ENGINE EFFICIENCY

For example, one cylinder of a four-cylinder aircraft engine has a bore, or diameter, of four inches. What is the area of the piston head?

Given:

Bore = 4 inches Area = 7tr²

= 3.14X2²

= 12.56 square inches

Once the area of one piston is known, total piston displacement is calculated with the formula:

Total Piston Displacement = A x L x N Where: : . ..,-..■■-.

A = area of piston head in square inches L = length of the stroke in inches N = number of cylinders

Using the example presented earlier, determine the total displacement if each of the four cylinders has a stroke of six inches.

Given:

Area = 12.56 square inches Stroke = 6 inches Number of cylinders = 4

Piston Displacement = A x L x N

; =12.56X6X4

= 301.44 cubic inches The total engine displacement is 301.44 cubic inches. Since the amount of work done by the expanding gases is determined in part by the piston area and the piston stroke, it should be evident that increasing either the cylinder bore or the piston stroke increases piston displacement.

ENGINE EFFICIENCY

Energy is the capacity for doing work and cannot be created or destroyed. However, energy can be trans-formed from potential, or stored energy into kinetic energy. Aircraft reciprocating engines transform the

potential, or chemical energy stored in fuel into heat energy during the combustion process. The heat energy is then converted to kinetic energy by mechanical means. Engine design and construction, fuel type, and environmental conditions all play a part in how efficiently an engine converts a fuel's potential energy. To determine how efficient an engine is, several factors must be examined, includ-ing an engine's thermal, volumetric, and mechani-cal efficiency.

THERMAL EFFICIENCY

An engine's thermal efficiency (TE) is a ratio of the amount of heat energy converted to useful work to the amount of heat energy contained in the fuel used to support combustion. In other words, thermal effi-ciency is a measure of the inefficiencies experienced when converting the heat energy in fuel to work. For example, consider two engines that produce the same amount of horsepower, but consume different amounts of fuel. The engine using less fuel converts a greater portion of the available energy into useful work and, therefore, has a higher thermal efficiency.

Thermal efficiency is found by the formula:

, , .,,,. . Horsepower x 33,000

Thermal Efficiency = --- FxBTTJxK ---

Where:

Horsepower = An engine's brake or indicated horse-power 33,000 = Number of foot-pounds of work per

minute in one horsepower F = Weight of fuel burned per minute BTU = Heat value of the fuel burned measured in BTU's

K = Constant representing the number of foot-pounds of work each BTU is capable of doing in one second.

Thermal efficiency can be calculated using either brake or indicated horsepower. If brake horsepower is used, the result is brake thermal efficiency (BTE), and if indicated horsepower is used, you get indi-cated thermal efficiency (ITE).

The constant, 33,000, is the number of foot-pounds of work per minute in one horsepower. Therefore, when horsepower is multiplied by 33,000, the out-put of an engine in foot-pounds per minute results.

Almost all engine performance data relating to fuel consumption is expressed in terms of gallons per hour. Therefore, you must be able to convert gallons your previous study of mathematics and the

discus-sion on indicated horsepower (PLANK), the area of a circle is calculated with the formula:

Reciprocating Engines 1-43

per hour to pounds per minute. For example, the weight of 100LL aviation gasoline is six pounds per gallon. If a particular engine burns 10 gallons per hour, you must multiply the gallons consumed per hour by six pounds and divide the product by 60, the number of minutes per hour. The resulting fuel burn is one pound per minute (10 x 6 -f- 60 = 1).

In the English system of measurement, the relation-ship between heat and work is the British Thermal Unit, or BTU, of heat energy. Each pound of aviation gasoline contains 20,000 BTU's of heat energy, therefore, the number 20,000 is typically used in the formula for determining thermal efficiency.

By multiplying the pounds per minute of fuel an engine burns by 20,000, you get the total number of BTU's, or total heat energy that is produced in a given engine. One BTU is capable of doing 778 foot-pounds of work. Therefore, when you multiply the total number of BTU's by the constant 778, both the top and bottom of the formula produce a product that is in foot-pounds.

Based on the information just presented, the for-mula used to calculate thermal efficiency can be simplified to read:

. , rr. . Horsepower x 33,000

Thermal efficiency = --- To check your understanding of this formula, deter-mine the brake thermal efficiency of a piston engine that produces 150 brake horsepower while burning 8 gallons of aviation gasoline per hour.

