Various types of starting mechanisms have been used with diesel engines. These range from hand cranking of small engines to compressed air starting of very large engines. Most automotive-type diesels use an electric and stopping a diesel are indicated below.
A diesel engine fitted with an electric starter motor would be started by means of starting
Switch. The accelerator pedal is held fully down (or the control lever to the high-speed position)
And starting switch turned to the “start” position to operate the starter. When the engine starts and runs evenly, the pedal is released to reduce engine speed. Holding the accelerator down provides maximum fuel, and in some cases excess fuel to the injectors during starting.
To stop the engine, fuel to the injectors is cut off completely. This is done by releasing the accelerator pedal and pulling the “stop” button, where this is fitted. The switch is turned to the “off” position to turn off all instruments and accessories.
In many instances, an electric solenoid is used to cut off the fuel instead of a stop button. The solenoid is controlled by the starting switch and so manual operation of a stop button is not necessary.
With turbocharged engines. The engine speed should not be rapidly increased and decreased immediately after starting. A warm-up period is necessary to ensure that the engine is operating smoothly and that the turbocharger is receiving adequate lubrication.
Engine with starting aid
To start the engine under cold conditions, the starting switch (Fig. 19.19) is first turned to the heat (H) position for about 15 seconds. A warning light may be fitted to the instrument panel to indicate that the air is hot enough. The switch is then turned to the heat and start (HS) position to operate the starter motor and start the engine. The accelerator is held down to provide maximum fuel for starting and released once the engine start. The switch is then turned to the run ® position.
To stop the engine, the switch is turned to the off (O) position. This operates the solenoid cut-off to stop the engine. If a stop button is fitted this is operated manually.
The illustration shows one arrangement only for switching and starting, but various other arrangements are used.
Devices to assist starting
Various devices are used to assist in starting diesel engines under cold conditions. Some of these provide additional fuel for starting; other devices provide heated air to promote combustion. Examples of these are given below.
In-line injection pumps are fitted with a maximum fuel stop, which limits the movement of the fuel control rod and so limits the maximum amount of fuel that the injection pump can deliver.
For starting purpose, the maximum fuel stop can be overridden to provide excess fuel. When the excess-fuel control is operated during starting, the control rod is allowed increased movement to engine to start. Once the engine starts, the excess-fuel device is no longer operative.
The thermostat starting aid is screwed into the inlet manifold and operates by running fuel in the manifold to provide heated air the cylinders.
The construction of the starting aid is shown in Figure 19.18. It is comprised of a central valve body (which supplies the fuel) surrounded by a heater coil which is extended to form an ignition coil. A needle valve holds a ball valve against its seat. The parts are surrounded by a metal shield.
When the unit is switched on, the valve body is heated by the coil and expands to open the ball valve. This allows vaporized fuel to be drawn into the manifold when the engine is cranked. The igniter coil and burns, thus heating the inlet air, ignite the fuel.
When the coil is switched off, the flow of air through the manifold cools the valve body and the valve closes to cut off the fuel supply.
These are small, electric heater plugs screwed into combustion chambers of the indirect-injection type. They provide heat during cold starting and so increase the temperature of the air being compressed in the cylinder. This assists with initial combustion of the fuel being sprayed into the combustion chamber. Glow plugs can be seen in the combustion chambers in Figures 4.6 and 4.8.
Some engines are fitted with decompression levers or cams which hold the intake or exhaust valves partly open. This reduces compression pressure and enables the engine to be turned fairly easily during starting. Before the engine fires, the decompression lever is released so that the cylinders have full compression. Relieving compression also enables the engine to be rotated during adjustments.