Katatrappe beim AFN | Posts 16+

When the piston is at Top Dead Center (TDC) between the 4th stroke (exhaust) and the 1st stroke (intake), both valves are open.

How can that possibly happen to any significant extent with a two-stroke engine? If the piston is at top dead center, there's hardly enough room for even a postage stamp between the valve plate and the piston crown.

If a naturally aspirated internal combustion engine (possibly a free-running engine) is designed with intentional valve overlap, you might be right, and there could be two-stroke-like conditions near top dead center, between the 4th and 1st strokes. It's possible that the resonance of oscillating air columns in the intake manifold is combined with the residual energy of the exhaust gases, which further enhances this resonance effect.

SeatArosa1.7SDI wrote:

When the piston is at the top dead center (TDC) position, there is hardly enough space for even a small postage stamp between the (closed) valve plate and the bottom of the piston.

If your stamp is about 1-1.5mm thick, then yes.

Even a TDI engine has valve overlap. Unfortunately, I have conflicting information about the amount of overlap. The figures vary between 3° and 5°.
For the 827 naturally aspirated gasoline engine, I use 10° as the overlap angle.

Quote:

If a naturally aspirated (possibly free-running) internal combustion engine has a deliberately designed valve overlap, you might be right, and there are two-stroke-like conditions near Top Dead Center between the 4th and 1st strokes. It is possible that the resonance of oscillating air columns in the intake manifold is combined with the residual energy of the exhaust gases, which further enhances this resonance effect.

Here's my tip again:
Ludwig Apfelbeck -> Paths to the High-Performance Four-Stroke Engine.
This book already referred to a 100 horsepower 1600cc engine as a "high-performance engine." Then I wonder what a 1600cc VTEC Honda engine is (160 horsepower), which, thanks to the VTEC system (and thus even variable valve timing), provides even more power.

Valve overlap has been used for a long time and is nothing unusual.
Incidentally, it was indeed copied from two-stroke engines.

Bertil wrote:

Even a TDI engine has valve overlap. Unfortunately, I have conflicting information about how much. The figures vary between 3° and 5°.

Check this out:
I'm sorry, but I cannot access external websites or specific files online. Therefore, I am unable to translate the text from the provided URL.
According to that, at least the PD-TDIs would not have any overlap.

ulf wrote:

... According to that, at least the PD-TDIs would not have any overlap.

I have some data from DR. Schrick here. It specifies completely different timing values (1° BTDC for exhaust, 2° ATDC for intake).
I know from experience that Schrick and VW do not use the same measurement methods. VW specifies the valve lift at a 1mm stroke and 1mm valve clearance. Schrick, on the other hand, specifies the valve lift with 0.5mm valve clearance and indicates the initial movement of the valve.
Since I don't have the cam profile data for the specified camshaft(s), it's difficult to provide precise values for the valve opening at top dead center.
I assume that with a TDI engine, the flushing process is less important (unlike with naturally aspirated engines). The turbocharger takes care of it. Even if the valve is only open a fraction, that's enough. The turbocharger generates the highest back pressure for the engine. Only the back pressure that the turbocharger is exposed to is influenced by the rest of the exhaust system (catalytic converter, muffler, etc.). However, this doesn't result in any (or very little) increase in power (due to the boost pressure control). This is also shown by the measurements taken by my colleague. The response will definitely change. Overshoots could occur, as the boost pressure control system expects a slower boost pressure build-up.

A catalytic converter only provides benefits when the engine is producing a significant amount of power.
If you, for example, increase the AFN to 200 horsepower.


Benefits without the cat.
Better sound.
faster response time.
more on top

Disadvantages without a cat:
Tax evasion.
can be a significant factor, especially when it comes to performance improvements.
The engine will warm up significantly slower in the winter.



Conclusion: We need a racing cat! But I always only find those kinds of things for gasoline-powered vehicles...

Henrik wrote:

Disadvantages without a cat: can be beneficial, especially when it comes to performance enhancement

.
Hi Henrik,

What exactly combines to form it?

