Well, the fact is engine braking IS possible, it is acheivable, so I think that is what this thread was started about.
Just take a look at a WR lap, in FXO or XRG and you will see how much they stretch the engine underbraking due to fast downchanging to gain more braking power. In real life, that is a big no no
There is no decompression or technically "significant" decompression at the moment of engine braking.
Creating decompression is same thing the creating power for an engine.
To do that you have to press accelerator pedal.
So we can not say that there are equal forces acting.
Secondly, I tried to share my own experiences here to help those want to know
what happened actually. I might be miss a point or so, its always possible.
I did not mention my educational level & studies to validate my point, and this is not necessary.
Its good to be in criticizing attitude, it makes clear out the discussion points.
For this thread, I can not say anything else.
BlueFlame, I agree with you. Thread started as you said.
And perhaps, ended right here for me.
Have a good day to you all.
It's called a jake brake. What it does is open a seperate valve in the cylinder while a piston is still close to top dead center, just after the compression cycle. This allows the compressed air to escape out of the cylinder instead of pressing back down on the piston to recover the energy lost during compression. So energy is consumed by compression, but is re-directed to accelerating air outside the cylinder instead of being recovered during the decompression cycle. Jake brakes are very load and this is why residents near highway downgrads complain about their usage.
Getting back to normal four stroke engines without jake brake valves, most of the engine braking is due to movement of air under the pistons and friction.
Depending on the car (if it's safe to do this), one way to test this is to drive on a long downgrade, shut off the engine, and notice that throttle position has virtually no effect on the amount of engine braking, only the rpms. Warning - I wouldn't do this on a carberated car since the gas fumes will fill the exhaust and could possibly blow a hole in the exhaust when the engine is turned back on.
I wonder how many of you actualy know how engine braking works.
Here is a better explination than you will find elsewhere.
Your driving in third gear and you let off the pedal, this creates a vacume in the intake manifold after the throttle, you start to slow down, but not much. This is because the kinetic energy driving your car forward is turning your transmission at a ratio that gives the road more leverage on your engine than what engine braking can retard effectively. You downshift to second gear and 2 things happen, first your transmission is now in a gear that gives your engine more leverage on the ground. Second your engine is going to want to speed up, however, your throttle is closed so the engine can not take in more air and it will increase the vacume in the manifold which will in effect increase engine braking by increasing drag on your entire driveline. Engine brakeing is a vacume in the intake manifold after the throttle.
Even though wikipedia says its due to compression, it can't be and who ever says it can obveously has never worked on an engine.
I did a test to verify my explination, I cut the fuel going to the engine on my dirt bike while I was driving in third gear, then I opened the throttle wide and dropped to second, no engine braking took place. If it was compression it would have not been effected by an open throttle because the valves still work. Usualy the bike nearly nosedives on the front suspension when I downshift without rev matching. compression has minimal, if any effect on engine braking.
Infact, on diesel engines, it can have a negative effect on the return from the compression stroke. Diesel engines don't have throttle plates and therefore require special equipment to engine brake. Wikipedia actualy explains that quite well and it does contridict there explination of engine braking on gasoline engines.
If you don't belive me, get a manual gearbox car with electronic fuel injection and cut fuel in third gear, then downshift to second with the throttle wide open, the engine will accelerate even though there isn't fuel to drive it and there will be no engine braking effect. You might notice a bit of a drag from the transmission and engine parts, but it won't chug down like it would with the throttle closed. If I can turn an engine by hand I don't think compression is going to stop 4000+ pounds of car.
Edit: I forgot to add, I work in a shop and I discussed this with my boss once and he agrees with me, and we are talking someone who went to university for this stuff.
I've done exactly this, and engine braking effect is virtually the same whether the throttle is open or closed. As previously posted, it's the movement of air under the pistons that accounts for most of the engine braking at higher rpms. Both pro stock motorcycles and cars use crankcase vacuum pumps.
This is something I've never heard of before, and it still doesn't seem to make sense. If it was air under the pistons it would still effect my motorcycle. Infact it would be more since my engine only has one piston, there's no where for the air to go exept out of the case through the breather, and at high RPM it still moves fast enough to produce almost no drag on the engine.
The only thing that could increase drag when you downshift with the throttle open is the engine and transmission components, and it shouldent be anything near engine braking. Unless you have some severe friction going on in the transmission or engine.
also just to add, my motorcycle has a close ratio gear box, the difference between 3rd and 2nd is only a fue thousand RPMs, not hardly enough to make the engine rev high when it's practicaly at idle in third, and I still get engine braking. It is also a carburated engine and it is perfectly safe to cut ignition and start it again because if the engine is tuned properly you should get no ignition in the exaust portion of the engine
You know that there are pistons moving vertically and/or laterally inside an engine, yes? Piston goes up, creates compressed air inside the cylinder, piston moves down, decompresses air inside the cylinder... Unless you have a 1 cyl.-engine, there are always cylinders compressing and always just as many others that decompress...
