Reciprocating engine

A reciprocating piston engine is an engine which converts the change in volume of a gas of a linearly moving piston via a connecting rod and a crank to a rotary movement. He is one of the piston engines.

Principle of operation

The expansion of the gas in a cylinder of a piston performs work which is transmitted through a connecting rod to the crankshaft. Thus, the oscillating movement of the piston is converted into a rotary movement. Reciprocating engines generally work after the two-stroke or four-stroke cycle. There are basically two kinematic configurations possible:

• The first is the ( conventional ) prior engine in which the cylinders are fixed, and the crankshaft rotates. This allows the piston work on the connecting rods to the crankshaft leave ( Fig. 1).
• The second form is known as a rotary engine. With this, the crankshaft is fixed and the crank pins and the cylinder are rotatably supported. The cylinders rotate in a different axis than the crank pin ( eccentric), whereby the stroke of the respective pistons in the cylinders is about.

Examples of reciprocating engines are:

• Combustion engines: Diesel engine ( compression ignition )
• Otto engine ( ignition)

Reciprocating engines are also divided according to the number and arrangement of the pistons each combustion chamber:

• Normal reciprocating engine having a piston each combustion chamber (by far the most common)
• Double- piston engine having two pistons in parallel cylinders, that are connected to each other
• Opposed-piston engine with two working against each other piston in the same cylinder

They are also divided according to the number and arrangement of cylinders:

• Line engine
• V-type engine
• Boxer engine
• Radial engine
• W motor (and others )

Mass forces

Due to the reciprocating motion of the pistons and connecting rods, as well as a result of the nonuniform transmission behavior of the crank mechanism occur inertia forces, which are supported in the motor bearings and stimulate adjacent structures to vibrate.

The inertia forces of the reciprocating parts of the crank drive ( oscillating masses ) can be calculated by the following formula approximation:

: Oscillating mass force: Oscillating mass: Crank radius: Angular velocity of the crankshaft: Ratio of connecting rod length to crank radius: Time since passing through the top dead center

Since it is the expression in the parenthesis are the first two terms of a series expansion is referred to as a mass force 1st order, 2nd order as a mass force.

Theoretically occur not only 1st and 2nd order, but an infinite number of integer orders, but due to their small size, most are negligible from the 4th order.

Mass balance

The rotating mass of the crank mechanism can be offset by counterweights on the crankshaft. Oscillating mass forces 1st and 2nd order can be in multi-cylinder engines avoided or reduced by a clever arrangement of the cylinders. In -line engines having less than 6 cylinders and V-engines with cylinders less than 8 times the balancer shafts are used. To compensate for inertial forces 2nd order, you need at least 6 -cylinder in-line engine or the 8 cylinders when V- motor, or balance shafts on which appropriate compensation imbalance with double crankshaft speed circulate (for example, Lanchester balancer (Fig. 2) ). Another possibility to achieve a total mass balance ( not only by the above-mentioned approximate calculation ), consists in the use of two counter-rotating crankshafts, such as the H - motor. So even 2- cylinder engines completely balance.

Nonuniformity

Since reciprocating engines such as turbines not run continuously, but through a split in different cycles process, it comes to the crankshaft to a speed and torque pulsation fluctuates around a stationary mean value ( Fig. 3).

The shape of the Drehunförmigkeit is determined by the number of cylinders, the pressure curve in the cylinder, the geometry and the mass of the engine components, as well as the combustion process (two -stroke or four-stroke cycle ) and the operating point (load / speed ) of the engine. The secondary camshaft drive and the secondary drive of auxiliary units may also have an impact.

These so-called rotational irregularity is the cause of torsional vibrations in the drive train downstream, which often lead to unpleasant engine noises. To reduce this, come dual mass flywheels or torsional vibration damper or dampers are used.