With emission norms getting tighter, it is us, the consumers who still want enough power to play with – while reaching from one place to another. Every vehicle, be it a moped, a scooter, a motorcycle, a car, a bus, a anything…is being worked upon to reduce emissions for a cleaner and greener planet. And. At the same time efforts are also being made to improve performance.
The industry has seriously gotten into hybrid energy to improve its game and match today’s ever increasing carbon emissions. But the initial attempts made back in the day led to the introduction of engine stratification. For understanding engine stratification we need to know the basic functioning of a spark ignition engine.
Let’s take a look at this injection here for understanding.
Imagine that the barrel of the injection represents the engine cylinder and the plunger represents the piston moving inside the cylinder.
The area where the red fluid is filled indicates the combustion chamber where a mixture of air & fuel is filled. A spark is provided to this mixture by say, the needle (spark plug). The spark triggers an explosion powerful enough to push the plunger (piston) out of the barrel (cylinder).
In reality, the piston does not move out of the cylinder because it is connected to a crankshaft on its outer end. The crankshaft’s roll is to convert the To & Fro motion of the piston into rotational motion and finally send that motion to the wheels of the vehicle.
The air/fuel mixture enters the combustion chamber via an inlet valve, combusts and the gases formed are vented out of the cylinder via the exhaust valve and then through the exhaust of the vehicle.
It takes 4 strokes of the piston from the intake of the air/fuel mixture, to its compression, to its combustion and to venting out the exhaust gases. Hence called a 4-stroke cycle.
The picture shows the intake stroke, compression stroke, power stroke and the exhaust stroke (from left to right). In short:
- Intake Stroke: Piston moves downwards, Inlet valve opens letting in air and fuel.
- Compression stroke: Piston moves upwards, Inlet valve closes (causing compression).
- Power stroke: At topmost position of piston, spark plug ignites mixture forcing piston downwards.
- Exhaust stroke: Piston moves upwards, Exhaust valve opens to vent out exhaust gases from combustion.
What is stratification?
The science of Efficiency depends entirely on the air: fuel ratio. The ideal air: fuel ratio for spark ignition engines is said to be 15:1. It means for every 15 particles of air, there is one particle of fuel. So, a 16:1 air: fuel ratio has more particles of air (Lean mixture) whereas a 14:1 air: fuel ratio has more particles of fuel (Rich mixture).
In a stratified engine, the air-fuel mixture immediately around the spark plug is rich and the rest of the combustion chamber is lean. The rich mixture around the spark helps in better ignition of air and fuel resulting in a better power stroke. The rest of the chamber need not have a rich mixture. This allows for lower amounts of fuel consumption and hence lower emissions.
How is this done?
- Piston head design:
The head of the piston is designed in a manner that it pushes the air-fuel mixture more towards the spark plug for better combustion. Here are some examples of stratified engine pistons.
2. Air-Fuel ratio:
For more controlled amounts of air: fuel ratios, Direct Fuel Injection is used. Here the inlet valve lets only air into the cylinder in the intake stroke. Fuel is separately injected via fuel injectors into the cylinder just before the compression stroke. These fuel injectors spray fuel directly towards the spark plug to attain a rich mixture in its immediate vicinity.
The image shows a stratified engine cylinder. Air particles are indicated in blue and fuel particles are indicated in yellow. The inlet valve is allowing air inside the cylinder whereas fuel is being sprayed directly into the cylinder just before compression stroke.
3. Placement of inlet and exhaust ports:
Inlet and exhaust ports are placed in different positions to obtain the maximum efficiency out of engines.
4. Spark Placement :
The position of the spark plug is altered to fire up the air and fuel to its advantage.
The play of inlet ports, exhaust ports, spark plug placement, piston head design and air-fuel ratio make engine stratification possible. R&D teams of different manufacturers have gone on a splurge to find the best possible combination for providing their most powerful and efficient engines.
Engine stratification has become so essential today that each and every vehicle has it. Vehicles fitted with supercharged and turbo charged engine right from the showroom have stratified their engines accordingly. For this reason it becomes really important to do some research before changing any component from your vehicle’s drivetrain, knowing how your vehicle’s output changes before applying it and finding the right engine tune for its optimum performance.