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When using engine breaking do you burn gas?

Ziggady

Ziggady

I Pwn Noobs
Joined
Apr 27, 2009
Location
San Francisco
Moto(s)
Aprilia Falco sl1000
Name
Nick
My brother and I are constantly arguing over whether or not you burn the same gas while using engine breaking as you would accelerating i think you dont.
thoughts? or knows
 
Not sure about motorcycles, but I would assume that their fuel injection is the same as cars. In modern cars, the fuel injection shuts off during engine braking/coasting in gear.
 
If you are breaking your engine, you have bigger problems than gas mileage.
 
I would think that whenever the engine is running, fuel is being consumed. Simple.
 
yes but is more being consumed because of higher rpm?
for instance if i am in cruse control at 7000 rpm then i down shift and you use engine breaking will my gas mileage suffer.
 
Ziggady said:
yes but is more being consumed because of higher rpm?
for instance if i am in cruse control at 7000 rpm then i down shift and you use engine breaking will my gas mileage suffer.
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In a fuel injected vehicle, the injectors would be shut off. In a carburated vehicle, the butterflys would be closed reducing the air intake and hence the fuel used. You would be using the same amount of fuel as you would be when parked and the engine is at idle.
 
I don't know about that: However, the 2nd law of thermodynamics stipulates that your gas mileage will suffer anytime you brake.
 
how do the injectors shut off? thats what ive heard to it just doesnt make sense, have you ever tried to start a car that wasnt getting any fuel? how does the engine stay running? so id say yes anytime the engine is running youre burning fuel but only equivalent to idle because the throttle is closed
 
kyles here said:
how do the injectors shut off? thats what ive heard to it just doesnt make sense, have you ever tried to start a car that wasnt getting any fuel? how does the engine stay running? so id say yes anytime the engine is running youre burning fuel but only equivalent to idle because the throttle is closed
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Easy. The ECU doesn't send an electrical pulse to them. The engine isn't 'running' in the true sense of the term. It is turning/spinning because the wheel(s) are turning it via the transmission. The ignition system is still operating, so when you use the throttle, fuel is delivered into the cylinder and internal combustion begins again.
 
Good question. I'd say it burns more than idle because the RPMS and high vaccum created on a carbed bike but never really thought about it. It may not though, just telling you what my gut says. You can runs some expirements and tell us what you find but it is hard to limit variables in the real world that would throw the testing off. If it is as others have stated it means your bike is running lean as hell with engine braking and that might be a bigger issue (heat mainly). I always downshift and guess I should have been thinking of this before.
 
well i kinda wanted to know because gas is expensive and its cheaper to change break pads then it is to be filling up more frequently
 
In a car, this is called DFCO.. de-celeration fuel cutoff. The fuel injectors are switched off when the throttle is closed.

I'd assume any FI bike acts similarly. Carbed bikes will continue to burn fuel. That said, the amount of fuel burned is pretty small, and I wouldn't worry about it too much. Keep in mind, with the throttle closed, we're really just talking about the idle jets, which aren't that
large.

Set of HH pads runs me about $30, but I have a drum rear brake... that buys me roughly 15 gallons here. That is 750 miles of riding, and probably much more than that in terms of pure dynamic braking potential, considering we'd be at idle.

I wouldn't trade off pads for fuel like that...I don't think it's a good bet. Besides, if you find yourself using the brakes a ton, you may be better served by practicing your throttle control.
 
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Fuel injection shuts off. Carbed bikes have slides/butterflies that block the intake so while there is some intake mixture, it is minimal. So the answer is very little to none.
 
Ziggady said:
well i kinda wanted to know because gas is expensive and its cheaper to change break pads then it is to be filling up more frequently
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The answer is whether carbed or FI you burn almost no gas. If you were burning gas the motor would, duhh, be powering up the wheels, not braking.
 
