Combustion chamber conditions

[MTMTDI]

Hello all,

About year ago I bought A4 Golf Tdi. I am more familiar with gas engines, so I have a question concerning the diesel burning process.

When a dsl engine starts to rumble and shake during acceleration what happens in the combustion chamber? Is the engine having too much air or fuel, or is it the opposite? (I mean too much boost and not enough fuel OR too much fuel and not enough boost).

Is this possible to happen in both cases?

 

[mickey]

It's hard to say, since I'm not sure exactly what you're talking about. The combustion process in a diesel engine does resemble an explosion more than it does in a gas engine, so the engine operation is generally "rougher." But if you're really jumping on the pedal at very low rpms you could be experiencing good old-fashioned "lugging." I think people have a tendency to take the "low rpm torque" thing a little too seriously. TDI's are tiny little engines and need to spin a little bit. Just because they tolerate low RPM lugging better than a similarly sized gas engine doesn't mean they LIKE it. Try to keep the engine RPMs in the higher gears up around 1900 or higher unless you are cruising very gently on level ground.

-mickey

 

[MTMTDI]

Mickey,

Let me clarify. I don't me the condition when you are accel. from very low rpms not that. I know that dsl engines operate with excess air etc. But what happens in combustion chamber when the engine starts to shake so badly that you really have hold the steering wheel. Is it "detonating" because too much boost or too much fuel so it doesn't burn well.

The thing I want to know. What is wrong with the burning process when the shaking occurs?

 

[fnj2]

I don't have a clue what you are talking about either. I don't get anything remotely like the "shaking" you describe under any conditions.

If you're really getting strong torque steer, the only thing I can think of is that you are accelerating so strongly that the front wheels are breaking their grip. This is not too difficult to attain when the road is wet or dusty.

Or perhaps something is way wrong in your drivetrain. Or your injection timing is way, way off.

A diesel can't preignite, because there is no fuel in the cylinder until the timed injection occurs, which is at exactly the point that you want it to ignite. And it can't really detonate either - or, if you prefer, it is sort of "detonating" all the time.

The diesel normally operates with large excess air. There is no misfire with any amount of excess air. This is because there is a local mixture right in the spray of fuel which is rich enough to burn. When the engine is idling, there is a vast amount of excess air and only a tiny trickle of fuel, and it is almost as smooth as a watch. This is in distinction to a gasoline engine (the excess air, not the smoothness) because a diesel has no throttle plate and the intake is wide open all the time!

As far as insufficient air (too much fuel), under these conditions you just get black smoke and soot out the exhaust, but not a misfire or rough running per se.

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Mutter at idle, growl at full throttle

 

[Turbo Steve]

You could have some faulty injector(s) or loose motor mounts. Also, low fuel pressure will more easily show-up under heavy acceleration. You might also want to check for excessive air in the fuel lines to the filter and injector pump. BTW - This reminds me of a situation similar to yours where the injector pump timing was off, but the A-4's should be computer controlled and trip a fault code if this was the case.

Steve

 

[ThinkDiesel]

It sounds as if you are simply asking why the diesel combustion process is noisier and causing more vibration then it does in a gasoline-powered car. Am I correct?

If so, the reason--for one, is that diesels have much higher compression, therefore combustion event is more violent, and more powerful (this is why diesel's have such prodigous low end torque)

As for having to hold on to the steering wheel while accelerating-- that's probably just torque steer.

What you are getting is a few powerful explosions (diesel) rather than lots of quick wimpy explosions in rapid succession in the same period of time (gasoline).

 

[Drivbiwire]

Hey Guys,

I'd like to add a perspective here, First off think of the diesel ignition process as a controlled burn without any possibility of preignition, detonation, or misfire.

TDI disels Burn fuel using pressurized air from a turbo, the air is then cooled to increase the desity of the air. The air is then routed and swirled into the combustion chamber and then compressed to a ratio of 19.5:1 (425psi@600F?) in a matter of a milisecond. This creates an atmosphere so intensly hot that as soon as the fuel is injected it immediately starts to burn. This is followed by a rapid increase in pressure, followed by a secondary larger burn that softens the pressure rise lowering noise and eliminating the "diesel" sounds often associated with larger less refined diesels.

The sounds and sensations that you are describing are the cylinders coming to life providing you with a massive pressure rise in each of the cylinders. In case you don't realize it you are burning "jet fuel" that elusive fuel that so many gas burners wish they could get ahold of. So press on, and let the ECM do the rest!!

DB

 

[mickey]

I agree with these guys. Unless you are just overly sensitive I'd say there's something wrong with your car. Is this "shaking" somethings that the car has been doing since you bought it, or is it a recent development?

