Diesel exercise... for ROB!

Diesel engines

Diesels are a lot different to petrol engines, yet they are so similar.  They both have cylinders, pistons, heads, manifolds, they both have four strokes and they both run in a similar way.  So what's different?   The fuel that it consumes.  Diesels (self explained) runs on diesel fuel while petrol runs on unleaded fuel.   But when you change something so important in an engine, you have to know the characteristics of it will effect everything that uses it.  Generally, diesel engines have a higher compression ratios (around 16-26:1) than petrol engines (8-12:1).  Diesels do not use spark plugs, rather the heat of the highly compressed mixture and pistons self ignites itself to combust.  Therefore, a non start diesel will check for fuel pressure, heat and air, rather than air, spark, fuel.  Diesels generally rev lower than petrol engines as they have very high torque power.  They are built of very heavy duty parts to handle the high stress of the engine.  They are usually found in SUV's, commercial trucks and construction site vehicles like diggers etc.

What is the purpose of glow plugs?

Glow plugs are a very important part of a diesel engine, as diesels combust the diesel-air mixture with the heat of the piston, cylinder and the high compression of the mixture (rather than using a spark plug.)  Like the triangle of fire, removing one of these will not allow the mixture to combust.  When an engine is turned on, usually it will be too cold to combust.  Diesels use a glow plug, which is a plug with an element on the end that heats up.  This glow plug heats up the combustion chamber to allow combustion.  Since diesels have high compression, they do not take as long to heat up.  The glow plug stays on when the key is turned for only around 10-15 seconds (depending the environment it is in). 

Glow plugs are a common fault in no-start diesel engines.  Servicing the glow plugs requires simple electronic diagnosis techniques with a multi-meter.  First check the resistance:

Make sure that the key is out in this test.  Set the multi-meter to ohms and calibrate the leads.  Place the negative lead on the housing of the glow plug.  If you cannot reach it, place it on the nearest bare metal to the plug.  Place the positive lead on the tip of the glow plug. this will have a cable attached to it leading to the other glow plugs.  Compare the resistance of each glow plug and check if there are any plugs that are significantly higher or lower in resistance to the other plugs.

Check the amperage running to the glow plugs.  I recommend using an amp reader rather than a multi-meter as they usually have a limit of 10A.  If we do the math, anything over 1.2 ohms will have amperage higher than 10A.   Clamp the amp reader over the cables running between the glow plugs for readings.  Again, check for any significantly higher or lower readings between the glow plugs.

Check the voltage running to the glow plug.  Place the terminals in the same place as checking resistance, then start the car.  Again, compare the readings of each glow plug.

If you must, remove all the glow plugs from their slots.  This will require removing the cables first.  Make sure while doing this, the key is out of the ignition.  Check each glow plug by using a jumper or a battery and jumper leads and connect the cables to the glow plugs directly.  Safety first, the glow plug will heat up very quickly so make sure it is safe to do so.  Put the glow plugs down on a safe place.

If you suspect a failing glow plug, in most cases, all of the glow plugs will need to be replaced. 

What to check on a non-start diesel engine?

·         Check the basics - Battery voltage, if you suspect electrical problems, check voltage drops to and from the alternator, etc.

·         run a multi-meter on the glow plugs checking voltage.  It should have 11-12V for around 10-15 seconds.

·         Check the fuses and cables. This can be done by opening the fuse box and pulling out each fuse.  If they are blow you will see a small wire split in the middle.  I recommend using a test light and placing the negative on ground.  Some of the fuses will not be on if the engine is not running.

·         Check the resistance of the glow plugs (above)

·         run a compression test - this will require a special gauge for diesel.  I recommend doing a dry test then doing a wet test. this is simply done by testing compression for all cylinders, then dropping a bit of oil into each cylinder and then repeating this process.  This will tell us if it is piston ring issues or something else like a blown head gasket.

Make sure that if you are replacing any parts, take the old and new part and compare them to see that they are the same. If in doubt, do not replace.

The diesel bug

The diesel bug is a virus in a diesel system that is formed over a period of time of running on a low fuel tank.  Algae forms from the moisture and condensation inside the fuel tank.  This algae passes through along with the diesel into the pumps, filters, injectors, etc. and forms  more algae where there is air available.  This can be prevented by running on high fuel levels or using additives in the fuel tank.

Since diesels have such a high compression ratio, they have very small clearance volumes.  This would limit the flow of fuel flow as it comes from the injector right above.  This problem is solved by re-shaping the piston head.  A petrol piston will generally be flat with a very shallow crown, dish or sometimes small slots which give it more clearance for the valves.  The diesel has a very deep dish which allows the atomized fuel to twirl around and mix with the air better.  Most diesel pistons look like this:

Diesel pistons are generally a lot stronger than petrol, as they are put under a lot more stress.  To achieve the higher compression ratios in diesels, we need to raise the stroke length, making the diesel piston travel longer at revs.  the distance:rev ratio will be a lot higher than a typical petrol car. You may also notice that the diesel piston has longer skirts than typical petrol pistons, this is for more stability to compensate with the longer stroke.  They are made from a typical cast process.  Although forged pistons can handle more stress overall, they tend to expand under high temperature.  Diesels are very high temperature so do not usually require forged pistons.