Engine Braking
 

Hi. So I was reading the article on wikipedia about engine braking.
First, I had always thought that engine braking was because of the friction to be overcome in the engine, not because of vacuum differences. Is that really the main reason for engine braking/loss?

Two, Wouldn't this mean that an engine that is idling uses a lot of power because the throttle plate is blocking airflow?

Three, how do automatic transmission cars avoid this problem, as autos in gear don't slow down as much without acceleration as manuals. Is this solely because of a lack of a direct connection?

Four, as far as I can tell, diesel engines completely avoid this form of engine braking. Why then are they not more popular and too much more fuel efficient? Is it because of higher compression ratios?

and Five, what effects does this have on hypermiling? It seems to reinforce the info found in BSFC charts and the P&G technique.

If anyone could expound on this subject, it would be highly appreciated. There seems to be almost no information about this on the web.

Engine braking only works for a small percentage of us who are trying to get good fuel economy. So far, I've only been able to achieve a little over 60% over EPA by using engine braking in my "driving without brakes" technique. Your mileage will vary.

1. The engine is still compressing air. If you were to kill your engine while driving and open the throttle you would find you get an increase in engine braking cause you are allowing the cylinders to fill completely.
2. You just described "pumping loss". One major inefficiency on the gas combustion engine.
3. Lack of direct connection is the main reason. Autos are also geared taller which results in lower engine speed during engine braking. The torque converter multiplies torque output to compensate for the taller ratio during power demand.
4.Just drove a Jetta TDI and I almost ended up hitting the windshield when I let off the accelerator. Trucks hauling heavy loads employ an exhaust brake. This is a throttle like valve in the exhaust collector that increases backpressure in the cylinders to amplify engine braking. Diesels burn far less fuel than gas motors at idle and partial throttle because they do not employ a throttle valve and therefore less pumping loss.
5.I use fuel cut decel alot. I am not a hypermiler but I do try drive conservatively.

Okay, so considering that diesels are much more efficient at idle and low throttle speeds than gasoline engines, would their BFSC charts have the sweet spot at a lower throttle amount, like say 20 or 30% than gasoline engines and therefore would P&G not be as fuel efficient with diesels?

Because friction is usually the biggest force. Pumping can exceed it on diesel compression brakes but friction is pretty huge. For peak efficiency the balance becomes how much fuel energy do you want to blow out the exhaust vs. increasing the proportion of fuel energy that is *not* going towards friction. Higher load produces more power but is thermodynamically less efficient in the absence of pumping work, but less power produced means a greater proportion of power went towards overcoming friction.

You're correct about engines consuming a lot of power at idle. The fuel flow rates look low until you realize actually going somewhere at moderate speed takes only maybe 2 times the fuel it takes to idle. Diesels consume quite a bit of fuel idling too because of friction, but since they don't have pumping to worry about it's quite a bit less.

Diesels having a lower peak efficiency load means that P&G is less effective, but it still beats running at very low loads.

As for how pumping (throttling) relates to friction, there are no real reliable friction figures for engines but ballpark numbers I've seen give that throttling is around the same magnitude of energy loss as friction when engine braking.

Still, engine braking is just like using your friction brakes except it doesn't wear anything down and you get a bit of free alternator charging, so you want to use it only to come to a stop.

Two, Wouldn't this mean that an engine that is idling uses a lot of power because the throttle plate is blocking airflow?

Idling uses a lot of fuel but makes little power. The plate is called a "throttle" for good reason--it's choking off the airflow...

13% of all the fuel used is wasted in idling engines in the US vehicle fleet.

Manifold restrictions in gasoline engines restrict the flow of air into the cylinders. The lower mass of air means lower compression in the cylinder which means much less power produced for the same amount of fuel consumed. This is because higher compression is a better "lever" to produce power. An engine produces power by compressing air and fuel then igniting the fuel, which expands the combustive mixture which produces pressure to push the piston and make power. If you reduce the in cylinder compression then you have much less power produced.

As was previously posted, in this thread, a diesel always has maximum compression. The diesel can run at air fuel mixtures as high as 50 to 1, so it will idle while using much less fuel than a gasoline engine becasue it can run at much leaner mixtures than a gas engine.

