Exploring Engine Braking and Compression Effects

[gmax137]

Yes its really tough to push because you have to do the work to compress the first intake charge, but notice how it suddenly gets really easy after that piston clears TDC?.

Maybe, if you're bump starting a single cylinder bike, but I really doubt you'll feel that on a four cylinder car (much less a six).

The intake charge is essentially an elastic medium in a closed system. Whomever said that the intake charge is acting like a spring is completely correct as I see it.

How do you figure it's a closed system? There will be some air pumped through. And even if not, the compression isn't adiabatic.

Finally, this idea about pumping the air around beneath the cylinders needs more thought - on a multicylinder engine I'd bet there isn't much change in that volume (since there are pistons rising & falling at the same time. Maybe the air pressure within the crankcase isn't rising & falling, but the air is being pushed about, leading to fluid friction. That would explain the drag racing vacuum pumps. But I'd bet the effect is small.

 

[Averagesupernova]

This thread seems to be going in different directions and those who are going in one direction are pointing at the wrong answers by those going in the other direction. In reality, none of this is apples to apples.
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The air beneath the cylinders issue: It's probably an issue at engine speeds above the average engine speed of most cars on the public roadways. Besides that, isn't doing much.
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Losing the spark plugs issue: Sure, taking them out makes an engine turn over like someone left out the conneting rods and pistons. But try turning over that engine that is missing its plugs at 3000 to 5000 RPM and you will find there is plenty of drag. Lots of it coming from dragging air in and out of the spark plug hole on the compression and power stroke but plenty from plain old friction of the engine parts too.
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REAL engine brakes (Jake brakes) open the exhaust at the top of the compression stroke. I believe this is only done on diesels. This allows the engine to absorb power on the compression stroke and release it out the exhaust valve at the top of said stroke instead of allowing the cushion of air built up to push the piston back down again during the power stroke. I would say that the engine braking on our everyday drivers comes from mainly friction inside the engine, drawing air into the engine against a very high vaccuum, and whatever restrictions and friction the air encounters during the exhaust stroke which might not be much considering not a lot of air is drawn in in the first place with a closed throttle. The compression and power stroke is a what-you-put-in-is-what-you-get-out type of thing.

 

[Gnosis]

This thread seems to be going in different directions and those who are going in one direction are pointing at the wrong answers by those going in the other direction. In reality, none of this is apples to apples.
-
The air beneath the cylinders issue: It's probably an issue at engine speeds above the average engine speed of most cars on the public roadways. Besides that, isn't doing much.
-
Losing the spark plugs issue: Sure, taking them out makes an engine turn over like someone left out the conneting rods and pistons. But try turning over that engine that is missing its plugs at 3000 to 5000 RPM and you will find there is plenty of drag. Lots of it coming from dragging air in and out of the spark plug hole on the compression and power stroke but plenty from plain old friction of the engine parts too.
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REAL engine brakes (Jake brakes) open the exhaust at the top of the compression stroke. I believe this is only done on diesels. This allows the engine to absorb power on the compression stroke and release it out the exhaust valve at the top of said stroke instead of allowing the cushion of air built up to push the piston back down again during the power stroke. I would say that the engine braking on our everyday drivers comes from mainly friction inside the engine, drawing air into the engine against a very high vaccuum, and whatever restrictions and friction the air encounters during the exhaust stroke which might not be much considering not a lot of air is drawn in in the first place with a closed throttle. The compression and power stroke is a what-you-put-in-is-what-you-get-out type of thing.

First off, this thread isn't about Jake Brakes. It’s about what causes the engine-braking on an automobile. Since production automobiles don't employ the use of Jake Brakes, they simply don’t apply here.

More importantly however, you've completely missed the point about what is proven by removing the spark plugs!

By removing the spark plugs, the engine-braking per a given RPM is INSTANTLY virtually eliminated! This demonstrates that “MAJOR ENGINE-BRAKING” ISN’T caused by the air being moved under the pistons!

If the air being moved under the pistons was the actual cause of “major engine-braking” (and it absolutely is NOT), then removing the spark plugs would fail to accomplish the elimination of the engine’s major engine-braking.

Additionally, with the spark plugs removed, engines can be spun up to high RPM without much resistance and I’ve demonstrated this on a number of engines in the past. A 3,000 pound car on a hill rotates the crankshaft with the greatest of ease when the engine’s spark plugs have been removed, and I’ve demonstrated this as well. For all practical purposes, major engine-braking is eliminated once the spark plugs have been removed, which demonstrates that major engine-braking is the result of compression related aspects of the combustion engine, which includes the energy required to draw in and exhaust air at very high rates. Consider how short the entire duration for air intake is at 1,000 RPM and 10,000 RPM.

1,000 RPM / 60 seconds = 16.666 crankshaft Revolutions Per Second

1 second / 16.666 RPS = .06 seconds per single crankshaft revolution

.06 seconds / 2 = .03 seconds (the actual piston down-stroke time per 1,000 RPM)

Naturally, 10,000 RPM yields a piston down-stroke time of a mere .003 seconds!

