What Are the Effects of Engine Braking and Compression on Car Performance?

[LeoKent]

I am starting to program a simple physics model of a car to help me understand how it works. I am certainly going to have to make quite a few simplifications as I'm only just about to start A levels.

Whilst researching how to simulate engine braking I came across the Racer (racing simulator) physics reference page which says...

When the engine is spinning, it has the tendency to slow down, mostly because of the compression effects in the cylinders.

Is this an accurate statement, and if so what are those compression effects? Why do they result in Engine Braking \propto Engine Speed?

Finally, the site suggests that braking torque = braking coefficient x rps, is this accurate and how can a braking coefficient be calculated or approximated?

Thanks,
Leo

 

[mgb_phys]

You know how a piston engine works?
Inject fuel, fuel burns, expands, pushes down piston. Piston comes back up and pushes exhaust gases out.
Now imagine it with no bang.
Air comes into the piston, the piston goes down trying to pull against the partial vacuum created, the piston then goes up trying to compress the gas in the cylinder.
(This might be oversimplified for modern fuel injection engine with electronically controlled valves)
The amount of torque needed to push the pistons around will depend on the capacity of the engine and the gear ratio but would be proportional to the engine RPM because that is simply the number of piston compressions per second.

 

[LeoKent]

Right OK, that's pretty obvious now I come to think of it! Thanks.

 

[rcgldr]

Most of the engine braking effect is due to the movement of air underneath the pistons, not above them. You can try this for yourself. Coast down a hill, turn off your engine, and vary the throttle settings, there will be no perceptible difference in engine braking. The air is reasonably elastic, so the losses above the piston are small compared to losses below the piston, where air is being moved back and forth between the pistons in an engine.

Pro-street drag motorcycles use a vacuum pump to evacuate the crankcase to reduce the loss of power from the movement of air underneath pistons.

This article on Jake Brakes explains it: "forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft"

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

Devices like Jake Brakes release compression at the right time to enhance engine braking, but few cars have this. On two stroke engines, just a compression release will increase engine braking (although the purpose is to make it easier to start the engine).

 

[mgb_phys]

Interesting - the friction of air flowing in the crankcase is greater than the compression of the air in the cylinders?

Coast down a hill, turn off your engine

Before trying this make sure you don't have a steering lock and you know how little braking/steering you have left without power!

 

[AlephZero]

I would have thought the largest engine braking effect was the friction between the piston rings and cylinders. Consider how rapidly an idling engine stops when the ignition is switched off. It's hard to believe that deceleration comes mainly from pushing a bit of air around under the pistons, at low RPM. Turning an engine over by hand (with the ignition OFF!) will show you how much torque is needed to overcome friction. Removing the spark plugs so there is no compression doesn't make much difference.

Of course if you want to screw the last 0.1HP out of your engine, reducing the crankcase pressure may help - I admit I know nothing about drag racing engines.

 

[Danger]

We used pan-evacuation systems on cars, too. It's worth a couple of percent increase in hp (not sure how much, but it's probably comparable to an H-pipe in the exhaust). Every little bit helps.

 

[rcgldr]

We used pan-evacuation systems on cars, too. It's worth a couple of percent increase in hp.

Pro stock bikes rev around 13,000 rpm, it's enough that Vance and Hines used 3 vacuum pumps on their bike until the rules limited the number of vacuum pumps to just one.

friction

Yes, there's a lot of friction. I was just pointing out that the movement of air back and forth below the pistons has much more braking effect than compressing air and letting it push back on the pistons.

 

[BobG]

Interesting - the friction of air flowing in the crankcase is greater than the compression of the air in the cylinders?

Coast down a hill, turn off your engine, and ...

Before trying this make sure you don't have a steering lock and you know how little braking/steering you have left without power!

Yes, definitely!

Periodically, we have tourists use their brakes exclusively to descend Pikes Peak (instead of using a lower gear that would provide the engine braking you're talking about). The sneaky ones manage to bypass the mandatory brake checkpoint a little over halfway down (you have to have the temperature of your brakes checked by a forest service technician).

