Hydraulic Motors

Hydraulic motors are rotary or mechanical actuators that operate by converting hydraulic pressure or fluid energy into torque and angular displacement.

Motor Displacement

Motor displacement is the volume of fluid required to turn on the motor’s motor output shaft through one revolution. Cubic inches and cubic centimeters per revolution are the common units that are used for motor displacement. Depending on the type of motor used and application, the displacement may be a fixed or variable quantity. In a fixed displacement motor the torque is constant. The speed of the motor can be varied when controlling the amount of input flow that gets into the motor. Variable torque and speed can be obtained if the motor used is a variable displacement motor.

Torque Output

The torque output of a motor can be expressed either in foot-pounds or inch-pounds. It’s a function of the pressure in a system and the motor displacement. The specific pressure drops in a motor can be evaluated based on motor torque ratings given by the manufacturer.

Starting Torque

Starting torque is the ability of a hydraulic motor to make a load start moving. The starting torque indicates the amount of torque that a hydraulic motor can develop to make a load start turning. It can be expressed as a fraction or percentage of the theoretical torque. The starting torque for piston, vane, and common gear motors is usually between 70% to 80% of the theoretical value.

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Diesel Engines & Pumps

Most modern diesel engines use the conventional cylinder and piston arrangement operated with a slider crank mechanism common to other internal combustion engines such as the gasoline engine. Considering this basic mechanism, there is very little difference between the basic structure of diesel and gasoline engines.

Conceptually, diesel engines operate by compressing air to high pressure/temperature and then injecting a small amount of fuel into this hot compressed air. The high temperature causes the small amount of highly atomized injected fuel to evaporate. Mixing with the hot surrounding air in the combustion chamber, the evaporated fuel reaches its auto-ignition temperature and burns to release the energy that is stored in that fuel [Heywood 1988].

The definition of the diesel engine has evolved over the years. For example, in the early 20th century, a distinction was made between a “true Diesel Engine” and one that shared some aspects of the diesel cycle but did not encompass all aspects considered to be part of the diesel cycle as then envisioned. One early definition of a “true Diesel Engine” is one having the following features [Purday 1919]:

  1. Compression sufficient to produce the temperature requisite for spontaneous combustion of the fuel.
  2. Injection of fuel by a blast of compressed air.
  3. A maximum cycle pressure (attained during combustion) not greatly exceeding the compression pressure, i.e., absence of pronounced explosive effect.
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Turbochargers

A turbocharger consists of a compressor wheel and exhaust gas turbine wheel coupled together by a solid shaft and that is used to boost the intake air pressure of an internal combustion engine. The exhaust gas turbine extracts energy from the exhaust gas and uses it to drive the compressor and overcome friction. In most automotive-type applications, both the compressor and turbine wheel are of the radial flow type. Some applications, such as medium- and low- speed diesel engines, can use an axial flow turbine wheel instead of a radial flow turbine.

With the exception of some large turbochargers for low-speed engines, the bearings that support the shaft are usually located between the wheels in an overhung position. This flexible rotor design ensures that the turbocharger will operate above its first, and possibly second, critical speeds and can therefore be subject to rotor dynamic conditions such as whirl and synchronous vibration.

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