ASME PTC 19.8-1970 — Measurement of Indicated Power (Instruments and Apparatus, Part 8)

📐 ASME PTC 19.8-1970 is Part 8 of the ASME Performance Test Codes (PTC) “Instruments and Apparatus” supplements, devoted to the measurement of indicated power. The original supplement, “Measurement of Indicated Horsepower,” was approved in prototype form in 1940, reviewed and reaffirmed in 1960, and revised from 1964 onward by a reorganized PTC Committee 19.8 to add coverage of engine and compressor indicators. In 1967 the title was changed to “Measurement of Indicated Power,” and the edition presented here was approved by the Performance Test Codes Committee in April 1970. Aimed at reciprocating engines and compressors, it systematically covers the selection, calibration, error control, and diagram-evaluation procedures for three classes of indicators — mechanical, balanced-pressure, and electronic — making it the fundamental technical reference for measuring in-cylinder power in piston-machinery performance tests.

1. Position and Scope of the Supplement 📌

This supplement deals directly with the measurement of indicated power of piston engines and compressors using the engine indicator. The principle is the determination of the integral ∮p·dV over a complete cycle. The devices that record data to permit evaluation of this integral are called indicators, and the value of the integral, in proper units, is the “indicated work per cycle,” converted to power by multiplying by the number of cycles per unit time — that is, the indicated power. Three boundary conditions apply:

• Indicated power is defined as the power exerted by, or expended upon, the working fluid within the cylinder of a piston engine or machine, determined from the indicator diagram.
• Only the determination of indicated power is treated; interpretation of capacity, volumetric efficiency, friction, or flow losses from the data is beyond scope and is referenced to PTC-9 and PTC-19.7.
• The indicator must sense or record pressure continuously or at many points through the cycle, and must relate the timing of pressure data to the corresponding cylinder volume — a synchronization requirement called indexing.

The overall selection principle is to pick the instrument best suited to the problem at hand, governed by the accuracy needed, accessibility of the quantity to be measured, and general test conditions such as available equipment. Once selected, calibration is performed as described, significant sources of error are evaluated, and qualified personnel execute all phases of the work.

2. Comparing the Three Indicator Classes 🔧

The standard divides indicators into three distinct classes and summarizes their basic characteristics:

Mechanical indicators drive a stylus through a spring-piston system to trace the pressure-travel curve on a rotating drum. They are simple and convenient for on-site rapid assessment, but the spring-mass system limits them to low speeds; lag and resonance distortion appear at higher speeds. Balanced-pressure indicators oppose the measured pressure against a known external pressure over a definite-area piston or diaphragm and detect the balance (zero) point, essentially eliminating spring hysteresis and nonlinearity — hence the highest accuracy, suited to careful tests at moderate speeds. Electronic indicators use piezoelectric or strain-gauge pressure transducers with amplifiers, oscilloscopes, or computing devices; their high frequency response and wide range cover 100,000 psi and speeds above 3000 rpm for high-speed, high-pressure applications.

3. Computing Indicated Power and IMEP 🧮

Indicated power is expressed by the equation:

P_i = p_i × L × A × n / 33,000

where P_i is the indicated power in horsepower (33,000 ft·lb/min per hp); p_i is the indicated mean effective pressure (IMEP) in lb/in²; L is the piston stroke in ft; A is the net cross-sectional area of the piston in in²; and n is the number of working strokes per minute per cylinder end — equal to rpm for single-acting two-stroke engines and to half the rpm for single-acting four-stroke engines. For double-acting engines and compressors, p_i and A are determined separately for the two ends of the cylinder, and the indicated power of the whole cylinder is the sum of the values for the two ends.

The indicated mean effective pressure (IMEP) is that pressure which, assumed to act at constant magnitude on the piston, would produce or absorb during one piston stroke an amount of work equal to that produced or absorbed during one complete working cycle by the actual variable pressure. Equivalently, it is the indicated work of one complete cycle divided by the piston displacement in which the work was done — dimensionally inch-pounds per cubic inch, i.e., pounds per square inch. Measurements of area A, stroke L, and speed n are handled elsewhere; this part deals only with the determination of IMEP.

4. Obtaining the Indicator Diagram: From Oscilloscope to P-V Plot 📈

The indicator diagram plots cylinder pressure as ordinate against crank angle, piston position (volume), or time as abscissa:

• Oscilloscope: displays the relationship between two or more variables on the face of a cathode ray tube. For calculating horsepower or keeping a permanent record of a P-t or P-V diagram, a photograph of the display may be taken.
• Oscillograph: produces a graphical trace on a strip chart of the same variables, but usually as a function of time only; to obtain indicated power these traces must be replotted as a P-V diagram.
• Digital or analog computer: computes intermediate or final results from input data signals according to programmed instructions and transmits them to display devices such as an X-Y plotter, printer, recording chart, or numerical display. Results may be mean time, crank angle, piston position, volume, or speed — instantaneous values or values averaged over a predetermined period or number of cycles.

Determination of dead center is essential for reliable data. The prescribed procedure: secure a pointer (stout wire or steel strip) to a stationary engine part, nearly touching the flywheel; turn the flywheel to a crank position about 5 degrees before dead center and mark the flywheel at the pointer tip; attach a dial indicator readable to 0.001 in to read piston or crosshead movement and set it to zero; continue turning past dead center until the crosshead or piston returns past the same position; then reverse direction and carefully bring it back to the zero reading; bisect the angle between the two flywheel marks — that position is true dead center.

5. Calibration, Error Control, and Engineering Practice 🛠️

• Error combination: error in IMEP measured with electronic equipment depends heavily on components, installation, and calibration. Systemic errors such as timing error add arithmetically from each source and must be eliminated wherever possible; random errors following a normal frequency distribution combine so that the square of the total error equals the sum of the squares of the individual source errors (root-sum-square).
• Calibration: after selecting the instrument, calibrate as prescribed — static and dynamic calibration of the pressure channel to verify sensitivity, linearity, and frequency response.
• Synchronized acquisition: cylinder pressure and dead-center (TDC) signals must be acquired and recorded synchronously, with the indexing method stated; shaft-angle encoders provide the crank-angle reference in modern practice.
• Personnel and planning: all phases should be executed by qualified personnel, with advance planning, agreement between parties on test items, instruments, and evaluation methods, and preliminary tests.

6. Frequently Asked Questions ❓