P&G utilises the peak BSFC of any engine by reducing to a minimum any partial compression operation. In a diesel P&G utilises the highest range of efficiency of that particular engine.

The most efficient hypermiler will eliminate all idling and acceleration will always be at peak efficiency. Measured by fuel consumption this means you are producing much more motive power for the same amount of fuel consumed.

regards
Mech

A dyno test I found showed 20HP of power for 1 unit of fuel at 20 HP for a 4 cylinder engine. When the load was increased to 50 HP the fuel consumed only rose by 50% while the power increased by 150%, at the same RPM.

This is because the in cylinder compression was increased from about 50% of atmospheric pressure to 100% of atmospheric pressure. With twice the actual compression and twice as much fuel, the engine produced 1.5 times as much power.

30 more horsepower on half again as much fuel. 50 HP on 1.5 or 2 HP on 1. The higher horsepower divided by the fuel consumption gives you 32.66 HP per unit, compared to 20 HP per unit at the lower load. Using the higher unit per fuel to add inertai to your vehicle means you can average the same speed while using a lot less fuel. The essence of P&G.

regards
Mech

Newer vehicles don't seem to engine brake as well. Vacuum on deceleration = NoX emissions.

Is this why people say to maximise the the mythical LOD thing on the SG2 which I still don't understand ? In which case I regard this posting as a potential magic key. I hope.


In what way - they stop sooner, no braking or no fuel shut off ? I noticed my TDI would shut off at anything over idle at all, the Aygo Petrol has to be over 1400 when you start engine braking for the ECU to detect it.

Most manual transmission vehicles i've driven with DBW throttles will not slow themselves down ... they just keep going.

the only way I can get my Focus to slow down without using the brakes, is to set the cruise control and hit the decelerate button on it - that shuts the throttle and lets the car slow down.

My focus cuts the injectors as DFCO, but does not shut the throttle

Why is it not then the case that the point of highest torque is the point of highest efficiency? I was discussing this with someone who thought this to be the case, and I found myself unable to effectively refute that except by saying that high rpm's are unefficient. I know that torque is technically ft.-lbs. but I have always thought of it as ft.-lbs per stroke of the engine. If torque is indeed the amount of work done per stroke of the engine, then the point of highest torque would be the point of highest compression ratio, and probably then the point of highest efficiency.

Is this right? I always thought that wasn't the case... I don't think NOx increases when you shut the throttle plate because nothing is being burned so there's no high temperature for NOx to form. However, leaving the throttle open under deceleration means that you're pumping cold air through the cats which cools them down, and that could be bad.

But it is the point of highest efficiency...
 

i think you answered your own question.

The peak torque that an engine generates does occur at the point of highest volumetric efficiency--when the airflow has the least restriction and the most power is made with the least amount of fuel. The engine speed that this occurs is the ratio of the power:torque.

Engines don't make speed, they make torque. Speed is what results when the load torques equal the generated torque from burning a certain amount of fuel.

Plot torque on the x-axis and speed on the y-axis of a BSFC chart with arcs of constant power to see this graphically.

Interesting, I haven't driven a DBW petrol engine (with a genuine throttle) to try this out - George is an old fashioned Fly By Pants throttle and is the first Petrol I've driven since 2002.

The TDIs would just shut off fuel because the demanded fuel was below the fuel needed for the engine to maintain its current speed - i.e. (for the ECU) it was on overrun.

I don't think the point of highest torque always lines up with the point of highest efficiency as seen on BSFC charts. For example, my brother's civic has a peak torque at 7000 RPMs. My car has a peak torque at 4750RPMs. Yet the common knowledge around here seems to be that operating your car over about 3000 rpms tends to be very fuel inefficient. What could account for the difference, or is there actually no difference and these car models really do produce power the most efficiently at these numbers.

Peak efficiency is pretty much never at peak torque since peak torque is when your volumetric efficiency is highest. More air, but also proportionally more fuel injected. Highest volumetric efficiency can be achieved at any point in the rpm range with cams and manifold/header tuning.

Peak thermal efficiency rpm is where lower cooling losses from running the engine faster balances out the increased friction and parasitic loss from running the engine faster. For "Otto cycle" engines (non-"Atkinson cycle") the gain in power from increasing volumetric efficiency/load doesn't balance out the increased amount of wasted pressure not captured by the expansion stroke that is simply blown out the exhaust (aka, more heat energy ending up in the exhaust).

