Practical Engine Airflow. John Baechtel

to the engine. A throttle linkage connected to the gas pedal operates the butterflies.

       Patrick Hale’s horsepower chain introduced three additional cycles to the traditional four-cycle engine model. They include intake ramming from the charge inertia effect, exhaust blowdown to account for the initial high-pressure exhaust evacuation, and the valve overlap period as a significant cycle affecting the intake and exhaust relationship. (Illustration Courtesy Scott Lozano)

      Our task as engine builders is remarkably similar. We want to understand the air mass condition and the various influences that act on it so we can manipulate it to improve efficiency and power output in the power range most useful to our application.

      Air moving through a running engine experiences a dramatic series of pressure changes before it finally exits the tailpipe and returns to atmospheric pressure. The seven cycles, or processes, identified by Hale define these pressure changes and how they combine to produce torque and horsepower. If you follow the air pump analogy and also think of the engine as an air processor, you can more accurately understand the major steps used to create power:

      1. Intake pumping

      2. Intake ramming

      3. Compression

      4. Fuel burning and expansion (power stroke)

      5. Exhaust blowdown

      6. Exhaust pumping

      7. Valve overlap

      The traditional four cycles are 1, 3, 4, and 6 on this list. These are what you have always had to work with, but as Hale points out, the major gains in engine output come from working with the three additional cycles that exert enormous influence on the overall process.

      You must also consider the negative pumping effects that accompany these processes, including the cumulative consequences of friction, mixture compression, and airflow resistance (more commonly referred to as pumping losses). Resistance to the motion of the rotating assembly and the free movement of air through the engine are also primary culprits. The air does not specifically require pumping except in the case of supercharged applications designed to boost and improve the normal characteristics of atmospheric cylinder filling, or natural aspiration. Instead, it reacts to pressure changes to fill the cylinders.

      One of the most important factors of the seven cycles is the close interrelationship among them. Each cycle represents a specific process inexorably linked and influenced by the cycle before it and the one following it. In Hale’s words, “The output from one process defines the input for the next.” They are inseparable. Each process affects the next in an unbroken circle or, as Hale calls them, links in the horsepower chain. It takes two revolutions of the crankshaft (720 degrees) of rotation to complete the seven processes for each cylinder. And then it begins again. Each process must be fully optimized to ensure maximum performance from the engine.

      A fault, or less than optimal performance, from each process affects every subsequent cycle and degrades the power process. Hence the inputs and outputs and what you do with them within each process define how well your engine performs within its operating environment.

      As Hale indicates, each of the processes adheres to a different set of physics. You can only manipulate their performance by changing the shapes, sizes, and various interactions of the components that make up the overall engine.

      For example, commercial exhaust headers are by necessity a compromise based on a broad range of engine sizes and requirements. Header size and length are largely determined by what fits a specific engine and chassis combination. It’s up to the engine builder to calculate and select the correct sizes. And to be honest, any full racing effort uses custom-built headers specifically tailored to that particular engine’s requirements and operational characteristics. If the wrong headers are used many links of the chain become compromised and less than optimal performance occurs.

      If residual exhaust gases remain in the combustion chamber through some failure of the exhaust blowdown or exhaust pumping process, they contaminate the fresh intake charge and seriously degrade the power potential. The contaminated charge then affects the entire process with a resultant power loss. That’s why each process must recognize and complement the subsequent process to ensure optimal performance.

      The individual seven cycles control the movement of air through the engine and ultimately influence the whole character of an engine’s performance potential. It is very important to visualize their effect on the high-speed air column as it moves through the engine. These cycles influence the airflow resulting from pressure changes that lead to superior power output.

      Intake Pumping

      The intake pumping process begins when the exhaust valve closes (EVC). This event initiates during the valve overlap period and slightly after TDC. At this point the intake valve has also opened (IVO). The intake valve is accelerating toward its full-open position. The piston is descending at some given rate dictated by the stroke and rod length, typically faster with shorter rods and slower with longer rods. In either case, this exposes cylinder volume to the intake port at some particular rate and offers a filling opportunity. The highest demand (or draw) typically occurs about 75 to 76 degrees after TDC, where the piston achieves its highest velocity (speed), thus creating the lowest pressure in the cylinder.

       High-velocity air in the intake ports gains inertia to help ram-fill the cylinder above and beyond that achievable by normal pressure recovery. This creates the ramming effect that Hale calls “intake ramming.” (Photo Courtesy Smithberg Racing)

       In Hale’s description, the descending piston (center) creates a depression (or low pressure) above the piston that tugs on the intake charge the hardest, somewhere in the neighborhood of 76 degrees after TDC. At this point air is rushing to fill the cylinder at maximum velocity. At BDC (left) the successful ramming process is still packing the cylinder to a density that exceeds the cylinder’s physical capacity, and pressure begins to rise prior to any actual compression activity. When the exhaust valve opens the cylinder is still under high pressure and the initial blowdown is very rapid. Following that, the piston pushes the remaining charge out of the cylinder as it once again rises to TDC (right).

      The piston descends and the intake valve opens farther, the flow rate (velocity × area) increases until the valve reaches maximum lift at about 108 degrees after TDC. This is the intake pumping process, or the rapid transfer of the air/fuel mixture into the cylinder in pumping fashion. It ends when the piston reaches BDC at the bottom of the stroke and begins to reverse direction.

      Thus begins the opening cycle of the traditional four-stroke internal combustion process. At this point, the intake valve is still open.

      As Hale said, the output of each process affects the input of the next process. In this case excessive camshaft overlap (during the valve overlap process) can allow residual exhaust gases from the still open exhaust valve to contaminate the fresh incoming charge from the intake valve. This means that the exhaust blowdown and exhaust pumping cycle has inadequately evacuated the spent cylinder gases or that valve overlap needs to be lessened to accommodate the inadequate scavenging effect.

      It is exceedingly difficult to manage the pressure changes: when they begin, when they end, and what happens as they change. In the presence of fixed piston motion, you can only alter valve timing or intake/exhaust flow characteristics to control each of the seven cycles.

      An ideal intake pumping process begins at EVC and ends when the piston briefly stops at BDC. Assuming a 100-percent fresh-intake charge, no contamination can occur once the exhaust valve closes. Under ideal conditions, the intake pumping process ends with the piston at BDC, and the same 100-percent fresh charge occupies the specific