Newer Models Ford F250 Gas Vs. F250 Diesel

[sbowman]

My 2 cents, 40 years of towing, 38 w/big block gasser's, last 2 w/Diesel. Worlds apart in how each performs and how one drives each one. Yes understand question was on Ford. Yes, my gasser's were Fords. For us, we have no plans to go back to gasser's.

Best regards and safe travels

[GCinSC2]

Originally Posted by Countryboy59

That’s funny. They pull cabs on those very often. You guy’s with 50,000 miles in 10 years wouldn’t know. Ask someone who actually works on trucks instead of just reading stuff on the web.

Yeah my neighbor thought it was pretty funny after his then new 6.7L had its cab off for a month shortly after new purchase.

He complained to dealer got cold shoulder and I found the valve guide TSB and suggested the dealer at least consider reviewing it. Cab came off soon after that.

My 2005 Dodge 5.9L averages 16K miles per year.

[jcl]

Originally Posted by Tuco

This is exactly what i was talking about as far as "Smoke and Mirrors" in the other thread. Do you honestly believe that drawing a vacuum behind the throttle plate is why a gas engine has engine braking? When you put your hand over the end of a vacuum cleaner hose is the motor working harder or does its speed increase indicating more rpm and less load? Both types of engines are four cycle engines (intake, compression, power, exhaust). What actually gives gas and diesel engines their ability to engine brake is their compression ratio. Diesel engines have a higher compression ratio than their gas counterparts. As you have stated before a diesel engine has no throttle plate. Each time the compression stroke comes around it has to compress a full air charge thus creating the work seen as engine braking. In a gas engine on each compression stroke the vacuum created in the manifold means less air is drawn into each cylinder to be compressed. This reduces the effective compression ratio of the engine. Which also reduces the amount of work seen as engine braking. The faster the engine turns the higher the manifold vacuum becomes thus reducing work done per compression cycle. That is not the case in a diesel engine. Now add in the exhaust brake which pressurizes the exhaust manifold. This causes the engine to work to push the air out of the cylinder on each exhaust stroke. This adds even more work seen as engine braking. The faster the engine turns the higher this exhaust pressure becomes. Which means more work is done at higher rpm. So to sum things up. A diesel engine has more engine braking than a gas engine in a normally aspirated configuration. Adding the exhaust brake into the mix on a diesel means you have added a second cycle to the equation which is seen as additional engine braking.

To use your analogy of putting your hand over the hose on a vacuum cleaner, it doesn't matter if you block off the inlet (like a gas engine does with a throttle, or the few diesels that have butterfly valves) or if you plug off the outlet (like a light duty diesel does with an exhaust brake. Exactly the same process. So yes, the vacuum behind the throttle plate, and more correctly, the pumping losses through the whole system, are what produce braking in a gas engine.

As has been noted, compression ratio in these engines doesn't enter in to it. It is irrelevant. It might be better to think of a single cylinder, though, and not introduce the multicylinder issue, to better see why that is.

Considering a single cylinder at a time now, you are completely right that work is done as the piston compresses what is in the cylinder. A diesel compresses it more, due to the higher compression ratio. What you have wrong though, is what happens next. The next stroke is the power stroke, not the exhaust stroke. On the power stroke, the exhaust valve is still closed, so the exhaust manifold is not part of the discussion yet. The compressed air in the cylinder works just like a spring (or an air bag, if you like) and pushes the piston back down, turning the crankshaft. The energy is recovered, less frictional losses, so no net braking effect from compression. Then the exhaust valve opens, and the uncompressed air goes out to the exhaust manifold. If there is an exhaust brake as on many light duty diesels (and some mid duty diesels), or a way of controlling the turbo geometry to produce restriction, then it takes work to push the air out, and that choke (throttling effect) results in pumping losses, which is what slows the engine, producing a retardation effect for the vehicle.

The exception to this is if one has an actual compression release brake. Note that it isn't a compression brake, but a compression release brake. In that case, when the piston is at TDC and the engine has done work to compress the air, a valve opens and exhausts that compressed air to atmosphere, releasing it. You can often hear it as a very loud noise on heavy trucks, which is why a "Jake Brake" is often prohibited in built up urban areas. Unless you have a way of varying the exhaust valve timing to release the compression, none of this is a factor.

In short, no light duty diesel has a compression release brake, so you are unable to take advantage of the higher compression ratio.

This isn't smoke and mirrors, it is engineering.