The diesel engine will vibrate more than a gasser, and under heavy acceleration you might see the shift lever shaking around a little bit. But "needing to grab the steering wheel" doesn't sound right to me.

-mickey

 

[MTMTDI]

This forum is absolutely great, but we are getting off the track on this topic. I'm familiar with torque steer and normal diesel noise and etc. This surely has something to do with with engine controls.

This started as the car had a chip installed, before that I tried the famous "VNT mod", but the engine started to shake after the chip mod.

I wanted to start this topic on a more general level as the fundamentals behind this problem is the key and most interesting part. As I found out what makes a diesel engine to shake and shiver under accel I am (maybe) able to sort out the problem.

So this shaking occurs when I'm accelerating from 1500-2300 rpms (only with 3rd,4th,5th gears). So the engine is under some load and boost hits the peak. On smaller gears this doesn't happen (the car literally "takes off").

Could the timing be wrong if there is too much boost too early (vnt mod)? As I can't see a lot of soot.

Let's keep on digging!

 

[Turbo Steve]

Question: What about lugging the engine with your foot in the throttle, rather than gradually easing into it?

I also forgot to mention in my earlier post that the piston to cylinder wall clearences in a diesel engine are 3 - 4 times greater than those in gas engines. I believe the newer TDI engine has closer tolerances than a normal diesel engine, being somewhere in between. This adds to the quietness of our engine.

 

[GoFaster]

OK, so this happened when you chipped the car. You might be maxing out the travel of the engine mounts. Skypup mentioned this happening.

Brian P.
'96 Passat TDI

 

[Powder Hound]

OK, to summarize:

Chipped car,
excessive noise and vibration under full acceleration between 1500-2300 rpms in 3rd, 4th, 5th gears.
Excessive noise did NOT occur before chip installed.
No observable soot.

What else can you tell us? What year, model? Any other mods (exhaust, air filter, VNT actuator adjustment)?
How does the noise differ from 'normal' diesel clatter?
Does the engine lose its smoothness and get rough (as if it is behaving like a gasser which is missing sparks)?
Fuel quality OK (have you tried a cetane boosting additive)?
Does this happen only when the engine is cold, hot, both?
Does this only happen in cold weather, warm, both?

Please provide any other details you can.

------------------
Always interested in steep & deep.
And now, a few great weeks of spring skiing...

 

[Sooty]

I hate to state the obvious, but have you tried it with the original chips back in?
Also you say that you’ve done the VNT mod, how many turns? And have you tried putting that back to standard?

 

[fnj2]

MTMTDI - yes, bad timing could conceivably cause this. Or I wonder if the clutch might be slipping. As well as the other suggestions, like a bad injector.

If it doesn't show up in 1st or 2nd, that might just be because the weight of the car isn't holding the engine back in quite the same way as the higher gears do.

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Mutter at idle, growl at full throttle

 

[Chip Master]

MTMTDI:

First, who's chip is it in the car? Curious because I see that you are from Finland. Second, to eliminate the obvious...install the stock chip back in and report back to us your results for further diagnosis.

2000 Jetta TDI auto and wetted of course
BMC Air Induction Kit
BBS 17x7 RC Wheels
Zender body kit
H&R Coil-over suspension
Supersprint downpipe back exhaust

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Vincent Braun
Autobraun Distributors Inc
Wetterauer Canada
www.autobraun.com

 

[MTMTDI]

Sorry, I have been away for couple of days.

Into the topic. A short summary first:

* No extra noise (don't know where this came from).
* Engine shakes under accel. between 1500-2300 rpm with 3rd, 4th and 5th gears. the shaking seems to be load related. If I push the gas gently it shakes more than if I dump the gas pedal. (Turbo Steve had a point here, PLEASE comment more on this)
* I did VNT mod but turned it back to stantard setting (was 1 turn). No other mods.
* The car is bought 5/99 as a new
* Fuel quality OK
* No matter of the weather
* Clutch not slipping

It seems to me that it is load and boost related in someway. WHY does it make a difference if I dump the gas or ease into it.
To change the original chip back is a good idea but very difficult to do.

 

[Powder Hound]

It sounds like you have an imbalance problem in what the injection pump delivers at low throttle. But it is strange that it happens in the higher gears but not 1 or 2.

From everything above, it sounds like you need to replace the original chip and see if the problem persists. If it does, you have an injector/pump problem the dealer needs to repair under warranty. If not, you have a bad chip.



------------------
Always interested in steep & deep.
And now, a few great weeks of spring skiing...