The reaction of a driver when their brake fluid boils on the way down a mountain can be poignantly pathetic, especially when the car is filled with the wife and kids of the driver. It's almost a haunting effect to be driving up the mountain and have a car come rolling silently down the mountain and to stare at the terror filled faces peering desperately out the window. Obviously, a driver couldn't possibly slam their car into park as it rolled down the hill at 45 - 50 mph. That would ruin their transmission. Better to shut off the ignition, lock up the steering wheel, and hope the car can withstand plunging off the side of a cliff once the first turn arrives.

 

[mgb_phys]

Used to work on an observatory that was about 2500m above and an hours drive from the city - you could do the run entirely in freewheel with practice.
One day somebody decided that they might as well turn the engine off since they weren't using it, first hair pin bend the steering lock went on -fortunately they went into the side of the mountian, not over the edge.

 

[Averagesupernova]

Can you explain to me how you can get the steering to lock without putting the vehicle in park? Any vehicle I've ever been in with an automatic transmission and a steering lock you CANNOT lock the steering without putting the vehicle in park just for the reasons discussed above.

 

[mgb_phys]

Who drives an automatic when your commute to work involves going up a 2500m mountain? Most manual cars go into parking lock as soon as you tunr the key past off and turn the wheel more than about 1/8 turn.

 

[Danger]

In any event, just the loss of the steering boost can be quite unnerving on a hill, even if it isn't actually locked.

 

[Averagesupernova]

Who drives an automatic when your commute to work involves going up a 2500m mountain? Most manual cars go into parking lock as soon as you tunr the key past off and turn the wheel more than about 1/8 turn.

I've never seen a stick-shift with a steering lock that doesn't have the little extra button on the opposite side of the steering column from the key which needs to be pushed in order to get the key to all the way back to the lock position.

 

[mgb_phys]

You must be in the USA - I remember having a hire car there once with so many safety interlocks ie. must be in park and the hand brake on and press the foot brake all the way to the floor while having both hands on the steering wheel to turn the engine on.

It is amazing how much steering and brake force you lose with the engine off as Danger said, I just moved my wife's new car a couple of m down the drive by putting it in neutral and letting it roll - my first experience of expensive car with power everything.

 

[rcgldr]

This is getting off topic. I've actually done this on a car with a stick shift. Since this is a thread about engine braking, I'm ignoring cars with limited engine braking, such as slipper clutches used on some race cars. If you're getting significant engine braking, then the engine rpms are reasonably high enough for power steering and power brakes to work just fine.

Regarding the steering lock, this depends on the car. Newer cars with keyless systems and just buttons for starting the engine don't have steering lock at all (for obvious safety reasons). Then again, I'm not sure you can turn off the engine on an all button car while the car is moving. For cars with steering locks, check to see if the engine can be turned off without engaging the steering lock while the car isn't moving.

So my suggested experiement is limited to those who own cars that don't have a steering lock issue. As an alternative a motorcycle could be used to try this experiement, or you can just take the word of the guys that did the wiki article about Jake Brakes that I linked to earlier.

Also if you try this experiment on a carbureted engine, you could fill the exhaust with gas fumes that could blow holes in the exhaust when you turn the engine back on. Stop quickly and restart the engine after waiting a bit.

 

[mgb_phys]

At the risk of getting on topic - is my understanding of modern engines correct?
In neutral with no load the engine runs normally at <1000rpm burning fuel.
Going downhill in gear but with no accelarator the engine turns at the same RPM that it would normally run at, in that gear at that speed.
No fuel enter the cylinders but the valves open and close on each cycle so the air is only compressed once then re-expands - since this is fairly elastic the main losses are friction in the engine and gearbox/transmission.
But in an old carburetor system the same amount of fuel would enter as on idle (the slow running jet) and the the spark plugs would still fire, so presumably this fuel would burn in a very dilute mixture, since the RPM is much higher.
What happens on a modern fuel injection car? Does it put any fuel in at all when in gear with no accelerator?

 

[rcgldr]

the main losses are friction in the engine and gearbox/transmission.

Plus all the movement of air back and forth between cylinders underneath the pistons. At higher rpms, most of the engine braking is due to this movement of air.

