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post #1 of (permalink) Old 03-20-2001, 11:53 PM Thread Starter
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Test Your engine knowledge!

Okay, I'm not sure that I know the whole story. When you are using "compression" braking, what exactly is happening? Are you burning the same amount of fuel? From what I understand, the throttle (on a carb) opens plates that allow for more air to flow through. This air passes over the venturis which then suck out the fuel. When you let off of the throttle, what happens? Do the plates shut all the way? If so, then what keeps them from being sucked back open my the motor? If they stay open, what keeps the engine from continuing? Just thinking after a long night in a class I don't want.

Any ideas?

Fritz

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post #2 of (permalink) Old 03-21-2001, 02:29 AM
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Re: Test Your engine knowledge!

Wow! You're bored!
No the plates don't close all the way, they stay open just a little bit. That's what you are adjusting when you play with the idle set screw.
If you look at your carb, you will see a large wound spring around the throttle plate shaft on the outside of the carb, this is why the engine vacuum doesn't open the plates. Also as the plate opens, the vacuum decreases so it would fight itself.
Compression braking... Uh... I think, the engine basicly starts to work against itself? This is making sence in my head. When you excellerate, more fuel and air go in and make the combustion in the chambers more powerful and the crank turns faster, and the output to the tranny is stronger than the rolling resistance at the wheels. When you compression brake the opposite happens, the rolling resistance forces the wheels to turn, turning the output and the crank. The pistons are now being forced up and down by the rolling of the wheels (sort of) and they meet resistance in the form of compression when the crank pushes the piston upward in the chamber. Until the valves open, the combustion pressure has nowhere to go and resists the crank. Make sence?
I must be bored too.


'88 Sahara, 4.2, auto.
post #3 of (permalink) Old 03-21-2001, 09:17 AM
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Re: Test Your engine knowledge!

[img]/wwwthreads_images/icons/laugh.gif[/img] WOAH! WOAH THERE BIG FELLA! Heh heh heh......you boys is jest a liddle off th' mark heah. Wot yer referring to as "Compression Braking" is actually VACUUM Braking. Yep, that's right. VACUUM does the slowin' down, not compression. That's why diesels are just like a roller skate downhill, there IS no intake manifold vacuum in a diesel so the compression stroke and the power stroke cancel out to ZERO. In gassers, the high intake manifold vacuum holds the vehicle, NOT compression, because if the truth were known, at idle, or with the throttle plates closed, there isn't much compression.[img]/wwwthreads_images/icons/tongue.gif[/img] In order to increase economy, Detroit has made cams which LOWER the low power setting, and idle manifold vacuum, so the cars coast better and get better fuel mileage.[img]/wwwthreads_images/icons/tongue.gif[/img] Now, if you want a real challenge to your newly engine-aware brain, try to determine how the Jacobs Engine Braking System in diesel trucks actually works. Heh heh....there'll be a quiz on Friday.[img]/wwwthreads_images/icons/laugh.gif[/img]

CJDave
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post #4 of (permalink) Old 03-21-2001, 10:18 AM
 
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Re: Test Your engine knowledge!

If I'm taking a quiz on Friday, I really want to know how the diesel brakes work. My dad's truck(98 Dodge ram, Cummins 12 valve diesel) has a Jacob's Exhaust Brake. This is really a strong butterfly valve downstream of the turbo in the exhaust pipe. To my understanding, this creates compression within the engine, and slows things down. I dont know if that is right or not, but I do know we put stronger exhaust valve springs in when we installed it. Funny thing is that with the newer 24 valve engines, they already have strong enough exhaust valve springs. hmmm.... I've also heard that some engine brakes cut off the air intake, so does this system work on vacuum and the exhaust brake work on compression? I like the E-brake on the Dodge, but it doesnt sound enough like the big trucks.

Andy

46 cj2a beater/future rebuild project
maybe a trail jeep?
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post #5 of (permalink) Old 03-21-2001, 10:29 AM
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Re: Test Your engine knowledge!

