Combustion System

[Brian Coat]

Hi

There's currently a lot of combustion system discussion across multiple threads with plenty of overlap/ duplication.

I thought I'd open a dedicated thread for ease of posting/reading.

Reading the debate so far, the 'aggregated rumour/discussion' seems to point towards:

* High overall pressure ratio
* Lean AFR
* Novel ingnition system - prechamber/jet/other
* Fuels optimised for energy and knock

[gruntguru]

Good idea Brian.

Combustion is the most critical avenue to success under the current formula. The fuel rate is limited to 100 kg/hr. This corresponds to a maximum heat energy rate of about 1240 kW. The combustion system will determine:

1. How much of that heat energy is actually released (ie by burning close to 100% of the fuel). Maximising this requires:
- A mix that is within combustible AFR limits throughout the chamber.
- A mix that is not quenched by cold metal (chamber walls) at the perimeter.
- A mix that is as lean as possible to reduce dissociation (reversion from burned to unburned products).

2. How much of that heat energy is converted into pushing the piston down. Maximising this requires:
- burning all the fuel/air as near to TDC as possible (requires rapid combustion).
- Keeping as much of the heat as possible in the gas itself.
- Eliminating detonation (which wastes energy via vibration and accelerated heat transfer to the walls due to shock waves disturbing the "insulating" boundary layer).
- Expanding the gas as much as possible before the exhaust valve releases the pressure.

3. How much of the remaining energy goes into exhaust heat (where it is available to drive the turbine) rather than lost to the chamber walls.

Note that "2." is much more important than "3."
- Of all the energy converted to "boundary work" - pushing the piston down, perhaps 90% is available at the flywheel to propel the vehicle.
- OTOH, of all the energy expelled to the exhaust, perhaps less than 20% can be recovered by the turbine.

[Brian Coat]

Thanks, GG!

Of course, the big deal with detonation is not the energy loss. The big deal is that to maximise (2) you need high cycle pressures and the resulting knock can break the engine (e.g. Top land fatigue).

To optimise (2) + (3) together you need to increase the boost (and hence AFR).

So dilution limit + knock limit become combined combustion system challenges.

Hence all the talk about novel rapid / stratified / other ignition methods.

The piston is a combustion system boundary severely clobbered by rapid or detonating combustion so this must be a key area of development too.

We should also remember that the fuel is part of the combustion sytem.

[Frank_]

re the ringland failure/det issues, what is the likelyhood of a piston combustion chamber ? with the perforated thimble prechamber protruding down into it, would it not absorb the peak pressure forces ?

[NL_Fer]

I was thinking about such an topic, but dedicated to Jet Ignition, since the all manufacturers are using it, or will be in the future or a comparable system.

But combustion is good also. And it is most important still, energy needs to come from combustion first, before it can be harvested by the hybrid system.

[godlameroso]

Note that "2." is much more important than "3."
- Of all the energy converted to "boundary work" - pushing the piston down, perhaps 90% is available at the flywheel to propel the vehicle.
- OTOH, of all the energy expelled to the exhaust, perhaps less than 20% can be recovered by the turbine.

There's ~400kw after all pumping losses friction and heat lost to the atmosphere in the exhaust gases, 20% of that is around 80kw, I think that number is a little higher, probably closer to 30%.

[gruntguru]

re the ringland failure/det issues, what is the likelyhood of a piston combustion chamber ? with the perforated thimble prechamber protruding down into it, would it not absorb the peak pressure forces ?

http://i64.tinypic.com/351xyt5.jpg

Such a chamber would absorb some shock wave impacts if detonation were occurring inside it. Peak combustion pressure would be felt by the entire volume above the piston.

It is unlikely there would be a chamber in the piston due to the difficulty in obtaining the desired (high) CR with a short stroke, pent-roof engine design.

[gruntguru]

Note that "2." is much more important than "3."
- Of all the energy converted to "boundary work" - pushing the piston down, perhaps 90% is available at the flywheel to propel the vehicle.
- OTOH, of all the energy expelled to the exhaust, perhaps less than 20% can be recovered by the turbine.

There's ~400kw after all pumping losses friction and heat lost to the atmosphere in the exhaust gases, 20% of that is around 80kw, I think that number is a little higher, probably closer to 30%.

I agree with your calculation. However a big chunk of that 30% is blowdown energy which won't increase as much as simple sensible heat in a the situation of less-efficient combustion. Lets say "about 20 or 30%"?

[Frank_]

re the ringland failure/det issues, what is the likelyhood of a piston combustion chamber ? with the perforated thimble prechamber protruding down into it, would it not absorb the peak pressure forces ?

http://i64.tinypic.com/351xyt5.jpg

Such a chamber would absorb some shock wave impacts if detonation were occurring inside it. Peak combustion pressure would be felt by the entire volume above the piston.

It is unlikely there would be a chamber in the piston due to the difficulty in obtaining the desired (high) CR with a short stroke, pent-roof engine design.

i envisaged a chamber-less head design, fuel limited/lower rpm/high boost avoids the need for long duration cams i guess (so minimal valve pockets too)
and diesels manage to achieve 20+ c/r with near square piston/stroke dimensions
with mercedes history of diesels (and honda,s lack of) i just wonder whether they attacked this engine formula from a diesel perspective ?

[gruntguru]

re the ringland failure/det issues, what is the likelyhood of a piston combustion chamber ? with the perforated thimble prechamber protruding down into it, would it not absorb the peak pressure forces ?

http://i64.tinypic.com/351xyt5.jpg

Such a chamber would absorb some shock wave impacts if detonation were occurring inside it. Peak combustion pressure would be felt by the entire volume above the piston.

It is unlikely there would be a chamber in the piston due to the difficulty in obtaining the desired (high) CR with a short stroke, pent-roof engine design.

. . and diesels manage to achieve 20+ c/r with near square piston/stroke dimensions

3 problems:
- F1 engine is nowhere near square
- Far more overlap and lift
- Pent-roof not flat head

[NL_Fer]

I think with Jet Ignition we are more back to a classic port injection like intake stroke. Where the better they mix up the air/fuel before compression (a homogenious mixture) the more efficient the combustion will be.

But since it is a single direct injector, they have to emulate the port injection intake stroke.

And about Mercedes pre knowledge: I know they hauled in some turbo experts from Mercedes' truck devision for help. But that's another topic i guess.

What i like to know for now is, how do they inject during intake and fill the pre chamber with only one DI.

[PlatinumZealot]

My theory is that the is a two injector loophole. Only one direct injector is allowed by the word of the regulations. However the prechamber injector is not a considered a direct injector but instead an "indirect" injector.

Anyway reasonable homogenous mixing can be achieved with today's direct injectors.

[NL_Fer]

5.10.2 There may only be one direct injector per cylinder and no injectors are permitted upstream of the intake valves or downstream of the exhaust valves. Only approved parts may be used and the list of parts approved by the FIA, and the approval procedure, may be found in the Appendix to the Technical Regulations.

It is not allowed. I think they are using pwm steering for different types of spraying.

[J.A.W.]

See here: http://www.insulatedpulseengine.com/sit ... /Fig25.jpg

Looks like a CI at 1st glance, but its an SI..

[R_GoWin]

Article came out today regarding TJI and the DI sitting in the main chamber and pre chamber fuelled by compression stroke.

http://www.motorsportmagazine.com/opini ... ine-update

Interesting that the link says ideas percolated from a core group of Cosworth engineers to Mahle and Mercedes AMG HPP. All 3 companies are roughly the same area close to each other in Northampton.