curious about engine breaking 1

[rwelk]

I don't know if anyone can answer this question. But I was just curious if engine braking a car (manual transmission) has any negative effect on the motor, or gearbox?

 

[NickE]

"No reason, for example, that brakes couldn't use active cooling via liqud coolant and a air-liquiid heat exchanger."

This is done in certain forms of racing. WRC Tarmac cars for example. I've also heard rumors that its done on the Semi-Truck racing in Europe.

 

[Rob45]

Evel -
Yes of course I meant John Surtees! Got his book "Speed" sitting on my bookshelf!

and Jay:
Disc brakes are coming on heavy trucks. They're being tested now.
Anything that adds weight, though, like the airflow or heat-exchanger ideas, would be flatly rejected by many truck owners, since weight reduces payoload, and payload is money.

 

[patprimmer]

Sorry

I can't resist it any longer.

If you are really curious about engine breaking, as you say in your heading, to experiment, you will need to use a very large hammer as engines tend to be very strong.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[DanEE]

Engine braking has its place.. especially in mountain driving.. and using it effectively might save your bacon some day. Anyone not using it is asking for trouble..

The most extreme I've done, and where heavy use is mandatory was the return trip back down Pikes Peak..

It's steep enough to require second gear (manual tranmission) and with no throttle it was reving the engine around 4000rpm most of the way, and in some places by adding the compressor load by turning the AC made the difference in not having to continuously ride the brakes..

They have a mandatory brake check stop after coming down the top third of the mountain.. The guy holds his hand about 2 feet from the brakes and if he feels heat at that distance, you are directed into the parking lot for a mandatory cool off..

Monarch Pass, Colorado is another good one.. about a 8 mile downhill grade at 9% slope most of the way.. It's straight enough to be doing 60mph in places and really spooky braking hard coming into a curve...

 

[sinkworks]

i think that there really is no more additional wear on the motor unless you do something drastic, such as 3rd to 1st, in reality you shouldn't even be needing to downshift for 1st except for from a stop. 2nd gear is far enough on the downshift unless you have a lamborghini which does like 90 in first!

 

[kurtis714]

Hi, I'm new here, but I thought I'd just add a bit of personal experience. Following on a little from the racing motorcycle mention earlier. I do a few track days on my Triumph over here in the UK, and as I've been learning my way quicker I've found engine braking to be very handy. I engine brake all the time when normal braking anyway. But when going into some corners it can be useful to roll off the throttle and weight up the front, giving steeper geometry, allowing a faster tip in. If you had to brake for the corner anyway this is obviously unnecessary.

Just a thought,

excellent forums by the way, some of you guys know more than I ever will!

Kurtis714

 

[EddyC]

I would rather rely on my brakes to slow down my car as they have the antilock feature. If I were to rely on the engine for braking, I would be concerned about going into a skid.

 

[NickE]

EddyC--

Actually when I was living in the great white north (UP Michigan) and coming down a really steep hill in an ice storm engine braking was a far better idea (at least it worked in this case).

Here's what happened when I just pumped the brakes: Immediate lockup, not a chance of braking effectively. (this is with full on winter tires (blizzaks) and no abs)

but when I downshifted the fact that the motor was still spinning the wheels prevented them from going in to full lockup. At this point I had to use all three pedals to keep my speed under control to make the sharp turn at the bottom.

*(for anyone familar with the area this was Quincy Hill, site of the deepest copper shaft in the area (9200'))

Now ABS is good and has saved my car once or twice. However I can stop my car faster on dry pavement if the ABS is turned off, I think because I can feel the threshold better. This also applies to gravel and lightly packed or fresh snow surfaces, there is a wedge of loose material that can build up underneath a locked wheel. ABS rarely shortens stopping distance, but it does allow you to push full on the brakes and still steer.

