Variable Intake Manifold Development trend and technology Author Taehwan Kim Managed Programs LLC (tkim@managed-programs.com) Abstract The automotive air intake manifold has been playing a critical role for engine performance and performance characteristics. Due to this, the appropriate intake manifold system development is regarded as one of the most critical factors for new engine development success. Now, with the growing market demand for high performance (low to high speed) and lower fuel consumption, the V.I.S (Variable Intake System) manifold is regarded as one of the solutions to achieve these goals. In the past, a V.I.S intake manifold has been regarded as a simple combination of long and short runners into one manifold, but recently with the adoption of many variable engine control devices such as variable valve timing and lift and spark timing control and together with plastic intake manifold concept, proper V.I.S system development has become more and more complicated and difficult to develop the manifold properly to meet all development goals. Below in the illustration, the recent trend with V.I.S intake manifold development is summarized briefly. 1 Introduction (1) Plastic Intake Manifold Development In the past, plastic intake manifold manufacturers were only supposed to develop the plastic manifold based on OEM-defined aluminum model. The main engineering objective was focused only on part cost and weight. In later years, to lower part cost and to enable easy engine assembly, many OEMs then developed the plastic intake manifold as an I.A.F.M (Integrated Air Fuel Module) System. This asked more knowledge and understanding on engine control system for supplier. Now as the V.I.S. system becomes a global trend for most internal combustion engines, recently the OEM engine development plan is modified to include the proper engineering support by the supplier to help the OEM not only with plastic manifold design, but also with proper system design for performance, packaging and fuel economy. This has become the biggest huddle for suppliers as well as the OEM, because those areas of engine development previously were skills possessed primarily by purely specialized engine companies. (2) Plastic Intake Manifold Goal The above illustrates the recent intake manifold devices for achieving OEM performance goals in comparison with past technology. In the past, the intake manifold was only developed as a simple passive manifold without a delicate variable intake system. Engineering issues focused on burst strength, NVH, weight and cost. Now, the recent demand for intake manifold performance goals has expanded and the plastic intake manifold should be developed for engine performance, fuel economy and emissions improvement. After some recent economy financial crises which has caused gas price surging and growing demand for low CO 2 emission, future demand is expected to mainly focus to implement improved fuel economy. With this new market demand on the plastic intake manifold, the plastic air intake manifold development not only includes plastic parts development, but also includes whole engine development work in cooperation with OEM. In order to achieve this new goal, strong knowledge and experience in engine development and engine control systems is highly requested of plastic air intake manifold suppliers to help meet the OEMs new performance goals previously discussed. 2 Variable Intake System general (1) V.I.S systems are classified based upon the variable feature types. Page 1 of 7
There are 3 types of variable intake systems widely used. Each of these types do not have to be standalone, and can be used together to meet for development goals. Variable runner systems and resonance systems are mainly for performance improvement and V.C.M. (Variable Charge Motion) systems are mainly for fuel economy or emission reduction even though V.C.M. also improves low speed torque with proper tuning. (2) V.I.S systems with different air fuel systems. Above chart shows the V.I.S intake system roles based on each type of air fuel system. The V.I.S system can work for both performance and fuel economy improvement with M.P.I or G.D.I engine. The contribution of V.I.S system is limited with the adoption of super charging system like turbo charger system. Therefore, for super- and turbocharged engines, normally the intake manifold is simply a passive manifold. Hybrid engines, like the Toyota Prius are usually working on the Atkinson Cycle, which has blow out process after the intake process. This makes the intake runner tuning effect reduced and the cycle is mainly focused on higher fuel economy. Due to this, V.I.S system, which is more focusing on performance improvement, is not necessary for hybrid engines. power and torque by optimizing runner length and diameter. ii. Low end torque Reflective wave tuning and intake resonance effect is utilized for optimization. iii. Drivability: Broad torque band. Torque and power runner length and diameter needs to be optimized to make the torque curves broad and flat across the engine speed range. iv. Emissions Variable Charge Motion system: SCV, Tumble, PDA, CBR,,,, HC emission reduction during cold start : Enable ignition retard for EPA SULEV v. Fuel economy Lean combustion - V.C.M: SCA, tumble, PDA, CBR,,,,, - Enable stratified flow in combustion chamber Low to mid end torque - Reduced gear ratio helps to get better fuel economy with the help of improved low to mid speed torque - Pumping loss reduction helps to get better fuel economy and properly designed V.I.S intake manifold can reduce pumping loss significantly. Reflective wave and resonance tuning is the key. (2) Variable Runner Intake System Variable runner intake system is to achieve both low to mid speed torque and high speed power simultaneously. Through system optimization for higher reflective wave tuning strength, pumping loss can be significantly reduced to implement higher fuel economy. The system is widely used for I3, I4, V8 and V10 engine configuration and high end V6 and I6 engine. 3 V.I.S Intake manifold Development Technology (1) Objectives of the variable intake system i. Performance (Max Torque + Max Power ) The manifold combines runners optimized for optimal Page 2 of 7
