Direct petrol injection is the latest technology
in injection systems – you’ll find it in models from Mazda, Audi/VW, BMW and
Alfa, amongst others. But how does it work?
As the name suggests, Direct Petrol Injection uses
injectors that add fuel directly to the combustion chamber. Like diesel engines,
the air/fuel mixing occurs inside the combustion chamber, rather than in the
inlet ports. Taking this approach gives far greater control over the combustion
process, allowing for a variety of combustion operating modes, including those
having ultra-lean air/fuel ratios.
This diagram shows the layout of the Bosch direct
injection system. Direct injection systems differ from
conventional port injection in several ways.
The fuel supply system uses two fuel pumps – a
conventional electrical fuel pressure pump (in the past dubbed a high pressure
pump but now referred to in this system as a low pressure pump) and a
mechanically-driven high pressure pump. The low pressure pump works at pressures
of 0.3 – 0.5 MPa while the high pressure pumps boost this very substantially to
5 – 12 MPa.
The high pressure fuel is stored in the fuel rail
that feeds the injectors. The fuel rail is made sufficiently large that pressure
fluctuations within it are minimised as each injector opens. The pressure of the
fuel in the injector supply rail is controlled by an electronically-controlled
bypass valve that can divert fuel from the high pressure pump outlet back to its
inlet. The fuel bypass valve is varied in flow by being pulse-width modulated by
the Electronic Control Unit (ECU). A fuel pressure sensor is used to monitor
fuel rail pressure.
This diagram shows a cross-sectional view of an
injector. Compared with a conventional port fuel injection system, the fuel
injectors must be capable of working with huge fuel pressures and also injecting
large amounts of fuel in very short periods. The reason for the much reduced
time in which the injection can be completed is due to the fact that all the
injection must sometimes occur within just a portion of the induction stroke.
Conventional port fuel injectors have two complete rotations of the crankshaft
in which to inject the fuel charge – at an engine speed of 6000 rpm, this
corresponds to 20 milliseconds. However, in some modes, direct fuel injectors
have only 5 milliseconds in which to inject the full-load fuel. The fuel
requirements at idle can drop the opening time to just 0.4 milliseconds. Direct
injection fuel droplets are on average only one-fifth the droplet size of
traditional injectors and one-third the diameter of a human hair.
The very lean air/fuel ratios at which direct
injection systems can operate results in the production of large quantities of
oxides of nitrogen (NOx). As a result, direct injected cars require both a
primary catalytic converter fitted close to the engine, and also a main
catalytic converter - incorporating a NOx accumulator - that is fitted further
So that’s the mechanical make-up – now, how does
the system work?
The really radical nature of direct fuel injection
can be seen when the different combustion modes are examined. There are at least
six different ways in which combustion can take place.
Stratified Charge Mode
At low torque output up to about 3000 rpm the
engine is operated in Stratified Charge Mode. In this mode the injector adds the
fuel during the compression stroke, just before the spark plug fires.
In the period between the injection finishing and
the sparkplug firing, the airflow movement within the combustion chamber
transports the air/fuel mixture into the vicinity of the sparkplug. This results
in a portion of relatively rich air/fuel mixture surrounding the sparkplug
electrode while the rest of the combustion chamber is relatively lean.
The gas filling the rest of the chamber often
comprises recirculated exhaust gases which results in a reduced combustion
temperature and so decreased NOx emissions.
In Bosch direct injection systems, the air/fuel
ratio within the whole combustion chamber can be as lean as 22:1 – 44:1.
Mitsubishi states that total combustion chamber air/fuel ratios of 35 – 55:1 can
be used. This can be compared with a conventional port fuel injected engine that
seldom uses an air/fuel ratio leaner than 14.7:1.
Homogenous Mode is used at high torque outputs and
at high engine speeds. Injection starts on the intake stroke so there is
sufficient time for the air/fuel mixture to be distributed throughout the
combustion chamber. In this mode Bosch systems use an air/fuel ratio of 14.7:1
(the same as with port fuel injection at light loads), while Mitsubishi use
air/fuel ratios from 13 – 24:1.
Homogenous Lean-Burn Mode
In the transition between Stratified and
Homogenous Modes the engine can be run with a homogenously lean air/fuel ratio.
Homogenous Stratified Charge
Initially, this mode doesn’t seem to make sense –
how can the combustion process be both homogenous and stratified? However, what
occurs is not one but two injection cycles.
The initial injection occurs during the
intake stroke, giving plenty of time for the fuel to mix with the air throughout
the combustion chamber. Then, during the compression stroke, a second
amount of fuel is injected. This leads to the creation of a rich zone around
the sparkplug. The rich zone easily ignites, which in turn ignites the leaner
air/fuel ratio within the remainder of the combustion chamber. Of the total fuel
addition, approximately 75 per cent occurs during the first injection and 25 per
cent in the second. The Homogenous Stratified Charge Mode is used during the
transition from Stratified Charge to Homogenous Modes.
