Exhaust Gas
Exhaust Gas
Recirculation Exhaust gas recirculation (EGR) has two advantages for diesel engine operation. EGR is usually used to reduce nitrogen oxide (NOx) emissions and this is true
EGR
for diesel engines. Additionally, a small quantity of hot exhaust gas in the air charge of a cold engine helps to reduce the delay period and the incidence of cold engine diesel knock.
Catalytic Converters Many modern diesel engined vehicles are fitted with oxidation catalytic converters that work in conjunction
with other emission components to reduce Inside a catalytic
converter
hydrocarbon and particulate emissions. Turbo charging, EGR and catalytic converters are described in the Air Supply, Exhaust and Emission Control learning programme.
Injection Pressures The fuel systems for direct and indirect injection are similar and vary only in injection pressures and injector types. Until
Bosch injection
recently, all light high speed diesel engines used pump rotary diesel fuel injection pumps. These pumps producing injection pressures of over 100 bar for indirect engines. However, these can rise up to 1000 bar at the pump outlet, for turbocharged direct injection engines.
Pressure Differential Injectors operate with a pulsing action at high pressure to break the fuel down into finely atomised parts. Atomisation is critical to good fuel distribution in the compressed air charge. The air charge pressure may be in excess of 60 bar. The pressure differential, between the fuel injection pressure and air charge pressure, must be sufficient to overcome the resistance during injection. This will also give good fuel atomisation and a shorter injection time.
Swirl An aid to good fuel distribution in the air charge is the swirl in the air flow induced in the inlet manifold. This is created by the combustion chamber design. Air flow into and out of the pre-combustion chamber produces a swirl in the chamber. These chambers are often referred to as swirl chambers. The ‘bowl in piston’ combustion chambers, of direct injection engines, are shaped to maintain the induction air swirl during compression and combustion.
Diesel Fuel Injection Components The main components of
a diesel fuel system provide for either the low pressure or the high pressure functions. The low pressure components are the fuel tank, the fuel feed and return pipes and hoses, a renewable fuel filter with a water trap and drain tap, and a priming or lift pump. Fuel heaters may be fitted in the filter housing to reduce the risk of paraffin separation and waxing at freezing temperatures.
System components
High Pressure Components The high pressure components are the fuel injector pump, the high pressure pipes and the injectors. Other
components provide for cold engine starting. Injection pump,
pipes and injectors
Electronically controlled systems include sensors, an electronic diesel control (EDC) module and actuators in the injection pump.
Low Pressure Components The fuel tank is a pressed steel sealed unit, treated both inside and out with anti-corrosion paint. The inside is
treated in order to resist corrosion from water Fuel tank that accumulates at the bottom of the tank.
Some modern tanks are manufactured from a plastic compound that is burst proof in an accident. They are also unaffected by the diesel fuel, which can attack some plastic materials.
Low Pressure Fuel Lines Low pressure fuel lines are steel or hard plastic and connections made with short hoses clamped at each end. New vehicles are using quick coupling connections for ease of service and assembly operations. The feed lines run from the tank to the filter and then onto the injection pump. A low pressure return line is used to maintain a fuel flow through the injection pump and the fuel injectors for lubrication and cooling. The return carries fuel back to the filter housing or the fuel tank.
Fuel Filter The fuel filter is a micro-porous paper element in a replaceable canister or detached filter bowl. The filter includes a water
and sediment trap and tap for draining the Filter and housing water. Many vehicles have a sensor in the water
trap. This completes a warning lamp circuit when water is detected above a certain level. All diesel fuel entering the injection pump and injectors must be fully filtered. The internal components of the pump and injectors are manufactured to very fine tolerances. Even very small particles of dirt could be damaging to these components.
Fuel Heating Fuel heating may be provided from the engine coolant or by an electric heater element in
Injection pump Heated fuel filter the filter housing. The fuel is
housing
lifted from the fuel tank to the injection pump by the transfer pump in the injection pump on some vehicles. This is possible where the distance and height of lift are of small dimensions.
Fuel Lift Pump For improved delivery and for priming the injection pump another pump may be necessary. A conventional fuel lift pump driven
from the engine camshaft is a common method. Fuel priming pump These pumps in some instances have an external
operating lever. Hand operated priming pumps are fitted for use when the vehicle runs out of fuel. They are also used for service operations such as when the filter is changed. Many modern injector pumps are self-priming.
