| Author |
Message |
Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 1:19 am: | |
I always hear the explanation of diesels giving no braking as opposed to gas engines because they have no vacuum in their intake manifold, thus do no "work" compressing while going downhill with the accelerator pedal up. Of course there are Jake brakes and there are exhaust brakes for turbo engines that make a diesel do "work" and slow things down.
So the dumb question, at least for a normally aspirated 4 stroke diesel: Why can't we just put a butterfly valve in the intake and shut it to create a vacuum in the intake manifold...it seems to me that while it wouldn't provide as much breaking action as a Jake, it would certainly act more like a gasoline engine going downhill, and definitely give some braking action...am I brainless here, or would this work?
Cheers
Gary |
Donn Reeves (67.193.140.91)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 8:29 am: | |
Gary,
I don't know where you herd that diesels have no engine braking,but it's not true.Engine braking is a result of the compresion in the cylinders not vacume.Conseqently,diesels have more braking ability than gas engines ,not less.I have been driving trucks for over 30 years,and gearing down on hills Definatly works.-Donn |
Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 9:13 am: | |
Donn,
In theory, they don't. The deal is this: in a diesel, (1) The intake valve opens and air is brought into the cylinder. (2) Air is compressed by the engine piston. The energy required to compress this air is produced by the vehicle's driving wheels.(3) When the piston passes over top dead center and begins its downward stroke, the compressed air pushes the piston down with the same force it took to compress it in the first place; the energy is returned to the piston (and to the driving wheels), and essentially no energy is absorbed- No net gain or loss of energy- no net retarding work is done.
In a gas powered engine however, since they have a throttle-butterfly valve, the engine becomes a vacuum pump when being motored with the throttle closed, and consumes energy. The butterfly cuts off air flow so the engine generates vacuum as the pistons are continually trying to draw air into the cylinders. This vacuum has a retarding effect on engine output and automatically provides some braking assistance on downhill grades when the driver's foot is not on the throttle.
That said, a big diesel does use some energy due to friction, alterator, fan, etc and will offer "some" braking, but if you put a same-size diesel and gas engine side by side in identical applications and make a comparison, the diesel will loose.
hence, my question still stands... |
Curious (65.190.11.25)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 2:20 pm: | |
More questions..
(1) The intake valve opens and air is brought into the cylinder.... Isn't the air blown in?
(2) Air is compressed by the engine piston. The energy required to compress this air is produced by the vehicle's driving wheels....Isn't the energy produduced by adjacent cylinders in various cycles of their power stroke?
(3) When the piston passes over top dead center and begins its downward stroke, the compressed air pushes the piston down with the same force it took to compress it in the first place.... Wouldn't the force pushing down be a factor of the amount of fuel injected?
(4) In a gas powered engine however, since they have a throttle-butterfly valve, the engine becomes a vacuum pump when being motored with the throttle closed, and consumes energy.... Doesn't the carberation and fuel pumped determine the speed in which the pistons travel?
(5) The butterfly cuts off air flow so the engine generates vacuum as the pistons are continually trying to draw air into the cylinders....Isn't the air flow used to mix the correct amount of air and fuel?
(6) If the fuel is determining how fast an engine is turning and it is regulated such that an idle speed is being maintained, isn't the weight of the vehicle trying to overcome this idleing speed? And, depening on the weight, speed and road incline, don't these factors influence the engine as to trying to speed it up(downhill)or slow it down(uphill)?
Clarke, you out there? |
Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 2:53 pm: | |
Hi "Curious"... (interesting name!!)
First, all of my statements are assuming you're coasting down a hill with foot off of the gas pedal. And if you do some searching on the web, notably sites that sell Jake Brakes, they confirm what I've said about how both types of engines behave going downhill being used as brakes.
So again, answering your questions;
(1) I'm talking about an "unblown" naturally aspirated diesel (no turbo or supercharger)
(2) No, the other pistons are not producing power- they are all doing the same thing...nothing...
(3) again, foot off gas pedal- essentially no fuel.
(4) Not when you're going downhill and using the engine as a brake. Actually these days modern cars cut the fuel off entirely in this scenario because not doing that results in a mixture that is too lean to burn and spews unburned hydrocarbons out the tailpipe when coasting downhill...ever notice how going down hills, the older cars and trucks stink something fierce but newer ones don't? That's because the fuel is not cut off in older engines during those conditions... and even when "idle" fuel is present in older engines, there is no combustion taking place because the mixture is too lean.
(5) and (6) Same answer as (4)...essentially no fuel there at all when using gas engines as brakes
Sooooo, again, my origional question still stands...
Cheers
Gary |
Don Peter (64.12.103.29)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 3:34 pm: | |
Ok, I'll take a stab at a guess. If you put a valve on the intake to prevent air flow into the cylinder on the downward stroke, the maximum retardation effort is going to come from the piston pushing against atmosphere in the crankcase. Since the crankcase isn't sealed, that pressure is a constant 14.7 psi at sea level.
I'm sure this will give some additional braking, but probably not nearly as much as a Jake.
As I understand it, Jakes open the exhaust valves at the top of the compression stroke so that the energy that was use to compress the air to a pressure much greater than atmosphere is not used to push the piston back down again.
Or not, this is just my best guess. -- Don |
johnwood (206.252.229.218)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 4:03 pm: | |
Jakes turn the diesel into a giant air compressor. On some aftermarket diesel brakes, they install a buterfly in the exahust. Gives the engine a "load" to work against without the complexity of a "jake" Any braking system has to turn momentum into some other form of energy. Brake shoes produce heat, tranny retarders produce heat. What would be really wonderful is a way to recuperate the forward momentum for use in the next acceleration. Some experimental city coaches are using flywheels and batteries to store the energy normally lost to heat.
