Would this give better cruise fuel economy in a turbo car

[ptuomov]

I’m wondering if opening the wastegate at cruise when there’s any manifold vacuum would improve the cruise fuel economy of a turbo car. The logic being reduced pumping losses wuth the throttle more open, lower pressure in the intake pipes upstream of the throttle, and lower exhaust manifold pressure.

[ptuomov]

I’m thinking a super simple setup like this:

784460B0-661F-4240-828D-C19CF235E1B3.jpeg

[cjperformance]

Interesting though and I really dont know. Off track a little but on a customers car, 4.0L inline 6 cylinder, 3spd C4 auto non lockup 1900 stall/stock convertor, 2.77 gears, 1430kg that is a daily driver that had done 5 years worth of doing aprox 700km per week with consistent fuel economy on LPG as its sole fuel, using closed loop fuel control. I then fitted a turbo to the same engine, no gearing changes, no fuel system changes, same intake manifold, exhaust system from transmission/bellhousing area back etc, no intercooler, same air filter as when non boosted. Mega squirt timing control before and with boost just timing map has been altered from around 3400 rpm up.
It has now done id say 3 years worth of the same driving. This car gets very slightly better economy when driven to achieve around the same acelleration and cruise speeds. He uses cruise control as much as possible and is very happy with the result. It does of course use more fuel when the extra hp is used.
Next time its in I'll have to see what intake pressure is at normal cruise speeds.
I think there is merit in what you are thinking under some driving conditions, probably already out there on something!, i can possibly see on light throttle cruise that when a little extra power is required having your system just start closing down the wastegate to turn the turbo a little more rather than using more throttle, interesting, keen to see others thoughts or experiences with this.
On the vehicle i used as example it has an internal WG, it would not be hard in my location to do a 50/60km light cruise economy test with WG closed and open, may provide a tiny insight.

[ptuomov]

Interesting though and I really dont know. Off track a little but on a customers car, 4.0L inline 6 cylinder, 3spd C4 auto non lockup 1900 stall/stock convertor, 2.77 gears, 1430kg that is a daily driver that had done 5 years worth of doing aprox 700km per week with consistent fuel economy on LPG as its sole fuel, using closed loop fuel control. I then fitted a turbo to the same engine, no gearing changes, no fuel system changes, same intake manifold, exhaust system from transmission/bellhousing area back etc, no intercooler, same air filter as when non boosted. Mega squirt timing control before and with boost just timing map has been altered from around 3400 rpm up.
It has now done id say 3 years worth of the same driving. This car gets very slightly better economy when driven to achieve around the same acelleration and cruise speeds. He uses cruise control as much as possible and is very happy with the result. It does of course use more fuel when the extra hp is used.
Next time its in I'll have to see what intake pressure is at normal cruise speeds.
I think there is merit in what you are thinking under some driving conditions, probably already out there on something!, i can possibly see on light throttle cruise that when a little extra power is required having your system just start closing down the wastegate to turn the turbo a little more rather than using more throttle, interesting, keen to see others thoughts or experiences with this.
On the vehicle i used as example it has an internal WG, it would not be hard in my location to do a 50/60km light cruise economy test with WG closed and open, may provide a tiny insight.

This would only work on a car and transmission that would otherwise build meaningful boost at cruise rpm. My car cruises 80 mph at 2400 rpm using 50-60 hp but can build 2 psi of boost already at 1400 rpm if loaded at constant rpm, which got me thinking. If it were a diesel, I don't think this would help at all to bypass the turbo, but since there's that damn throttle plate in my intake manifold, gears started turning in my brain, not just sure if in the right direction.

[ptuomov]

Nothing new under the sky:

http://blog.autospeed.com/2008/12/04/op ... l-economy/

...If boost is being sensed from the intake manifold, the wastegate will probably be closed – all the exhaust gas is being channelled through the turbo, even though boost in the intake manifold isn’t being developed or used.

If the boost is being sensed from the compressor outlet, the wastage will still be closed sufficiently to develop boost, but it won’t be as closed as the above scenario.

So, in either case, the turbo is working when it isn’t really needed – and so the turbine is developing back pressure on the engine.

This harms fuel economy.

It would therefore seem logical to sense when cruise is occurring, and to then open the wastegate and so reduce back-pressure, so improving fuel economy.

And some cars do just that.

But this approach has a downside. If the driver is cruising along but then suddenly slams the throttle open, boost will be delayed as the turbo will take time to come up to full speed.

On the other hand, if the turbo is already spinning fast, a sudden need for boost will be accomplished more speedily. In fact, because some boost will already be present on the turbo side of the throttle body, response can be very quick indeed.