Now I can believe that friction (of whatever parts) and/or vacuums in the manifold between throttle and engine(?) or under the cylinder (in a dry-sumped engine?) cause engine braking, but not compression inside the cylinders (which, all on it's own would cause the engine to run endlessly)...
The vacume is created between the throttle plate and the cylinders.
With downshifting to second from third you will need to drag the engine back up to speed regardless of throttle position, this could be percived as engine braking, but isn't, nor is it going to produce the effect of engine braking. Though it might still slow you down a bit.
So try this.
acclelerate to the top of second gear but do not shift.
Now let off the throttle, you will notice you still get engine braking.
Do it again but instead, cut ignition instead of letting off the throttle, you will NOT experiance anything like engine braking.
Edit: on some newer cars with EFI the throttle is electronic meaning that the throttle will close if the ignittion is turned off regardless of peddal position. The only reason I know this is because some of these cars have been known to accelerate on there own during electrical malfunctions. (nissans do it sometimes)
In that case, you're not going to get a lot of engine braking, since compression and expansion of air under the single piston will not involve a lot of energy consumption.
A flat engine layout isn't going to have a lot of engine braking effects either. My statements about energy consumption under the piston were in reference to engines like an inline 4 or 6, or a V2, V4, V6, or V8 type engine.
My experiements with varying the throttle input with the engine shut off were done on cars with V8 engines.
But it does have alot of engine braking, with the throttle off it nose dives on the forks when I downshift. That is ALOT of engine braking, infact, if your not ready it can throw you off the seat and onto the tank.
I had another talk with my boss today about engine braking, if it was air movement under the pistons that provided engine braking it would blow out anything that held in that pressure change wherever it occured (we are talking enough pressure to slow the car down), if it was on an end piston the pressure change could blow out the seals. It actualy happens sometimes, the breather tube gets blocked and you get a pressure spike causeing things to pop out.
I also discussed the compression theory, it doesn't float either.
As stated the compresson stroke would act as a spring and the only loss there is to friction and energy dissipation through heat.
however, he did say you will get an engine braking feel when downshifting on a big engine, even with the throttle on full, but all that comes from is the effort the weight of the car has to put in to speed up the engine again. It is not related to engine braking, and isn't going to give you the full effect of engine braking because the engine speed will still rise where as if the throttle is left closed the engine will still try to return to idle. engine braking can take place without shifting if you are in a low enough gear because when the throttle closes you still get a vacume that brings the engine to idle, or as close as it can before the vacume is no longer enough to slow down the vehicle.
Edit: Infact, all engines have crank breathers regardless of design, if they didn't they'd be explodeing all the time.
Edit2: what vehicle where you driving JeffR?
The key here is that it's air movement, not air pressure. Changes in pressure you get back, moving the air around corners (which is why this is engine dependent) is what's consuming most of the energy in the crankcase. Again, pro stock drag bikes and pro stock drag cars utilize crank case vacuum pumps. The bikes get more of a relative benefit because of the high rpms (13,000). There are other reasons for using vacuum pumps on crankcases for drag vehicles, but more power is one of the reasons for using them.
Several cars with V8's and fuel injection. The last time I did this was on 1997 Trans-Am WS6 about 5 years ago.
I just did more research into it because someone else would not belive me when I told them it was vacuum. They where intent on the fact it was compression.
"On deceleration or when descending a hill, the throttle will be closed and a low gear selected to control speed. The engine will be rotating fast due to the fact that the road wheels and transmission are moving quickly, but the butterfly valve will be fully closed. The flow of air through the engine is strongly restricted by the throttle, producing a strong vacuum on the engine side of the butterfly valve which will tend to limit the speed of the engine. This phenomenon, known as compression braking, is often used in engine braking to prevent acceleration or even to slow down with minimal or no brake usage (as when descending a long or steep hill). Note that although "compression braking" and "engine braking" are sometimes used to describe the same thing, "compression braking" here refers to the phenomenon itself while "engine braking" refers to the driver's usage of the phenomenon." ~quoted directly from an article on manifold vacuum.
interestingly, they solved the argument without knowing
No mater how you slice it, engine braking comes from a vacuum in the intake manifold, NOT from the crank case. In order for air to move that much to act as a resisting force in the crank case, it would have to build alot of pressure, and alot of pressure is bad inside a crank case.
The only reason you feel that resistance with the throttle open and the ignition off is because it still requires alot of energy to turn an engine that isn't provideing power of its own. Power that also negates any power lost to compression, which would end in engine stall. I've done my homework, I'm not just talking from experiance, but also from what I learned from my texts and my boss and teachers in auto mechanics.The reason I didn't mention engine drag so much before is because I was riding a small engine dirt bike, which doesn't provide that much engine drag, and air in the crank case has nothing to do with that.
Edit: a crank case vacuum pump doesn't reduce engine braking, it actualy reduces air drag on engine components (most notedly the crank shaft) which can provide an increase in top end performance. But that is still noted as a side effect, not the reason for using them. High performance engines experiance MORE engine braking than normal engines. Every high performance car I've ever seen has insane engine braking.
As stated, the only thing you get from the crank case is engine drag, which happens at any RPM regardless of engine type.