[QUOTE=Miller88;331536]Newer vehicles don't seem to engine brake as well. Vacuum on deceleration = NoX emissions.[/QUOTE

I have not found this to be true. every newer vehicle I have driven engine brakes great, even the autos if you tell it to down shift.

My worthless anecdotal evidence is my TSX. It's a DBW manual, and it does DFCO braking just fine down to about 1,000 RPM.

Okay today as I was coasting down to a toll booth I tested cutting the ignition and putting my foot all the way down (I have cable throttle) to reduce pumping. Braking effect was not noticably decreased at 3000rpm (but hard to tell from seat of pants dyno). So I think the charts I've seen on the internet are probably about right, friction is on the order of 100-150 kPa mean effective pressure, pumping is at most maybe 70.

My point about using the RPM speed of the torque peak as the shift point was meant as guidance for running the engine up into a high(er) efficiency region for acceleration in order to minimize fuel consumption.

To do this you will spend less time accelerating overall and do it at a high enough power level to be operating the engine in it's most fuel-efficient region.

The Bosch Automotive Handbook has a section of empirical graphs and formula for internal combustion engines both gasoline and diesel. The torque-power-speed curve for gasoline engines shows the peak torque occurring at the minimum specific fuel consumption point at WOT. In addition the formula for peak torque is engine displacement times the peak cylinder pressure (BMEP).

The speed of maximum volumetric efficiency occurs when the airflow has the least restrictions and the cylinder filling is greatest and the pressure thereby peaks. Hence the torque peak occurs at the point of maximum volumetric efficiency.

Most of the BSFC charts that i have seen seem to show a tiny island of the absolute minimum bsfc slightly below the WOT line and slightly to the left of the torque peak-- but the point to be made here is to get up into those lower bsfc bands at the higher torque regions during those times when you accelerate (the "pulse" phase). You can never operate your car when cruising down the road (the "glide" phase) in that tiny zone of lowest bsfc--the power level is too high for gliding...

BSFC charts are few and far between. The point of highest volumetric efficiency pretty much dictates the torque peak as I said, and volumetric efficiency and thermal efficiency are very very different. As you noticed, minimum BSFC is usually below the WOT line. This is because running at WOT has the lowest throttling loss, and also the highest mechanical efficiency, but the last bit of power comes at a greater cost because the extra heat can't be extracted efficiently by the piston.

The reason you're seeing lowest specific fuel consumption close to the torque peak is because most engines are made "street friendly" with a small rev range and low-medium torque peak usually in the 3000-5000rpm range. Coincidentally, the size of almost all street engines (per cylinder displacement range on most engines is like in the 0.35L to 0.7L range and bore/stroke has a similar range) makes it so that their peak efficiency occurs in the 2000-3500rpm range. However a torque peak in the 4000s is not uncommon at all, especially for smaller cars, so you really can't go by torque peak.

Again there's very few BSFC charts out there but the ideal shift point is probably usually in the low-mid 2000s to low 3000s, depending on engine. If the efficiency drops a lot below 2000 then shifting from 1st to 2nd gear should probably happen at a higher engine speed to avoid dropping too low.

Okay that makes a lot of sense. Thanks.
Personally, I base my shift point less on the RPMs and more on speed. I find the lowest speeds I can go in each gear without lugging and shift into that gear at that speed. That way I can operate the car at almost WOT and low rpm's for more of my acceleration time than normal.
Also, BSFC charts are few and far between, but is there any way that we can determine BSFC charts for our own engine? For example, run some tests and figure out fuel consumption at certain points of throttle and rpm, then plot them and come up with a rough bfsc chart where we can roughly determine the sweet spot to shoot for. Has anyone done this before, or is there no way to do this as an individual?

Need a dyno that can put a variable load on wheels. If you have the engine out you need less parts but it's still hard to do. Then you also need something that accurately measures the fuel flow.

