Troels Dyhr Pedersen.indd - Solid Mechanics

6 The basics of HCCI combustion
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6.1 Combustion principle
The HCCI combustion process concept is based on auto ignition in a lean premixed
fuel/air charge. The combustion event is therefore governed mainly by chemical kinetics,
which clearly separates it from the diffusion combustion in DI CI engines as well as the
flame propagation in SI engines.
The abbreviation HCCI has become a covering term for internal combustion technologies
that utilize lean premixed combustion at low temperatures. The temperature region for
low temperature combustion starts around 1500 K, which is the minimum temperature
required for satisfactory conversion of CO to CO2 in the power stroke. The highest
temperature allowed is usually determined by the amplitude of the engine knock.
Knocking is a high pitched sound caused by large pressure gradients in the combustion
chamber due to very rapid combustion. This limit is reached around 1800-2000 K with
DME as fuel, corresponding to an excess air ratio of approx. 2.5. At this temperature
level the formation of NO is also increasing notably, thereby reducing one of the
important advantages of lean premixed combustion.
6.2 Combustion characteristics
In general, the highly premixed fuel/air charges will burn to completion within a few
milliseconds near TDC. The ideal HCCI combustion process is therefore not much
different from a constant volume explosion due to the very short duration of the
combustion. The term explosion implies that the process is spatially uniform and hence
the pressure rise rate is also uniform. Although an explosion is usually connected with
destructive events, it does not have any damaging effect on the engine as long as the end
pressure is within the design limits of the engine.
With an excess air ratio of 3 or less, the majority of the heat release lies within less than
one millisecond in a well mixed charge. This results in a fast pressure rise rate, typically
above 10 bar per one tenth of a millisecond or higher. At this rate of combustion it is
common that not all of the charge reacts simultaneously, which can be caused by
temperature gradients and uneven fuel distribution in the chamber. As a result, large
pressure gradients are created in the combustion chamber. This causes the air to oscillate
with high amplitude in the chamber. The frequencies of the oscillations are identical to
the resonance frequencies, which may be calculated or determined by computer
simulations. The oscillations are similar to those observed in SI engine knock. HCCI
knock may - like SI engine knock - be harmful to the engine if the load is high enough;
otherwise it may run continuously at knocking conditions without any damaging effects
to the piston or cylinder walls. Knocking does however shorten the life of cylinder
pressure transducers, since the membranes of the transducer are typically loaded beyond
their design specifications when they are impacted by pressure waves.
Part of the energy contained in the pressure waves in knocking combustion is transferred
to the engine and causes the engine exterior surfaces to emit acoustic noise. As higher