Now consider the piston down-stroke time of a new Suzuki GSXR600 with its redline of 16,500 RPM! It only has .001818 seconds at that RPM to fill its cylinder with air. Its torque drops off considerably, so each A/F mixture releases less energy as RPM increases however, it produces 1,000’s more of these lesser energy releases per minute, which still makes the little bugger a bit of a terror.

 

[Averagesupernova]

Gnosis, lighten up.
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I didn't really miss the point of removing the plugs. I pointed out that what you say is true by removing the plugs and turning the engine over with a wrench shows that a lot of drag is eliminated and that the assumption is that most of the drag is compression related. What I ALSO pointed out is that I wasn't sure if the removed plugs have the same effect at 3000 RPM. So no, I didn't miss the point. But your experiments of rolling a car down a hill with plugs removed seem to indicate that drag is in fact significantly reduced.
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So my question to you is this: What does it sound like when an engine without plugs spins up to a couple thousand RPM being pushed by a car rolling down hill? And is there a difference with the throttle shut or open?

 

[xxChrisxx]

Engine braking is generally understood to be caused by compression which is end of discussion really.

Yes there are other factors acting at higher RPM that will tend to want want to slow the rotation of the crank but they are an irrelevence compared to the pumping loss. The throttle plate being open or shut will not make a bit of difference if there are gaping holes above the cylinder.

 

[Averagesupernova]

Thought this was relevant even though 'this thread isn't about Jake Brakes'. The article mentions gasoline engines as well.

http://en.wikipedia.org/wiki/Jake_brake

 

[Gnosis]

Gnosis, lighten up.
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I didn't really miss the point of removing the plugs. I pointed out that what you say is true by removing the plugs and turning the engine over with a wrench shows that a lot of drag is eliminated and that the assumption is that most of the drag is compression related. What I ALSO pointed out is that I wasn't sure if the removed plugs have the same effect at 3000 RPM. So no, I didn't miss the point. But your experiments of rolling a car down a hill with plugs removed seem to indicate that drag is in fact significantly reduced.
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So my question to you is this: What does it sound like when an engine without plugs spins up to a couple thousand RPM being pushed by a car rolling down hill? And is there a difference with the throttle shut or open?

Averagesupernova, try to relax. It wasn’t like I was jumping down your throat. In your statement that “a lot” of drag is eliminated, I realize that you’re still failing to grasp the magnitude of engine-braking that has been eliminated simply by removing the spark plugs. “A lot” as you put it, is a fairly subjective term. Some might consider 6 inmates out of 100 escaping from prison “a lot” whereas others might consider 6 inmates out of 10,000 “a lot”. I’m talking about the greater portion of engine-braking having been virtually eliminated simply by removing the spark plugs. Now do you see how very different the magnitude of your statement is from mine?

Now, to answer some of your questions…

Rolling down a hill while attempting to use engine-braking with the spark plugs removed proves to be an exercise in futility. Yes, the air can easily be heard moving in and out of the spark plug holes especially as crankshaft RPM increases to higher RPM. It sounds much like a steam engine on steroids.

Air Intake

Bear in mind that air isn’t being drawn into each cylinder solely through the spark plug holes. The intake valve is also opening during the intake down-stroke thereby providing an additional path to further reduce air intake resistance.

Spark Plug Size

Some spark plugs (especially prevalent in older car engines) have larger threaded bases than others, so some provide larger spark plug holes than others, which further alleviates cylinder pressure and reduces engine-braking.

Engine’s Compression Ratio

Typically, the higher performance your engine, the higher will be the cylinder compression ratio. Many old cars were only 9:1 compression when new and in good condition whereas my newer car makes use of an 11:1 compression ratio. The lower the compression ratio, the lesser will be the engine-braking even with the spark plugs removed.

Now, here’s an additional significant point that most will never realize on their own when using the engine for braking with the spark plugs removed...

…When the piston is traveling down on its power-stroke, it draws air in through the spark plug hole. At extremely high RPM, cylinder pressures will tend to drop during power-stroke (low cylinder pressure, as in heading toward vacuum). If the cylinder pressure were to drop below the pressure required to compress the valve springs, the low cylinder pressure can actually pull the exhaust and/or intake valves open to alleviate this low cylinder pressure condition, which further eliminates any significant engine-braking.

Think about it. There is no mechanism in the head (other than the valve spring tensions, which aren’t very high on production vehicles) to prevent the valves from being pulled open under these “low cylinder pressure/high crankshaft RPM” circumstances. Naturally, this doesn’t occur during normal engine operation. It only occurs due to the altered dynamics when the spark plugs have been removed and the engine is being used at extremely high RPM, as when used for engine-braking.

