Mechanical Injection System

Mechanical Injection System:

• Used to inject fuel into the combustion chamber.
• Fuel speed > air speed at delivery point to atomize fuel.
• Amount of fuel delivered into the air stream going into the engine is controlled by pump which forces the fuel under pressure.

A device used to initiate and control the combustion process is called Mechanical Injection System.

Functional Requirements of Injection System:

• Metering of Fuel: Used to control fuel to meet speed and load requirements of the engine.
• Injection Timing: Have proper injection timing to obtain max. power, fuel economy and proper burning. It is done by Camshaft or Push rod (Rocker Arm) Mechanism.
• Proper Control of Injection Rate: To get desired heat release pattern during combustion.
• Proper Atomization of Fuel: Proper making of droplets to make best Air/Fuel mixture.
• Proper Spray Pattern: Distribution of droplets to ensure rapid mixing of fuel & air.
• Uniform Distribution of Fuel: If distribution of fuel is not uniform throughout the combustion chamber, combustion is improper.

Classification Of Injection Systems:

There are two types of injection systems which are described as follows:

 1. Air Injection System:

Injection system in which fuel is forced into the cylinder by means of compressed air is called Air Injection System.

• Having good mixing of Air-Fuel mixture, Higher MEP, can utilize high viscosity fuel.
• The requirement of bulky multistage air compressor making air injection system obsolete.

2. Solid Injection System or Airless Mechanical Injection System:

Injection system in which liquid fuel is directly injected into the combustion chamber without compressed air is called Solid Injection System.

There are 4 types of Solid injection system which we are going to discuss.

A. Individual Pump & Nozzle System:

• Each cylinder is provided with one pump and injector are placed near to the cylinder (separate or cluster).
• Fuel is controlled by Plunger Effective Stroke.

B. Unit Injector System:

• It consist of housing containing HP pump and injector is provided on each cylinder.
• Fuel is brought up to the injector by a low pressure (LP) pump.
• Rocker arm is connected with plunger and thus injects the fuel into the cylinder.
• Fuel is controlled by plunger effective stroke.

C. Common Rail System:

• Hp pump supplies high pressure fuel to the header.
• Header forces the fuel into the injector.
• Rocker arm & push rod valve allows the fuel to enter the proper cylinder through nozzle.
• Fuel is controlled by the length of Push Rod Stroke.

D. Distributor System:

• Single pump is used to pressurized & meters fuel.
• Rotating distributor distributes fuel to each cylinder at proper time.
• Number of injection strokes per cycle for the pump is equal to the number of cylinders.
• Since it has only one metering element in each pump, a uniform distribution is automatically ensured.
• Cost efficient less than 2/3rd for individual pump system.

Components Of Mechanical Injection System:

1. Fuel Tank  ↔  contains fuel.
2. Fuel Feed Pump  ↔  to supply fuel from the main fuel tank to the injection system.
3. Injection Pump  ↔  to meter and pressurize the fuel.
4. Governors  ↔  to ensure that the amount of fuel injected with respect to the variation in load.
5. Injector  ↔  take fuel from the pump and distribute it into the combustion chamber by atomization into fine droplets.
6. Fuel Filters  ↔  prevent dust and abrasive particles from entering the pump and injector, minimizing wear & tear of components.

Fuel Feed Pump:

• Spring loaded plunger type pump.
• Plunger is actuated through a push rod from the camshaft.
• At minimum lift position of cam, rod moves down creates suction pressure, fuel comes into the pump.
• When cam is at maximum lift position, rod moves up and compress fuel, inlet valve close and open outlet valve.

