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windsprinter
08-20-2008, 01:20 AM
OK, besides accelerating slowly, keeping to about 90kmph/55mph, and reading the traffic ahead to try and maintain an even speed, what other techniques are involved with the sprinter? I just got my 08 144 high roof panel van, and I noticed that going down any hill, even steep, I couldn't let it accelerate in a high gear the same way I do in my manual tranny Dodge Dakota. I have a short steep hill near my house that I go down 1/2 way in second, engine braking in the Dakota, then if there is no traffic coming, I shift up to 4th or 5th and let it run out to the end of the road. I noticed right away in the Sprinter that if you backed off the fuel at the top of the hill, the engine braking actually slowed you down, and the manual upshift had no effect. You actually need to push the accelerator and give it fuel to go downhill. I suppose the zealot hypermilers would shift into neutral in that situation, which I won't do for safety reasons and I read in the manual that you can damage the transmission. But why can't you use gravity to help with these trucks? Is this just a difference of the diesel engine?

Any other tips?

sikwan
08-20-2008, 02:57 AM
I suppose the zealot hypermilers would shift into neutral in that situation, which I won't do for safety reasons

I would think with the vehicles that have electronic fuel injection, like the Sprinter, throwing it into neutral would actually use more fuel than leaving it in gear and making sure the rpm doesn't go below a certain value. :idunno:

jdcaples
08-20-2008, 03:15 AM
OK, besides accelerating slowly, keeping to about 90kmph/55mph, and reading the traffic ahead to try and maintain an even speed, what other techniques are involved with the sprinter?

I just got my 08 144 high roof panel van, and I noticed that going down any hill, even steep, I couldn't let it accelerate in a high gear the same way I do in my manual tranny Dodge Dakota.

I suppose the zealot hypermilers would shift into neutral in that situation, which I won't do for safety reasons and I read in the manual that you can damage the transmission. But why can't you use gravity to help with these trucks? Is this just a difference of the diesel engine?

Any other tips?

Shifting into neutral can damage the transmission? I don't remember reading that. I don't do that, but I am curious: Is that in the 08 owners manual or service manual, and if so, which page?

The first thing I noticed was the Sprinter's tendency to slow down once the accelerator was relaxed. I just figured it was the torque, but I'm not an expert.

One thing I do is drive by tachometer. I just use enough accelerator pedal to cause the vehicle to shift gears between 2000 and 2200 RPMs. I tend to drive the vehicle between 2000 and 2400/2500 RPM. I haven't had a need to approach - let alone sustain - 3000 RPM because of my driving habits.

The other thing I do on the highway is set the cruise control and tap the lever up and down to match my velocity to the speed limit.

-Jon

windsprinter
08-20-2008, 04:33 AM
Shifting into neutral can damage the transmission? I don't remember reading that. I don't do that, but I am curious: Is that in the 08 owners manual or service manual, and if so, which page?

The first thing I noticed was the Sprinter's tendency to slow down once the accelerator was relaxed. I just figured it was the torque, but I'm not an expert.

-Jon

Sorry, I had the truck for 3 days then have had to fly away for work for a month (must pay for the beast!), so I can't look up the page. It was in the owner's manual (the nearly 700 pg dissertation), I think under 'operating'.

I would have thought it was true about the torque, but this is at low speed going down a steep hill - you should be able to get a 'free' km or two from this! I think the throttle is down just a far going down as up!

Tx for the other tips. Don't have the cruise control, but I do the same with my vehicles that do have it.

Anyone know if its better for mileage to manually gear down when starting up a hill as soon as you feel lugging?: I suspect with the great torque of the diesel this isn't as much of an issue, but I know with my Dakota that I seem to be able to keep less throttle and maintain higher speed doing this, even though the 'gear up' light comes on on the dash.

cedarsanctum
08-20-2008, 04:58 AM
The slowing you experience is, i believe, the torque of the engine slowing it down. Gas engines don't do that as well. Higher compression gives better braking power.
But not to worry, seems it does OK coasting down hill. On our last trip, we started at the top of a hill 8 miles long, about 2000' of elevation drop, and averaged about 15 mph, the computer readout said 47.3 mpg at the bottom. All on gravel.
The rest of the trip was downhill from there, so to speak. We went from 4000 feet in the Cascades to the coast, and the computer got down to 24 mpg by the end of the trip. Amazing watching all that happen, and i think it helps keep the mileage #'s up to watch that number. Works in the Prius pretty well.

