There are several ways to present piston engine performances. Figure 10.41 shows a Lycoming IO-360 series 180-HP piston engine. Readers may obtain the appropriate engine chart from manufacturers of other engines, or this graph may be scaled for coursework.
Readers should note that the power ratings are given in rpm. A Lycoming IO – 360 series takeoff is conducted at a maximum 2,700 rpm, whereas a climb is conducted at 2,500 to 2,600 rpm and cruise at 2,100 to 2,400 rpm. A partial throttle descent can be accomplished at 1,800 to 2,000 rpm. Idle is below 1,800 rpm (i. e., around 1,200 to 1,400 rpm; not shown). A fuel-flow graph is shown separately in Figure 10.42.
Piston engine power depends on the amount of airmass inhaled, which is indicated by the rpm and manifold pressure, pmanifold, at a particular ambient condition. A throttle valve controls airmass aspiration; when it is closed, there is no power (i. e., pmanifold = 0). When it is fully open and the engine is running at full aspiration, suction is created and the pmanifold reads the highest suction values. If there is less propeller load at the same rpm, less power is generated and the valve could be partially closed to inhale less airmass in order to run at equilibrium. At a low rpm, the aspiration level is low and there is a limiting pmanifold line. Therefore, the variables affecting engine power are rpm, pmanifold, altitude, and atmospheric temperature (nonstandard days). If an engine is supercharged, then the graphs indicate the
effect. Figure 10.41 shows the parameters in graphical form; how to use the graphs is explained herein.
Figure 10.41 shows two graphs that must be used together. The left-hand graph provides the starting point for reading conditions at sea level, and the ISA day
Figure 10.42. Lycoming IO-360 series – fuel flow graph
that must be converted to the operating condition at any altitude and atmospheric temperature are shown in the right-hand graph. The given engine condition must be known to obtain the HP at the ambient condition. In the example, the task is to find the HP that the engine is producing at 2,100 rpm, at a 23.75-inch Hg manifold pressure operating at a 2,100-ft altitude with an ambient temperature of 19°F (the ISA day is 511.2 R). The stepwise approach is as follows:
1. In the left-hand graph, locate the point corresponding to 2,100 rpm and at 23.75-inch Hg manifold pressure. Next, the HP at the Y-axis is found to be 114.
2. Transfer the 114-HP point to the Y-axis of the right-hand graph. Then, join that point to the point corresponding to 2,100 rpm and at 23.75-inch Hg manifold pressure in the same graph.
3. From the 2,100-ft altitude at the X-axis, draw a vertical line to the line drawn in Step 2. This gives 119 HP on a standard day.
5. For a nonstandard day, use the expression [BHPact/BHPstd] = j(Tstd/Tact).
6. Figure 10.42 provides information about fuel flow and has two settings – one for best power and one for best economy – which are adjusted by a mixture-ratio lever. For higher power and rpm, the mixture setting is at best power; for cruise, it is at best economy. It is evident that 2,100 rpm results in best economy. For the worked-out example, it is 50 lbs per hour at a power rating of approximately 68%.