Procedimientos e instructivos internos de seguridad y salud en el trabajo

An internal combustion engine converts chemical energy into mechanical energy through fluid expansion by combusting fuel with an oxidizer. The effectiveness of this process is called the fuel conversion efficiency (ηf) shown in Eq. 1.31 The efficiency of an engine is related to the heating value of the fuel and the specific fuel consumption. The heating value (QHV) of a fuel is the amount of thermal energy released by the fuel during combustion. Typically, the lower heating value (QLHV) of a fuel is used in the equation, indicating all water products remain as vapor.32 The specific fuel consumption

(SFC), a measure of how efficiently an engine uses fuel to produce work, is the fuel flow rate ( ) per unit power output (P), as seen in Eq. 2.

(1)

(2) Reciprocating ICEs convert fluid pressure on a piston into rotating mechanical power on a driveshaft. Most reciprocating engines operate on a four-stroke (Otto) cycle. As the name implies, the cycle consists of four strokes of a piston: an intake stroke, a

compression stroke, a power stroke and an exhaust stroke. As shown in Fig. 5, the four- stroke engine requires two complete crankshaft revolutions for each power stroke.31 The

two-stroke engine was developed to obtain a higher power-to-weight ratio (Fig. 6).31 By

also using the piston as the inlet and exhaust valve, a two-cycle engine only needs one crankshaft revolution for each power stroke. By using this simplified fluid control method, the weight of the engine is reduced creating high power in a lightweight

package. While the two-stroke is superior to the four-stroke in power-to-weight ratio, the four-stroke compensates with greater fuel conversion efficiency and emission control.

Figure 6: Two-stroke operating cycle

Power (P) and torque (T) are the most common descriptors of engine

performance. These specifications are very useful, but are dependent on engine size. A useful relative performance measure is mean effective pressure (MEP), which is the work per cycle divided by the volume displaced per cycle.31 Equation 3 also shows MEP in

terms of the fuel conversion and volumetric efficiencies (ηv), the fuel heating value, the ambient air density (ρ∞) and the fuel-to-air ratio (F/A). Equations 4 and 5 show torque and power respectively in terms of MEP, the displacement volume (Vd), number of crank revolutions per power stroke (nR) and the crankshaft rotational speed (N).

(3)

(4)

(5) In order for an engine to be effective within a hybrid propulsion system, it must operate at or near its peak efficiency. To optimize fuel efficiency, an engine should run

at its minimum SFC. By plotting MEP against engine speed (RPM), a “performance map” is created showing lines of constant SFC. As shown in Fig. 7, the minimum SFC occurs at a partial throttle load. 31 The upper limit of the map demonstrates the full-

throttle performance curve. The goal of a hybrid system should be to maintain engine operation within the “efficiency island” created by the minimum SFC contour line. During periods of high engine demand, a controller must calculate the maximum SFC gradient between the desired and current states. The controller will then ramp up the engine and/or motor throttle input to provide the required power in the most efficient manner.

Figure 7: Typical four-stroke engine performance map

Currently, most small unmanned aircraft use two-stroke gasoline engines. The engines provide significant capability per dollar, but tend to have low endurance from high SFCs. Four-stroke gasoline engines provide lower SFCs at the expense of power-to-

weight. Theoretically, small diesel cycle engines could double the endurance of an aircraft with a two-stroke engine.10 Significant advances must be made, however, for

diesel engines to approach the power-to-weight ratios of gasoline engines. The high cylinder pressures and compression ratios required for the diesel cycle require materials not presently found in small ICEs. Weight reductions in ancillary components like turbochargers and cooling systems must also be achieved for use in aviation.

Since small UAS typically fly at low speeds and altitudes, several simplifying assumptions about engines are made. Power is reasonably constant with freestream air velocity due to negligible ram pressure at low airspeeds. For the same reason, SFC is also reasonably constant with freestream air velocity. SFC is also relatively insensitive to altitude changes associated with small ICE powered UAS.33 Power, on the other hand,

is directly affected by altitude change. As altitude increases, the shaft power output of an ICE decreases based on the approximation shown below, where P0 and ρ0 are sea level shaft output power and density respectively.33

(6) Despite the technological challenges, DoD is aggressively pursuing heavy fuels like diesel and JP-8 for all military engines. DoD Directive 4140.25 and NATO Standard Agreements (STANAGs) require using JP-8 (also known as F-34) as the common

battlefield fuel.34 Automotive and aviation gasoline are logistically difficult and

expensive to support. Gasoline has a lower flashpoint than heavy fuels, which makes it more susceptible to explosion. Since most military vehicles use JP-8, the logistical cost of supporting a secondary and more dangerous fuel are very high. AFRL and other military laboratories are researching cycle conversion technologies. By converting small

ICEs to run on heavy fuels, the engines will be more logistically supportable even though the SFCs may not improve. The author has chosen to investigate both commercially available heavy fuel and gasoline engines for the hybrid design. Heavy fuel engines are the ultimate goal for the DoD, but may not be presently viable.