[Tuco]

Originally Posted by jcl

To use your analogy of putting your hand over the hose on a vacuum cleaner, it doesn't matter if you block off the inlet (like a gas engine does with a throttle, or the few diesels that have butterfly valves) or if you plug off the outlet (like a light duty diesel does with an exhaust brake. Exactly the same process. So yes, the vacuum behind the throttle plate, and more correctly, the pumping losses through the whole system, are what produce braking in a gas engine.

As has been noted, compression ratio in these engines doesn't enter in to it. It is irrelevant. It might be better to think of a single cylinder, though, and not introduce the multicylinder issue, to better see why that is.

Considering a single cylinder at a time now, you are completely right that work is done as the piston compresses what is in the cylinder. A diesel compresses it more, due to the higher compression ratio. What you have wrong though, is what happens next. The next stroke is the power stroke, not the exhaust stroke. On the power stroke, the exhaust valve is still closed, so the exhaust manifold is not part of the discussion yet. The compressed air in the cylinder works just like a spring (or an air bag, if you like) and pushes the piston back down, turning the crankshaft. The energy is recovered, less frictional losses, so no net braking effect from compression. Then the exhaust valve opens, and the uncompressed air goes out to the exhaust manifold. If there is an exhaust brake as on many light duty diesels (and some mid duty diesels), or a way of controlling the turbo geometry to produce restriction, then it takes work to push the air out, and that choke (throttling effect) results in pumping losses, which is what slows the engine, producing a retardation effect for the vehicle.

The exception to this is if one has an actual compression release brake. Note that it isn't a compression brake, but a compression release brake. In that case, when the piston is at TDC and the engine has done work to compress the air, a valve opens and exhausts that compressed air to atmosphere, releasing it. You can often hear it as a very loud noise on heavy trucks, which is why a "Jake Brake" is often prohibited in built up urban areas. Unless you have a way of varying the exhaust valve timing to release the compression, none of this is a factor.

In short, no light duty diesel has a compression release brake, so you are unable to take advantage of the higher compression ratio.

This isn't smoke and mirrors, it is engineering.

So according to you, wikipedia and the youtube science guy when I let off the throttle in my truck it should just coast because energy is conserved. Please explain to me then why does the truck slow down just like a "gas motor"? Why when I down shift to the next gear do I get even more braking? This is reality not just smoke and mirrors on my part. All of this happens without me switching on the exhaust brake. Let me add something else to this discussion. When I was much younger I used to race 1/4 mile in my 440 Road Runner. I decided for whatever reason to add an electric full pump next to the tank to push fuel to the stock fuel pump on the engine. I didn't realize at the time that i needed to run a larger wire to that pump to minimize the resistive load of the wire run. The result was that the fuse blew not to long after that causing the pump to stop working. The mechanical pump could suck enough full to fill both carburetors at idle and and partial load, but at the track at about 100 mph both carbs ran dry and the car fell flat on its face. By this I mean the car was slowing...rapidly even though all 8 butterfly's were wide open (IE no vacuum). According to you and the internet my car should have just been coasting which it was not. So again please explain why that is.

[jcl]

Originally Posted by Tuco

So according to you, wikipedia and the youtube science guy when I let off the throttle in my truck it should just coast because energy is conserved. Please explain to me then why does the truck slow down just like a "gas motor"? Why when I down shift to the next gear do I get even more braking? ......So again please explain why that is.

The energy we are talking about being conserved is the related to the work done in compressing air in the cylinder on the compression stroke, and recovered on the power stroke.

The discussion above was about the effects of the higher compression ratio on a diesel engine, and how that produces no net braking effect. Of course, the engine still has friction from rotating surfaces and the pistons against the cylinder wall, (and more so at higher rpm, so more retardation effect in a lower gear) and the drivetrain has friction, and the vehicle is subject to drag from the air, so the truck will slow down. But not due to the compression ratio of the engine.

Does that help?

[Tuco]

Originally Posted by jcl

The energy we are talking about being conserved is the related to the work done in compressing air in the cylinder on the compression stroke, and recovered on the power stroke.

The discussion above was about the effects of the higher compression ratio on a diesel engine, and how that produces no net braking effect. Of course, the engine still has friction from rotating surfaces and the pistons against the cylinder wall, (and more so at higher rpm, so more retardation effect in a lower gear) and the drivetrain has friction, and the vehicle is subject to drag from the air, so the truck will slow down. But not due to the compression ratio of the engine.

Does that help?

