robp823 Posted March 16, 2009 Share Posted March 16, 2009 I hear people talking about a fuel knock in diesel engines alot.Can someone please explain to me what it is and what causes it? Quote Link to comment Share on other sites More sharing options...
Keith Browning Posted March 16, 2009 Share Posted March 16, 2009 I have not been able to find a definitive answer on this but here is how I understand "diesel knock" (abnormal loud knocking for a given engine operating normally) Diesel engines inherently knock because the fuel self ignites. When injection takes place it starts in several places within the combustion chamber, the burning charge is creating pressure and also being compressed at the same creating the knocking sound. But when we hear a diesel engine "knocking" it is usually a sign of over fueling, an injection timing error or a base engine concern causing incomplete combustion. These conditions will create laden fuel in the cylinder(s) that can ignite during the compression stroke before injection begins. The combination of that pre-ignition facing normal injection results creates abnormal combustion pressures and noise. What can cause this? Leaking fuel injectors, injection timing, bent connecting rods and valve train problems that cause low compression. Of course, this is not limited to diesel fuel, any failure that puts excessive engine lubricating oil in the cylinder can create pre-ignition and knocking as well. If I am totally wrong about this someone please correct me!!! Quote Link to comment Share on other sites More sharing options...
Damon Posted March 16, 2009 Share Posted March 16, 2009 I attribute diesel engine knock to the difference in time delay between the moment of injection and the moment of ignition. The bigger the delay, the bigger the knock. Quote Link to comment Share on other sites More sharing options...
robp823 Posted March 18, 2009 Author Share Posted March 18, 2009 It sounded pretty good to me ahahahah Quote Link to comment Share on other sites More sharing options...
Damon Posted March 18, 2009 Share Posted March 18, 2009 It sounded great until you realize the noisiest diesels are those that are running out of fuel. Quote Link to comment Share on other sites More sharing options...
robp823 Posted March 18, 2009 Author Share Posted March 18, 2009 Would this be considered a loud knock??? Quote Link to comment Share on other sites More sharing options...
Keith Browning Posted March 18, 2009 Share Posted March 18, 2009 You have never heard a 6.4L with an injector dumping fuel? Without a doubt the worst sound I have ever heard coming from a diesel engine!!! You are right about the fuel starving and noise though. Both 7.3L and 6.0L engines tend to clatter in a very characteristic way when fuel pressure drops. How much do you think low fuel charge pressure affects timing? Another example is the fuel "cackle" that the 7.3L had. I don't know where they come up with these terms but the few I ran into sounded like a knock in my opinion. That was due to variations in fuel pressure caused by the firing of the other injectors in bank 2 affecting cylinder #8. Personally, I think there are several possible conditions that can cause fuel knock but they all lead to the same thing... and I am looking for the right words... along the lines of "improper combustion." The more I find and read about this, I see terms like "abnormal combustion" "long ignition lag" "low cetane" "misfire" Good topic. Quote Link to comment Share on other sites More sharing options...
robp823 Posted March 18, 2009 Author Share Posted March 18, 2009 Actually i think i got my first 6.4 doing that today topic is in 6.4 forum and your right not a good sound. Quote Link to comment Share on other sites More sharing options...
Damon Posted March 18, 2009 Share Posted March 18, 2009 You have never heard a 6.4L with an injector dumping fuel? Without a doubt the worst sound I have ever heard coming from a diesel engine!!! Sure. If you consider a V8 as 8 single cylinder engines running in unison, you'll realize ANY engine running with cylinder imbalance will produce a terrible lugging sound as one cylinder tries to accelerate the other seven, and vice versa. Originally Posted By: Keith Browning You are right about the fuel starving and noise though. Both 7.3L and 6.0L engines tend to clatter in a very characteristic way when fuel pressure drops. For the same reasons. Originally Posted By: Keith Browning How much do you think low fuel charge pressure affects timing? It may not affect the timing of the delivered fuel in this engine, but it will affect the delay between the moment of injection and the moment of ignition, just as rate of injection alters the rate of pressure rise in the cylinder. Originally Posted By: Keith Browning Another example is the fuel "cackle" that the 7.3L had. I don't know where they come up with these terms but the few I ran into sounded like a knock in my opinion. That was due to variations in fuel pressure caused by the firing of the other injectors in bank 2 affecting cylinder #8. Again, this points to an internal conflict among imbalanced cylinders causing a similar noise but from a differnt cause. Take any diesl and start it off int he wrong gear, lug it down to idle speeds under a hard load. Lots of noises go on that are identical to the ones you're describing. IMHO, this should be considered differently than the abnormal noise a theoretical single cylinder diesel engine would make if it were modified in a way that would alter delivered quantity, rate of injection or timing.....agree? Originally Posted By: Keith Browning Personally, I think there are several possible conditions that can cause fuel knock but they all lead to the same thing... and I am looking for the right words... along the lines of "improper combustion." The more I find and read about this, I see terms like "abnormal combustion" "long ignition lag" "low cetane" "misfire" Good topic. I like the "log ignition lag" since "abnormal combustion", "low cetane", and "misfire" say the same thing in a roundabout way. "Normal combustion" is immediately upon injection without delay, so "abnormal combustion" must mean there's a delay. "Low cetane" is less flammable fuel, which means "delay" and "misfire" means....well that's another topic. Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted March 18, 2009 Share Posted March 18, 2009 In addition to all of this, I attribute fuel knock from an overfueling injector to one cylinder running at an extreme compression ratio from less air available and too much fuel in the cylinder. This is another way of saying it is near hydrolock. Good Luck! Quote Link to comment Share on other sites More sharing options...
Aaron Posted March 20, 2009 Share Posted March 20, 2009 Yeah, or it's a stank old DODGE with a Cummins in it. Quote Link to comment Share on other sites More sharing options...
Shlep Posted March 22, 2009 Share Posted March 22, 2009 In addition to all of this, I attribute fuel knock from an overfueling injector to one cylinder running at an extreme compression ratio from less air available and too much fuel in the cylinder. This is another way of saying it is near hydrolock. Good Luck! Also once you have heard what a fuel knock sounds like, you will never forget it... sounds almost like 2 large hammers being hit together. Bruce brought up a very good point about the injectors overfueling to the point of hydrolock. Both engines I had like that had at least one injector hung wide open and dumping fuel. Quote Link to comment Share on other sites More sharing options...
Brad Clayton Posted March 22, 2009 Share Posted March 22, 2009 I have replaced many a leaking fuel injectors on IDI's that were originally misdiagnosed by someone else as a rod knock. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.