Let's pick this up right where the formula left off — Page 24, Lesson 28

Let's pick this up right where the formula left off. We have the Indicated Horsepower formula, IHP = PLANE / 33,000, and I want to make sure you understand what each of those letters stands for, because they're the building blocks of everything we're about to discuss. P is the Indicated Mean Effective Pressure, measured in pounds per square inch. That's the average pressure pushing down on the piston during the power stroke. L is the Length of Stroke in feet — how far the piston travels. A is the Area of the cylinder in square inches. N is simply the number of cylinders. And E is the Effective working strokes per minute, which is essentially the engine speed in rpm. Now, why does this formula work? Because power is the rate of doing work, and work is done when a force is moved through a distance. So we have a force acting on the piston — that's the pressure, in pounds. The piston moves through the distance of the stroke — that's in feet. And it does this so many times a minute. When you multiply all that out, you get foot-pounds per minute — a rate of doing work. Now, here's the historical anchor. The inventor of the steam engine, James Watt, calculated that the average horse could move 1 pound a distance of 33,000 feet in 1 minute — that's 550 foot-pounds per second. That's why we divide the PLANE formula by the constant 33,000, and why we call the unit of power horsepower. In SI units, the unit of power is the watt, and 750 watts is approximately equal to 1 horsepower. But here's the crucial reality check: IHP is only a theoretical value of power. It's what the engine would produce if there were no losses. In reality, moving the piston and turning the crankshaft consumes power. That consumption is called Friction Horsepower, or FHP, and it must be deducted from the IHP. The power that's left over to do useful work — like driving a propeller — is called Brake Horsepower, or BHP. So the relationship is: IHP minus FHP equals BHP. Now let's move to a performance concept: Power to Weight Ratio, also called Specific Power Output. This is a comparison of an engine's power output per unit weight, expressed as a ratio. The units are kilowatts per kilogram, or horsepower per pound. For example: an engine weighing 1000 pounds — that's 450 kilograms — and producing 250 horsepower — that's 190 kilowatts — would produce a power-to-weight ratio of 0.42 kilowatts per kilogram, or 0.25 horsepower per pound. This ratio tells you how much performance you get for the weight you have to carry. Next, Specific Fuel Consumption, or SFC. The increase in energy given to the air comes from the heat released by burning the fuel, and that heat produces power in the engine. The weight of fuel burnt, in pounds, for the power produced — that's BHP — in unit time, in hours, is called the Specific Fuel Consumption. Engine designers strive to get as much power as possible from the engine for the minimum weight of fuel burnt. So during operation, if you get a reduction in power for the same weight of fuel burnt, that's defined as an Increase in Specific Fuel Consumption. Conversely, a reduction in fuel burnt for the same, or more, power is a Decrease in Specific Fuel Consumption. Now, SFC is affected by two things: engine design and pilot operation. Since the pilot has no control over design, correct operation of the engine is essential if the performance figures are to be attained. That's a direct call to you as the operator. Finally, let's talk about Engine Efficiencies. The engine is a machine that converts heat energy into mechanical energy. Sadly, there are losses in this transfer, and engine design tries to reduce those losses. As we said, the IHP developed in the engine is reduced by FHP, leaving BHP to do useful work. The term efficiency means simply a comparison of what is got out of a system with what is put into the system. The efficiency of any mechanical device must be less than unity — it's usual to express it as a ratio. And that brings us to Mechanical Efficiency, which is calculated as Output divided by Input, multiplied by 100 percent. That's the ratio of the useful power you get out — the BHP — to the total power you put in — the IHP. So, to tie it all together: IHP is the theoretical maximum, FHP is what's lost to friction, and BHP is what's left for useful work. Mechanical efficiency is the percentage of the input that actually becomes useful output. That's the core of how we evaluate an engine's performance.