When you turn the ignition key to the on position, the light will illuminate for a few seconds and then go out. You can then safely start your car by turning the key to the start position.
Diesel engines work a little differently from petrol (gas) engines. In a regular gas engine, the fuel and air are mixed in the cylinder, the piston compresses the mixture and at just the right time, a spark of electricity ignites the mixture.
Diesel engines don’t have sparks. There are no spark plugs. Instead, the diesel engine uses high compression to ignite the fuel. The compression on its own causes enough heat to ignite it diesel and air mixture in the cylinder.
However, the heat of the engine is also a factor in the fuel ignition. If the engine is hot, then the fuel will ignite at a lower compression compared to cold. If the fuel ignites too early, it would cause the engine to knock and this can damage the engine. So the engine is setup to run when hot.
This creates an issue for starting the car. If the engine is cold, it won’t get to the ignition point when compressed. To solve this, diesel cars have a small heating coil, called a glow plug, which warms up the cylinder for a few seconds before the engine is turned over. This ensures that the fuel ignites and the engine starts properly.
What happens if you don’t wait?
Many diesels will still start even if they are cold. Even though you didn’t wait, the glow plug will have warmed a little bit and it might catch. However, the car might just refuse to start. You might end up flooding the engine (too much fuel in the cylinder) which might prevent you form starting the car immediately.
Diesel engines rely on the air in the combustion chamber being heated to a temperature high enough to ignite the fuel when it is injected. Once they are running this heat is produced by the compression of the air in the cylinder as the piston rises during the compression stroke of the cycle, added to the overall warmth of the engine. That’s why the alternative name for these machines is compression-ignition engine.
But when the engine is cold, most diesels need some help igniting the fuel. Two techniques are used, often together in very cold weather. First, the combustion chamber can be heated by a small electrical heater (often called a glow plug) before starting is attempted; second, an easily ignited fuel (such as ether) can be added to the air being drawn into the cylinder as the starter is operated.
In extreme cold, more extreme measures may be needed. In Antarctica, we used special heaters to warm the whole engine to a temperature of perhaps -20C before attempting a start.
The warning light on the dashboard in a modern diesel car is typically a picture of a coil with a few turns. It comes on when the glow plugs are turned on, and goes off when the computer decides the engine is ready to start. On my VW TDI I usually see the warning light for less than a second. But I live in Texas, where really cold weather is rare.
I don’t think you will cause a problem with the engine if you don’t wait until the light goes out, but you will have to crank the engine longer, which might flatten the battery, and eventually could wear out the starter.
If a Diesel engine has Glowplugs for Cold Start purposes eg you are in the middle of winter then some engines may have a system that activates these heaters to aid starting. You will need to read the owners book to determine how cold start works on your particular model. Here in Australia where it does not get that cold, I simply turn the key and start the engine (Hyundai i30 Diesel). Some Diesel engines use fuel enrichment to aid cold starting and it will blow dark smoke for a few seconds on starting similar to older Petrol engines when you used a manual choke or a improperly set autochoke. The only delay that may be recommended and applies to all turbocharged engines whether it be Petrol or Diesel is if you have been driving hard and you stop at your destination then you should allow the the turbocharger to slow down and cool before turning the engine off. Some engines provide for this by having a gravity oil supply to the turbocharger or have a delayed turn off or have an auxilary pump to pump oil through the turbocharger for a timed period.
Diesel engines are not spark ignition engines, they are compression ignition engines. This means that they use compression to heat up the air in the cilinder to the point that the atomized diesel fuel (which is directly injected into the cilinder at the optimal time, just like a spark ignition engine, minus the spark. some older engines used inderect injection, but most modern diesels use direct injection) spontaneously combusts. This is fine for warmer temperatures, but in really cold weather the compression will actually not be adequate to reach auto-ignition temperatures, so some engiens use glow plugs to heat the air inside the cylinder so it's easier to reach the higher temperatures, but some engines use grid heaters to heat the air coming into the engine for the same effect. If you don't let things heat up adequately you can end up with an engine that is only firing on a few cylinders, which is no good, and that little light on the dash is on when the grid heater/glow plugs are on, and subsequently shuts of when the grid heater/glow plugs do. Hope this helps.
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