Principles of the Compression Engine

What is a compression engine?

• Needs no spark to ignite fuel/air mixture as in a gasoline spark ignition (SI) engine

• Combustion occurs spontaneously due to compression

• All diesel engines are compression ignition (CI) engines

Typical diesel engine

SI Engine with OHC

Camshaft

Intake valve

Rocker arm

Piston

Connecting rod

Crankshaft

Oil pump

Exhaust valve

Carburetor

Crank sprocketOil pickup

Timing belt

Cam sprocket

Air cleaner

Timing belttensor

Definitions

• Temperature /oC, F, K

• Volume /cm3 or cc

• Pressure / atm, psi, Pa

• See also glossary of terms

What is diesel?

• Crude oil fraction after gasoline (petrol)

• C9H20 – C12H26 nonane – dodecane

• Exact composition governed by various standards

The Chemistry

• Diesel is burnt:

CnH2n+2 + O2 H2O + CO2 +HEAT

• Inefficient combustion– CO, carbon monoxide– C, soot, diesel particulates– PCAs, benzene etc.

4-stroke operating cycle

• http://static.howstuffworks.com/flash/diesel.swf

Crank shaft

90o

180o

BC

TC0o

270o

Spark plug for SI engineFuel injector for CI engine

Top Center(TC)

BottomCenter(BC)

Valves

Clearancevolume

Cylinder wall

Piston

Stroke

1. Induction Stroke

• Engine pulls piston out of cylinder

• Low pressure inside cylinder (< 1 atm.)

• Atmospheric pressure pushes air into cylinder

• Engine does work on the gases during this stroke

2. Compression stroke

• Engine pushes piston into cylinder

• Air is compressed to high pressure and temperature (700psi, 540oC)

• Fuel injected as piston reaches top of stroke

• Engine does work on the gases during this stroke

3. Power Stroke

• Fuel/air mixture burns to form hot gases (2000oC, 1000psi)

• Gases push piston out of cylinder

• Gases expand to lower pressure and temperature

• Gases do work on engine during this stroke

4. Exhaust stroke

• Engine pushes piston into cylinder

• High pressure inside cylinder

• Pressure pushes burned gases out of cylinder

• Engine does work on the gases during this stroke

4-stroke cycle

• http://static.howstuffworks.com/flash/diesel.swf

Let’s take a closer look

The crankshaft and piston

Number of cylindersSingle-cylinder

…engine gives one power stroke per crank revolution (2 stroke) or two revolutions (4 stroke). The torque pulses are widely spaced, and engine vibration and smoothness are significant problems.Used in small engine applications where engine size is more important

Multi-cylinder

…engines spread out the displacement volume amongst multiple smaller cylinders. Increased frequency of power strokes produces smoother torque characteristics. Engine balance (inertia forcesassociated with accelerating and decelerating piston) better than singlecylinder.

Direct injection:quiescent chamber

Direct injection:swirl in chamber Indirect injection: turbulent

and swirl pre-chamber

Orifice -plate

Glow plug

Types of injection

Critical factors

• Compression must be high enough– Batteries– Worn piston/barrel– Valve seating

• Fuel and air must mix thoroughly– Injectors working correctly

• Glow plugs must work properly where fitted

Any questions?