MECHANIC
Page 40
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The compression ignition engine, 9
ON MANY diesel engines it is sufficient to provide excess fuel when starting from cold and the injection pump is provided with a means of supplying this excess fuel. In some cases special equipment is provided for heating the air under cold-start conditions.
On an indirect injection type engine, it is generally necessary to heat the air in the pre combustion chamber when starting the engine from cold. Electric heater plugs as shown in Figure 1 are usually employed. These plugs, which are screwed into the cylinder head, have a small wire element which glows when an electric current is passed through it.
When starting from cold, the heater plugs are switched on for about 30 seconds, or for whatever time is recommended by the vehicle manufacturer, before the starter motor cranks the engine. The air which has been warmed by the plugs materially assists in giving an easy start.
The plugs are of the two-volt type, and as is shown in Figure 2, they are wired in series. If the engine is a six-cylinder one, no external resistance will be required if a 12-volt battery is used.
On engines with less than six cylinders, or with a 24-volt battery, a suitable resistance must be incorporated in the circuit so that each plug is not subjected to a higher voltage than two volts.
In some cases a heater coil (which could be roughly described as an outsize edition of the heater plugs described above) is provided in the intake manifold to warm the air before the engine is turned over by the starter motor.
Figure 3 illustrates another cold-starting device, the CAV Thermostart which is fitted in the manifold of some engines. It consists of a tubular casing containing a heater coil and an extension. This becomes red-hot and ignites fuel from a small reservoir when the device is in operation. Fuel is supplied to the reservoir by a leak-off pipe from the main fuel system.
When the heater is not in use, a ball-valve is held against its seat by a stem; when the Thermostart is switched on, the coil heats up and expands, lifting the valve stem away from the ballvalve. Fuel is now allowed to flow into the heater, where it is vaporised by the heated coil and ignited by the glowing end. As soon as the unit is switched off, the flow of air in the manifold cools the valve body, which pushes the valve stem against the ball cutting off the fuel supply.
A completely different cold starting aid is to introduce ether into the main air-flow as the engine is being cranked. The simplest way of doing this is by the use of an ether aerosol-type spray. This is something of a hit-and-miss method and should not be overdone, since an excessive quantity will cause very high combustion chamber pressures which may cause damage. Some engines are fitted with a special device which sprays the starting fluid into the induction manifold.
More about compression ignition engines in the next article.
by Preceptor