Brake Torque With long runner With short runner 1000 2000 3000 4000 5000 6000 i. Flap valve system The flap valve system has been widely used for long time. It shows higher reflective tuning strength and through flow optimization, it achieves fairy good flow. 7000 the engine. Proper design and tolerance control with all components is critical to minimize the leak. As long runners and short runners are switched with V.I.S. valve to fulfill the desired long or short runner effect with the valve open or closed, robust sealing around the valve system is important. Proper valve system design and development with good sealing is the key to meet intended engine performance. In addition to the above mentioned leak and sealing importance, NVH and durability is also critical with VIS system. Therefore, careful and appropriate designs should be implemented to meet the OEM s NVH and durability requirements. iv. Recent V.I.S operation map Further, owing to the small valve space required, it is easy to fit into a small packaging space. But due to the necessity with straight valve alignment for all runners, valve position and runner length is limited by packaging space. ii. Barrel (Rotary) valve system The barrel valve system provides excellent flow performance but due to the bigger packaging space required for the barrel system, it has a packaging disadvantage compared to the flap valve system. iii. Technical difficulty A variable runner system consists of several components for valve system. This can cause air leakage past the valve system and this may cause difficulty for the ECU to properly control In the past, the V.I.S. system was controlled to use long runner for low-to-mid speed torque and short runner for high speed power. The main objective with the V.I.S. system was to get the highest value for maximum torque and maximum power. Recent market demand for engine performance has changed, now requesting 90~95% of maximum torque over 90% operating RPM range. In order to meet this, low-end and highend torque improvement is important as well. In addition, recently engine development is stressed to improve fuel economy at commonly used engine speeds for normal driving. So as to achieve these new goals of engine development, maximum utilization of reflective wave tuning and manifold design optimization, considering all manifold parameters is very important. With well optimized V.I.S. manifold system, the below V.I.S. operation control map can be achieved to secure a broad torque band and high fuel economy at commonly used engine operating speeds. Page 3 of 7
TOYOTA This fuel economy improvement can not be achieved by simple combination of long runner and short runner, rather only through manifold design optimization for performance tuning. (3) Variable Resonance Intake System The variable resonance system utilizes the intake manifold resonance effect at different operating speeds to improve engine performance. Because of engine configuration and firing frequency range by engine, variable resonance system is mostly used for V6 or I6 engine. Owing to V.I.S. system packaging space advantages, this is widely used for majority of V6 and I6 engine. NISSAN Just the same as with the variable runner system design, leak, sealing and NVH issues must be resolved to implement a robust plastic intake manifold concept. In order to achieve performance goals suggested by engine makers, all intake manifold design parameters should be carefully decided and optimized to achieve the intended performance characteristics. It is with not only optimization of runner length and diameter, but also with V.I.S. system layout, zip tube and plenum. Even with the same length of runner length and diameter, some manifolds can make low speed torque and others can make high speed power. Engine Cycle Simulation such as GT-Power, AVL-Boost may help to concept V.I.S. system, but as the manifold resonance effect has strong influence on combustion efficiency, analytic knowledge and experience in resonance tuning is critical for the development. (4) Variable Charge Motion System Variable Charge Motion system has been widely used in the past to improve low speed torque and to improve fuel economy through improved combustion efficiency. Porsche Page 4 of 7