That is an awful lot of friction in my book. So much so that I would assume the engine was blowing up. If I disengage the engine the truck rolls with little effort due to its mass. With the engine turning I'm getting 20 times the amount of rolling and wind resistance of the rest of the truck. No, somewhere inside that engine is another force being applied. Any other theory's as to what that might be?

[jcl]

Originally Posted by Tuco

That is an awful lot of friction in my book. So much so that I would assume the engine was blowing up. If I disengage the engine the truck rolls with little effort due to its mass. With the engine turning I'm getting 20 times the amount of rolling and wind resistance of the rest of the truck. No, somewhere inside that engine is another force being applied. Any other theory's as to what that might be?

How about we call it pumping losses? It should be less than the friction component, but it is still there.

My last two diesels (VW and Landrover) had pretty much no engine braking effect (no exhaust brake installed). Maybe your Cummins has more friction and pumping losses than you expect. What's your theory? Have we agreed that the power stroke follows the compression stroke, and that the exhaust valve is closed during that stroke, so that the compression ratio is not a differentiating factor?

[Countryboy59]

Originally Posted by GCinSC2

Yeah my neighbor thought it was pretty funny after his then new 6.7L had its cab off for a month shortly after new purchase.

He complained to dealer got cold shoulder and I found the valve guide TSB and suggested the dealer at least consider reviewing it. Cab came off soon after that.

My 2005 Dodge 5.9L averages 16K miles per year.

So what? They all can have issues. There is a Ram in the shop right now with the cab off. Doesn’t have to come off but saves quite a bit of labor if you know what you’re doing.

Cab off is not a difficult job.

[pteck]

Geebers. With the way these things need to get serviced, you'd think they're exotic cars. What happened to the maintainability? Trucks are supposed to be workhorses, easy to service. To maximize time in field. I would tolerate this for a sports car or exotic, but for a pickup??? One that has phallic front end, and a gaping hood???

With the way I like to outfit my cars, add accessories, now there has to be provisions for wire harness disconnects and such to ensure the cab can be separated.

What's the minimum laber addition for a procedure that requires cab off? Hope u guys don't have to deal with this ever out of warranty.

And in the other thread, the diesel guys are still trying to justify cost of ownership being equal to gas...

Or does gas variants also require cab off?

[Countryboy59]

Originally Posted by pteck

Geebers. With the way these things need to get serviced, you'd think they're exotic cars. What happened to the maintainability? Trucks are supposed to be workhorses, easy to service. To maximize time in field. I would tolerate this for a sports car or exotic, but for a pickup??? One that has phallic front end, and a gaping hood???

With the way I like to outfit my cars, add accessories, now there has to be provisions for wire harness disconnects and such to ensure the cab can be separated.

What's the minimum laber addition for a procedure that requires cab off? Hope u guys don't have to deal with this ever out of warranty.

And in the other thread, the diesel guys are still trying to justify cost of ownership being equal to gas...

Or does gas variants also require cab off?

I don’t know about the Ford, but the Ram adds about 4 hours to pull the cab. Work on the inline 6 is Much easier with the cab off.

The gas engine has a lot fewer head gasket changes. Also the EGR delete and other mods on the Powerstroke are much much easier with the cab off.

If we do a cab off this weekend (Ram or Ford) I’ll post pics. The one I mentioned earlier is already done!

[slowmover]

Originally Posted by mojo

My experience is as anecdotal as your opinion.

Six years towing with the F150 and experiencing close to overheating transmission (6 gears), close to overheating engine and brakes has shown me that even in 2nd gear on a 14% downgrade, a diesel with engine brake does a better job. And that's a fact!

Trailer brakes were incapable. Not fuel choice.

[jcl]

Originally Posted by slowmover

Trailer brakes were incapable. Not fuel choice.

Likely, at least with respect to the brake issue, but note that the transmission, engine, and brakes under discussion didn't overheat. They "came close to overheating" but we don't know what that means.

[pteck]

Originally Posted by Countryboy59

I don’t know about the Ford, but the Ram adds about 4 hours to pull the cab. Work on the inline 6 is Much easier with the cab off.

The gas engine has a lot fewer head gasket changes. Also the EGR delete and other mods on the Powerstroke are much much easier with the cab off.

If we do a cab off this weekend (Ram or Ford) I’ll post pics. The one I mentioned earlier is already done!

So serviceability in the field, let alone DIY, is a no go for common issues that previously were?

