Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Diesel Discussion

Reply
 
LinkBack Thread Tools Display Modes
  #16  
Old 11-11-2007, 11:16 PM
ForcedInduction
Guest
 
Posts: n/a
They use the same technique with a progressive throttle. It will move the throttle quickly for the first 1/2 travel.

Perception of performance, thats all it is.

Reply With Quote
  #17  
Old 11-12-2007, 11:02 AM
KarTek's Avatar
<- Ryuko of Kill La Kill
 
Join Date: Jun 2007
Location: Bahama/Eno Twp, NC
Posts: 3,258
On a somewhat side note, this topic is also kicked around quite a bit on the Diesel truck forums. Mainly due to the quest for more engine braking power when towing. Side by side, with identical transmissions, the gasser will provide more braking power because of the vacuum pulled against the throttle plate that is absent in the Diesel.

Various people have suggested things like putting plates in the intakes for more vacuum but most Diesels aren't designed to pull vacuum in the intake and you end up pulling oil out of the valve seals and from the turbo compressor labarinth seal. This is why the aftermarket simply puts a valve on the exhaust to gain the additional braking power.

On my truck and I think on my '98 as well, the injection is completely shut down by the PCM in a coast situation to further aid the braking action.
__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250
Reply With Quote
  #18  
Old 11-12-2007, 12:34 PM
BIGRED's Avatar
Registered User
 
Join Date: Mar 2003
Location: Coarsegold Ca
Posts: 693
How about some mini parachutes?
__________________
1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it
Reply With Quote
  #19  
Old 11-12-2007, 05:42 PM
Moderator
 
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,595
I find this discussion, limited as it is to automatic transmission equipped cars, a bit misleading.

I have a standard transmission in my 240D (it is my 3rd 240D, which were all preceded by a 1971 220D, also with a manual transmission-automatics should not be allowed within several hundred feet of one of those engines...). I have also driven many gasoline engine powered cars with manual transmissions, from VW's, to Fiats, Honda's, several 190E 2.3-16's, to our present 1988 300E with a five speed.

My experience, and my wife's experience differs from the generic conclusion reached in the prior posts.

The Diesel engines in the earlier incarnations (220D and 240D in the W114/115 chassis) had a flapper in the intake that closed off when the "idle" throttle position was reached. These cars had very peculiar feeling "coming off idle" performance as a result, and I believe this feature is controlled differently on the W123 240D as they are definitely different coming off idle, especially if the idle speed adjustment knob is turned up. I am not clear it exists on the 617 turbo engines at all.

Manual transmission equipped 220Ds, and 240Ds have pretty serious engine braking capacity, enough so that when you are driving on snow covered or icy roads, it it very ill advised to abruptly yank your foot off the throttle without first depressing the clutch.

I have personally spun these cars out coming down hills in challenging traction situations, and so has my wife. Just by taking my foot off the throttle. No need to touch the brakes. The rear wheels instantly came to their rolling speed at idle, which was less than the speed of the car and the front wheels, meaning the engine braking caused them to lose traction. And that was the beginning of a 360 degree approach to a stop sign in Girdwood, Alaska.

My wife was not so lucky in the 1982 240D, and backed into a guardrail (actually a cable supported by some rotten stumps that just broke off, so no real damage done). That same result couldn't be duplicated, and I tried in second instance noted, with one of the 190E's or in any other winter driving experiences I have had, which spans 4 decades.

I also think the net power of the Diesel engine in these cars at idle injection volumes and engine speeds other than idle speeds, is very low. At idle the freaking car won't even stay warmed up in really cold weather, and it is pretty easy to stall the engine unless the idle knob is turned up. Increasing speed, without increasing fuel flow will increase losses without a commensurate increase in engine output.

I think the braking "advantage" of the Diesel is a feature of its lower idle speed capability and is more apparent at lower speeds. At a trailing throttle, meaning just below the "maintain" speed/load position, the engine braking is not noticeably different than gas cars, which is good because it might otherwise make driving them smoothly a bigger chore.

I have no idea what the interface between the automatic and the throttle might be to make the automatic equipped versions have a weaker braking response. I used to figure it was the fact that there was a fluid connection for transmitting torque that might not be as efficient running "backwards" as it was running "forwards." But there may be other reasons too. All more motive to keep automatics away from my Diesels. Jim
__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
Reply With Quote
  #20  
Old 11-12-2007, 08:06 PM
Registered User
 
Join Date: Apr 2007
Location: UK
Posts: 357
Overrun clutch

Do MB automatic gearboxes have a one way or overrun clutch in their lower ratios like a Borg Warner 35? If so, in conjunction with the torque convertor would give very little engine braking.

On diesels with manual gearboxes in the UK, it was quite common to see them fitted with a throttle damper which reduced the rate that the fuel lever on the pump moved towards idle.

