When using engine breaking do you burn gas?

[Burning1]

On the old camry, the reason I could feel the injectors is that they would kick in as I approached idle speed, even with the car in gear. A car in 5th gear at idle speed is operating well below the minimum engine speed recommended by the manufacturer. It causes the engine to lug, which produces a harsh vibration. Most any vehicle will do this if you let the revs get low under load, and in fact, hiding the vibration from the operator would be a bad idea -- they should be alerted about the harm they are doing. It was noticeable exiting on-ramps, where I would come to a stop at freeway speeds. Obviously, that's not optimal driving technique -- I was messing around.

Regarding vacuum and airflow... The two are strongly related. How do you think a MAP sensor works?

Throttle plates to not create a perfect seal. The greater the vacuum, the more air the engine pulls in, even at closed throttle. IF the ECU is firing the injectors, and IF the ECU still attempts to create a stoichiometric fuel mixture at closed throttle, higher RPMs will mean slightly higher fuel consumption even at closed throttle.

Of course, with no load on the engine and very little combustion, there's no reason the ECU can't run a very lean mixture, so we do have to make some major assumptions here.

 

[arnoha]

On the old camry, the reason I could feel the injectors is that they would kick in as I approached idle speed, even with the car in gear. A car in 5th gear at idle speed is operating well below the minimum engine speed recommended by the manufacturer. It causes the engine to lug, which produces a harsh vibration. Most any vehicle will do this if you let the revs get low under load, and in fact, hiding the vibration from the operator would be a bad idea -- they should be alerted about the harm they are doing. It was noticeable exiting on-ramps, where I would come to a stop at freeway speeds. Obviously, that's not optimal driving technique -- I was messing around.

Regarding vacuum and airflow... The two are strongly related. How do you think a MAP sensor works?

Throttle plates to not create a perfect seal. The greater the vacuum, the more air the engine pulls in, even at closed throttle. IF the ECU is firing the injectors, and IF the ECU still attempts to create a stoichiometric fuel mixture at closed throttle, higher RPMs will mean slightly higher fuel consumption even at closed throttle.

Of course, with no load on the engine and very little combustion, there's no reason the ECU can't run a very lean mixture, so we do have to make some major assumptions here.

Eh, I think we're having communication issues, not technical issues.

In the end, it's fairly moot. Fuel consumption is minimal to zero at closed throttle, no matter the technology.

 

[teg916]

I can't speak for newer cars that may shut off the fuel during engine braking, bit I don't believe most cars do this.

As far as most cars go, you would burn more fuel during engine braking than you would just coasting to a stop and using only braking to slow you down.

If your car idles at 800 RPM, then it takes X amount of fuel to keep the engine running.

If you are engine braking say at 2400 RPM, fuel is used to keep the fire going in your engine at 3 times the rate of when you are idling because each cylinder fires 3 times as fast.

My numbers may not be 100% accurate because of ineficciencies during idling but it is a good round about number.

 

[afm199]

I can't speak for newer cars that may shut off the fuel during engine braking, bit I don't believe most cars do this.

As far as most cars go, you would burn more fuel during engine braking than you would just coasting to a stop and using only braking to slow you down.

If your car idles at 800 RPM, then it takes X amount of fuel to keep the engine running.

If you are engine braking say at 2400 RPM, fuel is used to keep the fire going in your engine at 3 times the rate of when you are idling because each cylinder fires 3 times as fast.

My numbers may not be 100% accurate because of ineficciencies during idling but it is a good round about number.

Both FI and carbed motors have butterflies or slides that close the intake tract, preventing almost any air fuel mix from entering the combustion chamber. By you method, if you downshift and engine brake at 13000 rpm, you will be using 10 time idle fuel. You won't. You will hardly use any. The carb slide is closed the needle fully in, and the fuel metering is only from the fuel screw or pilot jet. It's just does not work that way.

 

[Marlowe]

I can't speak for newer cars that may shut off the fuel during engine braking, bit I don't believe most cars do this.

DFCO has been present in FI cars for quite some time... trust me on this.

Quoting random people on the interwebs who may be wrong:

The 1957 Rochester FI units for Chevrolet and Pontiac tried this, for fuel savings only, but since this was a mechanical system, they could not get it to work quick enough for some instances. Therefore teh [sic] whole DFCO system was removed as a Technical Service Bulletin for all dealers to perform on the few early FI cars sold that year.

This is clearly a failed example, but the idea is not really that new, and I've driven plenty of 20 year old cars that had it. My 12 year old pickup definitely has it. Fuel injection is not all that new, either.

Does BARF really think it is smarter than all the automotive engineers out there?

 

[Burning1]

+1. My old 1986 Porsche 944 had it, as did my 1986 Toyota Camry (With multi-port batch fire injection! Yay!)

 

[Enchanter]

Just because the engine is spinning at X rpm's above idle speed does not mean that fuel is being spent to keep it at those rpm's. It is the momentum of the moving vehicle that is spinning the tires - transmission - engine. When the throttle is closed on a carburated vehicle, only the fuel required for idle is making it into the cylinders. Nothing more. It really is that simple. On a modern fuel injected car, the fuel is shut off a short time after the throttle is closed. Older fuel injection worked similar to carburetors mentioned above.

 

[mookiera]

I don't know about that: However, the 2nd law of thermodynamics stipulates that your gas mileage will suffer anytime you brake.

Pull in that clutch and let 'er coast!
Brakes do a good job of braking.
Engines are better suited for accelerating
and running @ constant speed.

