engine braking . . . how bad is it?

[bwilson4web]

I was reading this article about a BMW N20 engine when this popped up:

BMW's N20 engine--four cylinders, more power - AutoWeek Magazine

. . . although like most modern direct-injection units, there's a distinct lack of engine braking on a trailing throttle. The engine's powerful electronics package ensures maximum coasting potential is achieved by allowing the revs to fall away gradually.
. . .

A car descending a known grade at a constant speed checked by engine braking will dissipate energy per the change of kinetic energy. Having done this experiment in past in a manual transmission Camry, not instrumented but ordinary driving, I have memories of this technique being quite effective. But this also gives a clue about throttle plate losses or as a brake, effectiveness.

Bob Wilson

[pounsfos]

the toyota prius has a system installed in the gearbox.
the system automatically puts the gearbox into neutral when you take your foot off the accelerator

some holdens (or chevys as theyre called in america etc.. do it for a couple of seconds as well)

[Neddy Seagoon]

A lot depends whether it a low or high compression engine. My Vibe which is a fairly high compression engine does well in the engine braking dept, but my wife's Cooper S is a low compression engine until the supercharger kicks in and tends to rev a lot higher going down hills.

[serialk11r]

Dug up this thread from a few weeks ago...
Wouldn't programming the engine to "engine brake" with an open throttle allow a form of easy pulse and glide on a manual transmission?
The overall frictional loss would be equivalent to that of leaving it in gear, but operating it at a high load part of the time and then having the engine just pump air the rest of the time could (could being the keyword) be better than engine on coasting...

[Ken Fry]

Originally Posted by bwilson4web

A car descending a known grade at a constant speed checked by engine braking will dissipate energy per the change of kinetic energy. Having done this experiment in past in a manual transmission Camry, not instrumented but ordinary driving, I have memories of this technique being quite effective.

If the car is descending at a constant speed, then kinetic energy is constant. This is unlike coming to a stop on the flat, in which kinetic energy changes with speed.

The change occurring in descending a hill is in potential energy, so the heat given off by the brakes between point A and B represents the difference in potential energy between those 2 points (which changes with the change in height).

Assuming the engine cuts off injection during trailing throttle, engine braking vs friction brakes is an economic choice -- do you want to wear out the engine or wear out the brakes?

For fuel efficiency it is best to avoid either type of braking: pulse and glide with the glide using any form of braking becomes much less effective.

[ps2fixer]

I had the similar idea to serialk11r, except for my automatic. When it is coating in gear not dumping gas, I just wish I could floor it and not have it dump more gas so it could coast a bit further.

[Ladogaboy]

Originally Posted by Ken Fry

Assuming the engine cuts off injection during trailing throttle, engine braking vs friction brakes is an economic choice -- do you want to wear out the engine or wear out the brakes?

To get a bit technical, the wear includes far more components than just the engine and brakes. Compression braking puts wear on the engine, transmission, differentials, clutch, and any corresponding fluids (and maybe a few other items). Normal braking puts wear on the brakes (pads and rotor if you want to start breaking down cost) and tires (to a much greater extent than when compression braking).

But, that being said, I have never seen any research specifically stating how much wear is placed on those parts associated with compression braking (i.e., how does it affect the life of the motor oil or transmission fluid). We all know that brakes wear out, and they only wear when being used to brake. In essence, it's very easy to apply a cost/price to using brakes, while determining the cost of compression braking would be very difficult and probably not worth one's time.

[ps2fixer]

I don't think there is so much damage as people think there is when engine braking. Prime example is semi trucks use a jake brake to basically block of the exhaust to increase pressure in the engine and to slow down the truck faster. The same trucks typically go over a million miles... granted that is highway miles, but with a large load a large percent of the time.

[Ken Fry]

Originally Posted by ps2fixer

Prime example is semi trucks use a jake brake to basically block of the exhaust to increase pressure in the engine and to slow down the truck faster.

The classic Jake brake actually blows compressed air out the exhaust by opening the exhaust valve during compression -- thus the loud noise. This reduces the air spring effect (where the energy used for compression is returned when the air expands on the down stroke).

Exhaust brakes are more likely to be used on light duty diesels.

I agree that there is not much damage from engine braking with well-designed systems. An exception: On dirt bikes, compression releases used to be common, and they could suck in dirt and do some engine damage.

On ordinary spark ignition cars, meaningful braking requires downshifting, so engine rpm goes up, possibly increasing engine wear more than the brake wear reduction, but I doubt that studies have been done to really quantify the costs. Now engine braking (via downshifting) is not used even in racing, so it is probably a moot point.

For economy, it is best not to brake at all, regardless of the method... unless it's via regen, but even then, coasting is the better alternative if traffic allows.

[KY_Canyon]

For hypermiling purposes engine braking is still inefficient when compared to coasting in neutral......could be better or worse than using normal brakes, but it's still a loss of kinetic energy. In a manual transmission, you can simply avoid it with use of the clutch, but for those of us with automatics it can make a huge difference. Engine braking is a safety feature, not an efficiency feature.

My 2005 Canyon Auto uses DFCO under certain conditions, and while more efficient than traditional injector on engine braking it is still much less efficient than neutral coasting. In it the DFCO is programed to kick off somewhere below 1800 RPM and the truck begins to coast (free wheel). So for me, coasting down hills at speeds above 60 or so causes the DFCO to kick on (which drives me crazy). If I'm at 58 MPH then the coasting is almost like driving a manual, but I never have to press a clutch. I'm hoping that the DFCO parameters can be adjusted with a custom PCM reprogramming. I think my MPG would improve significantly if the engine DFCO only operated above 2100 RPM or so.