Adding a butterfly in the intake of a diesel may cause your engine to suck a lot of oil around valve seals and rings. Could be counter productive |
DonnReeves (67.193.140.61)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 4:43 pm: | |
Gary,
What you say may be true,but I think I'll continue to downshift for hills anyway.-Donn |
jmaxwell (66.42.93.116)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 8:32 pm: | |
Don (w/one n) is onto the trick by recognizing the presence and function of timed exhaust valves. Amazing how one little variable can blow theories right out the door. With timed valving, diesels do have compression braking abilities equivelant to the same size gas engine. Reed valved engines don't enjoy the same compression braking ability, but they still have some. |
dougthebonifiedbusnut (24.218.119.24)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 10:00 pm: | |
hey guys
i to worked driving heavy trucks for most of my working and i can tell you more trucks did not have engine brakes than did that i drove they had plenty of inherant braking vacumn is the result of bernulies law and it is the reson acarb works the ventury restricts the airflow going threw the carb the law sais that an increase in velocity causes a decrease in pressure or makes a low pressure area at the throat of the carb it is this restriction that causes vacumn you have no carb on a diesel thus no vacumn but the braking effect is the same as a gas engine because there is still combustion in the cylendar even when you let off the gas so the engine is still firing when you let off the throttle what happens when you turn on the engine is both the intake and the exhaust valve open and no combustion takes place the engine becomes an air compressor thats it in a nutshell |
dougthebonifiedbusnut (24.218.119.24)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 10:03 pm: | |
oh damn i meant to say when you turn the engine brake on |
Scott Whitney (24.205.232.249)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 04, 2002 - 11:08 pm: | |
One thing I can add to the conversation is that in Gary's model he talked about an 'unblown' 4-stroke engine. But if we area talking about a common 2-stroke DD bus engine (6V71, 8V71, 6V92 etc.), it will have a supercharger. If you totally shut off the air with a big butterfly (or by closing the emergnecy shut-off flap) while rolling down hill at a good RPM, it would probably suck the blower seals in? I tested my emergency shut-off at idle one time. No damage done. But the forces generated flying down a hill might be another story.
Scott |
Tom Caffrey (Pvcces) (12.146.33.4)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, May 05, 2002 - 12:53 am: | |
When the piston on a four stroke diesel comes all the way up from the bottom of the bore, the compression pressure goes up to around 400 psi or more and the temperature goes over 500 degrees.
On the way up and down, there are friction losses. On the way up, heat energy is lost to the block, and for some distance down.
That lost heat energy means that the piston is not pushed down as hard as it was to push it up, and the difference is worth more than the 15 psi possible if you were to draw a perfect vacuum under a carburetor. And 24 or 25 inches is more like it.
In any event, the argument that the vacuum is what holds engines back on compression overlooks the help that you get from the vacuum on the next stroke. You have to look at both engines for what happens on two strokes out of four.
My experience has been that a four stoke diesel has way more drag than a gas engine of comparable horsepower. If you want to get much compression braking out of a gas engine, it needs to be wound up to its maximum horsepower RPM.
Tom Caffrey PD4106-2576 |
Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, May 05, 2002 - 12:12 pm: | |
I think from all your great comments that my origional question may be answered... here's what I'm getting:
(1) Though technically diesels don't have as much drag as gas engines, in practical terms the usually have more, probably due to engine size, friction, heat-of-compression losses, etc
(2) My idea of butterflying the air inlet for additional braking drag is a good idea technically speaking, but diesel piston rings aren't set up for vacuum in the cylinders, and crankcase oil would probably be sucked in, making a real mess of the engine.. |
Tom Caffrey (Pvcces) (12.146.33.76)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, May 06, 2002 - 12:05 am: | |
Gary, you might be right about the diesel burning more oil if it ran a vacuum on the intake. However, Mercedes Benz did so sucessfully in their 180D models (1959 vintage) and they used the vacuum to control the rack.
They didn't run as much vacuum, maybe less than 10 inches, because the throttle plate had a dime sized port in it. Those engines, if worn, could go through the oil, but I had one rebuilt, and the oil consumption was no problem afterwards.
That combination had an interesting effect when being driven at highway speed. If I was going down a long grade and started to step on the throttle slowly as I neared the bottom, the drag from compression increased until I pressed down far enough to start the injection again.
The effect nearly doubled the slowing of the car on compression and I knew when it started injecting because I could hear and feel the engine firing.
The slowing from opening the throttle had to come from the engine pumping more air than when the throttle plate was closed. The vacuum under the throttle plate didn't increase the slowing from compression even though you might think it would.
I'm sure there's some information around about those engines if you want to check on them, but I think if the engine you want to try this on has good valve seals and rings, then oil consumption shouldn't be much of a problem.
If you should try it, remember that if the pump should inject when the throttle plate is closed, there will be no ignition of the fuel and you will get a white cloud out your exhaust pipe.
Likewise, if you should open the throttle slowly while it's injecting, you may get delayed firing as the pressure rises in the cylinder, which is very hard on an engine, much like pinging in a gas engine.
And if you open the throttle a little further, you will get black smoke from insufficient air for combustion.
Mercedes avoilded the complications that I mentioned above by making sure that injection didn't occur until there was enough air. That's why the dime sized hole in the throttle plate.
I hope this helps.
Tom Caffrey PD4106-2576 |
Dwight (67.213.8.3)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, May 11, 2002 - 9:38 am: | |
just one comment about the vacuume sucking oil into the diesel engine, some diesel engines (4 stroke)do not come with valve guide seals on the intake valves, though they can be installed.... |
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