In addition there are at least two more modes –
Homogenous Anti-Knock and Stratified Charge Cat-Heating. The first is used at
full throttle and the second to rapidly heat the catalytic converter to
operating temperature. A final mode – mentioned in only some of the literature –
is Rich Homogenous Mode, which is used to regenerate the NOx cat. (The NOx cat
deposits oxides of nitrogen in the form of NHO3 nitrates. When the cat is
regenerated, the nitrate, together with carbon monoxide, is reduced in the
exhaust to nitrogen and oxygen.)
This diagram shows the two primary combustion
modes – stratified charge and homogenous modes.
Electronic Control Systems
As was indicated earlier, the injectors must be
opened against very high fuel pressures. So that this can happen, a peak/hold
strategy is employed where the opening current is very high and the ‘hold’
current much reduced. A dedicated triggering module is used to control the
injectors, with a booster capacitor providing 50 – 90 volts to initially open
The sensing of how much gas is in the cylinder is
more complex on a direct injected engine than a conventional port injected
engine. This is because at times recirculated exhaust gas forms a major
component of the total cylinder charge. As a result, two cylinder charge sensors
are used. These comprise a conventional hot-film mass airflow sensor (ie similar
to a hot-wire airflow meter) and a manifold pressure sensor (MAP sensor). The
flow through the airflow meter is used as an input into the calculation of the
pressure within the intake manifold and this is then compared with the actual
intake manifold pressure measured by the MAP sensor. The difference between the
two indicates the mass flow of the recirculated exhaust gas.
As with many conventional engine management
systems, direct injection requires the use of an electronically-controlled
throttle. However, unlike conventional systems where the actual throttle opening
more or less follows the driver’s accelerator pedal torque request, in the case
of direct injected engines, for much of the time the throttle is fully open -
engine torque output is instead regulated by varying the fuel delivery, just
like a diesel.
This diagram shows how this occurs. During
Stratified Charge Mode the throttle (indicated by ‘a’) is held wide-open,
irrespective of the driver’s accelerator pedal input. When the torque request is
low, the air/fuel ratio is very lean (Bosch refer to this as an increased
‘excess air ratio’ – line ‘b’), with the air/fuel ratio gradually becoming
richer as more torque is required. At a certain point, which corresponds on an
engine-specific basis to engine speed and the amount of torque required, the
engine changes to Homogenous Mode. (For simplicity, the transitional Homogenous
Lean-Burn Mode is ignored in this diagram.) With the change in modes, the
throttle valve opening becomes related to the driver’s torque request and the
air/fuel ratio holds a constant stoichiometric air/fuel ratio (that is, 14.7:1
or Lambda = 1) across the rest of the engine load range.
The system incorporates an operating-mode
co-ordinator which maps operating mode against engine speed and torque request.
This diagram shows a schematic diagram of the functioning of this controller. As
can be seen, a 10-stage prioritisation is used when deciding on the required
operating mode. Before the selected combustion mode starts to occur, control
functions for exhaust-gas recirculation, fuel tank ventilation, charge-movement
flap (ie port tumble valves or variable length intake manifold), and electronic
throttle settings are initiated as required. The system waits for
acknowledgement that these actions have been carried out before altering fuel
injection and ignition timing.
The advantage of having the electronic throttle
valve fully open at low loads is a huge reduction in pumping losses – the engine
is no longer trying to breathe through the restriction of the nearly-closed
throttle. (A similar scheme is used in the latest Honda Civic but it is achieved
through valve timing changes.) However, the downside of this approach is that
the partial vacuum that is normally available for the brake booster is lacking.
To overcome this problem, a vacuum switch or pressure sensor monitors brake
booster vacuum and if it is necessary, the combustion mode is altered so that
vacuum again becomes available.
In addition to the reduction in pumping losses
occurring as a result of the throttle being wide open at low loads, during
Stratified Charge Mode thermodynamic efficiencies are also increased. This is
because the rich cloud of combustible air/fuel mixture around the sparkplug is
thermally insulated by the layer of air and recirculated exhaust gas that
Together with the much leaner air/fuel ratios than
can be used in a conventional port injected engine, the result is a fuel
efficiency improvement that can be up to 40 per cent at idle. Mitsubishi state
that at 35 km/h their direct injected engines use 35 per cent less fuel than a
comparably sized conventional engine and that in the Japanese 10-15 Urban
Driving Cycle (albeit a slow speed cycle), the direct injected engine uses less
fuel than even a comparable diesel engine.
During homogenous mode operation, both the use of
an air/fuel ratio that is never richer than 14.7:1 and the higher compression
ratios normally associated with direct injection engines result in a fuel saving
of about 5 per cent.
Mitsubishi has been building direct injection
petrol engines since 1996, but they have not been able to achieve worldwide
success with their designs due, it is said, to the engines’ reliance on high
quality fuel. Now Bosch has developed technology which is allowing direct
injected engines to be sold around the world. Combine direct injection
technology with the hybrid cars being widely developed – plus perhaps downsized
and turbocharged engines - and we’re certainly in for some interesting times
Starter Motor Starts!
advantage of Direct Injection is that it is possible to start a hot engine
without using the starter motor. This approach also reduces start-up hydrocarbon
emissions, making more attractive the use of engines that automatically switch
off once the car has been stationary for a period.