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Old 09-29-2012, 09:01 PM   #1 (permalink)
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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.

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Old 09-29-2012, 09:48 PM   #2 (permalink)
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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.
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Old 09-29-2012, 09:57 PM   #3 (permalink)
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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.
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Old 09-30-2012, 12:54 AM   #4 (permalink)
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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?
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Old 09-30-2012, 01:35 AM   #5 (permalink)
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Quote:
Originally Posted by wobombat View Post
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.
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Old 10-02-2012, 07:14 AM   #6 (permalink)
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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...
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Old 10-02-2012, 08:18 AM   #7 (permalink)
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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.

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Old 10-02-2012, 08:38 AM   #8 (permalink)
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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.

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Old 10-02-2012, 01:12 PM   #9 (permalink)
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Newer vehicles don't seem to engine brake as well. Vacuum on deceleration = NoX emissions.
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Old 10-02-2012, 02:43 PM   #10 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
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.

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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.


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

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