Diesel techies, what'ya mean a diesel has low compression?!

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.
See posts with no answers
Back2PA

Back2PA

Well-known member
Joined
Jul 26, 2015
Posts
5,766
A nagging question because inquiring minds want to know. (I'm asking for a friend.)

So here goes. What I learned about diesel engines (many years ago in a galaxy far, far away) was that they ignite the fuel not with a spark but just through compression. And it takes a fair amount of compression to get diesel fuel to spontaneously explode. I would think therefore that going downhill one would get significant compression braking, yet no. In fact the explanations I've read re the need for exhaust/Jake brakes is due to "the inherent low compression of Diesel engines". Say what?!

OK diesel boys (and girls) enlighten me. Your explanations will someday allow me, sitting around the campfire with a diesel newbie, to pass on this knowledge with the appearance of a grizzled diesel veteran ("young man, when I was a kid we had to refine our own fuel to drive these big rigs.... and let me tell you what I know about air brakes...")
 
I am certainly not a diesel engine "expert" but diesel engines do maintain combustion through high compression. From what I've read about our Jake-Brake, it adjusts the timing of the valves to create back-pressure in the cylinders when you engage it - hence the braking. My assumption is that the exhaust valves are otherwise operating normally, allowing no back-pressure to build when the Jake is not engaged.

Kev
 
Engine brakes (aka Jake Brakes) turn the engine into a compressor, releasing the compressed air (stored energy) via the exhaust valves near TDC on the compression stroke. Thus the pop, pop, pop noise you hear. More info on the Jacobs web site.

Unlike gasoline engines, diesels have no "shutoff" of air at zero throttle.

Edit: Lou beat me to it.
 
Think of compression as a spring.  While it takes energy to compress the air on the upstroke, if the pressure is not released most of the stored energy is returned as the compressed air pushes against the piston on the downstroke.

Compression braking is a misnomer - what causes braking in a gas engine is the force needed to suck air past the closed throttle plate in the air intake.  Diesel engines have a wide open air intake, so they don't generate the same amount of braking as a gas engine.

Diesels need an external mechanism to develop effective braking.  Either an exhaust brake that closes off the exhaust flow in the same way a gas engine's throttle plate closes off the air intake, or a Jake brake that opens a valve in the combustion chamber to release compression at the top of the compression stroke so the compressed air doesn't push on the piston during the power stroke.
 
from Wikipedia

The term 'engine braking' refers to the braking effect caused by the closed-throttle partial-vacuum in petrol (gasoline) engines when the accelerator pedal is released. While some of the braking force is due to friction in the drive train, this is negligible compared to the effect from the vacuum.

When the throttle is closed, the air flow to the intake manifold is greatly restricted. The concept can be illustrated by the amount of effort required to blow/suck through a narrow tube vs. a wider one. It is the work the engine has to do against this restricted air flow that provides the braking effect.

Diesel engines[edit]

Diesel engines do not have engine braking in the above sense. Unlike petrol engines, diesel engines vary fuel flow to control power, rather than throttling air intake and maintaining a constant fuel ratio as petrol engines do. As they do not maintain a throttle vacuum, they are not subjected to the same engine braking effects.
 
This description is a bit glib and glosses over the tech details, but I think it describes the concepts well enough. Hope Rudolf Diesel and Nikolaus Otto will forgive me!

High compression occurs in a diesel when all the valves are closed, which means after the fuel intake part of the cycle and before the power and exhaust strokes.  But with the engine at idle running downhill, the engine is effectively shut off. There is no fuel intake and the valves never close, so no compression takes place - unless an engine brake system, e.g. Jake Brake, is installed. Some diesels create braking force by closing off the exhaust manifold outside the engine, so the air in the cylinders cannot exit easily and thus produces back-pressure. That is called an exhaust brake.

Gas engines, however, are always running, burning at least a little fuel and opening and closing the valves. Air flow through the engine is controlled via the throttle plate and there is back-pressure in the cylinders.
 
Cummins 8.3 simply chokes the intake like a gas engine, not as effective as a full compression exhaust brake, but much quieter.

In theory, the cummins simply draws a vacuum on the intake stroke. as the exhaust brake restricts the exhaust.
 
Soon2,, you are also wrong about the firing of diesels,,yes a diesel depends on compression, but it also does NOT fire without fuel..
The fuel is injected AT THE SAME MOMENT THAT OPTIMUM COMPRESSION IS REACHED resulting in instant ignition of the mixture.. it also continues to inject fuel during the power stroke.. As you can see it takes more than compression alone.>>>Dan
 
Lou Schneider said:
Compression braking is a misnomer - what causes braking in a gas engine is the force needed to suck air past the closed throttle plate in the air intake.  Diesel engines have a wide open air intake, so they don't generate the same amount of braking as a gas engine.
Click to expand...

And so, around the campfire, I shall be saying, "Let me explain... compression braking is a misnomer..."
 
utahclaimjumper said:
Soon2,, you are also wrong about the firing of diesels,,yes a diesel depends on compression, but it also does NOT fire without fuel..
Click to expand...

Lots I don't know but that part I knew. As I said in my post "...they ignite the fuel not with a spark but just through compression....."

