Practical Engine Airflow. John Baechtel

Pressure decays as the piston descends, until the exhaust valve opens (EVO) around 120 degrees after TDC. You get what you get at this point, and there is little you can do to influence it except provide complementary fuel to control detonation and regulate ignition timing to optimize the burn characteristics of the combustion chamber. Remember that the piston top forms the floor of the combustion space, and its shape and rate of approach can also influence combustion characteristics to some degree.

      Exhaust Blowdown

      The discharge process can be divided into two separate events characterized by different physics and thermodynamic processes. The first is exhaust blowdown, which initiates at EVO. Cylinder pressure is still relatively high, and some of the gases may still be burning. In some cases, a combination of poor cam timing and late ignition may even cause burning to continue into the header pipe. For the most part, high cylinder pressure attempts to exit the cylinder as soon as the valve cracks open. Exiting gases briefly achieve supersonic flow until the valve opens farther.

      The blowdown cycle is the primary source of exhaust noise. The cylinder blows down rapidly because of the high-pressure gases escaping past the exhaust valve. Most of the exhaust exits the cylinder via its own high-pressure energy, and the event concludes when the piston reverses direction at approximately BDC. Depending on the timing, it’s possible that the cylinder pressure could still be higher than atmospheric, and the piston is moving very slowly, so it’s not precisely at BDC in every case.

      Exhaust Pumping

      This is the second discharge function, which begins when the piston begins to rise on what is traditionally called the exhaust stroke. Most of the cylinder has already blown down, but as the exhaust valve opens farther the rising piston pumps out residual gases. The valve reaches maximum lift at about 70 degrees after BDC and the piston achieves maximum velocity around 105 degrees after BDC.

      On the exhaust stroke, the piston is chasing the exhaust valve, which is trying to close before the rising piston catches it. The piston pumps the remaining gases out of the cylinder, completing the exhaust pumping process when the intake valve just begins to open (IVO) as the piston approaches TDC.

      The exhaust blowdown is a high-pressure self-induced evacuation of the cylinder. The exhaust pumping cycle is the forced expulsion of the remaining low-pressure gases by the rising piston (the mirror image of intake pumping).

      Exhaust Pumping Influences

       Here are some of the factors that affect the exhaust pumping cycle:

       • Rod/stroke ratio (influences piston speed and acceleration)

       • Exhaust-port steady-flow capacity (at low to moderate depression and moderate to high valve lift)

       • Exhaust system pressure-wave tuning

       • Maximum piston speed

       • Cylinder displacement

       • Bore area–to–exhaust throat area ratio

       • Initial pressure and temperature

      Important goals to accomplish during the exhaust-pumping event include minimizing the pumping effort and the residual mass of spent exhaust gases remaining in the cylinder at IVO. It’s also important to reduce the cylinder pressure at IVO to less than intake port pressure to prevent spent gases from flowing back up the induction path and contaminating the next charge.

      Valve Overlap

      For a brief period around TDC, both valves are open at the same time, and various things can occur depending on the strength of the inlet flow and the remaining exhaust pressure. This event is called the valve overlap period. It begins just after IVO, and slightly before TDC. Valve overlap doesn’t hit the piston because it is still up in the chamber at low lift. The exhaust valve is open but almost closed, and the piston does a drive-by of both valves at TDC. The overlap event ends when the exhaust valve finally closes and the intake pumping process is in full swing.

      A proper overlap event ensures that all spent gases are expelled from the cylinder. The ideal condition is to have the exhaust scavenging process, assisted by pressure wave tuning, provide a slight tug on the intake charge just before EVC. The overlap event is necessary, complicated, and even quite valuable when properly timed.

      Conclusion

      As described by Hale, the seven cycles, or processes, are very much interrelated and overlap each other with their input and output influences. The illustration on page 12 is especially useful in visualizing the ongoing cyclical process and how each event influences the next. It is useful to pay close attention to the overlapping portions of each cycle relative to the various valve events and piston positions on the inner circle of the diagram.

      For example, the compression cycle begins opposite the IVC point and ends at TDC. The fuel burning and expansion cycle overlaps compression to slightly before TDC to indicate the plug firing at whatever timing is set. Study these relationships closely to gain a solid perspective of how all these events interact. Then ponder the actual pressure changes and thermodynamic processes occurring within the chain every step of the way at mind-bending speed.

Torque and Horsepower

      Engine power comes from the chemical oxidation of the fuel that the engine burns. Burning more fuel provides the potential to increase power if the essential requirements of internal combustion are served adequately. Burning fuel requires an oxidizer to support combustion. In a stroke of extraordinarily good fortune, all IC engines enjoy a remarkably convenient and unlimited supply of oxidizer in the form of the earth’s atmosphere. The 21-percent oxygen content that keeps us all breathing easy also makes engines run by providing the oxidizing component that sustains the combustion process. Hence engine airflow is effectively the controlling factor of high-performance engine output.

      Overlap Influences

       The following either directly or indirectly affect the overlap period to varying degrees:

       • Configuration and influence of the combustion space

       • Rod/stroke ratio (influences piston speed and acceleration)

       • Exhaust system pressure-wave activity

       • Initial pressure and temperature in the cylinder and the intake and exhaust ports

       • Inlet system pressure-wave activity

       • Instantaneous plenum pressure

       • Intake and exhaust port flow capacity (at low-pressure ratios and low valve lifts on the flow bench)

      The goal of optimizing engine airflow is to produce performance and competition engines that deliver maximum torque and horsepower in an operational range most suited to the vehicle’s specific application. This is characterized by high VE and effective management of engine airflow within the RPM range that constitutes the desired operational power band. Maximum torque across an application-specific range of engine speed is the objective. Torque is the twisting force representing the potential to perform work. Engine torque is the force potential, or turning moment, applied to the crankshaft flange, or flywheel, when combustion pressure is transferred to the crank throws via the connecting rods. When the flywheel turns, torque is measured by the resistance to rotation. Once the flywheel is turning, torque applies over a period of time, and horsepower is calculated via this formula:

      HP = torque × RPM ÷ 5,252

      Where: 5,252 = mathematical constant

      Torque is the measure of an engine’s ability to perform work. It is characterized by high volumetric and combustion efficiency. Horsepower is the rate at which the work is performed. Torque accelerates the mass of a race car while horsepower, a derivative of torque, supports vehicle motion (speed) by maintaining the application of torque over time.

      Engine builders strive to produce torque rapidly over a pre-determined range of engine speed (RPM) chosen to support