Jef

autostaretx
08-20-2008, 12:23 PM
The "don't coast in neutral" is on page 202 of the 2008 manual.

Quoth: "Do not move the selector lever to N while the vehicle is in motion. The automatic transmission could otherwise be damaged."

In Ye Olden Days, "free wheeling" an automatic transmission for extended periods would cause them to overheat, since the oil (transmission fluid) wasn't being circulated through the radiator.

For hypermiling, coasting the diesel with the transmission engaged at engine RPM greater than 1000 rpm turns the injectors totally -off-. They only resume squirting when the rpm drops below 1000, and even then at a rate of only about 0.2 GPH (according to my scangauge on my 2005). If i coast in neutral, they're squirting at 0.4 to 0.6 GPH to keep the motor running.

have fun
--dick
p.s. *surrounding* that "don't pop it into Neutral" are these two paragraphs:
-------------
No power is transmitted from the engine to the drive wheels.
Releasing the brakes will allow you to move the vehicle freely, e.g. by pushing or towing.
-------------
If ASR is deactivated or ESP« has malfunctioned: only move the selector lever to N if the vehicle is in danger of skidding, e.g. on icy roads.
-------------------

...so you're allowed to be in Neutral if you're being towed (Ye Olden Days required removing the propellor shaft), or if you've lost ESP. So "Don't!" becomes "rarely".

bikergar
08-20-2008, 12:53 PM
I noticed right away in the Sprinter that if you backed off the fuel at the top of the hill, the engine braking actually slowed you down, and the manual upshift had no effect. You actually need to push the accelerator and give it fuel to go downhill. I suppose the zealot hypermilers would shift into neutral in that situation, which I won't do for safety reasons and I read in the manual that you can damage the transmission. But why can't you use gravity to help with these trucks? Is this just a difference of the diesel engine?

Any other tips?

Diesel engines typically have little or no back pressure, they have access to as much air as RPM, not nessecarily fuel (ie\ coasting down a hill) dictates. This is the reason for Jake Brakes and exhaust brakes on the larger diesel engines. In two different ways each adds backpressure slowing the engine RPM. There is another thread about variable geometry turbochargers doing similar things, but thats over my head.:thinking:

I suspect what you are feeling is drivetrain braking more than engine braking. Try feathering the throttle just enough to upshift. It seems mine will go into 5th between 35 and 40 mph and will stay there if I don't ask anything of the engine.

Search out threads on the board about Sangauge. They really helps you get aquainted with your engine.:professor:

gary

talkinghorse43
08-20-2008, 01:44 PM
OK, besides accelerating slowly, keeping to about 90kmph/55mph, and reading the traffic ahead to try and maintain an even speed, what other techniques are involved with the sprinter? I just got my 08 144 high roof panel van, and I noticed that going down any hill, even steep, I couldn't let it accelerate in a high gear the same way I do in my manual tranny Dodge Dakota. I have a short steep hill near my house that I go down 1/2 way in second, engine braking in the Dakota, then if there is no traffic coming, I shift up to 4th or 5th and let it run out to the end of the road. I noticed right away in the Sprinter that if you backed off the fuel at the top of the hill, the engine braking actually slowed you down, and the manual upshift had no effect. You actually need to push the accelerator and give it fuel to go downhill. I suppose the zealot hypermilers would shift into neutral in that situation, which I won't do for safety reasons and I read in the manual that you can damage the transmission. But why can't you use gravity to help with these trucks? Is this just a difference of the diesel engine?

Any other tips?

Regarding the downhill performance, my '02 acts the same. I have assumed that is due to the way the control logic is designed. It seems to me the designers assumed that whenever a driver would lift his/her foot from the accelerator, it would be to slow down. So, it seems that when you lift your foot the turbo vanes are closed (to act like an exhaust brake), the torque converter locks up to maximize engine rpms and maximize exhaust braking (if in 2nd - 5th gear), and the tranny stays in the present gear; all to help slow the vehicle down and help out the service brakes when/if they are applied. I've found that I can "communicate" with the control logic and get it to do what I want by giving it a "burp" of fuel to tell it that I want it to upshift. I haven't yet found another way to "communicate" that desire.

BaywoodBill
08-20-2008, 04:12 PM
Hi, Windsprinter, welcome to the forum.

My first thought is that your hill isn't very steep. In general, these things just love to go downhill and I've found it very necessary to anticipate moutain curves on downhill grades well ahead of time to keep from raising my "my god, can I do this" adrenaline level.

