Direct Petrol Injection.

 

 

 

 

 

 

 

 

 

 

The 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 (CO) emissions.

Getting better economy from a Petrol Engine.

Engineers for 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 Engines.

Modern engines 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 or more.
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.

Limitations of GD-i.

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.

Petrol engines 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 and fuel.

Some of the added benefits of GD-i.

The advantage 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.

Summary.

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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