What happens on a modern fuel injection car? Does it put any fuel in at all when in gear with no accelerator?

Depends on the car. My wife owned one such car, a 1989 Ford Probe that completely shut off the gas during engine braking with no throttle input, the instant fuel milage readout would suddenly transition to 99mpg, it's max reading, when the fuel was shut off.

Off topic, one other feature that the Probe had was a automatic tranny with an internal mechanical clutch to bypass the fluid clutch one the car was "cruising", it felt similar to having an extra gear in the car and improved gas milage (fluid clutches consume energy). I don't recall if the mechanical clutch remain engaged during engine braking. I also don't remember this lock up clutch feature an any of the cars she's since bought after the Probe. All of the cars I've owned since 1980 are stick shift.

 

[AlephZero]

What happens on a modern fuel injection car? Does it put any fuel in at all when in gear with no accelerator?

Mine does some of the time, at least. (UK, stick shift, small Vauxhall/GM built in 2000).

With no accelerator it tries to maintain the normal engine idle speed (about 800 RPM). That's quite useful when creeping along in traffic in 1st/2nd/3rd gear. Almost like low-speed cruise control.

I don't have a trip computer, but from the way the car behaves when decelerating slowly with no accelerator, I suspect it starts to feeding in fuel when the RPM drops to about 1500. There seems to be a step change in the rate of decel at about 1500RPM (though it's not noticeable with moderate or hard braking).

 

[BobG]

At the risk of getting on topic - is my understanding of modern engines correct?
In neutral with no load the engine runs normally at <1000rpm burning fuel.
Going downhill in gear but with no accelarator the engine turns at the same RPM that it would normally run at, in that gear at that speed.
No fuel enter the cylinders but the valves open and close on each cycle so the air is only compressed once then re-expands - since this is fairly elastic the main losses are friction in the engine and gearbox/transmission.
But in an old carburetor system the same amount of fuel would enter as on idle (the slow running jet) and the the spark plugs would still fire, so presumably this fuel would burn in a very dilute mixture, since the RPM is much higher.
What happens on a modern fuel injection car? Does it put any fuel in at all when in gear with no accelerator?

Actually, the engine RPM depends on the car and the gear. Going downhill and taking your foot off the accelerator, the car will noticeably slow (if the gearing is low enough), but it eventually starts to speed back up, although at a lot slower pace than if you were in neutral. The engine might try to maintain an idle speed, but it only resists acceleration due to gravity, not negates it. (It's probably more accurate to say the perception of the RPM depends on the gear. Low gears definitely resist acceleration a lot better than high gears.)

And even fuel injection cars inject enough fuel to at least maintain idle speed. In fact, mine will go uphill with no accelerator when in 1st gear and 4WD Low.

 

[mgb_phys]

In idle on carb systems there is a 'slow running jet' which leaks enough fuel into keep the engine ticking over at 500-800rpm and this will allow you to roll along in low gear.
I wondered if with fuel injection when you are going down hill and the engine is being dragged round by the gearing does the injection system 'know' that it isn't going to stall and so turn off the fuel completely?
Does a fuel injection system control the fuel flow or does it only handle the spark and valve timing?

My wife's new car is a 4WD subaru and it definielty doesn't free wheel as fast in gear - presumably because of the friction of the extra transmission+wheels.

 

[rcgldr]

I wondered if with fuel injection when you are going down hill and the engine is being dragged round by the gearing does the injection system 'know' that it isn't going to stall and so turn off the fuel completely?

As previously mentioned, it definitely shut off the fuel completely whenever there was moderate engine braking on my wife's 1989 Ford Probe. I haven't tested our current cars, both have an instantaneous fule milage readout, and it's clear that fuel is cut off when the gauges jump to the max reading.

 

[brewnog]

I wondered if with fuel injection when you are going down hill and the engine is being dragged round by the gearing does the injection system 'know' that it isn't going to stall and so turn off the fuel completely?

All modern fuel injection systems shut off the fuelling completely on the over-run. Only below (or approaching) idle speed will they fuel up to stop the engine cutting out. This is why using engine braking to roll up to a red light is more economical than coasting.

Does a fuel injection system control the fuel flow or does it only handle the spark and valve timing?

