ENGINES What is the famous “torque” (and how it works)


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In the world of engines, inboard and outboard, she always appears on brochures and model presentations. The word “couple”: it is often taken for granted that the reader knows what is being talked about, but a little review never hurts.

The engine is, by definition, a mechanical organ within which the transformation of energy from thermal to mechanical is accomplished. The thermal energy used is derived from the heat produced during the combustion phase, which is one of the crucial moments of this transformation. For a fuel to burn, it must be brought into contact with the oxidizer, which is oxygen.

In all engines, whether gasoline or diesel, the comburent used is air, with its normal oxygen content, but how the air comes into contact with the fuel is the major difference between a gasoline engine and a diesel. In fact, in the gasoline engine, air enters the cylinder already mixed with fuel. The combustible plus oxidizer assembly is then ignited by the spark between the poles of a candle due to the high electrical potential difference.

In the diesel engine, on the other hand, it is only air that enters the cylinder and is strongly compressed here by the piston stroke, an operation that increases its temperature. At this point, diesel fuel is injected, which, coming into a strongly overheated environment, immediately ignites, and it is during the burst that mechanical work production occurs.

In fact, in contrast to the pre-burst mixture, which has a strong liquid component, the combustion products are almost all gaseous: at the same temperature and pressure, the gases occupy a larger volume, which causes inside the blast chamber, a sudden increase in pressure. This newly created force pushes on the piston, which thus begins to move downward.

Considering for simplicity, a single-cylinder engine, the FC force generated by combustion can be regarded as composed of two forces: one with a direction perpendicular to the piston, and one along the connecting rod. FA is the force that will affect the walls of the cylinder, tending to ovalize it. FB, on the other hand, is the one that acts along the connecting rod to the crankshaft, giving rise properly broken down, to the torque or more commonly to the torque of the engine, that is, it sets all the gears in motion until it causes, in our case, the rotation of the propeller.

It should be kept in mind that FC is not constant throughout the gas expansion phase, but will be maximum a moment after the start of combustion and will decrease as the chamber volume increases as the piston descends. As a result, the driving force, i.e., the “torque” of the motor, will also not be constant throughout the operating cycle but will have a development like that shown in Figure 2.

When accelerating initially, the driving torque increases, so the RPM quickly rises to a higher value; at this point being the torque delivered greater than the resisting force, the boat undergoes acceleration. If we cease to accelerate, keeping the throttle lever stationary, the boat will also move to a constant speed, because drive torque and resisting torque will reach an equilibrium value.



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