Downshifting

[culver]

Downshifting doesn't use gas; My dad said when he was a trucker back in the day that the truckers said that engine braking was always better than using your actual brakes because it help saved fuel.

With heavy trucks things are different. It is very easy to overhead the brakes. Once the brakes are over heated you need to service the pads. I think they can resurface them but I'm not certain. Either way, it was better to use the engine braking most of the time to avoid the brake wear. Things are different in a car where the brakes have a lot more stopping capability with respect to the weight of the vehicle. It takes a lot more effort to over heat the brakes. A single hard stop won't do it.

[jcp123]

SOME cars shut the injectors off during a coast (manual transmission only), which nets great fuel economy gains, but not all (my Focus, for instance, keeps shooting fuel into the motor when coasting). Those that don't, as I understand it, keep injecting fuel proportionately to keep the engine's mixture stoichiometrically correct, in other words, they will consume more fuel, not less.

[Quiggs]

blocked intake (closed throttle) = no air, which = stall. The throttle plate will allow a small amount of air to enter the engine. for newer engines (efi enabled, not just fly by wire throttle) the fuel injectors shut down meaning no fuel on no throttle input. The air only mixture runs through engine being compressed and providing engine braking and no fuel consumption.

IAC valve ftw.

[jediali]

gotcha!

[ruim20]

So it depends on the engine, not all efi cars shut the fuel down when coasting? i allways put mine in neutral when coasting, i find it hard to believe that at 3000rpm the car isn't consuming more than at 800rpm.

[NSXType-R]

Downshifting doesn't use gas; My dad said when he was a trucker back in the day that the truckers said that engine braking was always better than using your actual brakes because it help saved fuel.

My father was telling me just yesterday that truckers shouldn't resort to brakes first because if they run out of air for the brakes they won't have them at all. So, engine brake first to reduce speed then use brakes. We were going down a steep bridge and he reminded me about that.

[Matra et Alpine]

So it depends on the engine, not all efi cars shut the fuel down when coasting? i allways put mine in neutral when coasting, i find it hard to believe that at 3000rpm the car isn't consuming more than at 800rpm.

When "coasting" with engine doing 300 revs the engine is being turned by the wheels ... so fuel is needed no matter HOW fast it's spinning. In neutral at 800 revs the engien HAS to burn fuel to KEEP it running.

[ruim20]

When "coasting" with engine doing 300 revs the engine is being turned by the wheels ... so fuel is needed no matter HOW fast it's spinning. In neutral at 800 revs the engien HAS to burn fuel to KEEP it running.

Could you clarify, i din't quite understand. Thanks.

so, it's the same amount of fuel that goes in the engine no matter at what speed it's turning and as long you don't open the throtle?

[Matra et Alpine]

WHen you are coasting with it in gear tt is the energy in the car moving forward turinging the wheels connected with the road, turing the diff, gearbox, clutch, flywheel and engine.
When you are "coasting" with the car in neutral it is still the energy in the car moving forward that keeps you going. BUT with the gearbbox in neutral there is nothing to turn the engine. Except for fuel that has to be delivered to use the combustion cycle to turn the engine over.
In the "in gear coasting" it is the kinetic energy of the car that is providing the energy to turn the engine and no requirement for petrol.

[ruim20]

WHen you are coasting with it in gear tt is the energy in the car moving forward turinging the wheels connected with the road, turing the diff, gearbox, clutch, flywheel and engine.
When you are "coasting" with the car in neutral it is still the energy in the car moving forward that keeps you going. BUT with the gearbbox in neutral there is nothing to turn the engine. Except for fuel that has to be delivered to use the combustion cycle to turn the engine over.
In the "in gear coasting" it is the kinetic energy of the car that is providing the energy to turn the engine and no requirement for petrol.

so the injectors stop "throwing" petrol inside the cilinders completly? if one had a trip only on an incline that would keep the car moving, 5th/6th gear in, it would have 0 consuption?

Also when we downshift to help the breaking, without using heal and toe, does it also have 0 consuption? same principle aplies? even on very brutal downshifts?

[Waugh-terfall]

I'm not sure but when going down a steep hill the Audi's Trip Computer suddenly hits 3figures, from around 24-36mpg to around 400-800mpg and sometimes it just reads '---MPG'

[hightower99]

sometimes it just reads '---MPG'

Infinite MPG FTW!!!

Perpetual motion anyone?

The thing is that if you go down hill with the engine still connected to the drivetrain you will be slowing down (unless you are on a really steep hill) and at some point fuel will be needed to maintain idle which will be the slowest speed you can go down the hill with the clutch engaged and no brakes.

[LandQuail]

Are jake brakes done with combustion or without?

I'm going to resort to my native Hillbilly to answer this:

Jake brakes? I'sll tell ya 'bout them-ole Jake brakes: ain't no dam 'bustion a'tall, boy, just a big ole flapper what flops down like in tha bottum a'tha shitter, cept'n this'un flaps down 'n'tha 'zhaust, sealin' 'er up tighter'n a dog a-shittin a peach seed. You's can gitcha onna them'n Jake brakes on dam-near any ole diesel pickup, what wit them not a-havin that engine brakin' of spark 'gnishuon motors.

An idea just came to me. Somebody correctly described an engine being run down through the gears at closed throttle as an air pump. Why hasn't anyone captured that pressurized air (and this would be especially useful on a turbocharged car) to provide a few seconds worth of forced (or more greatly forced) "boost" on demand?

I guess it's involve more solenoid- or vacuum-operated valves than it'd be worth...

[revetec]

On most electronic fuel injected cars there is a feature to reduce emissions. This feature cuts off all injection on high vacuum or over run situations such as hard decel. For example if you select a very low gear on over run and the throttle is totally closed and the revs are high producing high vacuum, injection ceases to minimize the incidence of unburnt fuel going out the exhaust. In this situation the cylinder does not fire normally. When the revs come down to a predetermined point such as 1,500rpm the injection restarts which is usually referred to as "tip in". This occurs for two reasons, generally at the lower RPM ignition of the cylinder will again occur normally, and there is a need to start injection earlier than idle to reduce a stall situation.

You can feel this happen if you force a situation to occur.

On decel, select a ratio that will provide a high rev decel situation (being careful not to over rev the engine) and with your foot totally off the accelerator let it coast right down in revs. you should either slightly feel the "tip in" or if you have a trip computer with instantaneous fuel usage, you will see it go to "0.0 litres" and "tip in" to a normal reading around the RPM point I referred to earlier. This feature is in manual and auto cars generally, and I can get my auto car to do it by manual selection of a low gear on decel.

This feature was added to most electronic fuel injection systems in the 80s-90s to reduce unburnt hydrocarbons coming out the exhaust. Vehicles that do not have this function will experience intermittent firing or popping in the exhaust. Cars with this feature will usually not pop. A throttle sensor not adjusted/positioned correctly will cause a popping from the exhaust as the sensor needs to be reading 0% to activate this feature. Most cars with a catalytic converter has this feature.

[CapnBoost]

Since i have a Power-FC & commander in my car I can tell you that when I engine brake my ECU is reporting 0.0% injector duty cycle. Today perhaps I will engine brake from 2nd to 1st and then let the car slow down so i can tell you when the commander reports that it's using throttle again.