The engine braking on a diesel is just not as good as on a gasoline engine that has a throat valve to cut off intake air.

windsprinter
08-20-2008, 05:04 PM
What a lot of great responses in a short time! Thanks :thumbup::thumbup: Please excuse my ignorance, I've never owned a diesel before and rarely driven them, I feel I have a steep learning curve ahead! So bear with some of my followup questions please!

Autostare: Tx for the page ref. Sorry, I don't follow completely. When you say the 'injectors are turned off' you are referring to fuel I presume not the circulation of the tranny fluid? When you say 'coasting' at >1K RPM, do you mean with the throttle backed off completely or pushed just hard enough to overcome the engine braking and maybe encourage upshifting? That is what Bikergar is talking about, I think?

I'll check out the sanguage (scanguage?) thread, sounds like I should spend some time there. I was hoping to get it to upshift to fifth at a much lower speed than that. The hill is steep (about 20%) and on my Dakota, if i shift to 5th at about 20 mph/35kmph, by the time I let out the clutch I'll be up to 800 RPM and a couple of seconds later I'll be doing 50-55 kmph, say just over 30 mph and about 1100 rpm at the bottom of the hill. The Sprinter by comparison will stay in 2nd gear and only do 25 kmph the whole way down if I don't give it throttle.

Talkinghorse, thanks, thats basically what I started doing, its too bad that doing an outward 'paddle' shift won't 'force' an upshift to higher gear at lower speed, seems like a waste of fuel to give it a burb.

BWB, thanks for the welcome, I've seen your name on a few threads looking around here ;) As I mentioned, the hill is steep, about 20% (too steep for the snowplows to get up during our rare snowfalls which can make life interesting), but you start down at very slow speed. I haven't driven it over anything steep and highwayish yet. One of my employees took it for its first run yesterday over the Malahat, a steep very long grade up and down a mountain that you have to traverse to get anywhere else on Vancouver Island outside Victoria. It was empty so no load momentum, I asked her how it went (she was nervous about driving it) by phone and she just said 'fine' so I guess it didn't want to runaway on her going down. I always thought engine braking, even without the Jake brake, was one of the benefits of the diesel over a gas engine? Or is that without the throat valve (don't think I've heard of them, either).

talkinghorse43
08-20-2008, 06:23 PM
Hi, Windsprinter, welcome to the forum.

My first thought is that your hill isn't very steep. In general, these things just love to go downhill and I've found it very necessary to anticipate moutain curves on downhill grades well ahead of time to keep from raising my "my god, can I do this" adrenaline level.

The engine braking on a diesel is just not as good as on a gasoline engine that has a throat valve to cut off intake air.

Probably you already know, but the key I've found with my Sprinter is to take it slow on the way down. When loaded near max, as I imagine you are, I heed the speed warnings posted for 18 wheelers. Maybe you don't like to go that slow (a lot of 35 mph limits as I remember), but if I gear down until the engine is near max rpm, mine will come down w/o using the service brakes.

To illustrate the effect of speed: say your GVW is 10k lbs and you're descending a 6% grade. Traveling at 35 mph, you'd need to apply 56 braking hp to stay away from a runaway condition (within the capacity of my '02 to supply). However, if you're traveling down at 60 mph, you'd need to apply 95 braking hp to avoid a runaway condition (my '02 couldn't do that and that would be a runaway condition - time to heat up the brakes)

bikergar
08-20-2008, 09:13 PM
my 08 will accelerate on very mild decents without any throttle input from me if I am 5th gear. This would indicate to me that there is little or no engine braking going on..... in my van at least.

gary

autostaretx
08-21-2008, 01:01 AM
What a lot of great responses in a short time! Thanks :thumbup::thumbup: Please excuse my ignorance, I've never owned a diesel before and rarely driven them, I feel I have a steep learning curve ahead! So bear with some of my followup questions please!

Diesels are different.

Autostare: Tx for the page ref. Sorry, I don't follow completely. When you say the 'injectors are turned off' you are referring to fuel I presume not the circulation of the tranny fluid? When you say 'coasting' at >1K RPM, do you mean with the throttle backed off completely or pushed just hard enough to overcome the engine braking and maybe encourage upshifting? That is what Bikergar is talking about, I think?

"injectors" are the devices which squirt fuel into the engine cylinders.
When you're coasting (i.e. foot *off the pedal*) in a Diesel Sprinter, the ECM turns off *all* fuel to the engine (it resumes squirting when the engine speed drops below 1K rpm).