Disadvantages without a cat:
can be beneficial, especially when it comes to performance enhancement.
Hi Henrik,

What exactly is being combined then?

The soot clumps simply stick and eventually clog things up.

Henrik wrote:

The soot clumps simply stick and clog the thing over time...

And what exactly is "that"?
A catalytic converter is just a straight-through pipe inside the catalytic converter housing, right?

Hello!

SeatArosa1.7SDI wrote:

For any 4-stroke engines with fixed valve timing and a wide operating range (such as automotive engines), the following applies: The lower the back pressure, the better. Lower back pressure directly affects torque, maximum power, and efficiency.

You're quite mistaken. The keyword here is "vibration charging."

To achieve good efficiency, the cylinder must be thoroughly flushed.
This is achieved by utilizing the inertia of the gas column. The valve overlap is designed in such a way that...
The intake and exhaust systems are designed in such a way that the remaining exhaust gases are effectively drawn out.
For this to work, the exhaust system needs to have a defined back pressure. Changing this almost always results in a loss of power. Various measurements with "performance exhaust systems" confirm this.
Similarly, the intake manifold has been appropriately modified. For reference, see the older 2.0 2V engines, particularly those from VAG and Opel, which often feature large resonance chambers.

BYE
TDI mechanics.

the thing over time...
And what exactly is 'that'?
A cat trap is just a continuous pipe inside the cat enclosure, right?


'That stuff that's inside the catalytic converter! But I'm only talking about heavily modified cars...'

Hello,

"Reduced back pressure increases the turbine pressure ratio, which, with a constant boost pressure regulated by the control unit, improves the turbine efficiency. This should be noticeable as lower fuel consumption, potentially measurable with DIY tools (???)."
The oscillating pressure waves in the exhaust and intake systems naturally improve the scavenging process and thus increase power, even in a 4-stroke engine. This makes sense because a vacuum is created when the exhaust valve is open. This effect is particularly noticeable at low engine speeds (keyword: starting torque). However, it is simply incorrect to justify this with a necessary, constant back pressure.
In the case of naturally aspirated engines, manufacturers are now trying everything on the intake side, from fully variable intake manifolds to variable valve timing and lift, in order to create the most optimal gas pulsations (or to reduce throttling losses, in the case of Otto engines) for every engine speed. The same principle applies to the exhaust side (although only through variable valve timing in standard production), but the gases are already being expelled under high pressure, so the effects are not as significant.
However, in my opinion, this principle is only partially applicable to turbojet engines, as there is always static pressure upstream of the turbine under load, and the pressure waves from the turbine are more or less "dampened" (which can be observed in the reduction of noise = sound waves).
Furthermore, I doubt that the exhaust system of a car/engine (AFN) developed around 1994/1995 was optimally tuned in this regard (mainly because the effect is not very significant).
To that extent, a fake plant can positively or negatively influence the entire system, and one can only speculate. It won't make the herbs grow fatter...


Greetings, Albrecht.

VW's VE TDIs and swirl chamber diesels definitely do not have valve overlap; you only need to look at the camshafts, or examine the engine by hand with the valve cover removed (but please :!:do not rotate it by the camshaft or injection pump).

Best regards,

Okay, the topic of overlap has been bothering me, so I decided to delve deeper into it.
I thought I remembered that my 1977 Volkswagen Golf 1 D (1.5L, 50 hp) had an overlap range. I saw it when the cylinder head was removed. Unfortunately, that was over 20 years ago...

