.. 80 Series Diesels What I've Learned thus far .. (1 Viewer)

Joined
Nov 20, 2020
Messages
20
Location
San Antonio, TX
Going to stir the pot here a little: some of ya'll aren't super familiar with physics, and it shows. Below are a couple helpful links with some info.

Engineering Explained - What Is Engine Braking? What Is A Jake Brake?

In short:

Gas engine braking comes from vacuum created by the throttle body when the piston is trying to pull air past the butterfly valve on the intake stroke (ie, creating vacuum)

Diesel engine braking comes ONLY WHEN either:
1. the "throttle body" from a gas engine is put in the exhaust pipe to restrict airflow through the engine. This causes work do be done when the piston has to push air past the butterfly valve on the exhaust stroke. Kinda like the old "banana in the tailpipe" trick in beverly hills cop or whatever it was.​
OR​
2. the exhaust valve is opened before the pressure built during the compression cycle can be used by the engine to push the piston back down. This means the piston is used to squish air on the compression stroke (doing a lot of work), then all that compressed air is released before the exhaust stroke (before the power stroke even).
Without either of those two things being involved, the diesel piston is simply squishing air, then that same squished air is pushing the piston back down.​
Agree completely. Pound for pound a gasoline engine will have better engine braking. I think there is a perception that diesel has better engine braking because many times they tend to be larger displacement engines, hence creating more friction, so friction braking is increased. But equal displacement gas/diesel, the gas will outbrake the diesel due to vacuum
 

mudgudgeon

SILVER Star
Joined
Dec 17, 2007
Messages
4,663
Location
Sydney, Australia
I think there is a perception that diesel has better engine braking because many times they tend to be larger displacement engines, hence creating more friction, so friction braking is increased. But equal displacement gas/diesel, the gas will outbrake the diesel due to vacuum

All my comments are based on my experiences with many different diesel, manual trans 4x4s.

I have owned two 4.2 litre diesel landcruisers. HD-T, and 1HZ engines. (18:1, & 22:1 compression respectively).
I currently have a 4.2litre, manual fzj80, so I have experienced apples for apples comparison.

I would choose the diesel over the gasser for offroad stuff any day of the week, and twice on Sundays.

I don't know how to do a meaningful calculation to scientifically compare the two.
In the diesel, each compression stroke is compression 700cc of air at 22:1

In the gasser, you'll get mininal compression because it's under vacuum. Under vacuum, it's still drawing some air into the cylinder, but not the full volume? Static compression ratio of 10:1, but partial vacuum drops the actual compression ratio.

No sure how vacuum helps?

Are you saying the resistance of the gas engine piston pulling a vacuum on the intake stroke outweighs the resistance of a diesel engine piston compressing a full pot of air to 20ish:1


The difference is notable on the street too.
Driving the fzj80 manual, you back off the throttle, it decelerates, but gently.
In a diesel, back off the throttle, they decelerate hard!
 

AussieHJCruza

SILVER Star
Joined
Jul 8, 2013
Messages
1,894
Location
New South Wales, Australia
All my comments are based on my experiences with many different diesel, manual trans 4x4s.

I have owned two 4.2 litre diesel landcruisers. HD-T, and 1HZ engines. (18:1, & 22:1 compression respectively).
I currently have a 4.2litre, manual fzj80, so I have experienced apples for apples comparison.

I would choose the diesel over the gasser for offroad stuff any day of the week, and twice on Sundays.

I don't know how to do a meaningful calculation to scientifically compare the two.
In the diesel, each compression stroke is compression 700cc of air at 22:1

In the gasser, you'll get mininal compression because it's under vacuum. Under vacuum, it's still drawing some air into the cylinder, but not the full volume? Static compression ratio of 10:1, but partial vacuum drops the actual compression ratio.

No sure how vacuum helps?

Are you saying the resistance of the gas engine piston pulling a vacuum on the intake stroke outweighs the resistance of a diesel engine piston compressing a full pot of air to 20ish:1


The difference is notable on the street too.
Driving the fzj80 manual, you back off the throttle, it decelerates, but gently.
In a diesel, back off the throttle, they decelerate hard!
Also having owned both FZJ80 and HDJ80 manuals this is 100% correct, the diesel engine-brakes significantly more than the FZ. You can drive a HZ without using the brakes fairly easily and to a lesser extent an HD the same
 
Last edited:
Joined
Nov 20, 2020
Messages
20
Location
San Antonio, TX
All my comments are based on my experiences with many different diesel, manual trans 4x4s.

