Diesel engines are subject to very high stresses during compression and ignition, and increasingly stringent emission standards have made better control of the diesel combustion process necessary.
Electronic controlled diesel systems give very precise control of the fuel injection and combustion process. Electronic controls have delivered other benefits besides a reduction in fuel consumption and emissions, such as an increase in power and torque; improved engine responsiveness; a reduction in engine noise and diesel knock; and improved and expanded diagnostic capabilities through the use of scan tools.
Diesel electronic control systems monitor and control many variables, including:
- Engine speed:
- to maintain a smooth functional idle,
- and to limit the maximum safe engine speed, power, and torque;
- and to keep the engine output to within safe limits.
- Fuel injector operation:
- including the timing, rate and volume of fuel injected.
- Glow plugs and heater elements:
- Control of pre-heating of the intake air to support quick cold starting and reduced cold run emissions.
- Exhaust emissions:
- Analysis of exhaust gas to determine combustion efficiency and pollutants.
- And the data bus:
- An electronic communications network that allows exchange of data between computers - necessary for efficient operation and fault diagnosis.
Other inputs monitored include:
- crankshaft position,
- throttle position,
- brake and clutch operation,
- battery voltage,
- cruise control request,
- air, oil fuel, exhaust and coolant temperatures,
- and intake air, oil and fuel pressures.
Common rail diesel injection system
The Common Rail Diesel Injection System delivers a more controlled quantity of atomized fuel, which leads to better fuel economy; a reduction in exhaust emissions; and a significant decrease in engine noise during operation.
In the Common Rail system, an accumulator, or rail, is used to create a common reservoir of fuel under a consistent controlled pressure that is separate from the fuel injection points.
A high-pressure pump increases the fuel pressure in the accumulator up to 1,600 bar or 23,200 PSI. The pressure is set by the engine control unit and is independent of the engine speed and quantity of fuel being injected into any of the cylinders. The fuel is then transferred through rigid pipes to the fuel injectors, which inject the correct amount of fuel into the combustion chambers.
The injectors used in Common Rail systems are triggered externally by an Electronic Diesel Control, or EDC unit, which controls all the engine injection parameters including the pressure in the fuel rail and the timing and duration of injection.
Diesel fuel injectors used in Common Rail injection systems operate differently to conventional fuel injectors used in the jerk pump system, where the plungers are controlled by the camshaft position and speed. Some common rail injectors are controlled by a magnetic solenoid on the injector. Hydraulic force from the pressure in the system is used to open and close the injector, but the available pressure is controlled by the solenoid triggered by the Electronic Diesel Control unit.
Some injectors use Piezo crystal wafers to actuate the injectors. These crystals expand rapidly when connected to an electric field. In a Piezo inline injector, the actuator is built into the injector body very close to the jet needle and uses no mechanical parts to switch injector needles.
The electronic diesel control unit precisely meters the amount of fuel injected, and improves atomization of the fuel by controlling the injector pulsations. This results in quieter, more fuel efficient engines; cleaner operation; and more power output.
HEUI diesel injection system
The Hydraulically actuated, Electronically controlled Unit Injector or HEUI system of diesel fuel injection operates by drawing fuel from the tank using a tandem high and low pressure fuel pump. The pump circulates fuel via the 'low' side of the pump at low pressure through a combination of fuel filter, water separator, and heater bowl, and then back through the 'high' side of the pump at high pressure into the fuel galleries located in the cylinder head, and through to the injector units.
The injectors are controlled by a Powertrain Control Module or PCM. Although the PCM controls the duration of fuel injection pulses based on a range of inputs, oil from a high-pressure oil pump hydraulically actuates the injectors.
By varying the oil pressure, injection can be controlled independently of the position or speed of the engine crankshaft or camshaft. A solenoid-actuated valve controls the high-pressure oil flow which is applied to the top of an intensifier piston in the injector. This can increase injection pressures to 1250 to 1800 Bar or 18,000 to 24,000 PSI.
The area of the head of the intensifier piston is approximately 7 times the area of its plunger. As a result a 7:1 pressure increase on the fuel beneath the plunger can be achieved.