Intake and Exhaust, the Creation of Pressure Zones

When a combustion engine is running normally, its air intake times are quite short in duration. Since a cylinder’s worth of air must be moved quite rapidly (in just .03 seconds at 1,000 RPM), the piston’s down-stroke speed is high in order to create a near instantaneous low cylinder pressure zone in which normal atmospheric pressure will flow to fill the cylinder. This creation of a near instantaneous low pressure zone requires energy. This contributes in robbing the engine of some of its kinetic energy and is part of the normal engine-braking component normally associated with compression related components.

At 10,000 RPM, the cylinder only has .003 seconds (10 times less) to create a near instantaneous low cylinder pressure to cause the same 14.7 PSI atmospheric pressure to flow into the cylinder with hopefully, the same volume. At this increased requirement to create a nearly instantaneous low pressure zone, additional energy is required and at this extreme RPM, additional kinetic energy is bled off in the form of engine-braking.

The pushing and pulling of air does require energy and this becomes significant as durations decrease significantly to move the same volume of air.

Likewise, the same is true for expelling the spent exhaust gases, so the up-stroke during the exhaust gas expulsion process also introduces a bit of engine-braking.

Summarizing:

Merely removing the spark plugs virtually eliminates “major engine-braking”.

 

[Ranger Mike]

Piston Drag - Consider that an engine with 85 percent mechanical efficiency loses 15 percent of the power produced in its cylinders to friction. In a 150 horsepower engine, that’s 22.5 horsepower that never reaches the flywheel. Last year we tried a "gapless" total seal piston ring combination and it was not the hot set up...we were down a whole bunch of H.P.If you can recover even a small percentage of these parasitic losses by minimizing friction (and windage..another subject), then you will have more net power to accelerate your race car. You don’t need to buy a new camshaft or a set of trick cylinder heads to realize these gains – you simply have to liberate more of the power that the engine already produces by improving its mechanical efficiency. The major sources of friction in an engine are piston skirts and piston rings. You can’t do much to affect the skirts, but you do have choices when selecting piston rings. When you rotate the crankshaft assembly in a short-block, you can feel just how much drag the piston rings produce

We use that second ring to fine tune the ring package. For example, if a motor needs just a little more oil control, we might install second rings that have been back-cut to a radial thickness of .175-inch instead of rings with .160-inch radial thickness. Often a small increase in second ring thickness (and a resulting increase in static tension) will dry up the engine with only a pound or two of additional drag. To get a comparable gain in oil control by increasing the oil ring tension could add five or more pounds of drag.

Another great friction-saver is the three-millimeter oil ring. Almost 10 ft-lb. less torque is required to spin the assembly by hand. You can check this figure with a torque wrench. While these rings offer terrific life (many stock production engines use them), their narrower radial dimension promotes improved cylinder conformity and oil control. It's important to remember that the No. 1 source of friction in an engine is piston ring drag. In a typical big-block V8 engine, each of the 24 rings is dragged up and down the cylinder walls more than a mile every minute. The top ring only performs useful work in the first few inches of the power stroke; the rest of the time, it's just soaking up power. The second and oil rings don't contribute to power at all - they're scraping oil at the cost of more friction. That's why reducing ring tension can dramatically increase engine output.
There can easily be a 30-horsepower difference between an off-the-shelf low-tension ring package and an optimized ring combination. If you install oil rings that pull 28 pounds of drag on a fish scale and full-width second rings, you can be assured that the engine is not going to smoke. However, it's also not going to make as much power as a motor with handpicked low-tension oil rings and back-cut second rings. That's 224 pounds drag for a V-8 full race mill!

We build our engines as close to the lower limit on ring tension as we can without stepping over the edge. A racing engine shouldn't put out enough blue smoke to kill every mosquito in the county, but it should be very close to the line. We religiously check the ring tension in every short-block we build; it's as critical as checking the bearing clearances. Unless you measure the ring drag in every cylinder, you can't be certain that a box of rings wasn't mislabeled or a set of expanders wasn't too stiff. One huge reason we fish scale every piston, ring after the rings are compressed and inserted into the cylinder, is to find out if we broke a piston ring during installation.

Depending upon the cubic inch, piston to wall clearance, type rings used a piston.ring assembly may take as much as 200 in/lbs. (17 ft/lbs.) to reach “break away torque”. In other words..it can take up to 17 ft/lbs. torque on the crankshaft bolt, with a torque wrench, to start to turn the SINGLE PISTON/RING assembly. That's 136 Ft/lbs. for a V-8.

Valvetrain - in a typical internal combustion engine comprises several moving components. Some are rotating and some are reciprocating. Valves that are operated by rocker arms or tappets, with valve springs used to return the valves to their seats. This is the main cause of valve train drag..the spring pressure. Parasitic power losses are major - power is wasted in accelerating and decelerating the components of the valvetrain. Friction of the camshaft, springs, cam belt or chain, robs HUGE H.P, Dynometer research tells us the power draw on the crankshaft to operate the conventional valve train is 5 to 10 percent of total power output.

So you can see. the mechanical draw on the engine is huge...with spark plugs installed , you got one big air brake...

hope this helps