do automatics shut off the injectors when coasting?

[BlahBlahYouToo]

apparently manuals do if you coast in gear.
is this also true of autos?

 

[Zenmervolt]

Yes. Although the slippage of the torque converter reduces the effectiveness of this technique in automatics. Most automatic transmissions are set up to disengage the lockup clutch when coasting in Drive which means the torque converter absorbs some of the force of the wheels and as a result the engine speed may drop below the threshold value for the injectors to start firing again.

ZV

 

[JulesMaximus]

Yes.

 

[radioouman]

Yes, but the accessories (alternator, AC compressor, power steering pump) require the crankshaft to be turning, so the engine will only drop to a certain RPM before the injectors start working again.

 

[JulesMaximus]

Originally posted by: radioouman
Yes, but the accessories (alternator, AC compressor, power steering pump) require the crankshaft to be turning, so the engine will only drop to a certain RPM before the injectors start working again.

No, as long as the transmission is in gear, coasting down a long hill the injectors will not be injecting fuel into the engine. Your accessories will continue to run as long as the engine is turning (wheels are turning the engine).

 

[exdeath]

Originally posted by: JulesMaximus

Originally posted by: radioouman
Yes, but the accessories (alternator, AC compressor, power steering pump) require the crankshaft to be turning, so the engine will only drop to a certain RPM before the injectors start working again.

No, as long as the transmission is in gear, coasting down a long hill the injectors will not be injecting fuel into the engine. Your accessories will continue to run as long as the engine is turning (wheels are turning the engine).

It doesn't quite work that way with an automatic. Torque converters don't transfer motion backwards.

 

[LTC8K6]

It doesn't quite work that way with an automatic. Torque converters don't transfer motion backwards.

Then why does the engine rev up when you downshift?

 

[Blunc]

when the injectors shut off the engine doesn't run.

 

[LTC8K6]

It spins, which is all that's necessary...

 

[thomsbrain]

Originally posted by: JulesMaximus

Originally posted by: radioouman
Yes, but the accessories (alternator, AC compressor, power steering pump) require the crankshaft to be turning, so the engine will only drop to a certain RPM before the injectors start working again.

No, as long as the transmission is in gear, coasting down a long hill the injectors will not be injecting fuel into the engine. Your accessories will continue to run as long as the engine is turning (wheels are turning the engine).

The computer will not allow RPM to drop below a certain point without resuming a minimum "idle" flow of gas. For most cars it will be a few hundred RPM over actual idle speed.

In my Accord, fuel shutoff continues down to roughly 1200 RPM, and the torque converter stays locked when coasting except in situations where locking it would result in sub-fuel-cutoff RPMs. So basically it stays locked down to about 1200 RPM, whereupon it unlocks and fuel flow begins.

I spend about two hours a day watching the intricacies of torque converter locking, throttle position readouts, fuel flow, and MPG because I commute up and over a 2000 foot mountain pass and have a Scangauge. If traffic permits, I sometimes coast for nearly 10 miles with only very brief periods of fuel flow because I have such long stretches of steep downhill driving.

By the way, an unlocked torque converter is murder on your MPG. Avoid it at all costs, even if it means carrying more speed.

 

[exdeath]

Originally posted by: LTC8K6

It doesn't quite work that way with an automatic. Torque converters don't transfer motion backwards.

Then why does the engine rev up when you downshift?

Either it's locked up or there is a load on the engine which allows the engine to rev up immediately when the output turbine spins faster than the input turbine as a result of the downshift (eg: momentary stall until it's 1:1 again).

Hence why to push start an automatic you have to get pushing to like 30+ mph to engage the hydraulic pumps and lockup mechanism. And even that only works if there is a secondary pump not part of the converter.

Though I could be wrong here, automatics aren't my thing, but I understand the turbines in a converter are curved to one direction and due to the way the oil flow works via centripetal force and all that, it's impossible for it to transmit power in reverse (from wheels to engine) without a lockup mechanism

 

[exdeath]

As far as the whole injectors shutting off thing (regardless of transmission) I think people just don't understand how much energy there is in a 4,000 lb object moving at 60 mph.

It's a case of grossly underestimating the energy in a coasting vehicle and vastly overestimating the frictional loss there actually is in an engine, and not understanding that an engine only needs fuel to accelerate.

 

[thedarkwolf]

You are wrong about them having to have a lockup mechanism to transmit power in reverse. Lockup converters are a fairly new thing. My 89 dodge caravan doesn't have a one and it certainly engine brakes. Mine however doesn't turn the injectors off but that is only because dodge didn't enable that feature in the computer. People have hacked the computer and turned it on without any problems while hacking other parts.

 

[LTC8K6]

Yes, and people unplug the lockup solenoids too.

 

[Zenmervolt]

Originally posted by: thedarkwolf
You are wrong about them having to have a lockup mechanism to transmit power in reverse. Lockup converters are a fairly new thing. My 89 dodge caravan doesn't have a one and it certainly engine brakes. Mine however doesn't turn the injectors off but that is only because dodge didn't enable that feature in the computer. People have hacked the computer and turned it on without any problems while hacking other parts.

True, however an un-locked torque converter is less efficient at transmitting power in reverse.

In my Volvo at least, the threshold value for the injectors coming back on is higher for automatic transmission vehicles than for the manual equivalent. IIRC in manual cars injector flow resumes at 1,200 RPM while on automatics the injectors kick in at 2,100 RPM.

ZV