Let’s pick this up right where the idea of efficiency ratios left off — Page 24, Lesson 30

Let’s pick this up right where the idea of efficiency ratios left off. I want to walk you through the two big efficiency figures you’ll see on a piston engine: mechanical efficiency and thermal efficiency. First, mechanical efficiency. The general idea of any efficiency is output divided by input, times 100%. For the engine, the input is the indicated horsepower, or IHP — that’s the power actually developed inside the cylinders by the expanding gases. The output is the brake horsepower, or BHP — that’s the power you actually get out at the propeller shaft, after all the internal friction and pumping losses have eaten into it. So mechanical efficiency equals BHP divided by IHP, times 100%. A typical value sits in the region of 80 to 85%. That means roughly 15 to 20% of the power made inside the cylinders is lost to friction and other mechanical losses before it ever reaches the prop. Now thermal efficiency. This one is about the fuel. Thermal efficiency is the efficiency at which the heat energy released by the combustion of the fuel is converted into work done in the engine. So it’s heat converted into work, divided by the heat energy available within the fuel, times 100%. Engine design and the use of correct fuels both work to increase thermal efficiency. A good value for thermal efficiency in an internal combustion engine would be 25 to 28%. That’s a striking contrast with mechanical efficiency — you see, the engine is only turning about a quarter of the fuel’s heat energy into useful work; the rest is lost as heat to the cooling system, exhaust, and so on. Now let’s tie this back to what’s actually happening inside the cylinders. As I said earlier, air is the working fluid within the engine. Added to that is fuel, so it’s actually a mixture of air and fuel that enters the cylinders. The power of the engine is determined by the maximum weight of that mixture — we call it the charge — that gets induced, and the subsequent rise in pressure during combustion. Here’s the key limitation: due to inertia and factors affecting the density of the mixture, it is not possible to fill the cylinder completely during the induction stroke. So the cylinder never gets a full charge — there’s always some loss, and that’s part of why the efficiencies are what they are. That manifold absolute pressure gauge you see there is how you read the density of the charge being induced — it’s your window into how well the cylinder is filling. So keep these three ideas together: mechanical efficiency is about friction losses between IHP and BHP, thermal efficiency is about how much fuel heat becomes work, and the charge weight — limited by inertia and mixture density — is what ultimately sets the engine’s power.