# Downshift vs fuel economy?



## bigluke (Aug 18, 2013)

Was discussing with a folk at work on jake brakes on bigger diesel engines and I'm questioning myself. Since there's no throttle butterfly in diesel engines and throttling the injectors only. When there's no load and downshifting no more fuel is injected. So regardless of which lower gear is selected to slow the car by downshift it does not affecting the diesel fuel consumption. But since we have a variable vane turbo, when the throttle is depressed the vanes close creating exhaust back pressure similar to a butterfly closing on a gasoline engine. I know that jakes are acting on exhaust valves on bigger engines to save breaks and downshift on our cruze seems to be the right thing to do. So now I'm wondering why if the throttle is depressed in auto mode to slow the car the dic shows 0.0l/100km but if I downshift in manual mode it shows 2.2 to 0.7 and then lowers to 0.2 and going back to 0.7 when downshift to a lower and lower gear? Anyone has a clue?


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## jblackburn (Apr 14, 2012)

I think it kicks the injectors back in temporarily while the transmission is clutching. You see the same thing in the manual versions - although an auto shifts much faster, there is some time it's still in a neutral state as well. And the DIC fuel consumption gauge typically has a second or two of lag.

The other possibility is that the car treats manual mode as a sport mode of sorts and won't enter DFCO because it thinks you're downshifting to get ready to power out of something. 

Does it ever go back to 0.0 if you stay in the gear for 10+ seconds?


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## bigluke (Aug 18, 2013)

Yes it goes to 0 in lower rpms. Example, from 3rd to 2nd , 4000 rpm it's like 2.2 and going down slowly to 0.7 approaching 2000 rpm or so and 0 if I stay like that till I stop completely. Weird.


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## ChevyGuy (Dec 13, 2014)

bigluke said:


> I know that jakes are acting on exhaust valves on bigger engines to save breaks and downshift on our cruze seems to be the right thing to do.


Depends on what we're talking about. A long downhill, yes, that's the right thing to do - save your brakes so you don't lose them and crash. Routine braking, I don't think so. You're adding wear to the engine and transmission just to save some expendable parts.

Yes, race drivers downshift. But that's because their brakes would fail if they didn't. And they probably rebuild the drive train once a season if not every race. Yes, trucks have jakes. But they're an industrial product designed for that, as well as for very long life. Not a consumer product like the Cruze.

If you want to do it because it's fun - it's your car. If you're doing it to save money - it's going to get real expensive if you guess wrong. I don't think the risk is worth it. Just my 2¢.


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## bigluke (Aug 18, 2013)

I'm using manual mode more often in winter to get better traction and sometimes for fun, it's just a thing that I've noticed and was questioning myself why if the pedal is depressed fuel is still injected as by the dic in manual mode but not in auto mode.


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## oilburner (Jun 13, 2013)

you manually downshift causing higher rpms, hence the reason for higher injector pulse width causing the dic to show fuel consumption. I think the
manual shift mode is a joke. most times won`t allow the shift.


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## obermd (Mar 3, 2012)

Just reread your post. When in full automatic the throttle position is nothing more than a hint to the ECU. The ECU will still do what it wants with regard to fuel and if this means not injecting fuel to maintain speed, that's what it will do. In manual mode the ECU is assuming you know what you're doing so pressing the throttle in manual mode will result in fuel being fed to the injectors.


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## bigluke (Aug 18, 2013)

Ok, so I'm missing something. Assuming you are in auto and rolling 60 miles and not applying any pressure to the throttle pedal you can see 0.0 on the dic as the car is slowing down an the transmission shifts at is defined rpm points till completely stoped. Now do the same thing in manual downshifting from same speed and same shift rpm as it would do in auto...you'll see the dic showing 2.2 and going down on each downshift gear. So it means that the ecu don't react the same way in auto and manual.


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## oilburner (Jun 13, 2013)

I guess the input to the ecm from manual select triggers more fuel delivery. I really can`t see any advantage in doing this. as chevy guy says ,I would rather wear out 50 dollar brake pads.


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## bigluke (Aug 18, 2013)

oilburner said:


> I guess the input to the ecm from manual select triggers more fuel delivery. I really can`t see any advantage in doing this. as chevy guy says ,I would rather wear out 50 dollar brake pads.


I agree, I'm not doing it to save 50 bucks pads,I just like the engine compression on higher rpm on downshift, and noticed this behavior and thought someone can confirm that in manual mode more fuel is used. Thanks for your inputs.


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## diesel (Jun 8, 2013)

This is interesting. I will have to experiment a bit with this while I observe the l/km. I am a fan of downshifting on steep downgrades.


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