I agree with the posters who say that you get a lot of the energy in compression back once the piston goes over top dead centre. But for any engine, petrol or diesel once the engine is producing less torque than is required to overcome the various sources of drag the vehicle will slow down.
Reply With Quote
  #21  
Old 11-12-2007, 08:12 PM
ForcedInduction
Guest
 
Posts: n/a
If you really want engine braking, look into installing an exhaust brake. My VNT turbo can function like an one (The same way the 6.7L Cummins' turbo does) and it makes very noticeable braking power.
Reply With Quote
  #22  
Old 11-12-2007, 09:52 PM
BIGRED's Avatar
Registered User
 
Join Date: Mar 2003
Location: Coarsegold Ca
Posts: 693
Re-thinking the topic....

The higher the compression ratio / compression psi the more effort it will take to rotate through compression. I know that the amount of force I need to use to rotate our 530it a 3 liter gas engine is a fraction of that which is required to rotate our diesel a 3 liter diesel.

The idle power isn't sufficient to offset the force created by the compression when the engine is pushed to a higher speed by the momentum of the slope of the road. No doubt you get something back, but not that which is required to maintain the speed traveled, thus the engine is braking.

A truck with a 40 to 50,000 lb load needs the additional braking provided by a Jake Brake which when you consider the mass being controlled is pretty amazing.

A compression release engine brake (Jake Brake) uses an extra lobe on the camshaft to open a second exhaust valve at the top of the compression stroke. This releases the compressed air in the cylinder preventing the energy from being used.

So a compression release.......release the compressed air and doesn't just open the cylinder to zero pressure at all times.
__________________
1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it
Reply With Quote
  #23  
Old 11-12-2007, 09:59 PM
Registered User
 
Join Date: Oct 2005
Posts: 4,263
Quote:
Originally Posted by BIGRED View Post
The higher the compression ratio / compression psi the more effort it will take to rotate through compression. I know that the amount of force I need to use to rotate our 530it a 3 liter gas engine is a fraction of that which is required to rotate our diesel a 3 liter diesel.
This is not relevant. If you rotate a one-cylinder engine, even in the absence of leakage you notice the compression force. If you rotate a six-cylinder engine fast enough, the expanding gas on the "power" stroke counteracts (nearly exactly) the compression strokes on other cylinders.

The power that it takes to rotate the engine to start doesn't mean anything, since at very low speeds, there is a lot of leakage and thus a lot more pumping loss.
Reply With Quote
  #24  
Old 11-12-2007, 10:14 PM
BIGRED's Avatar
Registered User
 
Join Date: Mar 2003
Location: Coarsegold Ca
Posts: 693
I think it is relevant. We are not talking about slow speeds. We are talking about engine braking. When I am heading down a hill at 2500 RPM and the fuel I am putting in supports 700. The example used was to show the amount of energy required to cause a rotation of different engines.

Visualize a 1 cylinder 5HP Briggs in a big rig being used for engine braking.
__________________
1980 300TD-T (82 Turbo and Trans) 159,000 Miles "Jackie-O"
1983 300SD 272,000 Miles "Aristotle"
1987 Jeep Wagoneer Limited - keeps the MB's off the ice and out of the snow
1994 BMW 530it
Reply With Quote
  #25  
Old 11-12-2007, 10:15 PM
ForcedInduction
Guest
 
Posts: n/a
Agreed w/Matt. Starting compression is not relevant. It takes a big motor to start the engine but it takes 1/5 of a drop of fuel per injection to keep it at idle.
Reply With Quote
  #26  
Old 11-12-2007, 10:20 PM
Banned
 
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396
Quote:
Originally Posted by BIGRED View Post
I think it is relevant. We are not talking about slow speeds. We are talking about engine braking. When I am heading down a hill at 2500 RPM and the fuel I am putting in supports 700. The example used was to show the amount of energy required to cause a rotation of different engines.

Visualize a 1 cylinder 5HP Briggs in a big rig being used for engine braking.
Clearly, the larger the engine, the more torque is required to rotate it without combustion. However, this is largely the result of internal friction in the engine rather then the resistance due to compression. Surely, some of the compressive force is lost on the expansion stroke and the high compression diesel might offer a slight advantage to an equivalent gasser if there were no pumping losses in the gasser. But, if the gasser has a closed throttle plate to work against and the diesel is unrestricted, the slight compression advantage is wiped out by the gasser's pumping losses.

BTW, this situation applies to manual transmissions as well. A manual will provide significantly better torque to slow the vehicle than any automatic because it's directly coupled to the rear wheels. Both the manual diesel and the manual gasser can easily cause the rear wheels to slide on pavement with poor friction characteristics.
Reply With Quote
  #27  
Old 11-12-2007, 11:51 PM
Moderator
 
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,595
Brian,

I will spot you those pumping losses on Diesels that really don't have a throttle plate for idle speed control, and for all of them at throttle positions other than idle. I know the early MB Diesels had them and on the 220D this actually caused the intake valve seal leakage to cake on the intake valves and make the car nearly impossible to start. I nearly threw the car away. Did a compression check with a cheesy NAPA compression tester and found out the valve leakage was so bad you couldn't tell if anything else was wrong. Off came the head. And the valves were literally covered in caked and baked on goo. Had the head disassembled and cooked in a vat of some nasty solution by a local machine shop, and ended up reusing all the valves. I did have to have two seats pressed into the head though. And, I had to reinstall the linkage for the intake flapper. The machine ran like it was half its age for another hundred thousand miles before my father in law traded it in on a 1980 240D. I think the 1982 240D had one too. At any throttle position other than "none" this plate is out of the way, and there are no restrictions.

