SI & CI Engines
4 Stroke Engine
This Engine cycle is the most common type present in vehicles today. Each stroke has a specific function. They are as follows:
( Please note
TDC - Top Dead Centre - top-most position head surface of the piston will travel to
BDC - Bottom Dead Centre - bottom-most position head surface of the piston will travel to )
1. Suction / Intake stroke
As the piston moves down from TDC to BDC, it creates a negative/low pressure zone inside the cylinder. The intake valve opens, and air which is at normal atmospheric pressure, will rush into the cylinder.
An analogy for the same will be how liquid/medicine is drawn into a syringe before injecting a patient
2. Compression stroke
Piston starts moving from BDC to TDC, both intake & exhaust valves are closed, forcing the air in the cylinder to compress into a smaller and smaller volume(space).
When piston reaches TDC, compression stroke is complete.
3. Power Stroke
As soon a piston reached TDC, fuel is ignited. This causes a flame to form inside the cylinder. In an ideal scenario, flame is started near the cylinder head and it travels towards the piston. As it moves towards the piston, it carries along with it a lot of pressure and energy, this release of explosive energy from the ignition of fuel forces the piston to move from TDC to BDC
4. Exhaust Stroke
The piston now moves from BDC to TDC, the exhaust valve opens. Gases produced due to combustion are pushed out of the cylinder, making space for fresh air to enter when to next suction stroke will start. Exhaust stroke is complete when piston reaches TDC and exhaust valve closes.
Intake stroke will start soon after, this cycle will be repeated as long as engine is running
Petrol Engines
- In petrol engines, an air-fuel mixture will enter the cylinder
- A spark plug will be present at at the head of the engine block which will ignite the air-fuel mixture to start power stroke.
- Since combustion is caused due to spark, these engines are called Spark Ignition (SI) Engines.
- In older vehicles a carburetor was used to create the air-fuel mixture. It was a mechanical device with limited scope for changes in air-fuel ratio while in operation.
- Carburetors are present just before the fuel intake manifold of the engine.
- Modern vehicles are equipped with fuel injectors which spray fuel at high pressure into the air stream to ensure fuel particles are atomized and distributed evenly in the mixture
- Injectors are preferred as you have more range for control of the air-fuel mixture.
- They are present just before the before the fuel intake manifold of the engine, like carburetors, also due to they compact packaging, they could be placed in the intake manifold assembly itself.
Diesel Engines
- Here, only air will enter the cylinder during intake stroke.
- This is done due to a property of fuels known as "Auto Ignition Point (AIP)", it is a specific point where the pressure and temperatures are such that no additional energy is required to start combustion of fuel. Thus, fuel at AIP will immediately start combustion and catch fire.
- The temperature and pressures found at TDC in diesel engines are at & above the AIP of Diesel Fuel.
- Hence instead of adding fuel to the air at intake, it is only introduced into the cylinder when piston reached TDC, just before power stroke is to begin.
- Since combustion is caused due to compression, these engines are called Compression Ignition (CI) Engines.
- No spark plug will be present, only fuel injector at head of assembly.
