ENGINEMAN 1 & C
Table 5-1.—Factors That Influence Engine Performance
1. Fuel characteristics
2. Compression ratio
3. Engine operating conditions
Combustion chamber design
Valve arrangements
Size of valves
Manifold arrangements
Hot spots (presence/absence)
Location of spark plugs
Number of spark plugs
4. Pressure and temperature of air in the engine cylinder
at start of compression
5. Pressure/temperature of the charge in the engine
cylinder at the start of compression
factors are the mean effective pressure, the length
of stroke, the cylinder bore, and the number of
revolutions per minute (piston speed) of the
engine. The latter, piston speed, is limited by the
frictional heat and by the inertia of the moving
parts.
Mean Effective Pressure
The mean effective pressure (MEP) is the
average pressure exerted on the piston during each
power stroke, and is determined from a formula
or by means of a planimeter. There are two kinds
of mep: indicated mean effective pressure (imep),
which is developed in the cylinder and can be
measured; and brake mean effective pressure
(bmep), which is computed from the brake
horsepower (bhp) delivered by the engine.
Diesel Engines
X
X
X
X
X
X
X
X
Gasoline Engines
X
X
X
X
X
X
X
X
X
X
Length of Stroke
The distance a piston travels between top and
bottom dead centers (TDC, BDC) is known as the
length of stroke. This distance is one of the fac-
tors that determines the piston speed. In some
modern diesel engines, piston speeds may reach
about 1600 feet per minute (fpm).
Cylinder Bore
Bore is used to identify the diameter of the
cylinder. The cylinder bore must be known in
order to compute the area of the piston crown
upon which the pressure acts to create the
driving force. This pressure is calculated and ex-
pressed for an area of one square inch as pounds
per square inch (psi).
5-2