Darn shame though, wish I could get it to run without fuel, would really help out the retirement budget  ;)
 
... wish I could get it to run without fuel, would really help out the retirement budget
Click to expand...

Scott, if/when you figure that one out, we'll be all ears  ;D
 
Sun2Retire said:
And so, around the campfire, I shall be saying, "Let me explain... compression braking is a misnomer..."
Click to expand...

Now you got it!

Read Lou's post again and commit that to memory.  If the campfire gang needs more help, recite that post to them.
 
And so, around the campfire, I shall be saying, "Let me explain... compression braking is a misnomer..."
Click to expand...
No, compression braking is not a misnomer. It just doesn't happen without either an exhaust brake or engine brake installed.  If the exhaust valves are closed (engine brake), or the exhaust manifold constricted (exhaust brake), the cylinder cannot vent adequately and the exhaust stroke becomes a moderate compression stroke instead.  Only it's not actually "exhaust" that is being vented - just plain old air.  With no fuel being burned, the diesel just sucks in air and pushes it back out the exhaust pipe.

Cummins 8.3 simply chokes the intake like a gas engine, not as effective as a full compression exhaust brake, but much quieter.
Click to expand...

I've not heard that before, Tom. Nor do I see anything about that feature in the ISC brochure.  Are you suggesting that the ISC 8.3 doesn't need an exhaust brake? Does it apply to the older mechanical 8.3, the newer electronic (ISC) engine, or all of them? 
 
Gary RV Roamer said:
No, compression braking is not a misnomer. It just doesn't happen without either an exhaust brake or engine brake installed.  If the exhaust valves are closed (engine brake), or the exhaust manifold constricted (exhaust brake), the cylinder cannot vent adequately and the exhaust stroke becomes a moderate compression stroke instead.  Only it's not actually "exhaust" that is being vented - just plain old air.  With no fuel being burned, the diesel just sucks in air and pushes it back out the exhaust pipe.

I've not heard that before, Tom. Nor do I see anything about that feature in the ISC brochure.  Are you suggesting that the ISC 8.3 doesn't need an exhaust brake? Does it apply to the older mechanical 8.3, the newer electronic (ISC) engine, or all of them?
Click to expand...
You only need to go look at a Cummins 8.3, none will have any equipment attached to the exhaust system. but you will see a slide valve installed between the turbo and the intake manifold.
This acts exactly the same as the throttle valve on any gas engine. Close it, and the intake vacuum goes way high.
the valves in Cummins are driven by a cam and lifter system just like any mechanical engine, they will open and close just just as they always do, even during the compression brake cycle.
Cat, they have a few old engines that the valves timing is changed.
Detroit. is a two cycle engine and is way different in the method of exhaust braking.

I any diesel there are three conditions in which the engine operates.

On Speed, = engine RPM & power is equal to the fuel flow given.
Under Speed, = the RPM & power is below what is required by fuel flow it is pulling, trying to catch up.
Over Speed, = the the fuel has stopped and the engine is reducing the RPM, or trying to.

Over Speed is when engine braking occurs, no fuel flow, so we add a mechanical device that will help stop air flow. either by restricting the exhaust or the intake air flow.
Cummins and Detroit chooses the intake side, Cat chose the exhaust. that is why the Cat will pop,pop,pop, each time the exhaust is relieved. Cummins and Detroit won't do this.
 
Tom, you need to distinguish between the early and late 8.3,, 12 valve or 24 valve,, pre 1998 or post 98,, they are different.>>>Dan
 
Compression does not work as you think it does to slow you.. The following numbers are not only thin air, but save for the first, taken out of thin air and are to be considered representive only

You suck in air at 14psi (roughly 1 atmosphere) pressure, You compress it to say 100 PSI. then you de-compress it back to 14PSI, in a frictionless world you recover all the energy of compression when you de-compress.  (This assumes NO fuel to the cylinder)

Then you open the exhaust, and push that air out, close the exhaust and open the intake and suck in fresh air.

All in all again in a frictionless worled, you have neither lost nor gained energy

On a Gasoline engine with a throttle plate. the air sucked in (During decelration) is at a much lower pressure, in fact the engine is fighting to suck, air in since the air flow is blocked,, In addition the timing changes (it is much advanced under high vacuum) so the engine provides braking... Diesels should not work that way from what I understand.

But high or low compression.. There needs to be something more to cause engine braking.. In the case of the diesel they basically throttle the exhaust instead of the intake.. Same result, different means.
 
That is a good video but it only covers one of the three auxiliary brake systems used on a motorhome.
1) Let me Youtube that for you https://www.youtube.com/watch?v=HkfjCJClWVA
2) The more common is the exhaust brake. http://www.jacobsvehiclesystems.com/technology/exhaust-brakes/
3) The transmission brake is another type. This is a good video showing how a automatic transmission works.http://youtubesob.com/download/video/Allison-Transmission-How-It-Works/v4AJ2BvUMG4
Billl
 
You must log in or register to reply here.

Latest posts

Members online

Total: 982 (members: 28, guests: 954)

Forum statistics

Threads
132,140
Posts
1,390,946
Members
137,860
Latest member
GeeRob
Back
Top Bottom