Enchanter said:
The ECU doesn't send an electrical pulse to them. The engine isn't 'running' in the true sense of the term.
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I noticed this while driving an old camry. If you coast to a stop in 5th gear (using the brakes or not) the engine would be smooth until it hit about 1500RPM, when the fuel injectors kicked back in. The moment fuel was injected, the engine would 'lug,' producing a lot of vibration. I'm sure this basic test could be performed with a modern sport bike.

Another test would be to use the kill switch under engine braking. If killing the engine noticeably reduces engine braking, it's clear that the injectors are still firing.

I suspect that the answer is somewhat dependent on the bike, although I will say that I suspect many bikes do not kill the injectors when the throttle is closed -- racers and stunters are known to raise the idle speed in order to reduce engine braking -- if the engine killed the fuel injectors when the throttle was closed, this trick shouldn't work (assuming that the riders aren't tricking the ECU with a mis-calibrated TPS -- if you raise the idle speed without recalibrating the TPS and the ECU never realizes that the throttle is closed.)

On my BMW, the fast idle switch noticeably reduces engine braking, so clearly the injectors are on. I'm not sure how the ECU behaves with the fast idle off -- I don't notice any difference between coasting with the engine on or off.

One other thing I've noticed is that vehicles with automatic transmissions will not kill the injectors the way a car with a manual transmission will. My cougar's trip computer would report fuel usage even when coasting.

Even if the injectors aren't killed, engine braking does not use fuel while engine braking the way it does when accelerating. While engine braking, the throttle is closed, and the bike doesn't consume very much fuel. It may consume more fuel than at idle, because an engine spinning fast produces a stronger vacuum than an idling engine... More vaccum means more air, which usually means more fuel is injected. But it certainly wouldn't consume fuel the way that an accelerating engine does.

With that said, a pair of brake pads is very inexpensive. It's better to brake using the brakes than to rely on the engine -- especially on a motorcycle, which is extremely unlikely to cook the pads under normal use, even on a steep descent.
 
Ziggady said:
well i kinda wanted to know because gas is expensive and its cheaper to change break pads then it is to be filling up more frequently
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If you want to save gas, don't go faster than 65 miles per hour on the freeway, and practice riding at the pace. My GSX-R got more than 200 miles to a tank when stuck in weekend traffic on Highway 1.

(On the way home, I spent most of my time riding > 100MPH. Fuel economy suffered.)
 
My wideband O2 sensor reads AIR in FI vehicles (Miata & STI) when undergoing decel in gear. :thumbup

Otherwise it stays close to stoichiometric unless I'm WOT, though the STI runs on the rich side all the time. :laughing
 
Burning1 said:
I noticed this while driving an old camry. If you coast to a stop in 5th gear (using the brakes or not) the engine would be smooth until it hit about 1500RPM, when the fuel injectors kicked back in. The moment fuel was injected, the engine would 'lug,' producing a lot of vibration. I'm sure this basic test could be performed with a modern sport bike.

...

Even if the injectors aren't killed, engine braking does not use fuel while engine braking the way it does when accelerating. While engine braking, the throttle is closed, and the bike doesn't consume very much fuel. It may consume more fuel than at idle, because an engine spinning fast produces a stronger vacuum than an idling engine... More vaccum means more air, which usually means more fuel is injected. But it certainly wouldn't consume fuel the way that an accelerating engine does.

...
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Couple notes: the sophistication of the ECU determines if you can feel the duty cycle on the injectors start again. An older Camry won't have that sophisticated a computer compared to a modern car. Motorcycles are all over the board. Some have really nice ECUs that rival those you find in a car. Some are really, really basic. Things like Power Commanders would also affect the outcome. There isn't really a general case for bikes, especially since fuel economy of bikes is not regulated like cars. The manufacturers only have market forces to push them to reduce usage.

I think you're a little confused on vacuum vs. airflow. Vacuum is more directly controlled by the throttle. Closed throttle produces a high vacuum. Closed throttle at high RPM produces a very high vacuum. WOT produces fairly minimal vacuum at any RPM.

- As you close the throttle, you get higher vacuum.
- As RPM come up, you get higher vacuum.
- As you open the throttle, you get more airflow.
- As RPM come up, you get more airflow.
 
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