All engines generate a variable torque curve that peaks at some point in the RPM range as determined by dimensional characteristics of the flow paths. This peak represents the most efficient point in the engine’s operating range and closely mimics the VE curve. The point of highest VE creates the torque peak. Various tuning and engine configuration techniques enable you to adjust the position of the torque peak to the most favorable spot in the power band and to reshape the curve around it for maximum performance benefit.
Pressure-Volume Diagram
Pressure-volume (PV) diagrams seem a little kooky to those not used to working with them, but they reveal a lot about the engine’s airflow characteristics, and they help pinpoint abnormalities that need correcting. A PV diagram is a visual representation of the fluctuating pressure and volume changes in a running engine. These are the same pressure changes you seek to influence in your efforts to alter the engine’s airflow characteristics to suit a particular application or purpose.
A PV diagram illustrates the same information displayed by a pressure crank angle diagram. A PV diagram consists of two primary loops that isolate work performed from work consumed, or wasted, in the process. Each loop is annotated by a plus sign (+) for positive work and a minus sign (–) for lost, or negative, work.
The area in the lower loop represents the pumping losses of the intake and exhaust strokes. It is commonly referred to as the pumping loop because it illustrates the intake and exhaust pumping cycles. The upper loop depicts work produced by compression, combustion, and the power stroke. Take note that the lower loop (–) follows a counterclockwise direction and the upper loop (+) follows a clockwise direction.
This drawing is numbered to indicate the various points where the cycles begin and end. As you follow the sequence, bear in mind the cylinder pressure and piston displacement, as indicated on the X and Y axes at each point along the way.
The action begins at the same point as on the pressure crank angle drawing: point 1, IVO. Between point 1 and point 2 is the intake stroke. The pressure drops below atmospheric through this part, as indicated by the sag in the loop. As the loop rises from its lowest point, it indicates “pressure recovery” in the cylinder as the incoming charge exits the valve and fills the cylinder.
From point 2 to point 3 is typically the compression stroke, but the pressure spike you see between point 2 and the other loop indicates the intake ramming cycle, or inertia charging, as the charge velocity continues to fill the cylinder. A fatter loop at this point indicates rising VE due to ramming.
At point 3, you can observe the cylinder pressure increasing and the charge volume decreasing due to compression. When ignition occurs at point 3, cylinder pressure quickly spikes to peak based on charge density and combustion efficiency. It then begins to decrease as it pushes the piston down the bore. The volume remains relatively constant at the pressure peak and then increases as the burning gases expand down the cylinder bore against the piston top.
Points 4 through 6 represent the power stroke. Point 5 is the pressure peak that occurs approximately 10 to 12 degrees after TDC.
The piston reaches BDC at point 7 but the area from point 6 to point 7 represents the exhaust blowdown cycle where high cylinder pressure at EVO rapidly expels the bulk of the spent gases.
From point 7 to where the loops cross again is the exhaust pumping cycle. The crossover point represents the overlap period when the higher pressure determines what happens while both valves are open. If the exhaust pressure is still greater than the intake pressure, it tends to push residual exhaust back up the intake flow path. If the intake pressure is too great, some of it rushes through and out the exhaust before the door shuts. In a perfectly matched system, the last remaining residual exhaust exits the cylinder and invites the new intake charge to follow, which it does until the valve closes and cylinder filling begins again.
As engine speed increases, the PV loops tend to move upward on the pressure scale; very slightly on the pumping loop, but considerably on the exhaust loop. This is caused by insufficient time to blow down the cylinder because exhaust cycles become shorter as engine speed increases. Pumping work increases due to higher residual cylinder pressures, and horsepower begins to fade.
The PV diagram looks different in a supercharged application where positive pressure in the inlet tract eliminates the sag in pressure between points 1 and 2, and the whole loop tends to rise and become fatter because of the constant increase in inlet pressure.
All of this can also be related to crank angle in a pressure crank angle diagram.
The pressure volume diagram illustrates the pressure and volume changes that occur within the cylinder and combustion space during a typical cycle. The loop follows a clockwise direction as the air-fuel charge moves into and out of the cylinder and processes into power. The horizontal line on the lower loop shows how the pressure is negative as the piston descends while the valve opens. As the valve closes, pressure begins to rise during compression until the ignition point at which it spikes; expansion pushes the piston down the bore.
Power is governed by the overall efficiency of the specific component mix. It’s relatively easy to supply enough fuel, but it is considerably more difficult to maximize airflow without the aid of a power adder. For any given collection of parts, an engine achieves a torque peak influenced predominantly by intake and exhaust tuning relative to its size (displacement) and engine speed. Stroke length, piston speed, and the valvetrain dictate the overall operating range while the intake cross section and ramming set the torque peak RPM. Within these parameters the engine’s displacement (particularly with large bore and short stroke), intake flow capacity, and intake valve diameter provide the greatest influence on peak torque.
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The engine generates a torque peak at its highest volumetric efficiency. Below this peak, torque trails VE because of insufficient intake velocity. Above the torque peak, torque falls off due to insufficient time to fill the cylinders.
Through attentive manipulation of these and contributing component hardware, the torque curve can be shaped and positioned to suit the engine’s final application. The trick lies in properly matching the math, and the component combination, to not just produce a torque peak at a desired RPM point, but also to fatten the curve below and above the peak by increasing VE. Hence, the major importance of the intake ramming process and supporting wave tuning is to pack the cylinders as full as possible throughout the effective RPM range.
This is the principal focus of all competent engine builders (designers), and it begins with the pursuit of VE relative to the engine’s static air capacity. The air mass component depends largely on available air density and the VE that a specific component mix is capable of generating. It is primarily governed by inlet and exhaust flow-path dynamics, combustion chamber efficiency, valve timing, and elements of the bottom end and valvetrain that dictate final RPM capability.
The shape of the torque curve closely mimics the VE curve at peak torque. This is the point of maximum engine efficiency,