I think ultimately, there's no need to worry too much about how you're accelerating, because on the road it's more important to pay attention to traffic and stuff. The best we can do is try to shift up whenever we don't need power and try to strike a compromise between ideal load/rpm and ease of controlling power. Small engines probably aren't super efficient under 2000rpm in general but being slightly less efficient is okay if the lower amount of power available makes it easier to fine tune your acceleration in traffic and reduce energy wasted to braking. At least that's how I drive.

There is some truth to that. For example, I never do P&G in traffic jams as that helps to stop up traffic and is extremely hard not to brake more. However, as traffic permits, I really do think it best to operate the engine as efficiently as possible. As long as you need the power and it wont be converted into brake heat, it doesn't significantly matter how long or in what matter you accelerate, but rather how efficiently the power is produced.

And concerning BSFC charts, it would be smart for dyno centers to offer to produce BSFC charts for vehicles they test. I bet it would cost a lot though.

Have retired, and have moved from Boston to Colorado. I have also gone from a TDI to a Chevy Cobalt. With all the steep hills around here, I have a hard time with this Cobalt. The engine will not hold me back, and I have been nailed for doing 12 over. Let me go but is watching. In Boston, down shifting, I went 250 K miles with out a brake change, Would tend to hold the speed down in town.

So what is new?

Right, but you can never predict what's going to happen, and the miniscule gains you make from staying in this gear instead of that gear for 0.5 seconds longer can easily be wiped out, so I think it's best to not sweat it too much.

Producing BSFC charts isn't just about measuring the power accurately, you also need to measure the fuel flow rate accurately! That's not that easy. 3 gallons per hour fuel flow rate for example is only 0.05 gallons per minute which is under 250 mL a minute, or ~4mL per second. Can't just run the engine for like 20 seconds at a time, because then you'd be sitting on that dyno for quite a while, as you'd have hundreds of points to test.

I have a local hill. 27 miles of hill and grade. I took a run, than put 45 lbs in the tires, then a dropped them to 25 lbs. Used Cruse Control to maintain the Posted. It was a Proof of Concept. Altitude is about 7,500 ft.

The other day I let a fellow get on I-25; flashed, and pulled over to make room. Just as I got there, I saw the DEER lying in my High Speed lane. Hit the lights, brakes, and swerved to straddle the line. I always drive with the Canadian Day Time Lights.

I always use the seat belts. If you don't, you walk. No Talk. You walk.

Bottom line, by the Grace of God, I am telling you about it.

Nice story. And you bring up a point that I would like to emphasize, although it isn't about engine efficiency.Daytime running lights. I tend to keep my lights on all the time as I feel it makes people slightly more aware of my car.

My understanding of Alternators is that they constantly draw power from the engine and constantly produce a certain amount of amps. If you go over that amp amount, then you'll slowly lose power and your battery will die, but otherwise, electricity is free because whether you use 1 amp or 50 amps you aren't changing the load the alternator draws from the engine. Yet some threads here seem to think that your electricity use effects your fuel economy. I am pretty sure that's wrong and I've seen tests done running the engine without any electrical things going, then running it with everything full blast, and there wasnt any noticable difference between the two in fuel use. So to me there's no reason to not have my lights on all the time because electricity in cars is essentially free.

Your "understanding" violates the laws of physics. The alternator generates enough power to maintain system voltage around 13.8 V with the ECU, radio, and whatever other electrical accessories operating. The power to drive the alternator is proportional to the electrical load placed on it. ANY additional electrical load places a mechanical load on the engine.
I actually DO have a degree in electrical engineering.

so how exactly does it work that the higher electrical load places a greater electrical load on the engine? Considering that the rotor in the alternator will spin at the same speed as the engine regardless of electrical load, how does the electrical load put a load on the rotor? Does it somehow put drag on it with the magnetic field or something? I'm confused as to how this would work.

For a motorcycle yes, but not for cages...
 

Your understanding of an alternator is correct for a permanent magnet alternator such as is used on motorsickles--they run all the time and dump excess current back into heat. But we are only talking ~30 amps. The magnet drag is probably 1-2 hp in order to make .5 hp.

In most cars the alternator uses a claw-pole rotor that can be controlled as needed to meet the electrical load demand. The higher your electrical load, then a higher current into the rotor and a stronger magnetic field is created, and more mechanical power is required to turn the pulley. And visa versa for lesser electrical loads--the rotor can be turned off and freewheel.

