Charge pressure limit

[roon]

What limits boost pressure in the 1.6L turbo engines? Poll above. I have left out the following:

Combustion (lean burn has been solved with injectors & ignition methods)
Compressor (likely not size limited)
MGUH (no power limit)

If I am wrong about the above, I will add them to the poll.

[Jolle]

How do you mean limits? It’s not the same as in the eighties where they tried so much boost until the engine said boom. It’s a case of efficiency now. Boost is at a optimum for the amount of fuel. More air (so even leaner burn) doesn’t mean more power. There is a sweet spot, somewhere

[Nickel]

i agree that the answer is fuel flow rate.

I'd expect the turbine to be much bigger than the compressor as a function of the need to extract energy from the exhaust placing a higher demand than the need to create charge pressure.

[roon]

That raises a similar question: what are the backpressure limits. If cam timing prevents valve overlap then exhaust manifold pressure much greater than intake manifold pressure could be maintained. If for example the pressure differential was greater than what wave tuning could attend to, such valve gating would be required. Perhaps such approaches could partially explain why Merc & Honda used log manifolds in their first iterations.

[subcritical71]

I picked thermodynamic cycle as I don’t believe the limit used is equal to the absolute maximum boost that could be produced. It Can be some lower value that makes sense from a thermodynamics view (With a prioritization on efficiency).

[Maritimer]

Believe the early log manifolds were just to get the gas to the turbine as fast as possible and package everything as tightly as they could with the turbo in the V, less concern for pulse timing at the impeller or back pressure. Full length tuned exhausts bring negligible gains for turbochargers outside of these electrified units used in F1, where they can spool the unit with a motor.

As far as charge pressure limits, there are none. As has been said its fuel flow that limits boost currently, jamming more air in is pointless if arent burning anything with it.

[saviour stivala]

“What limits boost pressure in the 1.6-litre turbo engine?”. None of the listed 7 options limits boost pressure. This 1.6-litre turbo engine is a fuel limited engine and not an air/boost limited engine.

[roon]

As has been said its fuel flow that limits boost currently, jamming more air in is pointless if arent burning anything with it.

These engines likely running beyond stoichiometry. Not all oxygen needs to combust. What matters is efficient combustion of the fuel and efficient heating of the combustion chamber charge. I would think an excess of oxygen aids combustion.

Regardless, the compressor helps drive the pistons down during the intake stroke, presuming low crankcase pressure. This could be enhanced with higher charge pressures.

Before the oil burning regs went into effect, the crankcase was exposed to the plenum between the air filter and the compressor inlet. Crankcase pressure below 1 bar may have been possible.

Thus, I think the boost limits are with the reciprocating hardware and the ES.

[henry]

As has been said its fuel flow that limits boost currently, jamming more air in is pointless if arent burning anything with it.

These engines likely running beyond stoichiometry. Not all oxygen needs to combust. What matters is efficient combustion of the fuel and efficient heating of the combustion chamber charge. I would think an excess of oxygen aids combustion.

Regardless, the compressor helps drive the pistons down during the intake stroke, presuming low crankcase pressure. This could be enhanced with higher charge pressures.

They do indeed run above stoichimetric. From the occasional boost numbers provided they are around 22:1. The extra boost does, as you say, drive the pistons on intake and the excess air mass collects the energy which then goes to the turbine to make MGU-H power, and drive the compressor.

This is another way that these PUs are unlike anything that has gone before and add many additional factors to the optimisation mix.

There have been lots of discussion of this subject. Try searching for “dilution” and user Tommy Cookers.

[saviour stivala]

As has been said its fuel flow that limits boost currently, jamming more air in is pointless if arent burning anything with it.

These engines likely running beyond stoichiometry. Not all oxygen needs to combust. What matters is efficient combustion of the fuel and efficient heating of the combustion chamber charge. I would think an excess of oxygen aids combustion.

Regardless, the compressor helps drive the pistons down during the intake stroke, presuming low crankcase pressure. This could be enhanced with higher charge pressures.