Injection Pump The injector pump shown is a rotary distributor type pump. These pumps are
filled with diesel fuel, which provides not only
Rotary injector
fuel for the engine, but also for full lubrication pump and cooling of the pump. These pumps are made from specially manufactured materials with surface treatments. Parts are lapped together to give very fine tolerances. Only clean and filtered fuel should be used to avoid damage to these parts.
Types of Pump There are two types of pump with different internal operation. Two major original designers make or license the manufacture of these pumps. The Lucas DP and Bosch VR pumps are
Bosch VR pump radial pistons
radial-piston designs. They use opposing pistons or plungers inside a cam ring to produce the high pressure. Bosch V series pumps are axial-piston designs having a roller ring and cam plate attached to an axial piston or plunger in the distributor head to generate the high pressure.
Pump Operation The operation of the two types of injector pump are quite different and explained separately further on in this section. Later versions of these pumps have
Bosch VE pump cam
electrical and electronic control. The latest plate and plunger versions have full electronic control.
High Pressure Outlets All types of pumps have delivery valves or pressure valves fitted to the high pressure outlets, which feed to each cylinder in turn. The delivery valves control the generation of pressure Delivery valves
waves in the high pressure pipes. They do this by giving initially a quick pressure drop and then by retaining a lower residual pressure. The delivery valve consists of a conical valve held closed by a compression spring and opened by hydraulic pressure when injection pressures are produced in the injection pump.
Delivery Valve Closure of the delivery valve when the injector pump pressure drops, allows a quick pressure drop in the pipe and injector. This means the injectors will close fully. Without the valve, pressure waves would oscillate in the pipe and force the injector to reopen. This would cause unwanted fuel to be injected. The retained low pressure helps to prevent fuel dribble from the injector nozzle during the non-injection period. It also aids lubrication through the leak-off pipes.
High Pressure Pipes The high pressure pipes are of double thickness steel construction and are all of the same length. This is so that the internal pressure rise characteristics are identical for all cylinders. The high pressure connections are made by rolled flanges on the pipe ends and threaded unions securing the rolled flanges to convex, or occasionally concave, seats in the delivery valves and injectors.
Fuel Injectors The fuel injectors are fitted into the cylinder head with the nozzle tip projecting into
DI injector
the pre-combustion (IDI) or combustion chamber (DI). The injectors for IDI injector indirect combustion are of
a pintle or ‘pintaux’ design and produce a conical spray pattern on injection. The injectors for direct injection (DI) are of a pencil type multi-hole design that produces a broad distribution of fuel on injection.
Injector Operation Fuel injectors are held closed by a compression spring. They are opened by hydraulic pressure when it is sufficient to overcome the spring force on the injector needle. The hydraulic pressure is applied to a face on the needle where it sits in a pressure chamber. The fuel pressure needed is in excess of 100 bar (1500 psi). This pressure lifts the needle and opens the nozzle, so that fuel is injected in a fine spray pattern into the combustion chamber.
Injector Spray The pressure drops when fuel is injected and the spring force on the needle closes the injector. This is immediately followed up by a build-up of pressure that again opens the nozzle. This results in a cycle of oscillations of the needle to give a finely atomised and almost continuous spray. The spray continues until the pump pressure is reduced at the end of the delivery stroke.
New Types of Injector The newer types of injector have two springs of different value in order to provide a small initial charge for ignition and then the main charge for controlled
burning. These injectors reduce diesel Two spring direct injection
injector
knock on direct injection engines and give a smoother engine performance. Fuel injectors are carefully matched to the type of engine and pump.
Glow Plugs There are two types of cold starting devices used on diesel engines. These are glow plugs and flame start devices. Glow plugs are used mainly on indirect injection engines although they are used on some small direct injection engines. Flame start devices are used on many, but not all, direct injection engines. Glow plugs are fitted in the combustion chamber. Their purpose is to help to ignite the fuel during injection.
Flame Start Devices
Flame start devices are fitted in the inlet
manifold. They preheat the intake air so that it achieves a high