A big drawback of turbo engines is the lag. Motoring enthusiasts love the immediate responses of an atmo. I wonder if you could do it a different way. If you had a big still air box with a one way valve at low revs the engine can draw on the atmosphere whilst the turbo end feeds into the box which increases the pressure as boost rises, as boost reaches a certain point the still air box is now a large over pressure chamber, the higher pressure closes the one way valve whilst opens another direct pathway via the turbo.

Sounds too simple to work and there are probably reasons why it isn’t done but I can’t really see any drawbacks.

AwesomeO said:

A big drawback of turbo engines is the lag. Motoring enthusiasts love the immediate responses of an atmo. I wonder if you could do it a different way. If you had a big still air box with a one way valve at low revs the engine can draw on the atmosphere whilst the turbo end feeds into the box which increases the pressure as boost rises, as boost reaches a certain point the still air box is now a large over pressure chamber, the higher pressure closes the one way valve whilst opens another direct pathway via the turbo.

Sounds too simple to work and there are probably reasons why it isn’t done but I can’t really see any drawbacks.

drawback would be bad yeah?

AwesomeO said:

A big drawback of turbo engines is the lag. Motoring enthusiasts love the immediate responses of an atmo. I wonder if you could do it a different way. If you had a big still air box with a one way valve at low revs the engine can draw on the atmosphere whilst the turbo end feeds into the box which increases the pressure as boost rises, as boost reaches a certain point the still air box is now a large over pressure chamber, the higher pressure closes the one way valve whilst opens another direct pathway via the turbo.

Sounds too simple to work and there are probably reasons why it isn’t done but I can’t really see any drawbacks.

Even simpler would be to start it electrically. No?

mollwollfumble said:

AwesomeO said:

A big drawback of turbo engines is the lag. Motoring enthusiasts love the immediate responses of an atmo. I wonder if you could do it a different way. If you had a big still air box with a one way valve at low revs the engine can draw on the atmosphere whilst the turbo end feeds into the box which increases the pressure as boost rises, as boost reaches a certain point the still air box is now a large over pressure chamber, the higher pressure closes the one way valve whilst opens another direct pathway via the turbo.

Sounds too simple to work and there are probably reasons why it isn’t done but I can’t really see any drawbacks.

Even simpler would be to start it electrically. No?

A box and some valves with plumbing is pretty simple.

AwesomeO said:

mollwollfumble said:

AwesomeO said:

A big drawback of turbo engines is the lag. Motoring enthusiasts love the immediate responses of an atmo. I wonder if you could do it a different way. If you had a big still air box with a one way valve at low revs the engine can draw on the atmosphere whilst the turbo end feeds into the box which increases the pressure as boost rises, as boost reaches a certain point the still air box is now a large over pressure chamber, the higher pressure closes the one way valve whilst opens another direct pathway via the turbo.

Sounds too simple to work and there are probably reasons why it isn’t done but I can’t really see any drawbacks.

Even simpler would be to start it electrically. No?

A box and some valves with plumbing is pretty simple.

Draw me a picture. I can’t visualise it.

At lower revs an intake takes from the atmosphere meanwhile where the line is the turbo is feeding into the air box pressuring it and also adding to the atmo intake, as the pressure increases to boost levels, the atmo valve closes off and the direct turbo inlet is redirected from pressuring the still air box to go direct into the intake manifold.

AwesomeO said:

A big drawback of turbo engines is the lag. Motoring enthusiasts love the immediate responses of an atmo. I wonder if you could do it a different way. If you had a big still air box with a one way valve at low revs the engine can draw on the atmosphere whilst the turbo end feeds into the box which increases the pressure as boost rises, as boost reaches a certain point the still air box is now a large over pressure chamber, the higher pressure closes the one way valve whilst opens another direct pathway via the turbo.

Sounds too simple to work and there are probably reasons why it isn’t done but I can’t really see any drawbacks.

Some manufacturers have got round the problem by fitting both a turbo & a supercharger.

AwesomeO said:

At lower revs an intake takes from the atmosphere meanwhile where the line is the turbo is feeding into the air box pressuring it and also adding to the atmo intake, as the pressure increases to boost levels, the atmo valve closes off and the direct turbo inlet is redirected from pressuring the still air box to go direct into the intake manifold.

