ENGINE INDICATOR

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Engine Indicator and Indicator Diagrams:

An engine indicator is used to record pressure/volume or indicator diagrams taken off engines, the areas of these indicator diagrams represent the work done per cycle of one unit.

There are two types of engine indicators:

1. Mechanical type: This records indicator diagrams on paper.

a)      Can record pressure within the engine cylinder at any part of the engine cycle.

b)      Not considered reliable of engine speed more than 150 rpm.

c)      Small lightweight models can be used for engines with speeds up to 350 rpm.

d)     Mean indicated pressure (m.e.p) from an indicator power diagram.

2. Pressure indicator type - this measures maximum combustion pressure only.

a)      Also known as maximum pressure indicator.

b)      Compression pressure is recorded with fuel cut off.

c)      No engine speed limitation.

d)     Often used on medium speed engines.

Does not record indicator diagrams on paper

Power Indicator Diagram.

Referring to the Indicated diagram (Power card), the area of the diagram divided by its length represents the mean pressure effectively pushing the piston forward and transmitting useful energy to the crank in one cycle. This, expressed in N/m², is termed the indicated mean effective pressure (p_m).

Power is the rate of doing work (basic unit is the Watt) or:

1 Watt = 1 J/s = Nm/s

Let:

p_m = mean effective pressure (N/m²).

A = area of piston (m²).

L = length of stroke (m).

N = Number of power stroke per second.

Then:

 Average force (N) on piston = p_m x A newtons.

Work done (J) in one power stroke = P_m x A x L newton-metres = joules.

Work per second (J/s = W) = p_m x A x L x n watts of power,

Therefore:

Indicated power = p_mALn.

This is the power indicated in one cylinder. The total power of a multi-cylinder engine is that multiplied by the number of cylinders, if the mean effective pressure is the same for all cylinders

Engine Indicator.

An engine indicator consists of a small bore cylinder containing a short stroke piston which is subjected to the same varying pressure that takes place inside the engine cylinder during one cycle of operations. This is done by connecting the indicator cylinder to the top of the engine cylinder in the case of single-acting engines, or through change over cocks and pipes leading to the top and bottom ends of the engine cylinder in the case of double-acting engines. The gas pressure pushes the indicator piston up against the resistance of a spring, a choice of specially scaled springs of different stiffness being available to suit the operating pressures within the cylinder and a reasonable height of diagram.

A spindle connects the indicator piston to a system of small levers designed to produce a vertical straight-line motion at the pencil on the end of the pencil lever, parallel (but magnified about six times) to the motion of the indicator piston. The “pencil” is often a brass point, or stylus, this is brought to press lightly on specially prepared indicator paper which is scrapped around a cylindrical drum and clipped to it. The drum, which has a built-in recoil spring, is actuated in a semi-rotary manner by a cord wrapped around a groove in the bottom of it; a hook at its lower end to a reduction lever system from the engine crosshead attaches the cord, passing over a guide pulley. Instead of the lever system from the crosshead, many engines are fitted with a special cam and tappet gear to reproduce the stroke of the engine piston to a small scale. The drum therefore turns part of a revolution when the engine piston moves down, and turns back again when the engine piston moves up, thus the pencil or stylus on the end of the indicator lever draws a diagram which is a record of the pressure in the engine cylinder during one complete cycle.