 

[mookiera]

You can easily test this in any of the modern cars that
have mpg readouts. Even using an automatic transmission.
Any time you let off the gas you will see that the mpg reading
increases rapidly and dramaticly, while the engine rpm decreases
slowly until reaching idle.
I haven't had the opportunity to watch an mpg readout using
a stick shift, but I'll bet the results are similar.

 

[teg916]

Both FI and carbed motors have butterflies or slides that close the intake tract, preventing almost any air fuel mix from entering the combustion chamber. By you method, if you downshift and engine brake at 13000 rpm, you will be using 10 time idle fuel. You won't. You will hardly use any. The carb slide is closed the needle fully in, and the fuel metering is only from the fuel screw or pilot jet. It's just does not work that way.

True, the butterflies close and reduce the airflow so you are not burning much. Those same butterflies are also closed during engine idling though. Your engine maintaitains a fuel mixture of about 15:1 regardless of what RPM you are running at (slightly richer at idle). So unless you have a fuel injection system that shuts off all fuel during engine braking, you are burning more fuel the faster your engine spins.

Of course it is still not all that much fuel.

 

[Enchanter]

So unless you have a fuel injection system that shuts off all fuel during engine braking, you are burning more fuel the faster your engine spins.

Not during engine braking, with the throttle closed.

You are failing to accept that when the throttle is closed, the only fuel being delivered to the engine is being done so through the idle circuit. No matter how fast the engine is spinning, it is not using more fuel than when it is at idle. It is impossible.

 

[teg916]

Higher RPM with the butterfly valve closed = lower manifold pressure because an internal combustion engine is a big air pump.

Lower manifold pressure = more suction on the idle circuit

more suction on the idle circuit = more fuel flow.

 

[Fonzie]

that's my instinct as you said teg916. I accept the buttterfly closes but thought that the vaccum would still suck in more fuel because more preasure at the pilot jet. If it gives same fuel as sitting idle then my new worry is it's running lean as hell since at high RPMS in acceleration the valves are wide open(letting in more fuel and air), and we now the plug is still firing. Lean means high engine heat. This means more wear on engine and oil costing more than brake pads. Proper riding always says to match gear to speed but during mellow economical street riding maybe downshifting to use engine breaking is a bad idea. As I said interesting question.

 

[afm199]

True, the butterflies close and reduce the airflow so you are not burning much. Those same butterflies are also closed during engine idling though. Your engine maintaitains a fuel mixture of about 15:1 regardless of what RPM you are running at (slightly richer at idle). So unless you have a fuel injection system that shuts off all fuel during engine braking, you are burning more fuel the faster your engine spins.

Of course it is still not all that much fuel.

The low speed jet and fuel screw meter fuel at a rate appropriate for low speed. They don't meter more at higher rpms, they meter the same after roughly 4k to 5k. Vacumn can only suck so much mixture.

 

[Fonzie]

Does it suck enough to keep it from running too lean though? Here's a situation, you are on an air cooled bike, it's 95 degrees, on a street with traffic and poorly timed lights, or even on twistys down hill and engine braking often equals heat, agreed? If engine runs lean during engine braking equals more heat. Too much heat? Too lean? It pulls a tiny amount more gas maybe but enough? Does the high rpm on oil pump compinsate? It's to determine if it's economicaly worth it when everything is factored including wear on bike, oil, gas, brake pads, etc. for me.

 

[Marlowe]

I regularly engine brake an air cooled, older Honda, in the Phoenix area, in direct sunlight, on days where the ambient temperature exceeds 115. I even do runs on the freeway, then come to a stop at a traffic light for several minutes in these conditions when I'm done.

And all my plugs look fine. You're really overthinking this. How long do you engine brake for, hours? Several minutes continuously? C'mon... let it go.

 

[Starshooter10]

yeah it's neg at best...

one would assume that the bike would only put in enought fuel to have the engine idle when your throttle is closed...

on a carb bike though it's differant since its fuel use is based ont he volume and vacum of the intake... closed butterflys means VERY little air but very high vaccum... that one always makes me wonder...

 

[Fonzie]

Overthinking yes but unleess you pull your plug and read it after engine breaking you are not reading its condition during engine breaking. You bike idles and read the plug that's your condition at idle, and so on.

 

[Marlowe]

I'm guessing that even if you did, a couple hundred feet of engine braking here and there wouldn't even be noticeable.

You're not going to bankrupt yourself in gas or kill the engine by compression braking. Really.

 

[afm199]

Does it suck enough to keep it from running too lean though? Here's a situation, you are on an air cooled bike, it's 95 degrees, on a street with traffic and poorly timed lights, or even on twistys down hill and engine braking often equals heat, agreed? If engine runs lean during engine braking equals more heat. Too much heat? Too lean? It pulls a tiny amount more gas maybe but enough? Does the high rpm on oil pump compinsate? It's to determine if it's economicaly worth it when everything is factored including wear on bike, oil, gas, brake pads, etc. for me.

No. Engine heat comes from combustion, not friction. There is so much oil and so much slick that it is easy to turn a motor over by hand. The ONLY heat that matters is combustion heat. The less combustion the less heat. (Actually if it is lean, it will run hotter at full throttle, because the air fuel mix is too lean, and enriching it does cool the engine AT large throttle openings. Note LARGE, not small)

During engine braking the little combustion that happens really is unimportant. Why do think engine braking CREATES heat? Reasoning, please.