Trailing Throttle
 

A while ago the light bulb came on and I realized that 'pumping losses', a presumed shortcoming of the gasoline engine is what I call 'compression braking', which I find a very useful thing. Diesel guys have to add a kludge, Clessie Cummin's Jake brake, to get what I get for free.

Or is it free? I drive a carburated, 4-cycle four cylinder engine with manual transmission. No power steering/brakes/anything. I typically descend moderate hills (these days) with clutch in and key on (for safety and ignition of any gas from the idle jet). In the South hills of Bluegene, it's clutch out and floating the valves against a closed throttle.

A throttle plate doesn't close completely, does it? Nothing to be gained from being in neutral except pain, right?

In town I use a closed throttle to respond to lights and changes in traffic, while reserving the brake.

Educumate me, or argue amongst yourself; whichever. :)

Closing the throttle pulls highest vacuum on the idle circuit, causing more fuel to n be pulled through it st higher RPM.

ENGINE OFF neutral coasting is the best option.

Clutch disengaged, trans in gear is quicker start up, but added drag. Trans in neutral is best.

When I was getting my license, coasting in neutral was legal for cars but illegal for trucks. Made it sound like a safety issue. So I guess you poke the brake once in a while to make sure it's working?

The added drag is only engine side the transmission, right? The other side is moving with the differential.

I guess my question is, on a hill too steep to coast down is it better to use compression braking, or disengage the engine (however) and retard it with the service brake?

My vote is for air brakes, like the perforated dive brakes on an F-86. :)

The good part is, you don't need to have the engine running for anything of the above, as it simply isn't there.
So you might as well shut it down.

Give it a try on some deserted road, and see how you feel about it.

Put the transmission in neutral as well, then you don't need to constantly pull the clutchplates apart. The clutch system is made to be engaged, if you constantly put a load on the mechanism to disengage it with the clutch pedal, that'll increase wear over time. Which may or may not show up.

It also saves some drag in the gearbox.


That's engine braking .
Good when you need to stop or control downhill speed.
In other circumstances, it's just as if you're using the friction brakes : wasting energy.

Being in neutral on moderate hills means you'll coast down a lot further and/or faster than while in gear.

But the engine is idling unless you shut it down.

Even with an idling engine, you should see a benefit from coasting in neutral over engine braking - simply due to the extra distance covered by not braking.

If you know how many gallons/h your car uses, you can calculate what your minimal coasting speeds with the engine on will be before it starts using more gas (as you don't cover much distance /h when you're going slow). Back in the day, that figure was in some cars' manuals.

That's good, but try to go one step better and coast instead of engine braking.
It means you'll have to look and plan even further ahead, and keep your initial speed low(er) as you'll coast a lot further.

If you're OK with shutting the engine off, that'll give the very best fuel economy.

It's a case where legislation that was based on old technology has been passed by progress, but the legislation has remained ...

Old cars didn't behave as nicely as modern ones when going around corners without traction.
Brake failure was not unusual, and brakes were $h!t in general (like drum brakes all around ...) so they were wary of runaway cars.


Why would they not work on an unassisted brake circuit ?

Engine off coasting with power-assisted brakes, you don't want to check the brakes once in a while because it'll reduce your remaining vacuum and thus reduce brake assist.

In gear but with the clutch disengaged, the gears in the gearbox are spinning with the wheels, adding drag.

In that case, you stay in gear and use engine braking and proper gear selection to control your speed.
Ideally, the service brake is then used for emergencies only.

You can of course use it to shift down if the chosen gear proves too high.
Or to slow down a bit more for a blind corner.

The amount of friction braking I've seen in the Cali hills, was worrying to me.
I may be a flatlander, but I hardly touch the brakes going downhill ;)

There just aren't as many deserted roads around as there were when I was a teenager. :)

Now this is what I was looking for; the part that I hadn't considered. But frankly, I've flogged some cars pretty badly over the years and never had to replace a clutch. The #3 exhaust valve always goes sooner.

That was me being sarcastic (and paranoid).

Being a Vet, I've got the thousand-yard stare down pretty well, but there're lights on demand and lane changes to contend with.

So if my initial speed is low(er) how do I coast farther?

I always wondered about drivers ahead of me that would tap the brakes *for no reason at all*. Then I drove something with an automatic transmission and realized they are doing it to rein those ponies in.

The other way around--you will coast farther, so to get back to coasting the same distance you have to start coasting at a lower speed.

You coast further because there is less friction with the transmission out of gear. No compression braking, just tire-to-road, wheel bearing, and transmission internal friction. Which is a lot smaller than the friction from compression braking.

Make sense?

Oh, and on an old carbureted car, I would not want to turn the engine off while coasting. I'd be concerned that it might not re-start gracefully, and it might use quite a lot of fuel when it does.

-soD

Lower initial speed because you're less engine drag from compression braking.

If you do compression brake on a carb'd engine, TURN THE KEY ON. You don't want raw fuel cycling through the cylinders from the idle circuit, it will wash the cylinders and damage the engine over time.

If you aren't constantly tapping your brakes to make sure they're working while driving, why would you be worried about it with the engine off? (I get that it was a joke)

If you've got the transmission in gear and you're coasting engine off with the clutch depressed (disengaged), you're also spinning the input shaft and the clutch plate, which is only a few thou of an inch away from the pressure plate and flywheel. There's still some friction, some heat being generated from it, and it's extra rotating mass.

Leave it in neutral and take your foot off the clutch. It's less drag, and it's not like you can't stomp the pedal, put it in gear and pop the clutch out to save your backside in an OHWHATTHEHOLYMOTHER situation.

Getting the engine MOVING, whether there's ignition or not, will provide drag to create engine braking. Actually, being ON lessens the effect of engine braking slightly because that fuel is still igniting and creating cylinder pressure.

If you want to continue using engine braking, create a throttle bypass that allows air in under the carb and open it any time you want to engine brake. This will prevent vacuum from pulling fuel into the engine so you can leave it off, still be compression braking, and not damaging your engine or burning fuel.

Also, if your engine has an electric choke that's actuated by alternator current, there's a good chance it will close with the key off if you engine brake without the engine on... if it does, it's really PULLING fuel through all the circuits, not just the idle circuit.

Yes. I went back and read it again. More efficient at coast-down = start sooner.

I trust a bump start at highway speeds is pretty reliable (I don't do Pulse & Glide unless I'm about to run out of gas).

I could probably reliably switch off at long lights. This car doesn't have the hot start problem the last one had.

Christ -- I figure if the key's off, it's in neutral. Maybe when I get into the Coast range. Another reason to get a boat-tail.

For most of my driving, it's compression braking when I first see the light [change] to drop just enough speed to where I can coast, clutch in and key on, until I'm rolling up on the last car at the light just as it pulls away. It always feels good when that plan comes together.