P ISTON E NGINE AND C OMBUSTION P ROCESS
6.1 Piston Engine
C H A P T E R
6
P ISTON E NGINE AND
FIGURE 34 Example of a four-cylinder horizontally opposed engine, fue l, magnetos, cylinders, and exhaust systems (drawings created using Solid Edge CAD tool).
FIGURE 35 Example of a four-cylinder horizontally opposed engine, cyli nders, valves, and propeller systems (drawings created using Solid Edge CAD tool).
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Cessna 172M, it is capable of generating 14 V and 60 A of DC power. A starter motor is connected to the engine crankshaft by a belt; the shaft is connected to the pistons via crankpins and connecting rods. The crankshaft moves the piston head inside the cylinder as well as the camshaft assembled on top of the central shaft, which operates the intake and exhaust valves synchronously with the piston motion, as in Figure 34 and Figure 35. As in all internal combustion engines, the four stages of intake, expansion, combustion, and exhaust take place. During the intake stage, the atomized fuel combined with air enters through the intake valves via a manifold. The Cessna 172M is equipped with four horizontally opposed cylinders that are air-cooled and drive the propeller directly. Each piston is capable of displacing air inside the cylinder by 320 in3. The engine generates about 150 to 165 hp (Lycoming O-320-E2D engine). The main shaft rotates the propeller at the desired Revolutions Per Minute (RPM). There are two different situations possible for engine RPM values: (a) static, in which the aircraft is stationary while on the ground (2,300 RPM–2,420 RPM); and (b) dynamic, which is the maximum RPM demonstrated during flight, where the fuel-to-air mixture ratio has been set to its maximum performance level (2,700 RPM).
6.1.2 Fuel System
The fuel system for a small aircraft (i.e., takeoff weight under 1,500 lb) typically consists of two internal wing tanks that are connected by a tube for venting air and keeping the fuel level on both sides, in addition to air vents that are built in the fuel caps, securing the fuel from exiting the tanks when performing maneuvers. Early versions of the Cessna 172 used a single air-vented fuel cap for the left tank; however, the right-tank fuel cap has also been air-vented since the mid-1980s. There is also a separate air vent located at the bottom of the left tank with an elongated L-shape; it is the main source of air venting. If the air passage is blocked for whatever reason, such as accumulation of ice and debris at the entrance of the air vent, fuel starvation may occur. Beryl Clutterbuck Markham’s aircraft, Vega Gull, suffered fuel starvation due to icing of the fuel tank vents, causing her to crash-land at the end of her solo flight west across the Atlantic. Note that when performing maneuvers such as landing, takeoff, descent, and ascent, the fuel regulator is to be set on both tanks. An exception is in emergency situations such as if one wing is on fire and the fuel feeding from that wing is to be turned off to avoid fueling the fire and to do damage control to the extent possible. This is done in addition to turning off the electrical devices associated with that wing such as landing, navigation, and taxi lights.
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Aircraft flight condition, such as level, ascending, or descending flight, is a determining factor in deciding on the optimum rate of fuel supply required to minimize fuel consumption. In a piston-engine aircraft, the internal combustion engine principles apply, and therefore the mixture of air and fuel is required in order for the combustion process to take place. Since the fuel is denser than the air by a factor of 589 on average at 15 °C (721 kg/m3 versus 1.225 kg/m3), one needs to vary the number of air molecules; for example, by leaning or enriching the fuel mixture, for the combustion process to proceed efficiently. The air-to-fuel mixture mass ratio may vary from 8:1 (rich) to 20:1 (lean). However, the complete combustion mixture ratio is approximately 12.5: 1—the chemically correct mixture for a stoichiometric process for an octane fuel.
The ideal process of burning fuel follows a stoichiometric combustion process in which the fuel belonging to a specific family—such as isooctane (2,2,4-trimethylpentane—C8H18) used as an Aviation Gasoline (AVGAS), versus the Motor Gasoline (MOGAS) used in some light aircraft—is mixed with air in a carburetor after being strained for small contaminant particles by means of gravity in high-wing aircraft and a fuel pump in low-wing aircraft.
