Diesel I Introduction
Heat Engines
Heat engines produce power by burning a
combustible fuel, for example such as coal,
fuel oil, diesel oil, gasoline, or jet fuel.
Cylinder displacement
{(π) (bore)2 / 4} x (stroke)
• Engine displacement =
swept volume of cylinder x number of cylinders
Two basic designs for engine frames:
Bedplate Construction
Monobloc Construction - also known
as:
Underslung
Two categories of pistons:
Crosshead-Type
► Also known as barrel pistons – archaic
Trunk-Type
► Utilized in Monobloc engines
Cam Parts
Base Circle
Flank
Ramp
Nose
Disadvantages of Diesel Engines
Greater weight for same power output as compared to
other heat engines
• Greater initial cost
• Roughness of operation at light loads
• Noise
Typical compression ratios
12:1 to 25:1
Welding advantageous
because it is:
– Lighter
– Repairable
– Flexible
Functions of a Trunk type piston:
To dissipate heat absorbed by the piston crown during
combustion and the early part of the expansion stroke
To seal the inside of the cylinder from the crankcase
Transmit gas pressure to the crankshaft via the connecting rod
Acts as a valve, controlling air inlet from scavenge ports
To take the side pressures due to the angularity of the
connecting rod
Camshaft speed
Camshaft speed in a 2-stroke cycle engine
matches the speed of the crankshaft
Camshaft speed in a 4-stroke cycle engine
is ½ the speed of the crankshaft
Advantages of Diesel Engines
The most efficient heat engine
• Durable
• Torque – nearly independent of speed
• Power output ranges from just a few HP to over
100,000 HP
• Able to burn a wide range of fuels
If compression ratio is too low
– Insufficient heat of compression, especially when engine is
cold
• Difficult starting
• Less efficient – higher fuel consumption
Crosshead
This also ensure the piston remains
central in the cylinder thus limiting wear
in the liner.
This Mexican hat shape helps
promote turbulence within the
cylinder
Cam Follower Types
Roller Type
Most common
Roller fabricated of case
hardened steel, ground to size
and a true circle
Flat or Mushroom Type
Used in smaller and high-speed
engines
Subject to wear due to side
thrust
Pivoted Follower
Advantage is that side thrust is
taken by lever arm pivot rather
than tappet
Pivoted Follower with Roller
Additional resistance to wear
provided by roller
Roller Flat Pivoted
Pivoted with Roller
The following scavenging methods are
used:
The following scavenging methods are
used:
– Cross-Flow or Direct Scavenging
– Loop or Return Scavenging
– Uniflow Scavenging
If compression ratio is to high:
– Theoretically elevates thermal efficiency and lowers fuel
consumption
• Increases stress and pressure on engine components
• Increases operating temperature of engine
• Requires stronger/heavier engine parts
• Reduces engine life
Tensioning tool fits over
the stud and nut
Piston cooling methods include:
Shaker method
Spray method
Circulation method
Valves – Nomenclature
Stem
Radius
Face
Rim
Cup
Flat
5 Two-Stroke Cycle Engine
Scavenging
Reciprocating mechanical pump
– Under-Piston
– Positive displacement rotary blower
– Centrifugal blower (electric motor
driven)
– Turbocharger
Supercharging
Increases
MEP (Mean Effective Pressure)
– Maximum firing pressures (aka “peak pressures)
– Maximum temperatures:
• Moderated somewhat by overlap and improved
scavenging
Crankcase Explosion Doors
Set to relieve at 1/15 Bar (about 1 psig)
Piston Rings
Step
Angle
Straight
Valve Seat Angle
Intake 30
Exhaust 45