My dad was also an electrical engineer. Was always saying that there was no FREE LUNCH. Then again he would laugh about it.

YIKES :eek:
Someone hurry and put some voltage converters on those things...1 hp for a few hundred grams is not a bad trade.

jonEmetro wrote:jake brakes

This is a Good question. Trouble is that Science is not taught in schools any more. Before World War 2, energy could not be created nor destroyed, but changed from one form to another. Mechanical to electrical to heat. If an engine is running, making mechanical, the alternator can change some into electricity, this can run the A/C, radio, lights, chemical, etc. The chemical is the battery, which can go, electrical, mechanical, (Running engine) Which is Chemical; Gas + Oxygen. You sort of have to think about it for a while.

You start with Gas, and you want to use as little as possible to get from here to there. You work at cutting your losses. The extreme is a bicycle, but that, this time a year is sometimes impractical.

Heat is a big waste; However in Boston, you might like to waste some and turn the Heater on. And so it goes.

Yes the alternator is harder to turn when putting out more power.

amps X volts = watts (a measurement of work/power)

RPM X torque / 5252 = horse power (a measurement of work/power)

So work done with electricity is replaced with work done by spinning the alternator.

If the RPM does not increase the torque has to goes up.

If the output of the alternator was the same all the time (would have to be it's peak rating) the extra energy given off would have to be dumped likely into a heat sink for a car running an 80 amp alternator at 14 volts makes a little over 1000 watts. A quick search for 1000 watt heat sink shows them to be 14 lbs of aluminum inches and finned to cool it for when you were not using all of the alternators capacity. The volume of such a heat sink is about 380 cubic inches, I have never seen such a heat sink on a car.

engine braking
 

Greetings: Although I am new to this forum, I am far from a Noob at automobile theory.

Engine braking in class A diesel engines employ a hydraulic device in the rocker arm box to open the exhaust valve at the moment of TDC compression to release the energy into the atmosphere.

Engine braking on a Medium duty diesel engine uses an exhaust pipe valve to prevent the exhaust gas flow thereby slowing the engine until released.

Both of these systems shut off the fuel during the braking event.

A gasoline engine is a little more difficult. It uses it's inherent operational friction, but it still pulls fuel from the idle circuits in the carb, or the engine idle function in the ECU, unless it was mapped differently.

It is a myth that automatic transmissions "free wheel" when engine braking. They are always coupled to the engine, albeit less than a manual transmission would be.

Most latter day automatics use a lock-up type converter to prevent the converter turbine from overheating when in an over drive ratio. They usually lock up in mid-third gear.

With the use of an aftermarket shift modifier, the shift points and lock-up can be controlled. Some even resort to a manual switch to lock the converter early to squeeze out better fuel economy.

My EV uses Regenerative motor braking to stop the vehicle. It is so impressive that I rarely use the hydraulic brakes at all. The side benefit is that I recapture about 10% of the energy I used to accelerate.

An early form of this was used in heavy haul trucks in the form of a "retarder". That was a field winding built into the flywheel area of the engine that slowed the truck by electric generation of current, but it was wasted by turning into heat and radiated into the atmosphere.

Miz

Your Dad was right. In fact, in modern consumerism, FREE is about the most expensive option :-)

True that about torque converters. The early one's I think Buick put them on their Roadmasters had a free wheeling TC. Don't know if that was hype or reality. Then, the early 2 speed TorqueFlites (Mopar) had a double pump so they could be push started to overcome the "can't push start" objection from the day. They later had to drop the pump to fit a 3rd gear. I heard, yet don't know, that TESLA was going to use a modified 2spd PowerGlide (GM) without a TC on one or more of their models. The PowerGlide, tho often called a "slushbox" uses, I've read 19HP, making the least power hungry of autos. Unfortunately the limited gear ratios make it less efficient. Don't know what a 3spd manual wastes in power if one includes shifting gears in the mix. They do get hot yet not nearly as hot as an automatic.

The "best" automatic I ever saw was the one VW put on the late run of Buses. It was something of a torque doubler getting going and had an internal fan in the TC. I saw some figures where it got better gas mileage than the average person using a clutch. Of course, the average person, ain't none of us, right?