Diesel knock and assembly error

[Azmio]

Hey guys,

I now have two cases of severe diesel knock that led to partial destruction of piston, head and block. Both the engines have the injectors removed and assembled again. Both engines have also accumulated quite a high mileage.

One engine is an IDI and another one is a DI. I just want to check with the experts out there on whether it is possible to cause diesel knock if the injectors were not properly assembled.

 

[TheBlacksmith]

Yes. Improper assembly can lead to incorrect timing due to high or low opening pressures or leaking.

 

[Azmio]

well, that's what I am afraid of. I suspect that the injectors were not properly installed. It led to compression loss that resulted ignition delay and diesel knock.

 

[IceStationZebra]

The VW TDI guys have to be careful to use the correct washer under the injectors. There are some almost fit washers being supplied with replacement injectors - the end result being that the injectors are too high and spray outside of the bowl. I can't recall the end result, but an example of a simple error with large consequences. (research TDIclub.com if you want more info)

ISZ

 

[Azmio]

ice,

thanks for your reply. the diesel injector assembly error is often overlooked and not many realized that it can easily lead to major engine damage.

Long while ago, a diesel engine guru from a Japanese OEM once told me to avoid taking out diesel injector once it is installed.

I didnt quite get it as I'm a spark ignition engine guy and we can take out injector whenever we like. Now that I have moved my focus from SI to CI, this whole thing started to make sense

 

[dicer]

So did you figure it out yet?
What type of injection pump?
What type of injectors?

The first thing to do is use the proper injector tester, and check pressure and pattern. Did you do that?

 

[Azmio]

dicer,

appreciate if you can clarify the relationship between the injector and the melted piston.

 

[dicer]

Spray pattern determines penetration and placement of fuel spray in combustion space, and that determines flame propagation and a/f ratio's in the various areas with in the whole cylinder. Just like detonation in an SI engine can cause piston damage, can see a similar effect in the CI engine. What I have seen more of though is, fuel washed cylinders and then bad scoring on skirts, galling and ring land failures. Do you have a photo?

 

[Azmio]

Good, i think we are getting there. can I ask you on how injector causes detonation to occur in a diesel engine?

I cant publish the photos because we got them from the testing that we conducted. All that i can say is that the piston and head melted. Bore has scoring.

 

[TheBlacksmith]

Diesel fuel is intended to ignite instantly upon injection; the measure of this property is the cetane rating. Gasoline is intended to resist ignition until the spark plug fires; the measure of this property is octane. In either type of engine, uncontrolled ignition with multiple flame fronts causes engine damage/knocking. In the case of diesel, if the fuel doesn't ignite upon injection, a pocket of fuel is formed that later ignites due to the pressure rise caused by the main burn. The results are the same as gasoline igniting from the pressue rise before the main burn spreads to it. The multiple flame fronts collide violently, you hear a knocking sound and the engine is subjected to enormous forces that cause piston, ring, valve and connecting rod damage. The weakest component breaks first.

 

[Majik]

Most IDI injectors just screw into position with a copper crush washer underneath them. It's about as technical as a sparkplug. DI's are a different ball of problems entirely.