Piston Engines - Performance and Power Augmentation — Page 153, Lesson 198

Let’s pick this up right where the engine checks begin. I want to walk you through the idea of a Reference rpm, because it’s the anchor for the whole power-check procedure. When a new engine is fitted, or after certain major work, the engineer establishes a Reference rpm. That’s the engine speed at which the manifold pressure should exactly equal the Static Boost figure. A note of that Reference rpm is made and placarded somewhere convenient in the cockpit — typically right on the relevant rpm gauge. Once it’s set, it should not change appreciably. If it does change, that’s a red flag: it indicates some form of malfunction. And importantly, a new Reference rpm has to be established every time a major engine component is changed — for example, a carburettor or a magneto. Now, before we even start the engine, we look at the pressure gauge. Depending on whether the system is British or American, that gauge is either the Boost Pressure Gauge or the Manifold Absolute Pressure Gauge — the MAP gauge. Before start, it will show approximately ambient atmospheric pressure. Let me be precise about the numbers. At exactly sea level pressure on an ISA day — that’s International Standard Atmosphere — the MAP gauge reads 29.92 inches of mercury, or the Boost Pressure Gauge reads Zero Boost. If the airfield is higher, the reading falls; if the ambient pressure is above ISA sea level, the reading rises. That pre-start reading is called Static Boost, and you must note it, because it’s your baseline for checking engine power output. Here’s the sequence once you start the engine. The pressure in the inlet manifold will drop below the Static Boost figure, and it probably won’t begin to rise until about 1600 or 1700 rpm is established. Then, with maximum rpm selected — that means the propeller on the Fine Pitch Stop — you progressively open the throttle. As you do, the inlet manifold pressure should regain the Static Boost figure at the Reference rpm, plus or minus a small tolerance of, say, 50 rpm. The Reference rpm varies with different engine models, but on average it’s approximately 2000 rpm. If the result is outside tolerance, you’ve got a fault to hunt. The possible causes are: a cylinder down on power, the ignition system malfunctioning, a carburettor maladjustment, or even an improperly set propeller low pitch stop. Now let’s shift to the comparison between the turbocharger and the supercharger — specifically the internal supercharger. The obvious question is, which is best? If you only cared about added performance at ground level for a given cost, the turbocharger would probably win. But there are other considerations. First: do we only want the added performance at ground level? Unavoidably, with an aircraft, the answer is no — you need performance at altitude too. That’s where the internal supercharger wins, because it can increase engine power with aircraft altitude. Second: do we require the response to throttle opening to be instant? If yes, the internal supercharger wins hands down. The turbocharger, for all that it’s the cheaper option, cannot with present-day technology respond to rapid throttle opening without suffering from turbo-lag — that’s the delay between throttle movement and the turbocharger spooling up to deliver boost. So, to tie it together: the Reference rpm and Static Boost give you a repeatable, placarded baseline to verify engine health on every flight, and the supercharger-versus-turbocharger choice comes down to altitude capability and throttle response, not just cost.