Four stroke cycle theory
Intake stroke
Piston moving down
Intake valve open
Exhaust valve closed
Copyright 2003 Gary Lewis - Dave Capitolo
Four stroke cycle theory
Compression stroke
Piston moving up
Intake valve closed
Exhaust valve closed
Four stroke cycle theory
Power stroke
Piston moving down
Intake valve closed
Exhaust valve closed
Four stroke cycle theory
Exhaust stroke
Piston moving up
Intake valve closed
Exhaust valve open
Four stroke cycle theory
Each stroke takes 180° of crankshaft rotation to complete
All cylinders fire in 720° of crankshaft rotation
720 divided by number of cylinders = firing interval
Odd fire V-6 engine (90° block with 120° rod journals)
Piston dwell time
Piston travel is at a minimum. . .
TDC and BDC
Crank moves horizontally
Piston velocity
Maximum when rod is 90° to crank
Acceleration
Maximum 30° earlier
Best VE is obtained by synchronizing valve opening
with piston speeds
Other engine cycles
Overlap
Both valves are open
End of exhaust & start of intake
Low pressure in exhaust port
Blowdown
Exhaust valve opens before BDC
To help evacuate cylinder before piston reverses
Pumping losses at end of exhaust stroke
Valve events
Intake valve opening
BTDC
Low pressure in cylinder
Intake valve closing
ABDC
Cylinder pressure is effected by timing
Exhaust valve opening
BBDC
Residual pressure helps blowdown
Exhaust valve closing
ATDC
Low pressure in exhaust port draws air in
Effects on valve timing
Intake valve opening
Late – Reduced VE
Early – Dilution of intake with exhaust
Intake valve closing
Late – Reduces cylinder pressure
Early – Increases cylinder pressure
Exhaust valve opening
Late – Pumping losses
Early – Power reduction
Exhaust valve closing
Late – Reduces vacuum
Early – Reduces VE
Combustion
Spark ignition
Maximum cylinder pressure 15° ATDC
Tumble and swirl
Motion reduces misfires
Excess motion inhibits flow
AFR 14.7:1 at part throttle, 12.5:1 under load
Compression ignition
18:1 direct injection
23:1 pre-chambers for better starting
Compression heats to 800-1200 °F
Diesel fuels
Cetane volatility numbers 50-55
Higher cetane #1 fuel for cold weather
Lower cetane #2 fuel for warm weather
Paraffin separates from fuel at 20°F
Valve trains
OHV (overhead valve)
Pushrod configuration
Many reciprocating parts
Higher valve spring pressure required
Compact engine size compared to OHC
Valve trains
OHC (overhead cam)
Fewer reciprocating parts
Reduced valve spring pressure required
Higher RPM capability
Cylinder head assemblies are taller
Valve trains
Cam-in-head
No pushrods
Use rocker arms
Valve lash compensators
Solid lifters
No internal parts
Periodic adjustment
Valve lash compensators
Hydraulic lifters
To maintain zero lash
Quieter
No periodic adjustment
Anti-scuff additives are required in oils
Hydraulic lifter operation
Valve closed
• Oil flows through lifter bore, &
past check valve
• Plunger return spring maintains
zero lash
Hydraulic lifter operation
Valve open
• Check valve seats and limits the slippage
• Now operates as a solid lifter
Hydraulic lifter operation
Return to valve closed
• New oil enters the lifter body
• This oil replaces oil that has leaked between
plunger and body (predetermined leakage)
Other lash compensators
Metering device
Metering valve meters the
oil flow to the pushrod
Timing sets
Gear sets
• Cam and crank rotate in opposite directions
• Noisy if not free of burrs
• Helical and spur cut gears
Timing sets
Timing chains
• Single and double roller
• Tensioners
Timing sets
Timing belts
• Require maintenance
• Quiet
Camshaft terminology
Cam lift (A-B)
Valve lift = Cam lift times
rocker ratio
Valve lift
.300” cam lift times
1.5 rocker ratio =
.450” valve opening
Engine oiling
Lubrication through pressure. . .
Engine oiling
and spray. . .
Engine oiling
Oil pan baffles
• To keep oil in sump during braking,
accelerating, and cornering
Engine oiling
Oil pan windage tray
• To prevent oil aeration in the sump
Engine oiling
Oil pumps
• Driven by distributors, gear on camshaft, or crankshaft
Engine oiling
Oil pumps with pressure relief valves
• Gear type pump
• Rotor type pump
Engine oiling
Full flow oil filtering system
• Oil pump output flows
through filter first
• Bypass circuit for restricted
filters will allow oil to
flow to engine
Engine oils
API, SAE, and ASTM
“S” - Spark ignition
“C” - Compression ignition
Engine oil additives
Viscosity index improvers
• To reduce viscosity change with heat
Detergents
• To dissolve varnish and sludge
Dispersants
• To keep sludge, carbon and other materials from
recombining and suspends them in oil to be drained
Scuff inhibitors
• To reduce friction and wear
Antifoam and antioxidants
• To prevent foaming and to slow oxidation in oil
Engine measurements
Bore
• Diameter of cylinder
Stroke
• Distance between TDC & BDC
Engine measurements
Displacement per cylinder
•  r² S
Displacement for the engine
• Disp per cylinder times the
Number of cylinders
Engine measurements
Compression ratio
D + CV
CV
To calculate clearance volume
D .