improved performance. Further, this acoustic tuning effect becomes even more dominant with the introduction of variable control features like variable valve timing and lift control systems. Next, several performance tuning factors are briefly introduced. Due to market demand for high power and maximum torque, previously intake manifolds were developed with VCM systems. In the case where the intake port and valve were desired to be tuned for both high torque and power, SCV or tumble valve system used to be a good solution to make up low speed torque by increasing air speed and by improving combustion efficiency with stratified flow in combustion chamber. However, SCV or tumble valve system usually causes some level of increased flow restriction which hurts engine performance and increases pumping loss. Because of this trade off, now the variable charge motion system is more in use for emission reductions or to enable lean burn combustion to improve fuel economy. 4 Performance tuning with intake manifold In the past, before we had a clear understanding of acoustic tuning and inertial behavior within the intake manifold, flow restriction reduction was regarded as the most important criteria for performance development. With the development of more accurate measurement systems, the pressure variation inside the intake manifold could be traced and it was observed that the tuning effect played a critical role in obtaining (1) Reflective wave tuning When the intake valve closes, the air suddenly is blocked by the air intake valve and a strong compression wave is generated and propagated back to the intake manifold at the local speed of sound relative to the flow velocity. When the compression wave reaches the manifold plenum, it reflects back towards intake valve as an expansion wave. If the timing is appropriate, the compression wave arrives at the intake valve at the beginning of the intake valve opening, raising the pressure above the nominal inlet pressure, which allows more air to be forced into the cylinder. With maximum utilization of reflective wave tuning, significant volumetric efficiency improvement (over 20%) is possible. Reflective wave tuning can raise the charged air pressure at the beginning of intake process and this can significantly reduce pumping loss by reducing FMEP. Further, significant fuel economy improvements are also possible with an intake manifold designed to optimize reflective wave tuning. The below illustration shows pressure fluctuation amplitudes for a demo engine showing the possible supercharging level improvements with reflective wave tuning. Page 5 of 7
With reflective wave tuning, optimized development focuses on enabling the manifold to intake air at the highest pressure level into the cylinder. (2) RAM effect For RAM effect, as air flows inside the runner, the runner plays the role of a velocity stack and the pressure keeps increasing with raised air velocity (high RPM) inside it. This is called the RAM effect. This effect becomes progressively more important with increasing engine speeds. which raises the pressure above the nominal inlet pressure, allowing more air to be charged. (4) Intake resonance effect Pressure pulsations inside the intake manifold can create very strong acoustic resonance modes. If the resonance effect is properly aligned with the engine firing frequency for performance tuning, this can help to improve engine performance as well. Resonance strength tends to be mitigated as it goes to high frequency, so this effect is useful for low- to mid-speed performance development. For I4 engine application and some of V6 or I6 engine configuration with split plenum concept, this approach is very helpful to improve low- to midspeed torque. The below illustration shows V6 engine torque curves with a variable resonance intake manifold. Through design optimization, all below torque curves can be tuned with proper concept and optimization. From the illustration above, with a larger runner diameter, we can easily notice that the maximum peak torque drops significantly. This is because air inertia momentum inside the runner is reduced significantly with the larger runner diameter due to increased flow area, which eventually lowers kinetic energy and pressure of the air mass inside cylinder. When using the RAM effect, runner length and diameter should be carefully decided to guarantee maximum kinetic energy and pressure at tuned speed. (3) Inertia charging effect When the intake valve opens, the air suddenly rushes into the cylinder and an expansion wave propagates back toward plenum at the local speed of sound relative to the flow velocity. When the expansion wave reaches plenum, it reflects back towards intake valve as a compression wave. The time it takes for the round trip depends on the length of the runner. If the timing is appropriate the compression wave arrives at the intake valve before the intake process completed The below illustrates I3 and I4 engine performance torque curves with different manifold parameters. A. Zip tube length effect B. Runner length effect Page 6 of 7
5 Conclusion Now with the increased market demand for performance and higher fuel economy and cost, V.I.S manifold is gaining more popularity. Until this time, most engine manufacturers and intake manifold suppliers have been developing the air intake manifold by simply combining both long runner and short runner without detail study of thermal and tuning behaviors. This resulted in major hurdles in achieving both performance and fuel economy goal together with cost. Now, to meet all engine development requirements, proper performance tuning technology for the air intake manifold is the key for program success and all tuning strategy should be carefully studied and adopted as appropriate. This is critical, as a poorly developed V.I.S system does nothing but increase development time and part cost. Page 7 of 7