Unlike a gasoline engine, there is no throttle plate in the intake system.
If the ECM (Engine Control Module) wants to limit the fuel (but still admit some) it must also limit the oxygen available, or the engine would run too lean (creating overheating and messing up air pollution). To reduce the oxygen, it uses the EGR (exhaust gas recirculation) to feed waste gas back into the engine to displace the volume that would otherwise be filled by fresh air.

A useful thing to buy (despite its $100 cost) is the vehicle Service Manual... it goes into -great- detail of how the various systems of the Sprinter operate. It's on a CD, and is therefore computer-searchable (at least my 2004 manual is). Very handy (although i'm told the 2008 manual has many pictures of the older 5-cylinder engine).

I'll check out the sanguage (scanguage?) thread, sounds like I should spend some time there. I was hoping to get it to upshift to fifth at a much lower speed than that. The hill is steep (about 20%) and on my Dakota, if i shift to 5th at about 20 mph/35kmph, by the time I let out the clutch I'll be up to 800 RPM and a couple of seconds later I'll be doing 50-55 kmph, say just over 30 mph and about 1100 rpm at the bottom of the hill. The Sprinter by comparison will stay in 2nd gear and only do 25 kmph the whole way down if I don't give it throttle.

I don't know the numbers for the 2007/2008 models, but on the older Sprinters, the peak power was in the 2000 to 2500 rpm range (i'm not near my books). Upshifting can be triggered by properly dancing feet, but there are some hills it just ain't gonna get fooled by.


I haven't driven it over anything steep and highwayish yet. One of my employees took it for its first run yesterday over the Malahat, a steep very long grade up and down a mountain that you have to traverse to get anywhere else on Vancouver Island outside Victoria. It was empty so no load momentum, I asked her how it went (she was nervous about driving it) by phone and she just said 'fine' so I guess it didn't want to runaway on her going down. I always thought engine braking, even without the Jake brake, was one of the benefits of the diesel over a gas engine? Or is that without the throat valve (don't think I've heard of them, either).

My dealer recommended *not* exploring high RPMS until many thousands of miles were under its belt. (he said 10,000 (16k km), but ...) No jackrabbits off-the-line, either. Be Gentle for the first bit.

have fun
--dick

DerFahrer
10-25-2008, 02:36 AM
I'm bumping an old thread, I know, but I wanted to comment on the engine braking thing.

When a gasoline engine coasts down a hill with no throttle application, pretty much no air is entering the engine. The only braking effect is simply from the friction of all the parts in the engine that are still moving (pistons, rods, crankshaft, valvetrain, etc.).

That is what allows the fuel cutoff that was mentioned before. If a car has a gasoline engine and a manual transmission, usually when it's coasting down a hill, it's not technically 'running.' The computer cuts off the injectors and just allows the momentum of the car to keep the engine spinning.

But, as Dick has pointed out, diesels are different because they lack throttle bodies.

Therein lies the reason why diesels have so much more of an engine braking effect than gasoline engines. Now, while coasting down a hill, they not only have the friction of the moving parts like the gasoline engine, but they're still taking in air and compressing it. So the extra engine braking effect of a diesel comes from the fact that it is compressing air while coasting.

Semi-truck drivers rely on this very heavily in driving with high elevation changes. If a semi has a full load and the driver tries to maintain a safe speed coming down a hill using only its service brakes, it will burn them up in short order. The driver must use engine braking in addition to maintain a safe speed without burning up the service brakes.

talkinghorse43
10-25-2008, 03:18 AM
I'm bumping an old thread, I know, but I wanted to comment on the engine braking thing.

When a gasoline engine coasts down a hill with no throttle application, pretty much no air is entering the engine. The only braking effect is simply from the friction of all the parts in the engine that are still moving (pistons, rods, crankshaft, valvetrain, etc.).

That is what allows the fuel cutoff that was mentioned before. If a car has a gasoline engine and a manual transmission, usually when it's coasting down a hill, it's not technically 'running.' The computer cuts off the injectors and just allows the momentum of the car to keep the engine spinning.

But, as Dick has pointed out, diesels are different because they lack throttle bodies.

Therein lies the reason why diesels have so much more of an engine braking effect than gasoline engines. Now, while coasting down a hill, they not only have the friction of the moving parts like the gasoline engine, but they're still taking in air and compressing it. So the extra engine braking effect of a diesel comes from the fact that it is compressing air while coasting.

