Iveco Marine Diesel Engines, Руководство по установке

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Piston displacement
The element which best distinguishes the engine is the “overall piston displacement” which represents
the total volume of air moved by the pistons during one complete turn of the drive shaft. It represents
also the theoretical quantity of air sucked by the cylinders during 2 revolutions of the drive shaft. It is
given by the formula:
in cm 3 , where:
• π : 3.1416
• d : cylinder diameter (bore) in cm
• c : piston travel in cm
• i : n° of engine cylinders
Real average pressure
It is the average value of the pressure inside the cylinders during the different operating phases of
the engine. It increases during the combustion phase and decreases during the exhaust and suction
phases. It is possible to consider it as an indicator of the engine stress since it represents the work
done per displacement unit. The real average pressure generates the driving torque and therefore
the engine power :
where:
• N : power [kW]
• p.m.e. : real average pressure [bar]
• V : total piston displacement [dm
3
]
• n : rotation speed [giri/min.]
From it you obtain:
With these formulas you obtain that:
■
the power is the linear function of the real average pressure and of the engine rotation speed;
■
with the same power and the same number of rpm, the engines with a higher piston displacement
are subject to a lower real average pressure
The power needed for a boat propulsion requires, if the operating rpm number is the same, the appro-
priate consideration about the engine to be used: an engine with a higher piston displacement is sub-
ject to a lighter mechanical load as shown by a lower value of the real average pressure and therefore
it will be possible to use it for heavy duties compared to the engine with a lower piston displacement.
Driving torque
It represents the thrust impressed by a piston through the connecting rod on the crank arm of the drive
shaft. It can be defined as the “rotating force” available to the engine flywheel; it depends on the real
average pressure and is strongly influenced by the volumetric efficiency of the engine, i.e. from its capac-
ity to suck as much air as possible. Other important factors to obtain a high driving torque and there-
fore power are the correct fuel intake and the perfect injection system setting.
MARCH 2004 INTRODUCTION
1.10
MARINE ENGINES INSTALLATION