Piston Engines - General — Page 24, Lesson 26

Let’s start with the big idea that governs everything in this part of the syllabus: valve timing is fixed for a given engine. It does not change with engine speed. So when you open the throttle or the engine spins faster, the valves still open and close at exactly the same points in the cycle. That’s a fixed mechanical fact. Now, if valve timing is fixed, how do we control power? The answer is by controlling the quantity of air that enters the cylinder. More air in means a greater pressure rise during combustion, which means more power. Less air means a smaller pressure rise and less power. The pilot controls that air quantity with a valve called the Throttle. So the throttle is not really a fuel control in the primary sense — it’s an air control. It varies the mass of air entering the cylinder, and that in turn varies the pressure rise during combustion. Next, I want to introduce a measuring device. The pressure variations inside the cylinder during the four strokes can be measured and shown graphically by a device that produces an Indicator diagram. This device plots pressure against volume, so the graph is also called a PV diagram. This is a small attachment fitted to research and experimental engines — not to the engines you’ll fly. It consists basically of a pressure transmitter fitted into the combustion chamber, in a similar manner to a sparking plug. That transmitter activates a moving pen which traces cylinder pressure variation against piston position. So you get a closed loop on the graph, and that loop is the indicator diagram. Now, what does that diagram tell us? The indicator diagram is used to plot the maximum pressures obtained, and that determines the shape and the area enclosed by the graph. That enclosed area is representative of the work done on the air and the power produced. So the bigger the area inside the loop, the more work is being done per cycle. There’s a version of the diagram opened out so the pressure areas under the curve can be compared and measured more easily. In that opened-out view, the area within the power column represents work done on the piston during the power stroke. The blue areas represent work done by the piston in compressing the charge and exhausting the cylinder against back pressure. So you have positive work from the power stroke, and negative work from compression and exhaust. When you average all of that out, you get an average reading of pressure on the piston during the working cycle. That average is called the Indicated Mean Effective Pressure, abbreviated IMEP. Remember that name precisely — Indicated Mean Effective Pressure. Now, here’s the practical point for the pilot. You are not given a cockpit display of IMEP. What you can be shown is manifold pressure, which is representative of cylinder pressure. That is displayed on the manifold pressure gauge. Opening the throttle increases manifold pressure; closing the throttle reduces it. The gauge is called the Manifold Absolute Pressure gauge, abbreviated MAP, and it is normally calibrated to read in inches of mercury. So when you see a MAP gauge, you’re reading inches of mercury, and that number is your proxy for what’s happening inside the cylinder. Finally, once you have the pressure in the cylinder, you can calculate the power of the engine. Using known constants — the area of the piston, which is the bore; the distance moved, which is the stroke; the number of cylinders; and time — you can calculate the Indicated Horsepower, abbreviated IHP. The formula is: IHP = (P × L × A × N × E) / 33,000 Let me unpack each symbol. P is the pressure, the mean effective pressure we just discussed. L is the length of stroke. A is the area of the piston, the bore area. N is the number of power strokes per minute — that’s the time factor. E is the number of cylinders. And the constant 33,000 is the number of foot-pounds per minute in one horsepower. So the numerator gives you the total work per minute, and dividing by 33,000 converts that into horsepower. That’s the Indicated Horsepower — the power developed inside the cylinders before any losses. So the chain is: throttle controls air quantity → that controls pressure rise → pressure is measured as manifold pressure in inches of mercury → and from pressure, bore, stroke, cylinder count, and time, you compute Indicated Horsepower. That’s the whole logic of this section.