Semi-truck drivers rely on this very heavily in driving with high elevation changes. If a semi has a full load and the driver tries to maintain a safe speed coming down a hill using only its service brakes, it will burn them up in short order. The driver must use engine braking in addition to maintain a safe speed without burning up the service brakes.

Not really - here's a good explanation:

http://dodgeram.org/ki4cy/exhbrake/exh_brake.htm

BBlessing
10-25-2008, 05:55 PM
i think i should mention here that you can manually shift these rigs also by tapping the shifter sideways. left for downshift and right for upshift. my2p

bb

Aqua Puttana
10-25-2008, 07:25 PM
I find my 2004 responds to left downshift once you're below where the TCM wants to be, upshift seems to wait until the TCM likes it. I don't think you can force upshift with right pushes.
i think i should mention here that you can manually shift these rigs also by tapping the shifter sideways. left for downshift and right for upshift. my2p

bb

DerFahrer
10-26-2008, 04:39 AM
Not really - here's a good explanation:

http://dodgeram.org/ki4cy/exhbrake/exh_brake.htm

Whoops. :thinking:

Well, thanks for clearing that up at least.

SteinarN
10-26-2008, 11:35 AM
Not really - here's a good explanation:

http://dodgeram.org/ki4cy/exhbrake/exh_brake.htm

This explanation is totally wrong.

The atmospheric pressure in the crankcase has no influence what so ever on the braking performance of any diesel or gasoline engine. The pressure in the crankcase is acting on the piston on the up stroke as well as on the down stroke making the net effect exactly zero.

It is a number of engine parameters responsible for determining the breaking performance of a resiprocating engine. The most important parameters is the compression ratio, valve timing, indirect or direct type engine (on a diesel) and closed throttle or open intake (difference between diesel and gasoline engine).

Any dedicated exhaust preaking system is not included in this description.

The breaking performance of a diesel engine is easy to explain. It consist mainly of the friction of the moving parts and the heat loss caused by the compressed air above the piston in the compression and expansion stroke loosing heat to the cylinder walls and the head. This heat loss makes the average temperature of the air in the cylinder to be less warm on the expansion stroke compared to the compression stroke. This decreased temperature on the expansion stroke causes a decreased pressure as well. The pressure/force acting on the piston on the compression stroke is therefore larger than the pressure/force acting on the piston on the expansion stroke. This difference in pressure is felt as a breaking effect exactly opposite to the "power" effect when the engine is running under load where the temperature/pressure is larger on the expansion stroke because of the combustion of fuel at the end of the compression stroke.

In an IDI diesel engine it is an additional parameter causing an increase in the engine breaking. Namely the pressure loss as the air is forced into and out of the pre chamber through a narrow channel. This pressure loss is adding up to the pressure loss caused by the air loosing some of it's heat.

EDIT:
Another factor contributing to the breaking power is the pumping losses. When the piston is moving down in the suction stroke, the pressure in the cylinder is somewhat lower than the outside atmospheric pressure. On the other hand, when the piston is moving up in the exhaust stroke the pressure in the cylinder is somewhat higher than the outside atmospheric pressure. This difference in pressure is caused by pressure loss, both static and dynamic, in the intake and exhaust channels in the head, intake and exhaust manifolds, exhaust system and intake/air filter system. This pumping loss contributes to the overall average difference in the cylinder pressure on the up strokes compared to the down strokes.
The pumping loss for a normal car engine at max power RPM and max load can be 5 to 10% of the total energy in the fuel.
END EDIT

The engine breaking mode of a throttled gasoline engine is completely different and more complicated. Assuming a completely closed throttle there isn't any air flowing through the engine, however air is flowing back and fort into the cylinders both at the intake and the exhaust side.

It is easy to imagine a vacuum develop in the cylinder at the intake stroke. This vacuum causes the intake manifold to be under a vacuum. Then imagine what happens at the end of the exhaust stroke. Both the intake and the exhauste valve is open at the same time making a path for the air at atmospheric pressure in the exhaust manifold to flow through the cylinder into the intake manifold for a short time. Also at the end of the combustion stroke there is a vacuum in the cylinder. When the exhaust valve then opens, air from the exhaust manifold, at atmospheric pressure, flows into the cylinder for a short moment before the piston start the exhaust stroke. At these two moments at each complete cycle air is flowing from an atmospheric pressure region into a vacuum pressure region for a short moment. This flow of air from a high pressure region to a low pressure region releases power in form of heat which is ejected into the cooling system. The required power to make the vacuum is felt as a breaking power. From a mecanical point of view the breaking power is caused by difference in the pressure in the cylinder at up strokes compared to down strokes. Detailed analysis of the pressures in the cylinder show that the pressure on average is smaler on down strokes than it is on up strokes.

