Project Title
Effect of Variable Length Intake Manifold on Performance of IC Engine
OR
Effect of intake manifold geometry on Performance of IC Engine
Abstract
Intake manifold is the breathing system of the car engine which supplies air to the engine cylinders
where the combustion of the fuel occurs. Engine power & torque are greatly affected by degree to
which cylinders are charged & the geometric aspects of intake manifold. Conventional intake
manifolds have fixed length & geometry which is optimized for specific engine speed. Current
work aims at studying & analyzing the effect of variable intake length on performance of IC
engine. This will be analyzed with the help of engine simulation software & experimentation on
an actual engine. Intake manifold with different intake lengths mounted on engine under test & the
results are analyzed. The volumetric efficiency of engine found to be increased by varying the
intake length. At lower engine speed intake length must be long for proper breathing of engine
while at higher speeds intake length must be short
Introduction
Intake system comprises of various complicated paths such as air cleaner, intake pipe, throttle
body, plenum & runners/resonators to each cylinder. Air flows through all these parts & finally
enters in to engine cylinder. The main parts of the intake system are plenum, & runners. Overall
length including runner & plenum & volume of plenum mainly affects the torque characteristics
of engine.
In case of naturally aspirated engines volumetric efficiency is still low which is in the range of 75-
80%. Another problem with intake system is that in conventional engines it is optimized for a
specific speed at which it gives maximum torque. Studies showed that longer intake manifolds
give peak torque at low engine rpm & shorter engine manifolds gives peak torque at higher engine
rpm. Conventional engines have intake manifolds which are compromised to get benefit of both.
Project Aims and problems
To test engine performance by mounting runners of different lengths accordance with the engine
speed & calculation of intake runner length by theoretical methods to increase the volumetric
efficiency.
Air flowing inside the intake manifold runner, past the intake valve and into the cylinder flows
alike until the intake valve closes. After the closure of valve air strikes on the closed valve & high-
pressure wave is created. This high-pressure wave oscillates in the inlet duct until the inlet valve
opens for next stroke. The time after which inlet valve opens again if matches to the time of arrival
of high-pressure wave of air at inlet then Ram effect occurs causing highest possible air pressure
at IVC. Tuning corresponds to adjusting the length of intake runner so that this pressure wave
reaches exactly at the time when the inlet valve opens. This effect is also called as inertial ram
effect and length is decided by Chryslers Ram Theory.
High torque requires an intake manifold with geometry different to one for high power output. A
medium intake manifold length with a medium diameter represents a compromise but it results in
the lower torque & power at very low & high speeds which ultimately results into degraded
performance & less fuel economy. That why variable intake manifold is an ideal alternative to
solve this problem.
But intake manifold has negative effect on engine performance beyond certain speed range.
Therefore, careful design is essential for getting maximum performance from the intake
tuning.
Intake and exhaust tuning are mutually independent and the net effect on the engine is a
summation of their individual contributions.
Multiple reflections of a positive pressure wave significantly enhance engine breathing. Even
though the magnitude of the positive wave decreased with each „bounce‟, shorter pipes with
multiple returns outperformed the primary return of a longer pipe due to flow friction. Because
of this, volumetric efficiency and torque were maximized with runners of shorter lengths.
Modelling of pipe pressure losses is another important factor it should be paid attention to. It
influences mainly volumetric efficiency and all other integral parameters which depend on it.
Frequency analysis of intake system pressure waves recommended that for maximum
volumetric efficiency tuned intake system develops 4th order fundamental frequency during
intake valve closed phase and 1st order fundamental frequency during suction stroke.
