ALWAYS use 4wd on hills!

[dieselscout80]

Diesels do have less compression braking than gas engines due to not having throttle plates that close (there are a few diesel that do). To generate engine braking in diesels in semi type trucks they use engine brakes one is the Jake Brake that causes the valve on the engine to close to generate the necessary braking affect.

 

[BeeferMan]

So only diesels trucks equipped with Jakes have any engine braking?


And not to be picky, but the Jakes provide extra engine braking by opening the exhaust valve, not closing it...

 

[Specop_007]

To think, I've been dodging death every day for years using a 2wd tractor on steep hills. As rjkobbeman mentioned, it has more to do with going straight down hill when possible, and going slow when conditions call for it. Common sense must be used no matter what technology you have.

Yep. 4wd? Not an option for me.

 

[TractorRob]

He he he.. I guess all my tractors have throttles that create a vacuum then, cause when I'm heading down a grade and I back off the engine rpms, I get pretty good engine braking. Maybe someone forgot to tell them they're diesels?

...But there are still powerful engine braking forces in a diesel engine, otherwise if one is going down a just a slight grade with a load on, why is it that backing off the throttle will usually provide enough braking? What is it that stops a diesel in this (non-severe) case from over-reving, potentially to destruction? Most anyone who has gone done a snow covered slope in a 2WD tractor and had the rear end start to slide has had the experience of having to push in the clutch to disconnect the engine from the drive train to get the wheels turning again... that's due to good engine braking...

When you back off the throttle of a diesel powered tractor, yes you will get braking, not only due to the engine but also the alternator, water pump, radiator fan, hydraulic pump, fuel pump, and anything else running off the engine. When people talk about "engine braking," they are usually talking about all of these forces together because they all do work together. In my comment I was talking about the engine alone because I was simply disputing the statement that a diesel would provide more braking than a similar gasoline engine. Even the diesel engine alone will provide some braking, but certainly a lot less than a similar gasoline engine.

Rob,
are you stating that creating a vacuum will give more braking than compression will?

Roy

Not exactly. Without a Jake brake then yes, the vacuum created by closing the throttle will give more braking than the compression in the cylinders. Most of the braking force that would be cause by the compression is countered by the compressed air pushing the cylinder back down after it reaches top dead center. When operating a Jake brake, however, when the cylinder is at or near top dead center an exhaust valve is opened to release the compressed air so that you do not get the force pushing the cylinder back down. In that case, I suspect the compression would create more braking than the vacuum.

 

[slan]

22:1 VRS 8:1
:confused2:
Ever tried to turn over similar sized small engines of each type???
:confused2::confused2::confused2::confused2::confused2:

 

[TractorRob]

22:1 VRS 8:1
:confused2:
Ever tried to turn over similar sized small engines of each type???
:confused2::confused2::confused2::confused2::confused2:

When on a compression stroke, the 22:1 will be harder to turn over; however, what happens when that cylinder gets past top dead center and into the power stroke? Even if the engine isn't firing, it will want to turn on it's own as that compressed air pushes the cylinder back down, and it will do that with more force on the 22:1 engine. As the engine is turning, every compression stroke will try to slow it down and every power stroke will try to speed it up. On a higher compression engine the compression strokes will try to slow it down with more force, but likewise, the power strokes will try to speed it up with more force.

 

[dodge man]

I'll throw my opinion into this question. I was not trying to start an argument when I asked about diesels not having any braking affect. I know that they just started adding engine brakes to a lot of diesels pickups in the last few years because diesels have poor engine braking.

One thing to take into account is that tractors have very low gears. Even large tractors will not run much over 30mph. These very low gears translate into coverting what engine braking that is available into a lot of force. I have a gas powered pickup, if I put it in 4 high and hold 1st gear, it has quite a bit of engine braking it, in 4 lo it will just about throw you through the windshield if you let off the gas pedal quickly.

Even my little Kubota will slow quite a bit if I close the throttle going down a hill and I let off the HST pedal, is this due to engine braking, a natural braking affect of the HST, or a combination of these two things and the fact that tractors have such low gears?

 

[BeeferMan]

You are right about the on the road trucks, they're using Jakes mainly when running at higher speeds, and thus higher gears. We usually killed the jakes when "around town", always when off-road. There was generally sufficient engine braking without them in those situations....

 

[dieselscout80]

So only diesels trucks equipped with Jakes have any engine braking?


And not to be picky, but the Jakes provide extra engine braking by opening the exhaust valve, not closing it...

I didn't say or mean diesels don't have any just less than a equally sized gas engine will.

 

[TractorRob]

...One thing to take into account is that tractors have very low gears. Even large tractors will not run much over 30mph. These very low gears translate into coverting what engine braking that is available into a lot of force. I have a gas powered pickup, if I put it in 4 high and hold 1st gear, it has quite a bit of engine braking it, in 4 lo it will just about throw you through the windshield if you let off the gas pedal quickly...

Good point, lower gearing will magnify whatever engine braking effect you do have.

...Even my little Kubota will slow quite a bit if I close the throttle going down a hill and I let off the HST pedal, is this due to engine braking, a natural braking affect of the HST, or a combination of these two things and the fact that tractors have such low gears?

When you let off of the hydrostat pedal, the hydrostat will act as a brake.