Would this give better cruise fuel economy in a turbo car

[Big Al]

As I see it , it could work to have the wastegate open to increase fuel economy but ONLY if the Wg pipe and wastegate is designed for it (designed same as a exhaust manifold for a NA car) and you only have a small RPM area that the car has that tune.
On my own car I can say definitely that it wouldn't work because the exhaust manifold is to short and wastegate to badly placed so it only could tune at too high RPM with my exhaust camshaft.

[user-23911]

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.

OK so we're at cruise with manifold vacuum?
The wastegate has been forced to be open by some means?
Why would the throttle be more open?
So your own logic has it that there's less exhaust restriction therefore the engine makes more power due to better VE, the throttle then needs closing a bit to keep the speed down.

Try again?
My own car like many others, it's got a recirculated BOV . At idle it's open, at low load it's open too, it only closes when there's a lack of manifold vacuum which only happens over a certain % of throttle.
So lets say we're at cruise but the BOV is still slightly open (very common).
That means that even though the turbo is spinning, it's not making boost, there's no load on the turbine so there's very little restriction to the exhaust due to lack of load.
The compressor heats the incoming air slightly, the intercooler cools it slightly and the throttle being slightly open cools the air too because it drops pressure?


But the big question is.......is the throttle going to be more or less open at light load if the wastegate is open?

Having an open wastegate at part throttle is pretty much the same as having an open BOV?
Yes or no?
Turbo speed is different but its not doing any work in either situation?

[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.

OK so we're at cruise with manifold vacuum?
The wastegate has been forced to be open by some means?
Why would the throttle be more open?
So your own logic has it that there's less exhaust restriction therefore the engine makes more power due to better VE, the throttle then needs closing a bit to keep the speed down.

Try again?
My own car like many others, it's got a recirculated BOV . At idle it's open, at low load it's open too, it only closes when there's a lack of manifold vacuum which only happens over a certain % of throttle.
So lets say we're at cruise but the BOV is still slightly open (very common).
That means that even though the turbo is spinning, it's not making boost, there's no load on the turbine so there's very little restriction to the exhaust due to lack of load.
The compressor heats the incoming air slightly, the intercooler cools it slightly and the throttle being slightly open cools the air too because it drops pressure?


But the big question is.......is the throttle going to be more or less open at light load if the wastegate is open?

Having an open wastegate at part throttle is pretty much the same as having an open BOV?
Yes or no?
Turbo speed is different but its not doing any work in either situation?

No, I don't think it is the same. In one situation, turbo is doing work to move air and in another it is not doing that work. I agree that not having a bypass valve open would make it even worse, slowing down the turbo and making it work even harder. But, in my opinion, the most fuel efficient setup would be to have the bypass valve only open on very high manifold vacuum and have the wastegate open already at slight manifold vacuum. I might be wrong though.

The car that I know opens wastegates at cruise, Cayenne Turbo, has very small turbos that come on at very low rpms and loads, so maybe that's why it actually makes a difference there.

[68corvette]

I have played around with variable nozzle turbos on diesel engines and opening the nozzle at steady state driving (dropping MAP and EMP to zero) makes great improvement on fuel economy.
EGR must be blocked as it depends on high EMP.

[vwchuck]

Yes it most definitely will help because now you have a very small displacement NA engine with the wastegate open. This makes your throttle open up and you have less pumping losses. If you can do EGR you can really get the fuel economy to skyrocket.

[ptuomov]

Yes it most definitely will help because now you have a very small displacement NA engine with the wastegate open. This makes your throttle open up and you have less pumping losses. If you can do EGR you can really get the fuel economy to skyrocket.

With variable valve timing and direction injection (that allows for stratified charge lean burn, for example) there are so many different ways that one can manage torque and fuel efficiency of the new engines. In contrast, I'm just thinking about tweaking or not tweaking my 31-year-old turbo conversion car per my primitive means.

In this particular case, I have decided to keep the wastegate closed and turbo spinning as much as possible, just to reduce lag. I don't really drive the car enough to care about fuel economy.

[gruntguru]

A simple test would be easy (preferably on the dyno).
Cruise it at 80 mph.
Check manifold pressure after the throttles.
Open the wastegate(s) with a bit of compressed air and reset the power required for 80 mph cruise.
Compare manifold pressure after the throttles for both situations.

I bet there is little or no difference. Conclusion - intake pumping work is unchanged. Exhaust pumping work will probably reduce slightly due to the reduction in EMP.

[user-23911]

You can't do it on a dyno because 80 MPH on a dyno is nothing like 80 MPH on the road due to the lack of drag.

80 MPH on the road at steady speed takes about 40 to 50 HP. 80 MPH on the dyno takes far less.


If you look at it from a slightly different point of view.....you're throttling the engine.
Either via the exhaust like an exhaust brake on a big truck.
Or the normal way on the intake manifold.

If you remove the throttling on the exhaust, you need to add it to the intake to keep the speed down.
So either way there's pumping losses.

[ptuomov]

If you look at it from a slightly different point of view.....you're throttling the engine.
Either via the exhaust like an exhaust brake on a big truck.
Or the normal way on the intake manifold.

If you remove the throttling on the exhaust, you need to add it to the intake to keep the speed down.
So either way there's pumping losses.

For the base engine that requires a given air-fuel ratio, yes. For the whole turbocharged system, no. If you let the turbo spin at part throttle and attempt to force air thru the engine, I believe you generate additional pumping losses. The exhaust is simultaneously throttled by the turbine with the intake being throttled by the throttle plate butterfly. You can reduce the total pumping losses by bypassing the turbine and reducing intake pressure usptream of the throttle.