ZKD Selection: Performance Variations Explained

Hello,

As far as I know, the compression ratio of an engine determines, among other things, its efficiency.

While browsing the repair manual for the Golf 3, I found that the three different head gaskets for VP 37 engines (to be used depending on the piston protrusion at top dead center) apparently differ in increments of 0.1 mm.

With a compression ratio of 19:1 and a stroke of 95.5 mm, the (assumed to be cylindrical) combustion chamber has a calculated height of approximately 5.31 mm.

Increasing (decreasing) the combustion chamber height by the specified ZKD grid of 0.1 mm changes the target compression ratio from 19.0 to approximately 18.69 (19.36), which is a change of about 1.7%.

This effect could become relevant, for example, when the piston protrusion measured during engine production is right at the "selection limit" of the head gasket (1, 2, or 3 notches...).
One mechanic might choose the thinner head gasket, resulting in a compression ratio of 1:19.16, while another might opt for the thicker gasket for a more material-friendly approach, building an engine with a compression ratio of 18.83.

If the cylinder head gasket is installed incorrectly (e.g., using the thinner gasket instead of the thicker one, or vice versa), compression ratios of approximately 19.7 or 18.4 may result, instead of the intended value of 19.0.

IMO, engines with such different compression ratios, if they otherwise have completely identical characteristics (and undergo the same break-in procedures), are likely to have different performance and fuel consumption - the question is, how significant would those differences be.

The answer might lie in the efficiency, but when searching the forum, I haven't found a direct relationship (formula) between (geometric) compression ratio and efficiency.

-> Mechanical engineers & students, step forward:
Who can calculate or reliably estimate the performance and consumption effects that the aforementioned compression deviations would have compared to an ideal value of 19.0?
Perhaps this provides a partial explanation for the performance variations observed in TDIs.

ulf wrote:

-> Mechanical engineers & students, to the front: Who can calculate or reliably estimate the performance and consumption effects that the aforementioned compression deviations would have compared to an ideal value of 19.0?

Okay, then I feel addressed. I've been meaning to take a look at a VKM script, especially because of the clam diagram from the VP.

The relationship between compression ratio and efficiency can only be described in an idealized theoretical comparison process.

The TDi engine approximately uses a homogeneous combustion process.
And in that case:

Efficiency = 1 - (Compression ratio raised to the power of (1-Kappa))

It's all very theoretical. I think how it actually works in practice can only be seen in a testing environment or something similar.

If the cylinder head gasket is installed incorrectly (e.g., using the thinner gasket instead of the thicker one, or vice versa), compression ratios of approximately 19.7 or 18.4 may result, instead of the intended value of 19.0.

IMO, engines with such different compression ratios, if they otherwise have completely identical characteristics (and undergo the same break-in procedures), should have different performance and fuel consumption - the question is, how significant would those differences be.


-> Mechanical engineers & students, step forward:
Who can calculate or reliably estimate the performance and consumption effects that the aforementioned compression deviations would have compared to an ideal value of 19.0?


Well, I'm not an engineer or a university graduate, but I'm a seasoned system administrator with a good intuition.
When I look at the compression values of broken-in engines...
...I think it's more likely that such differences are due to the ceramics .

It's probably impossible to precisely measure it, because while you can start the motors simultaneously, etc.
But it is impossible to produce them exactly the same (at least not with a reasonable amount of effort).

CU Gremlin.

Hi Ulf,

As far as I recall from thermodynamics, the practically achievable efficiency doesn't increase significantly above a compression ratio of 16:1. This is likely to be more noticeable when starting the engine in cold conditions. I'd like to reduce the compression ratio on my AFN engine, but lowering the cylinder head while it's still in good condition would be excessive.

faz wrote:

Efficiency = 1 - (Compression ratio "raised to the power of" (1-Kappa))

Hi,

Ultimately, I was looking for a kind of rule of thumb.
So, for example, does the efficiency behave proportionally?
-> for compaction.
-> or to the square of the compression ratio.
-> or to its root...

But it doesn't seem that simple to me.
What is this Kappa ?

Hello Ulf,

What was written by "faz" is fundamentally correct.

kappa is the ratio of specific heat at constant pressure to specific heat at constant volume, and its value for an ideal gas is 1.4.

Further prerequisites for the "Gleichraumprozess" (equal space process):

- No heat or gas losses, no residual gas.
- ideal gas (see above)
- infinitely fast heat transfer and removal.
- no flow losses.

Theoretically, an increase in the compression ratio from 18 to 19 would increase the thermal efficiency from 0.685 to 0.692.

However, the overall efficiency is also influenced by many other factors, some of which can affect each other. In practice, with diesel engines, it is often preferable to reduce the compression ratio, as this can lead to advantages that benefit the overall efficiency and durability.
The high compression ratio in older pre-combustion chamber engines is really only necessary for cold starting.

Best regards,
Albrecht.

Here's another diagram to illustrate this: it shows that an increase in compression ratio only benefits Otto engines.

Albrecht wrote:

Here's a diagram about it again: you can see that the increase in compression ratio only occurs in gasoline engines, which is beneficial.

Hi Albrecht,

a very nice diagram that perfectly answers my question

Therefore, it is clear that slight variations in compression caused by manufacturing tolerances in the range of 19 should not result in noticeable differences in performance.

Thank you!