On a spark ignition engine, only fuel flow is controlled by the fuel injection system. (Systems like Mitsubishi's GDI are obvious exceptions to this). Spark timing (and possibly duration and energy) is controlled by the engine management system. Valve timing is static on most cars, otherwise it is ECM controlled, as with VTEC, VVT and those fancy Ferrari engines.

 

[benr360]

I just found this in a Yahoo! search, I thought I would share my findings on this topic.

I am a hyper miler. For those of you that aren't familiar with the term, basically I put conscious effort into maximizing the fuel efficiency of my car at all times, which is really only fruitful with a manual transmission. Basically, put quite simply, the cornerstone of raising your fuel efficiency is to know, understand and implement the fact that a running engine in a stationary car is getting zero MPG, and a moving car with the engine shut off is theoretically getting infinite MPG, as you are simply raising the miles driven, and not the fuel used.

What I have experimented with, and have found, is that in a modern fuel injected car, your engine will not inject fuel if the throttle is closed(technically nine degrees open according to my Scangauge), so engine braking down a hill is getting you the same MPG as you will by coasting with the engine turned off. I have done a test on a very steep hill in my city, and the car does indeed gain more speed going down the hill in neutral than going down in any gear without touching the throttle. In fact it makes a difference of almost 10 MPH at the bottom of the hill, starting at the top at the same exact speed on back to back attempts. Also, you should be able to feel actual torque being made by the engine once it gets low enough to where the computer tells the injectors to start injecting again, which is pretty close to 1000 rpms. You will feel the car kind of lurch forward. You can also achieve the idle roll that automatic cars have. If you are on even ground, your engine will make enough horsepower at idle to keep the car moving forward; in first gear, it will not stall if you simply keep coasting.

Not only do hyper milers utilize this not-so commonly known function of modern cars, but also professional drivers do on road coarses, by way of heel-and-toe driving(operating the clutch with left foot, braking with right toes, and applying throttle with right heel during downshifting entering turns). Also, driving a manual car is beneficial for driving in snow. As I almost found out the hard way once, it does nothing on ice, but engine braking on snow that is deep enough to give some rolling resistance is so effective, that I have even stalled my engine by downshifting in snow, as the engine/wheels simply stop spinning initially if I let the clutch out quickly.

I would like to think that, bringing physics into the mix, there are a few different reasons for this, with pretty much all of them being listed already. The compression of simple air should theoretically impede the engine from spinning, and cause it to slow down, because there is no combustion to make the power stroke any more pressurized than the compression stroke. So, the variable here is the friction. I would think that even if you removed the valves from the engine, the engine would still slow down as fast, due to the friction of the pistons, rings and walls. You just wouldn't have the compression and power strokes canceling each other out.

 

[KLoux]

How do you determine whether or not the fuel injectors are injecting fuel into the engine? Are you measuring the injector pulse width, or are you just looking at your fuel milage gage? A maximum reading on a fuel mileage gage is not the same as infinite fuel mileage, which is what you would have if your engine wasn't burning any fuel, right?

I don't know for sure that fuel is not being injected into the engine if you are going downhill and engine breaking (it may depend on the car) but I suspect that fuel IS being injected, just a much smaller amount than normal.

I believe that the hypermiler in the above post is wrong on one point. When your throttle is "closed" there is still air getting into the engine, because some amount of power is required to overcome friction and all of this air moving about in the bottom half of the engine. To create power, this air needs to be mixed with fuel (a small amount) before it is ignited. There is a specific ratio of fuel to air that your engine's computers are trying to achieve, and it largely depends on what position your throttle is in (how much air is getting into the engine) but might also depend on things like air density and temperature on that particular day, the load on the engine, etc.

 

[brewnog]

I believe that the hypermiler in the above post is wrong on one point. When your throttle is "closed" there is still air getting into the engine, because some amount of power is required to overcome friction and all of this air moving about in the bottom half of the engine. To create power, this air needs to be mixed with fuel (a small amount) before it is ignited.

At idle, yes. On the over-run (closed throttle at higher engine speeds) no. Most systems completely shut off the fuelling until idle speed is resumed.