 

[JayMaechtlen]

Ref "additional wear":
I assume (ass/u/me?) that the additional engine revs induced by downshifting would create some additional wear.
Surely the total wear of a powertrain is a function of a number of things, including the sum of (torque*crankshaft rpm) and some number of other factors...
Without overrevving a motor to (near?) failure, you might not notice the additional wear- too many other variables, and modern motors last a long while anyway.
cheers
Jay

Jay Maechtlen

http://home.covad.net/~jmaechtlen/

 

[Tmoose]

"But I was just curious if engine braking a car (manual transmission).........."

"All of you guys that are against engine braking don't work in Heavy Truck.........."

--------------

Geographically challenged Eskimo mom, sending her teenage son off to vacation in Hawaii - "Nanook ! You promise me you will be sure and wear that parka and those mukluks"

 

[ivymike]

uh, coasting in neutral would give better fuel economy.

 

[dalcazar]

I don't see how it's true, but it's apparently true, I have read in places, and there are a few posts up there... that say that decelerating while in gear gives you the best possible fuel economy.

regarding engine braking, I use it for the most part on hills and freeways, I have to drive down a grade when coming home from work, and if I put it in neutral my car will easily pass 85 going down that hill, but put it into 5th and just lay off the gas and it does a perfect 70 mph, so I save gas and I save my brakes (and I save my kidneys too because I have a midly warped brake rotor I haven't gotten around to replacing)

I don't use it in regular city driving though.

Oh and regarding that post on downshifting into 1st, most manual tranny cars have a limiter built in that won't allow you to do this until you're under 15 or 10 miles an hour to prevent the engine from redlining, plus the collosal magnitude of the jerk it gives you is so bad you normally don't ever try it again anyway.

 

[patprimmer]

I have read in many places that people have been kidnapped by aliens.

I have never seen this downshift limiter on a manual transmission. How does it work.

It would be real handy if they also put it on reverse

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

I could give you a reasonably long list of manual gearboxes that do not have this feature, if it is anything more complex than baulky synchro.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ivymike]

I think it's a balky synchro. Every car I've had with a manual trans felt as though it was locking me out of first at relatively high speeds, but some persistence would usually overcome that (on a bad day when i couldn't find third).

 

[NickE]

Ingear 0% throttle give me teh best mileage in my car because the injector duty cycle also goes to 0%.

Greg-I think thats what hes talking about. That said I've never driven anything other than a synchro box and have never had any trouble w/ 2->1 shifts. Although double clutching isn't too hard.

 

[ivymike]

Ingear 0% throttle give me teh best mileage in my car because the injector duty cycle also goes to 0%.
...but you throw away X additional horsepower in engine pumping, friction, etc. losses as you spin the engine at 3000rpm instead of 800rpm...

 

[NickE]

ivymike- right thats where the "braking" effect comes from. but truly I will see lower fuel usage per unit time if coasting in 5th vs coasting in neutal.

 

[evelrod]

Trust me, guys...I come from a place and time when coasting down hills saved me mucho dinero. I can guarantee you that, in the long haul, staying in gear ALL the time WILL NOT be as efficient, vis a vis fuel conservation, as coasting in neutral! One trip in particilar from High Rolls, New Mexico to Las Cruces the 'Okie Overdrive' managed to give me >50 mpg in a 1949 Mercury V-8 !!!!! Gasoline was only $.25/gal but, then again, I only made $.50/hour mopping floors in a drug store, part time.

As to the 1st gear inhibitor...na

http://wwwww...but

the old Moss four speed in my '50 TD came close.

Rod

 

[NormPeterson]

As long as the ECU/PCM stops sending pulses to the injectors during overrun (but obviously not during conditions of idle), less fuel will be consumed than if the engine is allowed to rest in the 'idle' state while decelerating. This is completely consistent with any experiences involving 50+ year old carburetor technology, too.

I've been hoping for a description of the 'low gear inhibitor' and perhaps some 'performance tips' as well. I think one of mine is out of adjustment, and my shop manual doesn't seem to be of much help . . .

Norm