The exact valve timing is of significant importance for how large the breaking power will be. For example if the ovelapping of the simultaneously open intake and exhaust valve on the end of the exhaust stroke is decreased or eliminated completely the breaking power will be decreased. Such a valve timing will allow a smaler amount of air entering the intake manifold from the exhaust manifold at the end of the exhaust stroke, thus altering the pressure in the intake manifold and thereby the pressure in the cylinder will also be altered causing an decrease in the average pressure difference between the up and down strokes. This decreased pressure difference then leads to a decrease in the breaking power.

talkinghorse43
10-26-2008, 02:02 PM
This explanation is totally wrong.

The atmospheric pressure in the crankcase has no influence what so ever on the braking performance of any diesel or gasoline engine. The pressure in the crankcase is acting on the piston on the up stroke as well as on the down stroke making the net effect exactly zero.

It is a number of engine parameters responsible for determining the breaking performance of a resiprocating engine. The most important parameters is the compression ratio, valve timing, indirect or direct type engine (on a diesel) and closed throttle or open intake (difference between diesel and gasoline engine).

Any dedicated exhaust preaking system is not included in this description.

The breaking performance of a diesel engine is easy to explain. It consist mainly of the friction of the moving parts and the heat loss caused by the compressed air above the piston in the compression and expansion stroke loosing heat to the cylinder walls and the head. This heat loss makes the average temperature of the air in the cylinder to be less warm on the expansion stroke compared to the compression stroke. This decreased temperature on the expansion stroke causes a decreased pressure as well. The pressure/force acting on the piston on the compression stroke is therefore larger than the pressure/force acting on the piston on the expansion stroke. This difference in pressure is felt as a breaking effect exactly opposite to the "power" effect when the engine is running under load where the temperature/pressure is larger on the expansion stroke because of the combustion of fuel at the end of the compression stroke.

In an IDI diesel engine it is an additional parameter causing an increase in the engine breaking. Namely the pressure loss as the air is forced into and out of the pre chamber through a narrow channel. This pressure loss is adding up to the pressure loss caused by the air loosing some of it's heat.

The engine breaking mode of a throttled gasoline engine is completely different and more complicated. Assuming a completely closed throttle there isn't any air flowing through the engine, however air is flowing back and fort into the cylinders both at the intake and the exhaust side.

It is easy to imagine a vacuum develop in the cylinder at the intake stroke. This vacuum causes the intake manifold to be under a vacuum. Then imagine what happens at the end of the exhaust stroke. Both the intake and the exhauste valve is open at the same time making a path for the air at atmospheric pressure in the exhaust manifold to flow through the cylinder into the intake manifold for a short time. Also at the end of the combustion stroke there is a vacuum in the cylinder. When the exhaust valve then opens, air from the exhaust manifold, at atmospheric pressure, flows into the cylinder for a short moment before the piston start the exhaust stroke. At these two moments at each complete cycle air is flowing from an atmospheric pressure region into a vacuum pressure region for a short moment. This flow of air from a high pressure region to a low pressure region releases power in form of heat which is ejected into the cooling system. The required power to make the vacuum is felt as a breaking power. From a mecanical point of view the breaking power is caused by difference in the pressure in the cylinder at up strokes compared to down strokes. Detailed analysis of the pressures in the cylinder show that the pressure on average is smaler on down strokes than it is on up strokes.

The exact valve timing is of significant importance for how large the breaking power will be. For example if the ovelapping of the simultaneously open intake and exhaust valve on the end of the exhaust stroke is decreased or eliminated completely the breaking power will be decreased. Such a valve timing will allow a smaler amount of air entering the intake manifold from the exhaust manifold at the end of the exhaust stroke, thus altering the pressure in the intake manifold and thereby the pressure in the cylinder will also be altered causing an decrease in the average pressure difference between the up and down strokes. This decreased pressure difference then leads to a decrease in the breaking power.

Unusual hypotheses. Can you find support for your views anywhere? Does anyone else agree with you?

SteinarN
10-26-2008, 02:33 PM
talkinghorse43,

what exactly don't you agree on?

DerFahrer
10-27-2008, 12:45 AM
Again, my apologies for misleading anybody. I got a new wrinkle on my brain at least.

