Combustion in SI and CI Engines

Battery (coil) ignition system

It produces the high-voltage spark (10-25 kV) from a 6 V or 12 V battery.

 Battery --[Ignition switch]--[Ballast R]--+
                                           |
        +----------+----------------------+
        |  Primary |      Secondary       |
        |  (low)   |      (high)          |
        +--+--(coil)-----+----------------+
           |             |
     [Contact breaker]   |  [Distributor] --> plug 1,2,3,4
           |  = Condenser|
          Earth         Earth

Main parts: battery, ignition switch, ignition coil (primary of 200-300 turns of thick wire, secondary of 20 000-30 000 turns of fine wire on a soft iron core), contact breaker with cam, condenser, distributor and spark plugs.

Working

1. With the contacts closed the primary current (3-4 A) flows and builds a magnetic field in the core.
2. The cam opens the contacts. The field collapses very quickly, inducing about 250 V in the primary and 10-25 kV in the secondary because of its many turns.
3. The condenser across the contacts absorbs the primary surge, prevents arcing at the points and makes the field collapse faster.
4. The distributor sends the high voltage to the correct plug in firing order; the spark jumps the gap and ignites the mixture.
5. A centrifugal or vacuum advance mechanism changes spark timing with speed and load.

Comparison

Modern engines use transistorised, capacitor-discharge or fully electronic ignition with coil-on-plug units instead of contact breakers.

Combustion in an SI engine is a flame front that starts at the spark plug and spreads through the premixed charge. It occurs in three stages.

 p
 |                 C
 |               /   \
 |             /       \   firing
 |           /          '.....
 |         B  . . . . . . . . motoring
 |  . . . . .'
 |     A (spark)
 +--|------|-----|-----|-------- crank angle
    A      B    TDC    C

A to B: ignition lag; B to C: flame propagation; after C: after-burning.

Stages

1. Ignition lag (A to B): from the spark to the point where the pressure line leaves the motoring curve. A small nucleus of flame forms and a self-propagating flame is established. Chemical reactions dominate. Little pressure rise. Depends on mixture, temperature and pressure, typically 0.0015 s.
2. Flame propagation (B to C): the turbulent flame travels through the charge. Most of the heat is released, the pressure rises rapidly, and the maximum pressure occurs about 10-15 degrees after TDC. Flame speed is 15-30 m/s.
3. After-burning (beyond C): the piston is moving down and the last part of the charge burns slowly, with the pressure falling. Completion of combustion by this time is needed.

Factors affecting flame speed

• Turbulence: greatest effect; flame speed increases almost in proportion to turbulence, which rises with engine speed and with combustion chamber shape.
• Air-fuel ratio: the maximum flame speed is at a slightly rich mixture (about 10% rich); lean and very rich mixtures burn slowly.
• Compression ratio: higher ratio gives higher pressure and temperature, faster flame.
• Initial temperature and pressure of the charge: a higher value gives faster burning.
• Residual gas: more exhaust gas (exhaust gas recirculation, low load) slows the flame.
• Engine speed: flame speed rises with speed, since turbulence rises; ignition lag in degrees increases but the combustion time in degrees stays almost constant.
• Fuel type and the size and shape of the combustion chamber (short flame path helps).

Knocking

Knock is the metallic pinging sound caused by the auto-ignition of the end gas (the last part of the unburnt charge) before the normal flame front reaches it.

Process

The advancing flame compresses and heats the unburnt end gas by compression and by radiation from the flame. If the temperature and pressure are high enough for long enough, the end gas auto-ignites in several places and releases its energy almost instantly. This produces a high-frequency pressure wave (shock) that strikes the cylinder walls and causes the pressure-trace oscillations and noise.

Effects

• Loss of power and efficiency, rough running and noise.
• High local heat transfer, which can burn the piston crown and cause piston, ring or head damage.
• Overheating and increased fuel consumption.

Factors affecting knock

Control of knock

• Use a high-octane fuel or additives.
• Reduce the compression ratio, charge temperature and pressure; use intercooling in turbocharged engines.
• Retard the spark (knock sensors do this automatically).
• Design compact chambers with turbulence, short flame travel and the plug near the hot exhaust valve.
• Increase engine speed, use more cooling for the end gas, and enrich the mixture.

Knocking versus pre-ignition

In a CI engine fuel is injected into the hot compressed air near TDC. Combustion has four stages.

 p
 |                C-D
 |              /     \
 |          B /         \ . . .
 |   . . . . '  (motoring)
 |  A (injection starts)
 +--|-----|-----|-----|------- crank angle
    A     B    TDC    D

A to B: ignition delay; B to C: rapid combustion; C to D: controlled combustion; after D: after-burning.

Stages

1. Ignition delay (A to B): from the start of injection to the start of pressure rise. Fuel is atomised, vaporised, mixed with air and pre-flame reactions occur. Physical delay (atomisation, vaporisation) and chemical delay (pre-reactions) are included. It is about 0.001 s (10-15 degrees).
2. Rapid or uncontrolled combustion (B to C): the fuel injected during the delay burns suddenly. The pressure rises rapidly, and the rate of rise (bar per degree) determines the roughness.
3. Controlled combustion (C to D): the fuel injected afterwards burns as it is injected, at a rate set by the injection and mixing. The peak temperature occurs here.
4. After-burning: unburnt fuel burns on the expansion stroke, with a fall of pressure. Too much of it gives smoke and loss.

Diesel knock versus SI knock

The remedies are opposite.

Reducing diesel knock

• Use a high-cetane fuel and a high compression ratio.
• Increase the intake air temperature and pressure (turbocharging).
• Use a pilot injection or injection with a small initial quantity; correct injection timing.
• Increase swirl and turbulence, use fine atomisation and high injection pressure.
• Keep the jacket water temperature high.

Requirements

• High power output and thermal efficiency without knock (a high compression ratio possible).
• Compact shape with a small surface-to-volume ratio, so heat loss is small.
• Short flame travel: spark plug centrally placed or near the exhaust valve.
• High turbulence (squish) for fast, complete burning, but not so much that heat loss rises.
• Smooth pressure rise and low emissions.
• Cool exhaust valve and last-burning end gas near a cold surface.
• Good volumetric efficiency: large valves and smooth gas passages.

Types

1. T-head

 [ IV ]  [ EV ]
   \__piston__/

Valves on opposite sides, needing two camshafts; long flame travel, high knock tendency. Obsolete.

2. L-head (side-valve)

   plug
 ___|___
|  head | 
|  [V][V]  valves at the side
|__piston_|

Simple, one camshaft, easy to build and good turbulence from the squish area. But the chamber is not compact, flame travel is long and the compression ratio is low (about 6.5).

3. I-head (overhead valve)

   IV  plug  EV
    \   |   /
   ___\_|_/___
  |  piston   |

Valves in the head above the piston. Compact chamber, short flame path, high compression ratio (8-10) and large valve area. Pushrod or overhead cam drives are more complex. Most modern engines use it.

4. Hemispherical and pent-roof chamber (modern) with the plug in the centre, four valves per cylinder and central injection give the shortest flame travel and high efficiency.