12,448,000 .398 39.8

percent

Most reciprocating engines are between 30 and 40 percent efficient. The remaining heat is lost through the exhaust gases, the cooling system, and the fric-tion within the engine. In fact, of the total heat pro-duced in a reciprocating engine, 30 to 40 percent is

utilized for power output; 15 to 20 percent is lost in cooling; 5 to 10 percent is lost in overcoming fric-tion of moving parts; and 40 to 45 percent is lost through the exhaust.

VOLUMETRIC EFFICIENCY

Volumetric efficiency (VE) is the ratio of the volume of fuel and air an engine takes into its cylinders to the total piston displacement. For example, if an engine draws in a volume of fuel and air that is exactly equal to the engine's total piston displace-ment, volumetric efficiency would be 100 percent.

By the same token, if an engine draws in 288 cubic inches of fuel and air and has a total piston dis-placement of 320 cubic inches, the volumetric effi-ciency would be 90 percent.

Because the density of the air drawn into an engine varies with changes in atmospheric conditions, the only way to accurately calculate volumetric effi-ciency is to correct for nonstandard temperature and pressure. If you recall from your earlier studies, standard temperature and pressure at sea level is 59蚌 (15蚓) and 29.92 inches of mercury (1013.2 millibars) respectively. Based on this, the formula for determining volumetric efficiency is:

The volumetric efficiency of most normally aspi-rated engines is less than 100 percent. The reason for this is because bends, surface roughness, and obstructions inside the induction system slow the flow of air which, in turn, reduces the air pressure within the manifold. On the other hand, tur-bocharged engines compress the air before it enters the cylinders, and often have volumetric efficiencies greater than 100 percent.

Anything that decreases the density, or volume of air entering a cylinder decreases volumetric effi-ciency. Some of the typical factors that affect volu-metric efficiency of a non-turbocharged engine include:

1. Part throttle operation This restricts the vol ume of air that flows into the cylinders.

2. Long, small diameter, intake pipes As air flows through an induction system, friction slows the air flow, causing a decrease in air density. The amount of friction created is directly proportional to the length of the intake pipes and inversely proportional to their cross-sectional area. In other words, long, small diameter intake pipes create the most friction while short, large diameter intake pipes create less friction.

7-44 Reciprocating Engines

3. Induction systems with sharp bends Each time

intake air turns a corner in an induction system, air flow slows and less air enters the cylinders.

4. High carburetor air temperatures As the tem perature of the intake air increases, air density decreases. A lower air density means less air enters the cylinders. ■ /■■ . >

c . .■ u-

5. High cylinder head temperatures As the cylin der heads and corresponding combustion chambers heat up, air density in the cylinders decreases and volumetric efficiency decreases.

6. Incomplete scavenging If the valve overlap in an engine is incorrect, exhaust gases will displace some of the incoming fuel/air mixture. When this happens, less fuel and air is drawn into the cylin ders and a lower volumetric efficiency results.

7. Improper valve timing If the intake valve does not remain open long enough to allow a complete charge of fuel and air to enter a cylinder, volumetric efficiency drops.

8. Increases in altitude As an aircraft climbs, ambient air pressure drops and air density

decreases. As an engine draws the "thin" air into its cylinders, its volumetric efficiency drops.

This

problem can be overcome, to a certain degree, by turbocharging an engine. Turbocharging increases the induction air pressure above atmospheric pres sure which, in turn, increases the density of the fuel/air charge entering the cylinders.

MECHANICAL EFFICIENCY

Mechanical efficiency is the ratio of brake horse-power to indicated horsehorse-power and represents the percentage of power developed in the cylinders that reaches the propeller shaft. For example, if an engine develops 160 brake horsepower and 180 indicated horsepower, the ratio of brake horsepower to indicated horsepower is 160:180, which repre-sents a mechanical efficiency of 89 percent. Since aircraft engines are mechanically efficient, it is not unusual for ninety percent of the indicated horse-power to be converted into brake horsehorse-power.

The factor that has the greatest effect on mechanical efficiency is the friction within the engine itself. The friction between moving parts in an engine remains relatively consistent throughout an engine's speed range. Therefore, the mechanical efficiency of an engine is highest when the engine is running at an rpm that maximum brake horsepower is developed.