Practical Engine Airflow. John Baechtel

they encounter a closed valve and reflect positively while others encounter an open valve and reflect negatively. So the inlet can reflect a positive pulse that can help fill the cylinder. These are very high-speed pulsations, but their timing can be calculated and manipulated according to the length of the flow path. Because they move at supersonic speed you generally have to take advantage of every third or fourth reflection to suit the packaging constraints of the inlet paths.

      Inlet path length is typically calculated to take advantage of the third reflected wave for best power. The second wave works well for fuel-injected OEM applications, and most single 4-barrel race engines work best with the fourth wave. Builders no longer need to spend time calculating these lengths because it is done for them in a very affordable (less than 50 bucks) modeling program offered by Larry Meaux called PipeMax. Simply input your engine values and choose the recommended length from the calculated values. More on this later in Chapter 4.

      It is important to time the IVC point correctly. We used to refer to it as late intake closing for the purpose of continued filling, but it was never adequately explained that the additional filling occurs because of intake charge inertia, or ramming. Because the piston is now rising and starting to initiate a pressure buildup, you must close the intake valve at the point where the pressure rise in the cylinder equals the pressure of the intake flow to ensure maximum ramming. This is the opposite end of the combined intake events (pumping and ramming).

      At the beginning of the filling process, you have the potential for contamination because the intake valve opens before the exhaust valve closes (overlap). At the end of the filling process, you encounter a stalling effect to the flow if the valve is left open too long. Although minimal compression takes place in the early part of the compression stroke, it eventually reaches a point where it overcomes the strength of the incoming charge. This is the point at which you need to close the valve and count your blessings.

      The intake ramming process provides the VE increase above 100 percent plus clearance volume. In very refined applications such as a Pro Stock engine, it can approach 130 percent. The momentum effect of the ramming process and the pressure waves that assist it are engine speed dependent and thus most effective at certain engine speeds. Chapter 4 includes a discussion of how the momentum effects of the fast-moving inlet air column affect the flow direction, mixture quality, and pressure recovery characteristics of any particular inlet and combustion chamber combination; this means they must be contemplated as a total system.

      Compression

      The compression cycle begins at IVC. Prior to this, the piston has already begun to rise, but the intake valve is still open, and there is no appreciable pressure change until the inertia of the intake charge is overcome. Even then it is minimal, but in some cases it’s enough to dam up the intake process causing reversion, or the stacking up of the intake column to the point where a cloud of fuel vapor forms above the carburetor entry.

      The actual compression event begins after the piston is well on its way up the bore. And it ends before the piston reaches TDC because the spark plug fires at about 35 to 30 degrees before TDC, ending the compression process and initiating the next cycle: the fuel burning and expansion process. Depending on the particular geometry of the slider crank relationship the piston may still be anywhere from 1/8 to 1/4 inch down the bore when the compression process (stroke) ends.

       The bowl area below the valve serves as a conditioning space designed to help the flow transition as efficiently as possible around the entire circumference of the valve head. Valve throat diameter and the chamber wall and roof characteristics immediately around the valveseat largely determine the port’s flow efficiency.

       A Hemi head is less susceptible to shrouding because most of the valve circumference is not blocked by adjacent chambers walls as it is in a wedge-type chamber.

      Here, the efficiency of the combustion chamber and fast-burn characteristics can affect pumping losses. The plug fires before TDC to give the combustion kernel time to grow and begin building pressure. The earlier the plug fires (more timing, as in 42 degrees before TDC instead of 35 degrees before TDC), the more pressure builds ahead of TDC. Early pressure rise resists piston motion and incurs a pumping loss to overcome the resistance. Modern fast-burn chambers require much less initial spark timing and thus reduce this parasitic characteristic.

      The failure of the previous two processes to adequately fill the cylinder most often contributes to the faulty link that can occur in the compression process. Mixture density diminishes, and there is less trapped mass to compress before the firing sequence. It’s a vicious circle. If any player on the team stumbles it defeats the whole process and power suffers accordingly.

      The compression process is shorter than the actual physical stroke of the crankshaft. That’s because it doesn’t begin until IVC, and the piston is already part way up the cylinder. It ends when the plug fires, although the piston has not yet reached TDC. As an example, you might say that the compression portion of a 3-inch stroke only involves 2.2 inches of piston travel. These are arbitrary numbers, but you get the idea.

      The ideal compression process gains no heat from the cylinder walls and fully stratifies the fuel vapor charge across the entire combustion chamber to achieve optimal combustion. The maximum unfired cylinder pressure (to prevent detonation or pre-ignition) for the fuel type and air/fuel ratio is achieved by piston motion just prior to the plug firing.

      A higher static compression ratio translates to higher cylinder pressures depending on the IVC point. You can manipulate the rod-to-stroke ratio to alter piston speed and piston position at IVC. Plus, heat transfer can be partially controlled with thermal coatings, and you can select and modify piston domes and chambers to more favorable configurations to enhance the burn.

      Compression Event Influences

       Although ideal conditions are difficult to achieve, you can work with the following features to optimize the compression event:

       • Static compression ratio

       • IVC (determines the effective compression ratio)

       • Rod/stroke ratio

       • Initial temperature and pressure of the trapped air/fuel mass

       • Heat transfer from the piston top, cylinder walls, and combustion chamber

       • Piston dome and combustion chamber shapes and characteristics

       Custom intakes with tapered runners are used to build pressure and velocity in the inlet flow path. This supports the intake ramming cycle, which relies on charge inertia to continue filling the cylinder. The radiused inlets encourage smooth airflow into each runner.

      Fuel Burning and Expansion Cycle

      The expansion cycle is the money cycle, as they say, although many argue that the ramming cycle is the most important. The combustion cycle is wholly dependent on the success of the preceding cycles. Its contribution is also influenced by ignition quality and consistency, fuel quality and mixture consistency, chamber efficiency, engine load, cooling, and other factors that combine to influence the quality of the burn and the power derived from it.

      This cycle begins with the spark-induced ignition of the fuel mixture and subsequent burn and expansion of the gases. It is not an explosion, but pressure and temperature build rapidly, and the expanding gases push the piston downward. Combustion pressure multiplied by total piston area results in thousands of pounds of pressure exerted on the piston. Depending on the type of engine, pressure normally peaks about 12 degrees after TDC.

      The bulk of the work occurs here and trails off over