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Comment by Scoundreller | Hacker News Reader

Automatics are still too dumb to keep the clutch engaged while braking.

Officially automatics and manuals are a wash for mileage, but I think driving style can make manuals the winner.

All that kinetic energy getting dissipated by brakes instead of turning the engine and all of its accessories in autos.

I’m quite surprised auto manufacturers haven’t implemented engine braking for émissions, reducing brake wear, and quicker « free » cabin heat in cold conditions.

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Automatics disengage the driveshaft to save wear on engine parts. It's an active decision, not being "too dumb". Wearing out brakes is cheaper to fix than wearing out your brakes, and people like longer lasting, cheaper to fix, cars.

I figured they disengaged because they can’t know if you’re wanting to coast or wanting to brake when you step off the accelerator.

If they always assumed you wanted to brake, that would really hurt mileage.

And it would have to disengage anyway at some point to avoid stalling.

Engine braking is more RPMs in aggregate, but nothing particularly bad for an engine.

Automatics don't use a traditional clutch for transmissions, so it wouldn't actually stall, but your other points could be true.

Replying to myself here because I can't edit it after someone has responded?

I meant wearing our brakes is cheaper to fix that engine parts.

> but I think driving style can make manuals the winner.

Everything you can do, a machine can do better.

In theory. But I don’t have a much faith in auto transmissions (effectively a mechanical computer that has to handle many scenarios, probably none perfectly).

My hybrid's auto actually does a pretty good job (2013 V60 D6; diesel front, electric rear axle). And it's a simple 6 gear box, not a fancy 8 or 11 gear twin clutch. Picks a good gear, no hysteresis and the overall control intelligence seems to properly decide when to enter neutral and recuperate (on wildly shifting loads [steep uphill slopes] that "overall intelligence" sometimes tends to be too aggressive with shutting down the diesel altogether, but that's not the transmissions fault).

Most importantly: Your gearbox works the same regardless of your state of mind, regardless of you caring about mpg or not, regardless of you being focused on driving or not. It will just deliver.

Sure, a good driver in the manual version of a car might achieve better mpg than the auto version, but that requires skill and focus and effort. And realistically, a human driver won't deliver 100% of the time, so the auto version will beat the manual version over the long haul, no matter the driver.

I've never understood why it seems meaningful that an automatic can get a tiny bit better mileage. There are a lot of ways to reduce fuel use and/or the overall environmental impact of car operation. Using public transport, biking or walking 1/n trips would accomplish the same goal, and n probably wouldn't have to be very low. There's also the life cycle considerations of manufacturing and disposal of cars. Blahdy blah.

The reason to drive a stick is to drive. Anyone who's never tried it, try it! You become an essential part of the experience, both hands and feet involved and synthesizing related tasks, while you look ahead, anticipate, plan, breathe. You don't need music or the phone or a fridge in the glove box. It doesn't have to be a sports car, anything with a stick will do.

We are in a privileged moment. How long ago was it that cars didn't even exist? How long since they have been really excellent, dynamic, safe(er), powerful, reliable? I'd argue late 80s to about now is the entire window. Already, 2/5 of our appendages are useless to the task, and soon it will be 5/5.

Smoke 'em if you've got 'em!

> I've never understood why it seems meaningful that an automatic can get a tiny bit better mileage.

It used to be an argument for stick shifts, that they had better mileage. When I got my license 7 years ago, that "fact" was part of the course. Eco-driving is a mandatory part of both the written test and the practical test, so the "correct" answer if you wanted to pass the test was that you should prefer a stick shift over an automatic, because of fuel economy. Also, if you do your practical test with an automatic, you get a mark on your license saying you are not allowed to drive manuals.

These days, it's factually wrong, so driving students get told wrong things, and spend time learning and doing useless bullshit, and get told to buy the wrong type of car. That pissed me off like no other.

> The reason to drive a stick is to drive.

I agree. That's an honest argument for stick shift. The feeling of driving one. Even though a modern automatic will accelerate faster than you can ever do driving a stick shift. Because everything you can do, a machine can do better...

If that was true, you wouldn't be behind the wheel at all.

The transmission can't see the road ahead and prepare in advance.

Whenever there's an article here on Hacker News about self-driving cars, do you just... skip over them? Your eyes glaze over, and you're not seeing that part of the technological landscape? What do you think all the computer vision tech is doing in self-driving cars?

Sorry, I'm not as enlightened as you. Which self driving car do you own?

Put an automatic in "low gear" (I still remember when it used to be called "grade retard" on some cars, which was to be used for braking only and not acceleration) and you'll get engine braking.

and quicker « free » cabin heat in cold conditions.

Engine braking won't heat the engine much over just idling, in fact it may even cool it off more because the governor will cut off fuel completely.

It’s a good question re:heat that seems to be unanswered. While it is compression and decompression, it’s not 100% efficient. And in a gas car, it’s releasing hot compressed air throughout the system, including the cat converter. I figured enough heat during compression would radiate to the engine. Plus more RPMs = more stuff warming up.

I mean, the energy has to go somewhere, and the engine itself makes the most sense.

We’re doing the opposite of acceleration (sometimes faster), so that’s gotta be a lot more energy turned into heat than necessary to keep an engine idle.

If brake pads get to several hundred degrees while braking...

Normal engine braking (and even the compression-release "Jake brakes" on diesels, while more effective) doesn't develop anywhere near as much force as the wheel brakes. You're right that it does generate heat, but it's not a lot --- instead of purely compressing and releasing the compressed air, the engine is just acting as an air pump; the air gets compressed (and hotter) during the compression stroke, but instead of igniting, what would be the power stroke merely expands the air again to the same volume it had before, and then the exhaust stroke pushes it out with little restriction. I would bet that even if it's just idling, the heat of combustion will be far greater than whatever friction losses contribute to engine braking.

Reciprocating mass (pistons, rods, and valves constantly changing direction) is where a great deal of the energy is sunk when engine braking. It doesn't all go to heat. Accessories are also doing useful work like the alternator charging the battery and the water pump circulating coolant. Friction is significant and increases with rpm and oil pressure, but you aren't heating it nearly as much as when it's burning fuel.

Naw, diesels do all of that too and their engine braking is minimally effective without modifications. Trucks will have "Jake brakes" installed, but diesel cars get minimal effects from engine braking.

Gas vehicles effectively engine brake without modification, but dunno where all the energy is getting converted.

I do wish alternators would detect engine braking and ramp up electric cabin heat though.

Oh yeah, diesels suck air all the time whereas gas engines are creating a strong vacuum, which is a lot of effort. They turn at higher rpm than diesels, too.

I think they do keep it engaged, at least while coasting.

I know you said braking but I've seen it in both my 2002 and 2015 automatics when coasting.

If I'm coasting up to a red light I'm pretty sure I can brake lightly and still be coasting with no fuel consumption, and then it will only turn the fuel back on when it needs to creep.

I'll check it on the live MPG display next time I'm driving.

Typical automatics have comfortably passed out manuals for mileage on average, some time in the 2010s. Driving style "can" theoretically still make a given manual more efficient, but it requires an unusual attention to mileage by the manual driver and/or an unusually aggressive driver on the automatic.

My 2005 car will use engine braking to control hill descent speed automatically, and will downshift in some cases to assist braking.

> Automatics are still too dumb to keep the clutch engaged while braking.

Automatics don't have a clutch though. They have a torque converter, which is not the same thing.

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