In regards to head gaskets, that's an epic failure of engineering if it's a common issue. Especially if it's anywhere within 200k miles of service. Thought diesels were suppose to have way longer spans between major service intervals, but it's not looking like it with all the fuel filters, DEF reliability issues, injectors, high pressure fuel pumps, turbo's, headgaskets, and so on. People think diesels work less hard, but if they had any idea the comparable cylinder pressures compared to gas. It's no wonder head gasket failures are more common.

[Bill M.]

I pull a 6300 lb Airstream with a Cummins diesel with out a exhaust brake. They did not come on the truck the year I bought. I tried the combination on several different grades around 6% and determined that the engine does brake some. At least as much as the gas F150 with the little V8 I had before. There have been times out west when I really wished I had added the exhaust brake. But I did not know that I would be in this truck for such a long run.

Yes, I really believe that closing the throttle valve and making the engine a big vacuum pump provides braking. Yes, I think that closing off the exhaust can provide higher pressures and more braking. Therotically I would think that a closed off cylinder would give back as much energy on the down stroke as it took on the upstroke, minus some lost heat. But however it works the drive line does slow the rig down if you are properly downshifted at the top of the hill.

Why don't we see exhaust brakes on gas trucks?

[mojo]

Originally Posted by jcl

Likely, at least with respect to the brake issue, but note that the transmission, engine, and brakes under discussion didn't overheat. They "came close to overheating" but we don't know what that means.

What it means is that since you were not there, there are these things called temperature gauges that will tell you when you are reaching a condition that will switch on the MIL and put the vehicle into limp mode. Pulling over to let things cool down works well, but is time consuming. Also, an infrared temp monitor can display the brake temps. But enough of hijacking this thread, I'll let you armchair engineers continue your debate. Happy trails!

[pteck]

Originally Posted by Bill M.

<Snip>
Why don't we see exhaust brakes on gas trucks?

That's a telling question.

Jcl has done a great job at explaining. It is a complex mechanism.

Bottom line is that a gas engine architecture inherently has additional braking resistance beyond just engine drag. It's higher rpm ability is another asset in maximizing the braking leverage of the motor. Though not all motors create the same degree of braking and it does depend on design parameters of the specific motor, tranny, diff, etc.

Diesels with additional diesel brakes do likely have stronger engine brakes. Heck, if you're going to fit an additional mechanism, complexity, and cost for a specific function, it better do a great job.

[gypsydad]

Originally Posted by pteck

So serviceability in the field, let alone DIY, is a no go for common issues that previously were?

In regards to head gaskets, that's an epic failure of engineering if it's a common issue. Especially if it's anywhere within 200k miles of service. Thought diesels were suppose to have way longer spans between major service intervals, but it's not looking like it with all the fuel filters, DEF reliability issues, injectors, high pressure fuel pumps, turbo's, headgaskets, and so on. People think diesels work less hard, but if they had any idea the comparable cylinder pressures compared to gas. It's no wonder head gasket failures are more common.

Pteck- we get it; you love your SUV... But have you ever owned a 3/4T diesel PU for pulling an AS? Have you driven one for an extended period pulling a 27' or larger AS in recent years? If not, you may not get the point most of us owners are making, vs gas or other TV's. Just saying...the 3/4T diesels are pretty darn nicely equipped for the job of towing a larger AS...easier to pull, stop, control, and engine brake is heavenly...plus theres plenty payload with several people on board, a generator, firewood, gas, kayaks, camping gear, etc, etc, etc... We get it; you love your SUV...enjoy.
The Thread was gas vs diesel F250, I thought?

To repeat my earlier post; this is my first F250 Diesel 2017; big truck to drive compared to my F150 EB. Love it for the job it does pulling my 28'. 13mpg pulling; 17mpg without trailer. Service and fuel cost a bit more. Does the job exceptionally well pulling the 28' AS. Service manager at Ford I talked with last week said he has same year (2017) F250 6.2 gas; said his mileage is not close to mine pulling his 28' Jayco and wishes he had gotten the diesel. Hope this helps.

[jcl]

Originally Posted by Bill M.

Yes, I really believe that closing the throttle valve and making the engine a big vacuum pump provides braking. Yes, I think that closing off the exhaust can provide higher pressures and more braking. Therotically I would think that a closed off cylinder would give back as much energy on the down stroke as it took on the upstroke, minus some lost heat. But however it works the drive line does slow the rig down if you are properly downshifted at the top of the hill.

