Time for Camping?

daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa daa!!!!!!!!!!!!!!................

You can think of gas engine braking as the engine acting as a vacuum pump trying to suck air past the closed throttle, so the pressure differential is between vacuum on the intake side and atmospheric pressure on the exhaust side, which is about 15 psi at sea level. 

On a diesel with no jake or exhaust braking system there is no significant pressure differential between the intake and exhaust sides so no engine braking. Add a jacobs system and now its pumping against the compressed air in the cylinders up to the top of the compression stroke, which is then released by the exhaust valves opening. So the pressure diff can be what the cylinders and heads can stand.

On a diesel with exhaust braking a butterfly valve is typically added downstream of the turbo, kind like a throttle valve in reverse.  so now the engine is pumping against the pressure the valve is creating in the exhaust system. So the pressure diff can be what the exhaust system can stand.

Check out this video to see how all 3 systems work:

Notice that the gasoline engine brake system can only produce around 15psi pressure differential, while the diesel ones can produce more. In fact, either jake or exhaust brake systems can absorb power close to or higher than the hp rating of the engine itself. Why not even more? Because the engine internal components aren't designed for that. You'd break your crankshaft or con rods.

I couldn't find a reference but I would assume that the engine braking available in a normally aspirated gas engine could also be fairly close to its power rating. But in a gas turbo its going to be significantly less than that.

And podwerks you're right, there isn't any reason you couldn't add a jake or exhaust brake system to a a gasoline turbo and get more engine braking. No reason except cost and weight that is.....

The good news for us rpodders is that we really don't need that much engine braking in most situations because of the big parachute we're dragging around. Say you have a typical 8000 lb rig and you're driving down a highway with a 10% grade at 60 mph. You'll need to dissipate power at a rate of roughly 120hp. You'll already be losing about 40 hp from air drag and rolling resistance so the engine has to consume another 80 hp. Even the 2.7 liter engine ought to be able to do that I think. Now if you were hauling 9000 lbs of steel (the tow rating of the 2020 f150 2.7 ecoboost) on a low profile utility trailer you'd need to disspate more like 120-140 hp at 60mph. I'd be taking that hill at more like 30 mph in as low a gear as possible I think. 

What speed requires the most engine or friction braking to maintain? Looks like right around 70 mph for our rpod scenario. Braking requirement is about the same at 45 and 90 for example. So I guess its bad to take the grade at 70 if you don't have enough engine braking capacity. 

So say you ran out of engine braking and you're at 70 and still accelerating, but you haven't cooked off your friction brakes yet. You have a choice, either speed up to 90 or slow down to 45. As Dirty Harry would ask, are you feeling lucky? 

Us physics nerds don't let petty rules stand in our way of learning obscure stuff no normal person would care about..

Next up is to estimate how hot friction brakes would get on a 5000 ft descent at 60 mph towing an rpod with no engine braking. Anyone want to take a guess? 

Be sure not to activate those jake brakes around the kitty cats, might scare 'em into having seizures.,,,

We need to get rid of 80hp to maintain 60 mph. And we need to do that for about 10 minutes. 1 hp = about 2500 btu/hr so we will create 2500*80*10/60= about 30,000 btu. That BTU is going into about 100 lbs of steel in the form of brake discs and drums in the rig. The specific heat of steel is 0.12 btu/(lb-F) so the brakes will heat up by about 2500 degrees F. 

Of course, they wouldn't really heat up that much because (1) the hotter the brakes  get the more heat they will dissipate to the air and (2) they will quit doing anything to slow down the rig way before they get to 2500 degrees. 

Interestingly, the melting point of steel is also about 2500 degrees. So you can relax and not worry about literally melting your brakes as you go careening to your death. 

Moral of the story is ya gotta have engine braking on long downhill grades...