Diesel engines

Glow Plugs


Glow plugs are used when the engine is cold, it heats up the air in the cylinder so that the engine starts easier. The time that the glow plugs are on is controlled by the ECU. The time is calculated using the temperature measured  by the temperature sensor.

To check the glow plugs first see if they receive the correct voltage using a multimeter, then check the amps by connecting the multimeter (set to amps and make sure it can handle the amount of amps) in series to the glow plug circuit. If the reading on the ammeter is less than the specifications check each glow plug with a ohmeter to see if it is working. If the glow plugs are faulty they can be removed and tested on the bench with another battery. The glow plugs should heat up in 10 to 15 seconds and it should not start from the tip.

The difference between petrol and diesel engines


The fuel in a petrol engine is ignited by an electrical spark, in a diesel the fuel is ignited by heat created by compressed air. The speed of a diesel engine is controlled by the amount of fuel that is added and a petrol engine is controlled by the amount of air added. Diesels are more fuel efficient and have more power at lower engine speeds


Checks for a non start diesel engine


*Check that there is fuel in the system and that there is no air trapped in lines. The injectors can be loosened to check for fuel. The condition of the fuel should also be checked
*Make sure that the battery is able to crank the engine
*Check the compression on all the cylinders
*Check the glow plugs, the engine will have trouble starting when it is cold without glow plugs

Bleeding a diesel engine


To get the air out of the system loosen all the injectors then crank the engine until fuel starts coming out the side of the injectors the tighten them up again. If not all the air is out then do it again.

The diesel bug


When the fuel tank stands for a long time the fuel evaporates and condenses at the top of the fuel tank then the water mixes with the fuel that creates an environment for bacteria to grow the bacteria that grows in the fuel is known as the diesel bug. The "bug" can clog the fuel pump and injectors. To prevent getting a diesel bug its best to keep the fuel tank as full as possible, drain water from the system regularly and get fuel from suppliers that maintain their fuel well.

The diesel injector 


The mechanical operated injectors have spring loaded valves in them that are forced open when the fuel from the pump reaches a certain pressure therefor spraying the fuel at a high pressure into the cylinder. The electronic injectors on common rail diesel engines are controlled by the ECU it receives high pressured fuel from the common rail that is presurised by the fuel pump. Another type of fuel injector is the pump injector it receives fuel at low pressure and pressurizes it by the camshaft lobes pushing down at the top of the injector the pressure is then released by the ECU sending the injector a signal which then opens it via a solenoid.

Injector pump


The injector pump, pumps the fuel at high pressure to the injectors. The two commonly fitted pumps are in line and distributor pumps. A distributor pump is similar to the distributor in a petrol engine it supplies the fuel to each cylinder according to the timing.

Diesel pump timing


We set the timing for a Mazda diesel engine. First we loosened the bolt (in the middle of the four high pressure lines that go to the injectors) on the diesel pump and put the DTI gauge in its place. Then we turned the crankshaft to 30 degrees before T.D.C and zeroed the Gauge. Then we turned the crankshaft to 2 degrees after T.D.C. We then loosened the diesel pump so that it was able to pivot, moved it so that the reading on the Gauge matched up with specifications and then tightened the diesel pump again. We removed the DTI gauge and replaced the bolt. The flywheel has marks on it in degrees


Petrol and diesel pistons


Diesel engines generally have pistons with flat heads with concave areas in. This design creates a higher compression and the concave areas create an air flow which causes the fuel to burn better.


Turbo


A turbo is bolted to the exhaust pipe and has a turbine that is powered by the exhaust gasses. The turbine is connected directly to the a compressor that forces air into the engines intake.




Servicing a diesel


When replacing a diesel filter fill the new filter with diesel to reduce the amount of air in the system. Before starting it the system should be bled to get all the air out.