CR-1
Engine measurements
Deck clearance
• Top of piston to top of block deck
• Measured with dial indicator or depth mic
Engine measurements
Deck height
• Center line of crank to block deck
Fits and clearances
Running fit
• Clearance between bearing and shaft
• Clearance for oil
• Listed as diametral
Fits and clearances
Interference (press) fit
• OD is larger than ID
• Example is piston pin pressed into rod
Fits and clearances of pistons
Full floating
• .0003 - .0005 clearance in rod
• .0001 - .0003 clearance in piston
Press Fit
• .0008 - .0012 interference in rod
• .0003 - .0005 clearance in piston
Rod offset
• Beam offset to center of cylinder
• Enlarged chamfers to clear fillets
Pin offset
• Offset to major thrust side
• Quieter engine, less cylinder wear
Cooling system operation
Engine heat is transfered . . .
• through walls of the combustion chambers
• through the walls of cylinders
Coolant flows . . .
• to upper radiator hose
• through radiator
• to water pump
• through engine water jackets
• through thermostat
• back to radiator
Cooling system operation
Fans increase air flow through radiator
• Hydraulic fan clutches
• Hydraulic fans consume 6 to 8 HP
• Electric fans
Coolant (ethylene glycol)
• 50/50 mixture increases boiling point to 227°F
• pressurizing system to 15 PSI increases to 265°F
Coolant (propylene glycol)
• Less protection at the same temperatures
• Less toxic
Combustion efficiency
Under perfect conditions . . .
• Only byproducts would be carbon dioxide and water
• Iso-octane fuel is laboratory fuel
Because conditions are not perfect . . .
• Carbon monoxide and hydrocarbons are produced
• Oxides of nitrogen are produced from pressure & temp
Emission controls
• Catalytic converters – Convert CO & HC to
carbon dioxide & water
• O2 sensors – To monitor oxygen content in exhaust
• EGR – To reduce peak cylinder temperatures
Cooling system operation
Heat energy
• 1/3 usable power
• 1/3 released through exhaust system
• 1/3 released through cooling system
Engine temperature
• Cool enough to prevent part failure
• Warm enough to maximize engine efficiency
Four stroke diesel theory
Compression ignition
Uses high compression ratios instead of spark plugs
Engine components are more robust
Diesel fuel low has volatility
Four stroke diesel theory
Compression ignition fuel system
• Transfer pump from tank
• Injection pump to injectors
• Amount of fuel injected varies engine speed
• Diesels have no throttle (always WOT)
• AFR varies from 85:1 to 20:1
Four stroke diesel theory
Indirect Injection
• Indirect injection begins in a pre-chamber
• Initial combustion takes place there
• Slows the rate of combustion to reduce noise
• Glow plugs are needed to provide heat
Four stroke diesel theory
Direct Injection
• Fuel is injected directly into cylinder
• The piston has a chamber built into it
• More reliable than indirect
• More noisy than indirect
Four stroke diesel theory
Combustion
• Ignition is delayed after fuel is injected
• Rapid combustion when fuel 1st starts to burn
• Cylinder pressure rises quickly
• Engine knock (almost always detonating)
• Controlled combustion as injection continues
Four stroke diesel theory
Injection pumps
• Cam driven from front of engine
• Distributor driven
• High-pressure common rail
• 20,000 psi and computer controlled
• HEUI system (Ford Power Stroke)
• Hydraulically actuated with high pres oil
and PCM controlled
Four stroke diesel theory
Other diesel components
• Vacuum pump (crank driven)
• Electronic throttle
Four stroke diesel theory
Diesel advantages
Diesel disadvantages
• Higher engine torque
• Engine noise
• Better fuel economy
• Exhaust smell
• Long engine life
• Hard start cold
• Heavier
• Fuel availability
Rotary engine theory
Coolant Types
Traditional American coolants
• High silicates, should be changed every 24mos.
American low silicate coolant
• Longer life span
Heavy duty coolant
• Low silicate, require additive
Fully formulated (pre-charged)
• Same as above, but additives already in it
Coolant Types
Japanese coolants
• Many colors. No silicates. Uses other additives
instead of silicates
Organic Acid Technology
• Dexcool, European coolants
Hybrid coolants
• Aluminum and cavitation protection