Farm Power & Machinery

Engine performance

The different terms used in the engine system are :

Bore:
Bore is the diameter of the engine cylinder expressed in cm or metre.

Stroke:
It is the linear distance traveled by the piston from Top dead centre (TDC) to Bottom dead centre (BDC) and the distance travelled is called stroke length, expressed in cm or metre.

Stroke-bore ratio:
The ratio of length of stroke (L) and diameter of bore (D) of the cylinder is called stroke-bore ratio (L/D). In general, this ratio varies from 1 to 1.45 and for tractor engines, this ratio is about 1.25.

Swept volume:
It is the volume displaced by one stroke of the piston.
                                       Swept volume = A x L ……… m3
                                       Where,
                                                    A - Cross sectional area of piston in m2
                                                    L - Stroke length in m.

Compression ratio:
It is the ratio of the volume of the charge at the beginning of the compression stroke to the volume of the charge at the end of compression stroke, i.e. ratio of total cylinder volume to clearance volume. The compression ratio of diesel engine varies from 14:1 to 22:1 and that of spark ignition engine (carburetor type engine) varies from 4:1 to 8:1.

Indicated power (ip):
It is the total power generated in the engine cylinder by the combustion of the fuel and received by the piston. It is the power developed in a cylinder without friction or auxiliary unit. In SI system, the indicated power is expressed in terms of Kilowatts (kW) and the formula for determining is given by
                                                               PLAN               n
                                                   ip = ------------- x ----- ………… for four stroke engine
                                                               60 x 10¹²        2
                                                               PLAN     x    n
                                                  ip = ------------------ ………… for two stroke engine
                                                             60 x 10¹²
Where, P = mean effective pressure, Pa or N/m2
L = stroke length, mm
A = cross sectional area of piston, mm2
N = engine speed, rpm
n = number of cylinders

Brake power (bp):
It is the power delivered by the engine and is available at the end of the crankshaft. It is measured by using a suitable dynamometer.

Mechanical efficiency (?mech) :
It is the ratio of brake power to indicated power expressed in percentage.

Belt power:
It is the power of the engine, measured at the end of a suitable belt, receiving drive from the PTO shaft of the tractor.

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Power take-off horse power (Pto power) :
It is the power delivered by a tractor through its PTO shaft. In general, the belt and PTO horse power of a tractor will approximately be the same.

Drawbar power (db):
It is the power of a tractor measured at the end of the drawbar. It is that power which is available for pulling loads at the draw bar.

Frictional power (fp) :
It is the power required to run the engine at a given speed without producing any useful work. It represents the friction and pumping losses of an engine. It is the power lost due to the friction between the rubbing parts of the engine. ip = bp + fp

Mean effective pressure (mep):
It is the average pressure during the power stroke minus the average pressure during other strokes. This pressure actually forces the piston down during the power stroke.

Volumetric efficiency:
It is the ratio of actual weight of air introduced by the engine on the suction stroke to the theoretical weight of air that should have been introduced by filling the piston displacement volume with air at atmospheric pressure and temperature.

Torque :
A turning effect due to force applied on some point is called torque (T).
T = F x r ………… Kg-m or N-m
Where,
F = force in Kg or N
r = distance of the force from the centre of the shaft in meters.

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Piston displacement:
It is the volume displaced by one stroke (L) of the piston. It is also known as swept volume.
                                         Piston displacement = A x L ……….. m3
                                         Where,
                                                   A = cross sectional area in m2
                                                   L = length of stroke in meters.

Displacement volume:
It is the total swept volume of all pistons during power strokes occurring in one minute.
                                        Displacement volume = A x L x N x n/2 (for 4 stroke engine)
                                        Displacement volume = A x L x N X n/1 (for 2 stroke engine)
                                       Where,
                                                 A = cross sectional area of piston in m2
                                                 L = stroke length in meters
                                                N = speed in rpm
                                                 n = number of cylinders

Piston speed (Sp):
It is the total length of travel of the piston in a cylinder in one minute.
                                       Sp = 2 x Lx N

Specific fuel consumption:
It is the quantity of fuel consumed per kW– hr. in an engine. The unit of specific fuel consumption is Kg/ kW-hr.

Brake dynamometer:
The brake dynamometer is used to measure the brake power of the tractor engine. Different types of dynamometer are used to measure the power.

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