 

[benr360]

How do you determine whether or not the fuel injectors are injecting fuel into the engine? Are you measuring the injector pulse width, or are you just looking at your fuel milage gage? A maximum reading on a fuel mileage gage is not the same as infinite fuel mileage, which is what you would have if your engine wasn't burning any fuel, right?

I don't know for sure that fuel is not being injected into the engine if you are going downhill and engine breaking (it may depend on the car) but I suspect that fuel IS being injected, just a much smaller amount than normal.

I believe that the hypermiler in the above post is wrong on one point. When your throttle is "closed" there is still air getting into the engine, because some amount of power is required to overcome friction and all of this air moving about in the bottom half of the engine. To create power, this air needs to be mixed with fuel (a small amount) before it is ignited. There is a specific ratio of fuel to air that your engine's computers are trying to achieve, and it largely depends on what position your throttle is in (how much air is getting into the engine) but might also depend on things like air density and temperature on that particular day, the load on the engine, etc.

I can't find it, but in the past I have seen people post manufacturer information stating that the injectors completely shut off during engine braking. The Scangauge reading 9999 mpg is good enough for me :P Also, feeling the injectors kicking back in when the engine falls to the proper RPM is very distinct.

I believe that I also stated that the throttle is still open nine degrees when it is "closed" also. I'm not sure on overcoming the air in the bottom of the engine though. I would have thought that's what a positive crankcase ventilation valve is for. Maybe it is the pressure in the bottom of the engine that is largely responsible for slowing the car down, because if you're right, this pressure would cause the engine to lose speed gradually.

 

[mgb_phys]

Yes there is still air getting into the engine because even with the throttle closed the valves still operate - they are driven mechanically from the cam (unless you are driving an F1 car)

I worked out (from the rate of speed loss with no throttle) that my car takes around 30Hp to maintain highway speed - this is with permanent AWD so YMMV.

So if you are on a hill where you are going to have to brake at the bottom then it's best to be in gear and no throttle so you use no gas - but you have 30Hp of braking force.
If it's a hill where you are going to have to keep going at the bottom then it might be better to be in neutral (avoiding some of the friction drag) while burning a bit of fuel to keep the engine ticking over.

The other problem in Canada at this time of year, I live at the top of a hill so the first 5Km I don't touch the gas - which means the car is still freezing!
I used to have a tiny Citroen 1.2L diesel which was so efficient that in the winter after a 30min commute there was still no engine heat - you had to drive in hat/gloves/parka!

 

[KLoux]

Also, feeling the injectors kicking back in when the engine falls to the proper RPM is very distinct.

I'm intrigued - I'll have to give this a try on the way home today. My experience was working with programming a fuel system for a Honda CBR600 motorcycle, which definitely didn't shut the fuel off, but this is only one case.

I'm not sure on overcoming the air in the bottom of the engine though. I would have thought that's what a positive crankcase ventilation valve is for. Maybe it is the pressure in the bottom of the engine that is largely responsible for slowing the car down, because if you're right, this pressure would cause the engine to lose speed gradually.

Yes, I'm also not convinced about the air in the bottom being one of the driving factors behind engine losses - just noting what others have said above...

-Kerry

 

[brewnog]

Maybe it is the pressure in the bottom of the engine that is largely responsible for slowing the car down, because if you're right, this pressure would cause the engine to lose speed gradually.

Crankcase pressure is (should be) minimal, and really doesn't contribute to engine braking (which is mostly due to piston/bore friction, friction of rotating components, and pumping losses).

 

[Ranger Mike]

good link on the math to calculate brake coeffiecnt
http://answers.yahoo.com/question/index?qid=20070119075435AAQxhOG

and if you ever have had to push start a car with a dead battery ( manual transmission only) when the driver pops the clutch...it REALLY gets tuff to push...you are squeezing 8 to 9 atmosphers of pressure ( compression ratio) it used to be 11 to 13:1 with the old high compression engines
this is what slows up the car..the internal compression engine is one big air pump...think about it..typical compression tests on a fresh engine will mean about 190 to 210 psi in each piston cylinder

addto this all the associated parasitc drag from piston to wall clearenace, bearing drag, ring drag, oil pump hydralics, water pump drag..adds up

 

[Anskiere]

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? Thats because the intake charge is now doing work on the piston. Its hard to push overall because there is a drivetrain loss associated with any car.

