Fuel Stratified Injection

Homogeneous vs Heterogeneous mix.

In case of petrol engines the air and the fuel are already mixed in the carburetor and sent to the combustion chamber for combustion .so the air -fuel ratio is same at each and every point in the combustion chamber. That is a homogeneous air-fuel mixture is burnt in the combustion chamber. But in case of diesel engines, since fuel is directly injected in to the cylinder and only air alone is coming through intake, the mixing of air and fuel will not be proper .so a heterogeneous mixture is obtained with different air-fuel ratio at different points in the combustion chamber.

Evolution Of FSI

The term FSI denotes Fuel Stratified Injection .also called as direct injection stratified charge, a brilliant technical version of advanced engine technology. FSI deals with combustion of both homogeneous as well as heterogeneous air-fuel mixtures in the combustion chamber of petrol engines. How it is possible to have both homogeneous as well as heterogeneous mixtures at the same time in the combustion chamber? We’ll see now.

FSI technology uses a high-pressure direct-injection pump to create perfect synergy between power and fuel efficiency. It makes engines cleaner, more economical and delivers peppier, more dynamic performance. In direct injection systems, fuel is injected directly into the cylinder so that the timing and shape of the fuel mist can be precisely controlled. This allows higher compression ratios and more efficient fuel intake, which deliver higher performance with lower fuel consumption. Most of the petrol engines work on the principle that an even or 'homogeneous' - mixture of fuel and air at the correct proportion is ideal for combustion. Especially if full power is required, for example when accelerating. But how much of the time are we accelerating? What happens under a trailing throttle, for example when slowing down or holding in traffic?

Engineers discovered that under those conditions having a homogeneous fuel-air mixture could be wasting energy, and thus fuel. At such times the FSI engine switches to something called a ‘’stratified’’ charge, where the injector releases a precisely-timed and directed smaller charge of fuel, creating a variable mixture of fuel and air that is richest around the spark plug, for instantaneous combustion, but that becomes leaner further away from the spark. Result? Enough power for the situation and a clean, highly efficient burn. In simple terms, this means that the driver has all the power he or she needs, whenever it is needed, but the engine smoothly and seamlessly switches to a leaner burn mode whenever the car is in a steady-state cruise or is decelerating. 

Concept of FSI Engine Operation

Precise control over the amount of fuel injected and injection timings which are varied according to the load conditions is the norm in FSI engines. Basically, the engine management system continuously chooses between three different modes of combustion: ultra lean burn combustion, stoichiometric combustion and high power output mode. Each mode is characterized by the air-fuel ratio, the amount of fuel in the air-fuel mixture; the stoichiometric ratio for petrol is 14.7 to 1 by weight, but in ultra lean mode, it could be as high as 65 to 1. These leaner mixtures than those ever achieved in the conventional engines are desired because of reduced fuel consumption.

Ultra lean combustion mode is effective under normal running conditions, when little acceleration is required. The fuel is not injected at the intake stroke but rather at the latter stages of the compression stroke, so that the small amount of air-fuel mixture is optimally placed just near the spark plug. 　This stratified charge is surrounded by mostly air which keeps the fuel away from the cylinder walls for lowest emissions. The combustion takes place in a toroidal cavity on the piston's surface. This technique enables the usage of ultra lean mixtures with very high air-fuel ratio, impossible with traditional carburetors or even intake port injection.

Stoichiometric combustion mode is activated for moderate load conditions. In this mode, fuel is injected during the intake stroke. The air-fuel mixture is homogeneous with the stoichiometric rates necessary for the catalytic converter to remove the maximum number of major pollutants viz., CO and NO_X from the exhaust gas.

In full power mode, the air-fuel mixture is homogeneous as well but contains the maximum amount of fuel that is possible to ignite without knocking out, as defined by the compression ratio of the engine. The fuel is injected during the intake stroke. This mode is activated at high load conditions and provides maximum output and torque.

Mode of Operation of FSI 

The heart of FSI, lies in the adoption of a mode known as "stratified charge", which gives the fuel-air mixture in the cylinder an extra swirl to create an electric charge (much as a thunderstorm is created) at the point where it reaches the spark plug. This allows for a leaner fuel-air mixture, using less fuel, and the excess air forms an insulating layer in the cylinder to guard against heat energy losses. Exhaust gas recirculation systems divert some of the waste exhaust gases back up for re-combustion, working with catalytic converters which process the remaining surplus harmful nitrous oxides and convert them into inert nitrogen. Besides reducing such emissions, FSI technology can reduce fuel consumption by an estimated 15 percent². The fuel injector, which shoots fuel directly into the combustion chamber, can control injection to within thousandths of a second, at pressures of up to 110 bars. As a comparison, indirect injection operates at pressures of up to 8 bars. Higher the pressure betters the efficiency.

Pros and Cons of FSI

The Merits of FSI are reduced emission, reduced fuel consumption, power on demand and reduced heat loss. The only disadvantage is during ultra-lean combustion mode, the exhaust gas temperature is high enough to account for considerable NOx emissions. However, most of this is negated by adopting “Exhaust Gas Recirculation” technique.