Ive been thinking about it. I still don’t fully understand, but at least I’ve managed to split the problem into components.

There are two factors causing turbo lag. One is that there’s a time delay due to the timescale needed for gas compression. The bigger the volume of gas is between the cylinder exit and turbine and between the charger and cylinder intake the longer this timescale is. So adding a big air storage volume could make the lag worse, unless carefully designed.

The second factor causing turbo lag is the time required for the positive feedback loop. More accelerator is equivalent to more fuel. So you start with more fuel, possibility too much fuel if the computer control of the fuel injectors is poor. If computer control of the injectors is good then it inserts a time lag between accelerator pressing and fuel supply.

Anyway, back to the positive feedback loop. More engine power means higher exit pressure, which is bad in itself, but that speeds up the turbocharger which compresses the intake air which gives more engine power. This a positive feedback loop (except for the back pressure on the engine) so would require n cycles to reach steady state. That gives a calculable lag time of n times the timescale needed for gas compression.

OK, so that’s my understanding. One other factor I suppose, the rotational inertia of the turbocharger causes an extra time lag in addition to the gas compression time lag.

To overcome turbo lag, the main principle would have to be to speed up the turbo rapidly under computer control independently of the computer control of the fuel injectors. One way would be to initially speed up the turbo with an electric motor that would be either decoupled or used as a generator once the turbo lag has ended. A second way would be to temporarily clutch in the crankshaft, so the turbocharger ants as a supercharger for half a second or so. A third way, and we’re getting closer to the proposal in the OP here, is to feed a pulse of high pressure gas into the turbine inlet from a compressed gas storage.

Now feeding such a pulse of gas into the tubine inlet gas to be done in such a way as to avoid increasing the back pressure on the engine. I recommend an arrangement of plumbing similar to that of a jet pump.

Then there’s the issue of how to get the pressurised gas in the first place. It could be stored from the previous highest pressure in the system. But in that case turbo lag for the first time the accelerator is pressed will be bad. Oh wait, the pressure could be stored even when the car is not running. That’s what you’re getting at.

Issues to be overcome would be cooling of the gas store between uses. Overcoming the loss of power when the high pressure gas store is being recharged. And timing the high pressure gas release to match only when the engine is accelerating. Not impossible.

I have had a rethink about the plumbing as well and can think of a few more improvements. Basic principles are there are two air paths, one atmospheric and turbo. At lower revs the engine acts as atmos as is gets air direct via the air box as is done now. Meanwhile the exhaust is spinning up the turbo and spilling its air into the same atmo air box, as boost builds the atmo box gets more and more pressurised air, acting a bit like a ram air induction and at a point the turbo air starts going direct bypassing the air on and the air box returns to atmo pressure, when the turbo due to low exhaust gas starts spooling down, the engine starts feeding from both the atmo and turbo path.

Basically you are ensuring you have when the turbo is not working efficiently you have an intake that is not impeded by the turbo and as the turbo spools up a handover from an increasingly pressured airbox to turbo only intake.

AwesomeO said:

I have had a rethink about the plumbing as well and can think of a few more improvements. Basic principles are there are two air paths, one atmospheric and turbo. At lower revs the engine acts as atmos as is gets air direct via the air box as is done now. Meanwhile the exhaust is spinning up the turbo and spilling its air into the same atmo air box, as boost builds the atmo box gets more and more pressurised air, acting a bit like a ram air induction and at a point the turbo air starts going direct bypassing the air on and the air box returns to atmo pressure, when the turbo due to low exhaust gas starts spooling down, the engine starts feeding from both the atmo and turbo path.

Basically you are ensuring you have when the turbo is not working efficiently you have an intake that is not impeded by the turbo and as the turbo spools up a handover from an increasingly pressured airbox to turbo only intake.

why not run a smaller exhaust line?

worked on our EH

stumpy_seahorse said:

AwesomeO said:

I have had a rethink about the plumbing as well and can think of a few more improvements. Basic principles are there are two air paths, one atmospheric and turbo. At lower revs the engine acts as atmos as is gets air direct via the air box as is done now. Meanwhile the exhaust is spinning up the turbo and spilling its air into the same atmo air box, as boost builds the atmo box gets more and more pressurised air, acting a bit like a ram air induction and at a point the turbo air starts going direct bypassing the air on and the air box returns to atmo pressure, when the turbo due to low exhaust gas starts spooling down, the engine starts feeding from both the atmo and turbo path.

