differences between Petrol and Diesel.
It is commonly known that a Diesel engine of the same capacity as
its Petrol counterpart is more fuel efficient (approximately 10%).
The main reasons why a diesel returns better economy is because of
its ability to run very lean Air Fuel Ratios, better thermal efficiency
aided by its higher Compression Ratio (CR) and significantly less
pumping losses at part load due to the lack of a throttle valve.
Diesel engines are not that fussy about the measures of fuel they
receive, as long as they get some, they’ll burn it and produce
useable power. Petrol on the other hand is far more choosy. If the
Air Fuel Ratio (give or take a few ratios) isn’t around the
stoichometric value then it really doesn’t want to burn (Stoichometric
is the term that identifies the Air Fuel Ratio that offers the most
complete burn resulting in the lowest emissions for the hottest flame.
For unleaded petrol, it is 14.67:1, which is commonly rounded to 14.7:1.
The stochiometric value for other fuels varies with their energy content.)
Trying to run a petrol engine any leaner results in partially burnt
fuel, unstable combustion and high Hydro Carbon (HC) and Carbon Monoxide
better economy from a Petrol Engine.
years have tried to combine the economy of a Diesel engine with the
power of a Petrol Engine.
There are two main ways of achieving better economy with a petrol
engine. The first one is to get the engine to burn very lean mixtures
(lean burn engine) and the other is to create a localised stoichometric
cloud of mixture at the spark plug (stratified charge engine). The
goal of the stratified engine is to run at Wide Open Throttle (WOT)
and control the power in much the same as a Diesel by introducing
varying amounts of fuel. Under light load conditions it is possible
to run AFR’s as high as 60:1.
The stratified charge is not a new concept, Ricardo were experimenting
with the technology back in 1922. Early stratified engines used traditional
carburettors along with a separate mixing chamber to mix the chemically
correct AFR mixture which was then introduced into the ‘Clean
air’ in the combustion chamber before ignition.
Types of GD-i
are known as Gasoline Direct Injection (GD-i) and have benefited from
Diesel injector technology by injecting fuel direct into the combustion
chamber late into the compression stroke at a pressure of 150 Bar
There are two types of stratified engine, and these differ in the
way the air enters the combustion chamber. The swirl method is similar
to a Diesel concept in that air rushes into the combustion chamber
in an axial motion. This motion centralises the chemically correct
cloud of mixture towards the centre of the chamber in the vicinity
of the spark plug. The other method uses what is called reverse tumble.
The air entering the combustion chamber from the intake valve is deflected
in a circular motion in the opposite plane to the swirl motion. The
air hits the cylinder wall adjacent to the intake valve and then down
towards the piston. These engines use special ‘Ski jump’
shaped pistons to guide the air and fuel towards the spark plug.
Reverse tumble is probably the most suitable stratified charge delivery
system as this has already been successfully demonstrated on Mitsubishi’s
GD-i range of vehicles.
Even using modern
injection technology, it is still not possible to run in stratified
mode throughout the rev and load range of the engine. Thus, it is
only possible to run in stratified mode at part load. The engine switches
over to homogenous mode (early injection) at high speed conditions
because there is insufficient time to inject the fuel late into the
compression stroke and get the fuel to adequately mix into a cloud
of combustible mixture. Injecting the fuel too early when the piston
is near Bottom Dead Centre (BDC) results in the fuel missing the ‘ski
jump’ on the piston.
Attempting to increase the fuel pressure to deliver the same fuel
in less time, can result in the jet of fuel ‘hosing’ in
and hitting the ‘ski jump’ before it has a chance to adequately
mix and atomise with the air or produce a very rich portion of the
cloud that will fail to ignite at the spark plug.
High load conditions
are not possible in stratified mode either as injecting such a large
quantity of fuel will result in an ultra rich cloud of mixture at
the spark plug that wont burn. Attempting to continue Injecting fuel
very late into the compression stroke results in the cloud of mixture
hitting the piston when it is near to Top Dead Centre (TDC) that results
in the cloud of mixture overshooting the spark plug.
also have an optimum timing window when the ignition should ignite
the mixture. Too early and the engine will produce too many Oxides
of Nitrogen (Nox) and advanced even earlier will begin to ‘Knock’,
too late and you only get partial combustion and very high exhaust
temperatures. The perfect ignition timing is the Minimum advance for
Best Torque (MBT).
Stratified charge engine make the timing of the ignition even more
critical as the AFR at the spark plug changes as the cloud of chemically
correct mixture passes through it. Careful consideration has to be
given to the shape of the ramp on the piston as well as the injection
angle, pressure and timing in order to coincide with optimum ignition
timing. Sometimes throttling is needed at certain engine speeds in
order to create the necessary air velocity to adequately mix the air
Some of the added
benefits of GD-i.
of using direct injection in homogenous mode is that it brings with
it a charge cooling effect.
Charge cooling occurs when fuel is injected straight into the combustion
chamber whilst the intake valve is still open. The highly volatile
fuel ‘sucks’ heat from the incoming charge of air, therefore
increasing its density and allowing more air to enter. This increases
the engines volumetric efficiency. The cooler charge also means that
detonation is kept at bay. By pushing the Detonation Border Line (DBL)
limit out further, designers are able to increase the engines Compression
Ratio up to around 12.5:1 to 13:1. Increasing the Compression Ratio
improves the engines thermal efficiency. Lean burn strategies can
also be incorporated into Direct Injection engines. It is difficult
to initiate combustion in lean burn engines, therefore heavy duty
ignition coils provide a very powerful spark. Some engines also use
forced induction. By running a boosted lean mixture, it is possible
to run AFR’s as high as 24:1 or higher. However, such a high
pressure charge is inherently unstable, detonation and thermal runaway
can occur without much provocation.
In order to maintain
control of the engine, the ECU maps, control systems and exhaust after
treatment systems need to be quite complex in order to maintain stable
combustion and bring them inline with current EU emissions legislation.
Although GD-I engines do offer some advantage in economy over conventional
petrol engines they are still not as fuel efficient as an equivalent
Diesel. With the restrictive operating window, complex control systems
and exhaust after treatment, it does seem as though these engines
are twice as complex but only deliver half the benefit. For now, Stratified
engines only account for a tiny proportion of powerplant production
and look unlikely to become more viable unless some of the technical
obstacles are overcome.
However, don’t rule them out yet as most of the large automotive
manufacturers are still investing in stratified engine technology.
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