Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Identify the main components of a diesel fuel supply system.
• Describe the function of each component.
• Understand the layout and flow of fuel in the system.

Lesson summary

This lesson introduces the fuel supply system of a Compression Ignition (CI) or diesel engine. In Ghana, we see diesel engines everywhere – in the Ford Transit and Sprinter buses that form our tro-tro network, in the large cargo trucks that transport goods from Tema port across the country, in the tractors on our farms, and in the generators that power our homes and businesses during power outages. Understanding how fuel gets from the tank to the engine is a fundamental skill for any aspiring technician or engineer. A fault in this system is a common reason for a vehicle breakdown, so knowing its components is a very practical and valuable skill.

Lesson notes

The fuel supply system in a diesel engine has a very important job: to deliver a precise amount of clean, high-pressure fuel into the engine cylinder at the exact right moment. Unlike a petrol engine that uses a spark plug, a diesel engine ignites the fuel using the high temperature created by compressing air in the cylinder. This requires the fuel to be injected as a fine mist under extremely high pressure.

Let's break down the system component by component, following the path of the fuel. The Path of Diesel Fuel A simple way to remember the flow is: Tank → Lift Pump → Filters → Injection Pump → Injectors → Cylinder

Now, let's look at each component in detail. Component 1: Fuel Tank What it is: A secure container, usually made of metal or high-strength plastic, that stores the diesel fuel for the engine. Function: Its primary role is to hold a sufficient quantity of fuel for the vehicle or generator to operate for a reasonable period. It is designed to prevent leaks, reduce evaporation, and often contains baffles (internal walls) to stop the fuel from sloshing around too much. Ghanaian Context: Think of the large, often rectangular metal tanks you see on the side of a Man Diesel or KIA Rhino truck. For a generator, it might be a smaller tank located right on top of the engine assembly. Component 2: Fuel Lift Pump (or Transfer Pump) What it is: A low-pressure pump that draws fuel from the tank. It can be either mechanical (driven by the engine's camshaft) or electrical. Function: Its job is to pull fuel from the tank, which might be far away and at a lower level, and push it towards the fuel filters and the high-pressure injection pump. It ensures a constant, positive supply of fuel is available for the next stage. It operates at a relatively low pressure (e.g., 30-60 PSI). Analogy: The lift pump is like the person who fetches water from the big storage tank (Polytank) outside the house and brings it to the kitchen. It doesn't need to be high pressure, it just needs to move the liquid from storage to the point of use. Component 3: Fuel Filters What it is: These are critical filtering devices that clean the fuel. Diesel systems usually have at least two filters: Primary Filter (or Sedimenter/Water Separator): The first filter in the line. It's designed to remove larger particles of dirt, rust, and importantly, any water from the fuel. Water is heavier than diesel, so it settles at the bottom of the filter bowl where it can be drained off. Secondary Filter: This is a much finer filter located after the lift pump and before the injection pump. It removes very small microscopic particles that could damage the high-precision components of the injection pump and injectors. Function: To ensure that only perfectly clean fuel reaches the injection pump and injectors. Diesel injection components have extremely tight tolerances (the parts fit together very precisely), and even a tiny speck of dirt or a drop of water can cause serious and expensive damage. Ghanaian Context: Due to potential contamination in fuel storage and transportation, fuel filters are extremely important in Ghana. A mechanic changing a clogged, dark fuel filter on a tro-tro is a common sight. Skipping filter changes to save money often leads to a much more expensive injection pump failure later. Component 4: Fuel Injection Pump What it is: This is the "heart" of the diesel fuel system. It is a very complex, high-precision mechanical device driven by the engine. Function: It has two main jobs: Pressurisation: It takes the low-pressure fuel from the lift pump and pressurises it to extremely high levels (from 2,000 PSI to over 30,000 PSI in modern systems). Metering & Timing: It measures (meters) the exact amount of fuel needed for each combustion cycle and delivers that pressurised fuel to the correct injector at the precise moment it is needed. Analogy: If the lift pump fetches the water, the injection pump is like a high-pressure washer (like a Kärcher machine) that not only increases the pressure immensely but also directs the spray exactly where and when you want it. Component 5: High-Pressure Fuel Lines What it is: These are thick, strong steel tubes designed to withstand the massive pressure generated by the injection pump. Each injector has its own dedicated line from the injection pump. Function: To safely transport the high-pressure fuel from the injection pump outlet to each individual injector without bursting. Component 6: Fuel Injectors What it is: A spring-loaded nozzle located in the cylinder head of the engine. Function: This is the final component in the system. When the high-pressure pulse of fuel arrives from the injection pump, it forces the nozzle's valve open. The injector then sprays (atomises) the fuel into a very fine mist directly into the hot, compressed air in the combustion chamber. This fine mist mixes with the hot air and ignites instantly, pushing the piston down and creating power. Analogy: The injector works like a can of spray paint or insect repellent. It turns a stream of liquid into a fine, cone-shaped mist that can spread out and mix easily with the air. Simplified System Diagram (To be drawn on the board)

``` +----------------+ +-----------+ +-------------------+ +-------------------+ | | | | | | | | | FUEL TANK |----->| FUEL LIFT |----->| FUEL FILTERS |----->| FUEL INJECTION | | | | PUMP | | (Primary & Second)| | PUMP | +----------------+ +-----------+ +-------------------+ +-------------------+ | | High-Pressure Lines | /------->[INJECTOR 1] -> To Cylinder 1 / /-------->[INJECTOR 2] -> To Cylinder 2 / /--------->[INJECTOR 3] -> To Cylinder 3 / /----------->[INJECTOR 4] -> To Cylinder 4 ``` *(Note: Many systems also have a 'return line' that sends unused fuel from the injectors and pump back to the tank, but this diagram covers the main supply path.)*

Evaluation guide

• Describe the main components of fuel supply for compression ignition(C1)
• engines
• Group work/collaborative:
• Learners brainstorm in fair mixed groups to passionately discuss with respect to