[Vwdiesel] Diesel braking - again

[Roger Brown]

LBaird119 at aol.com wrote:
> 
> ok, i've been thinking about this. sorry to bring this
> up again, but i just can't get it off my mind. :)
> 
> I see it pretty much the same way you do.  However I've noticed that
> the gas DO engine brake better than diesels notieably but nothing
> to really brag about one way or t'other.  The compression should make
> the diesel better but pulling against a vacuum helps the gas.  Appears
> the vacuum is mightier than the pressure.
> 
> >    So why do you think they put those expensive Jake brakes on diesel
> trucks? They sure don't put them on gasser trucks.
> 
>   Probably because they don't MAKE them for gas engines.  Won't work
> without cylinder injection.  You'd suck raw fuel right through the engine.
> Wouldn't set well in the past and CERTAINLY wouldn't set well with
> the EPA!  Fuel could be cut but generally I'd assume and exhaust brake
> would make it comperable to a Jake'd diesel, and legal in many more
> places.  I've never seen an 80K gvw with a gas tho.  I heard they tried a
> few turbine models way back when. Talk about no engine braking!

Its not really a vacuum vs. pressure thing.  If you look at the thermodynamics, in either engine, you compress the air inside the cylinder on the compression stroke, then on the expansion stroke, the compressed air pushes back on the piston, so you get
most of the energy you put into compressing the air back.  Compressed air is like a spring.  What you lose in the process is heat, compressed air gets hot, compress it more and it gets hotter.  The cooling system in the engine is removing this heat and
releasing it to the atmosphere.  

Now, put a restriction on the intake and create a vacuum behind it to let the atmoshpere push air into the engine, there is a lot of energy lost in the expansion that occurs through the restriction.  The air is cooled as it expands due to the lost energy
(also explains why carburetor icing occurs) and the pressure drops (explains the high manifold vacuum you see under engine braking).  Its the crankshaft pulling against the manifold vacuum that creates the braking force, but since there was energy lost in
the expansion process, you don't get it back like you do with a free running compression/expansion cycle.  

Makes no difference if its a gas or diesel engine, assuming equal displacement and RPM, put an intake restriction in there and you'll have the same braking force, since you have the same volume of air moving through the same restriction.  However, there
is a limit, you only have 14.7 psi (at sea level) to work with so the most force you can get is the manifold pressure multiplied by the surface area of the piston that is pulling against the vacuum.  Also as the elevation increases, the atmospheric
pressure decreases, and so does the amount of braking force, good rule of thumb is about 3% loss for every 1000 feet of elevation gain.  I know most of the long, steep hills I drive on are in the mountains, which tend to be at a fairly high elevation,
thereby reducing the braking force just when you need it the most.

On a gas engine, the engine braking feature is free and it takes no extra equipment to make it work and its reasonably effective.  You can add it to a diesel, but that takes adding hardware/complexity to the engine, etc. and you would not get that much
more braking to really make it worthwhile, which is probably why its not done all that often.  Using compression braking, like a Jake Brake, is much more effective since you are working with much higher pressures.  You have many 100's of psi for the
piston to work against vs. 10-15 psi for vacuum.  

An exhaust brake is something you can add to a diesel (or gas) engine by placing a restriction in the exhaust plumbing to raise the back pressure and get the same expansion loss as the high pressure exhaust is expanded through the valve.  Its not as
effective as an engine/compression brake but is probably good enough for something like a full size pickup towing a trailer.  Adding a full Jake Brake to an engine, requires replacing internal parts like the exhaust valve train components to be able to
change the valve timing.  I would assume Jacobs Engineering works very closely with the major diesel engine mfgs. to make sure the engine design can accomodate the Jake brake components.  And I assume a setup like that adds a lot of $$$ to the cost of the
engine.

The other issue is the vehicle's power to weight ratio, put 50-200 HP engine into a 2000-8000 lb. vehicle, as is common in most passenger cars and light trucks, and you have adequate engine braking.  Make that vehicle weigh 10 times as much and only
double the engine power and you are severely underpowered (both accelerating and braking) compared to a light vehicle.  So instead of overdesigning the engine for the big rig, you just beef up the engine braking capabilities and rely on gearing in the
transmission to handle the braking needs.

I would imagine if you could rig up a Jake brake on a small VW diesel engine in a light, unibody Rabbit, it might lock up the wheels if you hit it at high RPM.  You would have so much braking force that you would not be able to use it on all but the
steepest of grades.  Full size pickups used for towing can benefit from an exhaust restriction type of brake, not as effective as a Jake brake, but probably good enough and assuming the engine design can handle the increased back pressure (stiffer valve
springs), installing a system is pretty much a bolt-on install, assuming the availability of a kit.

-- 
    Roger