Engine braking or drag?
 

While the wagon is getting some TLC, I have been driving the SD. Today when coming down a long hill (maybe 15 degrees) As always, I noticed that with my foot removed from the accelerator, the speed held steady. Curious to see what the cause might be, I slipped the trans into neutral and she started speeding up significantly.

That would seem to point to the transmission and or the engine that either by design or problem, is causing engine braking.

Given the high compression both in term of ratio and PSI, is it a normal engine braking effect?

Also noted on the SD as opposed to the TD, you can let your foot off the brake on level ground (after a complete stop) and it will not creep forward. The same holds true for being on an incline (nose up), the car holds steady if you take your foot off the accelerator.

If by moving your foot slowly you remove what feels like free play in the accelerator pedal (maybe 1/4" to 1/2") the car moves forward...release the pedal and you can feel the power to the wheels drop off. To me, this function seems to be designed / engineered into the trans and controlled by vacuum.

Thanks in advance for any input.
Don

You are correct, sir. Diesels "suck,".......literally. It's the compression, as you've surmised, giviing braking-action. Don'tcha know!

Enjoy ALL aspects of diesel-ownership! braking-suction, and ALL!:D

That is the Bowden Cable you are feeling. It means that the cable is adjusted right. I noticed that awhile ago in my 300.

This is incorrect. Compression itself is not a loss, as the energy is released on expansion. Any energy consumed by the engine on overrun is caused entirely by pumping loss. This is leakage and restrictions, but not the compression.

Right. Most of the energy absorbed on the compression stroke is released on the rebound (combustion) stroke.

Diesel engines continue to combust with foot off the pedal. I dont notice any engine-braking effect at all. Plus diesels are notoriously tougher on brakes than gasoline powered vehicles. Trucks are equiped with jake brakes that shut down valve activity to assist engine-braking, otherwise they never slow down on engine drag. Its a myth that high compression diesel engines stop dead in their tracks when taking foot off accelerator.

OK... then

Are you saying that coming down a downgrade your speed increases (not the case in either of my MB cars) and what do you think is causing mine cars to hold a steady speed on a downgrade with my foot off?

Thanks
Don

We've had this discussion many times in the past. There are folks that truly believe that the diesel will slow the vehicle down to an extraordinary degree when compared to a gasser due the the higher compression of the diesel. In reality, as FI noted, the energy required to compress the gas is largely released on expansion, so the answer cannot be "it's a diesel".

In fact, one of our members actually proved the case with the identical vehicle in both a diesel and a gasser. It was the gasser that provided greater engine braking.............not the diesel. This is largely due to the high vaccum that the gasser must fight to get a sufficient intake charge............something that the diesel gets without effort.

The answer to your question lies in the transmission. If you're familiar with most American vehicles with auto transmissions, when you come off the throttle the engine speed drops off dramatically. From a speed of 3000 rpm, the speed may drop to 1800 if you lift the pedal. Accordingly, you don't get much engine braking because the engine speed is far below the transmission/vehicle speed.

However, on the SD, if you lift your foot, the rpm drops from 3000 rpm to 2600 rpm and you feel significant engine braking. If you drop it into neutral, the engine speed drops to 700 and you feel the vehicle accelerate immediately. It's all related to the design of the torque converter.

Personally, I'm glad mine holds steady or slightly slows down going down the 8% grade on the way home from school. That's the steepest hill I know of.

Saves me having to use either pedal. Just set the car moving at 55 or so, take both feet back, and it holds just about perfectly. I'd rather do that than go down like all the people I see riding their brakes.

as noted, it's the transmission and the engine displacement relationship to the weight of the vehicle. that gives you engine braking. if your transmission has overdrive, you will get no engine braking. our older diesels have no overdrive, so we get engine braking. also, if you have a model that starts out in 2nd gear, you will get little forward movement on level ground at idle. if your starts in 1st, it will creep forward even on slight inclines.

Not complaining... just trying to insure it is operating as it should. The thought did cross my mind that there was an energy loss (MPG IMPACT) and while I watch prices getting close to $4.00 I would handle it ... if there is something to handle:)

Thanks
Don

Brian,

This is interesting......it would be nice to see an engineering report to corroborate your post... Apparently the gas engines have differing/different transmissions, from the diesels?

If an SD 's and a 6-cyl. SE's engine were turned off while driving, in an identical test. Which would have more braking action, if both were left in drive?

I believe it's the low stall speed of the torque converter.

In the end...I prefer the braking effect. I live in the Sierra's and almost exclusively drive mountain roads. I never have to downshift and or ride the brakes.

ALSO I am really glad I don't have more items to add to my MB to-do list.

Brian explained it right on. Its all in the transmission setup and torque converter. I love how you don't gain speed down most hills in these cars. In our minivan you always have to ride the brakes or it will speed up 20mph on a slope.

The design of the torque converter on American cars probably is also why when you slam the throttle down, they'll break your neck with acceleration, for a few seconds, then the engine is roaring the rest of the time and there isn't much acceleration. I've noticed that on many vehicles, mainly fords though. GM/Chrysler seem to have less.