Then, I expect the higher compression ratio causes significantly more blow by (to get the same leakage at more than twice the pressure would require a substantially better seal, which, using the same technology/ring geometry is unlikely) and greater surface friction (higher normal forces at the ring to cylinder interface due to the higher compression and, at a given rpm and displacement, higher surface speeds). Giving us a higher friction for a given displacement, and a poorer pumping efficiency. Add to that the generally more robust pistons, wrist pins, bearings and so on, the rest of the internal friction losses will be somewhat higher as well.

In the end the higher efficiency in miles per gallon we enjoy comes from the 15% higher energy content of the Diesel fuel, with the lower pumping losses doing their share to overcome the added internal friction, when the engine is off idle.

Jim
__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
Reply With Quote
  #28  
Old 11-13-2007, 12:08 AM
Banned
 
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396
Quote:
Originally Posted by JimSmith View Post
Then, I expect the higher compression ratio causes significantly more blow by (to get the same leakage at more than twice the pressure would require a substantially better seal, which, using the same technology/ring geometry is unlikely) and greater surface friction (higher normal forces at the ring to cylinder interface due to the higher compression and, at a given rpm and displacement, higher surface speeds). Giving us a higher friction for a given displacement, and a poorer pumping efficiency. Add to that the generally more robust pistons, wrist pins, bearings and so on, the rest of the internal friction losses will be somewhat higher as well.

In the end the higher efficiency in miles per gallon we enjoy comes from the 15% higher energy content of the Diesel fuel, with the lower pumping losses doing their share to overcome the added internal friction, when the engine is off idle.

Jim
The higher pumping losses have the potential to cause increased blowby as the engine wears. However, there are plenty of pristine examples of diesels with 150K on the clock with minimal blowby. Therefore, if we restrict our discussion to diesels that are in relatively good condition, the increase in pumping losses due to the increased compression won't be a significant factor in the discussion of engine braking.

It's far more likely that the increased friction due to the factors mentioned above serves to consume a measurable amount of additional energy on a diesel.

However, the pumping losses that are suffered by a typical SI engine are much greater than any of the aforementioned variables. The primary reason for the inefficiency of an SI engine as compared to a diesel is due to pumping losses. At WOT, the efficiency of the two engine types are very similar.........with the edge going to the SI engine because of reduced friction and the edge going to the diesel due to increased energy content of the fuel. It would be difficult to make a firm conclusion as to which engine is more efficient under such conditions.

But, under part throttle conditions, the restrictions on the SI engine due to pumping losses across the throttle plate is the primary reason for it's miserable efficiency.
Reply With Quote
  #29  
Old 11-13-2007, 01:37 AM
Moderator
 
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,595
I spot you the part throttle to full throttle discussion. When you are "engine braking" though, you most likely are at "no throttle" with your right foot ready to apply brakes. In that position on older Diesels, and I am not sure if this didn't exist up to the end of the 240D production, there is a throttle plate that blocks off the intake and produces a significant intake manifold vacuum. It made vacuum pump diaphragm failures at idle spectacular smoke events that were innocuous at any throttle position other than idle.

There is always blowby, from day one. For discussion purposes, lets restrict the blow by to the same leakage geometry since, for the most part the rings and cylinders and pistons are made with the same manufacturing methods. If both engines have an equivalent leak path from day one, the losses in the Diesel are than the gas engine because of the higher pressure differential.

What the figure is new, at 50k miles, 100k miles, 150k miles and so on is likely going to favor the gas engine as the miles build if for no other reason than they burn cleaner fuel. No soot and combustion by products preventing free movement of the rings or at least way, way fewer. So, you have to put more energy into the compression stroke and you get less back on the power stroke. That and a Diesel at idle barely tics over. In fact, at idle a gas car is significantly less efficient than a Diesel, one of the reasons they are so popular in city driving in Europe.

Anyway, this is another sum of many issues, not the least of which is the gearing of a Diesel. Jim
__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
Reply With Quote
  #30  
Old 11-13-2007, 04:04 PM
patbob's Avatar
Its a Whatsit
 
Join Date: Oct 2006
Location: Portland, OR
Posts: 837
Quote:
Originally Posted by BIGRED View Post
We are talking about engine braking. When I am heading down a hill at 2500 RPM and the fuel I am putting in supports 700.
When the engine is spinning at 2500 RPM, isn't it consuming a little fuel each time a cylinder fires? You'll only be breaking even fuelwise if the IP is injecting a little less than 1/3 the amount it injects when idling at 750 RPM.

The IP might go lower than the idle amount, but can it really go that low? If not, you're burning more fuel with the higher idle RPM.

__________________
'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -4. The time now is 02:54 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page