Clean Combustion Chamber

[hardingfv32]

From the Cosworth Ca F1 V8 article:

"A CA loses 9bhp through its life of 1365 miles. Much is down to keeping the combustion chamber and piston clean."

What principles are in play here? I understand keeping the valves and valve seats clean, but this is about the piston and combustion chamber. I would think a layer of carbon might have insulating properties or maybe decrease combustion chamber volume.

Brian

[Holm86]

The flame characteristics changes a lot on uneven surfaces I guess.

I read a some paragraphs in a study on ceramic thermal barrier coatings on piston crowns. Here they were comparing a piston crown surface with high roughness and one which was polished.

2.3.2 IC Engine
To date, studies examining the effects of surface roughness on heat transfer in IC engines have been very limited, with a review of the current literature revealing two primary studies. After having concluded that thick ceramic TBCs proved to be ineffective in reducing heat transfer based on the findings of previous research studies, Tsutsumi et al. [76] attempted to reduce the available surface area for heat transfer in a spark ignition (SI) engine by polishing the engine components to a mirror finish. Comparing standard to polished components (30 times smoother), it was seen that under full load at 3600rpm, decreases in piston surface temperature of up to 16oC were achieved, with an average overall decrease of 8oC. Increases in exhaust temperature of up to 10oC were also seen, as well as small improvements (1%-3%) in torque and fuel consumption. After allowing for the accumulation of carbon deposits on the piston crown, it was seen that though the effectiveness was reduced, the polished piston maintained some improvements in fuel consumption.
Similar results were seen by Marr et al. [77], which compared heat transfer measurements for plugs in an SI engine with four different surface finishes. Measurements were taken for smooth/grit blasted plugs, as well as polished and unpolished samples coated in a rough metallic TBC (Ra = 12-20m). In each case, the smoother of the two samples (smooth uncoated and polished TBC) showed surface temperatures approximately 5oC lower than the rough samples, and thus a reduction in heat transfer.

2.3.3 Summary: Roughness Studies
The results of the roughness studies discussed in Sections 2.3.1 and 2.3.2 provided a foundation for the argument that decreasing surface roughness plays a role in reducing SI engine heat transfer, which would provide benefits in terms of both performance and component life. Thus, it is clear that the effects of TBC roughness may prove to be a significant factor in determining the effectiveness of these coatings on reducing heat transfer and ultimately improving overall engine performance.

source

[Owen.C93]

Mixture swirl and flame development is also affected with uneven surfaces.

[riff_raff]

I don't know if I believe that claim about much of the HP loss of the engine coming from reduced cleanliness of the chamber and piston crown surfaces. Frankly, I think most of the HP loss is due to gas pressure leakage caused by fretting erosion at the valve sealing surfaces and wear of the piston ring faces, bore surfaces and ring grooves.

[tuj]

I guess one question is: how advanced are they running the timing? I would assume they are advancing spark as far as optimal. If that's the case, carbon deposits inside the combustion chamber could cause pre-ignition / detonation. But I agree with other posters that I would think the biggest losses would come from the piston rings wearing.

[Owen.C93]

I don't know if I believe that claim about much of the HP loss of the engine coming from reduced cleanliness of the chamber and piston crown surfaces. Frankly, I think most of the HP loss is due to gas pressure leakage caused by fretting erosion at the valve sealing surfaces and wear of the piston ring faces, bore surfaces and ring grooves.

Correct.

[olefud]

In a well-designed head, the intake flow over the valve seat is diffused efficiently over the head surface and even the piston top at near TDC. This efficiently trades port velocity for combustion chamber pressure with minimal turbulence and optimizes VE. It’s possible that deposits disrupt this flow and costs power.

As a separate aside, on street vehicles pulling heads with a few miles on them and decarbonizing the chamber would produce a noticeable power increase confirmed by ET’s. Since the carbon would increase CR and insulate the chamber, I suspected that it may also affect the combustion process adversely –perhaps by sopping up the energized oxygen like TET- but this is pure theory, if that.

[Tommy Cookers]

newly cleaned surfaces present bare metal to the radiated heat in the combustion chamber, so absorbs less of the heat ?
so more heat is available in the charge for expanding it and converting heat into work ?

with this belief, Stirling Moss and mechanic Alf Francis for each event internally cleaned their Cooper-JAP's (single cylinder) engine

[riff_raff]

Deposits on the aluminum chamber surfaces and piston crown would reduce the conductive heat transfer rate from the intake and combustion gases to these surfaces. The reduced heat transfer rate from the combustion gas to the piston crown or chamber surfaces would result in more energy remaining in the exhaust gas flow. The reduced heat transfer from the intake charge to the piston crown and chamber surfaces from compression heating just prior to ignition, would reduce the knock limit of the engine and would result in a lower BMEP.

It's a trade-off.