Why don't we see exhaust brakes on gas trucks?

pteck covered it. More fully, because it is an interesting story, the compression release brake was invented to rectify a deficiency when an early Cummins diesel was retrofitted into a highway truck, circa 1931, they overheated the service brakes on a descent, and Clessie Cummins described it as "we had escaped certain death by inches". The patent holder was Clessie Cummins, but he had left the Cummins engine company by that point, so looked for a manufacturer. He found the Jacobs manufacturing company, who were looking for products to make, and that is why we have what became known as the Jake Brake. Not the same mechanism as an exhaust brake like on pickups, but the same effect, and that is where it started.

Incidentally, that story, including the quote, and the full story of the rise of the diesel engine in transportation applications in North America, is told in the book "The Engine That Could" by Jeffrey L Cruikshank and David B. Sicilia, (1997) from the Harvard Business School Press. it is essentially the story of the Cummins Engine Company over a 75 year period, 1919-1994, but that story is intertwined with the development of diesels overall. I found it a very interesting read, so recommend it to anyone who is interested in how it all came about, particularly if you have an interest in diesel engines. Seems there are lots of posters here who do.

Jeff

[Tuco]

Originally Posted by jcl

How about we call it pumping losses? It should be less than the friction component, but it is still there.

My last two diesels (VW and Landrover) had pretty much no engine braking effect (no exhaust brake installed). Maybe your Cummins has more friction and pumping losses than you expect. What's your theory? Have we agreed that the power stroke follows the compression stroke, and that the exhaust valve is closed during that stroke, so that the compression ratio is not a differentiating factor?

I'm at a loss as to why you keep mentioning that the power stroke follows the compression stroke. Did I miss something here? Isn't that what we were taught as kids all those years ago? I should think that is a given. What we cant seem to find is all that engine braking that occurs in my truck (and all the other diesels I have driven). I agree that friction and pumping losses account for some of it, but not all of it. I gave you an example of what happens when a gas engine has a fuel starvation problem at WOT. Any hotrodder will tell you that they get thrown forward in their seat. There is plenty of engine braking available without a vacuum behind the throttle plate. From what I have seen you get more engine braking in the above scenario than when you just let off the throttle. I think we both agree that the intake and exhaust stroke do nothing for engine braking (let me know if you think otherwise). So if a gas engine doesn't need a vacuum to provide engine braking and a diesel without a throttle plate has engine braking. Then we need to take a closer look at the compression and power strokes to find additional work done.

Lets look at the compression stroke first. Air is compressed causing heat. Mechanical energy has been converted to thermal energy. Energy is also stored as compressed air. A small amount of fuel is then injected into the cylinder and starts to burn. This adds more thermal energy and pressure to the closed system. On the power stroke pressure is reduced adding energy back to the system and thermal energy is reduced due to expansion. Any excess energy is then expelled on the exhaust stroke. All the evidence seems to indicate that it takes more mechanical energy to perform these two cycles than is being returned. The next step would be to calculate the energy required to perform each of these "loads". Then find out what else we haven't thought of. Stopping here for input.

[jcl]

Originally Posted by Tuco

I'm at a loss as to why you keep mentioning that the power stroke follows the compression stroke. Did I miss something here? Isn't that what we were taught as kids all those years ago? I should think that is a given.

Because back in post #56 you postulated that the work done to compress the air during the compression stroke is what produces engine braking, and then went on to the exhaust stroke, leaving out the power stroke. I wanted to make sure we had agreement on the point that the power stroke returns energy due to the spring effect (not 100%, but largely) as I wasn’t sure that you had acknowledged it.

Originally Posted by Tuco

What we cant seem to find is all that engine braking that occurs in my truck (and all the other diesels I have driven)...... Stopping here for input.

Friction is a pretty big thing. Here is some reading on the breakdown of frictional losses.

https://www.researchgate.net/publica...ustion_Engines

You may also be interested in the book on Clessie Cummins that I mentioned in another post. It goes into why supplementary braking was required so as to be able to market Diesel engines when they were being promoted as an alternative to gasoline engines in early road transport applications. The lack of engine braking was a major commercial obstacle since service brakes were far less capable than they are today.

Finally, note that the point isn’t that Diesel engines have no inherent engine braking, just that they inherently have much less than gasoline engines, and that the higher compression ratio doesn’t create a retardation effect. There are ways of creating a retardation effect in the engine design. Consider that some diesels have throttle bodies that produce some restriction (whether to control runaway situations, or manage EGR for emissions, or to help with shut down), some have significant exhaust restriction even without an exhaust brake, and so on.