Here is info on Jacobs engine brakes. Interesting actually.

post #6 of (permalink) Old 03-21-2001, 01:57 PM
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Re: Test Your engine knowledge!

[img]/wwwthreads_images/icons/smile.gif[/img] There is no resemblence between the exhause brake on that dodge and the Jacobs engine braking system. What the Jacobs system does is DELAY the opening of the exhaust valves till the piston is very near the top then it POPS open the exhaust valve and the air squirts out. It uses about 90% of the exhaust stroke as an air compressor and uses up HP that way. It literally makes the engine into an air compressor on the exhause stroke. As I mentioned earlier, the compression stroke and the power stroke cancel out in a diesel on a downhill grade. The OTHER good thing about Jacobs braking is that it helps keep the engine WARM.......yes, I said WARM. In the "Old" days, diesel truck engines got so cold on LONG downgrades from handling all that air that the cylinder temps got too low to burn the fuel so when you got back on the throttle it knocked, smoked, and refused to pull.[img]/wwwthreads_images/icons/crazy.gif[/img]The Jacobs system helps get one more compression stroke in there to maintain temps from the heat of compression.[img]/wwwthreads_images/icons/tongue.gif[/img] Just a little note from days gone by[img]/wwwthreads_images/icons/crazy.gif[/img].

CJDave
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post #7 of (permalink) Old 03-21-2001, 03:52 PM
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Re: Test Your engine knowledge!

MAN, Dave, sometimes the automotive knowledge that is inside your brain is staggering! [img]/wwwthreads_images/icons/smile.gif[/img] Here I sit, thinking I know what an exhaust brake is, and here I get a whooooole lotta more knowledge on stuff from you.

I'll go back to my corner now, humbled like a little boy with his erector set after looking at a big truck. [img]/wwwthreads_images/icons/smile.gif[/img]

Pete

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post #8 of (permalink) Old 03-21-2001, 05:11 PM
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Re: Test Your engine knowledge!

[img]/wwwthreads_images/icons/tongue.gif[/img] Now I don't want anyone to get corn-fuzed about the difference between an exhaust brake and a Jacobs "Engine Brake" because they aren't the same thing. Interestingly enough, the Jacobs engine brake can be used on the two-stroke Detroit diesels as well. It does the same thing, just twice as many times per 100 revolutions.[img]/wwwthreads_images/icons/crazy.gif[/img] So....an exhaust brake is just a way of plugging up the exhaust pipe. The regular Jacobs engine brake is making the diesel engine into an air compressor and at the same time waking up anyone who may be asleep when you coast into town at 2:30 AM rapping the pipes.[img]/wwwthreads_images/icons/crazy.gif[/img]

CJDave
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post #9 of (permalink) Old 03-21-2001, 05:21 PM
 
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Re: Test Your engine knowledge!

Ok, then how can I make my truck sound like one of those big rigs? Is there anything for the small cummins that will use the jacobs engine braking system? Oh, by the way, the exhaust brake we use is made by jacobs for cummins.

Andy

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post #10 of (permalink) Old 03-21-2001, 06:24 PM
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Re: Test Your engine knowledge!

[img]/wwwthreads_images/icons/smile.gif[/img] I don't think that Jake ever made a regular "Engine Brake" setup for the seven liter Cummins. I could be wrong, but ah do not think them boahs ever did. You can see why there is a "rap" from the engine braking system can't you? It's the air popping out of the exhaust valves all at once at the top of the exhaust stroke.[img]/wwwthreads_images/icons/smile.gif[/img] It really is the best thing since sliced cheese because prior to that, the steep country trucks had 11" brake drums WITH two fifty gallon drums of water running on them for cooling, and EVEN THEN hauling logs in steep country was not for the faint of heart. Made lotsa widows.[img]/wwwthreads_images/icons/crazy.gif[/img] Before they had twin countershaft transmissions, the main boxes would really howl when that power went through them in the wrong direction, but Fuller changed all that with the 1,000,000 mile Roadranger series transmissions.[img]/wwwthreads_images/icons/smile.gif[/img]

CJDave
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