Basically you are ensuring you have when the turbo is not working efficiently you have an intake that is not impeded by the turbo and as the turbo spools up a handover from an increasingly pressured airbox to turbo only intake.

why not run a smaller exhaust line?

worked on our EH

You still get the lag and inertia from having basically an elastic air connection with the intake being restricted.

AwesomeO said:

stumpy_seahorse said:

AwesomeO said:

I have had a rethink about the plumbing as well and can think of a few more improvements. Basic principles are there are two air paths, one atmospheric and turbo. At lower revs the engine acts as atmos as is gets air direct via the air box as is done now. Meanwhile the exhaust is spinning up the turbo and spilling its air into the same atmo air box, as boost builds the atmo box gets more and more pressurised air, acting a bit like a ram air induction and at a point the turbo air starts going direct bypassing the air on and the air box returns to atmo pressure, when the turbo due to low exhaust gas starts spooling down, the engine starts feeding from both the atmo and turbo path.

Basically you are ensuring you have when the turbo is not working efficiently you have an intake that is not impeded by the turbo and as the turbo spools up a handover from an increasingly pressured airbox to turbo only intake.

why not run a smaller exhaust line?

worked on our EH

You still get the lag and inertia from having basically an elastic air connection with the intake being restricted.

yes, but much shorter lag (when doing 20 second laps, lag is very unwanted)

What sort of vehiclee are you looking at it for?

stumpy_seahorse said:

AwesomeO said:

stumpy_seahorse said:

why not run a smaller exhaust line?

worked on our EH

You still get the lag and inertia from having basically an elastic air connection with the intake being restricted.

yes, but much shorter lag (when doing 20 second laps, lag is very unwanted)

What sort of vehiclee are you looking at it for?

No vehicle, just a brain fart and wondering why not. If you are tootling along in your Evo and stamp on the go pedal nothing happens as the pressurised intake can’t react fast enough, but if you had an atmo path you would get an instant response, it wouldn’t bother the turbo as it is using the exhaust energy to pump its own air, it is working whilst the atmo is feeding the energy free of restriction and as the exhaust starts to power the turbo air intake fan it bleeds first into the airbox then as it gets on boost, into the normal turbo pressure circuit, the airbox has now been closed off by a valve at a certain pressure point.

AwesomeO said:

stumpy_seahorse said:

AwesomeO said:

You still get the lag and inertia from having basically an elastic air connection with the intake being restricted.

yes, but much shorter lag (when doing 20 second laps, lag is very unwanted)

What sort of vehiclee are you looking at it for?

No vehicle, just a brain fart and wondering why not. If you are tootling along in your Evo and stamp on the go pedal nothing happens as the pressurised intake can’t react fast enough, but if you had an atmo path you would get an instant response, it wouldn’t bother the turbo as it is using the exhaust energy to pump its own air, it is working whilst the atmo is feeding the energy free of restriction and as the exhaust starts to power the turbo air intake fan it bleeds first into the airbox then as it gets on boost, into the normal turbo pressure circuit, the airbox has now been closed off by a valve at a certain pressure point.

main problem I can see is it taking longer to pressurise, being a larger receptacle. so you’ll be waiting longer for the boost to kick in.
A lot of turbo design for cars goes into matching the powerband with the point the boost is effective to maximise the extra torque.
You’d have to choose a motor with a fairly high powerband for your design to suit

The boost wouldn’t take longer, the exhaust is still the same, it is just a matter of where that pressurised air is going. For that matter it could go direct into the intake manifold which at low revs is being fed both atmo and pressurised air. I just thought that handing off from an airbox might make for a less impeded path at everything below optimal boost pressure where the engine would become totally pressurised.

For some reason my usual browser on’t let me log on, so I’m using an alternate one.
Anyway …..

