were several more 4-valve cars introduced, such as the Lotus Esprit (1976), Chevrolet Cosworth Vega (1975, engine made by Cosworth), BMW M1 (1979) and Triumph Donomite Sprint. The latter introduced the first single-cam 4-valve engine, using rocker arms to drive valves.
In the early 80s, when Ferrari had just adopted Quattrovalvole V8, Honda was introducing 3-valve engines to its mainstream bread-and-butter models. In the mid-80s, both Honda and Toyota made 4-valve engines standard in virtually all mainstream models. The Western car makers did that some 10 years later !
Improving breathing is one of the keys for power enhancement. Unquestionably, in the 2-valve era valves used to be the bottleneck, hence the need for more valves.
The earliest mass production multi-valve engines were 3-valves because of its simple construction - it needs only a single camshaft to drive both intake valves and the exhaust valve of each cylinder. Today, there are still a few car cars using this cheap but inefficient design, such as Fiat Palio and all Mercedes V6 and V8 engines. Mercedes uses that because of emission rather than cost reason.
A typical 2-valve engine has just 1/3 combustion chamber head area covered by the valves, but a 4-valve head increases that to more than 50%, hence smoother and quicker breathing. 4-valve design also benefit a clean and effective combustion, because the spark plug can be placed in the middle.
4 valves are better to be driven by twin-cam, one for intake valves and one for exhaust valves. Honda and Mitsubishi models prefer to use sohc, driving the valves via rocker arms like the aforementioned Triumph. This could be a bit cheaper, but introduce more friction and hurt high speed power. Therefore the sportiest Honda and Mitsubishi still use dohc.
It is arguable that whether 5 valves per cylinder helps raising engine efficiency. Audi claimed it does, but fail to provide evidence to support. In fact, its 5V engines are no more powerful and torquey than its German rivals with 4 valves per cylinder.
Originally, 5-valve design doesn’t guarantee covering more head area than 4-valver. Nevertheless, if the head of combustion chamber is in irregular shape like the picture shown, the valves may cover larger area. Ferrari F355 make use of this to enhance high-speed breathing. Is there any disadvantage? Yes, faster breathing also harm low-speed torque if no counter measure is taken. Therefore it is more suitable to sports cars.
All existing 5-valve engines have 3 intake valves and 2 exhaust valves per cylinder, still arranged as cross-flow. The exhaust valves are larger, but in terms of total area intake valves are larger. In F355, by arranging the outer intake valves open 10° earlier than the center valve, it got the swirl needed for better air / fuel mixture, hence more efficient burning and cleaner emission.
The advantage of 5-valve engine is still under questioned. Not only few car makers used it (VW group, Ferrari and the bankrupted Bugatti), but Formula One cars also no longer favour it. Even the Ferrari F1 cars which was once famous for 5V engine has switched back to 4-valve design a few years ago.
Most early 4-valve engines were not good at low-to-middle speed torque, simply because the larger intake area resulted in slower air flow. Especially at low speed, the slow air flow in the intake manifold led to imperfect mixing of fuel and air, hence knocking and reduced power and torque. Therefore 4-valve engines were regarded as strong at top end but weak at the bottom end, until the technology of variable intake manifold became popular recently. The aforementioned Chevrolet Cosworth Vega performed particularly weak at low speed.
In response to this, Toyota introduced T-VIS (Toyota Variable Intake System) in the mid-80s. T-VIS accelerated low speed air flow to the manifold. The theory was quite simple: the intake manifold for each cylinder was split into two separate sub-manifold which joint together near the intake valves. A butterfly valve was added at one of the sub-manifold. At below 4,650 rpm the butterfly valve would be closed so that raising the velocity of air in the manifold. As a result, better mixing could be obtained at the manifold (excluding direct-injection engines, fuel injection always takes place in the manifold).