Injection Pump:

It is used to deliver metered fuel under high pressure of about 120-200 bars. There are two types of injection pumps which are discussed ahead:

a. Jerk Type Pumps:

• Reciprocating plunger inside a barrel driven by camshaft.
• Fuel is always available at intake.
• Axial movement (rotation) of plunger by camshaft (due to rack).
• When plunger is below intake opening, fuel get into barrel. As plunger rises, it closes intake port and fuel will flow out from the output port.
• As plunger rotates output port closes, it pressurized fuel and fuel will pass through the injection port. That's the beginning of injection called Plunger Effective Stroke. 
• As plunger rotates, output port uncovers and injection stops.
• Axial distance traversed between the time intake port is closed off and the time intake port is uncovered is called Plunger Effective Stroke.

b. Distributor Type Pump:

• Pump has only one single pumping element and fuel is distributed to each cylinder by means of a rotor.
• Rotor is provided by two holes. One for inlet and the other for outlet.
• Inlet port is connected to pump and outlet port is connected to injectors through delivery fuel lines.
• When rotor coincides with inlet port, fuel comes in and plunger moves away from each other.
• Fuel is pressurized by moving plunger towards each other.
• When rotor coincides with outlet port, fuel is delivered to the injectors.

 Injection Pump Governors:

When speed is increased or decreased, the A/F mixture should change. If speed is high (rich A/F mix), if speed is low (Lean A/F mix). If speed is suddenly increased or decreased A/F ratio cannot reduce with same proportion producing high emissions.

There are two types of Governors:

a. Mechanical Governors:

• When speed exceeds limit, centrifugal weights moves apart, causes lever to raise sleeves and operate the control lever in downward direction. 
• This actuates the control rack connected to fuel injection pump in direction which reduces the fuel. Lesser fuel causes the engine speed to decrease.

b. Pneumatic Governors:

• Amount of vacuum applied to diaphragm is controlled by accelerator pedal through the position of butterfly valve in venturi unit.
• Diaphragm is connected to a fuel pump control rack.
• Position of accelerator pedal also determines the position of the pump control rack and hence the amount of fuel injected.

   Fuel Injector Assembly:

• When fuel comes from injection pump, it exerts forces against the spring to open nozzle valve, fuel is sprayed into the combustion chamber.
• Atomization of fuel is done by passing fuel in combustion chamber through nozzle under high pressure.
• When fuel gets injected, pressure is released which closes the nozzle valve.
• For lubrication, fuel is leak off through the clearance between nozzle valve and it's guide.
• The spring tension (valve opening pressure) is controlled by adjusting screw on the top.

Fuel injector assembly consists of following parts:

1. A needle valve
2. A compression spring
3. A nozzle
4. An injector body

Nozzle:

Part of injector assembly by which fuel is sprayed into the combustion chamber is called a Nozzle.

Functions of Nozzle:

1. Atomization of Fuel  ↣  To obtain proper mixing of fuel and air in C/C.
2. Distribution of Fuel  ↣  Distribution of fuel increases with the increase in injection pressure, density of compressed air, physical properties of fuel (self-ignition temperature, vapor pressure, viscosity).
3. Prevention of Impingement on Walls  ↣  Reduce direct striking of fuel particles on walls.
4. Mixing  ↣  For non-turbulent type of C/C, nozzle should take care of proper mixing of air and fuel.

Types of Nozzle:

1. Pintle Nozzle  ↣  Stem nozzle is pin, low injection pressure 8-10MPa, cone angle is 60°, no dribbling effects.
2. Single Hole Nozzle  ↣  Cone angle is 15°, injection pressure 8-10MPa, dribbling effects.
3. Multi-hole Nozzle  ↣  multi-holes in tip of nozzle about 4-18, hole angle 20° upward, injection pressure 18MPa, very small hole so clogging does happen.
4. Pintaux Nozzle  ↣  Type of pintle nozzle with auxiliary hole in nozzle body. At low pressure, nozzle valve does not open, so fuel is injected through aux-hole.       

Note: Want to learn about Car Engine Operations then see the video given below:

And, 

References:

• University notes + my knowledge.
• Videos from YouTube.
• Images from Google Images.
• Book: IC Engines by V Ganesan (4th Edition).
• Class Lectures.