When engine braking, the RPM climbs, but does it increase fuel consumption on a fuel-injected engine?
On a carbureted engine, I imagine it does because fuel is forcibly drawn into the engine by engine vacuum.
No.
Depends on how the carbureted engine is set up. Many GM engines (and others) used a gulp valve that allowed fresh air into the intake manifold during deceleration.
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No
and...... on certain fuel-inj. engines, engine braking is a NECESSARY to the overall functionability of the drivetrain. Meaning the car won't work if the eng. brakeing isn't happening.
On the contrary, coasting in neutral wastes fuel on a fuel-injected engine.
If you coast in gear with a closed throttle, (many? most?) fuel-injected systems enter into mode called DFCO, or "deceleration fuel cut-off". The fuel injectors are turned off.
(In some, there is supposedly some of fuel delivered, for the sake of not over-oxygenating the catalytic converter).
But the throttle is closed. So fuel isn't drawn by the normal ventury jets but through those other openings on the inside of the throttle valve which are used to deliver fuel during idle. The fuel delivery through those openings is limited (according to my limited understanding of carbs). Carb engines don't always handle that situation gracefully, that's why they go "pop".
You're thinking of the diverter valve, it switched the air pump from pumping into the exhaust to pumping into the atmosphere. Sometimes called a decel valve.
These days? Probably not. More common to be used back in the 80s. (re: carburetors) One gulp valve, the asians called it an AB valve. GM had them Tee'd into the PCV hose, with a fresh air intake from the air cleaner, the third hose connection went to manifold vacuum (sense). Replaced hundreds on Chevy pick-ups for a no idle condition because they were leaking vacuum.
Unless it's into the transfer slots, my understanding is that the fuel/air mix delivered through the idle ports at idle is pre-mixed before it hits the bores, therefore the extra vacuum due to deceleration won't richen the mixture as much as you'd think it would. The throttle plates are all but closed at that point; a decent amount of the air being delivered is actually coming through the idle circuit. However, it will probably never go leaner than stoich; which you *can* do with EFI.
Now if the car in question has a "lumpy" cam, the throttle plates are probably cracked open a bit to get an acceptable idle; this doesn't help idle tuning any and likely contributes to the "snap crackle pop" that makes car guys' ears perk up :)
nate
I have a carbureted vehicle that has a DFCO solenoid. With the throttle closed and above 1500 RPM, the idle circuit in the carb is cut off.
"aarcuda69062" wrote
Yes. In some cases engine braking will increase fuel consumption, strictly speaking for a Honda. This is a flawed design. If you decide to coast downhill in neutral but decides to shift back into gear, the injectors will turn right back on no matter what rpm. You will have to speed up in order to reset this.
If you want to really save fuel then shut the engine off and coast down- hill. But this can burn out your brakes and land you on a ditch with a burning rubber.
The Honda will just cut off the fuel electronically.
Also, remember, the jets are in the venturi which is usually upstream of the throttle plate. The manifold vacuum is created downstream of the throttle. Thus, the trailing throttle does not cause as much 'sucking' as folks may assume. Choking is different. The choke is upstream of the jets and venturi.
Indeed, ANY form of braking wastes fuel. If you can coast, you save energy that was put in the car prior to braking. Regenerate braking certainly can help, but it is never 100% effective.
What bugs me these days with super-high cost of gas, is folks who roar up to a red light and slam on brakes. Their higher fuel consumption is affecting me by raising the price of gas at the pump. That kind of driving keeps demand high :-(
Indeed, ANY form of braking wastes fuel. If you can coast, you save energy that was put in the car prior to braking. Regenerate braking certainly can help, but it is never 100% effective.
What bugs me these days with super-high cost of gas, is folks who roar up to a red light and slam on brakes. Their higher fuel consumption is affecting me by raising the price of gas at the pump. That kind of driving keeps demand high :-(
Neutral is not engine braking.
Probably also illegal.
As will a number of other vehicles, which is why I said "depends on how it's set up."
Really?
Which carburetors have the metering jets in the venturi?
How was the engine idling in neutral with the injectors off?
"aarcuda69062" wrote
Neutral is what the driver was doing before he/she enter engine braking. If you drive normally, the injectors will turn off correctly. If you mess with the gear selector the injectors will go crazy.
Good question. Let me correct. If you decide to coast downhill in neutral but decides to shift back into gear, the injectors will [stay] on no matter what rpm.
And this fits the OPs question how?
Hmm.
What if you tap the accelerator a bit to match revs? Maybe the computer just needs to see a period of open throttle, followed by throttle closure above a certain RPM.
Anyway, why would you be coasting down hill in neutral? Where I live, it's even illegal.
A better question is, will the injectors stay /off/ if you select a different gear while going downhill. Say you are going in third, but the compression braking is too great for the grade, so you shift to fourth. Hopefully, at least if you don't let the RPM drop to idle, the injectors will stay off.
That depends on the software. Its possible to completely shut off the fuel flow on an EFI engine when the RPM shows to be significantly above idle but the throttle is closed, but doing so for extended periods (such as descending a mountain grade) would drop the engine below normal operating temperature, so the fuel might need to be periodically kicked back in to keep the engine hot for emissions control and driveability when you go back to "normal" operation. But I've noticed that a lot of engine control software actually *raises* the idle speed when the car is rolling forward. Definitely counter-intuitive, but it is done. I presume it reduces emissions by bumping the air flow up a little above minimum.
On a carbureted car the fuel flow rate is absolutely no different than at idle. Vacuum is maxed out anyway, so spinning the engine faster doesn't move any more air through the carb, hence no more fuel moves.
No.
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