Three values that determine power output and torque

It will help to remember the following definitions when identifying the character and performance of engines.

●Displacement

Displacement is how much air an engine can draw in. In a reciprocating engine, it equals the volume of the cylinder where the piston traverses x the number of cylinders. The larger the displacement, the more power output you will gain; however, rather than having a larger capacity per cylinder, it is easier to gain good combustion performance by having more cylinders.

●Maximum Output

1ps (horsepower) is the amount of work required for lifting a 75kg object one meter per second. In other words, a 100ps engine can lift a single-ton object 7.5 meters per second. Horsepower is gained by torque x number of revolutions, so even a small displacement engine can gain large output if you spin it fast enough. In today’s international standard, the unit of measure is in kilowatts (kW), where 1ps = 0.735kW.

Fig. 3-1 Maximum Output

●Maximum Torque

The torque when turning a nut one meter away with a 1m-long wrench with 1kg of force is 1kgm. In terms of an engine, this indicates the torque of the crankshaft that converts the reciprocating motion of the pistons into rotational movement. Having ‘more torque’ means that the engine has an ability to turn with stronger force. In international standards, the unit of measure used is ‘Nm’, where 1kgm = 9.80665Nm.

Fig. 3-2 Maximum Torque

●Bore/Stroke Ratio (inner diameter x stroke)

The value of the cylinder stroke (length of travel) divided by the cylinder bore (inner diameter) is called the bore/stroke ratio. Those with ratio values less than one are referred to as being short stroke, those with greater than one are long stroke. Those that are exactly one are referred to as being square.

Fig. 3-3 Bore/Stroke Ratio (inner diameter x stroke)

●Compression Ratio

This is a value which indicates how much the fuel/air mixture drawn in by the engine is compressed. This is an important variable that determines the thermal efficiency and output of the engine.

The compression ratio can be calculated by dividing the maximum volume of the cylinder when the piston is at its lowest position (also called bottom dead-center or BDC), with the minimum cylinder piston volume (also called the combustion chamber volume) when the piston is at the topmost position (also called top dead-center or TDC). For example, say that the displacement volume of a single cylinder in a 2000cc 4-cylinder engine without the combustion chamber is 500cc per cylinder. If the combustion chamber volume is 50cc at TDC, you would take 500+50=550cc and divide this by the 50cc combustion chamber volume, making the compression ratio 11.

Fig. 3-4 Compression Ratio