I have owned two 4.2 litre diesel landcruisers. HD-T, and 1HZ engines. (18:1, & 22:1 compression respectively).
I currently have a 4.2litre, manual fzj80, so I have experienced apples for apples comparison.

I would choose the diesel over the gasser for offroad stuff any day of the week, and twice on Sundays.

I don't know how to do a meaningful calculation to scientifically compare the two.
In the diesel, each compression stroke is compression 700cc of air at 22:1

In the gasser, you'll get mininal compression because it's under vacuum. Under vacuum, it's still drawing some air into the cylinder, but not the full volume? Static compression ratio of 10:1, but partial vacuum drops the actual compression ratio.

No sure how vacuum helps?

Are you saying the resistance of the gas engine piston pulling a vacuum on the intake stroke outweighs the resistance of a diesel engine piston compressing a full pot of air to 20ish:1


The difference is notable on the street too.
Driving the fzj80 manual, you back off the throttle, it decelerates, but gently.
In a diesel, back off the throttle, they decelerate hard!
I don’t disagree with you on the merits of a diesel vs gas off-road; I’d love to have a diesel! But for compression braking, I would argue if all things held equal, the gasoline would have better braking. Are the gear ratios equal (first gear and final ratio) between the HZ/D and FZ? If not, then it might convolute the comparison.

Yes, I would argue the resistance of the vacuum in a gas is the driving factor. In a diesel, the energy conserved in the compression stroke will be released in the expansion stroke, so despite the diesel having a harder time compressing the air, it will also have more “rebound” in the compression stroke (I do realize some of this energy will be lost as friction).

its also hard to argue with your real world results! :)
 

mudgudgeon

SILVER Star
Joined
Dec 17, 2007
Messages
4,663
Location
Sydney, Australia
Are the gear ratios equal (first gear and final ratio) between the HZ/D and FZ? If not, then it might convolute the comparison.

Similar, but with differences in transmisson type and tire size. Differences in final gearing were minor. Maybe 5% difference.
Both my diesels were driven with various gearing and tire combinations.
It's not so much about the gearing as the behaviour and engine characteristics.

To be fair, my three cruisers where owned at separate times over 15 year period. And the FZJ80 is still pretty damn good offroad with manual trans.

I'll never choose an auto for any vehicle, particularly not for wheeling.
 
Joined
Sep 30, 2014
Messages
468
Gas engine braking comes from vacuum created by the throttle body when the piston is trying to pull air past the butterfly valve on the intake stroke (ie, creating vacuum)

Diesel engine braking comes ONLY WHEN either:
1. the "throttle body" from a gas engine is put in the exhaust pipe to restrict airflow through the engine. This causes work do be done when the piston has to push air past the butterfly valve on the exhaust stroke. Kinda like the old "banana in the tailpipe" trick in beverly hills cop or whatever it was.​
OR​
2. the exhaust valve is opened before the pressure built during the compression cycle can be used by the engine to push the piston back down. This means the piston is used to squish air on the compression stroke (doing a lot of work), then all that compressed air is released before the exhaust stroke (before the power stroke even).
Without either of those two things being involved, the diesel piston is simply squishing air, then that same squished air is pushing the piston back down.​

So just remember, the "vaccum" the petrol engine has to suck against on the intake stroke then helps "suck" the piston back up during the compression stroke (the same way the compressed air in the diesel is then used to push the piston back down on the power stroke).

The difference is, in a diesel the energy used to compress air during the compression stroke is converted into heat, which is absorbed by the piston, cylinder wall and cylinder head. That's where the engine braking comes from... Forward momentum being converted into heat which is absorbed by the engine. As the air expands on the power stroke it rapidly cools and sucks some of the heat out of the engine block, piston and cylinder heads... But all of these conversions of heat are inefficient, meaning the "losses" are actually the momentum being converted into heat and staying in the metal masses. A petrol engine working with vacuum isn't producing anywhere near as much heat (compressing nothing doesn't generate heat compared to compressing a medium (eg air)).

And FYI, this isn't the first time I've seen information on Engineering Explained be COMPLETELY WRONG.

The theory can be as good as you like, but in the real world having two cruisers side by side (petrol vs diesel) the diesel has better engine braking. The main physics lesson to remember is you can't destroy energy (apart from converting it to matter), you can only convert it. Engine braking comes from converting kinetic energy to heat, nothing more.
 

swankstar

SILVER Star
Joined
Jul 20, 2017
Messages
475
Location
Boise, Idaho
1. All my comments are based on my experiences with many different diesel, manual trans 4x4s.