This must mean that Sprinters have exhaust brakes then, because my Sprinter engine brakes a lot better than any other vehicle I've ever driven...

talkinghorse43
10-27-2008, 02:43 AM
Again, my apologies for misleading anybody. I got a new wrinkle on my brain at least.

This must mean that Sprinters have exhaust brakes then, because my Sprinter engine brakes a lot better than any other vehicle I've ever driven...

I agree, reminds me of a gas engine with a manual tranny. I think the ECM is programmed to close the turbo inlet vanes when you lift your foot, thus restricting the flow of air out the exhaust and raising the pressure across the engine (pumping air "uphill" from intake to exhaust), resulting in engine braking.

SteinarN
10-27-2008, 05:15 AM
I think the ECM is programmed to close the turbo inlet vanes when you lift your foot, thus restricting the flow of air out the exhaust and raising the pressure across the engine (pumping air "uphill" from intake to exhaust), resulting in engine braking.

If this is correct, then it means the turbo must be spinning fast when engine breaking and thus producing a noticeable boost. Has any of you with a Scan Gauge noticed any boost when engine breaking?

talkinghorse43
10-27-2008, 01:38 PM
If this is correct, then it means the turbo must be spinning fast when engine breaking and thus producing a noticeable boost. Has any of you with a Scan Gauge noticed any boost when engine breaking?

There's info in this thread showing 5 psi boost @3500 rpm.

http://www.sprinter-source.com/forum/showthread.php?t=3714&highlight=map&page=9

Not a lot of boost, but not a lot of flow either (relatively cold air with no products of combustion) compared to the on power condition.

bikergar
10-27-2008, 06:57 PM
I preface by stating I am not of current technology!!! Having said that back in the day gas engines had throttle bodies that opened and closed. When closed manifold vacuum was at its max and this produced the most engine braking. Some accessories tapped this vacuum ie heater controls, power brakes, ignition timing to name a few.

Diesel engines on the other hand did not have throttle bodies...they gulped all the air they wanted...no manifold vacuum, therefore no engine braking. Some diesels had engine driven pumps to provide vacuum for accessories. Engine braking in these diesels was through exhaust brakes which blocked gases in the exhaust stream, Jake brakes which changed the exhaust valve timing (these are the noisy ones). More recently transmission retarders and systems that use the turbocharger in conjunction with exhaust valves are being used (I know nothing about the later).

I also know nothing about engine braking and the Sprinter so feel free to enlighten me.

Also where does the Sprinter acquire vacuum from?

gary

talkinghorse43
10-28-2008, 01:38 AM
[B][I]Also where does the Sprinter acquire vacuum from?

Don't know about your NCV3, but the T1Ns have a rotary vacuum pump mounted on the front of the engine and driven by one of the camshafts. I would bet your v6 has a similar pump.

jdcaples
10-28-2008, 03:59 PM
NCV3 vacuum Pump info attached

10208

windsprinter
10-30-2008, 04:37 AM
well, after reading this thread through again after 2 glasses of wine, I conclude that no-one really knows and its all magic!!:rolleyes:

My Sprinter can go down my very steep driveway without touching the brakes - something my manual Dakota will not do unless its in low range!

I'm using the tip given earlier by ...(I forget, sorry) to give the engine a blip of fuel and 'force' an upshift going down the next hill (after my driveway). Feels odd to 'floor it' at the top of a steep hill, but it does seem to work and the van will accelerate with the hill and 'coast'; though not as well as my Dakota or Mazda Tribute.

DerFahrer
10-31-2008, 12:50 AM
talkinghorse, tell me if I'm understanding this right...

The turbo closes the inlet vanes on the hot side, which apparently does two thing:

1) Creates a restriction in the exhaust
2) Creates positive intake manifold pressure at zero throttle (hence the 5psi at 3500rpm with zero throttle)

This essentially traps air in the engine by forcing more air into it and at the same time making it harder to escape.

Is that where the braking effect comes from?

When I mean my Sprinter engine brakes, I mean it engine brakes! I'd say better than a gas engine with a stickshift (except maybe for 1st gear)... in fact, I've practically done a moderate panic stop mostly with the engine braking and little use of the service brakes. I was astounded.

shanemac
01-31-2009, 04:13 AM
I'm not technical with vehicles but can say this... i too don't have to touch the brakes going down hills and i have plenty of them around hear some fairly steep grades. If anything i have to give it throttle to maintain the speed limit which after driving gas engines i actually prefer my diesles characteristics much more. With my old work van i would be riding or tapping the brakes much much more to maintain a constant speed.

