Transcript Engine Terminology - Centennial College

Engine Terminology
Engine Measurement
Lesson 8
March 2008
Engine Size Measurement
Engine
size is determined by the cylinder
diameter, the amount of piston travel (stroke)
and the number of cylinders.
Bore and Stroke
Cylinder Bore
Diameter
of the engine cylinder
Measured
across the cylinder, parallel with
the top of the block
Cylinder
bores vary in size, but typically
range from 3–4" (75–100 mm)
Piston Stroke
Distance
the piston moves from top dead
center (TDC) to bottom dead center (BDC)
Crankshaft
journal offset (throw) controls the
piston stroke
Stroke
varies from about 3–4"
(75–100 mm)
Changing Bore Size
What happens if the engine is bored?
Definition: Boring
Material
is moved from the cylinder walls and a
larger piston is installed
Displacement & compression ratio are both increased
when bored (AKA “bored out”)
How would the displacement of a stock 6 cylinder engine
with a bore of 4.000” and a stroke of 3.000” change if it
were bored 0.060”?
Changing Length of Stroke
What happens if the engine is stroked?
Definition: Stroked
Changing
the crankshaft offset (installing a new
crankshaft)
Displacement & compression ratio are both increased
when stroked
How would the displacement of a stock 6 cylinder
engine with a bore of 4.000” and a stroke of 3.000”
change if it were stroked 0.125”?
Change of Bore and Stroke
What happens if the engine is bored & stroked?
Changing
the crankshaft offset and boring the
cylinder
How would the displacement of a stock 6 cylinder
engine with a bore of 4.000” and a stroke of 3.000”
change if it were stroked 0.125” and the bore 0.060”?
Oversquare and Undersquare
How to increase engine output


How to increase engine output Increase volumetric efficiency

Better gas flow

Forced induction

Altered valve timing
Improve engine efficiency

Better combustion efficiency

Lower internal friction

Reduce heat loss

Better fuel
Piston Displacement
Volume
the piston displaces as it travels from BDC to
TDC Found by comparing cylinder diameter and piston
stroke
Piston
displacement formula:
piston
=
displacement
bore squared  3.14  stroke
4
OR
piston
=
displacement
bore  bore  0.7854  stroke
Example of Piston Displacement
Formula
If an engine has a bore of 4" and a stroke of 3", what is it’s
piston displacement?
piston displacement =
(42)  3.14  3
4
16  3.14  3
4
150.72
4
= 37.68 (cu. in.)
OR
4 x 4 x .7854 x 3 = 37.69
(cu.in.)
Volumetric Efficiency
Ratio
of air drawn into the cylinder and the maximum amount of air
that could enter the cylinder
• Simply put, it is the ability of the engine to breath
in on the Intake Stroke
Formula:
Volumetric Efficiency = actual volume of air taken into each cylinder
volume of the cylinder
Factors that impact Volumetric
Efficiency
Engines are capable of only 80–90% volumetric
efficiency.
Restrictions in the ports and around the valves limit
airflow.
Can you name some of the factors that would impede
air flow into and engine?
Is ‘volumetric efficiency’ the same as
cylinder volume?
8 to fill
BDC
1 to fill
TDC
Swept and Clearance Volume
Compression Ratio
Compares the cylinder volume with the piston at TDC to
the cylinder volume with the piston at BDC
Ratio of swept volume V1 divided by combustion
chamber volume V2

Compression Ratio – Gas Engine
Compression Ratio – Diesel Engine
High Compression Ratio
Pros and Cons
Higher compression gives greater
combustion efficiency, up to a
point

Efficiency goes down after a
certain point due to need to retard
ignition timing to avoid knock

Volumetric Efficiency and
Compression Ratio Relationship
Volumetric efficiency is:
Ratio
of air drawn into the cylinder (by the intake stroke) and the
maximum amount of air that could enter the cylinder (restricted by
design of intake passages, size of valve openings, valve timing and
overlap ).
Compression ratio is:
•Ratio between the cylinder volume when the piston is at TDC
compared to when it is at BDC.
Compression Pressure
Checking the Seals
How is it measured?
What unit of measurement is used?
(Answers on next slide)
Amount
of pressure in the cylinder on the compression stroke
Measured
Gasoline
in pounds per square inch (psi) or kilopascals (kPa)
engine compression pressure:
130–180
Diesel
psi (900–1200 kPa)
engine compression pressure:
250–400
psi (1700–2800 kPa)
Measuring Combustion Pressure
Used
to measure compression pressure
Using
a compression gauge:
screw
the gauge into the spark plug, injector, or
glow plug hole
crank
note
Gauge
the engine
compression gauge readings
readings are a good indicator of engine
mechanical condition (engine seal)
Brake Mean Effective Pressure
Not actually a measure of pressure, just
expressed in pressure units (N/m2 or psi)

A measure of work output from an
engine


BMEP =
Horsepower__________
(Swept volume Vs) ( rpm/120)
Examples of Engine Displacement
If one piston displaces 25 cu. in. and the engine has
four cylinders, what is the engine displacement?
25 cu. in.  4 = 100 cu. in.
If one piston displaces 500 cc and the engine has six
cylinders, what is the engine displacement?
500 cc  6 = 3000 cc
= 3.0 L
Conversion of Measurement
61.02 cubic inches = 1 litre
From litres  cubic inches
5.0L x 61.02 = 305CID
From cubic inches  litres
305 / 61.02 = 5.0L
1L = 1000cc
1 cu. in. =16.39cc
Engine Displacement
Volume
displaced by all the
pistons in an engine
piston
displacement multiplied
by the number of cylinders
Units
of engine displacement:
cubic
liters
inch displacement (CID)
(L)
bore  bore  0.7854  stroke x cylinder number