2. I would choose the diesel over the gasser for offroad stuff any day of the week, and twice on Sundays.

3. I don't know how to do a meaningful calculation to scientifically compare the two.
In the diesel, each compression stroke is compression 700cc of air at 22:1

4. In the gasser, you'll get mininal compression because it's under vacuum. Under vacuum, it's still drawing some air into the cylinder, but not the full volume? Static compression ratio of 10:1, but partial vacuum drops the actual compression ratio.

5. No sure how vacuum helps?

6. Are you saying the resistance of the gas engine piston pulling a vacuum on the intake stroke outweighs the resistance of a diesel engine piston compressing a full pot of air to 20ish:1


7. The difference is notable on the street too.
Driving the fzj80 manual, you back off the throttle, it decelerates, but gently.
In a diesel, back off the throttle, they decelerate hard!
I added some numbers to your comment so I can address individually. My comments match up below.

1. I am not trying to detract from your experience in any way

2. I would too, that’s why I’m doing a diesel swap in mine.

3. Calculations aren’t even needed for this, it’s higher level ideas that govern what’s going on here.

4. Compression ratio isn’t playing a role here in any way, shape, or form (for the engines we are talking about, if we were talking about 14L Cummins or Volvos that would be different because they have compression brakes added to them).

5. it’s not the vacuum itself that’s helping, it’s the work the engine has to do to create that vacuum that matters in this situation.

6. There are outside forces at work on the gas engine (in this case a restriction to air being allowed into the engine by the throttle plate, or whatever they are called) and none on the Diesel engine. It literally does not matter what the engine is doing inside of it in this scenario, with “this scenario” being one where no fuel is being added to the engine and your foot is off the skinny pedal.

Think about this example for a minute: say you are looking into a room with an electric outlet and a fan. Said fan is plugged into outlet, and fan is on. Work with me on this when I say “imagine this room is perfectly insulated”. This means ZERO energy of any kind can leave the room. What happens to the temp in this room? Up? Down? Same?​

7. Here, I have to disagree with you. I have owned diesel vehicles for over 20 years, worked in them, worked with them, worked on them, set world records with them, watched them runaway on the dyno from turning the fuel screw too much, hauled many a trailer over mountain passes, etc, etc

Coming down a mountain pass with a 5.9L diesel powered rig with manual transmission and a trailer in tow, if you take your foot off the brake, the vehicle will accelerate until there is so much inertial energy in the rotating components of the engine that it will come apart, all on its own. No if’s, ands, or buts. Does not matter in the least what gear you’re in. This is why semis are required to have compression brakes installed on them.

Come down the same pass with same trailer and a 6.0L cheby V8 in it with manual trans, drop a gear so you have high rpms, take your foot off the go pedal and that thing is going to slow the hell down.

This is why companies like PacBrake are in business. They make lots of things, but consumer grade exhaust brakes are what they are known for. This device is literally the same throttle plate from #6 above, but put in the exhaust pipe of a Diesel instead of the intake pipe of a gas engine. Instead of the gas engine sucking air through it, the Diesel engine has to push exhaust through the exact same device. Then, and only then, will you be able to take your foot off the brake pedal going down a hill with weight behind you.
 
Joined
Sep 30, 2014
Messages
468
I’m sorry, but that is not correct. Engine braking comes from external forces being applied to the rotating assembly.
And what are those forces? Where is the kinetic energy going?

The other reason why you may get good engine braking in a petrol in your example above, is the engine is spinning at higher RPM, creating more friction (and thus more heat).
 

swankstar

SILVER Star
Joined
Jul 20, 2017
Messages
475
Location
Boise, Idaho
And what are those forces? Where is the kinetic energy going?

The other reason why you may get good engine braking in a petrol in your example above, is the engine is spinning at higher RPM, creating more friction (and thus more heat).
See #6 in the post I was writing while you were writing your long one.

Friction in an ICE engine is close to negligent negligible in the big scheme of things. But, to answer your question, any heat generated by friction ends up either in the oil (and dissipated via the oil cooler, if equipped, or via the oil pan in most cases) or the water (and dissipated via the radiator).

An example to back that claim up is changing oil viscosities in a 500hp engine. Go from 50 weight oil as main lube oil to 0 weight, and I’m guessing you’ll see maybe 10hp increase, at max power level, which is going to be very high rpm.

As far as “not being able to destroy energy”, you are absolutely correct. What we are talking about in this discussion is work though, a very different concept.

Edit: a word
 
Last edited:

Users who are viewing this thread

Top Bottom