General Information - BMW 745i Repair Instructions

1. DESCRIPTION
1.1 General Information
The power output of an engine is governed by three factors, namely pressure, speed and the displace-
ment.
Applied formula:
p:::::: p
x n x
V H P ;:: Output
************
p ;:: Pressure
n
;:: Speed
VH
;:: Displacement
The power output of a specific engine can be increased by raising the speed or pressure. Since the
speed is limited by the occurence of inertia force, it is preferred to keep pressure "p" as high as pos-
sible.
Pressure "p" depends on the compression ratio and volumetric efficiency (cylinder charge). Since an
increase in compression ratio will only produce a relatively small rise in pressure, it is obviously better
to optimize the cylinder charge. This can be accomplished by precompression of the intake air . The
cylinder charged will be increased even more by also cooling the precompressed air. The precompres-
sion of intake air is known as supercharging. An engine can be supercharged with a compressor or an
exhaust gas turbocharger.
The application of a compressor is disadvantageous since operation of the compressor uses some of
the engine's power output. Supercharging with an exhaust gas turbocharger, on the other hand, uses
mecllanical energy converted from part of the energy contained in exhaust gases to drive the compres-
sor.
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