PISTON ENGINE CHARACTERISTICS

An aircraft piston engine is similar to an automobile engine with a few differences. First, an aircraft engine is designed with weight as a primary consideration. Thus, the weight-to-power ratio is generally lower for an aircraft engine when compared with an automobile engine of comparable size. Weight-to-power ratios for various aircraft piston engine configurations are presented in Figure 6.1 as a function of engine power. The ratio is seen to improve as the engine gets larger. Turboprop engines are also included in this figure and are seen to have weight-to-power ratios that are less than half of those for the piston engines. The turboprop engine will be discussed in more detail later. Notice for the piston engines that the weight-to-power ratio trend is about the same for all of the engine configurations. The horizontally opposed, air-cooled configuration appears to be somewhat better than the rest, but this may be because most of these engines were designed and built at a later date than the radial or in-line engines.

Today’s airplane piston engine is a very reliable piece of machinery. With recommended major overhaul periods of up to 2000 hr, one can get 400,000 to

300,0 km on an engine (depending on the cruising speed) before major components are replaced. An airplane engine has two spark plugs on each cylinder that are fired independently from engine-driven magnetos. Before taking off, a pilot checks to see that the engine will run smoothly on either “mag” alone. Many airplane, engine systems also incorporate an additional fuel pump that is electrically driven, independent of the engine.

Since it must operate over a range of density altitudes, an airplane engine has a manual mixture control. At low altitudes, the mixture is set relatively rich and is leaned at the higher altitudes, where the air is less dense. During a ‘continuous climb, where a large amount of power is required, the pilot will also set the mixture on the rich side in order to provide better cooling. The richer the mixture, the cooler the exhaust gases will be. Running too rich, however, can result in a loss of power and premature spark plug fouling. Therefore, many airplanes are equipped with an exhaust gas temperature (EGT) gage that allows the pilot to set the mixture control more accurately.

Carbureted airplane engines, as opposed to fuel-injected engines, have a carburetor heat control. In the “on” position, this control provides heated air to the carburetor in order to avoid the buildup of ice in the venturi. Since the air expands in the carburetor throat, the temperature in this region can be below freezing even when the outside air temperature is above freezing. If a
pilot is flying through rain or heavy clouds at temperatures close to freezing, he or she can experience carburetor icing with an attendant loss (possibly complete!) of power unless he or she pulls on the carburetor heat.

An airplane piston engine is operated with primary reference to two gages, the tachometer, which indicates the engine rpm and the manifold pressure gage, which measures the absolute pressure within the intake mani­fold. These two quantities, at a given density altitude, determine the engine power. To develop this point further, consider the characteristics of the Lycoming 0-360-A engine, which is installed in the Piper Cherokee 180 used previously as an example. A photograph of this engine is presented in Figure

This horizontally opposed, four-cylinder, air-cooled engine is rated at 180 bhp at 2700 rpm. On a relative basis, its performance is typical of piston engines and will be discussed now in some detail. Operating curves for this engine are presented in Figure 6.3. Its performance at sea level is on the left and the altitude performance is on the right. These curves are given in English units, as prepared by the manufacturer. Generally, these curves must be used in conjunction with each other in order to determine the engine power. Their use is best illustrated by an example. Suppose we are operating at a part throttle condition at 2400 rpm and a pressure altitude (altitude read on altimeter set to standard sea level pressure) of 1800 ft. The manifold air pressure (MAP) reads 23.2 in. Hg and the outside air temperature (OAT) is 25 °F. First we locate point В on the sea level curve at the operating manifold pressure and rpm. This point, which reads 129 ВНР, is then transferred to the altitude curve at sea level (point C). Next, point A is located on the full-throttle altitude curve for the operating manifold pressure and rpm. A and C are connected by a straight line and the power for standard temperature conditions read on this line at the operating pressure altitude. This point D gives a bhp of 133. Next we correct for the nonstandard OAT. At 1800 ft, the standard absolute temperature is 513 °R, whereas the actual OAT is 485 °R. The power varies inversely with the square root of the absolute temperature; thus,

bhp = bhp (standard) л/ ^iti, fld-d

(6.1)

Thus the engine is developing 137 bhp under these conditions. To deter­mine the fuel consumption, we enter the sea level curve with this power and the engine rpm (point F). We then read the fuel consumption at the same rpm and manifold pressure corresponding to point F. In this case, a consumption of 10.8 gph is determined.

These curves are typical of operating curves for nonsupercharged,

SPECIFICATIONS AND DESCRIPTION

TYPE—Four-cylinder, direct drive, horizontally opposed, wet sump, air-cooled engine

FAA Type certificate Takeoff rating, O-360-C2D only hp, rpm and manifold pressure Rated hp and rpm Cruising rpm, 75% rated 65% rated

Bore, in.

Stroke, in.

Displacement, in.3 Compression ratio Cylinder head temperature, max Cylinder base temperature, max Fuel octane, aviation grade, min Valve rocker clearance (hydraulic tappets collapsed) Oil sump capacity, qt Oil pressure, idling psi normal psi

start and warmup psi Spark occurs, deg BTC Spark plug gap, fine wire

massive wire Firing order

Standard engines (dry weight)

(Includes 12-V-20-amp generator and 12-volt starter) 285 lb

Figure 6.2 Lycoming Aircraft engine. Three-quarter right front view. (Courtesy, Lycoming Division, Avco Corp.)

To find actual horsepower from altitude rpm, manifold pressure and air inlet temperature.

. Locate A on full throttle altitude curve for given rpm manifold pressure

. Locate В on sea level curve for rpm and manifold pressure and transfer to C.

. Connect A & C by straight line and— read horsepower at given altitude D.

4. Modify horsepower at D for variation of air inlet temperature T from standard altitude temperature Г5 by formula hp at

18 19 20 21 22 23 24 25 26 27 28 29
Absolute manifold pressure, in. Hg

reciprocating engines. It is interesting to note that all wide-open throttle (WOT) power curves at a constant rpm decrease linearly with density ratio approaching zero at a a of approximately 0.1 corresponding to a standard altitude of approximately 18,000 m (59,000 ft).

Fuel consumption for a piston engine is frequently given as a brake specific fuel consumption (BSFC). This quantity is measured in pounds per brake horsepower hour. For the example just covered, since gasoline weighs 6 lb/gal,

Bspc = gph (lb/gal) bhp

(10-8X6)

137

= 0.47 lb/bhp-hr

Referring to Figure 6.4, the value 0.47 is seen to be reasonable. This figure presents BSFC as a function of engine size for both piston and turboshaft engines. Notice that BSFC tends to improve as engine power increases. Also observe that there is little difference in BSFC for the different piston engine types. Some gains

engines.

fuel injection. Turboshaft engines, to be discussed later, have specific fuel consumptions that are approximately 25% higher than those for a piston engine of the same power.

In the SI system of units, BSFC is expressed in Newtons per kilowatt – hour. In this system, the typical value of 0.5 lb/bhp-hr becomes 2.98 N/kWhr.