Autoelex Blog: Engine Combustion - Compression Ignition (Diesel):
Engine Combustion - Compression Ignition (Diesel)
In
a diesel or compression ignition engine, the first and major difference
compared to a spark ignition engine is the way that fuel and air is
prepared for combustion, also, the way combustion is initiated.
Diesel engines
induce air only during the intake stroke - the air charge is compressed
in the cylinder, heating it accordingly, the final temperature at the
end of the compression stroke is above the self ignition temperature of
the fuel and this factor is essential, as this initiates the combustion
event when the fuel meets with the hot air. The advantage of compressing
air only is that we don't have to consider self ignition of any
fuel/air during compression (as per gasoline engine) as at this point in
the engine cycle, there is no fuel to burn! The combustion process is
quite different to the gasoline engine, the timing and rate of
combustion is controlled via the introduction of fuel into the cylinder
(via the fuel injection system). The combustion process itself takes
place at the interface between the fuel and air. Therefore, sufficient
air motion in the cylinder (generally swirl in a diesel engine) is
essential to sweep away the products of combustion, ensuring that the
fuel charge always has sufficient oxygen at the flame interface to
prevent to formation of soot due to localised oxygen starvation.
Fig 1 - Air motion in a diesel engine is generally 'swirl'
The overall
volume of the combustion chamber itself has a variable air/fuel ratio
during operation, that is only chemically correct at the fuel to air
interface. In most operating conditions, the average air/fuel ratio in
the cylinder is considerably weak (compared to stoichiometric). The
engine power output is controlled by the amount of fuel injected, so
no throttling is needed and this improves efficiency at part load due to
the lack of pumping losses associated with restricting the airflow into
the engine. The technical term associated with diesel type combustion
is ‘diffusion’ combustion, as the fuel burning takes place at the
interface where fuel diffuses into the air, and vice-versa.
Due to the fact
that fuel and air have to be mixed during the compression/expansion
cycle (as opposed to pre-mixed, outside the cylinder) this reduces the
amount of time available to complete the whole mixing and combustion
process. Hence, generally speaking, diesel engines cannot rev as highly
as gasoline engine. Therefore, to get more power from a diesel engine
you increase the torque by turbocharging it! - common practice these
days. It’s notable though that the diesel engine combustion cycle, and
engine itself, is more efficient than gasoline for several reasons - the
higher compression ratio increases the cycle efficiency, the lack of a
throttle reduces pumping losses and the high precision, metered
injection system reduces cylinder-to-cylinder variation.
Fig 2 - A common rail diesel fuel injection (FIE) system
Diesel engines
have undergone considerable development over the last few years, mainly
in the area of fuel injection system technology. These developments have
enabled sophisticated, electronically controlled injection systems,
that can help reduce particulate emissions as well a engine noise
emissions. I think that anybody would agree that travelling in a modern
diesel engine car is no longer a noisy or unpleasant experience. Modern
diesels are very refined and smooth in operation!
Fig 3 - Direct and Indirect fuel injection - direct injection is predominant now!
All modern
diesel engines for passenger cars use direct injection technology (as
opposed to indirect). In the past, indirect injection - injecting fuel
into a pre-chamber - was technology used to create the required air
charge motion to speed up the combustion event, thus increasing the
maximum possible engine speed and power density. However, the increased
surface area of the combustion and pre-chamber increases heat losses and
reduces efficiency and has now been completely superseded by direct
injection systems for most applications. In a modern diesel engine, the
fuel injector nozzle sprays a complex, engineered spray pattern into the
hot , highly turbulent combustion chamber gases, to initiate the
combustion event at around TDC. The fuel is injected radially into the
combustion chamber, the liquid fuel vaporises and mixes with the air as
it travels away from the injector tip nozzles. The fuel self-ignites at
multiple ignition sites along each of the injection sprays. 
Fig 4 - Diesel spray pattern and combustion from a thermal image system
The design of
the combustion chamber, in the piston bowl, is critical to the
efficiency of the combustion event. This design creates the necessary
motion and energy in the cylinder charge to make sure that each tiny
droplet of fuel has sufficient oxygen for complete combustion, right
throughout the injection period.
Fig 5 - The 3 phases of diesel combustion
The initial
combustion takes a certain time period to establish, known as the delay
time, then the fuel will auto-ignite creating a very rapid energy
release and the flame spreads rapidly through the fuel that is exposed
to sufficient air for combustion. This creates a rapid rise in cylinder
pressure, forcing the piston down the cylinder. As the power (or
expansion) stroke continues, further mixing of fuel and air occurs,
accompanied by further, more controlled combustion period where energy
release is controlled by injection rate. Note that it is the rapid
release of energy, after the delay period, which causes the
characteristic combustion ‘knock’ associated with diesel engine.
Fig 6 - Common rail, electronic diesel systems allow multiple injection events with better control of the combustion process
Modern,
electronic fuel injection systems, with multiple injection events,
effectively reduce this noise via a more gradual introduction of the
fuel into the cylinder (via pre-injection events) as opposed to a
single-shot event, where all fuel is injected at once (causing rapid
pressure rise and noise). Note that single-shot injection strategies
were all that was possible with a simple rotary or in-line injector pump
in the past. In summary, the key points to consider with respect to the
compression ignition engine are:
- The fuel/air mixture is prepared internally in the cylinder, during the engine cycle and relies on self ignition
- The engine power is controlled via the quantity of fuel injected in each engine cycle.
- The
compression ratio is not limited by the fuel as the compressed charge is
just air, It is only limited by the strength of the engine design as
peak cylinder pressures are very high
- In operation, engine maximum torque is limited by peak pressures/mechanical loading
- Rapid pressure rise, generated by the self-ignition of the fuel, creates the diesel engine noise
