Pulse-Ram Induction Theory:

Pulse-ram induction is a method by which improved cylinder charging can be accomplished based solely on the geometry of the intake system. Commonly known as induction tuning, it uses pressure-pulse phenomena created by the piston motion to ‘ram’ a high-density charge of intake gas into the cylinder, thereby improving volumetric efficiency. Since it requires no engine-driven mechanisms, it is commonly considered a ‘free’ boost.  However, as explained later, proper tuning often requires large manifold dimensions. In many modern vehicles, where engine optimization is often subordinated to styling and packaging considerations, designers are unable to employ this principle to its full potential.

Induction-pulse phenomena has been recognized since the early development of IC engines. However, many suggested models have been published which approach quantification of the effect in different ways.  Some methods consider the pulse as a single event from generation until valve closing. Others involve modeling the effect as a standing wave using organ-pipe theory. Still others are based on experiment alone. The method chosen was a vibrational model of the induction system suggested by Benajes, Reyes, Galindo, and Peidro and based in part on Heisler. The model has been found to agree with empirical data.  Following is an abbreviated development of the method.

The pulse-ram process is best visualized by considering a simple intake pipe on a single cylinder.  The intake valve is opened slightly before the piston reaches top dead center (TDC), or 0° crankshaft angle

Simple intake pipe and wave superposition.

on the exhaust stroke. When the piston begins its downward travel on the intake stroke, a depression is created in the cylinder whose amplitude follows the sinusoidal profile of the piston speed. This depression propagates outward as a rarefaction wave along the intake pipe. When it reaches the atmosphere at the open end of the pipe, it is reflected as a compression wave. Of course, when the first reflected pulse reaches the intake port, some of it will be re-reflected towards the open end. However, each subsequent reflection will suffer an exponential reduction in amplitude due to attenuation. Therefore, it is desirous to tune the intake to capture the first and strongest reflected pulse. If properly timed, this first reflection can assist cylinder filling.  The above image also shows a representation of the initial and reflected pulses present at once in the cylinder, separated by a phase shift angle theta.  Also shown is the resultant cylinder pressure caused by superposition of both pulses. The phase shift angle theta is dependent on the length and speed of pulse travel, and on the number of pulses generated per unit time. Selection of the proper angle will determine the optimum design factors for the manifold.

The authors suggest a model of the pulse phenomena based on the acoustic wave model. Such modeling requires the assumption that the pressure pulse behaves as a sound wave of infinitesimal thickness. Actually, it is a finite amplitude wave of finite thickness. The authors contend, however, that such an assumption is valid since it is the speed of the wave’s center which is of interest. Air (as an ideal gas) is assumed to be the working fluid, and it can be shown that assuming no variations in local speed of sound will not reduce accuracy. This model leads to the determination of the manifold’s natural frequency of vibration, w.  Note that the system is now analogous to a spring-mass system where the mass of the air in the manifold vibrates against the ‘spring’ or the compressibility of the air in the cylinder. The natural frequency w can be related to the pipe length L by the following equation, where a is the speed of sound in air (340 m/s at standard conditions).

This is valid for the simple case of a single intake pipe with its open end at the atmosphere. For the general case of a multi-cylinder, multiple degree-of-freedom arrangement, a more complex equation is needed.  The equation below assumes a system consisting of Z1 primary pipes leading from the intake valves to a connecting plenum of volume V. From the plenum, Z2 secondary pipes lead to the open end at atmospheric conditions. L1, L2 and A1 , A2 are the length and cross-section of the primary and secondary pipes, respectively.

Multiple degree of freedom manifold.

Now it is apparent that a multi-cylinder manifold can be tuned for multiple boost points. The first is due to the simple ram-pipe effect, and the second is due to the vibration of the system as a whole. Such a system is often called a Helmholtz resonator, and is explained in detail by Heisler. Since the pulse-ram effect tapers off sharply above or below the tuning point (zero effect at about plus or minus 500 RPM), it was noted that a manifold tuned for maximum performance at multiple engine speeds would produce a flatter torque curve, and improve performance over a greater range than the simple tuned pipe.

     The authors suggest determination of the dimensionless parameter Q, given below, to relate the manifold natural frequency to the excitation provided by the engine. Note that a four-stroke engine produces one intake pulse every two revolutions.

Sources agree that a value of theta around 85° is optimum to maximize the cylinder pressure slightly after bottom dead center (BDC), just before intake valve closing. However, there are other factors, such as pipe end effects and variations in tract geometry through the carburetor and port, which may affect the optimum value of theta. In addition, the particular valve duration will affect how much of the wave is captured. Since these effects are difficult to quantify, it is widely suggested that experimental means be used to fine-tune the selected phase angle. The design team chose to determine experimentally the primary pipe length (L1) giving the maximum cylinder pressure at the target speeds. This would then determine the optimization parameter Q and the system natural frequency for possible designs.