100/100LL (blue) is the low-lead version of isooctane fuel types that is used in piston engines, and it performs better than the 80/87 (red) and 100/130 (green) fuel types due to the presence of lead, which improves the combustion process. Lead itself is poisonous to handle; therefore, wearing plastic gloves when filling the tanks or doing the walkaround fuel checks is recommended. During excess idling at the start of the flight while doing the routine checklist, the extra lead may deposit on the cylinder heads and therefore cause buildup and charring, which is the accumulation of the carbon black on the heads and spark plugs. The char may be formed in any part of the cylinder head, and due to its high heat absorption properties, it acts as a new spark plug, meaning that the created hot spots work as combustion nuclei, and as a result combustion takes place away from the spark plugs. This causes detonation, fouling of the spark plugs, and knocking to occur (a premature explosion which leads to banging noises due to the inappropriate contact between the piston and cylinder head). Tetraethyl Lead (TEL), first introduced in the 1920s, is the additive used to prevent engine knocking [134]. It allowed for an increase in engine compression and therefore engine performance. The United States Clean Air Act in the 1970s limited the use of TEL due to its negative environmental effects.
100LL (0.56 grams lead per liter, 1.2 to 2 grams TEL/US gallon) is allowed
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for 0.5TEL as an additive used for 100/130 (1.12 grams lead per liter, TEL) AVGAS. Earlier aircraft models run on a lower grade of AVGAS fuel such as 80/87 (0.14 grams lead per liter, TEL). China remains the largest TEL producer, followed by Algeria, Yemen, and Iraq.
6.1.3 Carburetor and Venturi Tube
The fuel carburetor mixes fuel and air to the required portion for the engine combustion process to take place. The ratio of the fuel and air is the determining factor for whether the combustion process is efficient. As the air is drawn into the carburetor by the movement of the cylinders (which work like an air pump), it passes by a narrowed opening which creates a venturi tube where the air accelerates. This creates a low pressure, which draws the fuel through small holes (called jets). The atomization process happens by forcing the fuel through these small jet openings under high pressure to break it into a fine misted spray (by primer)—similar to a perfume spray bottle mechanism. This mist is then mixed with air and becomes usable for the internal combustion engine. The flow of the air- and-fuel mixture is then controlled by a butterfly valve (throttle) connected to the throttle control lever. It nearly blocks the air flow when closed at idle and provides unrestricted flow when open at full power.
Turbine engines as well as diesel piston engines use jet fuel (kerosene- based). For an optimum stoichiometric combustion process to occur, a mixture of 12.5 molecules of air mixture to 1 molecule of fuel is required for a pure octane fuel—meaning the air-to-fuel ratio is 12.5 to 1, which will be discussed later in this section. This equals a 0.08 fuel-to-air ratio that the reader may come across in other scholarly resources.
You may have noticed a Venturi tube—the horn-shaped system which is used to operate vacuum-driven gyros in non-engine-driven vacuum systems, located on the side of older generations of aircraft such as the 612 kg single-piston-engine semi-monocoque Aeronca-7DC built in 1946, the first stick-controlled aircraft the author flew (Figure 36). In the constricted cross section of the tube, due to reduction in area and conservation of mass, fluid speed is increased, resulting in lower pressure to comply with a simplified form of conservation of energy as described by Bernoulli’s principle for cases of an incompressible flow. The flow then moves from the area of the higher pressure to that of the lower pressure (i.e., to the constricted area in a vacuum-driven gyro system). As the pressure drops, it will pull in the air from the gyro, causing the gyro to spin, as in Figure 37.
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FIGURE 3 6 Single-piston-engine semi-monocoque Aeronca-7DC built in 1946 (drawings created using Solid Edge CAD tool).
FIGURE 37 V enturi tube (drawings created using Solid Edge CAD tool).
Equation (86) shows the conservation of the mass inside a venturi tube (Figure 37). Mass flow is constant for each of the sections enclosed in a virtual control volume shown by dashed lines. If the flow is compressible (the density does not remain constant with temperature change or non- homogeneities), inviscid (viscosity is close to zero), and steady (the flow does not change with time), equation (87) is valid. Subscripts “i,” “c,” and
“o” are related to the properties for the inflow (flow entering the tube), the flow at the constricted area, and the outflow (flow exiting the tube), respectively. is fluid density, A is area, v is velocity, P is pressure, z is height with respect to a reference point, and m is the mass flow rate that is assumed constant throughout the tube.
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For an incompressible flow, where central locations of the inlet and constricted areas are at the same level, equation (87) may be simplified to equation (88)—assuming that the volumetric flow rate (Q) is constant throughout the tube.
i i i c c c o o o
m A v A v A v (86)
1 2
2 1
i c
c i
i c
c c
c c i i
P P
g z z
m A
A A
(87)
2
2 1
i c
c
c i
P P Q A
A A
(88)