Piston Engines - Fuel — Page 90, Lesson 112

I want to walk you through the fuel section of your piston engine knowledge. We're going to look at fuel additives, octane ratings, pre-ignition, and thermal efficiency. Let's start with the additives. Detonation is that violent, uncontrolled explosion of the fuel-air charge in the cylinder, and we can avoid it by putting small quantities of additives into the fuel. The principal one used is Tetra Ethyl-lead, which we call TEL. The action of TEL is to reduce the formation of peroxides, and those peroxides are the chemical compounds that would otherwise encourage detonation. So TEL is a chemical suppressor that stops the chain reaction leading to detonation. Now, here's an interesting historical point. In the course of time, fuels with better combustion characteristics than iso-octane were produced. Iso-octane was the standard reference fuel, rated at 100. To rate these new, better fuels, comparisons are made with iso-octane doped with TEL. But since the percentage rating of the iso-octane can no longer apply—because these fuels are better than pure iso-octane—an alternative scale is needed. That scale is provided by a range of performance numbers. So a rating above 100, for example 100/130 grade gasoline, is a performance number. In practice, though, the fuel would still be referred to as a 100 octane fuel. So the "100" is the octane rating, and the "130" is the performance number on that alternative scale. Now, why do we care about high octane, or anti-detonation, ratings? Better quality fuel permits two big advantages. First, increased compression ratios, which give you an increase in thermal efficiency, better fuel consumption, and an increase in engine power. Second, increased induction pressure and greatly increased power from a given engine by the use of a supercharger. Let me explain that power relationship. The power output of an engine is directly proportional to the weight of mixture burned in unit time. Increased induction pressure will increase this weight—more air and fuel crammed into the cylinder means more weight of charge burned per second, so more power. Although, basically, the quantity or weight of charge induced will still depend upon the position of the throttle butterfly. So the throttle still governs how much charge gets in; the supercharger just packs more into that space. Now let's look at pre-ignition, which is also known as running-on. Pre-ignition is the ignition of the charge before the spark occurs at the sparking plug. So the fuel-air mixture lights off early, before the spark plug fires. This is usually caused by a local hot-spot in the combustion chamber, such as incandescent carbon or very hot sparking plug points. The consequences are rough running, running-on, and loss of power. Finally, let's talk about thermal efficiency. The heat produced by the burning of one gallon of fuel is capable of producing a lot of work if the heat is fully utilized and none wasted. But in practice, a considerable amount of work is lost in the form of heat to the cylinder walls and the piston crowns. The exhaust gases also remove heat, because their temperature is still high when they are expelled from the cylinder during the exhaust stroke. Additional work is absorbed in overcoming the internal friction of the engine. The net result is that, under the best conditions, rather less than 30% of the heat value of the fuel is converted into useful work at the propeller shaft. So out of every gallon's heat energy, less than a third actually turns the prop—the rest is lost to heat and friction. And there's one more point about fuel volatility. If the fuel is very volatile, not only will there be excessive losses by evaporation in the aircraft's fuel tanks, but the fuel will tend to boil and vaporize at the depression, or inlet, side of the fuel pump. That causes cavitation—bubbles forming in the fuel around the pump impeller—and vapour. That's a problem we'll pick up on next.