So, how to balance the requirement for low backpressure cruise fuel economy (no turbo spinning wanted – exhaust flow fully bypassing turbine) with good throttle response (turbo already spinning, so developing boost upstream of the throttle).

One way to do it is to sense driving style. This can be achieved in a few ways: measuring the rate of change of throttle opening, measuring the frequency with which large throttle openings are used, or directly measuring car acceleration (eg longitudinal and lateral).

Depending on driver behaviour, the boost control system can then be automatically configured to be in ‘economy mode’ or ‘performance mode’.

Again, some cars – eg the Porsche Cayenne Turbo – do just that.

[ptuomov]

A little more engineering-oriented link:

https://ac.els-cdn.com/S147466701539936 ... 086a20bcd0

Page 287:

6.1 Layout 1: Fuel-optimal controller

Starting from low load the waste gate is fully open and the throttle is used to control the load. Only after the throttle is fully open the waste gate starts to close and controls the intake manifold pressures above ambient pressure.

6.2 Layout 2: Driveability-optimized controller

In current series production cars the time-optimal strategy is implemented. The goal of this strategy is to keep the turbocharger on the highest possible speed. This is due to the fact that the rotational dynamics of the turbocharger are the limiting factor for the time response. This is achieved by closing the waste gate as much as possible until an appropriate boost pressure after the compressor pc is reached.

[gruntguru]

I’m wondering if opening the wastegate at cruise when there’s any manifold vacuum would improve the cruise fuel economy of a turbo car. The logic being reduced pumping losses wuth the throttle more open, lower pressure in the intake pipes upstream of the throttle, and lower exhaust manifold pressure.

There is a lot of complication there for small gains (if any). If NOx emissions are not an issue - lean it out! You should be able to get it to cruise smoothly at lambda 1.2 - maybe more. (It will need a lot of extra spark advance.) You should see serious economy improvement.

[user-23911]

It's not going to make any difference at all. Need to fix the tune first.
A turbo car at cruise should give similar if not slightly better economy than the N/A equivalent.

Nor will you have the ability to do it without a source of compressed air and the right valves.

[JoePorting]

I wouldn't think it would matter.

[ptuomov]

I’m wondering if opening the wastegate at cruise when there’s any manifold vacuum would improve the cruise fuel economy of a turbo car. The logic being reduced pumping losses wuth the throttle more open, lower pressure in the intake pipes upstream of the throttle, and lower exhaust manifold pressure.

There is a lot of complication there for small gains (if any). If NOx emissions are not an issue - lean it out! You should be able to get it to cruise smoothly at lambda 1.2 - maybe more. (It will need a lot of extra spark advance.) You should see serious economy improvement.

That’s true (although I’m not sure exactly how lean one could go), but pumping losses are pumping losses and the fuel economy gains from opening the wastegate at cruise are multiplicative with any gains from lean combustion.

[ptuomov]

It's not going to make any difference at all. Need to fix the tune first.
A turbo car at cruise should give similar if not slightly better economy than the N/A equivalent.

Nor will you have the ability to do it without a source of compressed air and the right valves.

Reading more about it, it makes about 4% difference at 80 mph cruise and has theoretical maximum fuel economy benefit of about 10% when a theoretical engine is ideally suited to take advantage of it. That’s why many factory ECUs use this strategy in the economy mode and turn it off in sport mode.

You don’t need a separate compressed air source if you run a two-port wastegate. You can use manifold vacuum on one side. Electronic control of course makes sense, but simple solenoid bleeds should work with compressor outlet pressure on one side and check valves manifold vacuum on the other side of the wastegate.

[ptuomov]

I wouldn't think it would matter.

I don’t think that 4% fuel economy gain at 80 mph cruise matters that much for a hobby car, so I agree.

For the car factories working under the asinine fuel economy rules, it does matter, however.

[user-23911]

That’s why many factory ECUs use this strategy in the economy mode and turn it off in sport mode.

Which factory ECUs?
What strategy?


You're just making it all up.

[ptuomov]

That’s why many factory ECUs use this strategy in the economy mode and turn it off in sport mode.

Which factory ECUs?
What strategy?


You're just making it all up.

Keeping the wastegate open at cruise to reduce pumping losses. Porsche Cayenne Turbos do this, I believe. Haven’t personally looked at the code, so can’t be 100% certain, of course.

[gruntguru]

Pumping losses saved will be almost entirely on the exhaust side. MAP will need to be (almost) the same for your 80 mph cruise despite the throttle being more open, so intake pumping work will be very similar.

Leaning brings double gains - primarily from more complete combustion of the fuel in the excess-oxygen environment but also due to reduced intake pumping work due to a higher MAP at the same operating point. How lean? Lambda 1.2 and spark advance adjusted to MBT.