engine braking . . . how bad is it?

[ps2fixer]

Originally Posted by 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.

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.

[KY_Canyon]

Originally Posted by ps2fixer

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.

[ps2fixer]

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.