Bottom dead center

The intake valve is now closed as the piston moves from the bottom dead center (BDC) to top dead center (TDC), compressing the fuel/air mixture. At Point 3, just prior to TDC, a spark ignites the fuel/air mixture and the resulting combustion causes the pressure and temperature to begin a very rapid rise within the cylinder. 

As the piston begins its leftward stroke from bottom dead center (BDC), both inlet and exhaust ports are uncovered and air from some external source is flowing through the cylinder. Directional control is provided through port and/or piston design to ensure the most complete cylinder scavenging possible. 

Four-stroke cycle. TDC and BDC = top dead center and bottom dead center positions of the piston, respectively. Vj = displacement. V, = clearance volume. Compression ratio = (Vj -I-... 

The piston continues its travel toward the cylinder head. At some point it stops, and reverses its direction. This point is called bottom dead center, indicated by the dotted line in Fig. 29.1. Of course, bottom dead center cannot coincide with the end of the cylinder. The piston would have to travel past the valve ports for this to occur. If the piston travels past the discharge port, the compressed gas could not be pushed out of the cylinder into the discharge line. So bottom dead center must line up with the crank-end edge of the valve ports, as shown in Fig. 29.1. 

The piston reaches bottom dead center, and reverses its direction. 

The proper way to reduce the volumetric efficiency is to increase the starting volumetric clearance. This is done with an adjustable unloading pocket, as shown in Fig. 29.4. This device, also called the head-end unloader, works by increasing the starting volumetric clearance. This is known as the volume of gas trapped between the cylinder head and the piston, when the piston position is at bottom dead center, as explained in Fig. 29.2. 

Turning the wheel at the back end of the cylinder counterclockwise, pulls back a large internal plug in the head. Now, when the piston starts to withdraw toward the crank end of the cylinder, there is more gas left inside the cylinder to expand. The greater the volume of gas inside the cylinder when the piston is at bottom dead center, the closer the piston is to top dead center before the intake valve opens. The delay in the opening of the intake valve reduces the amount of gas drawn into the cylinder. This reduces the number of moles of gas compressed by the piston. Compression work also diminishes and the driver horsepower or amp load drops. 

Figure 192 represents eccentric or crank-type drives that convert rotary motion to linear, reciprocating movement. The mechanisms feature a small final rate of pressing speed (approaching bottom dead center) and high loading with low torque at maximum compression (at bottom dead center). The stroke can be adjusted on the eccentric cam or Pitman link. Normally, this method is used when force is applied from only one side and, typically, it drives the top punch. 

Nusselt [35] also analyzed this problem, assuming that the condensate drained from an npper tube at bottom dead center as a continuous sheet, falling on the next lower tube and flowing down and around that tube in undisturbed laminar flow. Under these highly idealized conditions, the average coefficient for a row of tnbes N tubes high is... 

FIGURE 8- la One 2-D plane of 19 through the water simulation of an internal combustion engine during the intake stroke when the piston is at bottom dead center (Reprinted by permission of Guezennec et al.)... 

DISPLACEMENT VOLUME - The volume displaced by the piston between top dead center and bottom dead center. 

Bottom dead center (bdc) stop Simple modification must be made. Inherently present... 

The press must be slightly altered to permit the appropriate dwell time at the bottom dead center (bdc) of the stroke. This assures complete rigidizing of the material in the mold. 

Compression ratio The ratio of the volume of cylinder space above the top piston ring with the piston at bottom dead center to the volume in the cylinder above the top ring when the piston is at top dead center. 

Bottom dead center The piston position in a cylinder when it is closest to the crank shaft. 

The Carnot engine operates on a two-stroke cycle that is called the Carnot cycle. We begin the cycle with the piston at top dead center and with the hot reservoir in contact with the cylinder. We break the expansion stroke into two steps. The first step is an isothermal reversible expansion of the system at the temperature of the hot reservoir. The final volume of the first step is chosen so that the second step, which is an adiabatic reversible expansion, ends with the system at the temperature of the cold reservoir and with the piston at bottom dead center. The compression stroke is also broken into two steps. The third step of the cyclic process is a reversible isothermal compression with... 

These pumps are often used for processing highly viscous adhesives. The design of a twin piston pump is schematically shown in Fig. 38.4. The special feature of a twin piston pump is that it transports materials in both movement directions. This is achieved due to the fact that the two chambers, which are separated by the piston, are kept in phase with each other by a nonreturn valve, and the cross section of the lower chamber is twice as large as the cross section of the upper chamber. When the pump is at the bottom dead center, both nonreturn valves close (= piston valve and foot valve). If the piston now moves upward, the foot valve opens and the lower chamber is filled. Simultaneously, the adhesive in the upper chamber is transported via the outlet into the connected pipe system. After reaching the top dead center and reversal of movement of the piston, the piston valve opens and the foot valve closes. The upward movement of the piston forces the volume of adhesive from the lower chamber... 

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