engine braking . . . how bad is it?
 

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

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

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

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)

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.

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...

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.

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.

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.

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.

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.

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.

My corolla is almost the exact opposite to your canyon, it does DFCO if I let off the gas enough at almost any speed including NEUTRAL (very short time of course)! I think it uses the TPS and engine load/MAP to determine when it go into DFCO mode. Going 45mph, I can coast about a mile and still be moving ~25mph before my engine kicks in and my scan gauge says around 80mpg. If I stick it in neutral at that point, my mpg drops... I don't know why. Coasting in neutral for me gives maybe 15-20% further coast, but uses fuel, about 140mpg down to the 70s when it gets down to 25mph. For my case, I think DFCO is the better pick for me.

Yeah, some cars it works better on than others, and each manufacturer / model has different conditions. My problem is that even if I start coasting at 68 then DFCO kicks in and I may be getting a huge boost in MPG, but speed decreases at such a rapid rate that my coast down is way too short. My commute is about 75% on a 70 MPH parkway. So, this doesn't work well for me as I'd like to be able to coast freely at idle for a longer distance with less drastic speed reduction. I refuse to shift in and out of neutral on the road, so I have no option but to continue to give throttle going down hill at my parkway speeds or the engine braking will robe my kinetic energy. If I'm traveling on a road less than 60, then I can let off and coast. So, it's not so much that the system doesn't work, it's that my needs are slightly different.

Yesterday I tested Neutral coasting vs letting off the gas in gear on a couple small hills I know fairly well now.

Letting off the gas the scan gauge would go to around 90-100mpg
Coasting in neutral it went to 130-160mpg

In both cases, I think the distance with coasting were about the same. So for my setup neutral coasting down hills (no stops afterward) really is a benefit, since I don't have any that I can DFCO down with out loosing speed.

Before the last mods (lower grill block, passenger mirror fold in, and mud flap delete), I couldn't coast in neutral down the same hills with out loosing speed.


Another thing I just though of for the main topic, since my car DFCOs so easy, I think it would be a good example of how much engine wear there is from engine braking in that mode. Car currently has 231k miles original engine/trans. My 92 camry has ~303k but I only drove it about 45k miles and it don't go into DFCO very often (it didn't like to idle down while coasting). The camry is on the original engine/trans as well, I just changed cars because it is a low base line to start from (27mpg) and the corolla I got super cheap, $700 + $500 in parts and a little time.

The only thing I can think that would have more wear would be gears in the trans mission since coasting is more or less like going in reverse (opposite side of the gear wearing compaired to normal driving). My camry has a sloppy feeling when letting off the gas and then resuming speed, and I suspect it to be the trans/axles, corolla is very tight yet.