Before the oil burning regs went into effect, the crankcase was exposed to the plenum between the air filter and the compressor inlet. Crankcase pressure below 1 bar may have been possible.

Thus, I think the boost limits are with the reciprocating hardware and the ES.

These engines crankcases runs in a partial vacuum. before the crankcase fumes outlet rule change, connection was from oil/air separator to planum.

[saviour stivala]

That raises a similar question: what are the backpressure limits. If cam timing prevents valve overlap then exhaust manifold pressure much greater than intake manifold pressure could be maintained. If for example the pressure differential was greater than what wave tuning could attend to, such valve gating would be required. Perhaps such approaches could partially explain why Merc & Honda used log manifolds in their first iterations.
[/quote
The modern turbocharger developments has resulted in the turbocharger equalizing the intake and exhaust pressures.

[Tommy Cookers]

.... These engines likely running beyond stoichiometry. Not all oxygen needs to combust. What matters is efficient combustion of the fuel and efficient heating of the combustion chamber charge. I would think an excess of oxygen aids combustion. ....

there's always a level of leaning beyond which combustion efficiency falls (relatively there's not 'more oxygen')
the limit of leaning varies greatly with the fuel eg with LNG fuel it's maybe 4 lambda, a bit less with LPG
such engines exist today eg for power generation
(and gas-fuelling of the prechamber/''TJI' etc will increase leaning possible in main chamber run on gasoline fuel)

this 'heat dilution' concept means that little heat needs to be taken (by coolant) for cooling the cylinders
so more heat (more gas mass but not a higher temperature) is available in-cylinder for conversion to power

we might see the F1 engine as at a point on the range of possibilities from the traditional to full heat dilution
a point marking the limit of HD available with SI and gasoline fuelling of both main and pre chambers
a point where there's significant availability of 'free' exhaust power recovery
I have no idea if so much 'free' recovery would be available at greater HD (F1 Honda has hinted that it wouldn't)

[63l8qrrfy6]

Surely the factor limiting excessive leaning is misfire? Whatever fancy pre chamber systems they use allow leaner mixtures but there still must be a point where the mixture is no longer ignitable.

Also the way I see it excess air will always increase the heat capacity of the trapped mixture leading to lower combustion temperatures as suggested by TC however maybe at some point the extra work done by the compressor (or piston or both) on the fluid offsets the gains in heat losses to combustion walls.

[gruntguru]

I didn't tick any because I agree with TC and Mudflap.

Previous formulae limited power by limiting oxygen flow to the engine (usually by limiting displacement, boost, rpm etc). AFR is free and designers would set the AFR by controlling the amount of fuel added to the available oxygen. The maximum amount of fuel they could add (lets call it the fuel flow limit) was actually an optimum value - too much fuel and power begins to fall due to over-rich mixture. It turns out the optimum AFR is on the rich side of stoichiometric - somewhere around 0.9 because excess fuel is needed just to ensure you burn as much of the limited oxygen as possible. (There are other factors in favour of rich AFR like flame speed, cooling, detonation etc which encourage mixtures even richer than 0.9)

The current formula limits power with a fuel flow limit rule. AFR is still free and designers set the AFR by controlling the boost - which controls the air/oxygen flow to the engine. The maximum amount of air they can add (roon has called it the charge pressure limit) is actually not a "limit" but an optimum value - too much air and the power begins to fall due to over-lean mixture. It turns out the optimum AFR is on the lean side of stoichiometric - somewhere around 1.2 because excess air needed just to ensure you burn as much of the limited fuel as possible. (There are other factors in favour of lean AFR like turbomachinery effectiveness, cooling, heat loss, detonation etc which favours mixtures even leaner than 1.2)

[roon]

AFR is a broad term here. It is often used in the context of a well mixed or homogenous fuel-air charge. I would be investigating stratified AFR within the combustion chamber. Surely the manufacturers have done so already. Use injector timing and fuel plume shape to optimize combustion within a locally-rich zone. Use the surrounding lean or dry charge air to absorb heat and convey greater energy to the piston and turbine.

If this is the case, I raise the original question again.