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.

 

[brewnog]

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.

Depends on the valve timing. If the exhaust valve opens before BDC (which it usually does) then you won't recover all the energy you put into compress the charge. With the spring analogy, you're fully compressing it, but then only allowing it to extend partially.

 

[Gnosis]

Most of the engine braking effect is due to the movement of air underneath the pistons, not above them. You can try this for yourself. Coast down a hill, turn off your engine, and vary the throttle settings, there will be no perceptible difference in engine braking. The air is reasonably elastic, so the losses above the piston are small compared to losses below the piston, where air is being moved back and forth between the pistons in an engine.

Pro-street drag motorcycles use a vacuum pump to evacuate the crankcase to reduce the loss of power from the movement of air underneath pistons.

This article on Jake Brakes explains it: "forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft"

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

Devices like Jake Brakes release compression at the right time to enhance engine braking, but few cars have this. On two stroke engines, just a compression release will increase engine braking (although the purpose is to make it easier to start the engine).

Jeff, you stated that the air being moved under the pistons is the “major cause of engine-braking”. Let me assure you that this is a major misconception on your behalf. It is NOT the “major cause of engine-braking” at all.

I’m also genuinely surprised by the general lack of knowledge concerning combustion engines by members on this board.

Gentlemen, it’s very simple to demonstrate what aspect of the engine actually produces the “major engine-braking”. This is easily and undeniably accomplished by simply removing the spark plugs from all cylinders! Once removed, the major engine-braking has been eliminated, as all compression of air has been eliminated! If you attempt to use engine-braking with the spark plugs removed, it’ll feel as if there’s no engine in your vehicle! Your car would roll down the hill nearly as fast as when placed in neutral. Any well-trained or highly knowledgeable combustion engine mechanic knows to remove the spark plugs to eliminate the major engine-braking to make it easy enough to turn the crankshaft by hand! Even the crankshaft of a big V8 engine can be turned by hand once the spark plugs are removed, in which case, it’s easy as pie when using a normal ½” drive ratchet, but requires tools with leverage to accomplish while all the spark plugs are installed.

Now, if the air movement under the pistons was genuinely the “major cause of engine-braking” (and it clearly is NOT), then the removal of all spark plugs couldn’t possibly have eliminated the major engine-braking as it in fact does, since all the same mechanisms/mechanics are still in place under the pistons after removing the spark plugs!

This proves beyond the shadow of a doubt that it is the compression of the engine that causes the major engine-braking, NOT the air movement under the pistons, nor is the major cause of engine-braking attributed to frictional losses.

Point proven, case closed.

Vacuum pumps used to evacuate the crankcase yield only miniscule gains in HP and they do so ONLY per extremely high crankshaft operation. At low to mid crankshaft RPM, they do not yield any measurable HP gains. Even with these pumps installed, the combustion engine still retains its inherent engine-braking at ALL RPM. These pumps are typically only used in drag racing to squeeze out every last bit of HP by reducing the crankcase to a virtual vacuum thereby eliminating air resistance on the pistons, which isn't all the significant. It's not really worth it to anyone except drag racers racing for serious financial return, where often the difference between winning and losing is a mere .01 seconds.

 

[Ranger Mike]

Just about all the hot dogs running super later model are running eva pumps..does many good things...creates vacuum in the crankcase, assisting ring seal, reduces blow by past the oil ring, any time you kee oil out of combustion chamber, better HP, reduces detonation, assists in oil control,,don't forget, all IC have controlled piston rod bearing clearence to oil the cylinder walls of the engine..typically at 60 psi and higher so there is a whole lot of oil being flung around, created parasitic drag on the HP
plus..with a lot of crankcase presure ( there is in a ful lrace engine..oil will blow thru the valeve stem seals and get into the combustion chamber from the valve side too.
you have to run a dry sump oil system for this to work properly,,by just about everyone who can, as rules permit..run a evac pump

 

[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.
-
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.
-
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