Charge pressure limit

[gruntguru]

There is no question that stratification is being implemented here. For a start the pre-chamber has a richer mixture than the main chamber. In addition it would be desirable to avoid proximity of fuel to the cylinder walls so it is likely the main chamber itself is also stratified.

However, in spite of the ignition system for the main charge being every vigorous (turbulent jets) there is still a lean limit and more importantly an optimum leanness for the main charge. Any leaner and the thermal efficiency suffers. This is the factor that limits charge pressure.

[roon]

...there is still a lean limit and more importantly an optimum leanness for the main charge. Any leaner and the thermal efficiency suffers.

If the region proximal to the spark and/or TJI is of the correct or specified local-AFR, then total-AFR no longer correlates to combustion efficiency.

[gruntguru]

The main charge still needs to support a flame front (multiple short ones to be correct). Too lean and a lot of fuel doesn't get burned.

[roon]

The main charge still needs to support a flame front (multiple short ones to be correct). Too lean and a lot of fuel doesn't get burned.

It may not need to. The region outside of the fuel plume could be totally dry. It seems you are claiming that the overall combustion chamber volume a:f determines the combustion properties of subregion(s) of the CC volume with richer a:f. What physical property would require this?

My presumption is they are aiming the injector nozzle at the spark plug from across the CC from in between the web between the exhaust valves and the head gasket, with or without a TJI apparatus present. Time and tune the fuel injection specifically to prevent dilution. Inject fuel during the combustion sequence to prevent dilution.

The dry, non-combustion air merely absorbs the heat, pressure, and radiation of combustion from the fueled region. This is why I claim overall a:f cannot be relied upon to make claims about combustion efficiency, in this context.

If this the case, then I offer the original question again.

[63l8qrrfy6]

Surely they don't aim all the fuel at the pre-chamber!

[roon]

Surely they don't aim all the fuel at the pre-chamber!

For reasons you can't articulate? Pre-chamber or not, the point being made is: preventing fuel dilution in a high a:f scenario.

[saviour stivala]

1955 Mercedes positioned their injector right through cylinder wall just below the inlet valve with the nozzle angled upwards 12.5 degrees. Fuel flow began 30 degrees ATDC on inlet stroke and continued for 160 degrees. Visiting “specifications of 50 famous racing engines” page 7 one can see through official drawing were the injector inclination was aimed too.

[63l8qrrfy6]

Surely they don't aim all the fuel at the pre-chamber!

For reasons you can't articulate? Pre-chamber or not, the point being made is: preventing fuel dilution in a high a:f scenario.

For reasons I though were obvious.
The whole point of the prechamber is to ignite a slightly rich mixture in order to generate highly energetic flame jets that would ignite a relatively lean main chamber in a short time. That implies that the pre-chamber volume is very small compared to the main chamber volume (such that flame propagation time within the pre-chamber is not significant).

If all of the fuel would go into the prechamber while the main chamber only had dry air the combustion would be very poor (prechamber would be too rich). Keep in mind that unlike a diesel diffusion flame the jets coming out of these prechambers need some pre-mixed air-fuel ahead of them to maintain combustion.

Secondly, the generation of the "turbulet jets" implies a large pressure drop across the pre-chamber nozzles. Chances are a large portion of the fuel energy within the pre-chamber goes into initiating the flame jets rather than contributing to significant work done on the piston. That is to say the power still comes from burning a pre-mixed charged within the main chamber.

[roon]

...

You've reviewed somewhat the function of TJI, but this is not specifically what I was posting about. Nor did I suggest a large prechamber, nor did I suggest trying to entrain all fuel into some sort of pre- or secondary chamber. Not sure what is being lost in communication.

[63l8qrrfy6]

...

You've reviewed somewhat the function of TJI, but this is not specifically what I was posting about. Nor did I suggest a large prechamber, nor did I suggest trying to entrain all fuel into some sort of pre- or secondary chamber. Not sure what is being lost in communication.

You've suggested that "The region outside of the fuel plume could be totally dry." which I don't agree with hence my comment that this would require a diesel-like diffusion flame.

My understanding of what you are proposing is that all fuel went into the pre-chamber where some of it would be ignited forcing out very rich plumes that would combust as they mix with the dry air in the main chamber.

[roon]

...

You've reviewed somewhat the function of TJI, but this is not specifically what I was posting about. Nor did I suggest a large prechamber, nor did I suggest trying to entrain all fuel into some sort of pre- or secondary chamber. Not sure what is being lost in communication.

You've suggested that "The region outside of the fuel plume could be totally dry." which I don't agree with hence my comment that this would require a diesel-like diffusion flame.

My understanding of what you are proposing is that all fuel went into the pre-chamber where some of it would be ignited forcing out very rich plumes that would combust as they mix with the dry air in the main chamber.

Assuming ever higher boost and a:f is beneficial, yes, it might resemble something like a diesel. My suggestion would be, TJI or not, late fuel injection and fuel injection through the combustion and initial expansion stroke. This might keep the flame region (combustion region) localized and out of contact with some or all of the CC. Time may be a limit.

