2 - Engine Design and Operating Parameters

2.1 Important Engine Characteristics
2.2 Geometrical Properties of Reciprocating Engines
2.3 Brake Torque and Power
2.4 Indicated Work Per Cycle
2.5 Mechanical Efficiency
2.6 Road-Load Power
2.7 Mean Effective Pressure
2.8 Specific Fuel Consumption and Efficiency
2.9 Air/Fuel and Fuel/Air Ratios
2.10 Volumetric Efficiency
2.11 Engine Specific Weight and Specific Volume
2.12 Correction Factors for Power and Volumetric Efficiency
2.13 Specific Emissions and Emissions Index
2.14 Relationships between Performance Parameters
2.15 Engine Design and Performance Data

2-1 Engine Parameters

For an engine with bore B, crank offset a, stroke length S, turning at engine speed of N :

S=2a ;

Up = 2SN ; average piston speed.
N is generally given in RPM (revolutions per minute); Up in m/sec (ft/sec).

Average piston speed for all engines will normally be in the range of 5 ~ 15 m/sec
(15 ~ 50 ft/sec), with large diesel engines on the low end and high-performance automobile
engines on the high end. There are two reasons why engines operate in this range.

First, this is about material strength of engine components.  For each revolution of the engine,
each piston is twice accelerated from stop to maximum speed and back to stop. At a typical
engine speed of 3000 rpm, each revolution last 0.02 sec (60/3000); (0.05 sec at 12,000 rpm).
If engines operates at higher speeds, there could be danger of material failure in the piston
and connecting rods as the piston is accelerated and decelerated during each stroke.

Internal Combustion Engine modeling using ANSYS FLUENT moving and deforming mesh
models and post-process using ANSYS CFD-Post software. ANSYS Inc.

The second reason why maximum average piston speed is limited is because of the gas flows
into and out of the cylinder. Piston speed determines the instantaneous flow rate of air-fuel into
the cylinder during intake and exhaust flow out of the cylinder during exhaust stroke. High piston speed would require larger valves to allow for higher flow rates. In most engines, valves are at
a maximum size with no room for enlargement.


Displacement.

For a given displacement volume, a longer stroke allows for a small bore (under-suqare),
resulting in less surface area in the combustion chamber and correspondingly less hear loss.
The increase thermal efficiency within the combustion chamber. However, the longer stroke results in higher piston speed and higher friction losses that reduce the output power which can be obtained off the crankshaft.

If the stroke is shortened, the bore must be increased and the engine must be over-square.
This decreases friction losses but increases heat transfer losses. Most modern automobile
engines are near square.

Clearance Volume:
The volume remaining in the cylinder when the piston is at the TDC.