I'm not a hyper miler but after driving my sprinter for close to 4 months its pretty much an art using throttle correctly to be in the right rpms and gear. For me i shudder when i can hear and feel the engine bog down until it finds the right gear and rev. to me that's been the challenge seeing i never drove a diesel before. Sluggish is a good term.

As of late i been using engine braking i guess to slow down for lights....I'm the type i want to maximize brake life if i could i would rather not touch the brakes, but i have surprised myself many times rolling up on red lights only touching the brakes so i don't idle through them. I usually only do the coasting up to lights when no car is behind me otherwise i would be pissing off people:lol:

The sprinter needs another gear maybe two more.

jdcaples
01-31-2009, 04:44 AM
The sprinter needs another gear maybe two more.

I agree with that. If the 7 speed auto found in other MB products is able to handle the torque, it would have been a good choice for North America.

I really doubt any of us could make that modification work, though.

-Jon

Benzi
11-02-2009, 06:43 PM
re: Engine Breaking, Cruise Control, and Optimum RPM...

I appreciate the various discussions on "engine breaking", but would encourage greater focus on "hypermiling on downhills" in Sprinters.

'Bumping' the accelerator to force it to upshift would temporarily use more fuel than necessary. If the engine is thereby forced into slower RMPs, this would suggest that less fuel is being used overall. However this isn't necessary if, when you lift the accelerator, no fuel whatsoever is being injected (above 1000 RPM). My observation is that when you lift the accelerator, the engine speed drops almost to an idle, which is a terrific way to save fuel.

Since there are many complexities in various engine designs (pumps, compressors, etc), the only way to know the actual fuel economy, is to "measure it in the real world". For this, I would suggest either using an onboard fuel economy computer, or directly measuring the amount of fuel used over a given distance. I encourage anyone who travels long downhills to try this and let us know. :professor:

Onto the topic of using "cruise control". Firstly, I agree that cruise control can be IMPROVE fuel economy on long, flat stretches. It overcomes the tendency of some drivers to speed up and slow down through inattention (unnecessary acceleration consumes fuel).

Where cruise control REDUCES fuel economy is in hilly or mountainous terrain. When going up a long hills, the cruise control forces the engine to work hard to maintain the set-speed. On downhills, it works again to reduce speed. The best way to save fuel is not to use the cruise control and slowly increase speed before the hill, then allow the vehicle to gradually slow down as it climbs. Traffic permitting, this isn't a problem because on the downhill, you can regain the ground you lost by letting the vehicle travel somewhat faster than normal as it coasts (curves, permitting). This concurs with the "Egg Rule of Hypermiling" (imagine an egg between your foot and the pedals).

Regarding "RPM", the optimum efficiency point of turbo-diesel engine is around 1750 rpm and 90% of full throttle. This provides high power for the engine as well as high efficiency and thus is useful for acceleration. However, for driving at a steady speed, one cannot choose any operating point for the engineŚrather there is a specific amount of power needed to maintain the chosen speed. In the turbo diesel, too low a gear will move the engine into a high-rpm, low-torque region in which the efficiency drops off rapidly, and thus best efficiency is achieved near the higher gear.

Generally, fuel economy is maximized when acceleration and braking are minimized. So a fuel-efficient strategy is to anticipate what is happening ahead, and drive in such a way so as to minimize acceleration and braking, and maximize coasting time.

NOTE: Diesel engines have energy efficiency of 45% and petrol engines of 30%. That's one of the reasons why diesels have better fuel efficiency than equivalent gasoline cars. A common margin is 40% more miles per gallon for an efficient turbodiesel.

david_42
11-04-2009, 05:23 PM
The problem with roller-coasting is most of hills are long enough that allowing the van to slow down would result in it downshifting or stopping before it got over the hill. It takes a fixed amount of energy to move a weight up a fixed height, so unless the "hills" are really small, the cruise control will do a very good job. I have yet to meet someone who gets better mpg than I do and I use cruise control almost all of the time I'm not in town.

cedarsanctum
11-04-2009, 08:27 PM
I don't believe cruise control can be beat for extracting the correct amount of power, at just the right time, over the longest distance and time. It is much more subtle than my foot at setting and keeping the power in the correct band. But then, i come from the theory that everything that can be automatic, should be. Frees me up to do the hard part, watching for the idiots that want to ruin my trip.
The one biggest thing i have found for keeping the fuel mileage good is keeping the speed down, but that is hard in the real world. When everyone else is going 65, it's tough to keep it at 55, or even 60.

Jef