If they can master this, then I think the limiting factors for boost will be engine hardware, intercooler capacity, & ES capacity to drive beyond what the turbine can give, particularly when the turbine gives nothing i.e. during WGO.

To your latter point, I don't think there's a practical method to throw all the fuel across the CC into the TJI subchamber through the tiny TJI holes. Generally, the F1 implementation of TJI seems hindered. But if they are using then it's good enough or better than I think it to be.

[henry]

.

If they can master this, then I think the limiting factors for boost will be engine hardware, intercooler capacity, & ES capacity to drive beyond what the turbine can give, particularly when the turbine gives nothing i.e. during WGO.

I would think that in high boost situations the only time that the ES might be a limiting factor is when the waste gates are open, e-boost. Mercedes admit to a maximum drain of 200kW, with the MGU-K taking 120 that would leave 80 for the MGU-H which would probably not be enough to drive the compressor at the sort of boost levels being run. With turbines open the turbine still delivers power from blow down energy, bringing the compressor power up to the 100 plus kW needed.

As well as the factors you mention I would think turbine and compressor efficiencies come in to play since this is a compounded PU. There are two modes in which max boost might be called for. The aforementioned e-boost, in which boost may be chosen for power and delivery time over a lap, and self-sustain in which boost will be chosen to maximise total PU crank power. There may be a third, overtake, mode like e-boost but without the need to manage optimum delivery over a lap.

[gruntguru]

Assuming ever higher boost and a:f is beneficial, yes, it might resemble something like a diesel. My suggestion would be, TJI or not, late fuel injection and fuel injection through the combustion and initial expansion stroke. This might keep the flame region (combustion region) localized and out of contact with some or all of the CC. Time may be a limit.

Injection during the expansion (power) stroke is bad for efficiency. The ideal is for combustion to end as close to TDC as possible. Injection during combustion (like a diesel) is not ideal. It takes time for the fuel droplets to mix and find oxygen molecules to burn. High speed engines need "pre-mixing" of the air and fuel if rapid combustion and high efficiency are to be achieved.

On the subject of stratification. If ultra-lean AFR's (beyond 2.0) are to be achieved by extreme stratification - say an inner core at lambda 1.4 surrounded by air - there must be a transition zone where the AFR is too lean for combustion under normal conditions. Remnants of unburned fuel in this zone would severely impact thermal efficiency which would be untenable unless this ultra-lean mix can be ignited by extreme temperature, pressure and turbulence from combustion in the core combined with high CR. I don't know enough about such combustion regimes to predict whether this is feasible - any experts care to comment?

[roon]

Injection during the expansion (power) stroke is bad for efficiency.

Full stop? Might be able to optimize combustion regime in question for a degree range ATDC via abnormally high boost. Rather than tuning for combustion at TDC.

Injection during combustion (like a diesel) is not ideal. It takes time for the fuel droplets to mix and find oxygen molecules to burn.

I suppose the time scale is important. Diesels make do, but at a much lower engine speed. Double, triple, quadruple the amount of time passing through TDC. Regardless some or ideally all mixing should occur in situ.

there must be a transition zone where the AFR is too lean for combustion under normal conditions. Remnants of unburned fuel in this zone would severely impact thermal efficiency which would be untenable unless this ultra-lean mix can be ignited by extreme temperature, pressure and turbulence from combustion in the core combined with high CR.

This is a key point for me and has been on my mind since proposing this schema. That periphery will be there as with any gaseous or vapor or aerosol plume. I think shock compression would be the phenomenon to rely upon. Similar to Mazda's spark-induced shockwave combustion scheme.

Mercedes admit to a maximum drain of 200kW, with the MGU-K taking 120 that would leave 80 for the MGU-H which would probably not be enough to drive the compressor at the sort of boost levels being run.

Perhaps, although, why would they publish accurate data at this time regarding this system?

With turbines open the turbine still delivers power from blow down energy, bringing the compressor power up to the 100 plus kW needed.

I have another pet untested hypothesis that says a MGU-driven-compressor-speed turbine will create too much backpressure to permit exhaust flow through it during WGO.

[Tommy Cookers]

.....via abnormally high boost.....

With turbines open the turbine still delivers power from blow down energy, bringing the compressor power up to the 100 plus kW needed.

I have another pet untested hypothesis that says a MGU-driven-compressor-speed turbine will create too much backpressure to permit exhaust flow through it during WGO.

doesn't the efficiency of the single stage compressor fall at 'abnormally' high boost ?
(isn't that what abnormally high boost is ?)

single stage supercharged aircraft (piston) engines at SL didn't exceed PR=2 and used less PR in sustained SL running
single stage compressor gas turbines similarly used rather low PRs in these situations ?
(I once booked to fly a Polish MiG 17)

what do actual compressor systems use ?
(NOTE to self - the only snippet I know reminds me of obligations via the Official Secrets Act)

PR improvement at an efficiency of c.80% became possible via research following the 70s energy crisis
PR=4 is available, even PR=5 at this efficiency
that's roughly how these F1 PUs can do what (we seem to believe at present) they do