First off, a likely correction – What almost everyone means when they say ‘lag’ is not lag but the engine/turbo combination being being below what’s called the boost threshold. That’s the point at which the turbo really starts to spin and hence make boost. Below that point they don’t do a great deal and so the engine feels a bit slow. Lag can certainly happen below the boost threshold, but it also happens above it as well. Lag is the time it takes for the turbo to stabilise from a lower rpm to a higher rpm, and to be fair that does happen a heck of a lot faster above the boost threshold.

With modern turbos (such like the later Borg Warner, Garret, etc) they come on boost far earlier and better than the older types. It’s really not much of a problem and if you feel the need to modify your engine (the point of this thread really) to make it nicer below the boost threshold then the easiest way by far is to simply fit a lighter flywheel. I’ve done it with my WRX and it transforms the engine quite a lot. There’s not much of a jump in power as the turbo comes on boost, nothing like a standard WRX has. The only downside is that it’s more difficult to get going smoothly from a standing start, as there’s much less rotational inertia in the engine, so it stalls quite easily until yo get used to it.

What the F1 cars are doing these days, and it’s going to trickle down to road cars some time soon-ish, is have an electric motor that spins up the turbo at lower revs so it makes boost far earlier than it otherwise would.

The most effective way is to just make the engine larger and use a smaller turbo. It’ll come on boost earlier and so drive a bit nicer. No trickery needed.

> At lower revs the engine acts as atmos as is gets air direct via the air box as is done now. Meanwhile the exhaust is spinning up the turbo and spilling its air into the same atmo air box, as boost builds the atmo box gets more and more pressurised air, acting a bit like a ram air induction and at a point the turbo air starts going direct bypassing the air on and the air box returns to atmo pressure,

That’s an excellent way to make the turbo lag much worse.

You want the compressed air out of the air box timed to coincide with the pressing of the accelerator, independent of the rev rate. Remember that compressing air creates its own time lag.

you could also have a pressure vessel that is constantly filled by the turbo, and a valve that works as a throttle body to supply the necessary air to the intake.
That way you always have an over-abundance of air, similar to the way you always have an over-abundance of fuel.

the problems with that could be the size of turbo needed and the safety issues associated with carrying around a pressurized tank..

engines suck an awful lot of air. how many litres of air does this tank hold? at what pressure? and how many cycles will this supply to the engine?

ChrispenEvan said:

engines suck an awful lot of air. how many litres of air does this tank hold? at what pressure? and how many cycles will this supply to the engine?

exactly.

AwesomeO was asking would it work, that is a suggestion for something that would be more suitable.

It doesn’t have to fit anything logistically because that has not been specified.

although, with a good/big enough turbo, how much compressed air could you squash into a 75L (standard car supply size) lpg tank?

stumpy_seahorse said:

ChrispenEvan said:

engines suck an awful lot of air. how many litres of air does this tank hold? at what pressure? and how many cycles will this supply to the engine?

exactly.

AwesomeO was asking would it work, that is a suggestion for something that would be more suitable.

It doesn’t have to fit anything logistically because that has not been specified.

although, with a good/big enough turbo, how much compressed air could you squash into a 75L (standard car supply size) lpg tank?

the turbo wont keep pressurising the tank as they are just a fan. so if the turbo boost by 2 the tank will get 150litres of air in at 14 psi. If my maths is right.

ChrispenEvan said:

stumpy_seahorse said:

ChrispenEvan said:

engines suck an awful lot of air. how many litres of air does this tank hold? at what pressure? and how many cycles will this supply to the engine?

exactly.

AwesomeO was asking would it work, that is a suggestion for something that would be more suitable.

It doesn’t have to fit anything logistically because that has not been specified.

although, with a good/big enough turbo, how much compressed air could you squash into a 75L (standard car supply size) lpg tank?

the turbo wont keep pressurising the tank as they are just a fan. so if the turbo boost by 2 the tank will get 150litres of air in at 14 psi. If my maths is right.

so, depends on the boost needed, but you’ll be needing roughly 12L of air per gram of fuel burnt, so you can burn 12.5 grams of fuel at 14psi, then it all depends on the refill rate

turbo or whatever doesn’t pump compression up (too high) at low RPM

airflows bumpier (lower frequency) too at lower RPM

Spiny Norman said:

The most effective way is to just make the engine larger and use a smaller turbo. It’ll come on boost earlier and so drive a bit nicer. No trickery needed.

You’re right. Wish I’d thought of that.