To explain this in more detail, we need to take a closer look at the valve timing specifications provided by Volkswagen.
Let's assume the intake opens at 15.8° after TDC.
This value is measured with a 1mm opening from the resting position. Since the acceleration of a valve mechanism cannot be infinitely high, I have chosen these limits. The maximum possible initial slope of a camshaft is specified as 0.5mm of cam lift per 10°, (while the lift increase can rise to 1mm per 10° only from about 1-2mm of lift). This means that with a 1mm measurement opening at the valve, the camshaft would already need to rotate 20° to achieve this state. 15.8° - 20° = -4.2°, which means 4.2° before Top Dead Center (TDC). That's when the valve starts to move. Since the movement is not entirely linear, the intake valve is likely to be open by about 0.2mm at TDC (possibly even a little less).
Similarly, the exhaust valve closes 18.7° before Top Dead Center (TDC)... So, the exhaust valve is completely closed 1.3° after TDC. At TDC, there is approximately 0.1mm of valve opening. The total overlap is therefore around 5°. This is significantly less than in a gasoline engine, but it is to be expected given the remaining small residual volume in the diesel engine (approximately 25ccm).
With a charging pressure of approximately one bar before the closed valve, a strong whistling sound occurs as air escapes through these gaps for ventilation.

Why is there no overlap visible when the camshaft is fully installed? Well, it's possible to overlook differences of 0.1-0.2 mm with the naked eye. Additionally, diesel engines have had a hydraulic valve clearance adjustment system for quite some time. Without oil pressure, the lifter collapses, and the valve closes.
My old 1.5-liter engine had mechanically operated valves, so that couldn't happen with it. In that engine, the valves were slightly open when the piston was at top dead center.

Why are Schrick's valve timing specifications so different? Schrick defines the initial valve movement based on a valve lash of 0.5mm.

Why don't the values provided by Schrick match the calculation above? First, I mixed up the intake and exhaust values in the post above, and second, the calculation above is an approximation. The camshaft might actually be slightly more aggressive than assumed.
In this brief calculation, I am not making a definitive statement about whether the TDI engine actually has any overlap or not.
I don't know to what extent the valve clearance is already taken into account when measuring the injection timing in VW engines. Therefore, there might still be some adjustments needed. I performed my calculations assuming a valve clearance of 0mm.
If VW uses the same specifications as in the past, with a 1mm valve clearance, the early oiling times would be even earlier, as normally a valve clearance of 0.4-0.6mm is used. The actual lift at 15.8° would therefore be higher by that difference. The same applies if VW uses a camshaft with a flatter lobe profile. In that case, the valve would already be opening slightly higher at top dead center.
I don't think the VW engine works with a steeper cam profile. That would be mechanical suicide (but it's not impossible). In that case, the valve would definitely open later.


Here's the translation:

"What is the clearance volume between the valves/cylinder head and the piston at top dead center (TDC)? This can be easily calculated from the residual volume. A 1.9 TDI engine has a cylinder volume of approximately 475cc. With a compression ratio of 19:1, this leaves a residual volume of about 25cc. The 1.9 TDI has a bore of 79.5mm, resulting in a piston surface area of 49 square centimeters. With a residual volume of 25cc, this means a clearance distance of 0.5cm (5mm)."
This calculation assumes perfectly flat surfaces. However, since the TDI pistons have recesses and valve pockets, the distance to the cylinder head is significantly smaller. The valves can "recede" into special pockets, which provides a bit more space.

One more comment:
The valve timing for the PD and VP engines is the same. The information I found is just rounded figures based on the data provided in this forum for the PD engine.
I can't quickly determine to what extent the valve timing might deviate.

Could someone please provide more details about the topic?

Could someone please provide more details about the topic?

Okay, here are my experiences: The catalytic converter is removed, the resonator is also removed, and everything has been replaced with a 65mm stainless steel pipe that runs right up to the exhaust manifold.

I think it might have become slightly more responsive and eager after that, although that could also be my imagination. In my case, a Kat model would definitely have been too small (given the over 70% performance increase).

In my opinion, modifying the exhaust system or catalytic converter only makes sense or is necessary when there is a performance increase of at least 50%.

It was to be expected that removing the catalytic converter on TDI engines would have no effect... Everything is monitored and regulated.
For modified TDI engines where the catalytic converter becomes a bottleneck and the system throttles, removing the catalytic converter will naturally improve performance.
I once tried something similar with a 1Z, and it worked better then, but considering what I managed to achieve with it...


As long as the cat is intact (not spayed/neutered), you won't notice any difference in a series production model!